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itari1985
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hi, i guess this is a noob question. But if speed of light is impossible to achieve, then how do the photons achieve it?
Hurkyl said:They're always at the speed of light, they never had to "achieve" it!
The theorem to which you're referring, incidentally, says that an object with nonzero mass cannot be accelerated to the speed of light.
There is a similar theorem which says that an object with zero mass can only travel at the speed of light.
rbj said:in other words, photons "acheive" a speed of c by definition.
just to be a pain in the arse, what Hurkyl means here is "rest mass" or "invariant mass". i still find myself in the old school of semantic that photons have a mass of [itex] m = h \nu / c^2 [/itex], but their rest mass or invariant mass is zero. i guess i wasn't paying attention in the last 20 or 30 years when it became conventional that "mass" without any qualifier meant "invariant mass" or "rest mass".
sporkstorms said:While you're all on the topic... I almost feel foolish for asking this (I'm a 4th year physics undergrad).. but I've never fully understood this, and never been able to find a satisfying answer:
If massless particles always travel at c, then how do they "slow down" in matter?
My educated guess and the only semi-decent explanation I've seen (although never in full, or completely hashed-out) is that it's not the same photon actually slowing down. But rather, the photon excites an atom, which emits another photon - and this occurs many times until a photon is emitted from the matter.
Am I on the right track? And if so, am I fully seeing the big picture, or is there more going on?
--
Edit: Maybe and/or the photon is being reflected back and forth inside the matter, until it finally escapes?
ZapperZ said:Read the Physics FAQ in the General Physics forum.
Zz.
sporkstorms said:Thank you. That is very well written, and answers my question far better than I've seen before.
Furthermore, now I know what to study more to get an even more detailed answer.
sporkstorms said:If massless particles always travel at c, then how do they "slow down" in matter?
selfAdjoint said:Photons can't have a rest mass, because they haven't ever got a rest frame. They always travel at c. And where did you get that [tex]h\nu/c^2[/tex] anyway? AFAIK photons have always been massless in quantum theory except for an early attempt by Feinmann to use massive virtual photons to correct the infinities in QFD, an attempt that came to nothing and which he abandoned to create his version of renormalization.
(Added) Oh I suppose you are using [tex]E = h\nu[/tex] and [tex]E = mc^2[/tex]. But the energy relation of relativity, got by doing "Minkowski-Pythagoras" on the components and magnitude of the momentum energy four-vector, is [tex]E^2 = p^2c^2 - m^2c^4[/tex], so when m = 0 energy only depends on momentum, not mass, and that's the situation with light.
Since [tex]e = mc^2[/tex] is derived from relativity, for massive bodies in their rest frames it's just wrong to use it to compute a fictitious mass for the photon.
Did you read it? He saysrbj said:it might currently not be the conventional usage of the term "mass" (to mean, in general, "relativistic mass") but there is still some semantic respect given it in the physics FAQ: http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html
That's not what I call respect. Redundancy is good engineering, but in physics theory, it means "not needed", so physicists turf it out.baez said:Relativistic mass is equivalent to energy so it is a redundant concept.
C refers to the speed of light, which is a constant in the universe and is approximately 299,792,458 meters per second.
According to the theory of relativity, nothing with mass can travel at the speed of light. As light has no mass, it is able to travel at the speed of light, but other objects cannot.
Light does not achieve C, it simply travels at the speed of C. This is because light has no mass and therefore is not restricted by the laws of relativity.
According to our current understanding of physics, no. The speed of light is considered the fastest possible speed in the universe and nothing has been observed to travel faster.
The speed of light is a fundamental constant that affects our understanding of time and space. It is used in many equations and theories, such as the theory of relativity, and plays a crucial role in our understanding of the universe.