I’m no physicist, but I thought the the staff article on this didn’t really explain it that well. The best explanation I’ve seen is in Richard Feynman’s QED.
I’ll see if I can explain it without mangling it too much:
Light consists of particles. Really. Not waves, despite what you’ve heard.
All we really know about one of these particles (called a photon) is that that it travels from point A (such as a light bulb) to point B (such as a spot on your eye’s retina, or on a photographic plate). What happens “in flight” is something of a mystery, because there’s no way to watch the photon move from A to B. (You can’t videotape a photon going by. All you can do is put something in the way to stop it, and stopping the photon in the middle changes the experiment.)
We do know that photons don’t move in straight lines like bullets. Feynman explained it as “the particle moves in every direction at once”. If a very dim light shines though a lens and individual photons are going by, you can’t point to a particular spot on the lens and say, “that photon went through the lens there.” The entire lens affects the destination of each individual photon. It’s only at the final destination (point B) that you can really say where the photon is.
Furthermore, photons move randomly. There is no way to predict exactly where a single photon will go. The best we can do is predict the probability distribution for where a photon will end up. If you shoot lots of photons through the lens, they start out as a just random dots on the screen, but when there are enough of them an image will appear based on the probability distribution.
When calculating the probability that a photon moves from point A to point B, physicists use wave equations. That’s where the wave comes in. It’s a mathematical construct. Physicists predict probability distributions for photons by pretending that there’s a wave starting from point A. But you can’t see the wave, you just see its effects in the form of photon arrivals.
sigh Because it is… it’s hard to accept… but it contains properties described BOTH described by particles and waves… there are experiments which prove this…
I frickin hate it! i hate that the SPEED of light is fixed, also! MY vision of the universe makes a lot more sense if it’s variable…
No. A photon moves in a completely described path… whether it is a simple straight one, straight at the Earth… or bending around a massive body (like our Sun…) the photon’s path is easily discernered…
Feynman’s approach is that every particle goes in every direction at once, forwards and backwards in time, and at every speed, including faster than the speed of light. However, all of these paths interfere with each other, and in most cases, they destructively interfere almost completely almost everywhere, leaving only one (or a small number) of paths where they constructively interfere. Interference patterns are an obvious case of this, but Feynman’s formulation is that all beams of particles produce interference patterns. It’s just that in most cases (as with a laser, for instance), the interference pattern only has one maximum.
And while it’s true that one can consider the particles to be “real” and the waves just a mathematical construct, one can equally validly consider the waves to be “real” and the particles just a mathematical construct, or consider both to be constructs, with the “reality” something as yet incomprehensible to us.
In a laser, the wavefunctions for the photons’ positions all coincide in space and time. But that’s still a probability wavefunction, not a precise position. It’s not totally random, in the sense that it’s equally likely to be anywhere, understand. It’s very likely for a given photon to reside at a given moment within a particular small box (defined in terms of its wavelength and Planck’s constant). In a laser, it’s the same small box for all the photons.
As I understand it, photons (plural) move in all directions at once. But any single individual photon is moving in only one direction at one time. However, trying to determine the direction of that one individual photon involves interfering with it, so it’s no longer moving as it was. And because photons (plural) are moving in all directions, you can’t pick out any single one and predict which direction it will move.
Compare this to people using a subway system:
People are moving in all directions at once. But any single person is moving in only one direction at a time. We can’t pick a random person from the crowd and predict what direction that person will travel. (Unless we use clues from their appearance, etc., because they look different (unlike photons).)
So I find that analogy to be useful in trying to understand this complex physics theory.
Light is neither a wave nor a particle. It is light… and light can behave like a wave or a particple, depending on the experiment you conduct with it… but light itself is neither a wave or a particle.
Unfortunately, if I understand QED correctly, actual reality is weirder than that. There are experiments (such as the famous two-slit experiment) that can be used to show that a single photon takes both paths and interferes with itself. You don’t need a lot of photons to cause interference - it works the same way even if photons are sent one at a time. (You do need a lot of photons to prove interference because each photon moves randomly, but they can be sent one at a time, not all at once.)
Another example is a mirror. When a light shines on a mirror and is reflected to your eye, usually we think of the photon hitting the mirror in one place and bouncing off, like a pool ball. This is because all the other paths cancel out. However, if parts of the mirror are scraped off in a certain pattern, the paths don’t cancel out, and the mirror becomes a diffraction grating. The diffraction grating works even if you send one photon at a time, which shows that the photon actually goes in all directions at once.
So a photon is in one place when it’s emitted and when it’s detected. In between, you can’t say where it is.
Probably. But the basic both-wave-and-particle is as weird as I can handle already. This admittedly crude analogy helps me unnderstand as much as I need to.
The language of modern physics isn’t english, it’s math. The math makes perfect sense. Try to translate it into words and it sounds stupid, because light is described by a mathematical function that’s kind of like a wave and kind of like a particle, but isn’t exactly either.
One thing I’ve never understood is how there seems to be no gaps between the light waves or particles as the move out from their source. Using shotgun pellets as an analogy for light particles, once they leave the gun’s barrel they begin to separate, and the further they travel, the further apart they are. Why doesn’t this happen with light waves and/or particles?
I’m not sure how a shotgun’s pellets separate. If you mean that they fan out, so the pattern of bullet holes is larger at longer range, that does indeed happen with light. A laser has a very tight spread, but even a laser beam will be a cone.
If you mean that the spread out radially, that is, that some pellets get further from the shotgun than others, that would be because the shotgun pellets are travelling at different speeds. But photons must all travel at the same speed, through vacuum (through anything other than vacuum, they’ll be slowed down, and different colors will be slowed by different amounts).
And there can be gaps between the photons, due to them not being emitted all at once. In a moderate-sized room bright enough to read in, there’s only about one visible light photon actually in the room at a time.
Thank you, Chronos. This answers elements of my question but the not the main part. I’m wondering why when light is emitted or reflected from any point, all of the object is visible in its entirety regardless of distance, provided you have a zoom lens, telescope, etc. to view it with. Let’s say I’m looking a car across the street. I assume a finite number of photons/light waves are being reflected off it at any given time. These photons/light waves are emitted out over a distance, even to another planet. If you have a strong enough telescope you could see the car perfectly even from another planet. You could also see it perfectly from any other planet having a direct line of sight. Why is it that the photons don’t spread further apart as they travel greater distances, as in the shotgun pellet analogy, creating gaps between the photons that render the object unrecognizable, and how is it that all the photons necessary to discern this detail are able to arrive at any and all points in every direction, and once again, without spreading apart as they do so?
Oh, I see. The answer is that there are those gaps, but usually, you have so very many photons that the gaps are all unnoticeably small. Even if you were looking at a very small, faint object very far away, you could still eventually get enough photons, by waiting long enough.
Chronos, thank you again. That makes sense. It’s amazing to me that the quality of light is such that I could shine a flashlight on the car I mentioned and that the light from that little flashlight goes out into every nook and cranny of space and could be observed from virtually any point in the universe (having a direct line of sight, of course) with a strong enough lens. This is just incredible to me. But, I digress. Thank you again.
It doesn’t take long, at the speed of light, for a photon to cross a room, so that about a hundred million photons can fly across a 10-foot room in one second without any of them being in the room at the same time.