Atom identity

[amidst]

Hi there are atoms of a paticular element say hydrogen for example exactly identcal or are they as individual as we are, in other words are there some very minor differences between them.

[Qfwfq]

They're indistinguishable and there are consequences of this which even have some macroscopic effects. For instance, the indistinguishibility of helium atoms is the reason for superfluidity.

[sanctus]

same applies to photons too, the thing called "teleportation" (which not like you would imagine from SF) works also because photons are indistinguishable...

[Qfwfq]

Of course, if we don't talk at just the atomic level, for the case of photons it is essential to the working of the laser, for the case of electrons it's the basis of superconductivity, ferromagnetism and... Pauli's exclusion principle without which there would be no such thing as chemistry!

[amidst]

Hi guys and thanks for your input. Now then i kinda understand what you mean but when you use the term indistinguishable do you mean that there is at present no known method or device to detect any difference. I just cant help feeling that no two things in the universe can be absolutely identical surely there must be some difference (or perhaps not).

[Qfwfq]

...but when you use the term indistinguishable do you mean that there is at present no known method or device to detect any difference.

If we found such a method or device, we'd have to look for totally new explanations of some things, including the very fundamental Pauli exclusion principle.

 

I just cant help feeling that no two things in the universe can be absolutely identical surely there must be some difference (or perhaps not).

It's a tricky thing to explain starting from scratch, but talking about two atoms of the same kind is very unlike talking about two ball bearings from the same production line.

[Little Bang]

The very fact that the proton and electron are totally indistinguishable from all other protons and electrons implies that we don't have a clue as to the true construction of either. The quark model wants to keep making the proton out of smaller and smaller pieces and like amidst says it's hard to believe there is no difference if the constituent parts keep getting smaller.

[Qfwfq]

You mean that two protons might be distinguished by counting their quarks. If this were possible, we'd have to say that superfluidity of helium is magic.

[Little Bang]

No I don't mean that. If protons are made of quarks then it seems reasonable to think that quarks are made of some smaller components. If you have two buckets of sand and each bucket contains exactly the same number of grains they would look identical from afar but you and I know if examined up close they would show differences.

[Qfwfq]

The single grains of sand are not indistinguishable.

 

Quarks of a feather flock together.

[Little Bang]

From ten feet away they appear identical. The point, if the components of the proton keep getting smaller we will be unable to tell just like the sand grains from ten feet.

[Qfwfq]

Appear but are not.

 

If it were only an appearance in the case of atoms, if it were enough that we can't tell, how would you explain superfluidity? Could it be that superfluididy is observed only by those that can't tell the difference between the individual atoms?

[Little Bang]

I don't have a clue. Apparently the discussion has gotten above my head.

[freeztar]

He's basically saying that there is observed physical phenomena that would not be possible under your premise. :)

[CraigD]

They're indistinguishable and there are consequences of this which even have some macroscopic effects. For instance, the indistinguishibility of helium atoms is the reason for superfluidity.

Qfwfq, could you explain this in more detail, or link us to some document that does? My understanding of superfluids and conductors – a very superficial appreciation of BCS theory – is that the phenomena have to do with bosonic interactions of quarks and electrons with quarks and electrons in their containers/barriers or conductors. While not requiring that quarks or electrons are distinguishable, this explanation does not preclude that they must be.

 

I’m aware that the exclusion principle can be stated, roughly, as “no two fermions may have the same quantum numbers”. This implies that, if two fermions were different in some way unrelated to position (ie: having differing in some undiscovered attribute), they could occupy the same space at the same instant. This is not observed, and would suggest dire “super-degenerate” consequences for the universe if it was. However, I’m unaware of any requirement by the formalism of quantum physics that implies that additional, undiscovered quantum numbers, exempt from the exclusion principle but detectable by some interaction, may not be added to the model – nor any requirement that such attributes must be added.

 

In short, I don’t believe that the formalism precludes its own modification.

 

This is pretty deep stuff. I’d appreciate guidance from people with better physics educations and comprehension than me.

The very fact that the proton and electron are totally indistinguishable from all other protons and electrons implies that we don't have a clue as to the true construction of either.

I don’t think this follows logically from the premise. The conclusion that electrons (which, unlike protons, are fundamental particles under the Standard Model) are indistinguishable can surely be credited as better than “a clue” as to their true construction. The claim that it is not seems akin to the statement “that I have stated a hypothesis which has not been experimentally demonstrated to be incorrect implies that the hypothesis must be incorrect, and an experimental demonstration of that just not yet performed”, a logical non sequitur.

 

I think Dirac commented brilliantly on the possibility that electrons really, ultimately, are indistinguishable in the sense we’re discussing, when he speculated that if we accept the formally defensible assertion that a positron ([math]e^+[/math], the antiparticle of an electron) is actually an electron traveling backward in time, and that through a variety of interactions, an electron may transform into a positron and vice versa, then there may actually be only one electron/positron in the universe! Though a frivolous speculation, it nonetheless conveys the bizarre nature of quantum physics.

The quark model wants to keep making the proton out of smaller and smaller pieces and like amidst says it's hard to believe there is no difference if the constituent parts keep getting smaller.

The quark model – part of the Standard Model – wants to say the proton, a baryon, is made of exactly 3 smaller fermions – 2 u and 1 d quarks – and a virtual (literally) zoo of bosons.

 

Given that the Standard Model describes the proton as such a menagerie of virtual particles in constant complex interaction with 3 real ones, it’s vexing to explain how we can make any claim that protons are indistinguishable from one another. Strictly speaking, I believe we can only say that protons are indistinguishable to every ordinary technique for distinguishing them. Some extraordinary techniques can – if this were not the case, we would have no experimental support of the quark model. Most of these extraordinary techniques consisted of accelerating particles such as electrons to very high speeds, colliding them with protons and neutrons, and measuring their velocity when the “emerged”. These experiments suggest that not only are individual protons different from one another, but the same proton differs from moment to moment from itself – reminiscent of Heraclitus’s famous river. :)

 

Finally, note that just because the Standard Model considers quarks and other fundamental particles fundamental, does not mean that no theoretical investigation of the possibility of them being composites of yet more fundamental particles doesn’t exist. String and preon theories are examples of such investigations. So far, however, these speculative theories have yet to produce any compellingly experimentally verified predictions, unlike the celebratedly successful Standard Model.

[Erasmus00]

is that the phenomena have to do with bosonic interactions of quarks and electrons with quarks and electrons in their containers/barriers or conductors. While not requiring that quarks or electrons are distinguishable, this explanation does not preclude that they must be.

 

Bosonic and fermionic statistics are expressions of indistinguishable particles. If particles were, even in principle, distinguishable, there wouldn't be bosons/fermions. Many experimental consequences follow from this. If I have more time a little later, I'll type up a longer explanation. But now the ALCS is on.

-Will

[sanctus]

ALCS?

 

Anyway, what about the virtual zoo of bosons? These gluons are always created and annihilated (and make up much of the proton's mass if I remember right). Saying that 2 protons are indistinguishable means that the statistical average of the the virtual gluons is the same or that they ALWAYS have the same number of virtual particles?