# Faster than light

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I have a question about faster than light speed.

For what i understood of relativity, it is impossible for anything to have a speed faster than light in a local frame, but the thing i dont understand is how does that make faster than light speed impossible globally ?

I hope i posed the question clearly ?

Mandrake

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i feel ur question is not valid one.sr is global because it's a flat geometry with the metric(1,-1,-1,-1).so the pridiction of limit for the speed to be maximum ofc will hold good for global too.i am not an expert ,i invite any corrections from ur side.

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>> I hope i posed the question clearly ?

sadly you didn´t. Velocity (and thus it´s magnitude speed) as I know it is defined at a point (as an tangential vector on the curve x(t), where x(t) is position at time t). I don´t really understand what you mean by being faster than light globally (of course I can be faster than light: Send a ray of light to the sun and then reflect it to a point 10m away from me ... I´ll be there faster).

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You both go from point A to point B and you will indeed be there first but that has nothing to do with actually travelling faster than lightspeed. You just took another route but never exceeded lightspeed. So you CAN get somewhere faster then light if you take another route but you will NEVER go faster than lightspeed.

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We don't know it's impossible to travel faster than light - we're still looking for things like tachyons, and although they haven't been found yet, it doesn't necessarily mean that they don't exist.

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It would also take enormous emounts of energy to travel as fast as light, more than we know to exist in our galexy. The weird thing to think about is since time slows downs the faster to the speed of light you travel, and in theory it stops at the speed of light, then doesn't that imply that it would reverse once you start traveling faster than light?

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This is intuitivly looking at light like we look at sound. If I shoot a gun and then travel near the speed of sound I will be behind the sound of the gun. If I travel at the speed of sound then the gun sound has stopped along side me. If I go faster than sound and wait the sound of the gun will be behind me and I will catch up to and hear things that happened before it was shot. Light can not be counted on to behave intuitivly.

Just aman

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Put your hand on a wall where there's a corner. Then move your hand to the opposite wall. (I'm assuming that the corner is a right corner.) The hand of your shadow can move faster than the actual hand. (I'm assuming this because it took your hand's shadow the same time to travel a greater distance than your hand in that same amount of time.) If you did this in a plane perpendicular to the source of light, then we can draw a right triangle ABC. c is the the distance your hand traveled. a + b is the distance the shadow of the hand traveled. a+b>c.

suppose that someone constructed this on a grander scale. c is a light-second. my hand travels at the speed of light (or very close to it). Will the shadow surpass the speed of light?

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Put your hand on a wall where there's a corner. Then move your hand to the opposite wall. (I'm assuming that the corner is a right corner.) The hand of your shadow can move faster than the actual hand. (I'm assuming this because it took your hand's shadow the same time to travel a greater distance than your hand in that same amount of time.) If you did this in a plane perpendicular to the source of light' date=' then we can draw a right triangle ABC. c is the the distance your hand traveled. a + b is the distance the shadow of the hand traveled. a+b>c.

suppose that someone constructed this on a grander scale. c is a light-second. my hand travels at the speed of light (or very close to it). Will the shadow surpass the speed of light?[/quote']

Yes. And no laws of physics would have been broken.

Wow, quantum.

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Put your hand on a wall where there's a corner. Then move your hand to the opposite wall. (I'm assuming that the corner is a right corner.) The hand of your shadow can move faster than the actual hand. (I'm assuming this because it took your hand's shadow the same time to travel a greater distance than your hand in that same amount of time.) If you did this in a plane perpendicular to the source of light' date=' then we can draw a right triangle ABC. c is the the distance your hand traveled. a + b is the distance the shadow of the hand traveled. a+b>c.

suppose that someone constructed this on a grander scale. c is a light-second. my hand travels at the speed of light (or very close to it). Will the shadow surpass the speed of light?[/quote']

Yeah. Well it could anyway, as long as the triangle you made was big enough. A shadow is not an object, its only lack of light. So nothing is actually moving, aside from the thing casting the shadow. Interesting question though.

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Thank you for all these replies. So if i understood correctly the answer would be that is a current debate whether or not it is possible to travel faster than lightspeed ?

For what i understood of the relativity theory, was that things should be looked at locally in comoving frames, and in such a frame it was easily deduced that something couldn't travel faster than light. But in the book i was reading it was equally written that such frames where just a local representation. Much like you can do locally euclidian stuff on a sphere if it is large enough or in some distored space with some regularity. So in fact i never quite understood why something would make it impossible to travel faster than light speed globally, all in respecting the fact that in each comoving frame you do not cross this speed limit.

I hope that this time my question is more clear ?

By the way what is a tachyon ? (Isn't a particle that appears in star trek stuff ?

It really exists ?, and what is it ?)

Mandrake

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A tachyon is a theoretical particle that travels faster than the speed of light. They're looking for it but it's not been found yet.

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So what is it made off this particle ?

And how theoretically (supposing this particle exists) could it travel faster than lightspeed ? (wouldnt it need mass zero in order to be able to do so ?, but then again what would it be made off)

Are you a physicist Dave ?

Mandrake

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No, I'm doing maths but I read up on the physics stuff from time to time

I'm not exactly sure what they're made up of, or even if anyone knows what they could be made of. A quick google turned up some interesting results though.

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Define "travel faster than light speed globally"! The answer is probably quite obvious, then.

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So what is it made off this particle ?

And how theoretically (supposing this particle exists) could it travel faster than lightspeed ? (wouldnt it need mass zero in order to be able to do so ?' date=' but then again what would it be made off)

[/quote']

If v>c then you have an imaginary solution to the relativity equations for things like mass and momentum. Now, there's nothing inherent in imaginary solutions that means they don't represent legitimate, physical processes or matter - imaginary solutions crop up in various models, and they have a valid interpretation (e.g. absorption vs. reflection for light) But tachyons would necessarily be different than the matter we are used to.

Since the solution has to be imaginary, one implication is that tachyons could never travel slower than c. There is also a matter of whether there would be any way to detect imaginary mass, if it exists.

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So, why do we even have an idea of tachyons if none have been seen. Did the idea come from the equations, or is it some unexplained experiment result that needs acounting for? There might be another reason, but what I'm trying to get at is: is there any unexplained evidence that leads to tachyons, or are they just what happens when v>c?

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So, why do we even have an idea of tachyons if none have been seen. Did the idea come from the equations, or is it some unexplained experiment result that needs acounting for? There might be another reason, but what I'm trying to get at is: is there any unexplained evidence that leads to tachyons, or are they just what happens when v>c?

I'm guessing it's just from the equations, and looking what happens if v > c, as you say.

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