Why do we assume that light travels at a universally constant speed?

Question

In physics, specifically relativity and Special Relativity, why do we assume the speed of light to be constant (for eg. in Einstein's formula E=mc^2 "c" is the speed of light which is considered constant) while also knowing that the speed of light changes when it passes from one medium to another (eg. water to air).

Answer 1

There are two things to point out here. The main point here is that we DISCOVERED light travels at a constant speed regardless an observer's motion. We found that out through the experiments of Michelson & Morley, whose grand experiment "failed" several times to measure the speed of light relative to the aether.

The second point is more subtle, and most people on yahoo answers don't grasp it, and will tell you, "Light always travels at c!" even though the index of refraction is defined by v=c/n, which surely indicates otherwise. The thing to understand in bulk media is that we are measuring the GROUP velocity of light passing through a medium, and not the PHASE velocity of light in a medium (which is still c). By having light pass through matter, the matter doesn't sit idly by, but instead, the electrons in the substance are affected by the alternating E field of the light wave, thus they oscillate as well. Oscillating charges PRODUCE light, so the light that passes through the medium is a sum of the original light wave, and the induced light from the medium. The resultant wave takes longer to convey information. An analogy would be a relay race. If each runner holds the baton for one second, before running at c to the next runner, the net effect is that the baton travels at less than c.

Answer 2

The constancy of the speed of light is more of a definition than an assumption. When we say we are measuring the speed of light, what we are actually measuring is our standards of length and time in relation to the speed of light. These three standards cannot all be constant when dealing with very fast relative motion. Einstein chose to define light speed as constant for two reasons. First because it is constant relative to natural clocks and natural objects which are moving with the observer. And second, because the resulting equations of transformation between frames of reference are symmetrical. The contraction of space and the dilation of time have a common factor.

Had he chosen, instead, to define the meter or the second as constant in all reference frames, the formulas of transformation would have been asymetrical. Besides, a meter stick which is constant between reference frames would not look like a constant length to observers in those reference frames. Likewise, a clock having a constant speed in different reference frames would seem to have different speeds to observers in different reference frames.

Answer 3

2 reasons

1) the most important to Einstein was that the postulate is necessary for a consistent understanding of electricity and magnetism. Maxwell's equations say light travels at a fixed speed. People were banging their heads against a wall trying to find the luminiferous ether--the reference frame with respect to which the speed was what Maxwell said it was. Einstein realized you could just drop the ether, postulate a constant light speed, and come up with a much nicer theory.

2) We've measured c to many decimal places. We've also measured general and relativistic effects predicted by Einstein's theories (based on the postulate that light speed is constant) and found agreement to many decimal places.

Answer 4

c is the speed of light in vacuum, which is constant. In a medium, as you know, light travels at less than c. But it's c, not the speed of light in a local medium that serves as the constant fastest possible speed in Relativity.

In other words, even though we call c "The Speed of Light," its fundamental property is not that light sometimes accidentally happens to travel at this speed. We should call c "The Fastest Possible Speed," which is its fundamental property.

Answer 5

gentle is persevering with which potential you may not outrun nor be on the comparable velocity with it. Your velocity would be relative to the cost of light. So in case you run at permit say 186 million miles according to 2d then gentle would be relative on your velocity. it is not two times as speedy yet your velocity would be like a 0 in a meter stick and the cost starts from there.

Answer 6

Because we have measured it, and that is what it does.

The speed of light has been measured to be independent of the motion of Earth in its orbit, and of stars receding from Earth.