The speed of light (299,792,458 m/s) is not merely a speed limit but a fundamental structural feature of spacetime that ensures causality is preserved; as objects approach light speed, relativistic effects like time dilation, length contraction, and mass increase become extreme, requiring infinite energy to reach exactly c, and any faster-than-light travel would violate causality by allowing effects to precede causes, making it fundamentally impossible within our current understanding of physics.
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What Science Thinks Happens Beyond Light Speed | Kip Thorne本站添加:
Now I want to ask you to imagine something with me. Not a thought experiment yet, just an image. You are standing by the side of a highway in the Utah desert somewhere outside of Salt Lake City on a clear summer night. The air is dry and still, the kind of still that only exists in the American West.
And the sky above you is the most complete darkness you've ever been. in no light pollution, no haze, just stars so dense and so sharp that the Milky Way looks almost solid. You've pulled over because you wanted to see this and you're standing there looking up. Now, the light hitting your eyes right now, the light from those stars left its sources years ago, decades, centuries.
Some of those photons have been traveling since before anyone alive today was born. They crossed the incomprehensible emptiness between the stars and they arrived at your retinas at the same speed they departed. 300,000 km/s.
Not approximately exactly. Every photon from every star, regardless of how far it traveled, regardless of what the star was doing when it emitted the photon, arrived at your eyes at the same speed.
That fact that all light travels at the same speed in vacuum always for every observer regardless of how that observer is moving is perhaps the single most revolutionary physical fact ever discovered. It is the seed from which Einstein grew the special theory of relativity in 1905 working in that patent office in burn 26 years old with no formal academic position and no collaborators.
And it is the bedrock on which everything I want to tell you today is built. Because here is the question that I find genuinely thrilling after a lifetime of thinking about it. What happens when you approach that speed?
What happens to space, to time, to mass, to energy as a massive object gets closer and closer to light speed? And what happens or what would happen or what would the universe have to do if something crossed that threshold? The answer, it turns out, is not simply nothing can go that fast. End of story.
The answer is much stranger, much richer, much more philosophically vertigenous than that. The answer involves the structure of spaceime itself, the nature of causality, the possibility of particles that have never traveled slower than light, the theoretical gymnastics of warp drives and wormholes and the uh extraordinary energy conditions that any faster than light mechanism would require. And it involves a subtle but profound idea that I want you to carry with you through this entire discussion. The possibility that the speed of light is not just a speed limit but a structural feature of reality, a seam in the fabric of spaceime that if punctured would unravel something fundamental about the logic of cause and effect. But before we go there, let me build the scaffolding because you can't understand what it means to exceed light speed without first understanding in some depth what happens as you approach it. And what happens as you approach light speed is one of the most extraordinary things in all of physics. Let's start with something that on the face of it seems entirely reasonable. You're in a spacecraft. You fire your engines. You accelerate. You go faster. you fire them again. You go faster still. There's no obvious reason if you could keep firing long enough why you couldn't eventually reach any speed you wanted. That's the Newtonian picture, the physics of Newton and Galileo. And it is perfectly adequate for everything you encounter in ordinary life. at the speeds of cars, of airplanes, of the fastest rockets we've ever built, Newton's physics is so accurate that the corrections from Einstein's theory are completely negligible. But when you start moving at velocities that are a meaningful fraction of the speed of light, something happens that Newton's equations don't account for. The mass of a moving object, the quantity that resists acceleration that we measure in kilograms on a scale is not a fixed unchanging property of that object. It increases with velocity. Uh specifically, it increases according to a factor that physicists call the Lorent factor after the Dutch physicist Hendrickk Lorent who worked it out before Einstein gave it its full physical interpretation.
The lorren factor, I'll call it gamma, which is the symbol we use, is equal to 1 / the square roo<unk> of the quantity 1 minus the square of the velocity divided by the square of the speed of light. When your velocity is much smaller than the speed of light, gamma is essentially one and everything is Newtonian. When your velocity reaches 10% of the speed of light, gamma is about 1.005, 005 barely different from one. So the relativistic corrections are less than 1%. When you reach 50% of the speed of light, gamma is about 1.155.
At 90% of light speed, gamma is about 2.29.
At 99% of light speed, gamma is about 7.09.
At 99.9% of light speed, gamma is about 22.4.
And here is the critical thing. Your effective inertial mass, the quantity that resists acceleration, scales with gamma. At 99% of light speed, it takes seven times the force to accelerate you by any given amount compared to doing the same acceleration at low speed. At 99.9% of light speed, it takes 22 times the force. As you approach the speed of light, gamma approaches infinity. And the force required to accelerate you any further approaches infinity. Now energy tells the same story. The kinetic energy of a moving object, the energy associated with its motion in Newtonian physics is 1/2* mass time velocity squared. Simple clean scaling with the square of the velocity. In relativistic physics, the kinetic energy is the lorren factor minus1 times the rest mass time the speed of light squared. And as gamma blows up toward infinity as velocity approaches light speed, so does the kinetic energy. Let me put this in terms of the large hadron collider because it's a real world demonstration that removes any suspicion that this is merely theoretical.
The LHC accelerates protons to 99.999991% of the speed of light. The protons are moving so fast that from the perspective of someone standing in the tunnel, they complete the 27 kilometer ring more than 11,000 times per second. Each individual proton at this speed carries a kinetic energy of roughly 6.5 TEV 6.5 trillion electron volts. To give you a concrete sense of that energy, a proton has a rest mass corresponding to about 0.938 Gev. So the proton in the LHC is carrying kinetic energy about 7,000 times its own rest mass energy. To accelerate that one proton a particle with a mass of 1.67 * 10 -27 kg about 1 over 1,836.
The mass of an electron to that speed requires an amount of energy comparable to the kinetic energy of a flying mosquito. That sounds trivial, a mosquito's worth of energy per proton.
But there are roughly 300 trillion protons per beam in the LHC. And the collider runs two beams simultaneously.
And uh the engineering required to deliver and store and steer this energy is uh is one of the great technical achievements of civilization. And still still the protons are not at the speed of light. They are at 99.99991%.
That last 0.00009% is the wall. And that wall doesn't merely get harder to push through as you approach it. It becomes literally and mathematically infinitely hard. You would need infinite energy to accelerate any massive object, any object with mass greater than zero to exactly the speed of light. This is not a statement about our current technology. It is not a statement about the limits of what we can engineer or what materials we can build. It is a statement about the structure of spacetime.
The speed of light is not a speed record waiting to be broken with a better engine. It is a geometric property of the universe woven into the fabric of spaceime in the same sense that the ratio of a circle's circumference to its diameter is pi. not approximately pi, not pi in a certain limit, but exactly pi as a consequence of the definitions of the objects involved. Now, I want to stop here and mention something because this is a natural moment in the discussion. If what I'm describing, the deep geometry of spacetime, the way that Einstein's equations encode physical reality, the mathematical structure underlying these extraordinary phenomena, if all of this is capturing your curiosity and making you want to go deeper, then I want to point you towards something. The books, the hidden physics of Kip Thorne and the science of Kip Thorne, the link is in the description below. These are books that take the kind of physics I've spent my career working on, the geometry of spacetime, the behavior of light, the nature of mass and energy, and present them with the depth that they deserve at a level that builds genuine understanding rather than surface familiarity.
The mathematics underneath these ideas is beautiful and these books don't hide from it. They approach it in a way that makes it accessible. The things in those books are not taught in ordinary physics courses. They are the things that working physicists think about presented clearly enough that anyone with genuine curiosity can follow. I recommend them without reservation. Now back to the speed of light and to a question that I think is genuinely one of the most interesting in physics. If nothing with mass can reach light speed, what is the experience of something without mass?
What does a photon traveling at exactly C actually experience? And here is where we have to be very careful because the question is subtler than it appears. The Lorent factor gamma which appears in all the time dilation and length contraction formulas takes a specific value for any observer with a specific velocity. For an observer at rest relative to some reference frame, gamma is one. For an observer moving at 50% of light speed, gamma is about 1.155.
For an observer at 99.9% of light speed, gamma is about 22.4.
But for a photon traveling at exactly light speed, gamma is formally infinite.
And this means that the formulas for time dilation and length contraction give infinite results. Specifically, they say that in a photon's reference frame, zero time passes during the journey and the spatial distance contracted to zero. A photon crossing the galaxy does not experience the passage of thousands of years. From the photon's perspective, the journey is instantaneous.
From the photon's perspective, the source and the destination are at the same location. But and this is crucial.
We need to be careful about talking about the uh photon's perspective as if a photon has a reference frame in the ordinary sense. The mathematical machinery of special relativity breaks down when you try to construct a reference frame for an observer moving at exactly C. The transformation equations produce undefined quantities.
There's no consistent well-defined reference frame for a photon. When physicists say that a photon experiences zero time, they are taking a limit the behavior of the formulas as velocity approaches C. And noting that in this limit, the elapsed time on the moving clock goes to zero. But a photon doesn't have a clock. The statement is really a statement about the mathematical limit, not a statement about the photon's experience in any normal sense. This is actually a subtle and important point.
The theory of special relativity says that no massive object can travel at C, but it doesn't construct a physics for observers at C. It simply doesn't apply in that case. Photons are not observers in the relativistic sense. They are particles described by quantum field theory moving through curved spaceime described by general relativity, but they don't have rest frames. Now, here is where the story gets genuinely interesting because so far I've been describing the wall from one side from below. What happens as you approach light speed from the slow side? But there is a question that has been asked seriously by physicists, not just by science fiction writers, and it is this.
What if there were particles that started above the speed of light that had never been slow? that rather than being prevented from reaching sea from below, were permanently confined to velocities above sea, unable to slow down to the speed of light from above.
These hypothetical particles have a name. They are called tachons from the Greek word for swift and they were first proposed seriously as a theoretical possibility by Gerald Fineberg in a 1967 paper in physical review. The idea works like this. The Lorent factor gamma for a velocity above C becomes imaginary the square root of a negative number. This doesn't automatically make tachons impossible. It means that if tachons exist, their mass must also be imaginary. What physicists call an imaginary mass. So that the combination of imaginary mass and imaginary lorren factor produces a real momentum and a real energy. Mathematically tachons can be described consistently within a modified framework. But here is where tachons become genuinely problematic.
And I want to walk through this carefully because the problem is not merely technical. It strikes at the deepest questions about the structure of physical law. The issue is causality.
Causality is the principle that causes preede effects. That if A causes B, then A happens before B in every reference frame. It is the principle that prevents you from being killed before you are born. It is the principle that makes the idea of the grandfather paradox genuinely paradoxical rather than merely strange.
Now, in special relativity, the ordering of events in time depends on the reference frame you're using. Two events that are simultaneous in one reference frame can be non-simultaneous in a different reference frame. But there is a crucial protection if two events are connected by a signal traveling at or below the speed of light. If the signal from event A to event B is a timelike or lightlike signal, then the ordering of A and B is the same in every reference frame. There's no reference frame in which B happens before A. If A and B are connected by a subluminal signal, this is the protection of causality. The speed of light is the maximum speed of a causal influence of information propagation and this ensures that the causal ordering of events is preserved across all reference frames. But if tachons exist, if signals can travel faster than light, this protection collapses. There exists configurations of reference frames in which a tachon signal appears to arrive before it was sent. The receiver gets the message before the sender sends it not as a paradox of interpretation but as a direct consequence of the Lorent transformation applied to super luminal signals. Let me make this concrete with a thought experiment because I think the abstract statement doesn't fully convey the strangeness of what's being said.
Suppose you have a device that can send a tachon message. a message that travels say at 10 times the speed of light. You send a message to a distant space station 10 lighty years away. In your reference frame, the message takes one year to arrive 10 lighty years at 10 times light speed. Now consider the reference frame of an observer moving at high velocity relative to you. In that reference frame, due to the Lorent transformation of time, the events message sent and message received are not separated by one year. They are separated by a different interval. And for certain choices of relative velocity, the interval is negative. That is in the moving observer's frame, the message is received before it is sent.
Now the moving observer in their frame sends a tachion message back to you also at 10 times light speed. Due to the same effect, the Lorent transformation creating a reversal of time ordering in your reference frame. This return message arrives before you sent the original. You receive an answer before you ask the question. You can use this to send yourself information from the future. You can construct in principle a closed causal loop, a loop of cause and effect that circles back on itself in which the future influences the past.
This is the tachionic anti- telephone first analyzed by the physicists G A Benford DL Book and WA Nukem in 1970.
And it represents the most fundamental physical objection to faster than light communication. not merely that it's practically difficult that it is logically corrosive that it would undermine the coherence of the causal structure of physics. Now I want to be careful here because the objection is sometimes stated too strongly. The tachionic anti- telephone shows that tachons combined with special relativities lorren invariance leads to causality violation. But one could potentially escape this conclusion by modifying one of the assumptions. If tachons exist but there is a preferred reference frame an absolute frame relative to which the direction of tachon signals is defined then causality could potentially be preserved. The lorren invariance of the universe would be only approximate with small violations at high energies. Some physicists have explored this possibility seriously. It connects to modern theories of quantum gravity which suggests that at the plank scale at energies of about 10 to the 19 GEV far above what any accelerator can reach the space-time continuum might break down in ways that modify the Lorent transformation and potentially allow for a preferred reference frame. But there is currently no experimental evidence for such violations. And the best measurements of Lorenten variants using gamma rays from distant gammaray bursts for example place extraordinarily tight limits on any deviations. The conclusion as far as we can tell is that tachons either don't exist or they can't be used to send information. And if they can't be used to send information, they are physically undetectable. kind of ghost that passes through the universe without leaving a mark we can measure. Now, let me turn to something quite different to the idea that rather than moving through space faster than light, you might be able to get around the speed limit by moving space itself. This is the idea behind the Alcubier drive first proposed by the Mexican physicist Miguel Alcubier in 1994.
And I want to say immediately Alabir's paper is a serious scientific paper published in classical and quantum gravity solving Einstein's field equations exactly. This is not speculation. It is mathematics.
Question is what the mathematics means and what it requires. The key insight behind the alubier drive is this.
Special relativity prohibits massive objects from moving through space faster than light. But general relativity, the theory of gravity and curved spacetime says nothing about how fast space itself can move. The expansion of the universe, for instance, causes uh distant galaxies to recede from us at speeds that appear by certain measures to exceed the speed of light. This is not a violation of special relativity because the galaxies are not moving through space. Space itself is expanding carrying them with it. There is no speed limit on the expansion of space. Alcabier asks, "Could we engineer space-time geometry so that space is contracted in front of a spacecraft and expanded behind it, creating a bubble of space that moves forward at any speed carrying the spacecraft inside it, the spacecraft inside its local bubble of spacetime would be at rest relative to the bubble, it would feel no acceleration. It would experience no time dilation relative to the external universe because the warp bubble moves through the external space time frame at whatever speed you choose while the interior is locally flat. In principle, by making the contraction and expansion happen faster, you could make the bubble move arbitrarily faster, faster than any speed you choose, and the spacecraft inside it would appear to an external observer to be traveling faster than light. The mathematics works. Uh Al Kubier wrote down a specific metric, a specific description of the space-time geometry for which this is a valid solution to Einstein's field equations. The solution exists, but here is what it requires. And this is where the Alcubier drive transitions from mathematically valid to almost certainly physically impossible. To create the space-time geometry that Alcubier's drive requires, you need matter with negative energy density. Not just negative kinetic energy or negative potential energy, negative total energy density. Matter that when you place it in a scale has negative weight. matter that violates what physicists call the weak energy condition, the requirement satisfied by all known forms of matter, that energy density measured by any observer is non- negative. Exotic matter with negative energy density is not known to exist in any form that would be sufficient for a macroscopic warp bubble. There is a quantum mechanical effect the kazmir effect in which the quantum vacuum between two very closely spaced conducting plates has negative energy density. The effect has been measured. It is real. But the magnitude is extraordinarily small comparable to the energy density of a small number of photons per cubic meter. And it requires specific geometric configurations of conductors to maintain. To produce the negative energy density required for an alcoir bubble would require quantities of exotic matter that are by various estimates on the order of the mass energy of Jupiter but negative. We have no idea how to produce even milligrams of such material. When I said this problem was first described by Alubier, I don't mean he hid it. He he acknowledged it explicitly in the original paper. He noted that negative energy densities violate the known energy conditions. He noted that this was a serious problem. He presented the result as a mathematical possibility, not as an engineering blueprint.
Subsequent work has refined the estimates. In the late 1990s and 2000s, Harold White at NASA's Johnson Space Center worked on modifications to the Alcubier metric that might reduce the energy requirements, changing the shape of the warp bubble from a sphere to a Taurus, for example, or working with oscillating bubble configurations.
Some of these modifications did on paper reduce the energy requirements substantially but substantially here means reducing from the mass energy of Jupiter to the mass energy of something smaller and the exotic matter requirement remains. No known physical mechanism produces the required negative energy density at macroscopic scales. There is also a separate and very serious problem. To create an alcubier warp bubble, you need to arrange matter in a specific configuration outside the bubble in the region of spaceime that the bubble is about to move through. But if the bubble is moving faster than light, no signal from inside the bubble can reach the region ahead of it in time to arrange anything. The interior of the bubble is causally disconnected from its leading edge. An observer inside an Alcub drive couldn't steer it, turn it on, or turn it off because none of their signals can propagate forward through the warp bubble at a speed sufficient to affect the geometry ahead. This is a profound practical problem. In addition to the exotic matter problem, even if you could solve the energy requirement, you couldn't control the device. And then there is the causality problem again. In 1994, the physicists Alan Everett and others showed that if an alcubier drive could be built, it could be used to construct a time machine. The argument follows the same logic as the tachionic anti-elephone. Two warp bubbles carefully arranged with appropriate relative motions can produce a closed timelike curve, a path through spaceime that circles back to its starting point.
An alcoir drive doesn't just allow faster than light travel. It allows time travel to the past. And here I think we reach the deepest thing I want to say about faster than light travel. Not the energy requirements, not the engineering difficulties, not even the causal paradoxes, though all of these matter.
The deepest thing is what this analysis suggests about the structure of physical law. But before I go there, let me pause again because this is the 20 minute mark. And I want to mention something that I think will genuinely serve you if this topic has got your mind working.
The books, the hidden physics of Kip Thorne and the science of Kip Thorne, the link is in the description. These are books about the deep physics that underlies everything we're discussing.
The geometry of spaceime, the meaning of causality, the structure of Einstein's field equations, the nature of energy conditions, and why they matter. What makes these books different from ordinary popular science is that they don't just tell you the results, they take you through the reasoning. They give you the scaffolding to understand not just what physicists have found, but why they found it and why it matters.
These books teach things that are almost never taught outside of graduate programs in physics. If you're the kind of person who finds yourself wanting to go deeper, and if you're still watching at the 20 minute mark, you probably are, then these are for you. The link is in the description. Now, the deepest thing, there is a pattern in physics that when you follow the argument long enough reveals itself as something more than a pattern. It is a theme. The universe seems to be structured in a way that protects causality.
Specifically, every time physicists have found a seemingly consistent mathematical framework that allows faster than light communication or travel, closer examination has revealed either that the framework requires unphysical matter or that the mechanism actually prohibits the transfer of information or that the mechanism contains a hidden flaw that removes the faster than light capability when correctly analyzed.
Let me give you several examples. The first quantum entanglement. You've probably heard that entangled particles are connected across arbitrary distances. That measuring one particle instantaneously affects the other no matter how far away. This is sometimes described as spooky action at a distance. Einstein's famous phrase. And it sounds like faster than light communication. It is not. When you measure an entangled particle, you get a random outcome. The particle in a distant location also gives a random outcome. And the two outcomes are correlated. That's the entanglement. But you cannot choose what outcome you get.
You cannot control the result of your measurement. And because you can't control your outcome, you cannot encode information in it. The correlation is only apparent when the two observers compare their results through ordinary sublight speed communication.
Entanglement does not allow faster than light signaling. This has been proved mathematically the no communication theorem and confirmed by every experiment. The second, the Hartman effect. In quantum mechanics, particles can tunnel through energy barriers. the famous quantum tunneling that underlies the operation of transistors and nuclear fusion in stars. And it turns out that quantum tunneling through a barrier can in some sense happen faster than the time it would take the particle to traverse the same distance at the speed of light. Experiments have measured tunneling times that appear by certain definitions to be super lumininal. Does this allow faster than light communication? No. The peak of a wave packet can move through a barrier in a time that appears to exceed C. But the wave packet is reshaped in the process.
The leading edge of the wave packet, which contains the information that a signal is coming, does not exceed C. The causally significant front, the boundary between before the signal and after the signal does not travel faster than light. The third phase velocity versus group velocity. Light waves in a medium can have a phase velocity. The speed at which the peaks and troughs of the wave move that exceeds C. In some exotic media, even the apparent group velocity can exceed C. This has been observed experimentally with laser pulses appearing to travel faster than light through certain specially prepared materials. Does this allow faster than light communication? No.
The signal velocity, the speed at which the front of the pulse propagates, never exceeds C, even when the peak of the pulse appears to move faster. The information encoded in the pulse cannot propagate faster than light. In every case, entanglement, tunneling, unusual media, phase velocity, anomalies, the universe seems to find a way to ensure that the causally significant quantity, the speed at which information actually propagates never exceeds C. There is always a subtlety, a correction, a distinction that preserves the causal ordering of events. Always. John Wheeler, who was my mentor at Princeton, one of the most creative theoretical physicists of the 20th century, the man who coined the term black hole and who had perhaps the most profound intuition about the relationship between physics and information. Wheeler used to talk about what he called the boundary of a boundary is uh zero. Uh uh it was his way of pointing to a recurring theme in physics that deep constraints on physical laws emerge from consistency requirements um from the requirement that the mathematical structure of the theory be internally coherent. The speed of light in this view is not just an arbitrary speed limit imposed by nature. It is a consequence of uh the requirement that physics be consistent, that the description of the universe by different observers be reconcilable, that cause and effect be preserved, that the laws of physics have the same form regardless of the reference frame you're using.
This is what makes the speed of light feel to me more like a feature of the logic of reality than a contingent fact about the universe. It is not this universe happens to have a maximum speed of 299,792,458 m/s.
It is any universe in which the laws of physics are the same for all inertial observers must have a maximum speed for the propagation of causal influences.
The Lorent symmetry of spacetime, the symmetry that relates the descriptions of physics in different inertial frames, logically implies the existence of a maximum signal speed. The specific value of that speed is the speed of light because light is the thing that travels at maximum speed. Take a moment to let that sink in. The speed of light is not fast because of some accident of nature.
It is maximum because of the logical structure of a universe with Lorent symmetric physics and Lorent symmetry is not an assumption made for convenience.
It is a principle supported by every precision experiment ever done from atomic clocks and satellites to the gyroscopes of gravity probe B to the collisions in the Large Hadron Collider.
Now let me turn to something that might be the most honest and interesting part of this entire discussion. which is what don't we know? Because everything I've said so far has been framed within special relativity and general relativity. The physics that has been experimentally confirmed to extraordinary precision. But these these theories uh as magnificent as they are are not complete. They do not incorporate quantum mechanics in a fully consistent way. They break down at the plank scale at energies of about 10^ the 19 Gev at distant scales of about 10^ the -35 m where the quantum fluctuations of spacetime itself become important. At those scales the classical picture of spacetime as a smooth continuum is expected to break down and some theory of quantum gravity must take over. We don't have that theory. It is the central open problem of fundamental physics. String theory, loop quantum gravity, causal dynamical triangulations, entropic gravity. These are all serious proposals, all with active research communities. None of them complete or definitively confirmed by experiment. And here is what is relevant to our discussion. Some approaches to quantum gravity suggest that at the plank scale Laurent symmetry might be violated. The maximum speed might not be exactly C for all particles at all energies. Uh there might be tiny uh energy dependent corrections to the speed of light that would be unmeasurable at ordinary energies but would become significant near the plank scale. This is called Lorent invariance violation or LIIV and it is actively tested by astronomers. When a gammaray burst occurs billions of light years away, a cataclysmic explosion producing photons across a wide range of energies.
The photons at different energies should if Lorent symmetry is exact arrive at Earth simultaneously, having been emitted at the same time by the same explosion. If there is any energy dependent variation in photon speed, neither energy photons might arrive slightly before or after the lower energy ones.
Measurements of gammaray bursts by the fermy gammaray space telescope have set extraordinarily tight limits on such energy dependent velocity variations.
The current constraints say that any Lawrence invariance violation at the plank scale must be so small as to be essentially negligible at any energies we can currently probe. This doesn't prove that Lorent symmetry is exact at all scales. We can't measure to plank scale precision, but it pushes any violation to a level that would be completely irrelevant for macroscopic faster than light travel. There is also the question of wormholes.
Wormholes are solutions to Einstein's field equations, tunnels through spaceime connecting distant regions that have been studied seriously by physicists for decades. The most famous analysis of traversible wormholes was done by Michael Morris, Kip Thorne, and Olvie Yurtzver in 1988. We specifically asked what would a traversible wormhole require? And the answer again was exotic matter negative energy density at the wormhole's throat. The same culprit as the alcubier drive and for the same reason the wormhole could be used for faster than light travel which means it could be used for time travel which means it poses the same causality problems. I want to say something honest about this because the analysis came from my own work. When Morris Yurtzver and I wrote that paper, we were not advocating for wormholes as a practical technology.
We were exploring the logical consequences of general relativity taken seriously asking what the theory itself permits and what it prohibits. The conclusion was that traversible wormholes uh if they could exist would require exotic matter and would create time machines. Whether they can exist, whether the required exotic matter can be produced in sufficient quantities remains an open question. One of the most interesting recent developments in this area is the connection between wormholes and quantum entanglement.
sometimes summarized by the equation ER equals EPR where ER refers to Einstein Rosen bridges wormholes and EPR refers to the Einstein Podilski Rosen paper on quantum entanglement. This connection first proposed seriously by Juan Maldescina and Leonard Suskin in 2013 suggests that entangled particles might be connected by microscopic wormholes.
that quantum entanglement and spatial connection through wormholes might be two descriptions of the same phenomenon.
This idea has generated enormous excitement in the theoretical physics community and is being actively explored. But I want to be clear about what this does and doesn't imply for faster than light travel. The microscopic wormholes in the ER equals EPR picture are not traversible. You cannot send information through them.
They do not allow faster than light signaling. They are a mathematical description of quantum correlations, not a transportation system. The no communication theorem still applies. So where does all of this leave us? It leaves us in a position that is, I think, genuinely wonderful if you approach it with the right frame of mind. On the one hand, everything we know about physics says that faster than light travel and communication in the ways we have explored them run into extraordinary and possibly insurmountable barriers. The energy requirements for warp drives uh are essentially unlimited. The exotic matter requirements are beyond any known physical mechanism. The causality problems are deep and fundamental. The universe seems with each passing decade of increasingly precise experiment to be more Lorent symmetric, more committed to the maximum signal speed of C, more insistent on preserving causal order. On the other hand, we know that our theories are incomplete. We know that at the plunk scale, physics is different in ways we don't yet understand. We know that the connection between quantum mechanics and gravity is one of the deepest unsolved problems in science and that the resolution of that problem may reveal structures and possibilities that we currently have no way to anticipate.
The history of physics is full of examples where a seemingly absolute limitation, the impossibility of heavier than air flight, the impossibility of splitting the atom turned out to be a limitation of current models rather than a limitation of reality. I don't know whether faster than light travel will ever be possible. I genuinely don't know. I have my opinion shaped by decades of working with the equations, by knowing where the problems are most severe and most likely to be genuine rather than merely technical. But I hold those opinions with appropriate humility. The universe has surprised me before. It has surprised better physicists than me. What I know with confidence is this. If faster than light travel ever becomes possible, it will require either that some of our most well-confirmed physical principles, Laurent invariance, the energy conditions, the causal structure of spaceime are wrong in ways that current experiments cannot detect or that some genuinely new physics operating at scales we have not yet accessed produces possibilities that are not visible from within our current theoretical framework. And that is, I think, a genuinely exciting place to be. Not a dead end, an open horizon with serious obstacles between us and it, but visible in outline, a place where the questions are real and the answers are not yet known. Now, let me spend some time on something that is both specific and I think underappreciated.
the question of what actually happens to the experience of time as you approach light speed because I've given you the mathematical picture gamma blowing up time on the moving clock running slower and slower. But I want to try to convey what this means at the level of human experience using the kind of concrete scenario that I think makes the physics feel real. Suppose you board a spacecraft capable of sustained high acceleration let's say 1g. the same acceleration as gravity at Earth's surface, which is coincidentally the most comfortable for human beings because it exactly mimics the feeling of standing on the ground. At 1g of acceleration, you would reach 50% of the speed of light in about 7 months. You would reach 90% of the speed of light in about a year and 3 months. You would reach 99.99% of the speed of light in about two years. Now the key feature is what happens to the clocks. Your clock on the spacecraft keeps ticking at its normal rate. From your perspective, you are experiencing time normally. You're getting older. You're having meals.
You're sleeping. You're watching the instrument panels. Time feels ordinary inside the spacecraft. But out there in the external universe, time is passing at a different rate. From the reference frame of a stationary observer, your clock is running slow. The faster you move, the slower your clock runs relative to theirs. By the time you've been accelerating for a year at 1G, reaching 99.99% of light speed, the time that has passed externally is enormously greater than the time that has passed on your clock.
The factor is exactly the Lorent factor.
Gamma at 99.99% of C gamma is about 70. So for every year that passes on your spacecraft, 70 years have passed on Earth. This is not a weird effect on measuring instruments.
It is not a trick of perception. It is a genuine difference in the amount of aging that occurs. If you return to Earth after a round trip in which your clocks measure 10 years and gamma is about 70 for most of the journey, then 700 years have passed on Earth. The people you left behind are dead. Their grandchildren's grandchildren are old.
The world has changed beyond recognition. This is the twin paradox and it is not a paradox at all. It is a genuine uh experimentally confirmed feature of relativity. The asymmetry between the two twins, one stays on Earth, one travels and returns, is real.
And it is produced by the acceleration that the traveling twin undergoes.
That asymmetry is what allows us to say without contradiction that the traveling twin aged less. In 1971, physicists Joseph Haleley and Richard Keading flew atomic clocks around the world on commercial airplanes and compared them to stationary atomic clocks. The clocks on the planes moving faster than the ground clocks were indeed slower, exactly by the amount predicted by special relativity. In 1976, the Gravity Probe A experiment measured the slowing of an atomic clock on a rocket compared to one on the ground, confirming the combined effects of special and general relativity at the level of one part in 100,000. Every GPS satellite in orbit carries atomic clocks that must be corrected for both the speed of the satellite, which slows the clocks, special relativity, and its altitude, which speeds the clocks.
General relativity. Without these corrections, GPS would accumulate errors of kilometers per day. The corrections work. The physics is right. And the physics says, approach light speed and time runs slow. Approach it closely enough and you can cross the galaxy in what feels like years while millennia pass on Earth. The time dilation is real. The universe is generous in its way with what it allows for massive objects that are willing to move fast.
What it is not generous with is the energy required to do so. Now, let me close by returning to the most fundamental question. Why does the speed of light have the specific value it has?
Why 299,792,458 m/s rather than some other number? Uh the answer I think is is one of the most important in in physics. The speed of light in meters/s doesn't have a deep significance.
It has the value it does because of how we define meters and seconds because of the arbitrary conventions of our measurement systems. What has deep significance is the dimensionless relationship between C and other fundamental constants, between C and the fine structure constant, between C and plank's constant, between C and Newton's gravitational constant. These dimensionless ratios, pure numbers independent of any choice of units, are the things that a complete theory of physics would predict from first principles. We don't have that theory yet. We don't know why these dimensionless constants have the values they do. The question of why the universe has these specific laws, these specific constants, this specific speed limit. This is the question of the fine-tuning problem. And it remains one of the deepest open questions in fundamental physics. But I want to say something about it that I find personally compelling after a lifetime of working with these equations. The speed of light, whatever its ultimate explanation, appears to be precisely the value it needs to be for the universe to have the structure that allows complexity, that allows chemistry, that allows stars and planets and life. If C were significantly different, if the ratio of C to other fundamental constants were significantly different, the physics of atoms, the stability of matter, the fusion reactions in stars would all change in ways that would make the universe unrecognizable, not necessarily unable to be interesting. We don't have the theoretical framework to explore all possibilities but different in ways that our understanding suggests would be catastrophic for the kind of complexity that we observe. Whether this is coincidence or selection or the consequence of some deeper principle we haven't yet discovered. I don't know.
But it is a question worth sitting with.
The speed of light is not just a number.
It is the maximum speed of causality itself. It is the speed of the universe's clock, the rate at which changes propagate from one place to another. It is the seam that holds the fabric of spaceime together. The thread that makes it possible for different observers in different states of motion to share a coherent description of the same reality. We will not cross it. We have no evidence that anything has crossed it. Every attempt to find a route around it has led back to the same set of obstacles, exotic matter, infinite energy, causal paradoxes, the fundamental protection of cause and effect. And yet the exploration of the question has not been wasted. In asking what happens at and beyond the speed of light, we have learned the geometry of spacetime, the nature of time dilation, the physics of wormholes and warp drives, the connection between quantum entanglement and uh spatial geometry. We have been led step by step into the deepest questions in physics. questions whose answers will require a theory of quantum gravity that we don't yet have.
The speed of light is the universe's most important number. Not because nothing can exceed it, but because asking what would happen if something did has revealed layer by layer the most extraordinary architecture in existence.
the highway in the Utah desert, the stars overhead, uh the light that left them years or centuries ago, arriving at your eyes at exactly 299,792,458 m/s.
Not because nature was lazy, because that speed is what holds the universe together. And the universe, it turns out, takes that job seriously. Now I want to take you into something that I think is genuinely underexplored in public discussions of this topic. The specific phenomenology of what a universe with faster than light travel would actually look like. Not the engineering of how you do it, but the physics of what you'd observe. Because I think working through the observational consequences is one of the best ways to understand why the causal structure of spaceime is so profoundly important. Let me start with uh something simple.
Imagine you're watching a spacecraft that somehow never mind how is traveling at twice the speed of light moving directly away from you. What do you see?
Here is the remarkable thing. You see nothing for a while. The spacecraft emits light in all directions as it moves. But the light it emits in the backward direction toward you is moving toward you at C while the spacecraft itself is moving away from you at 2C. So the spacecraft is outrunning its own light. The light it emits at time zero reaches you at time t. The light it emits at time t doesn't reach you until time 3t2 because it has to cover the extra distance the spacecraft created during that interval. The spacecraft is moved away by 2 C * T, but the light only gains on it at the speed of light minus 2C and it can't gain because the spacecraft is faster, the result is that you would see nothing for a while. And then when the spacecraft eventually decelerates below C or if it fires a strong enough beam back toward you, you would start seeing the images but in reverse order. The last image the spacecraft transmitted would reach you first. The first image would reach you last. The sequence of observations would be temporally inverted relative to the actual sequence of events. This is deeply important for understanding what it would mean to observe faster than light motion. The very concept of before and after in the observational record becomes scrambled. And this is not a limitation of our instruments. It is a consequence of the causal structure of spacetime. Information propagates at C.
If the object generating the information moves faster than C, the information arrives out of order. Now, let me go further and think about what you would see if the spacecraft came toward you at twice the speed of light. In this case, the spacecraft is approaching faster than the light it's emitting. So the light it emits when it's at distance 2 C * T reaches you at time T. The light it emits at time t2 when it's at distance C * T also reaches you at time T because that light only has to travel C * T. So two different photons emitted at different times arrive simultaneously.
What you would see is the spacecraft at two different positions simultaneously, a kind of visual echo. And the order in which you see events happening on the spacecraft would be reversed from the order they actually occurred. The spacecraft would appear to be running backward actions happening before their causes in your observation of the events. This is not a bug. It is one of the most direct physical consequences of faster than light travel and why causality concerns are not merely philosophical. If you could actually observe a faster than light spacecraft, your observation would show you would literally show you in your instruments events happening in the wrong causal order. The universe would look broken.
not in a confusing way, in a specifically, measurably, experimentally wrong way. And the reason this matters is that it connects directly to the grandfather paradox. If you can see effects preceding causes and if you can interact with what you see, you can intervene to prevent the cause. you have your time machine and you can use it to create closed causal loops uh situations where the future determines the past in ways that are logically contradictory.
Now there is a response to this that I want to address directly because it comes up regularly and represents a genuine attempt to find a way out of the causality problem. The response is, couldn't we simply choose not to use faster than light travel to create causal paradoxes? Couldn't there be a law of physics or a law of logic that prevents the paradoxical use of the technology without preventing the technology itself? Uh the philosophical literature calls this the Novikov self-consistency principle after the Russian physicist Igor Novikov who formalized it. The principle says that the only events that can occur are those that are self-consistent. That any closed causal loop that exists must be logically consistent.
So that um events in the loop don't contradict each other. In other words, if you travel back in time, you cannot change anything that didn't happen in the original timeline. The timeline is fixed. you are constrained to do exactly what you actually did. This is a self-consistent solution to the causality problem and it is logically coherent, but it has a troubling consequence. It removes free will from the picture entirely, at least within the causal loop. The events in the loop are fixed by the requirement of consistency. You cannot choose to do otherwise. If you could, the loop would be inconsistent.
And there is a more fundamental objection. The self-consistency principle doesn't follow from any known physical law. It is an additional assumption made specifically to prevent causality violation.
If we have to add extra assumptions beyond the normal laws of physics to prevent a technology from creating paradoxes, those extra assumptions are themselves suspicious. They suggest that the technology is in tension with the underlying physics and that making it work requires patching the theory in ways that wouldn't be necessary if the if the theory were on solid ground. John Wheeler had a beautiful way of putting things like this. He would say that physical theories when they are right are self-contained. They don't require external patching or additional consistency requirements to prevent them from generating nonsense. When a theory generates nonsense that requires external constraints to prevent, the theory is telling you something, it is telling you that it has been pushed outside its domain of validity or that the phenomenon it predicts doesn't actually occur. The the causality problems with faster than light travel are, I think, precisely this kind of signal. They are they are the theory's way of saying this doesn't happen. Not because anything prevents it from a technical standpoint, but because a universe in which it happened would be logically incoherent in a deep way. Let me now spend some time on something that is in a very specific way the most hopeful and the most realistic scenario for something like faster than light travel. Not warp bubbles, not tachion, something more subtle, more firmly rooted in physics we actually understand and more amenable to genuine scientific investigation. I am talking about the expansion of the universe itself. Here is a fact that is genuinely extraordinary and that I think most people who are interested in space don't fully absorb. The universe is expanding not just in the sense that galaxies are moving apart from each other in the deeper sense that the space between galaxies is literally increasing. The fabric of spaceime is stretching and the rate of this stretching multiplied by the distance gives a recession speed.
For galaxies that are far enough away, this recession speed exceeds the speed of light. There is a distance about 14 billion lightyear called the Hubble radius beyond which the recession velocity equals the speed of light.
Galaxies beyond the Hubble radius are receding from us at speeds greater than light. And this is not a violation of special relativity because the galaxies are not moving through space. Space itself is expanding. The local physics at any point measured by a local observer never exceeds C. This means that there are galaxies in the universe that are right now receding from us faster than light. We can still see them because the light they emitted billions of years ago when they were closer has been traveling toward us and has managed to reach us. But the light they are emitting right now today is being carried away from us by the expansion faster than it can travel toward us. It will never reach us. We have already seen the last light we will ever see from those galaxies. They are in the technical sense beyond our cosmic event horizon. The observable universe, the sphere of spaceime from which light has had time to reach us since the big bang is therefore not the same as the total universe. Beyond the observable universe, there is more universe, much more, possibly infinite, from which we will never receive any information because the expansion is carrying it away too fast. And here is the genuinely provocative thought. If the expansion of the universe can carry matter faster than light, is there a way to harness this to create a local version of cosmic expansion that moves a spacecraft? This is in essence the idea behind the Alcubier drive using a local distortion of spaceime analogous to the expansion of the universe to move a spacecraft at arbitrarily high apparent speed. The problem as I've described is the exotic matter required to create and maintain such a distortion. But here is what I find genuinely interesting about the expansion of the universe. In this context, the expansion is not driven by exotic matter with negative energy density. In the normal sense, it is driven by what physicists call dark energy, a property of the vacuum of spacetime itself, a kind of background energy density that exerts a negative pressure. The cosmological constant, which Einstein introduced in his equations and later called his greatest blunder, but which we now know to be nonzero based on the observed acceleration of the universe's expansion. Dark energy is real. We can measure its effects. The universe is accelerating its expansion because of it and it has negative pressure, not negative energy density, but negative pressure, which is a different thing.
Whether the energy conditions that rule out exotic matter for warp drives also rule out the kind of dark energy that drives cosmic expansion is a subtle and not entirely resolved question in general relativity. I am not saying this means warp drives are possible. I am saying that the connection between dark energy, cosmic expansion, and the theoretical requirements for warp drives is not fully worked out and that there may be things to learn in this area that we haven't yet learned. The research is ongoing. Let me end this section by describing what I think is the correct attitude toward faster than light travel. As a physicist who has thought carefully about it for a long time, the evidence strongly suggests that faster than light communication and travel in any form we have been able to analyze either violates causality or requires matter that doesn't exist. The speed of light as a maximum for the propagation of causal influences appears to be a fundamental feature of Lorent symmetric spacetime and every precision experiment confirms Lorent symmetry to uh extraordinary accuracy. At the same time our theories are incomplete. We don't have quantum gravity. We don't fully understand dark energy. We don't know whether the exotic matter requirements can be circumvented by some mechanism we haven't yet conceived. The history of physics is full of examples of barriers that turned out to be limitations of current theory rather than limitations of reality. The appropriate response to this situation is not certainty in either direction. It is not faster than light travel is impossible. Full stop.
We don't have a proof of that. And the incompleteness of our theories makes such certainty premature. It is also not faster than light travel will be possible eventually. Science always finds a way that is wishful thinking, not physics. The appropriate response is to continue doing the physics to probe Lorent's symmetry at higher energies to develop the theory of quantum gravity and understand what it says about the structure of spaceime at the plank scale. To think carefully about the energy conditions and whether any known or theoretically motivated physical process can violate them. To ask rigorously what the universe actually permits. And in the meantime to appreciate what we do know that the universe is built with a maximum speed built into its geometry. That this maximum speed is the same for all observers.
That this seemingly simple fact implies time dilation and length contraction and the equivalence of mass and energy and the curvature of spaceime by gravity and all the extraordinary richness of relativistic physics. that the speed of light is not a limitation on our ambitions. It is a window into the architecture of reality. The Utah desert on a clear night. Stars so dense the Milky Way looks solid. Every photon reaching your eye at exactly the same speed it departed. Regardless of what the star was doing, regardless of how you or the star is moving, that is the universe telling you as clearly as it can tell anything what it is made of.
Not matter, not energy, relationships, structures, the geometry of spacetime described by equations that Einstein worked out in a patent office in 1905 that have been confirmed by every experiment since. The speed of light is the thread that holds those equations together. And the universe, it turns out, is very serious about keeping it whole. What lies beyond light speed is not another country. It is the edge of the map. And the edge of every good map has written on it. In the tradition of the great cgraphers, here be dragons.
But dragons, in my experience, are not always what they first appear. And the map is still being drawn.
Now I want to talk about something that I find genuinely beautiful in the way that only deep physics can be beautiful and that is the connection between the speed of light and the nature of information itself because I don't think this connection gets sufficient attention in the usual discussions and I think it's central to understanding why C is not just a speed limit but something more fundamental. Let me start with a question that seems simple but isn't. What is information? Not in the colloquial sense, not the contents of a newspaper or the data on a hard drive.
In the physical sense, information as physicists understand it today, particularly in the wake of Claude Shannon's work in the 1940s and the subsequent development of quantum information theory is a physical quantity. It has units. It obeys conservation laws. It cannot be destroyed, only transformed. The connection between information and physics was made vivid by John Wheeler who spent the last decades of his long career developing what he called it from bit. The idea that the fundamental substrate of reality is not matter or energy but information. That every physical entity, every particle, every field derives its existence from the answers to binary yes or no questions.
It's a radical idea and it remains controversial, but it points to something that has become increasingly clear in modern physics. Information is not merely a description of physical systems. It is a physical thing with physical properties subject to physical laws. And one of the most fundamental physical laws that information obeys is this. It cannot be transmitted faster than light. This is more than just a restatement of what I've already said.
Let me say why. In quantum mechanics, there is a phenomenon that uh initially appears to allow instantaneous information transfer. When you measure a quantum system, say a particle spin, the act of measurement collapses the quantum state. If the particle is entangled with another particle far away, the distant particle's quantum state also changes instantaneously, not after a delay. Instantaneously.
This instantaneous change across arbitrary distances appears at first glance to be exactly the faster than light influence that special relativity prohibits. But the no communication theorem which I mentioned earlier shows that this instantaneous correlation cannot be used to transmit information.
The key is randomness. When you measure your particle, you get a random outcome.
You cannot choose what outcome you get.
And because you can't control the outcome, you can't encode a message in it. The instantaneous collapse of the distant particle state is correlated with your outcome, but only in a way that the distant observer can verify by comparing notes with you through ordinary sub lighteed communication. The universe seems to have arranged things so that the correlations that entanglement creates are precisely as strong as they can be, subject to the constraint that they cannot be used to send information faster than light. This is not a coincidence. It is a consequence of a deep principle called relativistic causality. The requirement that the laws of physics when expressed in terms of relativistic quantum field theory cannot produce super luminal information transfer. And this requirement is baked into the mathematical structure of quantum field theory from the outset through what are called the commutation relations of field operators at space-like separated points. In other words, quantum mechanics was built from the ground up to be consistent with special relativity. The two theories are not intention on this point. They agree deeply and precisely that information cannot travel faster than C. This is the key insight. The speed of light is not just the speed of photons. It is the speed of causality. It is the maximum rate at which any change in one part of the universe can influence another part.
It is the speed at which the news of any event propagates through the fabric of spacetime. And here is a consequence that I find genuinely striking. The universe is not fully connected at any instant. When we say the state of the universe at this moment, we are using a concept that is fundamentally observer dependent. Different observers moving at different velocities disagree about what now means for any two events at different locations.
There is no single observer independent before or after unless one of the events is in the causal past of the other.
Unless a light signal from one could reach the other. The universe is a collection of causally connected events.
But the causal connections are local.
What happens here can influence what happens there only after a delay of at least the light travel time between the two locations. The universe is built in patches in light cones and the global structure of spaceime emerges from the relationships between these local patches. Remove the speed of light limit and you remove the local structure. You replace causal order with chaos. You replace a coherent physical law with a situation where any event can potentially influence any other event at any time. This is why the speed of light is not a rule someone decided to impose.
It is a structural necessity of any physical universe in which the laws are the same for all observers and in which cause precedes effect. Without it, physics in any form we currently know collapses. Uh, I want to tell you about a specific experiment that I think makes this vivid in a way that mathematics alone can't. It's an experiment that has been done in many variations since the 1970s, testing uh what are called Bell's inequalities, inequalities derived by the Irish physicist John Bell in 1964, whose violation confirms the quantum mechanical prediction of entanglement and rules out so-called local hidden variable theories. The basic setup is this. You create pairs of entangled particles, photons usually, and send them to two distant detectors. At each detector, you choose a measurement setting independently, a choice of which property of the photon to measure. You record the outcomes over many trials, and then you calculate the correlations between the outcomes at the two detectors. Bell's theorem shows that if the universe were described by local hidden variables, if there were some predetermined property carried by each photon that determined the outcome set at the moment of the photon's creation, and if the choice at one detector couldn't instantaneously influence the other, then the correlations would satisfy a certain inequality. This is Bell's inequality. Quantum mechanics predicts that the correlations violate this inequality. The outcomes are more strongly correlated than any local hidden variable theory can produce. And the experiments confirm the quantum mechanical prediction. Bell's inequalities are violated. The universe does exhibit the stronger than classical correlations that quantum mechanics predicts. Now, here is the key question.
Does this mean the universe allows faster than light influences? The measurement choice at one detector seems to instantaneously affect the outcome at the other. Doesn't that violate relativity? The answer is beautifully subtle. The correlations are there. They are real, but they cannot be used to send information. The choice of measurement setting at one detector correlates with the outcome at the other, but neither outcome is deterministic. Both are random. The randomness is the key. It is precisely the randomness of quantum measurements that prevents the non-local correlations from being used for signaling. And this brings me to something that I think is one of the deepest insights in modern physics made explicit in the work of people like David Deutsch and Chris Fuches and Rutiger Shack. The randomness of quantum measurement outcomes is not a bug. It is a feature. Specifically, it is the feature that keeps quantum mechanics consistent with special relativity. Without the randomness, if quantum measurements were deterministic, you could use entanglement to signal faster than light. The universe uses randomness as a causal firewall. I find this beautiful. the uh apparent indeterminacy of quantum mechanics, the thing that so bothered Einstein who famously said God does not play dice turns out to be at least in part the mechanism by which the universe protects the causal structure of spaceime. God plays dice because the dice rolls prevent the universe from violating causality.
This is not, I should say, a fully established fact. The connection between quantum indeterminacy and relativistic causality is real and mathematically precise. But the interpretation whether the randomness is genuinely fundamental or merely appears so is still debated.
The multiverse interpretation of quantum mechanics for instance says that all outcomes occur simultaneously in branching universes and the apparent randomness is a consequence of branching rather than an inherent feature of reality. In this picture, the causal firewall works differently but still works. But in any interpretation, the bottom line is the same. Quantum mechanics and special relativity are consistent. And the speed of light remains the maximum speed of information transfer. Now, let me bring this full circle back to the question we started with. What does science think happens beyond light speed? The honest answer is science doesn't think anything happens beyond light speed because science doesn't believe anything can be beyond light speed in the relevant sense. Not that there is nothing moving at speeds greater than C. There are processes that by certain definitions appear to exceed C. The phase velocity of light in certain media. The speed at which certain quantum correlations propagate.
the recession velocity of distant galaxies, the speed at which the edge of a laser spot can sweep across a distant surface. All of these can exceed C. But none of them is faster than C in the sense that matters in the sense of information, causality, the propagation of a signal from one place to another.
The universe, as far as we can measure, is causally limited by C. The news of any event propagates outward through spaceime at the speed of light and the influence of any event propagates at the same speed. This is not a technological limitation. It is the architecture of cause time itself. What would happen if this were violated? The equations give an answer. If you could propagate information faster than light, you could violate causality. You could receive answers before questions were asked. You could influence the past. You could construct closed causal loops, circles of cause and effect with no beginning in which the physics is logically contradictory. The universe would break not metaphorically in the specific mathematical sense that its equations would admit self-contradictory solutions. And this I think is the deepest answer to the question beyond light speed. Physics as we know it doesn't just struggle, it fails. The mathematical machinery that describes the universe, the machinery that has been confirmed to extraordinary precision in every domain we have accessed, cannot describe a world with super luminal signaling without becoming internally inconsistent. This could mean that faster than light travel is truly impossible. Or it could mean that our current mathematical machinery is incomplete. That there is a deeper theory that encompasses what we know and adds new structures that allow for something we currently can't conceive of in a way that preserves causality or redefineses it. We don't know which.
What we know is that the speed of light is the universe's most precisely measured and most robustly protected constant. It is the same for all observers in all directions at all energies we have tested. It is the seam that holds special relativity together.
The foundation on which quantum field theory is built. The maximum speed of causality itself. Approaching it reveals the extraordinary geometry of spaceime.
The time dilation that lets you cross the galaxy in a subjective lifetime. the length contraction that compresses space in the direction of motion. The relativistic mass increase that makes the energy requirements for further acceleration blow toward infinity. These are not theoretical abstractions.
They are measured daily at every particle accelerator on Earth, confirmed in every GPS satellite, baked into every atomic clock that we have ever flown at high altitude. The speed of light is real. Its consequences are real. And the wall, it represents the asmtote that no massive object can cross is as real as anything in physics. But the question of what lies beyond it, whether the universe has left any gap, any crack, any loophole in the causal fabric, remains one of the most alive questions in fundamental physics. Not because there is any experimental evidence for a gap, but because the questions at the frontier of physics, quantum gravity, the nature of spaceime at the plank scale, the origin of the energy conditions remain open and open questions in physics mean possibilities.
The highway in the Utah desert, the stars in the sky, the photons arriving at your eyes at exactly 299 million 792,458 m/s after a journey of years or centuries or millennia through the cold between the stars. The universe in those photons is showing you everything it can show you, everything it has seen, everything that has happened within the light cone of your eye at this moment across the entire observable cosmos.
That is fast. That is in fact as fast as it gets. And yet in that limitation, in that specific geometric structural constraint on how quickly the universe can share itself with itself, there is something that I find genuinely wonderful. The universe is not only large, it is shaped. It has structure.
Its structure guarantees that cause precedes effect, that information propagates at a finite speed, that the laws are the same for every observer.
And these guarantees, these deep commitments of the universe to its own logical consistency are what make physics possible, what makes science possible, what make the standing beside a highway in the Utah desert and asking what is this made of possible. The speed of light is not a prison. It is the language the universe uses to remain coherent. and learning to speak that language, learning to read spaceime with instruments of of extraordinary sensitivity to measure the warping of space and time with kilometer scale interferometers and century scale observations and billiondoll particle accelerators. That is, I think, one of the most remarkable things our species has ever done. We have pressed our instruments up against the fundamental structure of reality and asked it to tell us its secrets. And it has answered at exactly 299,792,458 m/s. Not because that's as fast as nature could manage, because that is as fast as the universe needs to be. And no faster. Now, I realize I've spent a great deal of time on why the speed of light is a limit and what happens when you approach it, but I want to spend a few minutes and I think these are important minutes on the question of what physicists actually believe about the long-term future of our understanding because the honest position is more nuanced than it's impossible full stop. And I want to convey that nuance correctly. There is a distinction that I think is crucial and that gets blurred in popular discussions. The distinction between something being impossible given the laws of physics as we currently understand them and something being impossible given the true complete laws of physics that we have not yet discovered. General relativity and special relativity are extraordinarily well-confirmed theories. Their predictions have been tested to a precision of parts per billion in some cases, but they are not the final word.
They are the best theories we have for the physics of space, time, and gravity.
At the scales we have been able to probe at smaller scales, at the plank scale, 10 the negative 35 meters quantum effects of gravity become important and our current theories are known to be inadequate. The singularities at the centers of black holes and at the big bang where both general relativity and quantum mechanics are simultaneously important are the places where the theories literally break down where they produce infinite answers that physicists universally recognize as signals that the theory is being pushed outside its domain of validity. A theory of quantum gravity, one that correctly describes physics at the plank scale that resolves the singularities that unifies general relativity and quantum mechanics is the holy grail of theoretical physics. We don't have it. We have candidates string theory, loop quantum gravity, causal dynamical triangulations and others.
None of them is definitively confirmed by experiment and none of them fully agrees with all the others. Here is what is relevant for our discussion. Some approaches to quantum gravity suggest that spacetime at the plank scale is not a smooth continuum but something else entirely. a network of discrete quantum states, a foam of fluctuating geometry, a structure built from quantum information in ways we don't yet understand. And in these more fundamental pictures of spacetime, the speed of light, which in current physics is a property of the smooth continuous spacetime described by general relativity, might emerge as an effective approximate concept rather than an exact fundamental constant. I want to be careful not to overclaim here. The experimental evidence strongly suggests that Lorent invariance, the symmetry that makes C the same for all observers, is preserved to very high precision even at energies far above the electroeak scale. The constraints from gammaray burst observations which I mentioned earlier put the scale of any Lorent violation at or beyond the plon scale.
This is consistent with Lorent invariance being exact at all accessible scales and emerging from a more fundamental discrete structure. But it does mean that the statement nothing can ever travel faster than light should perhaps be phrased more carefully as nothing can ever travel faster than light given the laws of physics we currently understand applied within their domain of validity. Whether a complete theory of quantum gravity describing physics at and beyond the plank scale would preserve this conclusion or would reveal some deeper structure in which the light speeded limit is transcended is genuinely not known. And I find that genuinely exciting. Not because I expect faster than light travel to turn out to be possible. I don't based on everything I know about the physics. But because the question is alive and alive questions are where the most interesting science happens. Let me now say something about the broader context of these questions in the history of physics because I think the history is instructive about how to hold the right attitude toward claims of impossibility.
uh in 1895 the physicist Lord Kelvin gave a speech in which he said approximately that physics was essentially complete that the two remaining problems were small perturbations in an otherwise fully understood edifice. The two problems he identified were uh respectively the result that eventually gave rise to special relativity and the result that eventually gave rise to quantum mechanics. Within 10 years, the essentially complete edifice had been demolished and rebuilt from the foundation. The lesson is not that physics is never complete or that every claim of impossibility is a future paradigm shift waiting to happen. The lesson is that the appropriate attitude is not certainty. Quantum mechanics was not predicted by the classical physics of the 19th century. Special relativity was not predicted by Newtonian mechanics. The truly fundamental advances in physics have generally come from places that the prior framework could not anticipate. from anomalies in data, from mathematical inconsistencies pushed to their logical conclusions, from thought experiments that revealed hidden assumptions. Whether the speed of light will turn out to be a permanent fundamental feature of any physical theory, or whether some future framework will supersede it in ways we can't currently envision, I genuinely don't know. I suspect the former. Um but I hold that suspicion as a physicist who knows that uh his suspicions have occasionally been wrong. What I am confident about is the value of asking the question. The question of what lies beyond the speed of light has driven some of the most important physics of the last century. It led Einstein to special relativity in 1905.
It led to the analysis of wormholes and warp drives that while probably not practically realizable have taught us an enormous amount about the geometry of spaceime and the requirements of energy conditions. It led to the analysis of tachons that clarified the connection between super luminal signals and causality violation. It led through the Bell inequalities and quantum information theory to a deep understanding of how quantum mechanics preserves relativistic causality through quantum randomness. None of these insights was obvious before the question was asked. All of them have been scientifically fruitful. The boundaries of physics, the places where the laws seem to prohibit the most interesting things are where the most interesting physics gets done. Now let me address one more thing that I think deserves attention and that is the relationship between the speed of light and the structure of time because I've talked about time dilation. I've mentioned the twin paradox. I've described the Lorent factor and how it approaches infinity at C. But I want to say something more specific about what relativity says about the nature of time itself because I think it goes deeper than the standard popular science treatment. In Newtonian physics, time is absolute. There is a universal now a single moment that is simultaneous for all observers throughout the universe. The universe has a fixed observer independent present and the flow of time is the same for everyone everywhere.
Events happen at a specific time in this universal clock and that time is the same whether you measure it from earth or from alpha centauri or from anywhere else. Special relativity demolishes this picture completely. There is no universal now. There is no observer independent present. The question what is happening right now on Alpha Centator has no answer that is independent of the reference frame you use to define right now for an observer at rest relative to Earth. Right now on Alpha Centuri refers to events about four years in the past by Earth's clock. The light travel time delay for an observer moving at high velocity relative to Earth. right now on Alpha Centauri refers to a different set of events shifted forward or backward in time depending on the direction and speed of motion. This is not a measurement problem. It is not the case that there is a real now that we simply can't pin down due to the delay of light signals. It is that the concept of a global now, a single present moment shared by all observers throughout the universe, does not exist in a relativistic universe. What exists instead are events connected by causal relationships. Event A can cause event B if and only if a light signal from A can reach B before B occurs. The causal structure of spacetime, the web of light cones connecting events is the only observer independent structure. The time ordering of events that are causally connected is preserved in all reference frames. The time ordering of events that are not causally connected, events that are space-like separated, events that cannot influence each other even with a light signal is not observer independent. Different observers disagree about which one happened first.
This is the structure of spaceime that the speed of light enforces. Not a single thread of time running through the universe carrying all events in sequence. A network of causal connections defined by light cones in which only causally connected events have an unambiguous time ordering. And the remarkable thing is that this structure, strange as it sounds when you first hear it, is extraordinarily well adapted to the kind of universe we observe. It is consistent with the second law of thermodynamics, with quantum mechanics, with our everyday experience of time flowing in one direction. The causal structure, far from being a liability, is precisely the structure needed for a universe that has a coherent physical description. The speed of light is the architect of this structure. It determines which events can influence which other events, thereby determining what is real, what is physically meaningful in the universe. Without a finite speed of light, there would be no causal structure. Every event could potentially influence every other event. There would be no past and future in any observer independent sense. there would be only an undifferentiated present. I find this genuinely profound, not in a vague or handwavy sense, profoundly in the specific technical sense that the architecture of time, the aspect of the universe that makes it possible to tell a coherent story about what happened depends on the existence of a finite maximum speed for causal influences. The speed of light is not just fast. It is finite. And its finitness is as important as its value. Let me close with something personal because I think it's appropriate at the end of a discussion like this. I have spent my career at the intersection of gravity and time, specifically at the places where Einstein's equations describe extreme warping of spaceime. The places where the speed of light and the causal structure of spaceime show their most dramatic effects. Black holes, gravitational waves, wormholes, the collision of neutron stars and the space-time ripples they produce. These are the phenomena where the full power of Einstein's framework is on display.
Where the structure of spacetime is not a background assumption but the central actor. And in all of this work, what I have found again and again in calculation after calculation, in experimental result after experimental result, is that the universe is extraordinarily coherent. It fits together in a way that suggests deep design. Though I use that word cautiously because design implies a designer and I am talking about mathematics. What I mean is the laws of physics fit together in a way that is mutually supporting the speed of light is consistent with quantum mechanics.
Quantum mechanics is consistent with the second law of thermodynamics.
The second law is consistent with the arrow of time. The arrow of time is consistent with causality. Causality is consistent with the speed of light. It is a circle, a beautiful self-supporting circle. Each piece of the law depends on the others. And the center of the circle, the thing that holds it together is the finite speed of causal propagation, the maximum speed of C.
What science thinks happens beyond the speed of light is not much. Not because the universe is boring or limited in its imagination, but because beyond the speed of light, the circle breaks. the causality fails.
The physics becomes self-contradictory.
And self-contradictory physics is not physics at all. It is noise. The speed of light is not a wall because the universe couldn't build a door. It is a wall because there is nothing on the other side. Nothing that is consistent with the rest of what we know. Unless, and I return to this as I always return to it, unless what we know is not the whole story. Unless the plank scale physics that we have not yet accessed contains structures that we cannot currently anticipate. Unless the connection between quantum information and space-time geometry which is one of the most active and exciting frontiers in theoretical physics today reveals possibilities that seem impossible from within our current framework. I don't know. I genuinely don't know. But the question is alive and alive questions in physics are the most important questions there are. Go outside on a clear night, find a dark enough sky, look up at the stars. Every photon hitting your eye is traveling at 299,792,458 m/s has been the whole way from the nuclear reactions that created it across the emptiness between the stars through the Earth's atmosphere into your eye. And in that journey, in that speed, that constant, that beautiful and structurally necessary fact about the universe, there is more physics, more geometry, more depth of meaning than in almost anything else I know. The question of what lies beyond it is the question of whether the universe has a deeper architecture than we've found.
The answer so far is not that we can see, but we're still looking. And that I think is exactly
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