Is it like a bullet … going forward in a straight line?
Or like a snake … serpentining back and forth along the ground?
Or like a corkscrew … spiraling it’s way forward?
In empty space I visualize it going forward in a straight line, much as it does here on earth, unless something bends it.
Since a photon undergoes zero time from its frame of reference at lightspeed, as a sort of “corkcrew wave” that exists simultaneously at every point from its generation to its absorption.
Light travels in waves, which in many situations may be a more useful way of thinking about it than about “bullets”. The speed of propagation is finite, though, so it is not simultaneously “at every point”, but really it does not exist at various points in the classical sense; you should be thinking about quantum fields.
How about we say something like: Maxwell’s equations describe how a photon goes forward
Yeah…it depends on what reference frame is used.
Is it you watching the photon move across the room?
Or are you riding on the photon?
Each will have a very different perspective on what is happening.
A photon is a quantization of the electromagnetic field that connects the emission locus with the terminus, conveying information in the form of momentum, polarization, and spin. It exists in all reference frames where it can be observed in simultaneity, experiencing no passage of time. All classical interpretations of the photon are, at best, incomplete or misleading, albeit useful in many local and bulk contexts.
Stranger
In comic books in the 1960s they represented photons as perfect spheres, which has a pleasing elegance, but I think is misleading. Of course, you can’t see a photon – what would you be seeing it with? You’d have to bounce photons off its to make it visible, and photons scattering from other photons is a rare occurrence. But you also need a wavelength shorter than the size of the photon you’re seeing. Clearly, when The Atom or some other shrunken character “sees” a photon, it’s just fantasy.
Any macroscopic physical model you come up with for a photon an its behavior i going to fail ultimately, because light behaves both as a wave (which is an extended phenomenon) and a particle (which is localized). My instincts would be to picture the photon as a localized particle, although not a sharply-defined sphere. I would have it travel in straight lines, because light rays (as used in ray optics) essentially travel in straight lines. It would reflect from flat metal and dielectric surfaces much like rubber ball, because angles of incidence and reflection are equal. It would refract at a definite angle that depended upon the angle of incidence and the refractive indices of the substance it traveled from and the one it traveled into (although I can’t think of mechanism that makes that palatable. The only one I’ve encountered is the “lawn mower going from sidewalk onto grass” one that depends on the photon having a physical extent)
Of course, you run into problems when the wave-like “physical optics” phenomena come in. How do you account for things like interference and diffraction? The wav-like nature of light explains its ability to surge around boundaries and form things like Poisson’s/Arago’s spot.
In my mental quasi-physical model of the photon I imagine it not having a sharp edge, but trailing away from the center, the way the wavefunction of the s-orbital of a hydrogen atom exponentially decreases in size with distance, but never completely goes to zero. That would explain how a single photon “knows” how wide a slit it is passing through, and can thus create the appropriate diffraction pattern, which is determined by the overall slit width. If I imagine that the photon has potentially infinite extent, it can “sense” the slit width, even though it can easily pass through. In my mind, then, a photon is like a fuzzy dust bunny.
If you say that the diffraction pattern is the result of lots of photons passing through and making their contributions, I’ll point out that there have been expeiments done where the photon rate has been made so low that we can be certai that the pattern is being made by single photons passing through and making a cumulative effect, not pairs or multiple photons interfering with each other at the time. Yet the pattern built up by these individual photons shows “knowledge” of the slit width.
The same holds true for the double slit experiment – the question “which slit does the photon pass through”, in this model, is pointless. The photon passes through both slits. Or a diffraction grating – the photon “feels” each of the grooves, and “knows” the separation and width of each groove.
Of course, it’s a crude mechanical model, and it eventually breaks down. So you shouldn’t expect it to answer those simultaneity questions.
If I’m not thinking about it, the photon moves back and forth like a wave, but as soon as I start visualizing it, the wave function collapses and it moves in a straight line like a particle.
Sort of a Schroedinger’s Photon, then?
It usually does, but it’s worth noting the paradox that if detectors are placed to determine “which-path” information, the photon suddenly acts like a particle and passes through only one slit, and no interference pattern is observed.
The most astonishing paradox is observed in variations of the delayed-choice quantum eraser experiments. In a typical variation, individual photons pass through a half silvered mirror that has a 50% chance of directing the photon down one path or another one. The paths are then recombined to see whether an interference pattern builds up or not. The astonishing thing is that if there is any source of information recorded anywhere in the system about which paths the photons took, they act as particles and there is no interference pattern. If the information-recording source is removed, or even if it’s scrambled so that no usable information can be extracted, the photons act as waves, apparently taking both paths at once and producing an interference pattern. Quantum mechanics is weird!
First off, @CalMeacham thinks about photons a lot more than most of us do, and more even than most physicists do, since optics is his specific specialty. So listen to what he says.
How I visualize a photon depends on why I’m visualizing it. Ray optic is enough for a great many problems, and so for those, I visualize it as a dimensionless point. I only think of it as having an extent when ray optics doesn’t cut it, and then what sort of extent I visualize still depends on the problem.
You can, of course, describe light as a wave, but a wave is a whole bunch of photons, not any single one. It’s problematic to describe a single particle as a wave, because the wave-ness and particle-ness of light are directly at odds with each other. Certainly you shouldn’t imagine a photon as wiggling back and forth in space: The electric (or magnetic) fields that make up light do have a direction in space, but their amplitude is not an amplitude of distance.
So, in this context the individual photon is like a flying probability distribution (in 3D)? Or no? Does the diffraction pattern have a normal/gaussian distribution? Or something else? How precisely has it been measured?
The term “wave” has always eluded me. Ditto for “Field”. If we have this bunch of photons (to make things easier), is a wave a flying histogram/distribution of photons? That’s straightforward to imagine and could even exist at macro scales.
I don’t know but whatever it is, I’m picturing it with googly eyes.
The physics of photons are interesting but mainly I think of photons related to CGI. So a photon is a ray that exists for a moment in time between a 3D defined space and a virtual lens. It can be an entire plane occasionally.
Well, there is definitely a probabilistic aspect to it. A photon cannot be localized at a point (it has no mass, for starters), but it has electric and magnetic field distributions which are described by Maxwell’s equations, relevant for single photons even though classical electromagnetism predates quantum theory. [Also, if you recall all that Feynman stuff, what you can compute are things like probability amplitudes for a photon to go from one place and time to another place and time.] A photon is basically a (quantum) elementary excitation of the electromagnetic field.
How do you visualize a billiard ball traveling through space? Do you visualize a hard, smooth sphere with no relevant substructure? Probably. Or do you visualize a collection of zillions of atoms made up of subatomic particles obeying various quantum mechanical principles? Probably not.
No object has a single way to visualize it that is simultaneously universally useful and universally correct. If the billiard ball was going to crash into another billiard ball, then “hard, smooth sphere” is a quite reasonable approximation, so much so that we would happily say that the ball is a hard, smooth sphere. But, if the billiard ball were being struck by a high-energy electron from a particle accelerator, then the subatomic structure is going to dominate what happens.
A photon is no different in this regard. “How do you visualize a photon?” is a question that requires context. There’s no single answer. It might be useful and adequate to visualize it as a point particle, as an infinite plane wave, as a wave packet, etc. It may be useful to emphasize degree of locality, polarization, frequency, etc. in the visualization, or it may not.