Speed of light

[Guest chri]

Hi, I have this doubt about physics and This seems to be the right place to solve it

in many youtube videos from vsauce and others they show what would you see traveling at the speed of light, and they describe things as your visual field enlarging and light that was behind you reaching your eyes etc...but then I heard that the speed of light is the same in every reference frame, this means that if I'm traveling at c, or near c, light would still move at c relatively to me, and so I wouldn't experience all the cool things described before...so who's right?

I hope that the doubt is understandable despite my poor english skills..

[Awaras]

If you want to see the effects of moving at the speed of light, try this little game:

http://gamelab.mit.edu/games/a-slower-speed-of-light/

WARNING: Playing it for more than a few minutes at a time will likely make you slightly nauseous, and definitely DO NOT try it if you are prone to epilepsy seizures.

The fact that you always observe light to be moving at C no matter what your speed is what causes all those effects in the first place!

To put it simply, because the speed of light doesn't change other things like the flow of time, frequency of light (color) and the direction the light is coming from change instead...

Edited April 29, 2013 by Awaras

[Person012345]

Hi, I have this doubt about physics and This seems to be the right place to solve it

in many youtube videos from vsauce and others they show what would you see traveling at the speed of light, and they describe things as your visual field enlarging and light that was behind you reaching your eyes etc...but then I heard that the speed of light is the same in every reference frame, this means that if I'm traveling at c, or near c, light would still move at c relatively to me, and so I wouldn't experience all the cool things described before...so who's right?

I hope that the doubt is understandable despite my poor english skills..

I'm pretty sure all those videos describe what is happening at high fractions of c. When you hit c, nobody knows what it would look like, time would have stopped.

As for high fractions of c, you may be misunderstanding what a reference frame is. If someone fired a laser into space and you raced alongside it at near c, it wouldn't appear to rush ahead of you. Why? Because as far as your frame of reference is concerned, the entire universe is moving past you at near c, and the light has to be moving at c just to stay slightly ahead of you.

If you were inside a space ship traveling at near c however, a beam of light you fired within the space ship would appear to travel the length of the space ship in fractions of a second. Anything outside would have to be fighting the speed of the universe, and would appear to take it's time to travel the length of the ship.

The effects you mention describe the view peering from one reference frame (your ship) into another (the rest of the universe).

Or something like that. Apologies if I got that a bit wrong anywhere, I'm tired.

[Person012345]

For example, you would not notice time dilation on your own ship, it would appear to be the clocks of the rest of the universe that were ticking slower, not yours.

Long story short, both are correct.

[Nikolai]

If someone fired a laser into space and you raced alongside it at near c, it wouldn't appear to rush ahead of you.

Yes, it would. It would appear to you to race past you at c (exactly 299,792,458 m/s by definition), and it would appear to the one who fired the laser to be rushing away from her at c. Your definition of "meter" and "second" would be different from hers, though. You would also disagree on the color of the laser.

The speed of light in vacuum appears to be constant to every observer, regardless of where that light originated. Perception of spacetime itself changes to keep that true. There is no "speed of the universe" to fight. The whole point of relativity is that no reference frame represents the "true" frame of the universe.

[Person012345]

Yes, it would. It would appear to you to race past you at c (exactly 299,792,458 m/s by definition), and it would appear to the one who fired the laser to be rushing away from her at c. Your definition of "meter" and "second" would be different from hers, though. You would also disagree on the color of the laser.

The speed of light in vacuum appears to be constant to every observer, regardless of where that light originated. Perception of spacetime itself changes to keep that true. There is no "speed of the universe" to fight. The whole point of relativity is that no reference frame represents the "true" frame of the universe.

By speed of the universe, I mean speed of everything that isn't you.

You may be right in that if you were traveling near to c then you would be experiencing time dilation and length contraction that would alter your perception of space and time. I believe that was my mistake (hence my disclaimer at the end ), trying to think about it without those effects. Although I'm not really sure, or in a state to think about it too hard atm.

[Nikolai]

By speed of the universe, I mean speed of everything that isn't you.

I know. And there's no such thing.

EDIT: By which I mean, even things close to each other are moving with respect to each other. You might statistically reach a certain level of confidence that you're moving when you're traveling at a high fraction of c compared to most matter you can detect, but you can't tell when you're sitting still.

And more to the point, the rules work the same way whether you're rushing past the universe or the universe is rushing past you. Neither reference frame is "preferred".

Edited April 29, 2013 by Nikolai
Added more detail

[Person012345]

I know. And there's no such thing.

EDIT: By which I mean, even things close to each other are moving with respect to each other. You might statistically reach a certain level of confidence that you're moving when you're traveling at a high fraction of c compared to most matter you can detect, but you can't tell when you're sitting still.

And more to the point, the rules work the same way whether you're rushing past the universe or the universe is rushing past you. Neither reference frame is "preferred".

I'm aware of all that. "You might statistically reach a certain level of confidence that you're moving when you're traveling at a high fraction of c compared to most matter you can detect". That's essentially what I was talking about. As we move around the universe, it is just as valid to say the entire universe moves around us. In much the same way, as it's valid to say we are moving at a high fraction of c, it's equally valid to say that the rest of the universe is moving at a high fraction of c past (relative to) us. They're the same thing, so I'm not sure what you mean by "there's no such thing".

[Nikolai]

They're the same thing, so I'm not sure what you mean by "there's no such thing".

There's no such thing as "the speed of everything that isn't you". Not to put too fine a point on it, but you can't look outside and say, "Oh, everything else is moving at x" (where x is some singular velocity). Everything will be moving around at different velocities with respect to one another. You could take some kind of average, perhaps, or come up with some other mathematical scheme to come up with a reference frame you'll call "fixed" for convenience's sake. But it's not as if every object will be moving past your ship at some fixed speed; using the objects alone, you won't be able to tell what "stationary" is to resolution much better than a few hundred kilometers per second. And even if you could, the best you could really offer is "I'm moving at the same speed as these objects to within such-and-such a resolution".

I'm trying to point out that there's no sense in which things "outside" your ship "would have to be fighting the speed of the universe". There's no such thing for them to "fight".

[K^2]

as it's valid to say we are moving at a high fraction of c, it's equally valid to say that the rest of the universe is moving at a high fraction of c past (relative to) us. They're the same thing, so I'm not sure what you mean by "there's no such thing".

Neither of these are valid. You can't say, "You are traveling at high fraction of c". You can only say, "You are traveling at high fraction of c relative to ..." And the rest of the universe travels at very different speeds.

Now, what you can say is that you are traveling at high fraction of c relative to nearby stars. The stars within a few hundred light years, id est, all the visible ones, are not going to be moving very fast relative to each other, in general. So you can take their average velocity and measure speed relative to that. (Or the interstellar medium, which should be moving about as fast.) In that case you can say that if you are traveling at high fraction of c relative to these, they move at high fraction of c relative to you. And since these are going to be the only objects you see beside your ship, we can now talk about what sort of an effect this has on what you are going to see.

So that's the answer to the OP's question. It's not the fact that you're moving at high fraction of c that changes how things look. It's the fact these particular objects you look at are rushing by you at high fraction of c that makes them look that way. And that's an absolute regardless of your chosen frame of reference. So there is no contradiction with speed of light being a constant.

[Nikolai]

Neither of these are valid. You can't say, "You are traveling at high fraction of c". You can only say, "You are traveling at high fraction of c relative to ..." And the rest of the universe travels at very different speeds.

Thank you. You put that much better than I did.

[nyrath]

Yes, I agree with everything that has been said.

But for the original poster, just in case it was not clear: relativisitic time dialation, mass increase, and length shortening is a consequence of making the assumption that the speed of light in a vacuum is constant for all observers in all frames. Once you make that initial assumption, the other phenomenon are forced to occur by the logical consequence of how the laws of physics interact.

[Person012345]

There's no such thing as "the speed of everything that isn't you". Not to put too fine a point on it, but you can't look outside and say, "Oh, everything else is moving at x" (where x is some singular velocity). Everything will be moving around at different velocities with respect to one another. You could take some kind of average, perhaps, or come up with some other mathematical scheme to come up with a reference frame you'll call "fixed" for convenience's sake. But it's not as if every object will be moving past your ship at some fixed speed; using the objects alone, you won't be able to tell what "stationary" is to resolution much better than a few hundred kilometers per second. And even if you could, the best you could really offer is "I'm moving at the same speed as these objects to within such-and-such a resolution".

I'm trying to point out that there's no sense in which things "outside" your ship "would have to be fighting the speed of the universe". There's no such thing for them to "fight".

Everything would have it's own velocity relative to you... I'm not sure where we're disagreeing here... Maybe there has been some miscommunication?

But I already admitted my error on the other point so I don't know why you're still pushing that.

[Nikolai]

Maybe there has been some miscommunication?

That's possible.

But I already admitted my error on the other point so I don't know why you're still pushing that.

I read the admission as "Maybe I made a mistake" and not "I made a mistake" -- as if you weren't quite positive, and needed reassurance of what reality really acts like. But maybe I failed to understand properly. My apologies if I came off sounding boorish.

[Person012345]

I read the admission as "Maybe I made a mistake" and not "I made a mistake" -- as if you weren't quite positive, and needed reassurance of what reality really acts like. But maybe I failed to understand properly. My apologies if I came off sounding boorish.

Well, it kind of was "maybe I made a mistake", but only because I was tired and couldn't think about it properly at the time. I don't tend to state things in absolute terms very often anyway. Once I had gone to sleep and come back I was able to think properly again and realised more clearly why what I was saying was wrong. >.>

[Nikolai]

I don't tend to state things in absolute terms very often anyway.

Actually, that's very humble, noble, and responsible of you. Working things like that into your daily language is a good reminder that any knowledge anyone possesses is provisional, and that we can all be taught something.