Why doesn't light travel infinitely fast ?

In summary: Saying that the universe is homogeneous... means that there are no preferred locations in the universe; it looks the same no matter where you set up your telescope. Note the very important qualifier: the universe is isotropic and homogeneous on large scales. In this context, \large scales" means that the universe is only isotropic and homogeneous on scales of roughly 100Mpc or more.The isotropy of the universe is not immediately obvious. In fact, on small scales, the universe is blatantly anisotropic.
  • #36
And I was expecting some reference to "effective rest mass" in a superconductor, or the experimental limits on the photon mass.

I notice that "pounds" is not a unit of pressure here ... in the context of radiation pressure is not even a unit of mass, but iirc of force. Maybe that is the source of the misunderstanding.

1lb of force being the amount of force needed to accelerate 1 pound by 1 foot per second every second? Or is it to accelerate one pound at one gravity? <checks> Ah - it's the gravity one.

Should be written "lbF", shouldn't it, but people often leave the F off.
 
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  • #37
Simon Bridge said:
One of the things about this recurring question about the speed of light is that the person asking does not usually distinguish between the speed of electromagnetic waves in a vacuum and the limiting speed in relativity - which are the same. Very often the question can be interpreted as "why is it that the speed of EM waves happens to be the relativistic limiting speed?"
Start with [itex]\epsilon_0[/itex] and [itex]\mu_0[/itex], particularly the fact that they have the same values for everybody, regardless of how one is moving with respect to anything else. Because their values are universal, the speed of light is also universal. But to get observers moving relative to each other to measure the same speed for the same bit of light, you have to fiddle with lengths and times in different frames. That fiddling necessarily involves the speed of light, because that is what the fiddling is trying to get right. But the same fiddling has to apply to everything, not just light, because it would be ludicrous for lengths to be contracted when dealing with light but not when dealing everything else; how would the meter stick know to contract when measuring a wavelength, but not when measuring a football field? Since the length and time fiddling apply to everything, and since they imply the impossibility of a massive object reaching the speed of light, the speed of light limits everything.

I realize I am speaking a bit glibly, but I think this chain of reasoning makes sense, modulo the glibness.
 
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  • #38
It's fun isn't it.
What you end up with is an invariant speed in the equations that describe space-time geometry.

But the fool in his heart may still reason (hopefully in his head) that photons may not travel exactly at the invariant speed and, therefore, may have a very small mass. It may be the invariant speed is actually slightly larger than the observed speed for photons since, as you say, originally the invariant speed was postulated to be the photon speed and all the math worked out on that postulate.

Different observers would, indeed, measure a slightly different speed for light if only their equipment were sensitive enough, and relativity would still work. [*]

This would limit the mass of a photon to be certainly below 10-14eV/c2 (depending on the experiment).

The question in post #1 is certainly phrased in terms of the speed of photons - so maybe it can be answered by pointing out that no particle can be accelerated past the invariant speed of the universe, massive or massless, and so, since the slightest force would accelerate it to that speed, we should not be surprised to see all massless particles move at exactly the invariant speed. The name "the speed of light in a vacuum" is an historical accident.

Hmmm ... it's still glib isn't it?

-------------------------

[*] though there is the curious problem of how come all light is in a very narrow range of speeds ... which, in a way, would be the question in post #1.
 
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<h2>1. Why can't light travel infinitely fast?</h2><p>According to Einstein's theory of relativity, the speed of light is the fastest speed at which energy, matter, and information can travel. This is because as an object approaches the speed of light, its mass increases and it requires an infinite amount of energy to accelerate it further. Therefore, it is impossible for light to travel faster than its already incredibly high speed.</p><h2>2. How fast does light travel?</h2><p>The speed of light in a vacuum is approximately 299,792,458 meters per second, or about 670,616,629 miles per hour. This is the fastest speed at which anything can travel in the universe.</p><h2>3. Can anything travel faster than light?</h2><p>No, according to our current understanding of physics, nothing can travel faster than the speed of light. As mentioned before, as an object approaches the speed of light, its mass increases and it requires an infinite amount of energy to accelerate it further. This makes it impossible for anything to surpass the speed of light.</p><h2>4. Why is the speed of light considered a constant?</h2><p>The speed of light is considered a constant because it is the same for all observers regardless of their relative motion. This means that no matter how fast an observer is moving, they will always measure the speed of light to be the same. This is a fundamental principle of Einstein's theory of relativity.</p><h2>5. How does the speed of light affect time and space?</h2><p>According to Einstein's theory of relativity, the speed of light is a fundamental constant that affects the fabric of space and time. As an object approaches the speed of light, time slows down and space contracts. This phenomenon is known as time dilation and length contraction, and it has been proven through numerous experiments and observations.</p>

1. Why can't light travel infinitely fast?

According to Einstein's theory of relativity, the speed of light is the fastest speed at which energy, matter, and information can travel. This is because as an object approaches the speed of light, its mass increases and it requires an infinite amount of energy to accelerate it further. Therefore, it is impossible for light to travel faster than its already incredibly high speed.

2. How fast does light travel?

The speed of light in a vacuum is approximately 299,792,458 meters per second, or about 670,616,629 miles per hour. This is the fastest speed at which anything can travel in the universe.

3. Can anything travel faster than light?

No, according to our current understanding of physics, nothing can travel faster than the speed of light. As mentioned before, as an object approaches the speed of light, its mass increases and it requires an infinite amount of energy to accelerate it further. This makes it impossible for anything to surpass the speed of light.

4. Why is the speed of light considered a constant?

The speed of light is considered a constant because it is the same for all observers regardless of their relative motion. This means that no matter how fast an observer is moving, they will always measure the speed of light to be the same. This is a fundamental principle of Einstein's theory of relativity.

5. How does the speed of light affect time and space?

According to Einstein's theory of relativity, the speed of light is a fundamental constant that affects the fabric of space and time. As an object approaches the speed of light, time slows down and space contracts. This phenomenon is known as time dilation and length contraction, and it has been proven through numerous experiments and observations.

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