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If we fire one photon at a time, why don't all of the photons hit the barrier exactly in between the two slits? How come each photon goes in different directions? (some go through top slit, some go through bottom slit, and the rest hit the barrier at different altitudes) If the photon firing gun is stationary in between the slits (longitudinally) and always points perpendicular to the barrier, why doesn't 100% of the photons travel in a straight line and hit the barrier halfway between the two slits??

Fuad
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  • The problem here is the mental model that a photon is like a small object that gets fired by a gun-like emitter. Photons are not objects. They are quanta of energy. When we talk about an optical emission process, all we can say is that the source has added a certain amount of energy, momentum and angular momentum to the electromagnetic field. We can not say where that energy is at any given moment in time. We can, however, observe many such emission (and likewise absorption) processes and they will, on average, behave like a classical electromagnetic wave. – FlatterMann May 17 '23 at 07:15
  • @FlatterMann let's replace photon with electrons. I know electrons are physical "points" or "balls" or "moons" orbiting an atom, right? Since double slit experiment is valid for electrons, isn't firing electrons similar to firing a bullet? – Fuad May 17 '23 at 07:22
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    I know electrons are physical "points" or "balls" or "moons" orbiting an atom No they are not. – joseph h May 17 '23 at 07:32
  • @Fuad “What gets us into trouble is not what we don't know. It's what we know for sure that just ain't so.” ― Mark Twain. Electrons are just like photons. They are quanta of field energy that just happen to have a rest mass and an electric charge in addition. – FlatterMann May 17 '23 at 07:44
  • @FlatterMann how can something that isn't a physical object, have a charge or a mass? "quanta of field energy" is effectively to a common man the same as "chakra" or "spiritual energy" or "feng shui". If not, why not? how is it different? – Fuad May 17 '23 at 07:52
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    Hi Fuad and welcome to the Physics SE. It's an excellent question and touches on one of the things that makes QM to hard for people just starting to learn it. However the question has been asked before. Have a look at the answers to question I've linked and if you still have unresolved doubts please feel free to ask a new question to ask about the details that aren't clear. – John Rennie May 17 '23 at 08:31
  • In general the light beam is wider than the slits .... even if you focused the beam on the barrier there is no such thing as perfect focus .... so maybe 1 in a million finds a slit ... and if you watch for these, voila you see the pattern! – PhysicsDave May 17 '23 at 14:00
  • @Fuad The only reason why you think of mass as something special is because you have only been told about it from a classical angle in school. The current education system teaches like as if you are "the very model of a modern major general" (which is to say a 19th century person) right until you get into grad school. Only then are you being told the "truth" about nature that has emerged since roughly the 1930s. We are, educationally, roughly 100 years behind what we know. As a consequence a physics student has to literally "exorcise" all those poor mental models they were given in school. – FlatterMann May 17 '23 at 15:35
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    @JohnRennie the duped question is not the same as my question. it should be un-duped – Fuad May 18 '23 at 07:12
  • @Fuad I agree with you. The so called duplicate is nothing like your question and we should re-open yours. Its a simple question and I'm surprised no one bothered to answered it. Photons are emitted in random directions and that is what you want for a slit experiment. – Bill Alsept May 21 '23 at 07:13
  • Why? Because that's what we see in experiments. We then concoct models that capture what we see. But the models can't tell you "why". – John Doty May 22 '23 at 23:56
  • @JohnDoty My question was more to do with the details of the photon firing gun. If I aim it at a target, why are photos not hitting the target all the time. I think the double-slit experiment people do not explain properly that the photons are emitted in random directions, as opposed to a gun pointed at a target. – Fuad May 23 '23 at 02:03
  • @Fuad In order for a successful slit experiment, it is crucial for photons to completely cover the slits, from one edge to edge. It is important to remember that the photons diffract or scatter around the edges of the slit, resulting in the formation of a distinctive interference pattern for a single slit. Similarly, for the double slit experiment, it is necessary for photons to cover both slits entirely, extending from edge to edge. Even when dealing with one photon at a time, it is still essential for the photons to traverse through all sections of the slits. – Bill Alsept May 23 '23 at 06:45

2 Answers2

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People often interpret "particle" as a point-like being or something negligible in size. But photons' sizes definitely cannot be ignored in the double-slit experiment. Further, in quantum mechanics, the particle nature of photons refer to their energy come in packets and is not about their sizes. They are not point-like things. Each photon goes through both slits as a wave does.

Yuan John Jiang
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Your question is very specific. Let us ignore that your conception is actually wrong, and take your particle viewpoint to the extreme.

When a gun shoots a particle, one-by-one, there is recoil. There is vibration from the Earth that the gun has to be secured somehow. (And no, I am NOT invoking uncertainty principle.) There is no such thing as a gun that can fire particles exactly tracing the trajectory of an earlier particle. There is some actual spread.

Yes, if you have a very small spread, then it is likely to fire particles that hit the middle of two widely separated slits. When this happens, you get that the barrier with the slits is blocking the passage. This case actually defines what it means to be widely separated.

Given any small spread, the slits can be moved closer until the spread and the slits will overlap. Then, and only then, will the double slit experiment start to be discussed in the way that is envisioned to be discussed. You have simply ignored the point of discussing the experiment, and instead got worked up on a technicality.

Classically, light is a wave, and the size of the wave is necessarily huge compared to the slits. In fact, you can visibly see the how macroscopically large the resulting wavefunction is, on the viewing screen behind the slits. All those pictures are made with photons that are localised onto tiny atoms making up the viewing screen, much smaller than the macroscopically large wavefunction.

Then you can speak about how the wave is passing both slits at once. And confuse yourself over the particle trajectories and the interference pattern.

naturallyInconsistent
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