sol invictus
Philosopher
- Joined
- Oct 21, 2007
- Messages
- 8,613
Stupid question probably, but can you explain this in more detail? Specifically, suppose that the speed of light isn't constant; how do you determine the speed of the lab from that if you're inside the lab? Because it seems to me that even if the speed of light isn't constant, so to an outside observer the light is traveling at c + whatever mph (and therefore deduce the speed of the lab), it will be constant in relation to everything in the lab, and there won't be any way of measuring the difference.
Not a stupid question.
The easiest answer is that you could just build a scaled down version of your lab, inside your lab, then send it flying along. That would allow you to measure the speed of light from the "outside". If it changes, that would mean the speed you were measuring in your lab must also differ from the speed measured from the outside the big lab.
A slightly more detailed answer involves aether. Consider sound waves - they propagate at a fixed speed (call it v) with respect to the air they travel through. So if something (like a car) carries its own air with it, sound inside the car propagates at speed v + the speed of the car, with respect to the ground, or simply at speed v wrt to the car. But if the car has no roof sound propagates at v wrt the ground, and so as measured from inside the car it propagates at v - the speed of the car, and so you can measure the speed of the car that way.
What you're suggesting is that the lab could carry the some stuff (call it aether) with it like the sealed car, and that light propagates through that aether at fixed speed wrt it. That's a priori possible, but it's been ruled out by many experiments over the years - experiments similar to the "lab inside the lab" I proposed above. See here and (I haven't read it, but probably explains this in great detail) here.
A final answer is that if Maxwell's equations are correct, light always propagates at speed c in vacuum no matter what the velocity of the source is. Maxwell's eqs. describe electricity and magnetism, and are among the best tested laws of physics we know of. They're used constantly in everything from electrical engineering to physics. One of the great successes of 19th century physics was the realization that they describe light (as well as radio waves, x-rays, etc.). And if you take them seriously - which Einstein was the first to do - they also imply the above facts, along with all their bizarre consequences.
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