What does ‘time’ mean for light? What is the real lesson of the twin paradox? How can we all agree on the true age of the universe? Why is it all so technical? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
One of my favorite parts about the subject of time is that there are so many, well, parts. Of course, I just did a whole episode on why we think time has an arrow. Well, at least it was an episode on our best idea that everybody nonetheless agrees isn't all that fantastic in how we think time is sorta somehow maybe in a vague sense connected to the concept of entropy, and entropy comes from thermodynamics, which is the study of hot things like heat pumps and steam engines and work and all that stuff. But thermodynamics, or if you wanna be super precise, super jargony and nerdy about it, statistical mechanics, isn't the only lens through which we view nature. All of physics isn't inside of thermodynamics.
A part, a very important part, a part of nature is described by our laws of thermodynamics, and by the way, we need an episode just on that, so feel free to ask any time. But thermodynamics isn't the whole story of any system. It's like it's like your senses. Alright. We have we have different kinds of physics that we use to understand the world around us.
And just like you have different senses that you use to understand the world around us. And sometimes you just need your sight, sometimes you just need your sense of smell or touch, but usually you need all of them in combination to get a full, well, picture of the world. And so it's like that in physics. Like sometimes we only need to care about thermodynamics and we can ignore anything else. Sometimes we just need to care about special relativity.
Sometimes it's just high energy interactions, you know, etcetera, etcetera, etcetera. But most of the time, we need multiple inputs from multiple different kinds of physics in order to get a sense of what something in nature is doing. And sure, entropy and its part in thermodynamics may play a big, make large huge role in understanding time, don't get me wrong about that, but it's not the only player. It may not even be the starring role. What I'm trying to say is that when it comes to time, we have some unfinished business.
And you could very well consider this a part two to my earlier episode on time or a part one. I suppose it's a matter of perspective, folks. And today, I wanna look at time through the lens of Einstein, using that branch of physics to study time, that sense to try to wrap our minds around this concept of time. And I wanna use relativity, special relativity, and general relativity. And, yes, I know I did a whole episode way back in the dark ages on why it appears that we can't travel back in time even though general relativity technically allows it, you know, why wormholes and such don't seem to work.
And, yes, I know I did a whole episode, couple episodes, way back in the stone ages about special relativity and space and time and space time and time space, and that last one's not a thing, but maybe it should be. So I've talked about time a lot on this show, which isn't a surprise because it's kind of a big deal. But these kinds of broad episodes leave a lot of loose ends, a lot of new questions, which is fantastic because if it's one thing I love, it's cheese. If it's a second thing I love, it's hearing your questions, especially questions sparked by older episodes, and there are a lot of questions. We have Candy m on email asking, does light experience time?
Male e on email is, if time is different everywhere, how come the universe is 14,000,000,000 years old? Men b on Facebook, if time is relative, how can we measure the age of the universe? Carlo d on Twitter with the same question. Alan p on email, is now the same time in the whole universe? Derek m on Facebook, do photons experience time?
Rose j on email, if time flows different and how can we know the time to the big bang? Kirk b on Patreon, does light experience time? Kirk b on email, what's the deal with the actual twin paradox? Ruben Fries on email. Explain the twin paradox.
Chris f on email. What's going on with the twin paradox? Diego s on email. What is the solution to the twin paradox and special relativity in our scheduler on email? Twin paradox question mark.
So what we've got here is a grab bag episode about three, seemingly unrelated questions about time. Question number one, does light experience time? Question number two, how can two twins agree on who gets older if one is moving? And question number three, how can we all agree about the age of the universe, the time since the Big Bang? And I think all three of these questions are about the same thing, age.
It's a fundamental question that's connected with time, but but something we can wrap our minds. It's a it's a piece of the time puzzle that we can tackle and we can tackle through special and general relativity. And the question is, how do things in the universe age? Let's start with age of light, because the answer to this question is going to be the one you hate the most because the answer won't be satisfying at all, and you're going to feel like I'm just dodging the whole thing based on a technicality. But here's the thing.
This is physics. Physics is based on math. And math is, if you haven't noticed by now, kind of technical. Sometimes you're going to get an answer based on a technicality, and you're just going to have to live with it because the universe is under no obligation to make sense to our tiny little lizard brains. But let's not get ahead.
Relativity joke. The fundamental concept that we're gonna run into in relativity and its connection to time is that moving clocks run slow. Moving clocks run slow. If you pick up a clock and you start running, it will not tick at the same rate as a clock that you leave on the ground. It's just the way it is.
And in the special relativity episodes way back then, I explained why. For now, for this episode, we're just gonna take it as a bare fact of reality, and it I mean, we've measured it. It's a thing. It's a thing. Moving clocks run slow.
Everything that's moving runs slower. The perception of time in a moving frame of reference is slower than the perception of time in a stationary one. And the closer you get to the speed of light, the worse it gets. So if you start pumping and pumping and pumping and blasting your engines, blasting your engines, this difference in the clock rate on the moving thing and the clock rate on the stationary thing grows more and more dramatic. And, of course, you can't actually reach the speed of light.
You can get to 99, 90 nine point nine nine, 90 point nine nine nine nine nine nine nine nine nine. You can't get to 100% the speed of light. But the closer you get, the worse this time dilation factor, as we call it, gets. The slower it's like your your clock grinds to a halt as you approach the speed of light. So it's not, you know, not that weird to take it just one little step further.
You say, okay. I can push closer and closer to the speed of light. My clock goes slower and slower and slower and slower. What about light itself? Light itself isn't traveling at close to the speed of light.
It is, you know, exactly traveling at the speed of light. So if that's the case, does its clock not move at all? If you were to put a little watch on a little photon, a little bit of light as it lept from the surface of the sun and raced to the Earth from according to us here on the Earth, that bit of light takes eight minutes. But would eight minutes elapse on the watch attached to the photon, or would it be instant? Does light not experience time at all?
Well, folks, I'm sorry, but the question just doesn't make sense. I know. I know. And I know you're thinking like, well, I can I'm perfectly capable of understanding that that question, Paul, but it doesn't make sense in the math. What we've learned from relativity is that physics is the same in what we call inertial frames of reference.
Inertial frames of reference are ones that, you know, aren't accelerating or twisting or turning. They're just cruising along at the same speed. If you're in an inertial reference frame, if you're just cruising either staying still or cruising along, then physics is exactly the same. And all frames of reference measure the same speed of light. This is Maxwell's equation.
Maxwell's equations gives us the speed of light. And we can do it here stationary, like you can just sit in a laboratory and measure the speed of light and you get a number and it's a constant. It's just a number. And then you know from relativity because physics is the same everywhere that if you were to jump into another inertial reference frame, like if you were to accelerate and then start cruising along, well, the speed of light would appear to be constant for you too. Again, because it makes sense.
It's correct in one frame. It's correct in all frames. It's correct for one observer. It's correct for all observers. So according to our basic understanding of Maxwell's equations and our basic understanding of special relativity, all reference frames, all inertial reference frames in the universe must measure the same speed of light.
No matter what, you will measure c. It will be the same number. But if you're traveling at the speed of light, if you were really, really traveling at the speed of light, then what kind of speed of light you would measure? You would measure zero. You wouldn't measure a speed of light because you're you're chasing along with it.
You're with it. You're right next to that photon as it's racing through space. In that reference frame that races along with a bit of light, you don't measure the speed of light. You don't measure that photon zipping by you at the speed of light. You're it's right next to you.
You can hold its hand if you want to. And that means that's a signal. Wait a minute. Wait a minute. Einstein's relativity tells me that all inertial frames share the same laws of physics.
One of the laws of physics is that the speed of light is constant. If I'm in a frame, if I'm in a point of view where I'm not seeing the speed of light be constant, then special relativity breaks down. Special relativity does not apply at the speed of light. All the language, the mathematical language and the the English language that surrounds it that we use to describe clocks and time dilation and length contraction and all that simply don't apply. How could this I mean, it sounds like I'm speaking in riddles, but what I'm trying to get is that just because you can pose the question, does light experience time at all?
Just because you can pose it doesn't mean the question makes sense. Our understanding of time, our understanding of how the rate of change in time, of of how time flows for different observers simply doesn't work at the speed of light. It's the wrong language. It's the wrong toolkit for describing motion at the speed of light. So it's not that light experiences no time at all because that's certainly one way is that you can just say, no.
It's it's it's light doesn't experience time. I don't feel like that's the right answer. I don't feel like that's the right answer because the the very concept of time, at least as we understand it through relativity, just doesn't make sense for light. So does light experience time at all? Light doesn't even know what time is.
That's not a question you can ask. And I and I know I told you, I warned you, I totally warned you that you wouldn't like the answer, but too bad. Now what about the twins? The twins story is one of my favorite thought experiments from special relativity because this puts such a human face or it's possible to put such a human face on this ridiculous absurd phenomenon of time dilation, of moving clocks run slow. In the in the story here is, let's say, Jane and Jack, they're twins.
Jack builds a rocket ship. He blasts off. Jack travels really, really, really, really, really fast. Like, seriously, like 99.99 of the speed of light. We won't worry about how because it's a thought experiment.
We could just focus on the physics. And then he comes back. And say he cruises along for like five years. He goes out into deep space blasting along really fast, swings by. Oh, let's make it ten years so he can go out to Alpha Centauri, and then he comes back.
Well, for Jack, he aged ten years because, you know, he's Jack, and he's got a watch, and he knows how long time ago. So him on his rocket ship, he he thought he aged ten years. But Jane, his twin, aged a lot longer than ten years because Jane is sitting there on the Earth, stationary, watching Jack move around. Jack's clock is moving slow. According to Jane, Jack is stuck in slow motion.
And so Jane gets older and older and older. For one tick of Jack's clock, we get 10 or 20 or a hundred or a thousand ticks of Jane's clock. So by the time Jack comes back just ten years older, maybe Jane is dead. Maybe she's just really old. Maybe she's so far dead that, like, no one even remembers her.
That's pretty cool. Right? And that's just a consequence of relativity. It is just different observers have different experiences of the flow of time. That's all well and dandy and weird, but hold on a second.
Isn't movement relative? Jane says that she is stationary. She's sitting on the earth and Jack is the one who's moving. But if Jack is just coasting along, no acceleration, he can rightly claim that he's the stationary one, and it's Jane that's doing the moving. Right?
This is, again, this is relativity. Who's stationary and who's moving? Well, it depends. There's no absolute reference frame in our universe to to say, okay. Yep.
Yep. According to the absolute reference frame of the universe, Jane is sitting still and Jack is moving. No. There are no absolutes in our universe. There's no arbiter.
There's no consultant. There's no judge where you can say, well, I put forward a motion that Jack was the one who was moving. Motion corrected. There's nothing like that. There isn't.
If you are in an inertial frame of reference, which which means you're just coasting along, no acceleration, you're chill, then you can rightly claim that you are perfectly stationary and the rest of the universe is the one that's moving. So according to Jack, he's the stationary one. It's Jane that's moving. You know, maybe there's a big giant rocket engine attached to the Earth, less ignoring the boring engineering of the question. Focus on the fun physics.
No offense to engineers. But according to Jack's reference frame, Jane's the one that's moving. But if Jack can rightly claim that he stayed still this entire time, then it should be him who advances and age and dies while Jane skips forward in time. If Jane's the one that's moving, she's the slow mo one, not Jack. This is confusing, and this is a paradox.
And like Moe's paradoxes, the resolution is a technicality. It's absolutely true that while Jack is coasting away in his rocket ship on his outward journey, everything is symmetric. He thinks he's stationary while Jane is the one who's moving, and Jane thinks she's stationary while Jack is the one who's moving. Everything's perfectly symmetric, but then Jack does something weird. He slows down.
He stops. He turns around. He comes back. Uh-huh. Jack does something that Jane does not.
The symmetry, the the evenness between them is broken. The act of changing directions tells us which one is going to stay young, Jack, and which one is going to age. Sorry, Jane. Because Jane stays in the same reference frame for her whole story. Jane doesn't turn around.
Jane doesn't slow down and then speed up. She just stays being Jane. Jack is the one who changes. So what Jack sees on his outward journey as he's blasting away from the earth, Jane appears to be frozen in time. Right?
Because according to Jack, cruising along evenly, he's not moving. He's stationary. It's the rest of the universe that's moving. And Jane appears to be moving along with the universe, and she appears frozen in time. But when he slows down and stops and reverses, in almost a blink of an eye, he sees Jane rapidly, rapidly, rapidly age and jump forward.
Jack is is catching up with all the time he missed while he was traveling out as he slows down and stops and reverses. It like all piles up, and then when he turns around and reaccelerates and gets back up to cruising speed, once he's there cruising again back in an inertial reference frame, Jane once again appears to slow down, especially if she's dead by now. If Jack is watching the movie of Jane's life, it's like watching a movie on slow motion barely making it through the opening scene, then super triple fast forwarding through almost the entire thing, and then back to slow motion for the end credits. The lesson here with Jane and Jack and who's right and who's wrong is that a, technicalities exist, and b, we have to pay attention to what the characters in our stories do to make sense of everything. But enough about light and twins and all this small scale stuff, let's talk about the universe.
How many times on this show have I said with breezy confidence that the universe is 13,800,000,000 years old. A lot, I suspect, and I just said it once more, didn't I? But wait, you say? 13,800,000,000 years according to who? Or is it whom?
I I don't know so someone feel free to tell me. But the point is is didn't we just work really, really, really hard to wrap our minds around the fact that time is relative and that moving clocks run slow and different observers will have different measurements of time? How can I say that the universe is 13,800,000,000 years old? Is it according to us? Is it according to someone in another galaxy?
Isn't that kind of a big deal? Is the universe a different age for us than it is for someone in another galaxy moving at a different speed? The answer is, you guessed it, a technicality. This is actually a really magical technicality here because we're gonna make a jump from special relativity to general relativity. Special relativity is all about frames of reference and different observers and that there's no absolute frame of reference, like I said before.
There's no judge. There's no master ruler that we can all compare ourselves. You know, there's no master clock ticking away in the background or the supreme overlord meter stick, you know, all that. It just doesn't exist. But the expansion and history of the universe isn't the domain of special relativity.
It's the domain of general relativity. And in general relativity, we've found something remarkable. So remarkable that I'm about to remark upon it. We live in an expanding universe. Our universe gets bigger and bigger as it ages.
Our universe changes with time. Special relativity teaches us that space and time are woven together into this unified fabric of space time. But when we look at the whole hog of the universe, that symmetry breaks. It unweaves. Space and time are separated in the mathematical description of an expanding universe.
They are not treated equally when it comes to the whole entire universe. Because the universe changes with time, there is a master clock. And that clock is tied to the size of the universe. So you can say the time now is when the universe is this big, 90,000,000,000 light years across. And when the universe was, say, half its current size, it was at a specific time in the past.
And the same for a quarter its present size, a tenth its present size, a thousandth of its present size. There is one unique moment in time associated with every size of the universe. So you can trace back. You can say, okay, the universe is half half its current size. Here's how long ago it was.
There was a time marking that moment, and there's a time marking this moment when the universe is this big. And then tomorrow, the universe will be bigger. There's a it's a little bit fatter tomorrow, and there'll be a unique time associated with that size. There is a connection between time and size of the universe. So that fundamental special relativity weaving of space and time gets broken because the universe acts differently as it ages.
And this means that there was a finite time in the past when the universe had zero size at all, and that time was 13,800,000,000 years ago. Now, of course, this picture is only valid at cosmological scales, the whole entire universe, And we're moving around within that expanding universe. So we, us, in our Milky Way galaxy, we carry our own personal clock that will vary with speed. You know, so do you right now based on your motion. We'll measure a slightly different time since the big bang.
But no matter what, you can always figure out the true time since the big bang. True with with little quote y things around it. True time. The the actual time since the big bang. In the physics jargon, we call it the proper time, the proper time since the big bang.
It's like if if we have we have our measure, it's thirteen point eight billion years. And and, of course, years is human and subjective, but we could figure out the translation from our years to their their years. So it's not the number one three point eight that's important, but we all know that there's a finite time since the big bang and we can measure it. If there are aliens in another galaxy and they were trying to figure out the universe and they got their own Bedouin bubble and they got their own Albert Einstein, Albert Einstein would say, you need to go to Patreon. Patreon.com/pmslur is how you keep this show going.
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So anyway, if Albert Einstein in another galaxy could figure out general relativity, it could also figure out the time since the big bang. Yes. They'd be moving slightly slightly differently, but you can subtract your motion away from it. You can subtract your motion away, you know, separate your motion in the gal in your galaxy in the motion of your galaxy itself. You can separate that from the expansion of the universe itself.
And once you're able to note and measure the expansion of the universe, then you know that the universe changes with time, and you know that there is a unique moment in time associated with every size of the universe and you can work it out, run backwards to get thirteen point eight billion years. You might get a different number because your year might be a little bit different. You have a different way of counting time, but that's just a translation thing. That's a translation thing. We would all agree on the fundamental concept and we would all agree on the number.
We could translate. It's a universal number. That's another way to think about it. Thirteen point eight billion years is a universally known fact of our universe. Three questions, three technicalities.
Sorry, folks, but nature doesn't always play fair, especially when it comes to time. Thank you so much for listening. Remember, you can send your questions to ask a spaceman@gmail.com. You can hit me up on social media. All channels, that's at paul matt sudder.
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