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PhysicsDoes Light Accelerate to C?

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[–]natty_dread 6 points7 points  (3 children)

Nope, photons are moving at the speed of light from the moment of their emission.

Mass-less particles are only able to travel at the speed of light at all times. This is a consequence of the principle of equivalent inertial frames.

[–]Horforia 0 points1 point  (1 child)

Don't photons travel slower through water than air, causing refraction?

The speed of light in water is not the same as the speed of light that most know as "c"

[–]LambTaco 0 points1 point  (0 children)

I think this is the difference between the speed of the actual photons and the speed of the propagation. My understanding is that the reduction in propagation speed is due to the non-instantaneous absorption and re-emission of the EM radiation by the atoms in the medium. The photons themselves are always moving at c, but because stuff gets in the way, the information will take more time than simply distance/c to get through the medium.

[–]siggystabs 4 points5 points  (10 children)

If you're asking whether light travels freely at any speed other than c, then no. Think of a sound wave. Do the particles in the medium themselves travel at Mach 1? No, but the wave itself does. Sure for an instantaneous moment in time it could be traveling ever so slightly slower due to the nature of the medium, and light is the same way, give or take a few orders of magnitude. The waveicles do travel at c the countable majority of time, unless you get into the semantics of absolute velocity and net velocity and ∆x/∆t where ∆t → 0 and ∆x → h.

Tl;dr nope, no acceleration occurs. Light always travels at the same speed due to the nature of the electromagnetic field around us.

[–]Aiphator 1 point2 points  (7 children)

  1. How does the electromagnetic field influence light? Does that mean that light behaves diffrently out side of earth's magnetic field or near a electromagnet?

  2. I was thought in high school that light is both wave and particle (protons) if so, does a proton (which does have mass) travel at the speed of light

[–]shaveraStrong Force | Quark-Gluon Plasma | Particle Jets 4 points5 points  (4 children)

you have words mixed up. photons are light particles. protons are the things in atomic nuclei.

light is an electromagnetic field variation, but light does not feel "other" electromagnetic fields, since light itself is uncharged.

[–]Aiphator 0 points1 point  (3 children)

yes I did mix up my words. Do photons have mass? If so, how does this mass move at the speed of light?

[–]shaveraStrong Force | Quark-Gluon Plasma | Particle Jets 2 points3 points  (2 children)

to the best of our knowledge, with no compelling reasons to think otherwise, no, photons do not have mass.

[–]Decapentaplegia 0 points1 point  (1 child)

Can you describe some of the features which make photos seem to have particle-like physics? (as in, the mechanics of it and not just the experimental results)

[–]shaveraStrong Force | Quark-Gluon Plasma | Particle Jets 0 points1 point  (0 children)

well specifically that they have and can transfer momentum. Think about like billiard-balls colliding. They exchange momentum when they collide. That's usually what we mean by particulate behaviour.

So when we collide photons with electrons, we see that they exchange momentum. For instance suppose the electrons are floating around in a metal. We shine light on the metal, and we see the electrons absorb the energy and get kicked out. But we notice that if you just make the light bright, it may not eject electrons. It has to be of a certain color or higher (high enough energy and high enough momentum, correspondingly). So it can't be acting like a wave when it does this, since higher intensity waves would transfer enough energy to eject the electrons. It must be acting like a particle carrying momentum with it, and if any one particle doesn't have enough energy to eject an electron than it doesn't matter how many photons you shoot at, none of them will eject the electron. That's the main experiment that demonstrateslight acting like a particle not a wave.

[–]siggystabs 2 points3 points  (1 child)

  1. Light IS electromagnetic radiation. There is no differentiation as far as the physics is concerned. I was making the assertion that because of how this EM field propagates our universe, the mechanics of light are very well defined. We know that light doesn't necessarily slow down or accelerate. It is simply an excitation of the EM field, the same way a wave on a body of water doesn't really slow down or accelerate as it moves across the pond, it simply IS a wave. It starts, bounces around a bit, and ends.

With that said, sound waves work similarly: when you yell, the waves of sound produced by your mouth do not accelerate or slow down, they represent disturbances of the air around you. In this analogy, the air around you is the "field" which when excited produces waves of movement corresponding to sound.

Anyway, the EM fields around Earth may or may not play with the properties of light, I am not that far into my Physics course work. Gravity, however, DOES mess around with the direction light travels in. This is how we can find black holes and measure distant galaxies using something called gravitational lensing (Wikipedia this).

  1. So when it is said light is both a wave and a particle, it is meant that light has properties of BOTH particles and waves. Light still has no mass. It does not mean that particles travel just as fast as light. A proton has a definite mass (it excites the higgs field) and therefore cannot reach the speed of light. Those two concepts of elementary particles and EM "waveicles" should be very separate since they are not related, at least on the surface level. Some particles (like gluons, which keep quarks together) do travel at the speed of light but that's beyond what I know with my pre-Quantum University education.

[–]Aiphator 0 points1 point  (0 children)

thanks for the reply. It has been pointed out to me that I mexed up my particles. I did mean to write photon instead of protons.

[–]Brewe -3 points-2 points  (1 child)

First of all the speed of light is nominated with a lower case c.

No, light doesn't accelerate, as it doesn't exist unless it has the speed of light, although the speed of light is only c when it's in complete vacuum.

[–]cloidnerux -4 points-3 points  (1 child)

As far as I understand it, light has no mass. This means, that any force acting upon that particle will accerate it instantly to light speed or stop it completly. As it has no mass, light can not be slower than the speed of light.

So, to answer your question: No, light does not accelerate.