Talking About Dark Matter - Sixty Symbols

Added: 2026-05-06

[00:00:00]Professor C, let's go. What would you like to regail me with today? What are we talking about?

[00:00:05]>> So, it's it's as a result of a a conversation I've had recently with a very famous experimental physicist in Glasgow called Jim Huff. In fact, Professor Sir James Huff, who has been in charge of a group there which is responsible basically for the mirrors that go on LIGO, the intererometer that's measuring gravitational waves.

[00:00:26]And without those mirrors and the the precision of those mirrors, we wouldn't have detected anything. So, we were chatting and he was telling me about um Lord Kelvin and and that he was going to try and get a plaque for Lord Kelvin where he lived in Glasgow. He was a professor in Glasgow and he said Kelvin was amazing and he did all sorts of things including back in 1904 coming up with the idea of dark matter. And we don't associate dark matter a with Lord Kelvin. I don't. We'd normally associate it with a guy called Fritz Zviki who came up with it in 1933. And here's Jim telling me that Lord Kelvin who's famous for his work on thermodynamics, right?

[00:01:06]Kelvin, zero Kelvin is named after Kelvin, the temperature. So I was amazed and I thought I'd better find out a little bit about this. And indeed, it's not dark matter as we think of it today, which is a totally new particle. It's nothing to do with the particles that you and I are made of. But he did come up with the realization that there could be something within our galaxy that we couldn't see. He argued we needed it and he called it effectively dark matter.

[00:01:32]And the the the the technique he used to come up with it is a technique that was then adopted by people like Ziki. So you could in some sense indeed say that that Lord Kelvin in 1901 to 1904 came up with the idea of that of that there's this extra matter in the universe that we can't see and that so it's dark matter.

[00:01:56]He was born in 1824. He died in 1907. So at the end of the 19th century, right, we still didn't know the universe had had galaxies other than our own. We could see the Andromeda galaxy. They were these fuzzy blobs, but many people thought they were actually within the Milky Way. And at the turn of the century, they were beginning to see evidence of the stars. They could track the stars across the sky and they could see them moving with velocities and they were these velocities were about 20 50 of up to 100 kilometers a second that they'd see them going. They'd see them going across. Doppler shifts was in its infancy and they were beginning to see evidence of the stars moving away from us in on radial directions with the same sort of velocities. And so he asked this question he said well okay are these sensible type of speeds I mean this is what's remarkable to think about the many of these scientists that they they you know he's famous for his work on on thermodynamics. He's famous for his work on engineering and here he was turning his mind to stars moving across the sky.

[00:03:00]So he he he did a calculation. The calculation was motivated by his love of thermodynamics because in thermodynamics he'd consider particles and particles moving. And so he said, well, let's imagine our galaxy. It's made up of stars. I'll treat the stars as molecules. And I'll say that they're evenly distributed. So the density of the galaxy is uniform. And then I'll say that they start with sort of zero velocity. I'll just say they start like with a glass. And then I'll ask what happens to them? Well, they're being attracted by gravity, so they begin to move. And and what he he worked out was what would be the typical velocity that you would expect these stars to have.

[00:03:40]And so he was finding that in order to account for the the fact that you see velocities of between 20 to 100 kilometers a second, he needed of order a billion stars to be within what the the the region they could see. And the region they could see was about a kilop about 3,000 light years. So you consider a sphere of about 3,000 lighty years.

[00:04:07]you you put all the stars uniformly in that sphere and you just let them evolve under under gravitational pull and they sort of move through each other and back and forth and and he was he said if you're going to get velocities that are of what we see then he would have needed about a billion stars. Well, at the time they only saw about 10 20 million stars.

[00:04:33]He needed a billion stars. And so he said, if these the if these objects are moving with this velocity and the only stars in there are the ones we see, then they're moving too quickly. The gravity due to say the 50 million that you could see won't be enough to stop the stars shooting out. And so the Milky Way will just break up. But it hasn't broken up.

[00:04:55]There it is. And so he said therefore one way of accounting for this is there must be matter out there in the Milky Way that we can't see. So it's dark matter. And so this was the first time somebody had linked the idea of there being something dark that could still interact gravitationally.

[00:05:14]>> Apply the brakes >> to apply the brakes. It didn't need to be lit up to be affecting something gravitationally. Now, he didn't think of it as dark matter as we currently think of dark matter, which is a brand new type of particle that we haven't found yet. He thought of it maybe there were stars that had sort of burnt out and so they just gone too dim. There might be rogue, he called them rogue planets that are there, just massive planets that we just can't see. Other things that are just could be made up of the stuff that we're used to but that we just can't see because they're not lighting up. But he did another really important thing in doing this calculation. The principle he used was basically cons was energy energy conservation in that he said if a galaxy is to be stable then the kinetic energy of the stars. So the stars moving that kinetic energy must be less than the binding energy due to gravity. And so what you equate is the kinetic energy of the stars with the gravitational potential energy that's given you the binding energy. That relationship which he used is actually known as the viral theorem and that's what people like Vicki were using that you need to have this sort of equality between the kinetic energy and the potential energy and that was crucial to his calculation and that's what gave him his bounds and so even though the the actual details um are not right in the you know as as as time went on telescopes got better we realized that for example he understandably looking at stars that were nearer to to us than further away.

[00:06:46]So their results were biased by that. He could only see out to about a kilop sec.

[00:06:51]Well, it goes much further for the size of a galaxy. But the principle of this equipartition of the energy between the kinetic energy and the potential energy is still there and that was what was used by Vicki. So he wrote this up in 1901. It's a Nature article in which he talked about these they seem to be going too quickly compared to what kinetic theory would tell you they should be doing.

[00:07:13]>> The amount that things were out by the the mass of the darkness that he required for things to work presumably was different from what it is now.

[00:07:21]>> Yeah, that's right. So, for example, he was he did do one interesting thing. So, he said they need about a billion, right, to to to to account for the the velocities. And >> in fact, what is it? How much do >> it's about 100 billion 200 billion right? So he's out by a factor of 10.

[00:07:37]And in fact he had done he he did something else though he he sort of tried to get an an upper bound. He said imagine it's 10 billion I think is what he did. And he said if it's 10 billion then then the velocities will will just be too big and and it won't it won't won't have worked. And so he he he managed to sort of say well okay it's going to be of order a billion or so.

[00:07:57]And in doing so, he put a bound on on the size of the galaxy. But but it turns out, you know, that's just not quite right. It's order of 100 billion. The reason it's order 100 billion is that we've got all this dark matter. What has changed for us to know that it's not just a bunch of burntout stars and rocks causing it and it is in fact some exotic undiscovered particle? What has changed that has eliminated that as a possibility?

[00:08:21]>> Yeah. Well, that's a really good question. Cosmology has changed. I suppose cosmology has emerged which means that we now need to understand not just a single galaxy but the expansion of you know the role of all the galaxies. So that means we now need to understand the matter that's not just within one galaxy but within lots of galaxies and in particular we then need to understand the motion of one galaxy relative to another. galaxies are sort of not just randomly distributed. They they sort of cluster together. And so now rather than being killer par sex, thousands of parexs apart, they're mega parex, like millions of parexs apart.

[00:08:57]And yet they're moving relative to one another. And the it's known that the luminous amount of ma matter there wouldn't be enough to account for that.

[00:09:07]Not only that, what you can because the universe is evolving and it went through a radiation era into a matter dominated era. There's a an epoch called nucleioynthesis.

[00:09:17]Nucleiosynthesis is when the first nuclei form. The the proton connects a proton forms. For example, that's the nucleus of a of a hydrogen atom. From that epoch, you can constrain how much luminous material, barionic material there must be in the universe. And that's the thing that tells you there clearly isn't enough of that. Remember, he he was wanting to use that. He was calling them dead stars and >> boulders and things.

[00:09:44]>> Boulders and things, things you and I know of. So, it's known that that isn't enough to give me the motion of the clusters of galaxies. And that's what Zicki was measuring. He was using this idea of the viral theorem again to say, why are the galaxies sort of moving relative to one another with these large velocities? Why are they not shooting by? Because their velocities are too big compared to the gravitational pull of the luminous matter. And he then said, well, there must be something else. He called it dark matter. But it's known from nucleiosynthesis. It can't be the barionic matter. So it must be something new. And that that is what's changed really.

[00:10:16]>> So just to make sure I understand the reason we know it's not boulders and dead stars and things like is not that we would have seen them by now. It's that we know we couldn't have formed enough of them. Like we know enough about the formation of matter that we know we couldn't make. We know we know enough about how much matter there can be in the universe as a whole to know that the density of matter that you require is not enough for for for the clusters of galaxies is not enough to give me to give me that there was one thing that maybe I just add about Kelvin you know he was a pioneer of the kinetic theory of gases you know how gases move with respect to one another from that he was a pioneer in understanding the idea of entropy the second law of entropy he's the one that said we can actually use temperature as a way of talking about energies and it's so he's the one that said we can have an absolute temperature and that's where the Kelvin is zero Kelvin is your absolute zero for temperatures. So he did all of this but then it turned out he also did a whole load of stuff um from the engineering side. So for example, he was a big player in the laying of electrical lines through to the US and in particular he developed a way of controlling the distortion of signals going along the cables and he worked out ways of of making sure the signals propagated but he was extremely smart and canny and he so he patented stuff like this and so he became super super wealthy out of all of this. So he he became, you know, he bought mansions in in up in Glasgow and where he had his family. So he was just a multi-talented guy, but also very on the ball of the total energy density in the universe is made of stuff that we don't know.

[00:12:03]>> Well, it it's dark obviously. It's appropriate. It's chocolate.

[00:12:06]>> You know, they they they go around. and he was looking at the speed of rotation as you move away from the center of the galaxy.