# The Speed Of Light

#### Rainbow

One of the prostulates of the THEORY OF SPECIAL RELATIVITY states that the speed of light is constant, irrespective of the speed of the observor.

Can anybody help me understand the reason (or the logic) behind this?

Also, how did Einstein come to think of such kind of a thing. I mean, he couldn't have had a race with light.

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#### Janus

Staff Emeritus
Gold Member
Ah, but he could imagine what would happen if you did run a race with light.

It boils down to the Maxwell equations, which were a set of equations that describe the behavior of electromagnetic radiation, of which light is a member. One of the predictions of these equations was that the speed of such radiation in a vacuum depended on two properties of the vacuum alone. These two properties are physical constants that do not change depending on your movement. (if they did, it would lead to all kinds of weird results.) Now, if these two properties do not change with velocity and the speed of light depends on these two properties, the speed of light does not change with velocity.

Einstein took the Maxwell equations and used them to imagine what it would be like to ride along side a light beam. He came to the conclusion that the light beam would cease to exist for him. It also turns out that if you assume that you have any velocity relative to the light beam other than c, the light ceases to exist for you. (the Maxwell equations no longer predict the existance of an electromagnetic wave.

The practical upshot of this is that unless you were perfectly motionless with respect to a light bulb you would not see the light from it. (walk towards or away from it and it would disappear). You also would not be able to listen to the radio while the car is moving. etc.

This of course goes against what we experience in everyday life, so this cannot be the case. Since the equations that describe light say that the world would behave a lot differently than it does if we allow for the speed of light to vary from observer to observer, it is apparent that the speed of light does not change from observer to observer.

#### yogi

Einstein was convienced that it was impossible to detect motion relative to space - so it became paramount in his development of relativity that light would be isotropic - otherwise if light had a different velocity in different directions relative to the source or the observer, this difference could be used to determine absolute speed with respect to space.

#### autonomous

if one takes into account that if one was to reverse the cosmological constant, it would originate back to the origin pointof the universe; the 'big bang'. then would it be possible to messure the shift of light, in stars or of distant galaxies, to tell if it they were moving towards you, or away from you, could you then point in the direction of the cosmological constant by seeing which celestial objects in certain parts of the sky have the same amount of shift? if that would be possible one could then derive that the cosmological constant had a constant rate, or a non-constant rate (the rate of its acceleration, or decrease in speed)
also...

if you did find similarities in the shift of light in seperate parts of the sky, would the 2 regions of sky be at opposite poles to eachother, or if they were less then 180 degrees from eachother (or more) would the cosmological constant be bent? or would this be a mere bending of light from gravitational interference?

#### nabodit

why dont we try racing light with einstein himself! if he tavells in the speed of light at time 20:00 by his watch and pursues the lighe which left simultaneously with him at 20:00 then [applying newton's thoughts] einstein sees the light as a frozen thing in rest like himself is. now applying maxwells equations we see nothing such as a frozen light exists. einstein bloke just had the feeling that maxwell was right and not newtons frozen light. thus anyway u try u cant chase light and see it travel less than c.

#### mewhoexactlywhat

In that case, if you were to reach just less than c in space what would the stars look like? In movies the light from the stars stretches, and I thought that might have been inaccurate because it appears frozen, but if we are moving at constant velocity in what ways would the light look different?

#### pervect

Staff Emeritus
mewhoexactlywhat said:
In that case, if you were to reach just less than c in space what would the stars look like? In movies the light from the stars stretches, and I thought that might have been inaccurate because it appears frozen, but if we are moving at constant velocity in what ways would the light look different?
Looking with your eyes, you would see that the light from stars in front of you blue-shifts, while the light from stars to the rear red-shifts, and that stellar possitions abberate so that the stars appear to "move towards the front".

At extreme relativistic velocities, the universe would appear to be a point in front of you.

See for instance

http://www.exo.net/~pauld/stars/PD_images_relativ.html

Thanks

#### autonomous

you see, your getting me all wrong, i am not talking about light traveling away from us, im talking about reflected light off of object moving away from us, and if its moving away from is, there for the light reflected off of it would be redshifted, what im saying is could you measure that so see what is moving away from us, and by how much it is, by measuring the amount of redshift, now that you know im talking about reflected lightnot light traveling away from us, because your right llight is according to einstien isotropic.

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