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Laser fires single particles of light, called photons, through the slits. Even though only single photons of light are being fired through the slits and They create three pattern again. How single particles of light can create this wave pattern?

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marked as duplicate by Waffle's Crazy Peanut, Dilaton, Emilio Pisanty, BebopButUnsteady, Dan Aug 26 '13 at 21:00

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"how single particles of light can create this wave pattern?" Because there are many "single particles" (en.wikipedia.org/wiki/…). –  Cristi Stoica Sep 7 '12 at 7:51
@Cristi Stoica its just single photon not so many just single photon –  zeous Sep 7 '12 at 7:54
If it is only one photon, you will see only one spot on the screen. The pattern is formed by many such spots. But, the probability that the single photon lands on the screen at a given point is given by that pattern. Yet, to see the pattern, you have to wait for many photons to arrive and confirm the probabilities. –  Cristi Stoica Sep 7 '12 at 8:09
Wave-Particle duality (particles behave as waves). So, you should consider a single photon as a wave. See this video: youtube.com/watch?v=DfPeprQ7oGc –  cinico Aug 20 '13 at 22:50
@CristiStoica: If it is only one photon, you will see only one spot on the screen. The pattern is formed by many such spots. Even if I only do the experiment once, I can predict with absolute certainty that I will not get a photon in a place where there's complete destructive interference. –  Ben Crowell Aug 20 '13 at 23:06

4 Answers 4

A common misconception with the double slit experiment is that the particles are interfering with each other. In fact, the experiment is meant to show that considering photons as pointlike particles is only an approximation that in some cases breaks down spectacularly.

Rather than think about individual particles, think about a plane wave incident on the two slits (think about the wave being made of water if it helps). The amplitude (squared) of this wave corresponds to probabilities of finding photons - any photons, not just one or another. You wouldn't be surprised to see a water wave self-interfere when passing through slits, and indeed the wavefunction here does the same.

So where do the photons - the particles themselves - come in? Well, we don't detect infinitesimal amounts of energy everywhere, but rather a finite number of discrete packets. Their appearance is governed by the probability distribution of that wave. Going back to the water analogy, imagine you had an array of detectors that would trigger if the amplitude of the water wave became large, but that the triggering was probabilistic with greater probability coming from larger amplitudes. Depending on the detectors' sensitivity and the energy in the wave, you can expect a certain average rate of detections, but you can't say for certain where the next detection will be; you can only give a probability distribution. Nothing qualitatively changes if the wave amplitude is decreased to the point where you only get one detection on average every minute. The detections are temporally separated and don't interfere with one another - it is the underlying wave, which we are sampling with the detections - that is showing an interference pattern.

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Nice answer, +1. In very simple language, the point to understand is that the same photon goes through both slits. If that seems impossible, it's because we intuitively want to impute a trajectory to the photon. –  Ben Crowell Aug 20 '13 at 23:07
The biggest reason these problems arise is because people don't realize that waves have no physical meaning and that actual observation involves the collapse into eigenstates. Maybe the analogies with physical waves have got people confused? –  dj_mummy Aug 21 '13 at 10:04

First of all only particles (eigenstates of position or momentum ideally) are actually observed and interacted with. 'Waves' in this case are probability distributions for the presence of a 'single' photon.

When one has a lot of particles (brighter laser) the probability distribution into the total number of particles is almost identical to the actual observation (look up cross-sections in QFT). Obviously for low intensity diffraction there may not be any sharp pattern, there are many photographs showing this, you should look it up.

Finally, like all elementary particles, between 2 observations (causing collapse into eigenstates) photon number may not be conserved, but the net energy and momentum will be. The first observation/interaction here is that of the source emitting the photon. The second is that of the screen firing a photon at your eyes.

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parallel worlds. They're here, millimetres away. And they're being created all the time. This vision of reality says that any time we go to work, there'll be another universe where we stay at home. It's a disturbing idea, developed in the 1950s,

it's the best and only solution to the paradox at the heart of quantum reality. The big problem with quantum mechanics is that the little particles that we're all made of can be in multiple places at once

According to this theory, when the photon of light faces two slits it doesn't split in two. It splits the world in two. Every photon in the double slit experiment creates a new parallel world.

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The many-worlds interpretetation (MWI) of quantum mechanics dosn't explain things that the Copenhagen interpretation (CI) fails to. And it's not as though the double-slit experiment wasn't understood until the 1950's. –  Ben Crowell Aug 20 '13 at 23:03

Because they exhibit complacated trajectories like in this picture:

enter image description here


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Photons don't have trajectories. They propagate as waves. The intuitive belief that photons must have trajectories is in fact the misconception that makes the double-slit with single photons seem inexplicable to people. –  Ben Crowell Aug 20 '13 at 23:04
These are Bohmian trajectories (google this), in de Brogile-Bohm mechanics. We don't know that this is the explanation, but it may be an explanation (for the record, I don't think so). I don't see why was downvoted. –  Cristi Stoica Aug 21 '13 at 8:36
I have one objection. Regardless of whether this answer is correct or not, clearly is not proven, and also is not the standard explanation. This answer should have been put, IMO, in a form like "according to de Brogile-Bohm theory, ...". –  Cristi Stoica Aug 21 '13 at 8:51
@Annix: this is weak measurement. In the pdf you attached, the key of the picture states "The reconstructed average trajectories of an ensemble of single photons in the double-slit apparatus.". Weak measurement is not the same as detecting the position and obtain the trajectories. When you try to actually measure the position, the interference is destroyed, and the Bohmian trajectories are destroyed too. –  Cristi Stoica Aug 21 '13 at 9:45
A recent paper: Bohm Trajectories as Approximations to Properly Fluctuating Quantum Trajectories, arxiv.org/abs/1308.5021 –  Cristi Stoica Aug 26 '13 at 17:08

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