1
$\begingroup$

I thought up a slight modification to the classic two-slit experiment that would be fun to try, but I am certain that I am not the only person to consider it, and would like to know if it has already been done and what the results were. Here it is:

We insert an extremely thin opaque wall between the two slits, so that a photon which is destined to pass through, say, the right-hand slit is shielded from the left-hand slit at that instant when it enters the slit- and the same for a photon destined to pass through the left-hand slit. The idea here is to prevent the photon from "knowing" that the other slit is open as it passes through one of the slits.

I would expect that in this case the "interference" pattern would disappear just as if the second slit were blocked. Is this indeed what will happen? and how far "upstream" of the slits would that isolating wall have to protrude before the side lobes in the downstream pattern disappear and we get the one-slit result instead of the two-slit result?

PS please note that I am aware of the fact that the double slit experiment has been performed with single photons, electrons, and even buckyball molecules, and that the Mach-Zehnder interferometer is the preferred setup to do this with.

Improve this question
$\endgroup$
5
  • 1
    $\begingroup$ The photon doesn't exist in a light beam, or at least not in any useful sense. Specifically a light beam is not like a collection of little "balls of light". A photon is a unit of energy exchange between the light beam and its environment. The light beam really is just a wave, and it's perfectly capable of going through both slits even when you have a wall between them. $\endgroup$
    – John Rennie
    Commented Jun 25 at 15:39
  • $\begingroup$ @john rennie, what would the outcome be then in the single-photon case? $\endgroup$
    – niels nielsen
    Commented Jun 25 at 15:41
  • $\begingroup$ @nielsnielsen Single photon yields the same results. A number of experiments using a single photon at a time have been performed to demonstrate when interference will result and when it will not. Here is one of my favorites (admittedly a bit complex): sciencedemonstrations.fas.harvard.edu/files/… $\endgroup$
    – DrChinese
    Commented Jun 25 at 16:02
  • 1
    $\begingroup$ @nielsnielsen "single photon" just means we added one quantum of energy to the wave. It's still a wave and still diffracts like a wave. As Dr Chinese says above that experiment has been done. $\endgroup$
    – John Rennie
    Commented Jun 25 at 16:24
  • $\begingroup$ The real problem here is that linear optics experiments are completely independent of the "number of photons". Individual photons do not interact with each other. What we call "interference" is the absence of self-interaction of linear waves. In interacting theories interference doesn't happen. Instead we have strong scattering and self-focusing effects like kinks and solitons. $\endgroup$
    – FlatterMann
    Commented Jun 26 at 1:28

2 Answers 2

Reset to default
0
$\begingroup$

The photon knows nothing, the EM field knows everything. The field considers all electrons in the apparatus .... especially the excited electron before photon emission. The field sees 2 slits open, the field guides energy to some parts of the screen (bright bands) and no energy to the dark bands. Per the Feynman path integral photons like to travel paths that are multiples of the wavelength, i. e. the paths are resonant.

Improve this answer
$\endgroup$
5
  • $\begingroup$ The path integral is also an ensemble description. It is NOT the Santa Claus Sled interpretation of quantum mechanics. Path integrals tell us nothing about individual photons and they certainly don't suggest that photons have paths. $\endgroup$
    – FlatterMann
    Commented Jun 26 at 1:25
  • $\begingroup$ @FlatterMann if 1000 scientists each did the DSE wth only a single photon when they get together to compare/combine results they would indeed see the pattern! $\endgroup$
    – PhysicsDave
    Commented Jun 26 at 16:23
  • $\begingroup$ 1000 photons are the approximations of an ensemble. It's like estimating the probability distribution of dice using 1000 dice rolls. It still won't tell us anything about the outcome of any one individual roll. $\endgroup$
    – FlatterMann
    Commented Jun 26 at 16:32
  • $\begingroup$ @FlatterMann yes but with only one photon in the DSE I could make a lot of money predicting where it might fall on the screen ... it's a biased roulette game. $\endgroup$
    – PhysicsDave
    Commented Jun 26 at 16:37
  • $\begingroup$ A photon is a small amount of energy. There are no photons in equations, otherwise we could put a solar panel in front of a textbook and harvest free energy. $\endgroup$
    – FlatterMann
    Commented Jun 26 at 18:13
0
$\begingroup$

The double slit experiment creates an interference pattern on the screen behind the two slits. The standard explanation is that the wavefunction of the photon spreads out, passes through both slits, interferes with itself, and then collapses when the photon strikes the screen.

Mathematically, the total wavefunction on the wall is the sum of the partial wavefunction of each slit. Since wavefunctions are complex numbers, this causes both constructive and destructive interference, resulting in a fringe pattern.

Putting a wall between the two slits means that the two wavefunctions can't come in contact with each other. Hence the pattern should be the single slit pattern for both slits. (Slightly modified since the photons can't cross the barrier).

You also ask what would happen if the barrier doesn't extend all the way to the screen. Well, the wavefunctions would partially interfere. I would guess the result is a complicated pattern on the wall that would have some fringes in the middle, and the single slit clumps on the sides.

Improve this answer
$\endgroup$
2
  • $\begingroup$ A single photon doesn't have a wave function. A wave function is a description of a quantum mechanical ensemble, just like a probability distribution is a description of a statistical ensemble. So what is a single photon? It's just a small amount of energy, momentum and angular momentum that the electromagnetic field exchanges with an "external" system like a light source or a detector. Most importantly, wave functions are not physical entities. Nature knows nothing about them and there is no physical way to measure them. They are theoretical descriptions of certain (not all!) quantum systems. $\endgroup$
    – FlatterMann
    Commented Jun 26 at 1:23
  • $\begingroup$ Not a bad answer. But the wave function even exists before the photon energy enters the EM field, i.e. the EM field is already strongly affected by the excited electron in the source even before emission. Also the EM field still sees both slits, so interference occurs. Now if we build the wall all the way back to the source we still have interference as the 2 sources of light are highly correlated/coherent, if we had separate sources then no interference. Yes building a wall to the screen would reduce/eliminate interference. $\endgroup$
    – PhysicsDave
    Commented Jun 26 at 19:47

Not the answer you're looking for? Browse other questions tagged or ask your own question.