Everyone loves a theme. And this week we’ve collected together some of your questions about relativity. More light speed spacecraft, twin paradoxes, and the mixing up of gravity, time and mass. If you’ve got a question for the Astronomy Cast team, please email it in to firstname.lastname@example.org and we’ll try to tackle it for a future show. Please include your location and a way to pronounce your name.
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Relativity Tutorial — UCLA
General Relativity — Internet Encyclopedia of Science
What would happen if you’re traveling close to light speed through the solar system — would you mess it up?
- Traveling at light speed — Time magazine
- The Relativistic Mass Increase — U of Virginia
- The Physics of Flying High and Fast — Daily Camera
If I watched my friend accelerate close to speed of light, will I see him get younger or age slower?
Can parts of the Universe be traveling back in time?
- How Can Galaxies Recede Faster Than the Speed of Light? — Universe Today
- Paper: Varying Speeds of Light — arXiv
What form does relativistic mass increase take?
Why don’t photons have mass?
Twin paradox: Why is the “stay at home” twin’s frame of reference preferred?
- NOVA’s Time Traveler page — PBS
- Twin Paradox — Wiki
- Professor Solves Einstein’s Twin Paradox — PhysOrg
- Discussion of Twin Paradox — BAUT Forum
- Time Dilation — Wiki
Does gravity equal time?
- Why does gravity slow time? –Perimeter Institute
- Discussion of ball on a rubber sheet model of gravity — BAUT Forum
How can the state of an electromagnetic wave change when the wave exists outside of time?
How do we know the speed of light constant? Could it be different in different part of the universe?
- Why do we assume that the Universe for all observers is homogeneous and istropic? — Cornell University
- Homogeneity and Isotropy — UCLA
Transcript: Relativity, Relativity, and More Relativity
Fraser Cain: Welcome to the AstronomyCast Questions Show. This is where we answer your questions about Space and Astronomy – or in this case, Relativity. [Laughter]
Everyone loves a theme this week we’ve collected together some of your questions about Relativity, more Light Speed, Spacecraft, Twin Paradoxes and the mixing up of Gravity, Time and Mass. Pamela you had an announcement?
Dr. Pamela Gay: I’m going to London! I’m going to be heading to the other side of the Atlantic to go work on the International Year of Astronomy with a bunch of my Collaborators. On November 23rd I’m going to do a fan meet up for all you wonderful denizens of the United Kingdom and anyone who wants to travel across the Channel.
We don’t have an exact location yet but it’s probably going to be Sunday afternoon in Central London. I hope to meet you there. It will be myself and Chris Lintott one of the main guys behind Galaxy Zoo right now.
Fraser: Let’s get on with the questions. Brandon from Grand Island, Nebraska asks: “As I understand the closer you get to the speed of light, the greater your Mass. What would happen if you’re traveling at a significant portion of the speed of light and passed through the Solar System? Since your Mass would be much larger, would the Gravity from your spaceship mess up the system?” Well…
Pamela: This is a kinda complicated question because there are two different things that affect how much you can muck something up. One is how long you interact with that stuff and the other is how massive you are.
Fraser: But, by moving close to the speed of light, are you increasing your Mass? Could you reach the Mass of the Moon; could you reach the Mass of Jupiter and then the Mass of the Sun?
Pamela: Affectively yes.
Fraser: Right, just the faster you go the more massive you’re going to become.
Pamela: Yes. Now when it comes to mucking up the System though, since you’re flying through it so fast, there’s not enough time for you to really muck up the System.
If you were able to go close to the speed of light very slowly – like a little Spirograph through the Universe or something – you could probably do a lot more damage.
But if you just zip through in a straight line – and mind you the accelerations involved in doing a Spirograph through the Universe kinda make it impossible – super duper fast, through the center of the Solar System, the interaction times are so short that you’re not really going to do much harm.
Fraser: That’s kind of ironic, you’ve got a tremendous amount of Mass but because you’re going so fast, you’re just not around long enough to disrupt anything. As you mentioned, if you’re moving close to the speed of light very slowly [Laughter] so you’re just kind of jiggling back and forth at almost the speed of light but then you’re moving forward very slowly, then yeah you could cause some damage. But then if you’re just on a trajectory then no you wouldn’t.
Pamela: And as I said the whole acceleration involved in doing the Spirograph through the Solar System, really you’re not going to do it. It’s all good.
Fraser: The next question comes from Tyson Slater about the Twin Paradox. “If I were to watch my friend through a window in his spaceship as he accelerates from me at close to the speed of light, would I see him get younger or would I just see him age slower?”
Pamela: Time always ticks forward.
Fraser: Right so there’s no going back. There’s no getting younger.
Pamela: Right, exactly so yes, you can see him age slower. The only caveat is you sort of have to wait for the light to get to you which makes this even more complicated. Time has slowed down for him relative to you due to the difference in speeds.
But also you’re waiting for the light to travel from him as he gets progressively further and further away to reach you and so that makes him appear to age even slower. That’s kinda cool.
Fraser: Sure, but there’s no going backwards. There’s no hopping in your spaceship as an adult and then doing a quick flight and coming back as a baby. You’re going to be an adult.
Pamela: Backwards doesn’t happen.
Fraser: But the reality is that if I hop in my spaceship and move at close to the speed of light – this is where the Twin Paradox comes in – I may come back and only be a few years older but time on Earth may have moved forward thousands or even millions of years.
Pamela: Yes. Buck Rogers.
Fraser: Buck Rogers – yeah. Okay let’s move on. Danasks: “If there are parts of the Universe traveling away from us at speeds greater than the speed of light due to the expansion of the Universe and Dark Energy, are these parts traveling backwards in time relative to us?”
I can see how you might imagine that. As things move faster and faster, they appear to slow down from our frame of reference. We’ve explained in the past that the expansion of the Universe is the thing that can go faster than the speed of light. Two objects can be carried away from each other by the expansion of the Universe faster than the speed of light.
So, I can imagine you’re watching some distant object as it’s going faster and faster away from relative to you until it looks like the time has stopped. Yes, they’re going to go faster than the speed of light so do we start to see it go backwards in time?
Pamela: No. Sadly time, yet again only ticks forward.
Fraser: Then what would happen at the moment that some distant Galaxy started moving faster than the speed of light away from us?
Pamela: We’d stop being able to see it. Its light can’t get to us anymore. It’s kinda sad isn’t it?
Fraser: What do you mean by its light can’t get to us?
Pamela: If it is moving faster than the speed of light then it’s giving off light and that light is traveling toward us at the speed of light. So that light will eventually reach us but the Universe is expanding so the light is trying to get to us but the space that it’s trying to traverse is expanding faster than it’s moving.
It’s like you’re on a sidewalk that’s in the process of getting built. You’re running as fast as you can to try and get to the end of the sidewalk but the new sidewalk is getting built faster than you can walk; faster than you can run. If they’re building sidewalk faster than you can run, you can never get to the end of the sidewalk.
Fraser: Okay, I get it. It’s like the light is coming off that Galaxy trying to traverse the distance between you and it but there’s more Universe being placed in-between it so it’s almost like it’s being carried away with that Galaxy and it reaches us. So then what would you see?
Pamela: This is where we get back to things fade away to red and then disappear. The light coming off of an object as it’s disappearing across the horizon of a visible Universe that doesn’t have an edge defined by the Cosmic Microwave Background.
In that scenario we see an object over time and here we’re talking over thousands of years, millions or billions of years, we see an object fade away getting redder and redder and redder due to the red shifting of its light. Eventually it gets so red, so faded it disappears. Then we just stop getting light.
Fraser: Right because we’re just seeing the last Photons that were emitted before the Photons couldn’t cross the gulf because there was too much space being put in-between them. And so those last Photons just fade away.
Pamela: The closer the object gets to that horizon of no return the redder the light is going to be due to the Doppler shift of the Universe carrying things away from us.
Fraser: So we would even be stretched all the way out to the longest radio waves and then that’s that.
Pamela: And then no more.
Fraser: Hm.. This question comes from Brian : “As Relativity states as an object’s velocity increases its Relativistic Mass increases. I’m wondering what forms extra Mass takes on. For example, if you accelerate a ball of Iron to a high speed, the ball gets heavier as the velocity increases. Will that extra Mass the Iron gains just be more Iron or some unknown form of matter?”
So is there some kind of Mass that gets attached to objects as they move quickly different from what they were? Are there more Atoms popping in?
Pamela: I absolutely love this question. It’s the type of thing that somewhere along the line a lot of Physicists forgot to go “ha” and think about. [Laughter] So we don’t bring it up when we’re teaching.
Fraser: Yeah but a 4 year-old would just nail you with this question.
Pamela: Oh yeah, it’s really a wonderful question. The reality is that Mass and Energy are really just interchangeable things. One of the things I think I’ve said before on this show is Mass is really just frozen Energy.
When we start talking about an object’s Mass increasing as it moves faster and faster, what we’re talking about is its total Mass/Energy combined thing.
So, it’s still going to have the same number of Atoms and those Atoms are still going to be coupled to the Higgs Field, the field that nominally gives things Mass in the exact same way.
What’s changing is, as it’s moving faster and faster it’s getting more and more Kinetic Energy. We start to have to pay attention to the effects of that Kinetic Energy on how it interacts with the Universe around it.
We see that as a change in Mass. It’s the same way light which has no Mass is affected by Gravity. Energy itself can also, if you pile it up big enough, have its own affect-like Mass.
Fraser: Oh, and that’s why like when you have a Particle Accelerator and you’re taking some Proton and boosting it up to a significant fraction of the speed of light, and then you’re crashing it into something else, it’s the Kinetic Energy of the Proton that you’re turning into Mass.
This is kind of that same situation. Your spaceship made of Iron has more velocity, it’s got more energy in the System because it’s still a chunk of Iron it has to gain more Mass.
Pamela: So here what we’re talking about is its Rest Mass – the Mass that it gets from the number of Atoms it has and the number of Particles that make up all of those Atoms. That’s its Rest mass.
But then we have to add to that the Mass that it gets from its motion, its Kinetic Energy. All of that brought together is what gives us such a large effective Mass when this thing is moving so fast.
Fraser: So, if we could take this much Mass and crash it into let’s say an anti-Matter version of it moving at the same velocity in the opposite direction, would we get more energy out of the System?
Pamela: If you crash together at five kilometers per second a car and an anti-Matter car, you will get a gignormous explosion. If you crash together at nine-tenths the speed of light a car and an anti-Matter car you’re going to get way more energy out of that explosion; way more stuff going on.
But that’s true whether or not you use Matter or anti-Matter or just two Matter things. You’re just not going to have the same complete annihilation if you have Matter and Matter versus Matter and anti-Matter.
Fraser: Right, that’s why I love the anti-Matter because then you just get complete annihilation but the point is you get more energy out of the System. That’s a great question. I really like that one. Cool, thanks Brian.
This question comes from Ian Rose from Melbourne, Australia: “Can you explain why Photons have no Mass since it is Energy and according to e = mc squared it should have Mass even though it would be very small.”
Now, I’m not sure because e = this is where we go back to that equivalent right – that Energy and Mass are the same thing.
Fraser: It’s just that on one side you get Mass and on the other side you get Energy.
Pamela: And what we find is there are certain sets of parameters that lead to the creation of a Particle that quite happily has Mass and other combinations that leads you to things that have Energy.
For instances Photons, they are a pocket of Energy with a bunch of very specific properties but for whatever reason, they don’t couple to the Higgs Field so they don’t have any Mass.
At the same time, Gravitons – which we haven’t really detected yet but we’re pretty sure they’re out there – if Gravitons are out there they also will have a different set of very specific properties and they won’t have any Mass either.
What changes is some things are able to couple to the Higgs Field and experience having Mass. I wish I experienced this less. At the same time other things don’t couple to the Higgs Field and so they don’t have Mass.
In either case you can have Energy. Energy just is. It really doesn’t care if you couple to the Higgs Field or not. It is its own pure thing.
Fraser: Right, in theory.
Pamela: In theory.
Fraser: In theory because we haven’t detected the Higgs Boson yet. We haven’t been able to detect the Higgs Field. But I guess when you look at the calculation, it’s just that Energy on the one hand or you can have Mass multiplied by the speed of light squared.
Pamela: Some things just can’t take a form that allows them to have Mass. It’s just a matter of these are all different properties of how pockets of energy interact with the Universe.
One particular pocket of energy can have Mass and it might end up having all of the characteristics of an Electron. Another pocket of energy might freeze out to have all the properties of a Proton.
When you take the properties that are associated with Photons, the way the Universe builds Photons it doesn’t build them with Mass.
Fraser: As you mentioned before they are still affected by Gravity. And part of that is because it’s just like the total Mass or Energy in the System is still affected by Gravity.
Pamela: One of the really cool things is so far we have yet to find anything that isn’t affected by Gravity. And that’s just cool.
Fraser: Yeah, even the Black Hole, everything else gets sucked in but Gravity can get out.
Okay, next one comes fromand this one’s going to be a ‘head-scratcher’ [Laughter] so let’s see if we can go at it.
“According to Einstein there are no preferred frames of reference. So why isn’t it possible to consider that the twin in the spaceship is the stationery twin and the Earth and the ‘stay-at-home’ twin move away from the spaceship at relativistic speeds? Then it would be the ‘stay-at-home’ twin who remains young and the traveling twin who ages. Why is the ‘stay-at-home’s’ frame of reference to be the preferred one?”
This is that question, right, there’s no preferred place in the Universe. If I get in my spaceship and fly away at close to the speed of light, isn’t that essentially the same as you getting on the Earth and flying away from my spaceship at close to the speed of light? Why am I the one in my spaceship who ages more slowly and you’re the one back on Earth who ages more quickly?
Pamela: I have to admit this is one that has always broken me as well. It’s one of those “huh, that’s curious” types of things. I think what you want to do is think of it in terms of here we are on the Planet Earth; someone takes off and heads off to a distant Star. We relative to that distant Star aren’t moving them relative to that distant Star that’s moving quite quickly.
It’s this non-moving frame that includes us, the Star and everything else that’s in this case the Stationery Frame. It’s that distant Star that’s emitting the light that we’re both measuring our clock relative to. So you look at how are you moving relative to the light source.
It’s all relative to how you’re moving relative to that stream of light; relative to your clock of Photons. That doesn’t create a preferred frame of reference but it does give you something to set your clock by.
So take your light source, figure out how you’re moving relative to the beam of light. Whoever’s moving fastest relative to the beam of light, that’s the one who’s going to age slower? It’s highly unsatisfying.
Fraser: [Laughter] Okay, so let’s just kind of parse this. It doesn’t mean that there’s some underlying fabric of Space that you’re moving against.
Fraser: Right that it doesn’t. Okay and so if the Universe only consisted of two objects…
Pamela: And a beam of light – you need the beam of light.
Fraser: Well, what if we didn’t have a beam of light?
Pamela: Then you can’t really measure your clock so easily.
Fraser: No I understand but if [Laughter] you’re on the Earth and I hop on my spaceship and I fly away and I come back, you’re going to be the one who has aged more quickly than me and yet we’re the only things in the Universe, right? [Thundering Silence] Did we break you?
Pamela: I’m trying to figure out how to explain this in a sane way. It’s always relative to that stupid beam of light that isn’t really a preferred frame of reference but that’s how the human brain likes to default to.
If I’m standing there with my flashlight shining the direction for you to come home to, then you moving relative to that flashlight beam are going to age slower.
Then it always breaks though when I hand you the flashlight beam and you take off. Now it appears that I and the Earth are the ones moving. Then it starts to become a complicated dance of “oh dear how do I figure this out now”?
Fraser: So what you’re saying though is that it’s almost like you need to… if there was only two objects in the Universe, you know the Earth and the spaceship there really would be third points of view that would be able to view the whole thing.
Pamela: You have to start taking into consideration like which of you put in the energy to get yourself moving. If you and your rocket ship are the ones who put all the energy into accelerating yourself, to building up the energy necessary to increase your velocity and I’m just hanging out there, I’m not doing anything.
There clearly you’re the one doing who’s doing the motion. You have the acceleration; you have the additional Kinetic Energy that you’ve added to yourself.
Now if on the other hand, you and your spaceship watch me and the Planet Earth build this giant drive and accelerate ourselves away using this giant drive, then we’re clearly the ones putting out the energy. We’re doing the work to accelerate ourselves to close to the speed of light.
Fraser: But Russell already thought of that when he asked this question [Laughter] I didn’t include it in my read. He sort of said: “It’s not due to the acceleration because if you get in a rocket ship and you’re thrown to close the speed of light, you’re still going to experience a time dilation effect, right?”
Pamela: But the fact that you were thrown, that’s still someone has done work on you. You’ve still got the additional Kinetic Energy. You’ve still been accelerated.
Fraser: So whoever has been accelerated is the one who will experience the time dilation.
Pamela: Yeah. It hurts.
Fraser: Well, I know, we can all deal with this. [Laughter] Well I think, I’m hoping this will keep Russell happy but if not Russell, hit us back again. Now Matthew Peacock has a question and I’m not entirely sure I understand it. So, I’m going to try and read it and then we’ll go from there.
“Is it possible that Gravity equals Time? The idea of Einstein’s Relativity, widely accepted by Scientists, we accept that the analogy of the fabric of Space Time being warped as things move around. Then if we think of Space Time being a rubber-like surface or a T-shirt like surface and drop a ball in the center we can get other things to move around it and balance. My question therefore is why is there a need for the idea of Gravitons? Why can’t what we perceive to be Gravity just be Time?”
Does that make sense?
Pamela: Yeah, okay. Gravity can’t be equal to Time but I can see where he’s trying to go. Time is its own happy little thing off in the corner happily ticking forward and always moving in one direction. Although the rate at which it’s moving does vary with velocity.
Gravity on the other hand is one of these things that just keeps mucking with our paradime. Einstein came along and had this great way of looking at it which is that typical rubber skin with weights dropped into in the Earth is rolling around in the Divot made by the Sun.
We’ve all seen the models. We’ve all seen the Cosmos episode or the Planetarium show or the Planetarium demo or whatever that shows us this model.
Great, it worked, the math worked, everything was lovely until we figured out Particle Physics. So then we started building this crazy Particle Physics-based view of the Universe in which now we have Particles exchanging all of the Forces.
So we have Photons going out and explaining; “now we have Electro-magnetism because of the Photons.” We have Gravitons explaining Gravity. Suddenly we have a Particle Physics view. In the Particle Physics view, we no longer need to warp Space.
But we don’t know if it’s right! It kind of works mathematically. It breaks in the centers of Black Holes. It breaks in the first moments of the Universe. We don’t really understand what to do when you start hitting really high energies and you start hitting really high mass-density relationships.
Time just ticks forward. We’re not sure if we can explain the Universe using purely geometry or if we need to invoke a Particle Physics model. This is where things break. Gravity could simply be nothing more than the shape of Space or it could be complex interactions between particles.
Fraser: Right, but I guess the question then is Gravity equal to Time? Gravity changes Time, doesn’t it?
Pamela: It’s not so much the Gravity that’s changing Time. We think of Relativity as strictly the theory of Gravity. But Relativity actually takes on: how does everything change when you start looking at the energies and you start looking at the velocities and when you start looking at the Mass?
How do all of these different things interplay? When it comes to your Mass changing as you accelerate that actually has to do with the energy of the System changing. It’s not purely Gravity.
Fraser: Okay so I guess it’s sort of like they do two completely different things in the Universe. I guess for me it doesn’t feel like it’s a question that really makes a lot of sense. They have such different properties that you can’t kind of say that they’re the same thing.
Pamela: Right, it’s in a way like asking “is running Time?” No, Time is still there even if you’re not running.
Fraser: Right, right there we go. I like that one, just a couple more. This question comes from Eric Mars “My understanding is that light is an electromagnetic wave that constantly changes between electromagnetic energy and light energy. Time does not exist for objects traveling the speed of light and by definition, change requires Time. So how can the state of an electromagnetic wave change when the wave exists outside of Time?”
Okay so I guess what Eric is getting at is that light is part of the electromagnetic force. Photons are emitted and so you can have certain situations where it is a magnetic field, it is electromagnetic radiation. You can even have situations where the energy levels change.
It starts out as one thing and then it can be stretched out to another thing. Or a Photon can slam into a Particle and get boosted up to a higher energy level. Yet it experiences no Time – so how is that possible?
Pamela: Quantum Mechanics – okay that was a snarky answer. But [Laughter] light is just cool. One of the fascinating characteristics about it is a packet of light can essentially change depending on the observation.
One of the really cool demos that we used to do at Harvard is you send light through a polarized filter and – you only have a certain amount of light left – and then you send it through a different polarized filter and you can end up with each observation changing how much light gets through even though it’s the exact same beam.
That was a hard to understand experiment. Let me clarify that information. Light is affected by observations. There is essentially a whole packet of different probabilities for what a Photon could/should/might do as it passes through from one situation to another. It’s in the act of observing the Photon that we take and force it to be one thing instead of another.
Fraser: Then it’s almost like from the Photon’s point of view it’s always been the same. It always will be the same and there is no concept of Time.
But from our point of view the Photon is whatever it needs to be depending on the situation of the Universe where it’s at.
Pamela: This is where Quantum Mechanics starts to sound like magic.
Fraser: Right I know [Laughter] Quantum Mechanics says crazy stuff about what’s possible and yet predicts them with unerring accuracy.
Pamela: And at a certain level you can change the light. One of the cool things about the expanding Universe is as the Universe expands with this wave of light within it the light gets redder with the expansions. You’re essentially taking an imaginary wave and stretching it and changing its color.
Fraser: And the wave never experienced when it started out as a Photon of ultra-violet and somehow ended up in the Microwave. It just has no concept of Time from its point of view.
Yet for us if viewed at different times that Photon was a ultra-violet ray and now it’s a microwave and that’s just fine.
Pamela: And that’s just the way the Universe works. Light is just cool.
Fraser: I think the answer to this is that light is whatever it needs to be from our point of view, whatever the situation is appropriate for it.
And even if it goes into Mass and then comes back into Energy [Laughter] and all of that it’s just our perspective defines what state it’s in.
Pamela: One Photon isn’t going to randomly become Mass but …
Fraser: Right, but from its point of view it’s always been the same.
Fraser: Ohhhhhhhh – that’s so cool. [Laughter] Alright, this is the last question and this was sort of asked by two different people soand asked kind of the same question.
“We know that the speed of light is a constant but how do we know? Can it have some value in one part of the Universe and a different value somewhere else? Or why does it not vary in Time? For example, could the speed of light have had some value when the Milky Way Galaxy was formed and a different value at the present and yet another value at some point in the future?”
Could the speed of light be different in different parts of the Universe and at different times?
Pamela: We start from the premise of the Universe is homogeneous and isotropic. This basically says it’s the same everywhere. It’s basically the same stuff everywhere in the same distribution.
From that idea we end up with at all times light should be behaving exactly the same.
Fraser: Now is this an assumption or something that’s been experimentally proven?
Pamela: It’s something that every observation we’ve ever made proves out.
Fraser: So, if we see light say going from one Galaxy to another it seems to be moving at the same speed of light that we measure here in the lab here on Earth.
Pamela: Yes and people have explored the concept of maybe light moved at a different speed in the past but all observations that we’ve made they all disagree. Light is light. Light just keeps going at the exact same rate.
So, every observation we have points to light is light is light moving at the same speed at all times in all places. So we’re happy.
Fraser: One of the ideas that might help to explain the expansion of the Universe is that it may not be that the Universe is accelerating. It’s that the speed of light is changing. But that’s been kind of cleared up with better calculations with Dark Energy.
It appears that light moves at the same speed everywhere in the Universe and has always moved at the same speed in the past and will always move at the same speed in the future.
Pamela: And time always moves forward.
Fraser: And time always moves forward. So unless there’s some new fabulous discovery you can kind of count on that.
I think that wraps it up. We’ll talk to you next week.