"Speed of Light"

[Umbran]

Could there be a "terminal velocity of mass-less particles" that is some 300,000,000[+/- a few] meters/second? Just as there is a terminal velocity in Earth atmosphere that is slower then in the vacume of space based on things such as wind resistance of the object, could there be a terminal velocity of light and similar particles? Am I making sense?

I think I understand what you mean.

"Terminal velocity" is what you get when an object has a force upon it, and it feeling some resistance. The object accelerates to the point that the motive force is balanced by resistance, and that's its terminal velocity.

Problem is, photons aren't being pushed by a motive force, and aren't feeling a resistance. Specifically we used to think they moved through the "aether" that might provide resistance, and experiments have been done to show there is no aether. They don't start slow and speed up. As far as can be told, they just *always* move that fast. So, the analogy doesn't seem to apply.

 

[GMMichael]

If the universe fails to impress you, it might be that you're a tad of a tough sell.

Guilty as charged. I'm picky about everything - music, beer, coffee, RPGs, and movies too. You know Mikey, he hates everything.

Problem is, photons aren't being pushed by a motive force, and aren't feeling a resistance. Specifically we used to think they moved through the "aether" that might provide resistance, and experiments have been done to show there is no aether. They don't start slow and speed up. As far as can be told, they just *always* move that fast. So, the analogy doesn't seem to apply.

Aether is reminding me of something I read once: on a subatomic level, all sorts of tiny little particles are springing into, and out of, existence. These tiny particles can lend energy to more permanent particles, so long as that energy gets paid back.

Is this theory still plausible? Does this version of an aether not affect photons because photons are, relatively speaking, too big to be affected?

 

[tomBitonti]

Guilty as charged. I'm picky about everything - music, beer, coffee, RPGs, and movies too. You know Mikey, he hates everything.



Aether is reminding me of something I read once: on a subatomic level, all sorts of tiny little particles are springing into, and out of, existence. These tiny particles can lend energy to more permanent particles, so long as that energy gets paid back.

Is this theory still plausible? Does this version of an aether not affect photons because photons are, relatively speaking, too big to be affected?

That swarm of particles -- virtual particles -- in not only virtual electrons, and such, but also virtual photons. And apparently there is some interaction of virtual particles and non-virtual particles.

I found this:

http://physics.stackexchange.com/questions/76068/some-questions-about-virtual-particles

Concerning the last question, yes, virtual particles may interfere with the real ones. For example, if we study processes in an external electric field create by many coherent long-wavelength photons, there will still be Feynman diagrams with virtual photons in them. The amplitudes from these diagrams have to be added to the amplitudes with the real classical electric field, and only the result (sum) is squared in absolute value. That's what we mean by interference.

I did find this:

http://arstechnica.com/civis/viewtopic.php?f=26&t=1220047

Yes, photons participate in pair production. Yes it effects their speed, no the effect is not large enough to effect any of our deep space observations.

But!

http://physics.stackexchange.com/questions/41834/speed-of-light-and-virtual-particles

Yes, light can interact with "virtual particles". It can also interact with itself via virtual particle interactions (see Delbruck Scattering), although I believe direct observation of this effect is currently outside of our experimental capability.

Edit: Just realized I didn't address the second part. When a photon propagates, the propagation receives contributions from its splitting into an electron-positron pair which recombine etc. These processes contribute corrections to the photon propagator, but they do so in such a way that the propagator pole remains in the same place, which in turn means that the propagation speed is unaffected.

The link to Delbruk Scattering has:

http://en.wikipedia.org/wiki/Delbruck_scattering

Delbrück scattering, the deflection of high-energy photons in the Coulomb field of nuclei as a consequence of vacuum polarization has been observed. However, the process of scattering of light by light, has not been observed.[1] In both cases, it is a process described by Quantum Electrodynamics (QED).

And ... I'm way out of my depth. Can anyone provide a clearer answer?

Thx!

TomB

 

[Mustrum_Ridcully]

Virtual particles as an explanation why light is experiencing resistance is still just a version of the aether. Remember the part where Umbran wrote that no matter how fast you move or how you move relatively to a light source, you will always measure the light moving at c? And if there are two people with two different directions and speed, they will also all agree that the light is moving at c.

If virtual particles slow provide resistance to slow down the light, you are also moving through these virtual particles, and have a movement relative to them, and so, you should also measure a relative speed difference depending on your own velocity and direction towards these virtual particle sea. But we don't.

 

[Bagpuss]

Has anybody mentioned that c is the speed of light in a vacuum yet? If not, maybe I should mention it!

Does it matter if it is one of those new Dyson DC54s? With the bag-less cyclone technology they can really suck, that's got to make the light travel faster surely.

 

[freyar]

I love when these threads come up on ENWorld. I wish I had more time available to answer them more often! In any event, I teach a university course each year on relativity and use it every day in my physics life, so I have way too much to say on the subject for a single post. I'll just try to mention a few things here.

1) What happens if c becomes infinite? Well, besides breaking electromagnetism and therefore us, it does change the notion of past & future in our universe. But it would actually change it to the notion most people have already: there would be a definite "present" that is the same everywhere, plus a definite past and future the same everywhere. In Einstein's relativity, that's not true. Your future is made up only of the events you can travel to at speed c or less. The reverse is true for the past.

2) As noted, the speed of light in materials may be different than c. This isn't due to the "ultimate speed limit" changing --- c is the same --- but due to the material affecting light. In terms of photons, yes, you can think of the photons bouncing off of electrons in atoms. That's not quite accurate, though, as the "photons" are large compared to atoms for visible light, microwaves, radio, etc. It's better to think of light as waves in this case, and the change in speed is due to the response of the wave to the material's response to the wave.

3) Virtual particles do not affect the speed at which light travels. If they did, it would violate relativity!* It is true that we can think of virtual particles as having measurable effects, but that's not one. Some of tomBitoni's links mention effects where you can think of virtual particles acting like matter and changing the speed at which light travels, but that's always in the presence of some other matter, like a nucleus or metal plates. That doesn't violate relativity because of the other stuff present.

*I should note that you can make theories of physics that violate relativity in small, subtle ways that would not have yet been detected by experiments. In those theories, you could ask about virtual particles changing the speed of light, but you could also ask about whether light always travels at speed c even before you think about virtual particles!

 

[Bullgrit]

Does a photon have a measurable lifespan, relative to itself? I mean, a photon that travels 1,000 light years has a lifespan of 1,000 years relative to us, the observer. But the photon traveled at c, so ... instantaneous/simultaneous beginning and end, relative to itself?

It seems to me, that if it has a measurable lifespan relative to itself, time for it isn't "stopped." But if it doesn't have a measurable lifespan relative to itself, can it be said to exist?

Also, is there any proof that time reverses at speeds beyond c? Is it just theory, (can't be proved, right)? Could it be that faster than c speed won't create a paradox?

Bullgrit

 

[Bullgrit]

But if it doesn't have a measurable lifespan relative to itself, can it be said to exist?

Having said this, and then thought a few moments past clicking the Submit button, I guess this is like questioning whether a black hole singularity can be said to exist.

Bullgrit

 

[Morrus]

It seems to me, that if it has a measurable lifespan relative to itself, time for it isn't "stopped." But if it doesn't have a measurable lifespan relative to itself, can it be said to exist?

Well, they exist because we can observe them, and we can use them.

 

[tomBitonti]

An aside, this has a nice historical view of our understanding of light:

http://www.olympusmicro.com/primer/lightandcolor/particleorwave.html

---

But, we don't actually observe the singularity. Isn't observation limited to up to (but not quite including) the event horizon?

From one point of view, that could be turned into an argument that the event horizon is the entire structure of the black hole. The history of particles which have fallen into the hole seem to be contained in the topology of the surface.

Whether the singularity exists becomes a pragmatic choice: Thinking about black holes is a lot simpler if a particular interior structure is presumed, and, generally, scientists rather prefer to keep models as simple as possible.

A question that I've had about light transmission is whether we could model this as discrete events, with the only physical events being the emission and absorption, with the absorption delayed according to the intervening distance, much as we would do if running a discrete event simulator, and sort-of what is done when rendering a scene: We compute the paths that light will follow, and render only the final point reached by light.

Thx!

TomB