Lasers, Why doesn’t a photon go through the same slit every time?

I am trying to understand the setup of a double slit experiment. When a laser fires photons through the double slit, wouldn’t the laser be accurate enough that all photons just go through one slit that is being aimed at all the time? And assuming that the answer is that a beam has a diameter, and the slit is cutting half of the beam, wouldn’t the photons in the left part of the beam go through the left slit 100% of the time and likewise the right side?

Is a laser like a hose of water and you’re filling up two buckets of water touching each other. If you aim the hose right, half the water goes In the left bucket, half in the right bucket?

I guess I am asking where the randomness comes from? Or why does a laser generate Randomness? If there is randomness in the angle of trajectory the photon leaves the source, it is deterministic which slot it will go through. Eg. at 0.001 degrees left it’s going to go through the left slit.

wouldn’t the photons in the left part of the beam go through the left slit 100% of the time and likewise the right side?

The trick is that photons don't travel like bullets in straight lines.

They propagate according to Maxwell's equations.

That means if they're emitted from a laser aperture, they diffract just like classical light waves do. And the narrower the laser aperture, the wider the angle that the waves (or photons) diffract.

So you can't say "this photon is in the left half of the beam", and "this other photon is in the right half of the beam". The photons themselves are propagating in a way that is spread out across the whole beam.

And the way we know this is (partly) because of the way laser beams interact and diffract when passing through a double slit aperture as in the experiment you're doing.

If there is randomness in the angle of trajectory the photon leaves the source, it is deterministic which slot it will go through.

As mentioned above, the concept of "trajectory" just doesn't apply to photons. They are not bullets or drops of water. They're quanta of electromagnetic radiation, and they propagate according to Maxwell's equations, not according to Newtonian mechanics.

• The other "trick" is that the pictures that appear in popular media have the transverse scale intentionally borked so the viewer can see the structureof the experiment. For a typical classroom demo approach the slits are much less than a milimeter wide and less than a milimeter apart, so even without a beam-spreader the beam envelope naturaly intersects both slits. – dmckee --- ex-moderator kitten Nov 13 '19 at 21:59
• If photons are waves and their wave spreads over the width of the beam (or supposedly over whole universe according to the non-zero values of a wave function), Why does it ever make sense to say it goes one path or another? It’s a wave and so propagates all the space. Is it possible the particle like effect is just because something dampens the wave in one of the paths? – Luke Nov 13 '19 at 22:05
• @Luke, usually it doesn't make sense to say a photon travels by a particular path. – The Photon Nov 13 '19 at 22:06
• I don’t know what you mean that it doesn’t make sense which path it takes, since they add detectors to measure that very thing. – Luke Nov 13 '19 at 22:19
• @ Luke, for example, if you put a detector after a two-slit aperture, the detector only tells you the photon got to the detector, it doesn't tell you which slit it went through to get there. And in fact there is no way to tell, nor does it even really make sense to say the photon went through one slit or the other. If it went through only one slit, it wouldn't make the interference pattern that we observe when it gets to the detector. – The Photon Nov 13 '19 at 22:25

You are basically asking whether the photon goes through one of the slits. And you are asking whether if you shoot 100 photons after each other, and 40 will go through the left slit, 60 the right, and we repeat the experiment, with another 100 photons, will the same amount of 40 on the left and 60 on the right go through? The answer is no. You are asking whether the randomness of this comes from the direction of the laser beam. The answer is no. The randomness comes from the QM phenomenon called photon, and that our universe is basically QM. There are a few things that are important to understand:

1. photon are not localized spatially between emission and absorption

The photon position is only well defined when we interact with it and collapse its wave function. This interaction would normally be with the detector. If we interact with the photon, to define its position, before it reaches the slits then the diffraction pattern disappears.

1. each photon (in case there is a pattern) goes through both slits

The photons do not have a well defined trajectory. The diagram shows them as if they were little balls travelling along a well defined path, however the photons are delocalised and don't have a specific position or direction of motion. The photon is basically a fuzzy sphere expanding away from the source and overlapping both slits. That's why it goes through both slits.

Shooting a single photon through a double slit

1. if you want to know which slit the photon went through, you will not see the pattern anymore, and you need to interact with the photon, that is, the detector on the slits will inelastically scatter the photon, changing its energy, and phase, and thus spatially localizing the photon. The reason you see a bright dot on the screen when the detector on one of the slits interacted with the photon, is that only the untouched slit will create the dot.

A detector after one of the slits intercepting the photon, changes the boundary conditions to a different system, and thus a different Ψ∗Ψ. It is no longer the same experimental setup. It should be obvious that if the detecting instrument after the slit , absorbs the photon like the screen does, only the untouched slit will give a signal on the far screen, which could not interfere with itself .( A sophisticated experiment with electrons which tries to minimally show the effect came to the conclusion that the detecting level acts as a point source for the electrons going through it, i.e. a different Ψ∗Ψ for the electron which is no longer coherent so as to show the interference pattern.) Detection at the screen has picked ("collapsed ")an instance of (x,y,z) of the original wavefunction and removed that photon from the final screen. In general after the detection of "which slit" the photons are in a different wave function with new boundary conditions.

Double Slit Experiment. What effect does the detector actually cause?

1. each and every photon (that was shot) will leave a bright spot on the screen

In a double slit experiment, does each and every photon leave a dot on the screen in the bright area?

You are asking about the randomness, which slit the photon goes through (actually which slit we will detect it at), and it comes from the QM phenomenon of the photon itself, being not localized as it travels through space.

What actually waves is the wave function of the photon, spreading as a Gaussian wave.

Do photons oscillate or not?

You are basically asking whether the randomness of the photons going through (being detected at) certain slits is because of the setup of the laser, and the original direction. The answer is no. The randomness comes along the way as the photon travels through space (and is not localized) before actually being absorbed at the screen creating a bright spot.

• why the downvote? – Árpád Szendrei Nov 14 '19 at 2:33

The double slit interference pattern occurs for an incoming plane wave of wavelength comparable to the dimension of the slit. This assumption is not valid for a laser with a spot smaller than the distance between the slits and no interference pattern is observed in this case.