Yes, there are higher-order terms in quantum electrodynamics that describe photon-photon interactions, although they result in very small forces that are practically insignificant. Same goes for higher-order effects we see when we add-in in quantum chromodynamics. When one calculates how a photon goes from A to B one can include the effects of how the photon interacts with the vacuum along its trajectory. Such interactions are essential and “real” in the sense that so far the observed behavior of photons is well-described by the calculations.
Because from photon’s perspective the entire universe is Lorentz-contracted to a 2-D plane. So the distance between its origin and destination is zero.
It pretty much has to be like that. If it takes the photon zero time to reach its destination, its source and destination must be the same location – otherwise it would be traveling faster than the speed of light, which is impossible
True, but the entire universe is much smaller. The resulting ratio of distance/time will be c (again, taking appropriate limits where required).
I’ve been thinking about photons recently, because it occurred to me that its properties are extremely weird, and that an alert undergraduate could, like a kindergartener perpetually asking “Why?” in response to an adult’s explanation about, say, why the sky is blue, reduce a graduate student or a professor to gibbering idiocy. Photons really are weird things, and defy your attempts to give simple answers about them. How big is a photon? What is its frequency spectrum?* How can something with a wavelength many, many times greater than an atom be emitted from that atom?
I was intrigued to see that others have been considering this question, as well. An irresistable book on the topic, gathering articles from the previous decade or so, came out in 2008 from CRC Press. It’s called The Nature of Light: What is a Photon? If you’re really curious, and have too much free time on your hands, it’s probably worth the $122:
*If a photon has no spectrum, then it consists of a single wavelength, which means the photon is a single monochromatic wave that has always been on and will be on for all eternity. For a photon to have a beginning and an end – for that wave to only exist in a limited region of space, between the atom that emitted it and the atom that absorbed it, requires that it be made up of a range of wavelengths that, superimposed, produce an almost monochromatic wave train of limited extent.
But Special Relativity doesn’t allow for an inertial frame to be moving at c, does it?
I think it’s you who are misinterpreting what Pasta wrote.
In the first paragraph, the particles involved travel under c, therefore have mass and are not photons. It’s correct to say for a massed particle that “Rather, it sees itself covering just under 300,000 km in every second.”
Only in the second paragraph are photons referred to. But that one’s correct as well.
Based on these explanations, ‘spooky action at a distance’ doesn’t seem so spooky after all. It sounds like photons seperated by great distances seem to be in the same place. Kind of like photons being somewhere and everywhere at the same time.
So back to the OPs question, what in a photon’s frame of reference is the difference between interacting with something else and not? Is the time referred to only related to a photon traveling, rather than the passage of time itself, i.e., would that ‘photon clock’ tick at all?
Sorry, I’m not very good at phrasing questions like this, maybe someone has an answer to a comprehensible question which addresses this. I don’t intend this to be a hijack.
Back to the OP… You have a photon source at point A and a photon detector at point B. say photons are emitted every second (in the source/detector frame).
It is true that there is a certain (asymptotically defined) reference frame in which points A and B are separated by zero distance. Thus, emission, transmission, and detection are simultaneous. But…
Imagine that an interloper, stationed midway between A and B, occasionally inserts an angled mirror[li] in the way. A photon would now experience emission at A but not detection at B. Thus, those two events are certainly not two parts of a single process. They can be separated.[/li]
While you can define a pathological reference frame that makes some quantities go to zero, it doesn’t eliminate the reality of objects. Indeed, in the pathological reference frame, nothing happens. To anything. Anywhere. There are probably lots of linguistic tricks one could play in that frame, but I’m not sure they get you anywhere.
If the mirror is unsatisfying, perhaps have the interloper move a massive object near the path so that the photon is deflected enough to miss the detector at B.
You’ve merely replaced one event with a different event.
When a photon “travels” from A to B, A and B are the same location.
When a photon “travels” from A to the block, A and whatever chunk of the block absorbed the photon are the same location.
In both cases, from the perspective of the photon the time and the distance traveled were zero.
It seems to me that this implies that 3-D space is merely a familiar convenience. Although it seems to us that light involves a photon moving from here to there over the course of some duration, it’s just as valid to view light as an instantaneous event between two adjacent particles. And hence photons don’t really exist. They’re just a computational convenience for working in 3-D space.
It’s more than a familiar convenience, though. It’s the only reality we have. Why would you choose a particular (inaccessible) reference frame to decide the photon’s existence when there are an infinite number of other (accessible) references frames, all of which see the photon as traveling some non-zero distance in some non-zero time?
I could equivalently imagine myself in a reference frame moving at c parallel to your upright body standing in a field. I would see you as having zero height. Does that mean you don’t exist in any volumetric way? No, it just means I’m viewing you from a reference frame in which things break down. Since I can never actually be in that reference frame anyway, it isn’t a problem.
This is why I mentioned language. One is free to define existence based on how things would appear in a particular (and, in this case, pathological) frame, but I’m not sure what mileage just re-definition gets you. Nothing happens in that frame at all.
An analogy: consider a triangle viewed face-on, so that you see all three legs. If you pick just the right different vantage point, you can view the triangle edge-on and see only a straight line – no triangle at all. The fact that you can do this doesn’t mean that triangles don’t exist. It just means that you can choose a particular view to make certain aspects of the triangle become degenerate. But that view is only one of an infinite number of others.
This calls to mind a speculative article (or was it a science fiction story?) I read once which depended on the idea that existence is relative to reference frame. It may be that there are reference frames in which the particles making up the pen in my mouth* aren’t moving in a way that it would make sense to describe as cohering as a single object–meaning that from those reference frames, in a sense, the pen doesn’t exist. And it may be, by the same token, that there are countless entities composed of the very material around us which cohere into objects we’ve never witnessed, from reference frames other than our own.
Each of us human beings, of course, exists in the reference frame determined by his or her own worldline. But couldn’t it be (this article/story speculated) that there are entities that exist in some reference frames but which, on the reference frame determined by their own worldlines, don’t exist?
What if these entities were, from some reference frames, composed such as to cohere in that particular way we call “sentient”? They appear to exist and appear to be sentient from some reference frames. But from “their own” reference frame, they don’t exist.
Would such entities have phenomonological awareness?
Not intended as a GQ question, just relating the thrust of the thing I vaguely remember having read, because the idea always fascinated me but I’ve never been sure the speculation is a sensible one according to GR.
*TMI I know, sorry!
I think the easiest way to answer this is just to say a photon doesn’t have a reference frame. Taking the limit as v goes to c of the Lorenz transformation you’re not really constructing a new spacetime coordinate system* (in relatvity spacetime coordinate system and frame of reference are pretty much synonymous), so even in the limit I think it’s wrong to talk about a photon’s frame of reference. What’s really happening is that you’re measuring the spacetime seperation along the photons worldline, not constructing a frame of reference for it.
*or if you’re are you’re constructing an extremely badly-behaved one.
In my limited understanding of relativity, I though the problem was you couldn’t compute time or distance in the frame of reference of a photon because everything seems the same distance away, and nothing can happen that is the equivalent of clock to measure time with. Is that anywhere near correct?
The statement that “everything is the same distance away” suggests that one can calculate distance. (More precisely, all distances along the direction of motion are zero. Transverse distances are not contracted.) You are just forced to calculate things as limits, but that’s okay. You perform your calculations for a frame that is moving at speed v different from c, and then, when done, you take the limit as v → c. The frame of reference still isn’t physically realizable, but the math of it is well-defined.
For the clock: if you calculated the observed rate from within the frame moving at v, you’d get one second per second. If you then took the limit as v → c, you’d still get one second per second.
Sorry to be a pedant, but no, a photon does not exist. It is an idealization. A photon in free space is infinite in extent and stationary (i.e. only its quantum mechanical phase evolves in time, no measurable quantity changes in time). Any real event involves a superposition of photons.
A photon is a very useful idealization, however. While in practice you can’t have a single photon, you can have a very close approximation to it. The emission of light from a quantum transition is approximately represented by a photon, but not quite, since a stationary state cannot describe the emission process. A conventional laser does not produce photons, it produces a Poisson distribution of photons, which lookes very classical at high amplitudes.
Do croutons exist?
Lots of folks would disagree with this. If single photons do not exist, then experiments made with light intensity filtered down to extremely low levels so that only one photon at a time interacts with material are absurd. There are also single photon sources available:
D.A. Baylor, T.D. Lamb, K.W. Yau, “Response of retinal rods to single photons.” Journal of Physiology, Lond. 288, 613-634 (1979)
Am. J. Phys. Vol. 64, No. 2, Feb 1996, Pages 184-188
A lecture demonstration of single photon interference
Wolfgang Rueckner and Paul Titcomb
http://www.toshiba-europe.com/research/crl/qig/singlephotondetection.html
And, yes, these really are experiments that assert that they are observing single photons, not merely “low light level”
I think the folks contributing to the volume I cite above might take issue with the claim that “photons don’t exist, and are just an idealization”
Consider the following experiment. A source of electrons and a source of positrons sits inside a box with thick, lead walls. One wall has a hole in it, and some distance away from that hole is a detector. Picture:
________________________
| |
| L ____________________|
| E |
| A | == e+ source ==
| D | |
| | |
| | v ||
| | || detector
| | ^ ||
| | |
| L | |
| E | == e- source ==
| A |____________________
| D |
|________________________|
On cue, the two sources spit out a single particle each, and these collide at the center, say at time T[sub]0[/sub]. The detector on the right will sometimes find energy deposited in its detection medium a certain time after T[sub]0[/sub], say at time T[sub]d[/sub]. This is reproducible. During some of the trials, imagine inserting a sufficiently thick lead panel between the interaction point and the detector. Imagine further that this insertion occurs after T[sub]0[/sub] but before T[sub]d[/sub]. On these trials, the detector doesn’t report anything.
The positron/electron annihilation is producing something that is traveling at some finite speed (since we can block it after T[sub]0[/sub]) from the interaction point to the detector, carrying with it some energy. That “something” is called a “photon”. You can say it doesn’t exist, but why would you do that?
I think he was claiming that there’s no such thing as one single photon by itself, only the abstraction called a quanta that comes out of the equations for a distribution of wave functions (if I’m saying that right).