Why Light-beams Don't Go On Forever

Added: 2026-05-07

[00:00:00]Chuck.

[00:00:00]>> Yes. I got a cool explainer.

[00:00:02]>> Uh-oh. You're excited. That doesn't happen normally.

[00:00:04]>> Oh, okay.

[00:00:05]>> So, this must be a good one.

[00:00:07]>> Extra good.

[00:00:07]>> Yeah.

[00:00:08]>> Okay. Ready? What is it?

[00:00:09]>> Okay. You ever see Hollywood Search Lights?

[00:00:10]>> Yes.

[00:00:11]>> Okay.

[00:00:11]>> Because they they they shoot a shaft of light into the air >> as they do, right?

[00:00:16]>> Very poetically put.

[00:00:17]>> Yes.

[00:00:18]>> Okay. And in fact, >> which we used to use to spot bombers.

[00:00:23]>> Yes. Yes.

[00:00:23]>> And Yeah. And then all of a sudden they were like, "Well, what are we going to do with all these lights?

[00:00:29]Now that THE WAR IS OVER, WHAT DO WE DO WITH THESE LIGHTS?

[00:00:32]>> Shoot them up in the air anyway."

[00:00:34]>> And in fact, uh, in 20th Century Fox, >> right, >> uh, now called 20th Century.

[00:00:40]>> Yes.

[00:00:41]>> Their fanfare at the beginning are are search lights with the with the big >> There you go.

[00:00:48]>> Yeah. That was Yeah.

[00:00:49]>> All right. So, have you ever looked at one? A search light.

[00:00:54]>> Yes.

[00:00:54]>> No.

[00:00:55]>> No. Not directly in it. Okay. From the side.

[00:00:58]>> From the distance. Yeah. There.

[00:00:59]>> You look from the side and it goes up and up and up. Then it stops.

[00:01:04]>> Yes.

[00:01:05]>> Have you ever noticed this?

[00:01:07]>> Yeah. Even from a distance, what happens is cuz they do it every year for 911.

[00:01:12]>> Yes, we do. Yes. What they do is for people who don't know on 911 in honor of 911 because this is New York City, we're here at my office at the Hayden Planetarium. Uh we're now four, five miles north of ground zero. They have two huge spotlights.

[00:01:31]>> Yes.

[00:01:31]>> And each go vertically up, >> right?

[00:01:34]>> Creating >> creating a ghost of the towers that went down.

[00:01:37]>> Yes. Yes. And so just as an homage >> to each of the towers. And so I live in Jersey and I live pretty much on the river where you can see the the the installation. And what's cool is there's these two shafts of light that go up.

[00:01:56]>> Uhhuh.

[00:01:57]>> But then they disappear into nothing at the very top.

[00:02:03]>> Right. There is a top of >> to the column of light. Right.

[00:02:07]>> Yeah. Which is, you know, kind of wild.

[00:02:09]>> It's kind of wild.

[00:02:10]>> Yeah.

[00:02:10]>> Okay. There's a reason for that.

[00:02:12]>> Oh, really?

[00:02:12]>> There's a physics reason for that.

[00:02:14]>> Oh, okay. I'm interested now.

[00:02:16]>> Okay. Are you ready?

[00:02:17]>> Let's go.

[00:02:18]>> All right.

[00:02:19]>> God hates light.

[00:02:22]>> STOP ILLUMINATING.

[00:02:24]>> JESUS. YOU SEE, I'M TRYING TO SEE.

[00:02:26]>> NO. NO.

[00:02:28]>> If I put you 20 miles away on the horizon, >> okay, >> and I aimed the beam of light at you, >> all right, >> you would have no trouble seeing it.

[00:02:36]>> Nope. Not at all. Especially a search light. Whereas, if I point it straight up and you're down here and you're looking, it has an edge. That's wild.

[00:02:45]>> Okay, it's just physics. Okay, you ready?

[00:02:47]>> Okay, >> here we go. So, first of all, you only see the beam because there's stuff in the air, pollen, dust, water vapor that reflects light out of the beam to your sight line.

[00:02:59]>> Okay, >> think about that.

[00:03:00]>> That's That's great.

[00:03:01]>> This is why the the phasers in Star Trek >> Mhm. You always see the side view of the Enterprise sending phasers out front.

[00:03:11]>> Mhm.

[00:03:11]>> By the way, I never knew why the phasers weren't parallel to each other. They were at an angle. That was a little weird to me. But if you're looking at a light beam go sideways, you can't see the light beam because the light beam is going the other direction and it is empty space.

[00:03:26]>> Mhm. They turn on the phasers, it would be nothing for you to see from the side unless they they get chalk dust, right?

[00:03:33]And pop, then you'd see the beam because the dust particles are reflecting it back to you out the side of the beam.

[00:03:40]>> Okay.

[00:03:41]>> Okay.

[00:03:41]>> Gotcha.

[00:03:42]>> There's a scene in >> Star Wars Force Awakens where they have the diabolical Death Star that can kill eight planets at once or whatever rather than just one. Yeah.

[00:03:52]>> Okay. The engine revs up and it goes And these like light light beams come out, laser power light beams, each aimed for a planet and it takes out all the planets in this star system. Okay. They show people on each of those planets watching the light come to them, >> right?

[00:04:09]>> No, they would have no idea.

[00:04:11]>> They would have no idea it was on route.

[00:04:13]>> Mhm.

[00:04:14]>> You cannot anticipate light that is about to kill you cuz it's going the speed of light.

[00:04:19]>> Right.

[00:04:19]>> There's no light ahead of it saying I'm coming.

[00:04:21]>> You don't get to say IT'S COMING.

[00:04:24]>> RIGHT. RIGHT. RIGHT. OKAY. So that's just a it's a movie trivia fact. All right. So that's why you can see the beam at all. If we were in a total vacuum, >> you would not see anything and search lights would be pointless.

[00:04:36]>> Interesting.

[00:04:37]>> Okay. By the way, in the desert where there's less water vapor and less pollen, there's still sort of desert dust and so you can work with desert dust. But if the dust were very calm, then the beam would be hardly visible going up. You'd see the the the base of the light, but the beam would be very hard to see. All right. There's something called the inverse square law of light. So, how bright is a light? If you're twice as far away, it's 1/4 as bright.

[00:05:03]>> Gotcha.

[00:05:04]>> Three times as far away, 1 nth as bright.

[00:05:06]>> Gotcha.

[00:05:07]>> Four times as far away, >> 1/16th.

[00:05:09]>> 1/16th. Okay. I call this when I teach this in school, I call this the the the butter gun experiment.

[00:05:16]>> Butter gun.

[00:05:17]>> Butter gun. The longer story, I don't know if I have time to tell this, is you're hired as a short order cook in a restaurant.

[00:05:24]>> And you're buttering the toast to toast it to bring it out.

[00:05:27]>> Okay.

[00:05:27]>> And the management says you're using too much butter.

[00:05:30]>> Okay.

[00:05:30]>> Okay. But you want the toast to be evenly spread with the butter.

[00:05:34]>> Exactly.

[00:05:35]>> All right. If you if there's not enough butter, it'll be You can't No one wants that. Dry spots to >> You can't have dry spots on your toast.

[00:05:40]>> Exactly.

[00:05:41]>> So, this person is clever animals.

[00:05:43]>> This person is clever and took Astro 101.

[00:05:45]>> Okay.

[00:05:46]>> And said, "I'm going to make a butter gun." Gotcha.

[00:05:48]>> You squeeze the gun. It melts the butter and sprays it onto the slice of toast.

[00:05:52]>> Okay.

[00:05:53]>> Okay, there it is. Now, let's make the toast twice as far away.

[00:06:00]>> Uh-huh.

[00:06:01]>> The cone of butter spreads.

[00:06:05]>> Right.

[00:06:05]>> Now, I can fit four slices, >> right, >> in the beam.

[00:06:09]>> Yes.

[00:06:10]>> Using the same amount of butter.

[00:06:12]>> Okay. Each toast has 1/4 of butter, but I'm covering the whole surface.

[00:06:15]>> Right. Now I put it three times as far away. The same squeeze will now cover nine slices of bread of bread. Okay.

[00:06:22]>> 16 slices of bread. I'm still covering the bread. I'm using less butter. I'm saving the restaurant money. I get a raise and everybody gets a fully buttered slice of toast.

[00:06:30]>> Butter for everybody.

[00:06:32]>> You get butter. You get butter.

[00:06:34]>> So the intensity of the butter is dropping.

[00:06:36]>> Right.

[00:06:37]>> But you're still covering.

[00:06:38]>> But you're covering it.

[00:06:39]>> Okay.

[00:06:40]>> Right.

[00:06:40]>> Light. The intensity of the light drops even though there's still light there.

[00:06:44]>> Gotcha. Now >> I see the butter, but it just don't seem like it seems like it's not as much butter on here.

[00:06:51]>> So this beam is dropping off in intensity as one over distance squared.

[00:06:57]>> Gotcha.

[00:06:58]>> From the ground, from the ground from its source, the light gets twice as high up, >> right?

[00:07:02]>> It's only 1/4 as bright at that point.

[00:07:05]If you're looking down at it, it's 1/4 as bright, 1 nth as bright, 1/16th as bright, 125th, 136th. It's getting dimmer and dimmer and dimmer as it ascends.

[00:07:16]>> Right?

[00:07:16]>> Okay.

[00:07:18]>> However, >> you're not looking directly at the light.

[00:07:23]>> You're looking at light that reflects back to you from the crap that's in the atmosphere, >> right?

[00:07:29]>> So, the light coming back to you also drops off as 1 over distance squared.

[00:07:36]combine them, the brightness of the beam drops as one over distance to the fourth power.

[00:07:43]>> Holy crap.

[00:07:44]>> Holy crap. So if it's three times higher up, that light is 1 nth as bright there and it's 1 nth of that 1 nth as bright to you on the ground, right?

[00:07:55]>> It's 181st as bright.

[00:07:57]>> Look at that.

[00:07:58]>> So on the ground, the light is dropping off exponentially. And there's a point where your cone can't detect it.

[00:08:06]>> Can't. We can't. Right.

[00:08:07]>> There's light there. You're just not going to be able to see it.

[00:08:09]>> Correct. Because you're relying on the light reflecting off of crap coming back to you.

[00:08:15]>> Whereas the person in the beam, it's a simple one over. It's still getting dimmer and dimmer, of course, but it doesn't have this added effect of having to reflect back to you. So all search beams look like they come to an abrupt stop. It rapidly drops below the detection limit of the rods in your retina.

[00:08:32]>> Wow.

[00:08:33]>> Every beam of light going up will look like it has a I have a laser that I use to point out the night sky. It looks like it stops. Yet, if you're 60 mi away, you'll be able to see the laser, >> right?

[00:08:43]>> But from on the ground, it looks like it has an edge. So, there you have it.

[00:08:47]>> That is very >> It's not the one over r 2 law. It's the 1 / r 4th.

[00:08:51]>> I love it. Yeah. So, now the next time I see the 9/11 memorial, uh, I'll be like, "Hey, look." And they'll be like, "You're a little too excited >> for this. LET ME EXPLAIN."

[00:09:03]>> EXACTLY.

[00:09:04]>> I KNOW WHY.

[00:09:05]>> GUESS WHAT, GUYS? They were like, "Dude, it was a tragedy. Just take it easy.

[00:09:08]Let's leave it that way. Get two flashlights and explain."

[00:09:12]>> All right, that's all we got time for in this explainer.

[00:09:15]>> That was good.

[00:09:16]>> From the Start Talk verse, Neil Degrass Tyson. Chuck, good to have you.

[00:09:19]>> Always a pleasure.

[00:09:21]>> As always, keep looking up. What would you do if an alien actually showed up?

[00:09:27]Would you shake its hand or run? Does it even have a hand to shake? In my latest book, Take Me to Your Leader, I explore not only how they might have gotten here, but what they might want and how you should respond. Because the real question is not are we alone, it's are we ready? By the way, I also narrated Take Me to Your Leader. And I'm duly informed that you can get a copy of that book or the audio book now wherever books are sold. You should probably get the book sooner rather than later. You don't want to have a first alien encounter and not be ready for it. I'm just saying