Is the speed of light constant even relative to a moving body? And if not...?

Question

I realize that you might dispute the use of "moving body" in the aforementioned, however I will address that soon. very soon.

This sentence denotes that this paragraph explains "relative to a moving body". I mean that because we are on a moving planet, then if I were to point two flashlights in opposite directions, and record the distance the beams of light had traveled from me after x picoseconds, one would be farther due to the fact that I, being a moving body, would have moved (however insignificantly) in the direction of one of the beams of light, correct? That is what I mean by my question.

If the speed of light is NOT CONSTANT relative to an object in motion, then that would imply that there is such a thing as universal spatial reference (you can judge speed without referencing other objects, contrary to relativity). This concept would be fundamentally contrary to the theory of relativity, would it not? And other implications?

If the speed of light IS CONSTANT relative to a moving observer, ie bipedal mammals on a moving planet, then either:

c is subject to modification due to the velocity of its source, just like a ball thrown off a moving truck (contrary to everything I believe about physics)

OR (less likely, but logic dictates that it is a possibility)

The speed is defined by the defined by the observer's motion (not the same as first option, let me explain...) meaning that two observers with different velocities both of whom carry their instruments on their person would both measure the same velocity of the same photon... *head explodes due to user's disregard for all logic*

Please help me understand how c can, no must, exist in a relative universe!!!!!

Answer 1

The speed of light is constant if your are in the same reference frame of the light.   When you move with constant velocity relative to the light you will still be in the same reference frame.   This phenomenon can be explained by the idea of time contraction.   The faster you move the slower time will progress for you.  To make the idea have more of an impact I will us  an extreme case Lets say that you move close to the speed of light and throw a ball ahead of you.  To you the ball will move away from you just fine and you can measure it a 50 mph or something.  To some one in your same reference frame but staying stationary on the ground they see you flying by at near light speed and you will appear to be basically frozen in time.   In essence they see you as throwing the ball at a very very small velocity so small that it appears to be still.  Therefore to the observer your speed will still be near light speed and your ball will not be moving relative to you...it will be going at roughly the velocity as you.      

This is all explained in special relativity.

On the other hand.  General relativity is another type of beast.  If you are changing reference frames while measuring the light speed (i.e you accelerate with respect to the light) you can indeed wee speeds larger than the speed of light, It will no longer be constant.

Answer 2

It is one of the defining phenomenon of our universe that the speed of light is constant for ANY observer, no matter what they or anyone else thinks their 'speed' is; that is, in any reference frame.

This is still more complicated since there is no objective reference frame. As you have noted, if you could pin down the speed of light as a relative speed, you could also find an objective reference frame. Einstein shocked the world in 1905 with his theory of Special Relativity, since it was the 'last straw'  of the 'flat earth' ideas that made us, here, special, central, and above all important in all of the universe.

The key to the conundrum is that the speed of light defines our relationship to time itself.

Answer 3

Wow, quite a thorny question - especially cause I cant discern a clear question...but I'll try address the last part. I can atleast give my understanding of how c exist in a "relative universe" and maybe it'll help.

First I must point out a possible flaw of the "two opposing flashlights" experiment. If both you and the flashlights were on Earth, then the flashlights are traveling with you - with Earth. There would be no difference in the speeds. But - if the flashlights are in empty space shining on you and the earth- like stars, for intance, - then we would see just doppler shifts in the frequency of light, but the speed of light for both flash lights would remain the same.

Why?

Er...well (ahem)...no one knows *why* light speed is the same for all reference frames, but we do know that it is true.

In his special relativity, Einstein starts of with two postulates that lead to relativity, and one of the postulates is the speed of light is constant in uniformly moving frames of reference.

So the constantcy of speed of light in not a consequence, but an axiom, an assumption, of relativity theory.

All we have is a bunch of experiments, that showed no matter what speed one was going the speed measured was always the same.

No one knows why though. Some say it's because a photon is massless and all massless objects go at the speed of light. But again - why?

No one can show how a constant c exists in a relative universe because we have to assume a constant c to get relativity theory!

The only way we know c is constant is through a plethora of experiment - the most famous being the Michaelson-Moreley experiment that works similarly to the thought-experiment you described above.

Hope that helps.