If figure 4 and the surrounding theorising is correct: The photons in figure 4 didn't start off identical (they had different momentums). So your experiment made them identical.

I am troubled by your assertion that two photons can interfere with each other, since this slide suggests that even with billions of photons from the same light source, that they only interfere with each other.

My best guess is that the slide is just wrong. I can't think of any reason for each photon to interfere only with itself under those conditions.

Photons certainly can all amplitude-add with each other in principle. That's how a laser works in the first place.

Any physicists want to chime in?

See also e.g. Wikipedia on Identical Particles, though rest assured, I'm not getting all my information from Wikipedia.

Okay, I'm looking this up, and it seems Dirac said that "Each photon then interferes only with itself; interference between two different photons never occurs", saying this because, apparently, the amplitude gives the probability for a photon to be in a particular state, not the probability for a number of photons to be in a particular state.

It also seems that this statement is not to be interpreted in the intuitive way, because beams of light from two different lasers can interfere.

I know that there can be amplitudes for more than one particle b... (read more)

Dirac was saying that you still have interference effects even when there is only one photon; not that you don't have multi-particle interference effects when there is more than one.

OK, the problem is that second sentence from Dirac, "interference between two different photons never occurs". Google that and the name "glauber" and go to Google Books and you will get the beginning of an explanation. He is talking about branches of the single-particle wavefunctions which enter into a multi-particle wavefunction (tensored together and then symmetrized), and saying that you don't have a branch from one particle interfering with a branch from another particle. Dirac was not writing from a configuration-centric perspective, so his idioms are different. But I think his point could be reexpressed in your language should it prove necessary.

A configuration-centric way to put it might be as follows. Consider a particular association of amplitudes with all possible two-particle configurations. If one assumes indiscernibility, then the configuration space is not R^3 x R^3, it's that divided in half, by an equivalence relation which equates (x0,x1) with (x1,x0) (the xs are three-vectors). So working the other way, if you start in that truncated configuration space as the real configuration space, and expand out to R^3 x R^3, you end up with a symmetrical function (since you've just copied the amp... (read more)

If nature creates trillions and gazillion snowflakes, each of which is not identical to one another, then it certainly is not impossible that electrons would behave the same way or have similar characteristics. With the naked eye, snowflakes look all the same, and only with the invention of a powerful tool such as as microscope did we finally realise that they are all unique.... so for me as neither a philosopher nor a physicists, I guess it seems plausible...

So why are philosophers always the bad guys in your examples? Surely lots of other academics also often say foolish things and presume to know more than they do. Even ... physicists!

For once, I agree with Hopefully Anonymous - what was the point of the philosopher?

never mind, I see it...

"Kant's conclusion that space, by its very nature, was flat. Turned out, Kant was just reproducing an invisible assumption built into how his parietal cortex was modeling space. Kant's imaginings were evidence only about his imagination - grist for cognitive science, not physics."

Wasn't Kant explicitly taking about which things were possible to imagine? Your point still stands, though, since certainly plenty of people seem to have read him the way you did prior to Gauss, Lobachevsky, Einstein, etc.

Eliezer: I really like this post, but it seems to me that empirically it was substantially a cultural practice in philosophy, including Kant etc, that enabled those early 20th century Germans (and only those people, in that particular culture, with that particular philosophical tradition) to seem, vaguely a significant subset of those assumptions that they did know existed but that other philosophers and lay people didn't know existed. That philosophy also lead them down some wrong roads, such as towards thinking mind was fundamental rather than emergent... (read more)

Now I'm curious about the historical question: is there any philosopher in the pre-quantum era who actually made Bob's argument? I don't doubt that if you asked the question, a philosopher might have responded much as Bob has. But did the question actually occur to anyone?

Scott: Charles Peirce maybe? http://en.wikipedia.org/wiki/Charles_Peirce

Eliezer, I may be missing something here, but it seems you did not really disprove the philosophers argument. Yes according to physics status-quo of understanding, there might be a way to prove absolutely that two particles are the same, but who says the status-quo is complete, and how can you ever know that it is? That is the philosphers point I believe.

So can the same be said for two atoms whose constituent particles are indistinguishable from each other, and then molecules made up of said atoms, etc.? In other words, could one follow this chain until you had two rocks which you would assert were the same object, even though you held one in each hand?

It's easy enough to see where Eliezer is going with this, but the foundation being laid isn't terribly strong.

The dualist case is built for the possibility that--in this particular instance--scientific reductionism will fail. So to argue that reduction often works in other fields or changes the way we look at the world, etc. is perfectly valid, but totally redundant. Chalmers, for example, admits that right off the bat, with gusto. Not only does he accept that some things can be reductively explained, he argues that nearly everything--effectively everyt... (read more)

Wiseman: there might be a way to prove absolutely that two particles are the same.

Yes, by observing the macroscopic statistics of experiments. Current physics understanding can only explain the results if the particles are indistinguishable in principle, not just indistinguishable with the current experiments we happen to have thought of so far.

Sure, all of physics may be overturned tomorrow with a new Einstein. But in so far as current physics theories mean anything, they require that the particles can never be distinguished in any future experiment. T... (read more)

Scott, isn't the point here that even seemingly iron-clad philosophical reasoning can be invalidated by empirical physical discoveries? That physics, in effect, supercedes logic, rather than vice versa?

Some philosophers, I think including Chalmers, argue that no conceivable physical evidence could make the "hard problem" of consciousness go away. They suggest that there is a limit on the kinds of information we can receive from experiments on the physical world, and that no matter what specific information we receive from those experiments, their... (read more)

What Hal Finney said.

I add only that, in the case of the question as to whether experiments in physics or more likely cognitive science, could upend philosophical reasoning about consciousness being irreducible, there are a number of particular reasons for you to be suspicious; it is not simply a generic "might someday see why you were wrong". So as soon as you appreciate how fragile "impeccable philosophical reasoning" really is, especially while the reasoned-about quantities still remain confusing, you suddenly begin to doubt impeccable philosophical reasoning about consciousness a great deal.

We can see where their argument went wrong right now. We don't need to wait for 'someday'.

Chalmers does not make an argument founded in our current understanding, he makes an argument founded on a hypothetical - one that happens to be incoherent. So if his argument held, which it does not, it would only apply if its hypothetical foundation held. Which it, to the best of our knowledge, does not.

Well then, if philosophers must be more cautious about their philosophies, because observable evidence might prove them wrong, then this goes equally for the physicist's arguments as well: Observable evidence might prove him wrong in the future. Since it is always true that "You might be wrong", then it is never valid to say you can prove that two particles are exactly the same, since future theories or evidence may show there are properties of a particle we just don't know about yet, and how to test for them. Therefore Eliezer's argument against... (read more)

Wiseman:

Acceptance of an objective rule is compulsory. Until we find a test that makes it fail; then the new rule becomes fact.

If there is an unknowable probability of a statement being false and a mountain of empirical data saying it is true, the scale seems to be clearly tipping toward the direction of the data. Choosing to focus on the unknowable but ever-present chance that something is wrong is not going to be very fruitful.

So if someone says something is 'proven,' there is a tacit understanding that they really mean "the data gathered thus far indicates this is so." You could say it is relatively absolutely true.

Wiseman, I fear that I have not yet managed to convey how quantum physics works. I'll keep trying.

There can be properties of the particles we don't know about yet, but our existing experiments already show those new properties are also identical, unless the observed universe is a lie.

What does "the universe is a lie" mean technically? Otherwise, I think I understand.

Eliezer: There can be properties of the particles we don't know about yet, but our existing experiments already show those new properties are also identical

According to a specific theory, the experiments do, yes. But again I beg to know why you have 100% confidence that right now you think our understanding of sub-atomic particles is totally complete, such that there can't possibly be anything about particles that we haven't taken into account in our experiments so far. More specifically, I really doubt that any experiment will show two particles are exact... (read more)

"What does "the universe is a lie" mean technically? Otherwise, I think I understand."

I believe Eli's referring to the usual "Matrix" scenario, where nothing we see actually exists, and it's all an illusion carefully created by the deliberate manipulation of our neurons.

Wiseman, you say rather dismissively that, yes, "according to a specific theory" the particles are identical. But that's already a huge deal! For me the point is that, before quantum mechanics, no one had even imagined a theoretical framework that could force two particles to be identical in all respects. (If you don't understand how QM actually does this, reread Eliezer's posts.) Obviously, if QM were overthrown then we'd have to revisit all these questions -- but even the fact that a framework like QM is possible represents a major philosophical discovery that came to us by way of physics.

Scott, I can't imagine any possible overthrow of QM that would resurrect the idea of two electrons having distinct individual identities.

Suppose we discovered our universe was being simulated on a classical computer. Then, at the fundamental level, there would be particles with individual identities. But the "electrons" we see today, would still be computed as amplitude flows between simulated configurations - they would not have identity as fundamental particles.

It wouldn't be enough to discover something new underneath QM - discover a new lev... (read more)

Ok...

So we aren't dealing with probability distributions because that implies that reality acknowledges individual identity in particles? And that can't be true because the experimental results show that the configurations are only concerned with having a particle in the right position, regardless of the particles origin? And therefore the "2" particles are "1" by existing on the same amplitude pattern? Is any of that remotely close?

I read the joint configuration post 3 times and I still feel like I'm missing something. The problem is probably on my end. =+)

Scott, I'm not dismissing QM's accomplishment, because yes it's significant, the point is simply that it's still just a theory, and so long as that's what it is, dismissing the possibility that it is incomplete, or wrong, is not scientific.

Eliezer, I get that you are highly confident in QM. Obviously, QM has a lot going for it. But that still doesn't mean that QM can't be incomplete, or even wrong. Of course, reality is what it is, but our mental representation of it can be arbitrarily accurate or innacurate, and we can continue to fool our selfs into thin... (read more)

I've read this post several times, as well as the previous three posts, and I still can't see how the theory guarantees the indistinguishability of any two particles. Admittedly, I'm weak on the math, so maybe there was a really clear mathematical explanation that was wasted on me (I take it that the amplitude equations were supposed to rule out distinguishability in virtue of difference in, say, spin values at the 100th decimal place, but I couldn't tell for sure).

But, much like our stubborn philosopher Bob, I still think that the theory is insufficientl... (read more)

On the idea that there might be extra properties, presently unknown, which make identical particles distinguishable:

First of all we need to be clear on what 'indistinguishability' means here. In a configuration with one particle at x0 and another particle at x1, the particles are indeed distinguishable in a sense: they have different positions. One is over here, the other is over there.

The essence of indistinguishability is this: that if you were to get the two particles and move them around so that they occupied the other position, that would count as the... (read more)

Scott, I can't imagine any possible overthrow of QM that would resurrect the idea of two electrons having distinct individual identities.

Nor can I! Wise Bayes-Master, I was simply trying to follow your own dictum that an inability to imagine something is a fact about us and not the world.

(For technical reasons set out elsewhere, I have difficulty imagining any theory superseding QM -- so once I'm asked to condition on that happening, there's very little I'm willing to say about what the new theory might entail.)

Scott: Nor can I! Wise Bayes-Master, I was simply trying to follow your own dictum that an inability to imagine something is a fact about us and not the world.

Great NP-Lord, we have to draw the boundary somewhere. I draw the line at imagining that atoms are not made of nucleons, that apples are fundamental, or that electrons have individual identities.

Okay, I realize that's probably a little too extreme... but only a little. I think it's worth distinguishing different degrees of unimaginability; between unimaginable surprising new facts, and unimaginabl... (read more)

The thing that bothers me about quantum physics is that I would like to do total simulations of things using universal laws, all at the same level. This is how I have come to understand things. So I want to model the half-silvered mirror quantumly and the detectors as well.

It is deeply disturbing to have half silvered mirrors perform operations on things, with no way of determining what happened to them in return, because they are not modelled at the same level, but at a higher level.

This goes back to my problem with the conservation of momentum on half-si... (read more)

Mitchell, the essence of indistinguishability is NOT "that if you were to get the two particles and move them around so that they occupied the other position, that would count as the same configuration, quantum mechanically." Bob doesn't care about a version of indistinguishability that restricts the relevant properties to those important for QM. QM-indistinguishability is not indistinguishability. So if that's the notion of indistinguishability at play here, then Bob could accept that QM can determine that two electrons are QM-indistinguishab... (read more)

Let me give another concrete example of where a physical system might be indistinguishable by one experiment, but has to be distinguishable regarding another.

Imagine heating a bose-einstein condensate with a laser, if all the atoms were truly indistinguishable, then there is no basis upon which an atom could be excited to a higher energy level while another one shouldn't be. Trying to get one particle to be in a different state to the rest seems to be like trying to separate sand by sieving it, when all the sand particles are the same size.

I'm still thoroughly puzzled by quantum.

Ben Jones: Well that's just plain wrong.... QM is the most experimentally validated theory we have, but one of its implications is the relative identity of quanta.

The experiments show specific results, but it may be possible that some properties of the particles aren't interacting with any aspect of the experiment, thus QM would still be correct in the explanation for the original experiments, but not complete, as they don't explain the additional properties. So it is entirely possible.

The generality "invalidating one aspect of a theory can't invalida... (read more)

I take it that the theory doesn't tell us determinately that any given particle absolutely lacks any more fundamental structure. How could it, even in principle?

Paul N., you're right that QM can't rule out the electron having a more fundamental structure -- but it can tell us that whatever that structure might be, it's the same from one electron to the next! Why? Because we're talking about a theory in which whether two states of the universe are "the same" or "different" is a primitive with testable consequences, and this is true not... (read more)

Bob doesn't care about a version of indistinguishability that restricts the relevant properties to those important for QM.

This doesn't seem right. Distinguishability should entail empirical distinguishability. If two particles are distinguishable in any sort of way that Bob cares about, I should be able to send them through a device that shows a green light for tagged particle A and a red light for tagged particle B. But "emitting green light" and "emitting red light" are, obviously, different quantum configurations.

What is an example of a distinguishability that is not allowed to entail quantum distinguishability, and why should Bob care?

if all the atoms were truly indistinguishable, then there is no basis upon which an atom could be excited to a higher energy level while another one shouldn't be.

Referring to "an atom" versus "another one" here is just begging the question on the identity of atoms. Why is a BEC containing N indistinguishable atoms not allowed to evolve into a BEC of N-1 indistinguishable atoms and an excited atom?

I think Eliezer wasn't specific enough what he meant by "indistinguishable". In the QM and thermodynamics/chemistry sense the particles are indistinguishable iff they are completely described by their quantum states - which are just finite sets of numbers. The properties of statistics change a lot when moving from distinc to non-distinct particles - think of permutations versus combinations.

"But the "electrons" we see today, would still be computed as amplitude flows between simulated configuration"

- Eliezer, the argument being posted against you is that the MODEL could be wrong. Remember, it's a mathematical model - it describes, it doesn't define.

Remember, there are quite a few models of quantum physics that describe the behavior of quantum "particles" - and that presumes on the particles' very existence. It is quite possible to invent a model which describes physics perfectly but which omits the existence of... (read more)

watch this vid for some interesting ideas.video.google.com/videoplay?docid=8503156790716142769

"Referring to "an atom" versus "another one" here is just begging the question on the identity of atoms. Why is a BEC containing N indistinguishable atoms not allowed to evolve into a BEC of N-1 indistinguishable atoms and an excited atom?"

I think Eliezer and possibly the physicists are using "identical" and "indistinguishable" differently to me. Identity and identical is typified by things like 4 = 4. Under some circumstances two different numbers can be indistinguishable, without being identical. If you h... (read more)

And if you apply an operation that excites one atom to a BEC in the ground state, you always get the same result: a BEC with one excited atom. I assume you can't consider the atoms separately and be physically realistic.

Will -

The reasoning is better understood in terms of in wave mechanics; if the particle states diverged in the least, then the cancellation wouldn't be complete, and the experimental results would differ.

That is, they must be identical, not indistinguishable, for wave cancellation to operate. (sin-1(sin(x) +.0000000000001) isn't x.

However, again, this depends upon a particular mathematical definition of the particles - in particular, a model which has already defined that particles have no discrete existence. Eliezer is by far my favorite author here, bu... (read more)

Adirian - Thanks, I think I see what you are saying.

I find the following quote from Wikipedia article for QFT far more to my liking to this talk of identical things somehow differentiating.

"Many physicists prefer to take the converse interpretation, which is that quantum field theory explains what identical particles are. In ordinary quantum mechanics, there is not much theoretical motivation for using symmetric (bosonic) or antisymmetric (fermionic) states, and the need for such states is simply regarded as an empirical fact. From the point of view ... (read more)

Will - field theory is pretty good, yup, although...

We're basically at the same point in physics we were a little more than a century ago. Back then, there were two major camps - the atomicists, and the energists. The energists' position was essentially that everything was made of energy, the atomicists' position was that there were these tiny particles we hadn't seen yet, but they were in fact real.

Now, at the time, both camps had equally valid positions, although the energists had the stronger support - but there was a very interesting distinction betw... (read more)

"Anybody who is proposing we know all the fundamentals of a field should arouse your instant suspicions - this is a hubris from which men have fallen every time they've mounted it. It's a very seductive idea to those who chase order. It is also a mindkiller."

I'm not too worried about fundementals myself. I just want something that makes sense of the experimental data that I have heard of.

I can see why Eliezer is. He is trying to prove things about a computer system (whether it is FAI or a machine to build FAI), for this he needs to have a firm ba... (read more)

Isn't location in space-time a characteristic of a thing? If it is, then it can be used to distinguish non-integral spin [particles], and since integral spin [particles] are [interactions] between distinguishable non-integral [particles], then they can be distinguished.

I wonder if there is any redeeming value in my previous comment....

This is the crucial point:

"In our universe, the individually and fundamentally real entities are configurations of multiple particles, and the amplitude flows between them."

This idea is the basis of Mermin's Ithaca Interpretation of QM, aka correlation without correlata, aka zero worlds hypothesis. Here are some sample quotes:

"Correlations have physical reality; that which they correlate does not."
What is QM trying to Tell Us ?

"... the first pillar of the Ithaca Interpretation is that correlations are the only fundamental an

Correct me if I'm wrong but it sounds like you and Bob might both be wrong. Bob says the 2 separate particles can't be shown to be identical. You say that the 2 separate particles are shown to be equivalent. But I think QM shows that there aren't 2 separate particles. Maybe you could say something weaker, "like this configuration has a particality of 2".

There is, of course, also the observation that on Feynmann diagrams, positrons behave like electrons moving backwards in time, whatever that means. This suggests a hypothesis that, perhaps, there is only one electron, moving forward and backward in time, interacting with itself...

From what I understand, what's known is that a given configuration will have exactly the same probability as the same configuration with two particles swapped. This is enough to show you can't test it. If you had a detector go off when you show it photon a, it would have to also go off when you switch it out with photon b, or it would break the symmetry laws. This means that, if there are multiple photons, it's an epiphenomenon. If you believe that they can, in principle exist, like a particle that doesn't interact with any of the other particles and isn't... (read more)

The reason why we are supposed to be able to prove, that two particles are identical, is because quantum mechanics allows us to make an experiment, which has a certain outcome only! if the two particles taking part in the experiment are identical (the amplitudes add up). However, there is no prove that the amplitudes of "P1 at L1, P2 at L2" and "P1 at L2, P2 at L1" can't add up, even if P1 and P2 are different particles. Its only what quantum mechanics tells us. Because, although quantum mechanics is a brilliant and most successful mode... (read more)

This experiment does not prove that the electrons are indistinguishable. It merely proves that to the limit of our ability to measure, interchanging two electrons has the effect of negating the wave function. This could be achieved by anti-symmetrizing any configuration, even if the electrons were distinct. Furthermore, if the electrons had nearly identical physical properties, this property would maintain itself over time.

What is this based on? Also, what do you mean by "A ball or an electron can only spin in one direction in relation to a previously defined direction''? This seems trivially false - a ball on a given axis can spin in two directions - so perhaps I have misunderstood?

Spin is qualitative. Qm is dealing with a degree of spin (spin up or spin down) which is quantitative.

I believe the distinction you want is "continuous" vs. "discrete", rather than "quantitative" vs. "qualitative".

In the macro scale, spin (ie rotation) is definitely quantitative - any object is rotating at a particular rate about a particular axis. This can be measured, integrated to yield (change in) orientation, etc.

In QM, my understanding is that (much like "flavor" and "color") the term is just re-purposed for something else.

Suppose you were in a universe where particles really did have tags and you really could check them. How do you prove that no two particles have the same tag, implying they are truly interchangeable? As with everything else, there is no true proof either way. (Just evidence)

"Certainty" is an algorithmic optimization your brain performs (and evolution performed on brains) to avoid computing the effects of unlikely scenarios. It is an implementation detail of a decision making algorithm, not some guaranteed facet of decision theory delivered from o... (read more)

I think quantum physicists here are making the same mistake that lead to the Gibbs paradox in classical phyiscs. Of course, my textbook in classical thermodynamics tried to sweep the Gibbs paradox under the quantum rug, and completely missed the point of what it was telling us about the subjective nature of classical entropy. Quantum physics is another deterministic reversible state-machine, so I don't see why it is different in principle from a "classical world".

While it is true that a wavefunction or something very much like it must be what the... (read more)

Unless I'm totally confused, it seems like almost all the debate about this post is just about competing intuitions around the word "prove." If "prove" means "establish with probability exactly equal to 1," then Philosopher Bob is right for exactly the reasons he says he is: probabilities are never exactly 1, and you don't need to know the details of any specific theory to understand this. It's a fact of epistemology. If "prove" means "establish with the same level of credence we assign to things we generally take for granted" (like the sun rising tomorrow), then Bob is wrong, and he needs to take some QM classes. I think I hear a tree falling...

I am no physicist, but doesn't energy come in discrete and indivisible packets called quanta? This is the reason for the election rings/levels I see in diagrams of atoms. Would this not be the threshold for an exact measurement? Even if it was found that particles differ, would this difference would apply to quanta? Would quanta be indistinct from other quanta, identical even?

To tell you the truth, I find the idea of infinity divisible universe a bit maddening, anti-intuitive. But, I know that means nothing, when it comes to the truth of things. So I ask, in earnest. Am I way off base here?