Quantum: Which improbable macroscopic events are possible?

Question

Basically, the title. Web search had not found pages in top results with similar QA.

E.g. I understand nuclear blast can just end at any time because random chain-reaction has probability of not continuing. Or anyone can go blind for any arbitrary amount of time because none of incoming photons will interact with retina. Am I correct about the above?

What I'm not sure, e.g. is it possible to fall through the floor (body mostly intact) due to some probability of electro-magnetic repulsion of boots to floor (and other layers of the body) go null?

Is there a paper discussing my question in general? Otherwise please answer for situations above. TIA

Answer 1

What follows is an intuitive answer (i.e. it is low on rigour).

The informal (and slightly satirical) totalitarian principle states that any process that doesn't violate a finite number of conservation laws is "compulsory", i.e. will inevitably happen if you wait long enough.

Strictly speaking this is only true for quantum processes, i.e. things you can assign a probability amplitude to in a path integral.

For the examples you mention I am tempted to suggest that, even if we naively accept the totalitarian principle, the number of particles involved makes the likelihood of such events so unlikely as to be practically impossible. However, this is a classical statistical argument like one would use in situations involving the second law of thermodynamics, and not rigorous.

From the perspective of the density matrix formulation of QM, we expect the the off-diagonal elements to go to zero as the macroscopic object decoheres. In a hand-wavey sense this is equivalent to "taking the limit as $\hbar$ goes to 0" in the path integral formulation, where one regains the classical result, i.e. we ignore all the small probability amplitudes one associates with unlikely (but potentially possible) paths.

See also the answer to this question: Macroscopic quantum phenomena and tunneling

Answer 2

In quantum theory the different possible values of a measurable quantity can interfere with one another. The state of any object including the keyboard on which I am typing this isn't perfectly localised at one position or momentum: it has a spread of positions and momenta that are narrow on the scale of everyday life. This approximate localisation is a result of the fact that when information about a measurable quantity spreads to other systems interference is suppressed: an effect called decoherence:

https://arxiv.org/abs/quant-ph/0306072

This effect is heightened for conserved quantities and leads to the probabilities for histories of a macroscopic system being strongly peaked around classical histories:

https://arxiv.org/abs/0903.1802

So quantum theory very strongly suppresses "impossible" histories like the ones you describe to the point where for all practical purposes they are impossible.

UPDATE The original poster made a comment clarifying his question:

The question was not how improbable (expected to be observed) they were, but is something possible at all or not. Another E.g. I point a laser pointer on the wall. Is it possible NOW the point appears on Mars from that cause photons are not full localized?

This question raises some fundamental issues that I didn't think should be discussed in a comment. Photons are indistinguishable from one another: if you permute two photons of the same energy the probabilities of all measurements on the resulting system remain unchanged. This is because a photon is a particular kind of state of the electromagnetic field, which can sometimes be described as having a certain number of units of photon energy in a region: a photon is just one of those units. So there is no such thing as whether a photon on Mars is the same photon as a photon on Earth.

The electromagnetic field's equations of motion are local so a change in the field in one place propagates elsewhere at a finite speed. So a photon emitted from a laser on Earth can only affect the field on Mars after changes in the field on Earth have propagated there.

If a photon appears on Mars at the same time as you switch on your laser on Earth, then that photon is not causally connected to a photon in your laser on Earth.

A more detailed discussion of localisation can be found in Section 5 of this paper:

https://arxiv.org/abs/quant-ph/0112148

As a result of all this, if you want to discuss causality you end up talking about changes of local densities of fields over time and that gets you into discussions of histories, as in the paper I linked in my original answer. The probability of a history where light shines on your retina and doesn't interact with it are practically indistinguishable from zero and drop at least exponentially with the number of non-interactions.

Answer 3

You ask, what is it that is logically possible (according to current physical theory), no matter how improbable. Well, that is hard to answer in a comprehensive way. Even if we exclude physically possible but thermodynamically improbable events, like the proverbial broken glass that spontaneously assembles itself into an unbroken cup that vaults onto a table top and gently lands there, the variety of "events in the world that are possible" is still extremely broad! Numerous outlandish permutations and combinations are physically possible.

Also, if we really were trying to list "everything that is possible" according to current theory, the vast majority of the possibilities would be random-looking arrangements of particles which do differ from each other, but not in any way that is meaningful to a human mind. Humans are especially interested only in highly specific possibilities like "what if all the molecules of air in the room rushed into one corner, leaving me in a vacuum" or "what if lifeless particles happened to coalesce into a 'Boltzmann brain' possessing fake but detailed memories" or "what if the light was still on but none of the photons reaching my eye happened to trigger the light-perceiving cells".

All these (and many other grotesque and outlandish scenarios) are possible, but they are also so unlikely that we would not expect any of them to ever occur, not just in our own universe, but in a googol universes that were governed by the same laws as our own. I mention this because people sometimes get hung up on these possibilities, especially in the context of many-worlds theories or a spatially infinite universe. It will bother them that there are branches of the universal wavefunction where insane things happen or they randomly perform heinous acts, or they will attach especial significance to the idea that they might have an atomically exact doppelganger a googol light-years from here, or they will be bothered by the idea that all possible things happen and all possible choices are made, in a sufficiently vast universe or multiverse.

These topics may be worthy of some philosophical reflection, but in a practical sense, they should occupy a fraction of your attention comparable to their frequency of occurring, which is to say, about a googolth of your time and thought.

If we really are trying to answer the question "what is possible", from a pure physics perspective - it comes down to stating the ontological constituents of the physical universe, and the laws that govern them, and the variety of behaviors that those laws make possible. E.g. one might be interested in the extent to which basic physical properties can apparently fluctuate, and the way that the probability of such a fluctuation depends on the magnitude of the fluctuation.

Answer 4

All modern computers and smartphones utilize quantum tunneling to store data. When you take a photo with your smartphone, in order for the data to be stored the electrical charges representing the data tunnel into a charge trap through a barrier that, from classical physics alone, they could not overcome. This transition happens with a "low but nonzero probability" purely due to the quantum mechanical nature of charge, which is actually harnessed by design as part of the device's function. So every time you take a photo or store a file, you are utilizing this phenomenon.