This article purports to be someone carrying out an experiment described in David Deutsch’s The Fabric of the Cosmos. Since I don’t have a copy and the unversity’s copy seems to be out, I was hoping someone here had seen this.
a) The article is slightly unclear. Are the four holes supposed to be in a square shape or in one long line?
b) My more pressing question: The article states
To which I say: why not? I suppose I could try to work out the intensity of the quadruple-slit experiment in analogy to the back-of-the-envelope calculation that gives the positions and intensities of the fringes in the two-slit experiment, but on the one had I don’t know what configuration to use (see part a) and on the other I’m a lazy mathematician. Has anyone seen this before and done some calculations? I’m sure that either the author of the article misread something or Deutsch made some horrific assumption when working out that it “can’t be [a new interference pattern]”, but without the book I don’t know what that could be.
a) I read the article, and I agree it’s unclear. I think the pinholes should all be in a row. I base this on the author sometimes using “slit” instead of “pinhole” towards the end, and apparently Deutsch also used “slits” instead of “pinholes” in the quote in the sixth paragraph from the end. Using pinholes, a square arrangement might make sense, but not with slits. (The slits would be arranged like | | | |, not like _ _ _ _ )
b) “Why not?” indeed. I haven’t done the math either. If Deutsch’s experiment contradicted the QM prediction as badly as he implies, I’d think he could write it up in a real journal somewhere, be famous, and probably collect a Nobel.
This is basically my take on it, but IANAP. Does anyone actually know anything directly about Deutsch? Is he holding Oxford’s equivalent of the Josiah S. Carberry chair?
Sigh. I suppose it would have to be me that points out that Deutsch’s book is The Fabric of Reality.
The Fabric of the Cosmos is by Brian Greene.
I agree that the description appears to be of four dots in a line. Why dots? Reading between the lines, so to speak, I assume that he’s using pinholes rather than slits because he can’t make slits small enough on an ordinary piece of paper.
Without the book, the article doesn’t give me enough info even to guess at what Deutsch is trying to say. He is a believer in the multiverse.
That’s from his new website, but most of his info is still on his old site.
Gr, I saw I made that mistake and thought I changed it in preview…
It’s obvious he believes in a many-worlds interpretation or a “multiverse” of some sort, but I still don’t see an argument that the interference pattern that arises from a quadruple-slit experiment is inconsistent with quantum photodynamics as it stands, which is what the article asserts his book asserts.
I’ve not read the book and the page linked to seems confused, so I can’t comment on that, but …
He’s legit. Indeed his 1985 paper on quantum computing pretty much kicked off that entire field.
There’s a perception - amongst British physicists at least - that he’s become overly philosophical since, but also a feeling that it’s useful to have someone arguing his adopted position, if only as the convenient example.
I finally got around to trying the suggested experiment last night, and I don’t see any “missing shadows.” I used a green laser pointer, with about 2.5m from the laser to the pinholes and another 2m to the screen. I tried four pinholes in a row and got a pattern of bright light bars with about the same spacing as with two pinholes (though the pattern was more complicated; the bars appeared “split,” with various less-bright bars in between the bright ones). Four pinholes in a square produces a pretty latticework pattern, but I’m pretty sure that’s not what the author had in mind.
I also tried using razor blades to make a pattern of slits on soot-smoked glass, but either I have defective soot or I was caking it on too thick, so I couldn’t get a nice clean pattern (the soot tended to stick to the blade and make jagged slits instead of nice clean ones).
Theoretically what you expect as you add more pinholes or slits with equal spacing is for the bars of light to become narrower, with more dim bars of light between the bright ones. (For a large number of slits you get a diffraction grating, with very narrow bright bars.) The spacing between the brightest “primary” bars of light should be constant if the spacing between the pinholes is kept constant. There is an overall envelope imposed on the intensity of the whole pattern by the diffraction pattern of an individual pinhole. There are not in general exactly five bright bars of light, but the number of bars of light depends on the ratio of the pinhole separation to the pinhole diameter, and if you try to keep the holes as close together as possible you’ll usually be able to see three or five bars in the primary lobe of the one-hole diffraction pattern. (The secondary lobes will also have bars, but these are all pretty dim and harder to see.) As you make the individual holes smaller, relative to the spacing between them, the main lobe will have more and more bright bars of light.
In the four-hole case, the (narrower) bars near the edge of the diffraction lobe might be a little harder to see than in the two-hole case, so perhaps these were just missed by the author of the article. Alternately, if the author didn’t control the distance from pinholes to screen, he may have confused the dim secondary bars between the now-narrower primary bars for “missing” bars. That’s all I can think of. Has anyone read the actual book to know what Deutsch is actually saying?