One thing I never understood was how is motion of an object always relative to another object but somehow the speed of light is always constant. If you have something moving you determine this by its relation to another object.
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Well, it is a hard concept to understand. Let me make an attempt and hopefully successfully so:
(1) Light travel
One thing to remember is that light is not a particle that moves, light is an oscillation. Electric field induces a Magnetic field; as M field increases, E field is being used up. And, the newly created M field in turn induce a new E field as the M field will uses itself up to create the new E field. E->M, M->E, E->M...so on, so on. This cycles continues indefinitely oscillating between E and M. This oscillation is EM wave. Light is EM wave.
Thus, you can see that the oscillation has its own native frequency within any particular medium. For example, light in water, how fast in water can the E field induce the M field to the extend that the E field is totally used up and only M field is left, and then how fast can the M field induced a new E field in water and used itself up to zero M energy and all E energy. It all depends on how the EM interacts with the water medium - that is, it depends on the property of the medium.
(2) Particle travel
re: " But a massive object can never approach the speed of light as it becomes too massive."Not just massive object gains mass. A very low mass particle traveling will gain mass. As long as the object is not mass-less, it will gain mass as it travels.
The faster the particle travels, the more kinetic energy it contains, the more mass that particle has. You can thus view the added mass as the added kinetic energy.
(3) Putting togetherSo, if you are a person in a medium... The rate EM self-induce and propagates depends on the medium. How fast E can induce M and M can induce E is fixed by the property of that medium. You in that medium can run, and your speed is affected by the medium as well, but how fast you move (or not move) will have nothing to do with how E and M field interacts with that medium. So, EM wave (light) is constant in that medium.
Your girl-friend and me are both with you in that medium and timing you as you run.I can see you start only when the light showing to me "you started" gets to me. So, how I see you see is affected by the speed of the EM wave in that medium.
The question "how quickly do I see you start?" is answered by "how much time does it take for the light (of you starting) gets from you to me." Say you are just one meter from me so the time is X.
Now you begin running away from me and I am standing still. Now, correspondingly, "how much time it took you to move one million meter?" It would take more time for the light to get back to me this time. At your finish, the light has to travel 1,000,001 meters to get to me this time. The light would take (1,000,001*X) from your finish to get to me. What you timed as YOUR speed will be different than what I timed as your speed. Time dilated between you and me.
Now imagine your girl-friend is also doing the timing and she is running at some lower speed behind you. What time she marked as your start/finish will depends on how fast she is running behind you.
All the while, how fast that light got to me has constant speed depend solely on the medium. You can be standing still or running at 1/2C, the photon from you (EM wave) has constant speed because the speed that EM wave propagates only depends on how E and M interacts with that medium,
Your girl friend timed you differently (your speed is relative to her), and I time you differently (your speed is relative to me). Yet the photon that from you to her, or from you to me, are all traveling at the speed dictated by how EM interacts with that medium.
I hope this explanation helps your understanding...
Rick