Of Light and Double-Slit Experiments

I’ve been reading a lot about of quantum theory to the point where things get so weird, there is no reference point to call “reality”! But, I digress. I took a step back to review what I know from the ground up, including the classic, double slit experiment.

In the early days, light was assumed to act like a particle until the double-slit experiment demonstrated that light, passing through a pair of slits in close proximity, (i.e.,: the double slit) will manifest an interference pattern - classic wave behavior. Later, as I understand it, the experiment was repeated with a beam of electrons, the same interference pattern appeared [allegedly] demonstrating that all particles can behave as a wave, too. Last, I read in the same article that the experiment was later repeated again, but this time with a stream of photons*. This time, the interference pattern supposedly proved the photons were [per the experts] interfering with themselves; thus, a photon could be in two places at once - one of many weird behaviors of quantum physics.

My questions are:
a) Based on my basic understanding of quantum theory, it is random whether a particle or wave will manifest. Yet, in all these cases, a wave’s interference pattern appeared every time. And, I would assume the results would be the same ad infinitum. …Yet, shouldn’t the result be totally random? (Maybe we expect a wave, so we see a wave pattern! :eek:)

b) In a stream of photons flowing through the double-split, how can it be said say for sure that one photon is interfering with itself (this, being two places at once)? Is it because the stream is precision-aimed to pass through only one slit? Are they sure there is one and only one photon passing through the double-slits?

Bonus! c) Related to this, Einstein discovered the photovoltaic effect first by increasing the brightness of a light, and then increasing the light’s frequency. The goal was to give the photons more energy (to free electrons in a conductor and create a current), but only some higher frequency would work. Yet, doesn’t a brighter light emit a stream of photons of higher energy, too**?

*Isn’t a light beam and a stream of photons the same thing?

**I realize a light can be made brighter by increasing the intensity by moving the light closer; thus, the light would produce a richer field of photons. However, a higher wattage bulb also produces a brighter light…so wouldn’t the higher wattage bulb produce a stream of higher energy photons, too? …eventually becoming high enough to match those photons of the higher frequency (if one keeps increasing the wattage)?

a) Whether you detect a wave or particle is determined by the type of experiment you run. It is not random. If you look for waves, you will see waves. If you look for particles, you will see particles.

b) We say a photon interferes with itself because the interference pattern appears even when you only send one photon at a time through the slits. Send one photon. Wait ten minutes. Send another photon. Repeat. You will still see an interference pattern in the points where photons were detected.

c) A brighter light of the same wavelength just has more photons. Each photon has the same energy as in the dimmer light.

Look up the pilot wave interpretation of quantum mechanics. Preferably the veritasium video on YouTube. This is an excellent and intuitive explanation of how something can be both a wave and a particle at the same time.
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It makes a lot of sense when you think about platonic forms, or the five elements. They don’t actually exist, right? But to suggest that back then would confuse the tar out of people back then. It was a core concept without which they couldn’t fathom how the world could possibly operate, so of course they had to exist.

Particles are the same thing. They don’t exist. Everything is a wave, or more precisely a fluctuation in a set of fields, that can sometimes more easily be conceptualized as a particle. But the particle is just a concept, like a platonic form, that isn’t actually an accurate description of reality.

1: They can measure after each photon. A single photon doesn’t actually tell you anything. It just lands somewhere on the detector. It is after you fire many protons that the pattern of distribution becomes clear, and displays a classic interference pattern.

2: A “particle” can only pass through one slit without being in two places at once (which would be an odd thing for a “particle” to do). It is precision aimed, but such that the “particle” must pass through either one or the other, with an even probability of doing so.

3: Again, see 1.

No. Brightness is a measure of amplitude (as “particles” that’d be how many photons). Color/Wavelength determines how energetic each photon is. Think of a gamma ray as having very short wavelengths - a lot of energy all in a small space - while radio waves have very, very long wavelengths - very little energy per unit space.

NOTE: I’m not a fan of “quantum mechanics.” I personally lend a lot of weight to Quantum Field Theory and its variations. It gets rid of a lot of the confusing stuff the Copanhagen Interpretation foisted upon the scientific community. I’m also not a particular fan of string theory. Fans of those other theories will have radically different interpretations of fact (that, ultimately result in an equivalent phenomenon).

The experiment I’m familiar with is a stream of ELECTRONS. Since there is an exactly calculable (Schrodinger’s Wave Equation) chance that an electron will appear at any point on its wave at any given moment, they register as an interference pattern instead of the two solid impact points you would expect from ordinary solid objects. The ability of an electron to “appear” and “disappear” at any point on it’s wave front gives rise to what is known as the Quantum Tunneling Effect. Amazingly, the lowly Light Emitting Diode is a purely Quantum Mechanical device because it operates in just that way.

You can reduce the intensity of the light until it is so low that you can be reasonably certain that only one photon at a time is passing through your two-slit “mask”. That being the case, if you see an interference pattern – as you will – then the photon is interfering with itself.

As I’;ve frequently stated on this Board, you don’t need TWO slits to get caught up in the quantum “particle vs. wave” weirdness. All you need is one slit. There is a characteristic pattern to the light that passes through a slit. It’s not just bright at the center, gradually tailing off. If you have only one wavelength* of light, you will see the pattern drop to zero, leaving a dark patch one either side, after which it will rise again, although to a much lower value than the center peak, before plunging to zero, giving you a dark patch again, then do the same over and over. For a squared-off aperture, this is called a sinc function (actually, a sinc function squared, since you see the square of the amplitude). The distance betweenm the closest dark patch on each side is inversely proportional to the width of the slit.

So do your single slit experiment at very low intensity. One photon at a time goes through, but the resulting pattern of light is determined by the overall slit width. But wait – isn’t a photon really tiny? How does it “know” how wide the slit is? And how can there be such an interference pattern at all unless the photon is effectively interfering with itself?

This all seems counterintuitive if a photon is a tiny ball of a particle with no knowledge of the world outside itself. So clearly your mental picture of the photon can’t be correct. If the light is a wave, then you can easily see how it interferes with itself and “senses” the edges. If you want to mentally picture the photon – always dangerousd, because it can be mi8sleading – imagine it more like a dust bunny, with a concentrated center, but hich tails off over very large distances, so it can “sense” slits much larger than you’d think.


Sorry about that – got interrupted.

It depends upon which kind of light you’re using. If I had a tunable laser, for instance, it would emit only light of about one wavewlength (there’s alwqays a bit of spread in frequencies). I could use it to shoot light of one frequency, or of another. If I simply cranked up the intensity, the light would remain at that frequency.

If I had a blackbody source (an incandescent light is pretty close to a blackbody in its output), I will have a broad range of frequencies in a characteristic shape. If I were to increase the output by making ity hotter, I would change both the overall photon outp[ut and change the shape of the spectrum.
I don’t need to use a laser to get single-frequency light. I could use a bandpass filter in front of my light bulb, or pass it through a monochromator (which uses a prism or a diffraction grating to split my light into its constuituent frequencies) and select only one wavelength to fal on my slits.

‘Quantum mechanics’ can mean two things here. First, in the ‘narrow’ sense, it’s a theory of objects represented by wave functions obeying the Schrödinger equation. In this sense, it actually is a quantum field theory, as well: albeit one in a 0+1 dimensional, non-relativistic spacetime (the fields depend on one parameter, time, and are just the particle positions).

On the other reading, quantum mechanics is a framework; quantum field theory is a quantum mechanical theory of fields (typically, relativistic fields, but there are also non-relativistic quantum field theories that are used in condensed matter, for example).

On neither reading can you really draw a line between quantum mechanics and quantum field theory and profess trust in one, but not the other—they’re really both kind of the same thing. Also, the interpretational riddles of QM don’t really get any easier in QFT; indeed, if anything, they get more difficult, because now you also have to think about things like the meaning of indistinguishable particles and so on.

One doesn’t encounter the measurement problem quite as often in the context of QFT, however; but the reason for that is merely that the actual quantum state is rarely the object of interest. Most of the time, QFT instead is concerned with the operators acting on the states (indeed, the quantum fields themselves are such operators, creating and annihilating particles).

Not really. Certainly there are well defined quantum mechanics as described in numerous specific theories. And then there are “quantum mechanics” (note the quotes), that nebulous hodgepodge of theories, speculation, misconception, and flawed inference usually hinging on Copanhagen and a very dismal understanding of anything related to quantum theory. Outside of a very small number of people “quantum mechanics” is about all you get. I don’t claim to be particularly well educated in the field (and I’m suspicious of anyone who does), but I have an elementary understanding sufficient to form a very negative opinion on the psuedoscience that gets thrown about as actual scientific theory.

I don’t want to speak out of my depth, but at least one scientist a lot better versed than I disagrees: https://arxiv.org/pdf/1311.0205. It’d take a lot longer than I’m willing to spend to gather up a decent collection of arguments, but it is my understanding that a lot of the "problems’ stem from conceptual hold-overs from Copanhagen that are obviated by QFT.

Quantum mechanics is an entirely well-defined theory with a rigorous mathematical formalism that produces predictions in spectacular agreement with experiment. Copenhagen is one attempt to come to grips with the (again, very clear and well-defined) mathematical formalism of quantum mechanics; there are others, like many worlds theory, Bohmian mechanics, consistent histories (considered by many to be a more rigorous refinement of Copenhagen), and so on.

The theory of quantum mechanics is itself entirely independent of these interpretations, and you can just as well ‘shut up and calculate’, if you so choose. You need never waste a moments thought on Copenhagen if you want to do quantum mechanics.

Note that this isn’t a published paper, but a preprint that’s apparently never appeared in a peer-reviewed journal, by a single author who doesn’t seem to have produced any other work in the area. I would not put too high a credence on it.

And again, if the measurement problem were solved in quantum field theory, it would likewise be solved in quantum mechanics.

I just finished a well-written book called “What is real?” by Adam Becker that explains a lot of this essentially in layman’s terms. Even including Bell’s theorem and Aspect’s experimental verification of it.

I can’t answer your points better than they’ve already been, but I want to correct a mistake in the OP. Einstein did not discover the photoelectric effect. I think it was Hertz, though I’m not sure. What Einstein did was explain it, using Planck’s brand-new concept of energy quanta. As far as I know, Einstein never did any experimentation whatsoever.

I think he did actually do a little bit of experimentation, but on the subject of refrigeration, of all things.


Apparently his only experiment involved a capacitor, and flux…

I second the recommendation for the book. It’s best long explanation of the various “interpretations” of QM that I’ve read.

I appreciate the replies. It will take me quite awhile to digest this. I’m in for more than I bargained for!

The book is well written, and does an OK job on many topics, but I would be a little cautious regarding some of its claims.

First of all, it plays the ‘lone rogue takes on the scientific establishment’-card a bit too often—but I guess you need a bit of drama to sell the story. But it’s also relaying the story from a bit of a biased point of view, and while it devotes lots of space to discussing the shortcomings of the ‘Copenhagen’ interpretation (most of which really boils down to the fact that the ‘Copenhagen’-label was adapted by many physicists who simply didn’t pay the question of interpretation much mind), it fails to properly discuss the shortcomings of other interpretations, like the many-worlds and Bohmian pictures. The resulting image of physics at large just sort of following the Copenhagen herd while ignoring ‘better’ alternatives is at least a little skewed (albeit not totally false). In reality, interpretation of quantum mechanics is a controversial field because it’s really hard, and lots of people who were excellent scientists otherwise but hadn’t really gotten into the thick of that particular issue have failed to appreciate its subtleties.

It also gets things dead wrong at times. Most egregiously, I think, are the EPR experiment and (not unrelatedly) the question of locality in quantum mechanics. Now, in particular the latter has been subject to recent controversies, I’ll grant, with one side claiming that Bell actually proved that quantum mechanics is nonlocal (he didn’t, even if he himself occasionally seems to say so), but the book doesn’t even really present the controversy, but flat-out states that QM’s nonlocality is indeed what Bell proved.

So, yeah—it’s a great read, but tread carefully.

Noted. Is there a better recent popular book?

I have heard good things about Philipp Ball’s Beyond Weird, but I haven’t yet had a look at it. If and when I do get around to reading that one, I’ll try and write up my thoughts about it (under the presumptuous assumption that anyone cares).

Bell took a set of assumptions which each seem reasonable and intuitive, and showed that that set of assumptions leads to conclusions which are at odds with our observations of reality. From this we can conclude that at least one of those assumptions, despite seeming reasonable and intuitive, must actually be wrong. We can’t, however, conclude which one of them is wrong. We can make judgements about which of them seem to be “more intuitive” or “more reasonable” than the others, and say that the least-intuitive or least-reasonable one is the wrong one, but that’s entirely subjective, and if two different physicists (or philosophers) come to different conclusions on that question, we have no way of settling who’s right.