A train going with the speed of light

[Insinuator]

The other thing is you wouldn't be able to walk up the train, because then you'd be going faster than the speed of light, which is impossible.

Actually, untrue. This is one of the fascinating qualities of our universe and is reflected in the theory of Special Relativity. If the train is moving at close to the speed of light, it is doing so in relation to some point outside itself. However, a passenger sitting in the train is not moving at all in relation to the train. If he walks forward or backwards, it is only in relation to the train, not to an outside point.

We can see the proof of this in the movement of celestial bodies, particularly galaxies. The universe is expanding, and very quickly. At our current time, it expands at near the speed of light. So, if you think about Galaxy A moving along a line parallel with but away from Galaxy B at, say, .9c, the combined velocities are 1.8c. But that can't be, right? That's when Special Relativity steps in with it's reference points. You can't reference two moving bodies from a single reference point, basically.

[Frogger5]

So, your saying that if there was a superhuman on the train, then they could run along the train near the speed of light?

[powershot]

The other thing is you wouldn't be able to walk up the train, because then you'd be going faster than the speed of light, which is impossible.

Actually, untrue. This is one of the fascinating qualities of our universe and is reflected in the theory of Special Relativity. If the train is moving at close to the speed of light, it is doing so in relation to some point outside itself. However, a passenger sitting in the train is not moving at all in relation to the train. If he walks forward or backwards, it is only in relation to the train, not to an outside point.

We can see the proof of this in the movement of celestial bodies, particularly galaxies. The universe is expanding, and very quickly. At our current time, it expands at near the speed of light. So, if you think about Galaxy A moving along a line parallel with but away from Galaxy B at, say, .9c, the combined velocities are 1.8c. But that can't be, right? That's when Special Relativity steps in with it's reference points. You can't reference two moving bodies from a single reference point, basically.

Your absolutely right, the movement of another object far away doesn't affect the speed of another unless it has gravity.

[johndh]

I just had an interesting thought... launch a rocket through space at .6c, and that rockets launches another rocket at .6c. The second rocket should be moving at 1.2c relative to the rest of the universe, right?

Sorry, I've only thought about this for a couple seconds, so there could be some colossal hole in it that I'm missing.

[Frogger5]

But in order for a rocket to launch another rocket, it would slow down lots from the thrust of the second rocket launching. Especially seeing as the second rocket is launching at the speed of the first rocket. Also, the second rocket would have to use about twice the amount of fuel to launch, because of the force of the first rocket moving, it would need twice the thrust to achieve .6c.

Im no physics expert, but would guess no, it would not work.

But as i stated just before, speed doesn't have an effect, only acceleration. I think that it might work for a few seconds, but after it is far away enough from the first rocket, it would lose speed. (unless you pumped more fuel into the engines.)

[Insinuator]

I just had an interesting thought... launch a rocket through space at .6c, and that rockets launches another rocket at .6c. The second rocket should be moving at 1.2c relative to the rest of the universe, right?

Sorry, I've only thought about this for a couple seconds, so there could be some colossal hole in it that I'm missing.

No. Your reference points are messed up. Each rocket is moving at .6c from a central point. That point is the reference for their speed.

Think about it this way: Why do you perceive them as moving at .6c? Take, for instance, that rocket fired from the surface of Earth. Now, you could say that it's already moving faster than .6c because the Earth is whipping about the Sun at 30Km/sec. But wait, the Sun is orbiting the Milky Way at 220Km/sec! And faster still, our galaxy is zipping about the universe! But these DO NOT accumulate velocities because where does it end? The universe is almost certainly infinite, so where is the "Ultimate reference point"? According to current scientific understanding(and how I'm relaying it ), there isn't one.

[Frogger5]

So, i suppose if your were to add all these velocities together in a simple equation, say, from the whole universe to a person. Then surely the number you come out with is really really bloody high?

[Insinuator]

Exactly. And the result really would be meaningless.

[Dunno]

Hypothetically, there wouldn't be any blur - you'd see the light from everything ahead of you condense into a disc in front of you, and the light from everything behind you condense into a disc behind you, everything in front dramatically red-shifted, everything behind dramatically blue-shifted, as you approached c.

Actually, untrue. This is one of the fascinating qualities of our universe and is reflected in the theory of Special Relativity. If the train is moving at close to the speed of light, it is doing so in relation to some point outside itself. However, a passenger sitting in the train is not moving at all in relation to the train. If he walks forward or backwards, it is only in relation to the train, not to an outside point.

So, after we stop accelerating, everything inside should behave normal, according to what Insinuator said. So what happens with those "discs" TSI mentioned? And what happens if you turn on a flashlight and point it outside the window?

[Frogger5]

My guess is this:
Well, think of it this way. Say your driving in a car down a straight highway. When you look out the side window, you see everything whizzing past. When you look out the front or back, the effect it much more reduced. If your going near the speed of light, theoretically, the effect would be magnified. As for your flashlight, i think you'd be able to see its effects inside, (probably the reflection on the window) but nothing outside.

[thespaceinvader]

Hypothetically, there wouldn't be any blur - you'd see the light from everything ahead of you condense into a disc in front of you, and the light from everything behind you condense into a disc behind you, everything in front dramatically red-shifted, everything behind dramatically blue-shifted, as you approached c.

Actually, untrue. This is one of the fascinating qualities of our universe and is reflected in the theory of Special Relativity. If the train is moving at close to the speed of light, it is doing so in relation to some point outside itself. However, a passenger sitting in the train is not moving at all in relation to the train. If he walks forward or backwards, it is only in relation to the train, not to an outside point.

So, after we stop accelerating, everything inside should behave normal, according to what Insinuator said. So what happens with those "discs" TSI mentioned? And what happens if you turn on a flashlight and point it outside the window?

IIRC from the book, and IIUC from my own knowledge, things would zip past infinitely fast from front to back.

[powershot]

I think(from this interesting thread) that making a train go the speed of light is crazy, dangerous, and basically impossible. And as they said before the flashlight wouldn't do a thing.

Off topic: This is one of the most interesting thread I have encountered. And it grew tremendously fast.

[Captain_Wrathbow]

I think that making a train go the speed of light is crazy, dangerous, and basically impossible.

Great observation.

[powershot]

I think that making a train go the speed of light is crazy, dangerous, and basically impossible.

Great observation.

[pauxlo]

I just had an interesting thought... launch a rocket through space at .6c, and that rockets launches another rocket at .6c. The second rocket should be moving at 1.2c relative to the rest of the universe, right?

Sorry, I've only thought about this for a couple seconds, so there could be some colossal hole in it that I'm missing.

The missing hole is that velocities are not simply additive. They are (approximately) for low velocities like we have on earth, but they are not if we start to compare with c.

To restate the problem:

We have an observer A, and a rocket B which flies relative to A with velocity v. Then we have a rocket C which flies relative to B with velocity u (in same direction, to simplify things). The question is, what velocity of C will A measure?

The formula is s = (v + u) / (1+v*u/c^2) (with c being the speed of light).

If we put 0.6c for v and u, we receive s = (0.6c + 0.6c)/(1+ 0.36c^2/c^2) = 1.2c/1.36 ≈ 0.88c.


Anyone who wants to discuss this I recommend first reading some articles about the special theory of relativity.