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# Physics/double-slit experiment

Question
QUESTION: Expert,
I have seen numerous references to variations of the famous "double-slit experiment."  The question that keeps occurring to me is:  Since we don't really know what light is, how can we know that we are sending exactly one photon at the target at a time?
Thanks,
Paul"

ANSWER: > how can we know that we are sending exactly one photon at the target at a time?

Because we can reduce the light intensity reaching the slits to such a low point that the detectors actually record individual photon hits, one at a time.

http://www.physics.brown.edu/physics/demopages/Demo/modern/demo/7a5520.htm
http://www.teachspin.com/instruments/two_slit/experiments.shtml

When we record the pattern created by a series of individual photons reaching our detectors, one at a time, we STILL get an interference pattern!

[an error occurred while processing this directive]---------- FOLLOW-UP ----------

QUESTION: Expert,
Right, thanks, but then the next question is: How, if we don't really know what light is, can we build a detector that knows exactly what is one photon of light?  Do we know for sure that one hit equals one photon? Also, how can we say for sure that the photon in indivisible, when we don't really know what a photon is?
Thanks,
Paul

ANSWER: > Do we know for sure that one hit equals one photon?

Yes, because the detector has a specific reaction to one photon, and a very different one to two photons. Specifically, the detectors record a cascade current based upon the number of photons that hit them. More than one photon, a greater current. The experimenter describes how this works.

Recall that the energy intensity of light is defined as the amount of energy hitting the slits each second, and that each individual photon has a specific amount of energy. Thus, if we send monochromatic light into the slits, once we know the energy intensity at the slits, we can know that how many photons are going there each second. By reducing that number to just one every tenth of a second or so (or even just one a second or so), we can know that only one photon is reaching the slits at a time.

> how can we say for sure that the photon in indivisible

If one photon -- one chunk of light energy -- became two such photons, that would require a doubling of energy (and also momentum and angular momentum, which are also conserved quantities). If the photon became two photons, each having half the energy of the original, then that would change the frequency of the light. Light doesn't spontaneously cut its frequency in half.

> when we don't really know what a photon is?

Now sure what you mean by that. We KNOW that photons are chunks of light energy, and that, for a given frequency of light, each such chunk has energy equal to that frequency times Planck's Constant. This fact allows us to easily explain a wide variety of phenomena in our Universe, that are impossible to explain without that fact -- or at least to require magic elves that do tricks on our experiments.

---------- FOLLOW-UP ----------

QUESTION: Expert,

My question about dividing a photon wasn't to suggest we humans could make two out of one.  It was to ask how do we know that the photon isn't made up of smaller somethings?

By "when we don't really know what a photon is," I mean that we don't really know what a photon is, do we?  Yes, we define it as a chunk of light energy, the smallest we can detect, but if you say we don't know if it's a particle or wave, for instance, can't I say that means that we don't know what it is? And how then do we know that it's not made of smaller stuff?

I am aware of how robust quantum theory is in making verifiable predictions, but that doesn't necessarily preclude anything that is (for the present)beyond our observations, does it, including strings, magic elves, or anything else?

Thanks,
Paul

> wasn't to suggest we humans could make two out of one.
> how do we know that the photon isn't made up of smaller somethings?

Note that I was careful not to say, "Humans can't take one photon and make two of them," I instead said, "Light doesn't spontaneously cut its frequency in half." In other words, the breaking apart of one photon of a certain frequency into two photons of a lower frequency doesn't happen in our Universe.

> how then do we know that it's not made of smaller stuff?

If by "know" you mean, "Is it absolutely impossible that any intelligence in our Universe will EVER discover that photons can break down into constituent parts?", I'll have to answer, "No, we don't KNOW that." We (or some other intelligence in our Universe) may, in the future, find (or maybe has already found) that photons can break down into smaller parts. Or that magic elves have been tricking us this whole time.
What I WILL say is, "There is not one shred of evidence, either in experiment or in the theoretical structures we have developed (and tested rigorously for decades), that photons are anything but fundamental particles." In other words, to the same extent that we "know" that our Earth goes around our Sun, we "know" that photons have no constituent parts.

> if you say we don't know if it's a particle

Ah, here's where you may be going wrong. We DO know (again, to the same extent we know our Earth goes around our Sun) that photons are particles. Point-size particles, to be exact, according to the Standard Model -- the most robustly tested theoretical structure for sub-atomic physics EVER. Indeed, when we make it possible for us to measure which of the two slits the photon goes through, then photons behave EXACTLY like particles -- with no interference pattern whatsoever.

"So," you may ask, "how come we get an interference pattern when we send individual photons towards a double slit?" That is a question that scientists are STILL working on! The BEST answer that we have as of today -- ie, the one that best fits experiments -- is that our concept of a singular photon going through one or the other slits is just wrong! To the same extent that Nicolaus Copernicus was WRONG when he conceived of the planets as moving in perfectly circular orbits (he couldn't imagine any other way for them to move!), we are wrong to think that, at the quantum level, photons "exist" when they are not being observed. Experiments over the last couple decades have irrefutably shown that our Universe does not follow "local realism." Either objects at the quantum level have no meaningful existence when they are not being measured OR objects can convey information to other objects at a speed faster than light. (There are scientist who insist that this is too strong a statement, but they are a distinct minority).
http://scienceblogs.com/startswithabang/2011/07/01/people-keep-making-einsteins-

Don't like this fact about our Universe, the same way Copernicus didn't like non-circular planetary orbits? Sorry -- this is the Universe you and I are stuck in, no matter how much we may dislike how it operates.

It is hoped that future scientists -- preferably ones on our planet -- will be able to resolve the problem. The ones that do so, and create a testable hypothesis that ends up matching experiment -- will undoubtedly win a Nobel Prize.
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B.A. in Physics (with honors) from University of California at Berkeley.M.A. in Physics (with honors) from University of Texas Austin.