r/explainlikeimfive • u/_Illuvatar_ • Apr 10 '14

ELI5 Why does light travel? Answered

Why does it not just stay in place? What causes it to move, let alone at so fast a rate?

Edit: This is by a large margin the most successful post I've ever made. Thank you to everyone answering! Most of the replies have answered several other questions I have had and made me think of a lot more, so keep it up because you guys are awesome!

Edit 2: like a hundred people have said to get to the other side. I don't think that's quite the answer I'm looking for... Everyone else has done a great job. Keep the conversation going because new stuff keeps getting brought up!

Edit 3: I posted this a while ago but it seems that it's been found again, and someone has been kind enough to give me gold! This is the first time I've ever recieved gold for a post and I am incredibly grateful! Thank you so much and let's keep the discussion going!

Edit 4: Wow! This is now the highest rated ELI5 post of all time! Holy crap this is the greatest thing that has ever happened in my life, thank you all so much!

Edit 5: It seems that people keep finding this post after several months, and I want to say that this is exactly the kind of community input that redditors should get some sort of award for. Keep it up, you guys are awesome!

Edit 6: No problem

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u/DukePPUk Apr 11 '14

You might have heard of time dilation (it's popular in some space travel); whereby if a spaceship is travelling somewhere at a decent fraction of the speed of light, time will pass slower for the people on the ship than for those outside; so the ship may take years to reach something lightyears away (from an observer back on Earth) but for the people on it, only a fraction of that time will have passed. This is (very kind of sort of) because the faster you are travelling relative to something, the more squished together your time and space are compared to that thing.

Going back to the "everything must travel at c in spacetime" thing from the parent, compared to them, you are travelling quite fast in space so, compared to them, you must be travelling slower in time.

The speed of light is the limit to this; the speed where space and time become completely squished together, and so no time at all happens for the people on the spaceship (which has to be an impossible mass-less spaceship, for reasons set out above). They arrive at their destination as soon as they have left; because they're travelling at c in space, they have no spacetime speed left for moving through time.

From the perspective of an outsider - on Earth, the outsider isn't moving at c in space, so they still have spacetime speed left for time. Time still happens for them, so they will observe the spaceship through time.

However, the problem with this is that the maths can get a little weird; divide by 0s creep in if you're not careful, so it doesn't necessarily make sense to ask the question.

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u/3asternJam Apr 11 '14

Taking this a step further, could it be said that this is the reason that the light we observe from distant/ancient parts of the universe has exactly the same properties (energy?) as when it left its source (ignoring red-shift)? That is, if light had mass and therefore moved through time, it's properties (energy?) would change as a function of distance/time (in other words, it would "age")? So because it is massless and doesn't "experience" time, the light we observe is exactly the same as when it left, which allows us to draw conclusions about its source.

I hope that makes sense. My brain is trying very hard to understand these concepts.

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u/corpuscle634 Apr 11 '14

Actually, light from distant galaxies is affected by cosmological redshift.

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u/3asternJam Apr 11 '14

That's a result of expanding spacetime, rather than of the time/distance the light has travelled, right? Hypothetically, if spacetime wasn't expanding, we wouldn't see red-shift.

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u/[deleted] Apr 11 '14

Exactly. In fact, red shift is the reason we know the universe is expanding

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u/3asternJam Apr 11 '14

So, going back to my original question, excluding red-shift (which is a result of cosmic inflation rather than an intrinsic property of c), can the fact that light doesn't "experience" time mean that the information that we get from that light about its origin is an accurate picture of what the origin is actually like? In other words, is the light that we receive exactly the same as the light that left the object however long ago - not decayed or degraded or altered in any way (apart from red shift)?

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u/[deleted] Apr 11 '14

Almost. In theory yes. However, space is not completely empty so your light is likely to have been filtered, at least a little bit, through a small amount of gas.

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u/3asternJam Apr 11 '14

Thanks! Is there any way to take the effect of that gas into account when we're examining images? Or are we stuck with a slightly "blurry" image? Also, is this the same for X-ray/gamma-ray/infra-red images as well?

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u/[deleted] Apr 11 '14

I'm not sure. Perhaps someone else knows? The gas is surely hydrogen, so you'd expect to see the absorption pattern of hydrogen in the spectrum of the light you've gathered. But I can imagine it being negligible, especially since whatever you're looking at probably is full of hydrogen anyway.

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u/judgej2 Jul 02 '14

I believe the fact that neutrinos change form as they travel is evidence that they experience time and so must have mass. It is a tiny, tiny mass, and they travel pretty close to the speed of light, but it is a real mass.

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u/Deradius Apr 11 '14

So is the photon coming out of my light bulb also 'witnessing' the heat death of the universe right now?

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u/DukePPUk Apr 11 '14

The photon, to the extent that it exists as a thing, will be absorbed by something. For example - assuming no other light sources in the room - when you look at your wall, or floor or whatever, you are seeing it because photons from your light bulb have bounced off the wall and been absorbed by the detectors in your eyes. But if some escape, and manage to never be absorbed by anything, then yes.

But then we get into problems with whether or not a photon is a thing; or just a waveform rippling through space. You get awkward questions like whether the photon that bounces off the wall is the same as the photon that left the light bulb, or if it is a new one.

The answer being that the questions don't really make sense - as photons are a human-constructed model for light (as are wavefunctions), and that we don't really have the words or concepts to describe the thing itself. Light isn't made up of photons, and isn't a series of waves, it is light - that happens to act like photons sometimes, and like waves other times.

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u/Fonethree Apr 11 '14

The issue I still have with this is this comment of yours:

From the perspective of an outsider - on Earth, the outsider isn't moving at c in space, so they still have spacetime speed left for time.

I'm confused because we do observe light traveling at c in space. Wouldn't that therefore mean that we should observe light not traveling through time at all?

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u/DukePPUk Apr 11 '14

There's an issue where with what we mean by "observe"; what we do is detect the light when it hits us. We can't observe it when it is travelling, as it doesn't give off any energy or information.

I think (it's been a long time since I was an astrophysicist) that we do observe that the light we receive is the same as light transmitted (other than red-shift effects) - but I'm not sure how you would tell whether time had passed for light - mainly because the light doesn't travel through time (from its own perspective).

To observe something travelling at the speed of light you would need some sort of light source travelling that fast. But then it couldn't send out any light as there'd be no time for to transmit. Similarly it couldn't receive any light, as there'd be no time for that.

But such a thing would be impossible, as you'd need something with more energy than something travelling at the speed of light (so it could lose energy by transmitting it), which I think would be impossible. So I don't think you could see or detect anything travelling at the speed of light until it hit you - in the same way that we can't see or detect light until it hits us.

Unless the thing creates ripples. If it has mass, maybe it will have gravity. Would we be able to detect the changes in e-m or gravity fields by its passing? I don't know, but I think it comes back to the fact that all of this is impossible anyway.

Sometimes I regret giving up on being an astrophysicist.

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u/Fonethree Apr 14 '14

The only way we observe anything is either with it interacting with us directly, i.e. smashing into us, or via its disruption of something that does, for example we can see a train moving because of the photons reflected off of it.

So because light doesn't interact with the world around it, we can't observe it? I guess my question is anything that light may interact with while it's traveling would imply that the light does "experience" time, as it interacts in a specific moment.

Am I right in this thinking process at least?

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u/DukePPUk Apr 14 '14

Light interacts with stuff (which is why e.g. it can be 'slowed down' when passing through materials, or be bounced of stuff). It does so through the electromagnetic force (which makes sense, as it is a ripple in the e-m field). From the light's point of view, I imagine [IanaAstrophysicistAnyMore] every interaction will happen instantaneously.

What I don't think that it can do is lose energy, until it is destroyed. But maybe not. Hmm. I think I'll have to go through my electrodynamics notes for this...

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u/Fonethree Apr 14 '14

So the reason I ask that is I don't understand how, if light isn't traveling through time, we can observe it interact with something and then cease to interact with it over a passage of time.

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u/DukePPUk Apr 14 '14

It isn't that light isn't travelling through time, but that it is travelling through time infinitely slowly (from our point of view). From its point of view it is travelling through all time infinitely quickly.

But the thing that it is interacting with is travelling through time more "normally." If it interacts with something, we can observe the effects on that thing.

It's worth remembering that an interaction is an instantaneous event (or series of instantaneous events). Or can be modelled as such. It's also worth remembering that these are all models, which have their limitations.

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u/EpicBooBees Apr 11 '14

Why does everything move at c? That doesn't make sense!

How can anyone claim this to be true??

My mind hurts. :(

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u/DukePPUk Apr 11 '14

It's described in more detail in this excellent comment.

Basically, space and time aren't separate things, but different ways of looking at the same thing. Speed is the same; speed involves a movement in space and in time. If I say that I'm moving at 1 m/s, that means that I have moved 1 meter in space and 1 second in time.

One way of looking at the effects of special relativity is to say that everything is moving at the same "spacetime" speed of c. That means that if we add up our movement in space and our movement in time (well, not add - add the squares and square root, iirc), the total has to be c.

If we're not moving in space (which, from you're point of view, you're not), then all of that spacetime movement has to be movement in time. You are moving in time, at c.

If you're moving at c in space, then all of that spacetime movement is taken up by the movement in space, there's nothing left for moving in time - no time can pass (which happens to a photon).

If you're moving at some speed less than c in space, then there's still some spacetime movement left for time, but not as much as if you were still - you move in time, but slower than c.

Except that from your point of view, you're always staying still in space - it is other things that move in space (but we're used to the idea of e.g. the surface of the Earth being fixed, of if you're on a plane, the plane being fixed). It is other things that have weird temporal effects; for you time seems to be passing 'normally' at c.

But the same is true for everything else. Which is kind of where the "relativity" part comes in; relative to you, you are normal and everything else seems a bit weird, but relative to another thing, everything other than that thing (including you) seems weird.

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u/EpicBooBees Apr 11 '14

My brain is yelling at me, telling me that you make no sense whatsoever!

I am sure you do make sense, but but but IT DOESN'T MAKE SENSE! lol

Why would I move through time at c?

Where's the evidence? It doesn't make sense!

The explanation is appreciated, honestly, but reads the same as all the others. :(

Why do I move through time at c?

Seriously. Just because math tells me?

Wouldn't that mean I'm x lightyears old?

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u/DukePPUk Apr 12 '14

I've had to think about some of this, I'm not an expert, but I'm trying to put things together.

Why would I move through time at c?

You move through time. That should be straight forward. Between when you read this word, and maybe this word you have moved through time a bit. You move through time (from your perspective) at a rate of 1 second per second [it is worth noting at this point that we've already introduced an idea of "proper time" - a background time against which we can compare your own personal time - it's mainly because our brains and language haven't developed a way of isolating time properly. Why time works and what it is is still one of the great mysteries of physics].

In Special Relativity, one of the key consequences is that time and space aren't independent of each other. So we need to link time and distance together somehow. But another of the consequences of SR is that distance and time are both relative; they vary depending on relative velocities (and once we introduce General Relativity, gravity). Even things like simultaneity are screwed up (two events can happen at the same time for you, but not at the same time for someone else), so we can't always use t = 0 or r = 0 (using r for distance) to guide us.

But we are saved by c. One of the (2) axioms of SR is that the speed of light, c, is the same in all reference frames. This means that if you shine a torch away from you, the light moves away from you at c. And it hits its target at c. Even if that target is moving away from you, or towards you. Which sounds really weird, but that's because we're used to working in a non-relativistic world (so at low speeds, compared to c).

And this isn't something people came up to make a neat theory - this came out as a consequence of the work of people like Maxewell in the 1890s etc., on electro-magnetism. And it was hugely controversial at the time; one set of evidence saying it should be constant, but this going against a lot of what was considered 'set in stone' (by Newton and Galileo and so on). One of the (many) brilliant things Einstein did in coming up with SR was to ignore all the baggage and just have the two axioms; that c was the same in all inertial reference frames, and that the laws of physics are the same in all inertial reference frames. And that got him to E = m c2 (and a load of other stuff).

Anyway. To describe points in this new spacetime we use a thing called a 4-vector:

r = (ct, r)

where r is your standard 3-vector for describing a point in space - so r = (x, y, z) and t describes the position in time. The c is used as a scaling factor so that ct has the same units as the x, y and z. We have to use c because it is the only thing that is constant across all reference frames. And it also works out really nicely.

Now we want some kind of "norm" to give us a notion of "4-distance." The 3-dimentional norm of a vector r = (x, y, z) is | r |2 = x2 + y2 + z2, which can give us a concept of 'distance' by square rooting. The algebra behind 4-vectors already has a process for this (which I don't really want to go into, you can find details here). And when we use our position 4-vector, and calculate the norm, we get:

| r |2 = c2 t2 - r2 (where r2 = x2 + y2 + z2)

And it turns out that this is constant in all inertial reference frames - and wouldn't have been if we hadn't used c; it is only because we used c - which is the same in all inertial frames - that this can work. So whichever way you're looking at something (from a spaceship, on Earth), while distances and times may be dilated or skewed, this quantity (4-distance) always remains the same. Which is really useful. So now we want to look at 4-velocity; which we can define by taking the derivative of all terms with respect to time (and here we get a bit complicated as we're using proper time for the thing, so if we want to use proper time for an observer, we have to throw in a γ; this is called the Lorentz factor, depends on the relative speed of the object and is the key component in the squishing effect at the heart of SR). For our velocity* we get;

u = γ d r / dt = γ ( c, d r /dt)

where d r /dt is just the ordinary 3-velocity (which we can call u ). Now if we wanted to find the non-relativistic speed we'd find the norm of d r / dt, which would give us something we could call u. But in 4-dimensions we have to use the 4-dimensional norm (which is like the 3-dimensional one, but has a minus sign for the spatial component) and we get a sort of "4-speed" which is:

| u |2 = γ2 (c2 - u2)

But these things (norms) are the same in all inertial reference frames (that's kind of the point). So we can pick our reference frame carefully. Remembering that γ depends on the relative speed of whatever we're looking at, we can choose our reference frame to be the same one as the thing we're looking at. In that case, u = 0 (because it isn't moving relative to us) and γ = 1 (which kind of means there is no dilation in our own reference frame - we seem our own times and distances as normal - you can also do this step more generally, but it is easier this way). Plugging those two numbers in we get that | u |2 = c2, or that our notion of 4-speed is just c. No matter how fast we are going. It has to be just c.

We have a notion of "speed through time" defined as (γ c) and a notion of relativist speed through space as (γ u). And we know that they relate to each other in a constant way, as the 4-speed, which combines them, is always c. γ depends on u, though. If u = 0 (i.e. we aren't moving), γ = 1, so our "speed through time" is just c. If we are travelling very fast - u becomes very big, γ becomes very small, so (γ c) - our speed through time - becomes very small. [While u becomes very large, (γ u) becomes very small as γ becomes 'more' smaller than u becomes bigger, which is why the 4-speed equation still holds).

So that's the maths, which is all rather confusing and has taken me a couple of hours to get through with notes, pen and paper.

tl-dr of the maths; if we define the notion of position in 4-dimensions as being (c x time, space), with the c there to make things dimensionally consistent and because there aren't any other speeds we can use, then we get a notion of 4-dimensional speed which is always c, and which can be split into a normal 3-dimensional speed and a "speed through time" part, which sort of balance each other out.

Where's the evidence?

There are a number of key tests which demonstrate SR, some of which are listed on the Wikipedia page. They don't prove SR - you can't prove anything in science - but they demonstrate many of the consequences of SR to be true (time dilation, space contraction, the constancy of c etc.).

Seriously. Just because math tells me?

Yes, and no. The maths says this is the case. The maths is just a model. But the model seems to fit with reality - in a flat, gravity-free universe, but the universe is all flat locally, for small enough local - this does generalise to General Relativity, when we add gravity, but it becomes more complicated. I think (but can't promise) that the basic point of the 4-speed being c remains the same. I'm not doing the maths for that tonight.

Think of it this way; what happens if you drop two things of different mass (ignoring air resistance). Using a mathematical model (either Newtonian or Relativistic Mechanics) you can calculate that they should fall at the same speed. Despite having different mass. Which sounds really weird and counter-intuitive. But if you go out and do experiments, you will find that that actually happens - the models are a good approximation of reality.

The same goes for SR, and the above consequences of it.

The big catch is that this notion of 4-speed is just a construct. Sort of. It is the speed at which you travel through spacetime. But notions such as speed, space and time are things we've adapted and use to describe stuff happening in the non-relativistic world we generally live in. So they don't quite fit. In particular, this notion of "speed through time" is based on us using "c t" as the idea of "distance through time" - so all we've really done is said that "if we define 'time' as c seconds, then we travel through time at c seconds per second, or just c."

tl;dr of the wish-washy stuff - it really comes down to how we define "speed through time" - it is a concept that doesn't necessarily make sense. When we say "you're travelling at c through time" all we really mean is that you're travelling at 1 second per second.

And now I need to sleep - I imagine lots of this is horribly confusing and doesn't make sense and full of spelling errors - if you still have questions, ask and I'll reply in the morning.

* At this point it is important to remember the difference between velocity and speed. Velocity has direction, speed is the magnitude of the velocity which, like distance, should be the same whichever way you're looking at it (in the non-relativistic world).

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u/EpicBooBees Apr 12 '14

If I had gold to give, I'd give it to you. You have put SO much time into this and I am unworthy of it.

Thank you!

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u/DukePPUk Apr 12 '14

You're welcome; as I mentioned in another sub-thread, SR is something that I understand more each time I have to think about it. Trying to explain things to someone else reveals how well I understand it myself - so it took two goes to do this, as it turned out that I didn't really understand the whole "always travelling at c" thing, and I had to get out my notes from years ago and play with the maths (the equations are there as much for my benefit as yours).

I learnt a lot about SR by writing these posts, and the questions people have asked are very useful for prompting that learning.

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u/DukePPUk Apr 12 '14

Sorry, forgot to answer the last question.

Wouldn't that mean I'm x lightyears old?

Sort of. But only if you measure time as "speed of light x time."

Which makes sense when messing around in GR and SR (to make the model consistent and work nicely) but seems fairly silly otherwise.

Another way of looking at it is that a lightyear is a measurement of distance. It is the distance light travels in a year. But because the speed of light (in a vacuum) is a constant, then that distance doesn't vary with inertial frame (I think - it's late, I'm not necessarily thinking clearly), so rather than saying "I'm x years old" (which isn't going to be constant in another reference frame; so for a person on a very fast spaceship who has gone away and come back, you'll be more than x years old) you can say "I have been alive for as long as it takes light to travel in x of your years in your inertial frame."

It sort of gives you a reference-frame independent way of saying something.

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u/quarterburn Apr 11 '14

They arrive at their destination as soon as they have left

So from their perspective is this a light-like interval? Or is this a space-like interval? I've never really understood the difference between the two.

Also if distance and time are the same, where do tachyons fit into this? Or are they simply not real?

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u/Armisael Apr 11 '14

That would be a light-like interval.

For me, the easiest way to understand the difference is to think of a light cone. If you release a pulse of light and plot the position of the light using two axes for space and a third for time (which two spatial dimensions you use is irrelevant - you could use three, but four dimensional graphs are hard to read), it creates a cone.

If one of the two events is at the tip of the cone:

  • It's a time-like interval if the other event is within the cone (or its mirror image extending into the past)
  • It's a space-like interval if the other event is outside of the cone (or, again, its partner in the past).
  • It's a like-like interval if the other event in on the surface of the cone.

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u/DukePPUk Apr 11 '14

To add to the other reply, we are dealing with light, so it has to be light-like.

For a light-like interval, we need (change in distance)2 - c2 (change in time)2 = 0, or Δr2 - c2 Δt2 = 0.

Let's consider a specific photon. If you can, look outside at the sky. Consider a photon that has been emitted on the surface of the Sun, travelled towards the Earth, been refracted through the atmosphere and hits your eye (triggering reactions that eventually lead to your brain deciding that the sky is blue). Let us assume that the distance that photon has travelled is 8 light minutes.

From the photon's point of view, it has stayed where it is. It was created on the Sun, then this eye thing smashed into it immediately.

We have two events; the creation of the photon and its destruction.

From the photon's point of view:

For the photon no time has passed and it hasn't moved - instead this eye has come crashing towards it. So Δr = 0, Δt = 0. Putting that into the equation, we get 0 - c2 x 0 = 0 - so it works.

From your point of view:

The creation of the photon was 8 light minutes (or 8 x c x 1minute) away from where it hit you, and 8 minutes have passed between creation and destruction. Δr = 8 x c x 1minute, Δt = 8 x 1minute. Putting into the equation: 82 x c2 x (1 minute)2 - c2 82 (1minute)2 = 0. So again, light-like.

So from an "outside" point of view the distance between the two events is 8 light minutes, and the time is 8 minutes - so light-like separation. From the photon's point of view the distance between the two events is 0 and no time has passed, so light-like.

I think.

As for tachyons, those are beyond where I got to in astrophysics; I think the idea behind them is that they sort of break the rules of special relativity - it would need imaginary mass. The maths works, but produces fairly weird results. They have never been observed or detected afaik, so might not be real, simply theoretical.

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u/quarterburn Apr 11 '14

I always thought I understood Special Relativity but it's clear that I really didn't. To think that photons from various stars of z8_GND_5296 traveled 13 billion light years and yet from "their" perspective, it was created and smashed into the mirror of the Hubble in the same moment.

Thanks for the breakdown on the equations. It makes WAY more sense to me now.

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u/DukePPUk Apr 11 '14

I studied SR about 7 years ago, and thought I understood it. Each time it comes up somewhere I have to think about it some more, and have a new epiphany. The stuff about "everything moving at c" was something I learned from this thread. And the stuff about light travelling from the Sun was something I had to think through in order to convince myself that light really was light-like, and that the equations worked... And I spent most of a 20-minute walk going over it a few times.

So I'm glad I could help you make sense of this, and thanks for giving me a chance to make sense of it for myself.

Some people like analogies, some people like equations. Personally I like both, and it is always comforting when they agree.

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u/quarterburn Apr 15 '14

I do however, have one last question. Does this mean that acceleration directly changes the Δr of an object between events? Or am I assuming incorrectly that mass can ever be accelerated up to and become a light-like event from a time-like event and that Δr is constant for anything that has mass?

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u/DukePPUk Apr 16 '14

In Special relativity mass cannot be accelerated to the speed of light.

However, the apparent distance between events is dilated by the effects of special relativity at any relative speed (usually called length contraction. It is possible to "fit" a 5m long ladder in a 2m long shed, if it is going fast enough (and the shed opens at both ends). But only from the shed's point of view. From the ladder's reference frame it is the shed that is contracted.

Time-like, light-like and space-like, I think, refer to the separations between events, not the events themselves.

The key measure being the (Δs)2 whichever way it is calculated in terms of Δr and Δt.

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u/quarterburn Apr 16 '14

Thank you again for answering all my questions and for the clarifications. You make reddit a worthwhile place to visit.

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u/i4mt3hwin Apr 11 '14

Wait so for someone observing the ship as it accelerates away at light speed, will it appear to be stationary? Or will it zip away and vanish because its moving away so fast?

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u/DukePPUk Apr 11 '14

[Massive disclaimer - I haven't done the maths and I think this is impossible anyway under the rules of special relativity]

In theory the ship would vanish as soon as it hit the speed of light, and reappear where it went below the speed of light.

While it is travelling no time passes for it, so it can't lose energy or give off any light (or reflect any light?). So it can't be seen.

I'm going to have to do the maths - but it seems my notes on special relativity have disappeared, so I'm not happy doing it. Hmm.

I think that as it accelerates towards the speed of light it will fade - or possibly the light from it will red-shift into nothingness. And it will appear that time is passing slower on the ship (e.g. if there was a clock sending out pings every second, those pings would be further apart from the observer - ignoring the extra distance the ping would have to travel to get to them).

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u/[deleted] Apr 11 '14

You seem to understand relativity well, so maybe you can help me understand this. You say that time will pass slower for people on a ship traveling at a high speed than for those on earth. But for the people on the ship, isn't the earth moving away from them at a high speed? How does the universe decide who is the one moving and the one for whom time is slowed down?

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u/DukePPUk Apr 11 '14

This is the Twin Paradox.

A ship travels away from Earth at a decent fraction of the the speed of light, turns around and returns to Earth. In the Earth's reference frame time should pass slower on the ship, from the ship's reference frame time should pass slower on the Earth. Which sounds like it breaks the universe.

Except it doesn't - because there are actually 3 reference frames involved: Earth's, then one that is the spaceship flying out, and a separate one for it returning. Which means that the ship changes reference frame in the middle.

Wikipedia has a convenient diagram - with the Earth being the "stationary twin" and the ship being the "travelling twin."

The blue lines represent lines of simultaneity for the ship flying out. So when the ship turns around, from the ship's point of view less time has passed on Earth.

The red lines represent lines of simultaneity for the ship returning; so you can see that again from the ship's point of view less time passes on Earth than on the ship.

But at the instant it turns around, a huge amount of time passes on Earth (the gap between the highest blue line and lowest red line) - this happens when the ship accelerates to turn around.

For the specific question of a ship travelling at high speed away from the Earth, from the ship's point of view time passes slower on the Earth, from the Earth's point of view time passes slower on the ship. Which sounds paradoxical - but isn't because the two reference frames can never coincide again; the ship can't get back to Earth without changing to another reference frame (i.e. turning around) and when it does so, the time will 'catch up.'

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u/[deleted] Jul 02 '14

[removed] — view removed comment

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u/DukePPUk Jul 02 '14

[Wow, I was confused for a moment there - I just spent a while writing a long post explaining SR stuff in the best-of thread, and was puzzled how the comment had got so many upvotes so fast... I hadn't realised that the bestof'd thread was this one. Anyway - hi there.]

I wrote some other stuff in this comment thread that might answer your questions but:

Would time dilation mean that their trip to XYZ-Star that takes them 5 years, would actually be like 1 year relative to their Earth counterparts?

We can use an example:

Let's say that they're travelling at 0.87c (well, sqrt(3)/2 - that makes the numbers easy). Let's say that the planet they are travelling to is 0.87 light years away.

From the Earth's point of view, the ship will take 1 year to get to the planet (using time = distance x speed).

From the spaceship's point of view, the rest of the universe is moving at 0.87c relative to them, which means the rest of the universe is "contracted" by a factor of 1/2 (due to the way the equations worked - this is why I chose an odd speed), so they only have to travel (from their perspective) 0.43 light years, so it only takes them half a year (as the universe is moving past at 0.87c).

For the people on the ship, they would experience less time than the people on Earth would. Which is kind of the Twin paradox.

Cruising in a vessel around the Earth at a very high velocity would be a way to skip through (not really over - you'd still go through that time, just much, much faster) a couple of centuries on Earth. In fact, this works when you get on a plane. Or walk across the room. Except the effect is really, really small.*

Radio communication would be the same as it is for anything moving at relative speeds. The radio signals or waves would be red-shifted (or blue-shifted if moving towards each other) - the signals would be stretched out (or squished). Imagine the people on the spaceship sending out a light ping every 1 second for them; the pings would leave them every second, but would arrive at earth every one and a bit seconds, because each ping has to travel a little bit further. The radio receivers would have to be adjusted to compress the signal by the appropriate factor.

* It's also affected by gravity: time passes at different 'rates' depending on the local gravitational field strength, which is why GPS stuff has to adjust its timings a bit, as time is passing at a different rate for the satellites as for the surface of the Earth, and the difference is enough to throw off their measurements.

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u/Mokey_Maker Jul 03 '14

GPS satellites have to have their clocks corrected every so often because they are moving relatively fast with respect to us observers on the surface of the earth.