Phenomena only explicable via QM

[mathew3]

Can someone list some of the phenomena only explicable via QM?

 

[berkeman]

Can someone list some of the phenomena only explicable via QM?

What is the context of the question? Is this for schoolwork?

 

[mathew3]

What is the context of the question? Is this for schoolwork?

No. Not at all. I just want to see if I can come up with a classical counterpart or where the classical counterpart fails.

 

[berkeman]

No. Not at all. I just want to see if I can come up with a classical counterpart or where the classical counterpart fails.

How about tunneling? I'm not aware of any classical explanation for that.

 

[akhmeteli]

How about tunneling? I'm not aware of any classical explanation for that.

I am not sure about "classical explanation", but there seems to be at least a "classical counterpart" of tunneling. As I wrote earlier, "I suspect theoretically you can jump over a classical barrier having lesser kinetic energy than the potential energy your mass would have at the top of the barrier. In fact, in the course of a high jump you can bend over the barrier in such a way that your center of gravity will be outside of your body and pass under the barrier." According to Wikipedia (http://en.wikipedia.org/wiki/Fosbury_Flop ), high jumpers do achieve this, but I did not check the Wikipedia source.

 

[berkeman]

I am not sure about "classical explanation", but there seems to be at least a "classical counterpart" of tunneling. As I wrote earlier, "I suspect theoretically you can jump over a classical barrier having lesser kinetic energy than the potential energy your mass would have at the top of the barrier. In fact, in the course of a high jump you can bend over the barrier in such a way that your center of gravity will be outside of your body and pass under the barrier." According to Wikipedia (http://en.wikipedia.org/wiki/Fosbury_Flop ), high jumpers do achieve this, but I did not check the Wikipedia source.

That's not what QM tunneling is.

 

[akhmeteli]

That's not what QM tunneling is.

If you don't think this is a counterpart of quantum tunneling, what are your arguments?

 

[Bill_K]

Can someone list some of the phenomena only explicable via QM?

1) The universe

That's only partly in jest, because more seriously what cannot be explained without quantum mechanics:

2) Atoms
3) Electromagnetism
which does not leave much.

By (3) I mean more specifically black body radiation, which is finite only because of the Planck distribution.
But that's Ok, because here's something else that could not exist:

4) Thermodynamics. Without the correct counting of states as given by quantum mechanics, the Gibbs paradox would make a definition of entropy impossible.

 

[Drakkith]

If you don't think this is a counterpart of quantum tunneling, what are your arguments?

The "barrier" referred to in quantum tunneling isn't a physical barrier like a wall you can jump over. It refers to the amount of energy needed to overcome the strength of a repulsive or attractive force. For example, fusion in the sun could not occur in classical physics because the energy of the particles in the core is not sufficient to bring them together in range of the strong force to fuse. Quantum tunneling describes the mechanism that allows protons to overcome the repulsion and fuse if they have "almost" enough energy. Similarly this allows alpha particles to overcome the strong force in heavy elements like uranium or plutonium and is the mechanism behind fission.

 

[DaveC426913]

I am not sure about "classical explanation", but there seems to be at least a "classical counterpart" of tunneling. As I wrote earlier, "I suspect theoretically you can jump over a classical barrier having lesser kinetic energy than the potential energy your mass would have at the top of the barrier. In fact, in the course of a high jump you can bend over the barrier in such a way that your center of gravity will be outside of your body and pass under the barrier." According to Wikipedia (http://en.wikipedia.org/wiki/Fosbury_Flop ), high jumpers do achieve this, but I did not check the Wikipedia source.

akhmeteli, having a classical explanation for something does not simply mean you can come up with an analogy that might form the start of a plausible description, and then leave the details of the math to the eggheads.

With a classical description of subatomic particles, there is no known way a particle can jump from one side of an energy barrier to another. The particles are essentially little balls - there is no "bending in the middle".

To posit that particles can "bend in the middle" is to toss out classical mechanics and start proposing quantum mechanical explanations.

 

[mathew3]

Thanks all.
I knew about tunneling, and double slit, and electron orbits. I was just wondering if there were a few more.

 

[Dickfore]

Discrete emission spectra.

 

[akhmeteli]

The "barrier" referred to in quantum tunneling isn't a physical barrier like a wall you can jump over. It refers to the amount of energy needed to overcome the strength of a repulsive or attractive force. For example, fusion in the sun could not occur in classical physics because the energy of the particles in the core is not sufficient to bring them together in range of the strong force to fuse. Quantum tunneling describes the mechanism that allows protons to overcome the repulsion and fuse if they have "almost" enough energy. Similarly this allows alpha particles to overcome the strong force in heavy elements like uranium or plutonium and is the mechanism behind fission.

I think this is still a "counterpart" of quantum tunneling. Indeed, what is "tunneling", in the first place? This is a way to pass through a high hill, and this is what is done (or theoretically can be done) in high jump without any "tunnel" under the bar. I agree that the analogy is not complete, as quantum tunneling can occur in one dimension, whereas high jump needs at least two dimensions. This does not seem very important though, as you can "rewrite" my example for one dimension for some elastic forces instead of gravity. The main point still stands: an extended object can pass through a high potential energy area, having kinetic energy below that required for the entire object to be at the top of the potential energy hill. The only problem is "esthetic": such an example would not be as graphic and natural as that of high jump, but the principle is the same.

 

[DaveC426913]

I think this is still a "counterpart" of quantum tunneling. Indeed, what is "tunneling", in the first place? This is a way to pass through a high hill, and this is what is done (or theoretically can be done) in high jump without any "tunnel" under the bar. I agree that the analogy is not complete, as quantum tunneling can occur in one dimension, whereas high jump needs at least two dimensions. This does not seem very important though, as you can "rewrite" my example for one dimension for some elastic forces instead of gravity. The main point still stands: an extended object can pass through a high potential energy area, having kinetic energy below that required for the entire object to be at the top of the potential energy hill. The only problem is "esthetic": such an example would not be as graphic and natural as that of high jump, but the principle is the same.

The trouble, as explained before, is that, just because you describe something that is plausible does not make it possible. Classical phyiscs does not predict tunneling.

In our classical description of particles they simply do not have the properties you ascribe to them. And you can't just patch it up by tacking on an extra variable to an equation here or there.

 

[akhmeteli]

The trouble, as explained before, is that, just because you describe something that is plausible does not make it possible. Classical phyiscs does not predict tunneling.

In our classical description of particles they simply do not have the properties you ascribe to them. And you can't just patch it up by tacking on an extra variable to an equation here or there.

Right now I just wanted to say that this was perhaps the fastest reply I've ever received at physicsforums:-)

I'll try to answer your posts "in the order of their appearance".

 

[akhmeteli]

akhmeteli, having a classical explanation for something does not simply mean you can come up with an analogy that might form the start of a plausible description, and then leave the details of the math to the eggheads.

DaveC426913, I don't quite understand why my post seems problematic to you. The OP said "I just want to see if I can come up with a classical counterpart or where the classical counterpart fails." Another poster mentioned quantum tunneling, so I just offered a "classical counterpart." What's wrong with that? It was certainly relevant, could be interesting for some people, and did not challenge any principles of physics. So I just don't understand why I cannot "come up with an analogy".

Furthermore, how do you know I "leave the details of the math to the eggheads"?

With a classical description of subatomic particles, there is no known way a particle can jump from one side of an energy barrier to another. The particles are essentially little balls - there is no "bending in the middle".

To posit that particles can "bend in the middle" is to toss out classical mechanics and start proposing quantum mechanical explanations.

I don't quite agree with everything you're saying (for example, why classical description cannot use anything but "little balls"?), but first things first: how is this relevant? Where did I "posit" anything? The OP wanted a classical analogy, and I just offered that.

 

[Drakkith]

The OP wanted things which do NOT have classical counterparts. Quantum tunneling is one of them. Your analogy, while certainly applicable under other circumstances, is not correct here. If there was a classical counterpart we wouldn't have to use analogies in the first place!

 

[akhmeteli]

The trouble, as explained before, is that, just because you describe something that is plausible does not make it possible.

I believe my analogy is legitimate. What plausibility/possibility are you talking about and how is it relevant? If you think that in real life a high jumper cannot always have his/her center of gravity below the bar, maybe you're right, but, first, what are your arguments, and, second, is it really important for physics? If you are saying that quantum theory cannot be emulated by a classical theory, are you sure you want to open this can of worms in this thread? I am not sure OP’s questions or my humble analogy warrant that.

Classical phyiscs does not predict tunneling.

As my analogy shows, in some sense it does.

In our classical description of particles they simply do not have the properties you ascribe to them.

Did I mention any particles, let alone ascribe anything to them?

And you can't just patch it up by tacking on an extra variable to an equation here or there.

Again, it seems to me that you are pushing a disproportionate expansion of the topic of this thread. Are you sure you really want that? Let me just say that once I started a thread dealing with these issues (https://www.physicsforums.com/showpost.php?p=2530114&postcount=1 ), and there are almost 750 posts there:-)

 

[akhmeteli]

The OP wanted things which do NOT have classical counterparts.

Yes, but another poster mentioned quantum tunneling, and I just tried to say that, in my opinion, this is not what the OP wanted, as it does have a classical counterpart.

Quantum tunneling is one of them.

As I believe I offered a classical counterpart, I have some doubts about your statement.

Your analogy, while certainly applicable under other circumstances, is not correct here. If there was a classical counterpart we wouldn't have to use analogies in the first place!

Look, Drakkith, English is probably your native tongue, but not mine, so I had to look up “counterpart” in Webster. Some of the meanings I found there:

“one remarkably similar to another”
“one having the same function or characteristics as another”

I believe my ”analogy” fits these meanings.

 

[Drakkith]

Sorry, your example is just plain incorrect. A person jumping over a barrier and shifting their weight is using energy to move their body. ALSO, a person must have enough energy on the jump to get enough of their body over the pole in the first place, otherwise shifting their weight will not work. The key is that a particle does NOT have to have enough energy to move past the barrier while the person does.

And no, your analogy does not fit those two definitions, as it is not remarkably similar nor does it have the same function or characteristics.

 

[akhmeteli]

Sorry, your example is just plain incorrect. A person jumping over a barrier and shifting their weight is using energy to move their body.

In principle, this does not require much energy (more than (the difference between the height of the bar and the maximum height of the center of gravity) times mg) to be relevant. Again, what happens or does not happen in real-life high jump is not critical for physics. The principle still stands. I believe my example is correct, so we disagree.

ALSO, a person must have enough energy on the jump to get enough of their body over the pole in the first place, otherwise shifting their weight will not work. The key is that a particle does NOT have to have enough energy to move past the barrier while the person does."

I don't understand that. I believe the person can have less kinetic energy than mgh (where h is the height of the bar) and still move past the barrier. Do you disagree?

And no, your analogy does not fit those two definitions, as it is not remarkably similar nor does it have the same function or characteristics.

It is certainly "similar", whether it is "remarkably similar", is a matter of opinion, I admit, but it seems to have the same function: pass the barrier that is higher than the available energy would normally allow.

 

[DaveC426913]

I believe my analogy is legitimate.

Except that classical particles do not behave that way.

What plausibility/possibility are you talking about and how is it relevant?

Does your analogy map onto to actual behavior of classical particles? No.

The mathematical description of a classical particle does not include any properties that could cause it to bend around a barrier. For one, classical particles are points.

If you think that in real life a high jumper cannot always have his/her center of gravity below the bar, maybe you're right, but, first, what are your arguments, and, second, is it really important for physics?

I don't doubt the high jumper. I only refute its relevance.

If you are saying that quantum theory cannot be emulated by a classical theory, are you sure you want to open this can of worms in this thread?

That is the entire purpose of this thread.

Are there phenomena that can only be described by QM, and not by classical physics?

Indeed there are. Electron tunneling is one.

Did I mention any particles, let alone ascribe anything to them?

We are talking about the quantum tunneling of particles through a barrier. Have you forgotten?

Again, it seems to me that you are pushing a disproportionate expansion of the topic of this thread.

Please re-read the subject line and most of the OP's posts after that.

Are you sure you really want that?

No, the OP does.

akhmeteli, please, before posting again, please review the thread from the beginning. You seem to have missed the entire point of the thread.

 

[mathew3]

Akhmeteli and Davec426913

Let me thank the both of you very much. As usual in the forum both of you are correct. Dave you are correct in saying I initially asked for phenomena classical physics couldn't couldn't explain. But in #3 I explained that I wanted this to explore classical counterparts, if any and where they would fail. The both of you have provided a great discussion on just that topic. I could not have asked for a better pro and con discussion. Thanks again. Your input has been invaluable.

 

[akhmeteli]

Except that classical particles do not behave that way.

I don’t want to open this discussion without necessity, but what’s important is that classical objects do behave this way. I reject your “particle” constraint. The OP did not introduce such a constraint, and quantum tunneling is not necessarily “quantum tunneling of particles”, see, e.g., http://prc.aps.org/abstract/PRC/v82/i6/e064607. In principle, quantum tunneling is possible for anything: particles, objects, and what not.

Does your analogy map onto to actual behavior of classical particles? No.

See above.

The mathematical description of a classical particle does not include any properties that could cause it to bend around a barrier. For one, classical particles are points.

See above.

I don't doubt the high jumper. I only refute its relevance.

With all due respect, the OP is a better judge than we are of what is relevant, and he seems to welcome my input (see post 23 in this thread).

That is the entire purpose of this thread.
Are there phenomena that can only be described by QM, and not by classical physics?

With all due respect, the OP is a better judge than we are of what is “the entire purpose of this thread”, and he says very clearly:

Akhmeteli and Davec426913

Let me thank the both of you very much. As usual in the forum both of you are correct. Dave you are correct in saying I initially asked for phenomena classical physics couldn't couldn't explain. But in #3 I explained that I wanted this to explore classical counterparts, if any and where they would fail.

My conclusion: you are right that “That is the … purpose of this thread”, but no, that is not “the entire purpose of this thread”, as the OP is interested in “counterparts” as well. Let me note that I started posting in this thread after OP’s post #3.

Indeed there are.

And I question your “indeed”. I cannot accept your statement without any arguments.

Electron tunneling is one.

First let me note (or reiterate :-) ) that this a new angle, which you introduced. Previously I only wrote on “quantum tunneling” in general. However, I question this new statement as well. For example, in my article published in IJQI (see the postprint at http://akhmeteli.org/akh-prepr-ws-ijqi2.pdf) I showed that the Klein-Gordon-Maxwell electrodynamics (with conserved external currents, if necessary) is pretty much equivalent to modified classical electrodynamics (I am cutting some corners here). The Klein-Gordon equation, which is part of this theory, does describe electron tunneling. The extensions to the Dirac-Maxwell electrodynamics are mentioned in https://www.physicsforums.com/showpost.php?p=3544588&postcount=73

We are talking about the quantum tunneling of particles through a barrier. Have you forgotten?

See above

Please re-read the subject line and most of the OP's posts after that.

In #3, the OP seemed to qualify his request: “I just want to see if I can come up with a classical counterpart or where the classical counterpart fails.” (the emphasis is mine). That is why I wrote what you quoted. Since then the OP gave an explanation in #23, and I stand corrected. Again, he is a better judge of what he wants.

No, the OP does.

akhmeteli, please, before posting again, please review the thread from the beginning. You seem to have missed the entire point of the thread.

As I said, I had some reasons to think the topic is more narrow, but, according to post #23, I was wrong. I apologize.

I outlined my approach to this difficult issue in my comment on your “electron tunneling”.

 

[Drakkith]

Unfortunately I don't believe your article counts as "Classical Physics". Nowhere at any point in time before QM was developed was there a method of explaining tunneling. Your example is exactly correct for something that is not explainable in classical physics. Note that there are specific rules on PF about what is and isn't considered "mainstream" and I doubt your modification of Klein-Gordon-Maxwell electrodynamics would be. But if I'm wrong then please correct me.

Edit: Also, realize that your example primarily fails because it simply isn't even talking about the same concepts. A "counterpart" to tunneling would be to say that Classical Physics shows how a proton or electron or whatever gets through a barrier without the required energy. Replacing a proton with a whole person is like comparing the orbit of the Sun around the galaxy with a child on a merry-go-round. The two are similar only in a few basic ways and neither are a counterpart of each other. We can argue all day long on how relevant it is to compare the two, because they are comparable in certain ways, but in the end quantum tunneling cannot be explained by mainstream classical physics, which is exactly the point.

 

[mathew3]

but in the end quantum tunneling cannot be explained by mainstream classical physics, which is exactly the point.

I don't think this is carved in stone. For instance take F=iV/c where i is current V is voltage and c is the speed of light. Current and voltage are indirectly proportional. As i decreases due to the imposition of the energy barrier it's accompanying voltage V increases boosting it over the barrier. In a closed system, which an electrical circuit may be considered as, power in must equal power out. Now that is a perfectly reasonable classical explanation. Is it true? Has it been experimentally verified? Who knows if its true but one could make the argument that tunneling is its experimental verification. The point is there is a classical explanation. Whether or not it is accepted as THE explanation is quite another matter.

 

[Drakkith]

I don't think this is carved in stone. For instance take F=iV/c where i is current V is voltage and c is the speed of light. Current and voltage are indirectly proportional. As i decreases due to the imposition of the energy barrier it's accompanying voltage V increases boosting it over the barrier. In a closed system, which an electrical circuit may be considered as, power in must equal power out. Now that is a perfectly reasonable classical explanation. Is it true? Has it been experimentally verified? Who knows if its true but one could make the argument that tunneling is its experimental verification. The point is there is a classical explanation. Whether or not it is accepted as THE explanation is quite another matter.

No, the point is that nowhere (that I know of) in mainstream classical physics can you ever explain or calculate the effect of quantum tunneling. Just because one person claims he can by using a modified version of classical physics doesn't mean that A: it is accepted, B: that it is true, and C: that it would be considered "Mainstream Classical Physics" in the first place.

However, I will admit that this really kind of depends on what you want to know and how we define classical physics. It is true that before QM the effect of tunneling could not be explained. If someone has found a way by modifying classical rules does that really still count as being classical? The biggest reason that QM is successful is that it predicts and agrees with different effects that are NOT reproducible using classical physics rules.

 

[DaveC426913]

I don’t want to open this discussion without necessity, but what’s important is that classical objects do behave this way.

I'll call you on that. Please describe, using classical physics, how tunneling works.

I reject your “particle” constraint. The OP did not introduce such a constraint, and quantum tunneling is not necessarily “quantum tunneling of particles”, see, e.g., http://prc.aps.org/abstract/PRC/v82/i6/e064607. In principle, quantum tunneling is possible for anything: particles, objects, and what not.

It is not a constraint*. I have provided an example. The OP asked for examples. Examples are not exclusive or constraining and do not constitute an exhaustive list.

*You sure you're using that word right? If he had asked for "examples of man-made pollution", and I offered up "acid rain" as an example, would you be accusing me of a "rain" constraint?

See above.

You said nothing above, so you still have not made your point.

With all due respect, the OP is a better judge than we are of what is relevant

If the OP were the best judge of what answers he'd get, he would not need to ask the question. The nature of a question is that it solicits input about things one does not already know about.

... he seems to welcome my input (see post 23 in this thread).

The fact that the OP welcome you input does not make your input right. I am bot trying to shut you down; I am simply ensuring that any assertions you do make are within he bounds of accepted science.

I'm still waiting for that classical description of quantum tunneling.

 

[DaveC426913]

I initially asked for phenomena classical physics couldn't couldn't explain. But in #3 I explained that I wanted this to explore classical counterparts, if any and where they would fail.

What is the difference between these two statements?

If classical physics can't explain a phenomenon then it is a description that fails.

 

[mathew3]

No, the point is that nowhere (that I know of) in mainstream classical physics can you ever explain or calculate the effect of quantum tunneling.

I agree that mainstream classical physics has yet to explain the effect of quantum tunneling. It does not necessarily follow that "mainstream" classical physics cannot explain or calculate the effect of quantum tunneling.

Just because one person claims he can by using a modified version of classical physics doesn't mean that A: it is accepted, B: that it is true, and C: that it would be considered "Mainstream Classical Physics" in the first place.

For a scientist, particularly a research scientist, only B, i.e., that it is true- should be of concern.

The biggest reason that QM is successful is that it predicts and agrees with different effects that are NOT reproducible using classical physics rules.

Not quite. The biggest reason that QM is successful is that it predicts and agrees with different effects that are NOT reproducible using some of the available classical physics rules.

No one is questioning the predictive majesty and power of QM.