# I Schrödinger's cat and gravity

#### jon4444

At a high/abstract level, I'm trying to understand how QM treats gravity. What if you set up a Schrödinger's cat experiment so that the cat lost some of its mass when in the "dead" state. So, someone outside the box could "observe" the cat being alive or dead based on change in gravitational force they're experiencing.
From a QM perspective, is this consistent with an observation being made or is QM silent as regards "interaction" based on measurement of gravity?

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#### hilbert2

Gold Member
The box can't lose mass if it is a closed system. If it's not closed, there are many other ways in addition to the mass change how information gets out of the box. The thought experiment requires a complete prevention of information getting outside.

#### Nugatory

Mentor
At a high/abstract level, I'm trying to understand how QM treats gravity. What if you set up a Schrödinger's cat experiment so that the cat lost some of its mass when in the "dead" state. So, someone outside the box could "observe" the cat being alive or dead based on change in gravitational force they're experiencing.
From a QM perspective, is this consistent with an observation being made or is QM silent as regards "interaction" based on measurement of gravity?
As long as the box is sealed so that nothing can enter or leave, conservation of mass and energy assures us that the weight of everything inside it will not change. Conversely, if anything big enough to notice enters or leaves the box, even if it is neither noticed or observed, that interaction with the outside world counts as an observation.

In fact, the detector that opens the vial of poison is itself a sufficiently complex physical system that the interaction between it and the decaying radioactive atom is enough to count as an observation and produce the effect of wave function collapse. Thus, we don't need to complicate the situation by considering weird states in which the cat is neither/both dead and alive - for all practical purposes either the cat has died or it hasn't and we'll find out which when and if we look inside the box. The situation is no more weird than if I were to toss a coin and not look to see how it landed - it's either heads or tails and I don't know which because I haven't looked. For an explanation of how this happens, you could try David Lindley's layman-friendly book "Where does the weirdness go?" or google for "quantum decoherence" (although the math will be somewhat daunting).

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#### StevieTNZ

In fact, the detector that opens the vial of poison is itself a sufficiently complex physical system that the interaction between it and the decaying radioactive atom is enough to count as an observation and produce the effect of wave function collapse.
Qualified, of course, with the remark for all practical purposes (FAPP). In principle the cat is in a superposition. I thought I'd add that, even though you say
for all practical purposes either the cat has died or it hasn't and we'll find out which when and if we look inside the box.

#### bahamagreen

The change in the cat mass could be just a change of position... the live cat is standing or sitting upright with an elevated center of mass, whereas the dead cat is lying on the floor of the box (the cat could be prepared by feeding it some coffee so it would not lie down to nap and confound that position with being dead).

Surely the position of the center of mass of the whole box is something that can be detected externally, so couldn't a well defined change in the vertical component of the center of mass (low to the floor and staying there beyond a certain period) be associated with the live/dead condition of the cat?

#### Nugatory

Mentor
Surely the position of the center of mass of the whole box is something that can be detected externally, so couldn't a well defined change in the vertical component of the center of mass (low to the floor and staying there beyond a certain period) be associated with the live/dead condition of the cat?
Yes, if the center of mass of the box changes in a way that can be detected from outside the box (for example, the distribution of stresses in the surface supporting the box changes) that interaction would be sufficient to cause decoherence and counts as an observation. However, in the case of Schrodinger's cat, this is something of a red herring because the detector, the vial of poison, and the cat are all macroscopic systems that will have decohered whether the center of mass of the box changes or not. Thus, we should think of the center of mass measurement as just an alternative to opening up the box and looking at the cat.

#### Mentz114

Gold Member
Qualified, of course, with the remark for all practical purposes (FAPP). In principle the cat is in a superposition. I thought I'd add that, even though you say
No it is not in a superposition. Fatal poisoning is thermodynamically irreversible in any living organism.

#### StevieTNZ

No it is not in a superposition. Fatal poisoning is thermodynamically irreversible in any living organism.
Aren't you assuming the cat has been poisoned, rather than what QM predicts - which is the poison vial breaks and doesn't break?

Unless decoherence solves the measurement problem so people shouldn't invest any further time because it has been answered? Please advise me of the current situation.

#### Mentz114

Gold Member
Aren't you assuming the cat has been poisoned, rather than what QM predicts - which is the poison vial breaks and doesn't break?

Unless decoherence solves the measurement problem so people shouldn't invest any further time because it has been answered? Please advise me of the current situation.
As Nugatory pointed out, there are equally irreversable processes before the poison is released so it does not affect the argument.

I don't suffer from the measurement problem because to expect unitary evolution to predict the outcome of a measurement that has not been performed is like asking for a formula that tells me whether the coin on my desk will come up heads or tails next time I toss it.
Neither is possible without knowing the entire state of the universe up to the point of tossing the coin or making the measurement.
What happens in a measurement is dynamic and depends on the precursor state. It is not the evolution of probabilities.

#### StevieTNZ

As Nugatory pointed out, there are equally irreversable processes before the poison is released so it does not affect the argument.
Only for practical purposes, though, as admitted in that post.

EDIT: edited to clarify comment.

#### Mentz114

Gold Member
Only for FAPP, though, as admitted in that post.
If something is not 'FAPP' ('for all practical purposes') is it anything but speculation or misinterpretation ?
Anyhow I've given my opinion and this argument can't be resolved by experiment (by its very nature) so further discussion is probably pointless.

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#### StevieTNZ

If something is not 'FAPP' is it anything but speculation or misinterpretation ?
Anyhow I've given my opinion and this argument can't be resolved by experiment (by its very nature) so further discussion is probably pointless.
It is certainly an interesting discussion to have.

#### Demystifier

2018 Award
From a QM perspective, is this consistent with an observation being made
It's consistent.

#### Lord Jestocost

Gold Member
2018 Award
Thus, we don't need to complicate the situation by considering weird states in which the cat is neither/both dead and alive - for all practical purposes either the cat has died or it hasn't and we'll find out which when and if we look inside the box. The situation is no more weird than if I were to toss a coin and not look to see how it landed
One shouldn’t water down the consequences of quantum mechanics. There is a fundamental difference between quantum ignorance – the desired information simply doesn’t exist – and classical ignorance – the desired information exists but is hidden. The knowledge one lacks about the state of the cat before an observation is simply not there to be known, viz. a superposition remains a superposition. The cat is only known to the observer to be either dead or alive, with certain probabilities predicted by quantum mechanics. The assumption that one of the answers is "objectively" realized – in a classical sense - in between the measurement is simply impossible.

#### bhobba

Mentor
In principle the cat is in a superposition. I thought I'd add that, even though you say
Even in principle it isn't. Its entangled with the radioactive source. You work through the math of entanglement and you see its in a mixed state - its either dead or alive - not in a superposition of dead and alive.

This is at the I level, I will presume you can do some simple bra/ket calculations. The following paper does it in a more general context:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

See the section on proper and improper mixtures. Since its in an entangled state the cat is not in a pure state, its in something else if you observe just it. It turns out it is in a mixture - the interference between alive and dead is gone by the interpretation of mixed states (you could even say by their definition), but it certainly follows from what I call the Born Rule E(O) = trace (OP) where O is the observation to determine if the cat is dead or alive. P is the state its in, which is a mixed state of dead or alive.

Now does this explain the measurement problem? That is another thread all by itself - but basically the answer is no - although some may say it does. Read the entire paper I linked to for that one.

Thanks
Bill

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#### bhobba

Mentor
One shouldn’t water down the consequences of quantum mechanics. There is a fundamental difference between quantum ignorance – the desired information simply doesn’t exist – and classical ignorance – the desired information exists but is hidden. The knowledge one lacks about the state of the cat before an observation is simply not there to be known,
Correct so far.

viz. a superposition remains a superposition.
Also correct - but it is entangled with the atomic source - that is what is in superposition - this is a crucial point.

The cat is only known to the observer to be either dead or alive, with certain probabilities predicted by quantum mechanics. The assumption that one of the answers is "objectively" realized – in a classical sense - in between the measurement is simply impossible.
That is an issue you and I have discussed before. You take the view quite legitimately that we do not know anything outside of an observation. I too take that view. But for ease of understanding since it is in a mixed state, under the usual interpretation of mixed states, it is either dead or alive - not in some kind of superposition.

Thanks
Bill

#### StevieTNZ

Even in principle it isn't. Its entangled with the radioactive source. You work through the math of entanglement and you see its in a mixed state - its either dead or alive - not in a superposition of dead and alive.
My apologies, I should have been clearer.

When I say the cat is in a superposition, I take the whole set up -- cat + poison + environment etc as entangled and being in a superposition.

If you only look at the cat in the calculation, then yes, you get a mixed state. But then one must ask are they probabilities of what exists -- Bruce Rosenblum and Fred Kuttner say no, in 'Quantum Enigma'.

#### edmund cavendish

Penrose Diosi suggest the collapse of tbe wave function is gravitationally ( observer independently reduced). Surely the poison is never more than illustrative? For me the issue is of the degree of separation between observer and observed..i find it hard to believe observation is physically instrumental in affecting outcomes more that it is instrumental in which outcome is discovered.

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