# Is the Speed of Light Relative to A or B?

Discussion in 'Physics & Math' started by Athelwulf, Aug 26, 2004.

1. ### AthelwulfRest in peace Kurt...Registered Senior Member

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Here's a scenario I thought up when I was very young (maybe 10 or so). I often wondered what would happen. Anyway, here's my scenario.

An object (A) is sitting in one spot in the universe completely stationary, as is an observer (B). A emits light which B can see.

Suppose that A suddenly jumps to the speed of light, headed away from B. Assume that this acceleration happens within an infinitesimally small period of time.

Does the light from A travel away from A as if A were stationary, thus enabling B to see the acceleration? Or is the speed of the light which is headed away from A relative to A's speed, which would make A seem to disappear to B?

And if the latter is the case (and I feel it is), what about the light going away from, but ahead of, A? Is that light moving at twice the speed of light?

I sure hope I put that in language which is understandable, and that ya get what I'm asking.

Thanks for yer imput! Peace, Love, Health, and Happiness to all!

&Acirc;&eth;elwulf

3. ### John ConnellanValued Senior Member

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Acceleration of what? The light will travel at c wrt A and B!

Again the light will travel at c in that direction. How does it do this without creating a paradox? By the modification of space-time.

5. ### Quantum QuackLife's a tease...Valued Senior Member

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I think Lorenze contraction comes to play here as well.....

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8. ### Quantum QuackLife's a tease...Valued Senior Member

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A...... becomes light

9. ### (Q)Encephaloid MartiniValued Senior Member

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Does the light from A travel away from A as if A were stationary, thus enabling B to see the acceleration?

This question doesn't make sense. The light will travel at the same speed to observer (B) regardless of what object (A) is doing. If object (A) accelerates away from observer (B) the light will appear redshifted.

Or is the speed of the light which is headed away from A relative to A's speed, which would make A seem to disappear to B?

The speed of light is not relative to either (A) or (B). (A) would simply appear redshifted to (B) and vice versa.

Is that light moving at twice the speed of light?

No, it is moving at c because the speed of light is NOT relative to (A) or (B).

10. ### PeteIt's not rocket surgeryRegistered Senior Member

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Hi Athelwulf,
I'm going to reword your scenario a bit to make it physically possible.

It means nothing to say that A is completely stationary... "stationary" only means something when compared to something else. So, I'm changing this sentence to one of the following (they share identical meaning).

A and B are stationary with respect to each other, or
A and B are comoving

No problem there. A is sending a signal to B.

This is breaking the rules. It is impossible to accelerate to the speed of light (according to tried and tested models of how the Universe works). Can't be done. There is no way to go from a *timelike path* to a *lightlike path*. If something has light speed, it had light speed as long as it existed, it has no rest mass, and (I think) it can't emit any signal until it is destroyed.

However, in order to answer your question we can use limits... to begin, we'll simply assume that A accelerates very quickly to a very high speed.

A accelerates away from B such that in B's reference frame, A achieves an infinetisimal fraction less than light-speed within an infinitesimal time.

Now we can answer the question.
The light emitted from A moves at light speed relative to both A and B. However, in B's reference frame the light from A is redshifted. It's as though it is stretched out by A zipping away. The redshift is calculated according to the following formula (c = speed of light, v = relative speed of A and B):
For example, if A was going at 0.99c relative to B, B would see A's signal redshifted by a factor of about 14 (14 times longer wavelength, 14 times less energy received) deep into the infrared (assuming a visible light original signal). If A was going at 0.99999999c relative to B, the redshift factor would be about 14000, and B would need very sensitive equipment and a very large antenna to read A's signal which is now very weak and of longer wavelength than radio waves. If A is within an inifinitesimal fraction of light speed, the redshift factor approaches infinity, and the signal received by B becomes undetectable.

So, the short answer is that A seems to disappear - not because the signal is not reaching B, but because it is stretched into indetectability (although still moving past B at light speed).

Let's say that A is now moving toward C, who is comoving with B.
C will see A's signal moving at light speed, but the signal will be strongly blue-shifted (sort of bunched up). The formula is similar to the reshift formula, except the wavelength is divided instead of multiplied.

At a blueshift factor of 14, A's signal is shifted into the ultraviolet as received by C. At 14000, C is bathed in gamma rays. In each case, the signal is going past C at light speed - no faster.

Peace to you too! :m:

11. ### RawThinkTankBannedBanned

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Athelwulf

U just scraped out all the theories of relativity to dust.

Light traveling from B to A should also remain constant for A when A suddenly jumps to speed of light. But as we know light will not travel faster than light hence …

12. ### gentleRegistered Senior Member

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light

Minor point -If you are traveling as fast as light, you are light there would be no difference. In that case see the light, be the light. Only light can be light and travel as light.

Last edited: Sep 5, 2004
13. ### Brandon9000Registered Senior Member

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Since a material object cannot travel at the speed of light, it is meaningless to describe what you would experience if you did so.

14. ### cckieranHighSchool Phys/Chem studentRegistered Senior Member

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Isn't the contancy of the speed of light restricted to inertial reference frames?