Is quantum coherence a random event?

[arnell7]

TL;DR Summary

I am a lay person with a question regarding quantum coherence.

How we should understand the randomness of quantum events in the context of the significant role that they apparently play in our macroscopic world. Using processes as superconductivity, super-fluidity, and in Bose-Einstein Condensates researchers have been able to produce macroscopic quantum effects. In addition to these and some other specific laboratory conditions which can produce macroscopic quantum effects there is an ever increasing understanding that macroscopic quantum phenomenon seem to be all around us- in birds, in photosynthesis, and in other biological functions, as well as in cosmology - as in the activity that produces the sun’s energy.

Despite having an indeterminate origin, once a macroscopic quantum event occurs it obviously becomes a deterministic effect that will cause infinite subsequent effects. However, if such quantum macroscopic effects play as crucial a role in our biology as we are discovering, how can they result from an entirely random process? Would not a randomly generated effect be too unreliable to play a key role in the way a living organism functions?

Do macroscopic quantum events indicate that normal quantum decoherence has been overcome? And when such coherence does occur has a non-random event has taken place?

 

[anuttarasammyak]

Summary:: I am a lay person with a question regarding quantum coherence.

Despite having an indeterminate origin, once a macroscopic quantum event occurs it obviously becomes a deterministic effect that will cause infinite subsequent effects. However, if such quantum macroscopic effects play as crucial a role in our biology as we are discovering, how can they result from an entirely random process? Would not a randomly generated effect be too unreliable to play a key role in the way a living organism functions?

Ehrenfest theorem https://en.wikipedia.org/wiki/Ehrenfest_theorem would answer your question to some exent.

 

[arnell7]

Ehrenfest theorem https://en.wikipedia.org/wiki/Ehrenfest_theorem would answer your question to some exent.

I appreciate you responding to my post however I am a lay person and the page you directed me to is essentially a page of equations, in which I am not literate, or perhaps the more correct word is numerate. So unfortunately I have no idea what the Ehrenfest theorem is saying. If you or someone could translate for me that would be great.

 

[PeterDonis]

how can they result from an entirely random process?

"Random" refers to individual quantum events, but the kinds of macroscopic processes you are referring to don't depend on the results of individual quantum events. They only depend on the statistics of very large numbers of quantum events. Those statistics are highly determinate and predictable even if the individual random quantum events are not.

Do macroscopic quantum events indicate that normal quantum decoherence has been overcome?

If by "macroscopic quantum events" you mean things like superconductivity and Bose-Einstein condensation (which are really forms of the same thing), no, in these cases decoherence has not been "overcome", it has been (at least for some period of time) prevented from occurring, by keeping the system sufficiently isolated from its environment.

In most macroscopic events, however, such as biological processes or nuclear fusion in the Sun, decoherence is taking place constantly. These processes, as I said above, rely only on the statistics of large numbers of quantum events, and those events will involve interactions with the environment, which will in turn involve decoherence.

 

[PeterDonis]

If you or someone could translate for me that would be great.

The Ehrenfest theorem basically says that, even if individual quantum events are random and unpredictable, the statistics of large numbers of them will be determinate and predictable, and will, under appropriate conditions, produce behavior that looks "classical"--for example, a thrown baseball is made of a huge number of quantum particles, but its trajectory is still predictable because the randomness of the individual quantum events cancels out and the overall behavior is that of a classical object with a definite position and momentum obeying classical equations of motion.

 

[arnell7]

"Random" refers to individual quantum events, but the kinds of macroscopic processes you are referring to don't depend on the results of individual quantum events. They only depend on the statistics of very large numbers of quantum events. Those statistics are highly determinate and predictable even if the individual random quantum events are not.If by "macroscopic quantum events" you mean things like superconductivity and Bose-Einstein condensation (which are really forms of the same thing), no, in these cases decoherence has not been "overcome", it has been (at least for some period of time) prevented from occurring, by keeping the system sufficiently isolated from its environment.

In most macroscopic events, however, such as biological processes or nuclear fusion in the Sun, decoherence is taking place constantly. These processes, as I said above, rely only on the statistics of large numbers of quantum events, and those events will involve interactions with the environment, which will in turn involve decoherence.

Thank you so much Peter for taking the time to answer my question. Your response has me a bit confused though. Isn't what makes a macroscopic quantum event possible in biological processes and in the nuclear fusion taking place in the sun quantum coherence? I have repeatedly read that for a quantum effect to be seen on a macroscopic level there must be sufficient coherence among the quantum constituents. Decoherence makes no sense to me. Please clarify.

 

[arnell7]

Also Peter thanks for explaining the Ehrenfest theorem too! Isn't that effect commonly referred to as quantum decoherence?

 

[PeterDonis]

Isn't what makes a macroscopic quantum event possible in biological processes and in the nuclear fusion taking place in the sun quantum coherence?

No.

I have repeatedly read that for a quantum effect to be seen on a macroscopic level there must be sufficient coherence among the quantum constituents.

By "quantum effect" they mean something like superconductivity or Bose-Einstein condensation. They do not mean biological processes or nuclear fusion in the Sun.

 

[PeterDonis]

Isn't that effect commonly referred to as quantum decoherence?

Not quite. The effect referred to in the Ehrenfest theorem is just statistical averaging over large numbers of individual quantum events.

Decoherence involves a quantum system interacting with its environment and losing quantum coherence because of the randomness of those interactions. Not quite the same thing, although it is true that for the Ehrenfest theorem to apply there cannot be quantum coherence between the individual quantum events that are being averaged.

 

[arnell7]

"... a thrown baseball is made of a huge number of quantum particles, but its trajectory is still predictable because the randomness of the individual quantum events cancels out and the overall behavior"

"Random" refers to individual quantum events, but the kinds of macroscopic processes you are referring to don't depend on the results of individual quantum events. They only depend on the statistics of very large numbers of quantum events. Those statistics are highly determinate and predictable even if the individual random quantum events are not.

This is very confusing to me. It sounds like in the two quotes above the law of large numbers are being attributed as both the cause of such powerful effects as macroscopic events and also as resulting in ineffective randomness.

 

[Jimster41]

If by "macroscopic quantum events" you mean things like superconductivity and Bose-Einstein condensation (which are really forms of the same thing), no, in these cases decoherence has not been "overcome", it has been (at least for some period of time) prevented from occurring, by keeping the system sufficiently isolated from its environment.

Are you saying quantum coherence is a purely human creation that nature only destroys?

 

[PeterDonis]

It sounds like in the two quotes above the law of large numbers are being attributed as both the cause of such powerful effects as macroscopic events and also as resulting in ineffective randomness.

You keep confusing two different kinds of "macroscopic events". A thrown baseball is a very different kind of thing from a Bose-Einstein condensate. The law of large numbers is only useful in understanding the behavior of the baseball. It is not at all helpful in understanding the behavior of a Bose-Einstein condensate.

 

[PeterDonis]

Are you saying quantum coherence is a purely human creation that nature only destroys?

No.

 

[anuttarasammyak]

unfortunately I have no idea what the Ehrenfest theorem is saying. If you or someone could translate for me that would be great.

Deterministic classical physics is used in various scenes of biology, e.g., motion of individual birds, insects, etc. Probabilistic Quantum physics, through quantum chemistry, is also used, e.g. cell biology, epidemiology, metabolism, etc.
Ehrenfest showed an interpretation how deterministic classical physics are derived from base probabilistic quantum physics. Though there surely is probabilistic nature in physics, it had been tiny so to be disregarded in our daily experiences up to the beginning of the last century.

 

[docnet]

Not quite. The effect referred to in the Ehrenfest theorem is just statistical averaging over large numbers of individual quantum events.

Decoherence involves a quantum system interacting with its environment and losing quantum coherence because of the randomness of those interactions. Not quite the same thing, although it is true that for the Ehrenfest theorem to apply there cannot be quantum coherence between the individual quantum events that are being averaged.

The bolded part is difficult to understand.. could you please explain in layman's terms?

 

[arnell7]

You keep confusing two different kinds of "macroscopic events". A thrown baseball is a very different kind of thing from a Bose-Einstein condensate. The law of large numbers is only useful in understanding the behavior of the baseball. It is not at all helpful in understanding the behavior of a Bose-Einstein condensate.

Yes Peter I now understand that laboratory controlled conditions which produce macroscopic quantum events are different from those naturally occurring events as in solar nuclear fusion and quantum biology. But it seems as if you are accounting for the canceling out of quantum effects due to the large numbers of particles canceling each other out - and also attributing those naturally occurring quantum macroscopic events to the large number of quantum participants. If I am right about this then - how could large numbers account both for quantum macroscopic effects as well as the lack of those effects?

 

[PeterDonis]

The bolded part is difficult to understand.. could you please explain in layman's terms?

If a thrown baseball, for example, maintained quantum coherence between all of its atoms, it wouldn't behave as it does. It would be something like a Bose-Einstein condensate or a superconductor or a superfluid, not a baseball. In other words, it wouldn't behave like an ordinary classical object; it would behave like something that had no analogue at all in classical physics.

The Ehrenfest theorem only applies to things like actual baseballs, that do have analogues in classical physics. In other words, it only applies to things that, because they do not maintain quantum coherence between their parts, behave like ordinary classical objects, like baseballs.

 

[PeterDonis]

it seems as if you are accounting for the canceling out of quantum effects due to the large numbers of particles canceling each other out - and also attributing those naturally occurring quantum macroscopic events to the large number of quantum participants.

Not at all. What you are calling "quantum macroscopic" events do not have to be macroscopic. You can make a Bose-Einstein condensate out of a microscopic number of bosons. (Indeed, just two is enough, strictly speaking.) There is nothing inherently "macroscopic" about these states; the only reason Bose-Einstein condensates of macroscopic size are talked about is that it's a lot harder to maintain quantum coherence when the number of bosons is very, very large, so producing macroscopic Bose-Einstein condensates is a much more impressive technical achievement than producing microscopic or sub-microscopic ones.

By contrast, to get classical behavior at all for an object like a baseball requires the object to be macroscopic--there has to be a huge number of quantum particles involved for classical behavior to be produced by the canceling out of quantum effects due to lack of quantum coherence. You can't make a classical object out of a small number of quantum particles at all. So the two cases are very different.

 

[arnell7]

I really appreciate you taking the time to keep responding. Sadly I don't feel as if I'm much closer to understanding how random quantum behavior can reliably be counted on to produce macroscopic effects in living systems. In fact I'm actually more confused than at the outset when I was under the impression that what accounted for such events was quantum coherence- since I think you have mentioned that is not the case. Perhaps my confusion is a positive sign that learning may be taking place- I don't know. I do wish that there was an accessible way for me to understand how trillions of randomly behaving particles can produce macroscopic effects- not those that laboratory induced, but in the natural world.

 

[EPR]

Several basic points that you absolutely must take into account before delving into specifics:

1. The world is entirely quantum in nature.
2. It looks more classical with size(but it's not!).
3. The quantum nature of everything cannot be avoided, as most processes( depend on this very quantum nature - stability of atoms, Sun's fusion, etc.)
4. Quantum explanations are exact and accurate. Classical ones - a crude approximation under specific conditions. They make sense with big beings like us who don't notice quantum effects or irregularities.
5. Decoherence is work in progress. Not everyone agrees that it explains the transition from quantum to classical.
6. Quantum fields are currently the best available description of how the macro reality works from micro to macro scales.
7. These topics generally don't have a very satisfying resolution for everyone as it is a field of active research.
8. There will be very good answers and arguments that will not make sense to you, due to you being a lay person.

Would not a randomly generated effect be too unreliable to play a key role in the way a living organism functions?

As has been pointed out, the averaging out of many degrees of freedom, produces a classical like outcome. By far, most outcomes stay within the classical like values. It is a crude explanation but FAPP this is true and suitable for non professionals.

When you ask how can it be so, do remember that all systems(even biological) stay entirely quantum the entire time. It is a relatively new framework with new discoveries being made almost daily(all of them confirming the validity of the framework). If it looks like it is classical(due to loss of phase coherence or anything else) - it is not. It's still quantum.

 

[vanhees71]

No.By "quantum effect" they mean something like superconductivity or Bose-Einstein condensation. They do not mean biological processes or nuclear fusion in the Sun.

Well, that's a pretty interesting topic. Of course you have these now classical examples for the quantum behavior of macroscopic objects like Bose-Einstein condensates (which were however realized experimentally only in the 1990ies, leading to a Nobel prize for Ketterle et al) or superconductivity (explained by Bardeen, Cooper, and Shriever finally as a macroscopic quantum effect too) and superfluidity.

In recent years however the observation of other quantum effects for pretty large objects has become possible. Examples are the quantum interference a la double-slit experiments with very large molecules, quantum noise of the mirrors of the LIGO gravitational wave detector, entanglement between vibration modes of diamonds even at room temperature (!).

There's a very nice new article on Quanta on it:

https://www.quantamagazine.org/how-...orld-be-physicists-probe-the-limits-20210818/

As said there, these experiments address the fundamental question, whether quantum theory applies to systems of any size or whether there is some limit. The big problem is of course to avoid decoherence, which destroys the "quantum behavior" of macroscopic objects very efficiently making them behave classical. The really fundamental question is of course, whether quantum theory breaks down at some point when gravity becomes relevant, as said in the article. On the other hand simple predicted quantum effects where gravity is relevant have already been observed confirming these predictions of QT (like the famous textbook problem of neutrons in the gravitational field of the Earth).

 

[PeterDonis]

I don't feel as if I'm much closer to understanding how random quantum behavior can reliably be counted on to produce macroscopic effects in living systems.

The "macroscopic effects" in living systems do not rely on quantum behavior. They are classical effects. The kind of macroscopic effects that "rely on quantum behavior" are things like Bose-Einstein condensates that do not occur in living systems.

If you want to get closer to understanding, you need to pay attention to the above statement, which I have made a number of times now in different ways, and you have said you understand it, but you apparently don't.

I'm actually more confused than at the outset when I was under the impression that what accounted for such events was quantum coherence

There isn't quantum coherence in living systems. Living systems are not Bose-Einstein condensates. You need to stop confusing these very, very different kinds of things.

I do wish that there was an accessible way for me to understand how trillions of randomly behaving particles can produce macroscopic effects- not those that laboratory induced, but in the natural world.

There are possible "natural world" examples of macroscopic quantum effects like Bose-Einstein condensates--for example, some speculations about the deep interiors of neutron stars--but they are very rare, because quantum coherence on a macroscopic scale in the natural world is very rare. The vast majority of things in the "natural world" are classical objects and do not exhibit the kind of "macroscopic effects" you are talking about. So you do not need to understand how such "macroscopic effects" are produced in, say, rocks, because they're not.

 

[PeterDonis]

quantum interference a la double-slit experiments with very large molecules

"Very large" compared to previously experiments, but not compared to macroscopic scales. These molecules are still many orders of magnitude smaller than macroscopic scales.

quantum noise of the mirrors of the LIGO gravitational wave detector, entanglement between vibration modes of diamonds even at room temperature (!).

AFAIK these are very indirect observations, much more indirect than observing an interference pattern in a double slit experiment. By this definition, the existence of macroscopic objects at all is a "quantum" effect, since QM is required to explain why atoms exist and repel each other on small enough scales, so objects composed of large numbers of atoms don't simply implode.

 

[vanhees71]

That's of course true too. Given the atomistic structure of matter we couldn't even explain that stable matter exists at all without quantum theory. That, however, doesn't contradict the fact that this matter almost always behaves as described by classical physics. It's because in almost all cases we are not able to prepare a macroscopic system accurately enough and isolate it from "the environment" sufficiently to observe quantum effects related to coherent superpositions.

When would you say, the experimentalists have demonstrated (or maybe excluded) double-slit interference for a macroscopic object? I'd say if they succeed with these nanoparticles sized $10^8$ atoms, it's pretty close to being "macroscopic".