The Home Parallel Universe Test

Sam Sachdev writes "David Deutsch, a physcicist at Oxford, has designed a home test for parallel universes. Using a pin, a red laser pointer, a piece of paper, and a relatively dark room, he claims that the results from this experiment confirm the existence of parallel universes." Okay, so it may not really be proof of parallel universes, but it's a fun trick to try with a laser pointer nonetheless.

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A Home Test for Parallel Universes
by Sam Sachdev
March, 2004

When you think of a parallel universe, do you think of a universe, or a world, similar to ours but different in some fundamental quality. Bill Clinton, for instance, is a happily celibate priest. Or George W. Bush delights his fellow Mensa members, at parties, with his verbal games. Or, perhaps, you only have a science-fiction quality vagueness to what you think of a parallel universe: pointed ears, warp-drive through worm holes, and form fitting Lycra body suits on a thin, well-groomed crew. A parallel universe, it may surprise you to learn, is actually detectable in your own home, office, or almost anywhere indoors. All that's required is a red laser pointer, a pin, and a piece of paper.

With the aid of David Deutsch, a physicist at Oxford University and his excellent book "The Fabric of Reality", the experiment, in a step-by-step process, is going to be set-up and, then, it's going to be explained why this magic-like result from this experiment is indeed proof of a parallel universe.

First, a red laser pointer is needed. I found one at Radio Shack for $19, not including the triple A batteries that were needed. The red color of the laser pointer is important. The red light, unlike the white light of a flashlight, which is a composite of many colors, doesn't fray as white light does. The red light, specifically, of the laser pointer casts more specific shadows - which is what this experiment does. A flashlight, according to Deutsch, can probably be substituted. A filter, however, is going to have to be placed over the white beam. The filter, can only be red colored glass; paper or any other filter won't work.

Next, a relatively large, dark room is needed. The room should be large enough to set up the laser pointer on, say, a table, and have it cast its light on a wall about one and a half meters, or about five feet away for my metrically challenged Americans. At first, this humble journalist tried to do the experiment, during the bright light of a Washington, DC winter day, in a walk-in closet and a bathroom. Both weren't large enough. My dining room, when the sun had set, was.

David Deutsch recommends a room that's almost totally dark. I found, however, that this was too dark. The experiment requires enough light to manipulate the laser pointer. What I did was have a light on in another room, which provided enough light to see what I was doing but dark enough to see the shadow cast by the laser pointer.

The experiment is best done with done with two people, with one handling the laser pointer and the other observing the pattern on the wall. The positions can then be switched. Be careful, however, not to shine the laser light into the other's eyes.

If you don't have two people, this is what I recommend. Fold a piece of paper in half and place it on the table, so that one half is perpendicular to the table. Then, using a book, or anything to set the laser pointer on, aim the pointer at the paper. Mark where the red light hits the paper. Using a pin (and only a pin, not a tack, the holes have to be as small as possible) punch two holes, on the mark, as close to each other as you can. Then, aiming the laser pointer at the two small holes, a shadow of five slits should be cast on the wall. That is, there's going to be one large red dot cast on the wall. In the dot, there should be five distinct shadows cast by the two holes. If this doesn't work, the most common problem I found was that there wasn't enough distance between the paper and the wall. If possible, increase the distance. David Deutsch recommends about five meters, or fifteen feet, but I found about five feet, or a meter and a half, was enough to observe the pattern.

Why, you may be wondering, are there five slits of shadows when there are only two holes? That's because light, as you

Idiots

[Lord Kano]

Hawking describes this type of thing in A Brief History of Time. This is NOT proof of a paralell universe, it's proof that light travels as a wave as well as a particle.

LK

Crap article, just plain optic diffraction!

[Kjellander]

This kind of pseudoscientific articles are one of the worst things on the internet!

This is a classic optics experiment to show that light has wave properties, and it has NOTHING to do with parallell universes. It is all explained here:

diffraction [wikipedia.org]

And if you want to show any quantum mechanical effects you need to make sure that only one photon leaves the laser at any given moment, and that is not happening here.

Re:Isn't this just the double-slit experiment?

[MacroRex]

Yeah, here [uoregon.edu] is a nice summary with pictures.

I'm not a quantum physicist, but I think I have a idea what this is about; the light waves just interfere differently with four slits. Since this Deutsch guy draws wildly different conclusions about the result, I guess he's either much stupider or much smarter than me. And since he's the university physicist and not me, I feel bad for him if it's the former.

Re:Since I can't see air it must be another univer

[CyberDruid]

You have understood nothing. The phenomenon is real and one of the strangest and most spooky things in physics. It shows that it it possible to get a particle (in this case a photon) to interfere with itself.

The only question is how you interpret it. The first interpretation, created by Einstein, Bohr and other dignitaries of the time, was the "Copenhagen Interpretation" which requires an "observer".

The "Many-worlds interpretation", first thought of in the late fifties gets rid of the need for a mystical observer by introducing parallell universes, where entangled particles can still interfere with each other.

This interpretation is championed by many of the leading physicists. For example Deutsch and Murray Gell-Mann.

I believe Feynman has a strange third interpretation involving particles travelling backwards in time, that cancel out the waves of forward travelling particles at specific points in space-time.

Re:Isn't this just the double-slit experiment?

[DrLudicrous]

And by old, bravehamster means OLD. Like over 200 years old. See this link [wolfram.com] for more details on Young's double slit experiment.

Basically, light behaves as a wave, and since waves can constructively and destructively interfere with one another (cast two stones simultaneously in a pond and oberve the resulting interference pattern) light will form a funny looking pattern that one would not intuitively expect on a screen some distance from the slit.

Re:Fabric of Reality??

[Anonymous Coward]

I hate to break it to you, but that's not a valid interpretation; a single photon can and does interfere with itself. Where you think of a particle, QM sees that a "photon" is a localized wave packet, represented by a probability wave that has useful values in a small volume (because it still looks like a particle) but exists everywhere. This probability wave can and does go through the different holes, and what you get out is effectively an interference of the photon with itself. This is the basic idea behind Feynman's "sum of all histories" (properly, the path integral formulation of QM) approach, that looks at all possible paths - in this case, all the holes.

Re:What's the photon/proton thing about

[Anonymous Coward]

No, if you're careful, you see that he starts talking about protons. Protons - and neutrons and electrons and all particles - have wave properties just like photons. Given the proper setup, you can conduct this same experiment with protons, and that's what he's talking about.

Re:Fabric of Reality??

[Ckwop]

It depends on your Interpretation of the underlying quantum mechanics.

The normal double slit experiment doesn't actually tell you very much. It's when you do the double slit experiment with *single* photons that the truth becomes spooky.

The reason being that even with single photons you get the same pattern on the wall. The question is did the photon interfere with itself or was there a 'ghost' photon that went through the second hole that interfered with our photon but this ghost exists in a Parallel universe?

Well, if you read the Feynman lectures in physics he does a good thought experiment to clear this up a bit. Imagine we have a second single photon beam. The idea is that we measure the photon going through the slits to see which slit it actually goes through. At first the frequency is too high and it destroys the interference pattern.

As we turn the frequency down the pattern begins to reappear but at the precise moment that the pattern does reappear we are unable to view which slit the photon went through. The frequency of the light is too low to clearly resolve the slits and hense which slit the particle went through - they've blurred into one slit.

So the question of which intepretation is correct is more a point of philosophy. We can't decide which one is correct because quantum mechanics wont let us take a measurement.

Simon.

Re:Crap article, just plain optic diffraction!

[BZ]

The point (briefly and ineptly mentioned in the article) is that if you _do_ have only one photon leaving at a time (such experiments have in fact been performed) you get the same diffraction pattern.

So a single photon somehow passes through all four slits at once and interferes with itself.

Unless you try to detect which slit it's going through -- then the pattern disappears.

Now all this can be explained in terms of wave functions, state superposition, and wave function collapse when a measurement occurs. But the point is, that "wave function collapse when a measurement occurs" and "parallel universe with shadow photons" are about equally bizarre phenomena. And assuming they give the same predictions for results of experiments, neither is more "correct" than the other.

Of course this article doesn't cover the question of whether the two theories give the same predictions... which is where the pseudo-scientific part comes in.

Re:Fabric of Reality??

[DrLudicrous]

This is because what is reaching the last filter in the second case does not have the same polarization that it had in the first case.

After passing through the first filter the light has been polarized in some direction- all of the perpendicular components have been removed by the filter.

In the first case, there is only one other filter, oriented at 90 to the first one. This will only allow the components of light polarized 90 to the first screen's orientation to pass. But all of those components were removed, so nothing gets thru. In the second case, your filter is not oriented at 90, but at 45 to the first filter. Hence it will only components of light at 45 to the first filter's orientation to pass. But using vector analysis, we can break that orientation up into two vector components (that match up with the orientations of the first and third screens), and see that some light will get thru. How much? Well, cosine of 45 is 1/sqrt[2], and the intensity of the transmitted light goes as the square of that, so 1/2 of the light coming from the first filter gets through the 2nd filter.

Also, 1/2 of the original light went thru the first filter- this assumes a random distribution of polarizations of incoming light, i.e. unpolarized light.

Since the third filter is oriented at 45 to the second, we get another factor of 1/2. Totaling up all 3, we get 1/8 of the original intensity. I hope this makes sense. It probably won't unless you are comfortable with vectors.

QM and single photons

[steveha]

This demonstrates that light can act like a wave, and have a diffraction pattern.

The "parallel universe" part comes in to explain why it still works if you fire single photons, but since you can't fire single photons (or easily check the results if you could), this isn't really a "home test" of any use.

The fact that single photons can make a diffraction pattern, seemingly interfering with themselves, is a truly weird feature of quantum mechanics (but then, I repeat myself -- quantum mechanics is always truly weird!). And one of the explanations proposed is that light in parallel universes is somehow causing the interference with the single photons in this universe.

Another explanation is that light sometimes acts as a particle, and sometimes as a wave, and when you detect a single photon coming through a slit, you are forcing that photon to act like a particle, and it will not throw a diffraction pattern; but if there is no measurement to decide which slit the photon passed through, the light can act as a wave instead of a particle, and can have an interference pattern.

http://www.starlight-pub.com/UnitNatureofMatter/Pa rtIII/III2QuantumEnergy.html [starlight-pub.com]

This page lists various explanations of why the single-photon two-slit experiment behaves as it does. One of the explanations is the parallel-universes one.

http://members.aol.com/jmtsgibbs/TwoSlit.htm [aol.com]

Here's just the part with the "Many-Worlds Interpretation":

There are two sets of universes, each containing a version of our photon, one set in which the photon passes through the left slit and one set in which it passes through the right. (Actually there are an even greater number of universes in which the experiment is never carried out in the first place, but we are ignoring those.) The photons are particles that carry a property called "quantum phase" which oscillates as they travel. Two universes which are identical except for the photon arriving at a certain point on the film with opposite phases, cancel each other out. Neither one is "real". Maybe it is more correct to say that the multiverse cannot contain two such contradictory universes in the first place, rather than to imagine them existing, and then meeting and going "poof".

steveha

Re:Fabric of Reality??

[tehdaemon]

In english, The photon is not a point, only when it hits something does it act like a point, as it only hits one point. A photon is a weird fuzzy thing that is mostly here, and partly here and over there, but a little bit everywhere else. It interferes with itself because it squeezed through both holes, and because it squished itself through both holes, it's shape (places where it mostly and partly was) changed, and so there are some places that it is more likely to hit, and some places that it can't hit.

Re:Wavicles are fun

[Anonymous Coward]

In my opinion that theory is not needed here as we already have an excellent model (the quantum physics)

The many worlds idea is not a new theory, it is a way to explain the laws of quantum mechanics. Some people have problems accepting the idea that things are not what they are until they are measured, which is the idea of the Copenhagen interpretation.

So it's not a matter of wrong predictions, it's people being uncomfortable with the fundamental aspects of the theory.

No other theory other than quantum theory matches as exactly with the experimental results (up to 10 to the power of -9)!

I think might want to add a couple of zero's. I think the most accurate spectroscopic today is on the 1s-2s transition of hydrogen. These currently have an accuracy of a couple of Hz (look for T.W. Hansch on google). Given the fact that the 1s-2s transition sits at a frequency of 2466 THz, you can do the math on the accuracy. All this experimental work has not yielded any errors in quantum mechanics.

opinion

[Anonymous Coward]

I think that this physicist must be wrong. It is obvious that that this experiment only shows photons acting as both particle and waves. See my high school physics book proves me right. They should give me a Phd right now just for reading it. In 10 years when this theory is proved to be at least probably wrong, I will say I told everyone so.

Re:Fabric of Reality??

[Ckwop]

In my understading of the multiverse theory, there are infinitely many parallel universes. This article seems to be talking about probability, which in the face of an infinite number of chances, is moot. It's guaranteed to happen. Obviously, this isn't the case. Someone want to clear this up for me?

Let's go back to our trusty two slit experiement. We have a pattern on our wall. That pattern is a distribution of photons. The question you have to ask is: "Can we predict where a photon is going to land on our pattern?"

Well, where there is darkness, we know the probability of a photon landing there is very small. In the bright areas its a rather good probability but other than that we can't say much. We can't before the time tell which bright fringe a photon will land at.

The many worlds theory explains this by saying that there is a different universe in which the photon lands in each of the bright strips. We see it land in whichever strip because we happen to be in one of those universes

Simon

Re:There is nothing new here

[pyrrhonist]

Even the idea that it is proof of parallel universes is not original. Michael Crichton made that claim in his book Timeline.

Is that the only book you've read? I ask only because there's been a great many books before Timeline that have made that claim. In fact, many of them were Deutsch's own books.

Re:Isn't this just the double-slit experiment?

[jfern]

The difference is that there are very simple tests for the helio-centric model, and there aren't any obvious experimental tests for the many world hypothesis (and seems like there are likely none), but that appearantly hasn't stopped people from trying to find a test [arxiv.org].

Many Worlds Theory Invalidated

[arevos]

Firstly, as others have pointed out, this is essencially the double slit experiment. In this case, because he's just using laser light, this simply demonstrates the interference of waves.

More interesting results come from when you pass through single electrons or photons one at a time, and they show the same behaviour, but this experiment does not demonstrate this. Nor is the only explanation for this to assume parallel universes. The so-called "Many Worlds" theory.

In fact, according to this [kathryncramer.com], the Many Worlds theory has been invalidated by a recent experiment.

So not only does this laser-pen experiment not prove the existence of parallel universes, but the Many Worlds explanation of the phenomenon has been potentially been already disproven.

Re:Since I can't see air it must be another univer

[rokzy]

Feyman's idea is to treat antiparticles as particles travelling back in time. it's mostly just a trick to make calculations easier (this of course means it must be a valid interpretation).

Sensational rubbish!

[bcmm]

Ok, this proves quantum theory. Saying that parralel universes therefore follow is either gross oversimplification or just forcing your metaphysical opinions of a physical theory on others. And yes, Young's double slits, old news. Also, this doesn't show that light is a wave. This demonstrates that a photon can be placed in superposition. This experiment has also been done with electron beams, whole atoms, and (IIRC) C60 (buckminsterfullerene), and they make interference paterns. Now atoms are definitely particles.

Re:Incompetent drivel

[haxor.dk]

"First off, the author can't keep straight the difference between a photon (a boson) and a proton (fermion)."

Sorry to be bitching, but a proton is a Baryon, not Fermion.

Re:Isn't this just the double-slit experiment?

[forgotmypassword]

Just to give perspective, I am a physicist who thinks that the Many Worlds interpretation (along with other things like the anthropic principle) is not only incorrect but is bad science.

It fills in a small interpretational gap but creates much larger (unanswered) questions and confusion.

If you think universe splitting occurs whenever a measurement is made, then I believe that you have a very poor understanding of what measurement is. First of all collapse is not some special/magical process and secondly you can't arbitrarily seperate the universe into observed and observer. If you are going to have splitting, it's got to happen always/everywhere and with every state basis. I would say that a preferred splitting is far more egocentric than only wanting to have 1 universe.

And assuming there is no unsplitting/suicide (and maybe even if there is) then there will universes with no measurable physics, or even worse - measurements that give a false physics. And there is no reason for us to not be in one of those universes, other than probability. Of course this could (improbable) happen here, but the point is that according to MWI it does happen somewhere. Infact, Everett proved that the crazy universes will have zero norm on the Hilbert space only if infinite measurements are taken.

But enough of my rambling, just go here for some information against MWI [univie.ac.at]

The principle of Entanglement

[alphakappa]

is neatly explained in this stanford page [stanford.edu].

Dunno if anyone mentioned it, but Michael Crichton's Timeline was based on time travel using the concept of parallel universes. Crichton neatly details an experiment to show the principle of entanglement. (sad that the movie did not deal with the science at all) Read the book for some nice fun with this concept.

Re:Many Worlds Theory Invalidated

[Prune]

Another point: regarding your link to the blog, I mention the following comment from Physics Forums [physicsforums.com]: The blog did not indicate two things: (1) that there's no references to the Afshar experiment and (2) that it is having problems in the refereeing stage. In fact, unconfirmed reports have indicated that the Afshar experiment report that was uploaded to the e-print archive was removed, something that is unheard of for arXiv. Until peer review of this supposed invalidation, lack of skepticism is silly.

Re:untold universes and wave function collapse?

[Anonymous Coward]

Why can't some of the things take a different path down the slope and interfere with me later.

So your hypothesis is that lots of invisible things we can't detect in any other way come together to produce just this effect? An effect that can perfectly adequately be explained using wave theory, which has 400 years of theory and experiment behind it? Your theory, with no mathematical basis and no predictive power, up against a precise mathematical formulation with very accurate predictive power?

I think I know which one I prefer.

Secondly, if a single photon creates the effect, what to say that the effect isn't the accumulation of a lot of photons creating there own little effect.

Because a SINGLE PHOTON displays the effect. The reason that SINGLE PHOTON displays the effect cannot be due to an "accumulation of a lot of photons" because there AREN'T a lot of photons in the case where the SINGLE PHOTON displays the effect.

NOT about double-slit experiment! RTFA!

[Prune]

Contary to numerous posts by people too lazy to read the whole article, this is not about the double-slit experiment. Halfway through the article we have the following:

Next to the two holes you've punched, make two more.

Deutsch interprets the results of this four slit experiment as evidence for parallel universes. A critique of this specific argument can be found for example here [henrysturman.com].

Re:Many Worlds Theory Invalidated

[Prune]

Oh, it gets better. Gotta love this comment [livejournal.com] for example. Using Google reveals a lot of similar responses. In other words, it's nonsense and we can all forget about it. Too bad that comment got moderated so high and now people that didn't bother to doublecheck will actually think that some QM interpretations have been invalidated.

Re:Fabric of Reality??

[eddie can read]

photons sounded like they were riding a wave. The wave passes through both slits, but the thing we measure as a photon only goes through one. Since the wave is now interfering with itself, it affects where the photon lands

The major interpretations disagree with your account. However, David Bohm has elaborated an interpretation right along the lines that you have suggested. The wave in his theory is called the "pilot wave" and it guides the particle, which is a separate entity from the wave.

If you search Google with these terms:

David Bohm pilot wave

(i.e. http://www.google.com/search?q=David+Bohm+pilot+wa ve [google.com])

then you will get some good information. But a warning: his interpretation is very much a minority interpretation. The major interpretations do not treat the wave and particle the way Bohm does.

Re:Isn't this just the double-slit experiment?

[MCraigW]

I don't believe that it is possible to do this specific experiment with water waves, because not all the holes can be at the water's surface, where the waves are. You could probably do a wave simulation using a computer though.

It also seems to me that they *are* using light for this experiment, it just happens to be red laser light. I believe that this is just a complex wave interfernce pattern... but a parallel universe is far more sexy.

Re:Fabric of Reality??

[nine-times]

Well, the double slit experiment, technically, shows that light behaves as a wave. It's the double slit experiment that had people convinced for a couple hundred years that light was a wave. The weirdness comes in when you send out what we believe to be a "particle" of light, and it still behaves like a wave by demonstrating interference patterns, which implies that the "particle" has somehow travelled through both slits.

It actually gets weirder than that: If you, in any way, detect which slit the photon went through, it stops showing interference patterns. In other words, a "single particle" simultatiously goes through two holes, unless you watch it to see which hole it goes through, in which case it only goes through one (the one you see it go through).

Neat experiment, but this "Fabric of Reality" and "multiverse" stuff is pretty unfounded, and probably nonsense.

Re:Actually it was on Star Trek: TNG

[drik00]

IANAQP either, but to the best of my understanding, the notion of parallel universes is actually a fairly logical result of empirical quantum physics experiments.

Experiments have shown that subatomic particles act very funny when you try to describe or figure them out. Basically, these particles act like particles half the time, and like waves the other half of the time, but never both at the same time. Certain well known experiments (like the banding described in the article, which are due to wave interference of light particles) have shown that particles can somehow seem to act is if they are in multiple places at once, yet they cannot be observed in multiple places at once. This has led a lot of physicists to surmise that there are 'multiple parallel universes' where that exist simultaneously. The rationale is that since the inherent particles that make up our universe are in multiple states at the same time, these inherently MPD (multiple personality disorder) particles make up a sort of multi-verse that exists at the same time in different states, thereby creating different realities/parallel universes.

...and I will reiterate, IANAQP, but it seems to me that there is a lot of going from A, B, C to X, Y, Z with nothing in the middle with that notion. We cant observe the quantum weirdness at our human-sized perceivable universe, and to assume that this quantum weirdness can cause other realities where GWBush is in Mensa seems to be a far step of logic.

If there are any quantum experts out there, and see a problem with my reasoning, or just want to educate the ignorant masses (please leave out the math, its just boring), I urge you to help.

--J

Re:Isn't this just the double-slit experiment?

[dpilot]

One thing worth mentioning about 'measurement' that I saw a few years back.

Sometimes people want consciousness to be involved in measurement, and that makes an absolutely stunning comment about the universe and our place in it. But a few years back, I read about an experiment involving interferometry, and placing a detector where it would destroy the wave nature. There have been numerous experiments involving this 'illegal' measurement, including 'destroying' the measurment and getting the wave nature back.

In this case, they simply unplugged the detector, leaving it in-place. The wave nature was still missing, the experiment still showed particles. The physical presence of the detector was sufficient for 'measurment', and the universe left more physically consistent, though less mystical.

Re:Since I can't see air it must be another univer

[Phurd Phlegm]

You have understood nothing. The phenomenon is real and one of the strangest and most spooky things in physics. It shows that it it possible to get a particle (in this case a photon) to interfere with itself.

While the interference effect is certainly present for a single photon, there's no way you're seeing single-photon interactions with a friggin' laser pointer. I agree with the earlier posters--there's nothing any more mysterious going on here with the four-hole case than with the two-hole one. The interference patterns obviously look different when you have a different configuration of holes. If someone credible (e.g., someone that isn't claiming that Spock with a beard is holding his hand over the holes) says that the pattern is different than that predicted by theory, then I'll be interested.

Note that saying you'd expect the pattern to look the same with four holes as with two is nonsense--you'd expect the pattern to look the same with two holes as with one, if you didn't know about interference effects.

Copenhagen Interpretation

[pinopino]

As it happens I am a super-Copenhagen believer, that is, our function, as conscious entities, is to observe the many possible universes and 'select' the real one. Um, no. The Copenhagen interpretation is one of strict Logical Positivism. It states that QM is a complete description of probabalistic outcomes of the experiments only, not of the objective reality of the 'objects'. The main problem that people have interpreting QM is they try to treat the 'objects' (e.g. an electron) like a macroscopic object (e.g. an apple). The Concious Observer framework was championed by Wigner, who although although he was a brilliant scientist, went a bit loony toward the end of his life. That happens to a lot of Quantum Physicists. I wonder why ..... ;-)

Re:Actually it was on Star Trek: TNG

[joethebastard]

IAAQP, and while I've not read this man's book, I'd be skeptical. Most physicists subscribe to the "Copenhagen" interpretation of quantum mechanics, which does not deal with parallel universes. The so-called "many-universe" theory has its followers but doesn't get much attention, for one reason: introducing the extra idea of multiple universes doesn't add anything to the descriptive power of the theory.

Before quantum theory was developed, most phenomena in nature were considered to be either particles or waves. This classification system broke down when particles were shown to diffract and waves were shown to be quantized. So nothing is really particle or wave, but everything has a particle or wave nature.

The canonical example is electron diffraction: shine a beam of electrons through two slits, and get an interference pattern on your photographic plate. Woo. Repeat with one electron at a time, recording each result.... and you still get an interference pattern. This presents a problem: each electron must have gone through both slits and interfered with itself. On the other hand, you can never measure the electron to be in two places at once, so we need to construct a third option. This is the idea of a superposition state: the electron is in a superposition of places (again, not actually "in" them); this superposition has wavelike properties and can interfere. When a measurement is made (by the photographic plate) the superposition "collapses" to one location.

This is where many-universe theory (to my understanding) comes in: how does the particle know which state to collapse to? Copenhagen Qm says it's random, but weighted by the superposition; i.e. quantum mechanics predicts probabilities only. Many-universe theory says that when that collapse occurs, the universe splits into a bunch of new ones, one for each outcome of the measurement. I've not yet read a good explanation (anyone have a link to one? I'd love to) of why measurement (that is, a phase-randomizing interaction with a larger system) should create a new universe.

Anyway, I hope this helps! If you are curious about QM, there's an inexpensive book by Isham that has really wonderful discussion (and even mentions many-universe). Feynman's book "QED" takes his path-integral approach and is a great layman's introduction; just don't try solving any problems with that method later (the math is rough).

Re:My problem with Schrödingers cat

[Dastardly]

The thing that always made Schrödingers cat seird to me is that the cat is not counted as an observer of itself. I may not have seen if the cat is dead or not, but the cat sure has. So hasn't it collapsed the wave function? Would it be different if it was a human in the box?

That is because Schroedinger's cat is a simplification to try and explain the phenomenon using more commonly known objects. Air interacting with the cat collapses the wave function. Air interacting with the released poison collapses the wave function. The cats material interacts with itself collapsing the various wave functions of the cat's particles. Whatever mechanism detect the decaying particle in order to release the poison collapses the wave function.

Re:Isn't this just the double-slit experiment?

[Too Much Noise]

There is nothing new here. Just a redistribution of intensity to the interference maxima. Gee, if I make 100 holes, the 2-hole pattern disappears (only the central maximum remains), but I get a bunch of other dots further apart. What a big surprise!!! [*]

It's a freaking 1st year optics problem. If the book says there's some shadow interference here, the book is so wrong the author should be ashamed of being a physicist. Otherwise, the guy writing the article has unbelievably low reading abilities.

[*] for people not having taken/remembering optics from college, the interference (a.k.a. principal) maxima are suppressed by the diffraction envelope (which has its own maxima) of each slit. The more holes you add, the further apart the interference maxima move. Different numbers of slits have different interference patterns.

Re: Isn't this just the double-slit experiment?

[gidds]

That's not my understanding at all.

I'm no physicist (though I did get an 'A' grade for A-Level physics), but from general reading, I think that the way light behaves is pretty well understood and explained by the standard model -- in particular, by the branch of Quantum Theory known as Quantum Electrodynamics (QED). The problem isn't that we don't know what photons do, but that what they do seems so different from what our normal intuition about the world expects them to!

(A great book on QED for the interested layman is that [amazon.com] by Richard Feynman, one of the theory's originators. Not much maths, but goes into lots of detail and manages to make it fascinating.)

Remember that the 'many-worlds' interpretation of QM isn't the only one, nor even the prevailing one. Another way of understanding the Young's Slits experiment is to think of the photon interfering with all the other possible paths it could take. In reality, as far as we can tell, the photon doesn't actually take a single path anyway until it impinges on the observer -- in some sense, photons taking all possible paths half-exist before that point, so interfering with each other makes some vague sort of sense...

Anyway, the point is that the Young's Slits experiment is one of the few which are simple enough to set up in a living room, clear enough that you can see the results with the naked eye, apparently obvious in classical-physics terms, and yet (once you know that light is made of particle-like things) bizarre and inexplicable without the deep mathematics of QM.

Re:Actually it was on Star Trek: TNG

[DavidTC]

The Copenhagen theory is basically 'we only know what we know' which is a cop-out if I ever heard one.

There are other interpetations besides Many Worlds, like transactional, which basically says that QM is really non-local. It has invisible waves propagating backwards in time from everywhere to everywhere, and photons and other waves happen when there's a forward wave in the opposite direction on top of invisble backwards waves.

I.e., for the slit experiment, it's not photons interfering with photons in other universes, like Many Worlds, or 'probablity waves' interfering with other waves, it's the advanced waves that interfere with each other (While, of course, going backwards in time), and to have a photon, it has to be following one of these backwards waves.

And anyone who knows anything about electromagnetic radiation is nodding their head at this point, because they already learned about these hypothetical 'advanced waves' when learning Maxwell's equations, which do not take QP into account. The transactional interpetation just says that even non-existent waves have advanced waves going the other way, and it's those waves that are interfering with each other.