There’s a nice little physicsworld article dating back to 2002. It was written by then-editor Peter Rodgers, and it started by asking “What is the most beautiful experiment in physics?” The answer was, of course, the double slit experiment, which was first performed by Thomas Young in 1801:
Double slit experiment image from the curiosity makes you smarter article by Ashley Hamer
People refer to the double slit experiment as an example of the weirdness of quantum physics. Or to promote weird ideas such as the multiverse. See for example Tim Radford’s 2010 Guardian article on the fabric of reality by David Deutsch. That’s where Deutsch referred to the interference pattern from a single photon and inferred the existence of a “prodigiously complicated, hidden world of shadow photons”. He also inferred “a huge number of parallel universes, each similar in composition to the tangible one”. This is David Deutsch the quantum computing pioneer who also believes in the many-worlds interpretation of quantum mechanics. There is a mention of that in the Wikipedia article on the double slit experiment. However it’s fairly curt, and says “some scientists are skeptical of this claim”. You don’t say.
Particles are waves
The Wikipedia article also says the double slit experiment belongs to a general class of double-path experiments, in which “a wave is split into two separate waves that later combine into a single wave. Changes in the path lengths of both waves result in a phase shift, creating an interference pattern”. I think this is good, because I know the photon has an E=hf wave nature. What’s not so good is that the Wikipedia article says the double-slit experiment is “a demonstration that light and matter can display characteristics of both classically defined waves and particles”. I say that because I’ve read Pascual Jordan’s resolution of the conundrum of the wave-particle duality of light by Anthony Duncan and Michel Janssen. Pascual Jordan solved the mystery of wave-particle duality in 1925. Particles are waves, even matter particles like the electron. Hence the Aharonov-Bohm effect, which was first predicted by Ehrenberg and Siday in their 1949 paper on the refractive index in electron optics and the principles of dynamics.
The facts of the matter
Something else I don’t like in the Wikipedia article is this: “moreover, it displays the fundamentally probabilistic nature of quantum mechanical phenomena”. Photons and electrons don’t have a fundamentally probabilistic nature. They have a fundamentally wave nature. That’s why it’s the wave nature of matter rather than the probabilistic nature of matter. That’s why we can diffract electrons, as per the Davisson-Germer experiment:
Image from Rod Nave’s hyperphysics
This is only to be expected since we make electrons (and positrons) out of light in pair production. Knowing all this, let’s look further at the Wikipedia article: “The wave nature of light causes the light waves passing through the two slits to interfere, producing bright and dark bands on the screen – a result that would not be expected if light consisted of classical particles”. No problem with that. Like I said, photons have an E=hf wave nature. What’s next? This: “However, the light is always found to be absorbed at the screen at discrete points, as individual particles (not waves), the interference pattern appearing via the varying density of these particle hits on the screen”. There’s a non-sequitur there, and it’s a big one. Why should a dot on the screen change your view about the photon’s wave nature? Didn’t you ever fry ants with a magnifying glass when you were a kid? Next comes this: “Furthermore, versions of the experiment that include detectors at the slits find that each detected photon passes through one slit (as would a classical particle), and not through both slits (as would a wave)”. So what? As above. Next: “However, such experiments demonstrate that particles do not form the interference pattern if one detects which slit they pass through”. Ditto. And finally there’s this: “These results demonstrate the principle of wave–particle duality”. No they don’t. Let me reiterate: particles are waves. We’ve known that for the thick end of a hundred years. And we have had timely reminders more recently.
The secret lives of photons revealed
Take a look at Physics World reveals its top 10 breakthroughs for 2011. It said this: “after much debate among the Physics World editorial team, this year’s honour goes to Aephraim Steinberg and colleagues from the University of Toronto in Canada for their experimental work on the fundamentals of quantum mechanics”. It also said this: “Using an emerging technique called ‘weak measurement’, the team is the first to track the average paths of single photons passing through a Young’s double-slit experiment – something that Steinberg says physicists had been ‘brainwashed’ into thinking is impossible”. I was pleased about that, because I’d read the relevant article earlier that year. It was the secret lives of photons revealed:
3D plot of a single photon showing wavelike behaviour, image from physicsworld
It said Steinberg and his team were inspired by work in 2007 by Howard Wiseman of Griffith University in Australia. It also said weak measurement was “first proposed in 1988 and developed by physicist Yakir Aharonov and his group at Tel Aviv University”. You can read more about it in the paper on observing the average trajectories of single photons in a two-Slit Interferometer, and in the article furtive approach rolls back the limits of quantum uncertainty.
Rewriting the textbooks
The latter says researchers have used weak measurement to resolve apparent paradoxes posed by quantum mechanics. That’s good. It also says they’ve used weak measurement “to probe things previously thought impossible to probe directly, such as the quantum wave, or ‘wave function’, that describes a particle”. That’s good too. What’s not so good is that the article talks about an atom spinning in two opposite directions at once, and about negative probabilities and waves going back in time. Hence I would recommend that you focus on the rewriting the textbooks section, which talks about the two slit experiment. There’s nothing wrong with this: “light shines through two parallel vertical slits in a thin plate and onto a distant screen (see figure, below). The waves emerging from the slits overlap on the screen to create bright stripes where the waves reinforce each other and dark stripes where they cancel each other”:
Double-slit image from furtive approach rolls back the limits of quantum uncertainty in ScienceMag
The article also says the “interference pattern appears even if the photons pass through the slits one by one. So each particle literally must go through both slits at once and interfere with itself”. There’s nothing wrong with that either. Or this: “The experiment, reported recently in Science (3 June, p. 1170), doesn’t violate quantum mechanics, Steinberg says; each individual photon still goes through both slits”. See Steinberg’s uToronto web page for more.
Catching sight of the elusive wavefunction
The article tells how in the same month, Jeff Lundeen and colleagues reported that they’d “used weak measurement to measure directly the wave function of photons emerging from an optical fiber”. And that this was something that generations of physicists have learned cannot be done. Jeff Lundeen’s group were runners-up in the Physics World top 10 breakthroughs for 2011. See the relevant article from earlier in the year. It’s called catching sight of the elusive wavefunction and it refers to their Nature paper on the direct measurement of the quantum wavefunction.
Artists impression of wavefunction from catching sight of the elusive wavefunction
The article says the team “used weak measurement to map out the wavefunction of an ensemble of identical photons without actually destroying any of them. Quantum tomography, in contrast, maps out the wavefunction at the expense of destroying the state”. It’s good stuff. So is the uOttawa Lundeen lab website. See the past research page, and then take a look at the semi-technical explanation. It says Niels Bohr’s view was that the wavefunction was merely a mathematical tool, and that this the Copenhagen Interpretation ended up being accepted by most physicists. It also says this: “with weak measurements, it’s possible to learn something about the wavefunction without completely destroying it”. And this: “We hope that the scientific community can now improve upon the Copenhagen Interpretation, and redefine the wavefunction so that it is no longer just a mathematical tool, but rather something that can be directly measured in the laboratory”. What they’re saying is wavefunction is real.
Coconuts and corks
See the non-technical explanation for more: “The wavefunction embodies the idea that every particle is also a wave. This wave is much like the set of ripples travelling out from a pebble dropped in a pool. And the shape of this set of ripples is what is analogous to the wavefunction. A feature of quantum mechanics is that, unlike a water wave, the very act of observing the wavefunction changes it, making it a slippery object to measure”. It goes on to say ordinary measurement is “like placing a coconut at a particular position in the pool to see if the ripples would cause it to bob. As well as bobbing, the coconut would have the unwanted side effect of reflecting the incoming ripples and sending them in every which direction”. It then talks about weak measurement thus: “The obvious solution to this problem, to use a lighter float such as a wine cork”. And that “one does not get much information about the particle’s position”. Hence “The trick is to repeat the gentle position measurement followed by the normal velocity measurement over and over again on many identical wavefunctions until one has enough information to say what the average result of the position measurement is. This average is equal to the wavefunction itself”. You plot out the wavefunction with weak measurement.
Wavefunction is real
So, the takeaway from all this is that wavefunction is real. The current Wikipedia wavefunction article will tell you that wavefunction “is a mathematical description of the quantum state of an isolated quantum system”. It isn’t. It’s a real thing. It isn’t some “complex-valued probability amplitude”. It’s the thing that goes through the double slit. Think of it as something like the pilot wave, but with an important difference: it isn’t piloting anything. The photon isn’t some billiard-ball thing that makes a wave as it bullets through space. It’s a wave in space, and an extended entity like a seismic wave is an extended entity. That’s why like Steinberg said, each individual photon goes through both slits. The mistake is to think a photon is some billiard-ball thing, and then think it’s a miracle that it can go through both slits at once. It isn’t a miracle. There is no magic. The same applies for the electron. It isn’t some billiard-ball thing that has a field, it is field, with no outer edge. It’s an extended entity too. Because it’s a self-trapped 511keV photon in a spin ½ configuration that looks like a standing wave. Hence similar rules apply. That’s why you can refract it and diffract it.
We choose to examine a phenomenon which is impossible, absolutely impossible
So, what happens at the screen? This: “the light is always found to be absorbed at the screen at discrete points”. The same applies for an electron. Look again at the 2002 physicsworld article on the double slit experiment. It says the most beautiful experiment in physics is Young’s double-slit experiment applied to the interference of single electrons going through the slit one at a time. It also says the first double-slit experiment with single electrons was performed by Pier Giorgio Merli, GianFranco Missiroli, and Giulio Pozzi in Bologna in 1974. Their paper was on the statistical aspect of electron interference phenomena. This was some 15 years before the better-known work by Akira Tonomura and colleagues at Hitachi. Their paper was the demonstration of single‐electron buildup of an interference pattern:
CCASA image uploaded by Belsazar with permission of Dr. Tonomura, rearranged by me
The physicsworld article says most discussions of double-slit experiments with particles refer to what Feynman said: “We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery”. But there is no mystery. Instead there’s kids with magnifying glasses, and there’s a trick of the light.
That’s the trick
We know light has a wave nature, and we know matter has a wave nature too. Photons have a wave nature, and we make electrons out of photons in gamma-gamma pair production. So we can be confident that both photons and electrons go through both slits. And yet when we detect them on the screen, we see dots. Why?
Image from The Fourier Transform at Work: Young’s Experiment by P J Bevel
Have you ever read anything about the optical Fourier transform? Steven Lehar wrote an article about it called an intuitive explanation of Fourier theory. He said a simple lens can perform a Fourier transform in real time. He also said this: “place an image, for example a slide transparency, at the focal length of the lens, and illuminate that slide with coherent light, like a collimated laser beam. At the other focus of the lens place a frosted glass screen. That’s it!” The input image is converted into something pointlike:
Image from Steven Lehar’s intuitive explanation of Fourier theory
When you detect an electron on the screen, what you see is a dot. Even though it went through both slits and interfered with itself. Why do you see a dot? Read the Hitachi paper by Tonomura et al. On journal page 119 you can read that the distance from the source to the screen is 1.5m. You can also read that the length of the electron wave packet is as short as ~1μm, so “there is very little chance for two electrons to be present simultaneously between the source and the detector”. Note the reference to the electron wavepacket. When an electron interacts with the detector, a fluorescent screen emits circa 500 photons. These excite a photo cathode which emits photo-electrons. These are accelerated to 3kV through the electrostatic lens, and then “the point image of electrons is formed at the upper surface of the multichannel plate”. That dot on the screen is not an image of the electron, any more than the eye of the storm is an image of the hurricane. What’s really happening is that an extended entity called an electron is interacting with other extended entities in the fluorescent screen. Just as an extended- entity input image interacts with an extended entity called a lens. What you see is a dot. Now, you might talk of wavefunction interacting with wavefunction and wavefunction collapse and wavefunction squared, written as |ψ|². But I think it’s better to keep it simple and say detection involves something akin to an optical Fourier transform. That’s why you see a dot on the screen. I think it really is that simple. I think that’s the trick.
No many-worlds multiverse is required
I went to a wedding a while back. At the reception, my wife and I were sitting at one of the big round tables drinking and chatting and laughing with other guests. Then a magician appeared. He performed a variety of party tricks with cards and coins and other things. He was good. However for one trick, he bent a spoon. I just happened to see the way he held it in both hands as he waved it up then down and quickly snapped his hands to do the deed. He did it fast, almost too fast for the human eye, but not too fast for my human eye. The other eleven people on my table were wowed and amazed, but I sat there with my mouth open and my brow raised. After the magician moved on to the next table, I spoke to the guy sitting next to me: “Didn’t you see that? He bent that spoon right in front of your nose”. He hadn’t seen it, and nor had anybody else. I was amazed that I was the only one who spotted the trick. It feels similar for the double slit experiment. When you detect the electron at the screen, you perform something akin to an optical Fourier transform on it, so you convert it into something pointlike. Then when you detect the electron at one of the slits, you perform something akin to an optical Fourier transform on it, so you convert it into something pointlike. So it goes through that slit only. So the interference patterns disappears. That’s the trick There is no magic. It’s that simple. It isn’t some phenomenon which is impossible, absolutely impossible. It is mundane, and no many-worlds multiverse is required.
Note 3rd Aug 2019: Art Hobson gave a similar explanation of the double slit experiment in his 2013 paper There are no particles, there are only fields. I’m surprised I haven’t heard about it before.