Suppose a photon is emitted in a vacuum (for example) and so we perceive it to be moving at C (whether as a particle or a propagating wave form).
As I understand it, the speed of the photon would be C immediately, so to speak. Photons and certain other “particles” have a constant speed and do not “accelerate” to that speed–i.e., go from a speed of zero up to C, for instance.
So doesn’t this seem to indicate that a (massless, at least) “particle” is not a particle moving through “space” but is instead, the propagation of some kind of perturbation of space itself?
‘Particle’ in the sense of ‘solid ball of material’ is a definition that breaks down for nearly everything, once you look closely. So yes, it’s a phenomenon based on fields, distortion of spacetime and [insert explanation here] that has particle-like properties, but so is ordinary matter.
Gross oversimplification but particles that are “massless” must travel at the speed of light, and they do not experience time.
From the photons perspective it is emitted and absorbed as a single event. It does not matter if that photon traveled 14 billion light years or just nanometers it is instantaneous from the photons perspective.
It is impossible for it to travel slower than the speed of causality or the speed of light. (using the particle state as a reference)
But what about a photon that is brought to a standstill as in this German experiment from 2013 in which they managed to trap light in a crystal and immobilize it for a full minute? It mentions in that article that the experiment may “give experts clues on accelerating light beyond the universal speed limit.”
OK, I realize it’s the Daily Mail and that stuff about pushing light beyond lightspeed sounds totally off the wall but I presume the experiment was real. It also mentions a previous successful attempt by physicists in 1999 to slow light down to 17 metres per second and then completely halt it for a fraction of a second two years later.
If this stuff is real and not just the Mail’s usual claptrap then I assume if they can slow a photon down and even stop it then it should be possible to accelerate it?
Missed the edit window to add this. If the photon is motionless for one minute can it not be said to experience time in some fashion? I know I’m way out of my depth here and I would welcome guidance from those who actually know what they’re talking about!
The light was not stopped, it was bouncing around at the speed of light within the crystal for that time.
As an example a photon emitted in the center of the sun could take around 170,000 years to even reach the surface but it is always traveling at the speed of light (speed of causality)
But it is important to understand that the photon “wave” in this case does not slow down.
But I am intentionally being obtuse to avoid bringing up the wave–particle duality issue.
As in the case of the sun above the real answer is far more complicated because it is not really that an individual photon keeps it’s identity in that case, but that is also true of the above article.
Thank you, rat avatar, that wasn’t made clear in the article.
I think I understand. And I shall certainly give that video a watch, I love stuff in which these subjects are discussed in terms accessible to a layman. It’s tough to grasp the concepts involved when your highest mathematical summit is a rudimentary knowledge of calculus!
Then you already know more than you need to know to grasp Special Relativity, which is all you need here. You can derive everything you need from first principles using high school algebra.
But what does bouncing around mean? Any time you “bounce” off of something, several things happen, you impart some level of energy no matter how small from the bouncer to the object your reflecting off of, which seems to imply that as some point the particle would lose the energy of its acceleration with enough bounces wouldn’t it? Also a bounce normally implies a change in both velocity and direction, if you bounce a photon back 180 degrees how do you have a change of direction without a particle reaching a theoretical stop point before it goes the other way?
Massless particles travel at c. (Note that I’m talking about individual particles here. The velocity of a light wave, or what happens when, say, photons are repeatedly absorbed and re-emitted in a medium are separate things.) Special relativity (SR) claims that a particle traveling at c in one inertial system also travels at c in every other inertial system; if you work out the math, that gives you the entirety of SR from that one statement.
In SR, there is no rest frame of the photon. It doesn’t quite make sense in SR to ask what the universe looks like from the vantage of the photon. On the other hand, we can say that (at least two of the three genrations of the) neutrino have mass due to their flavor oscillation. For comparison, think about all the very short-lived particles like kaons and pions that we record on earth. Their lifetimes are on the order of nanoseonds, but we can still record them because in their frame, it only takes them that long to reach the surface (through, depending on which frame you use, either time dilation or length contraction).
No, why would it? They have and transfer momentum; they’re spewed out of a quantum field; they fit nicely into QED (photons) or QCD (gluons) formalism; they can be detected by standard equipment (in the case of a photon, at least); and so on.
How do different materials fit into this? Light travels slower through water or glass than through air. Any of these have slower speeds of light than in a vacuum.
Do photons re-accelerate after passing through glass, or do they stay at the reduced speed?
Please correct me if I am wrong. As a poor mathematician I have only a layman’s knowledge of physics. The reason light travels slower in a medium like glass or water (or hydrogen clouds in the galaxy) than in a vacuum is that it is constantly being absorbed and re-emitted by the atoms of the medium. More precisely, it hits an atom, an electron is raised to a higher energy level, then, after a small, but non-zero time, the electron falls back to its ground level and emits a photon of the same wavelength since that depends only on the change in energy level. Is it the same photon? That is a meaningless question. And different wavelengths are slowed by different amounts, hence the spectrum.
And the fact that neutrinos can change flavor implies that time passes for them, hence they are not traveling at the speed of light, hence must have mass.
See what Hari Seldon said. For purposes of message boards, you can consider the photon being delayed by the constant need to be re-emitted to be a single photon traveling more slowly, but it’s really speedoflight stop speedoflight stop speedoflight with the stops being more numerous in some media.
There is no point in time that a photon exists that is moving at a speed other than c. This is important because our understanding of photons indicates that’s their only speed. Referring to the phenomenon as “slowing” implies that there is some time where an individual photon is moving at less than c, whereas “delay” allows all photons to always move at c when they are moving, which is a better verbal descriptor.
Referring to the phenomenon as slowing may be a reasonable first order approximation when you’re concerned with how long it takes for a signal to pass through some medium, but it introduces confusion when the discussion is of what’s happening to individual photons.
This os not really something that intuition will work for and this really gets into post doctoral physics.
For something to be “clear” like a crystal or glass the photon is not “absorbed” and it does not interact with an electron in a that strong fashion when passing through. For “clear” substances you have to think of light in the wave function and not as a particle or individual photon.
It is important to understand that light is electro-magnetic radiation, and that magnetism and electric forces are the exact same thing, just viewed from a different perspective.
The wave of light is not absorbed/emitted as an individual photon, but the wave does weakly interact with the electrons, for a lack of a better term in this case lets say it giggles the electrons that it does not have enough energy to kick to a higher energy state.
That wave of light, like a dropped pebble in a pool interacts with all of the electrons in a medium like a crystal just as the wave in the water does. The wave that “bounces” or travels through and is emitted from the other side or surface is the product of all of those movements and interactions similar to how the wave you see on a beach is the product of all of the ripples etc… on a pond.
Basically when functioning as a wave light will take all paths through a piece of glass or crystal, and it is the product of those interactions that result in that electro magnetic wave being observed on the other side that is delayed.
Diffraction and reflection and other phenomenon relate to the phase velocity, and it is that phase that we say is going slower than the speed of light. But that phenomenon is a product of all of those weak interactions but the photons are still traveling at the speed of light.
This is an incredibly deep rabbit hole, and I doubt that I can add any more except creating more questions, and will hit my limits of understanding or our collective understanding but the core idea is that a photon never accelerates and it never goes slower than the speed of light.
This is correct, but is the masslessness essential to your subsequent question? An electron emitted from a decay process also has a fixed speed that is acquired immediately, just like a photon. For the photon the speed is always c whereas for an electron or other massive particle it is something else depending on the kinematics of the process.
Does it help simplify your question to remove the complication of speed-of-light propagation?
(I ask this because I don’t understand what you’re getting at in your OP. This is an attempt at peeling a layer off.)
Thank you for those links. I actually managed to follow the math of the derivation of E=mc2 and got a great charge from doing so. The stupendous consequences of one man’s thought experiment are staggering, The derivation of the Lorentz transformations I’m going to take some time over, that one didn’t go as smoothly for me but I’ll get there!