Time and space travel at light speed

Algr

Little Ted might have an old map that _says_ that the planet is 5 light years away, but everything he might do on the ship will tell him that the planet was never that far away. The only way to calculate a speed faster then light would be to use the ship's clock, but ignore the ship's telescopes in favor of the old map. I'm sure little Ted would find that procedure flawed.

But now my own question:

When the ship is at rest relative to the destination, the star will appear 9 light years away. Then the ship accelerates and the distances contract. At traveling speed, the star appears to be 0.9 light years away. Wouldn't observations during the acceleration phase appear to show the destination star approaching faster then light?

mfb

That is an effect of general relativity, and the speed depends on your choice of a coordinate system. You don't have the same issue in special relativity.

None. We are for some reference frames, and we are not for others.

If you move quick relative to the cosmic microwave background (CMB), the universe looks like it has a different age in different directions.

mike777

From the perspective of light itself would it be travelling at almost infinity speed if we removed any other variables?

jbriggs444

Yes and no.

Direct observations are not going to show an immediate and dramatic shift.

But a calculation of how far away the [moving] far-away star is "right now" is going to change dramatically during the acceleration because the standard of synchronization is changing. The notion of "right now" is not fixed.

The speed of the star is sub-light in any inertial frame of reference. The fact that its "speed" is greater than light speed in a particular accelerating frame does not violate any physical law. One way of thinking about it is that this speed is more a matter of moving coordinates past the star than moving the star past coordinates.

mfb

That is not possible in relativity.

Algr

I'm not sure, but it sounds like you are saying "Yes".

(I'm presuming that the start and end locations are not moving relative to each other.) Any measure of speed must involve two points in time. If an object goes from 9 light years away to .9 light years away in anything less then 8.1 years, then by definition it has moved faster then light relative to the observer. You seem to be saying that this IS possible, provided that the observer & clock are accelerating or in a gravity well?

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Another post mentions "Seeing galaxies receding at faster then light." Due to the expansion of the universe. Is this really possible? I thought that all light from such galaxies would be red shifted to zero, and thus would never reach Earth. Such galaxies have ceased to exist from Earth's perspective because no information from them will ever reach us. (Unless the universe turns around and starts contracting!)

jbriggs444

Yes, it is possible for objects to move faster than light -- if you are not careful about what you mean by "move".

For instance, take a rotating coordinate system in which the Earth is motionless. In this coordinate system, Alpha Centauri revolves around the Earth once every 23 hours 56 minutes.

That's a lot faster than light. But it's not what you or I would think about as "really" being faster than light. Like I said before, that's more like moving coordinates past the object and not so much moving the object past coordinates.

An accelerating frame in Special Relativity is the equivalent of a rotating coordinate system. It's just that you are rotating in four dimensions and it's a hyperbolic rotation instead of a circular rotation.

DaleSpam

Big Ted should teach Little Ted the definition of velocity. It is dx/dt and not dx'/dt. I.e. Both x and t need to be measured in the same inertial reference frame or it simply isn't velocity.

phinds

Everything in the univere IS moving at close to the speed of light, including you right now as you read this. You just have to pick a frame of reference in which that statement is true for the object you are considering.

In other words, your statement "everything in the universe can be moving close to the speed of light" is meaningless becasue you have not said RELATIVE TO WHAT? Speed is always relative but you have mentioned it as though it is absolute.