# Why does anyone think gravity might collapse wave function?

• B
• HomesliceMMA
In summary, the double slit experiment shows that collapse is most closely analogous to whether or not the item at issue (for example, an electron going through a double slit) is measured or not. If its measured, it takes one path (it is collapsed), if it is not measured, it takes all possible paths (wave). This is irrespective of gravity.

#### HomesliceMMA

TL;DR Summary
Gravity cannot be what logically causes collapse of wave function
Why on Earth does anyone, let along Roger Penrose, think gravity might be what causes the wave function to collapse? The most basic experiment in quantum physics, the double slit experiment, shows that collapse is most closely analogous to whether or not the item at issue (for example, an electron going through a double slit) is measured or not. If its measured, it takes one path (it is collapsed), if it is not measured, it takes all possible paths (wave). This is irrespective of gravity.

Why on Earth do people think this?

vanhees71 and Demystifier
HomesliceMMA said:
Why on Earth does anyone, let along Roger Penrose, think gravity might be what causes the wave function to collapse?
Do you have a source for this by chance?

Hi Drakkith! I heard him say it in a debate on youtube, I'll see if I can find that. It sounded so strange to me when I heard it, didn't make any sense. Just googling right now, and found this:

https://www.quantamagazine.org/physics-experiments-spell-doom-for-quantum-collapse-theory-20221020/

You can search for his name - its in paragraph right before the "The Test of Truth" section title. I mean, it would have been totally cool if that was the answer, but I just don't see how anyone could have thought that given the clear correlation between measurement (NOT gravity) and wave collapse.

vanhees71
HomesliceMMA said:
shows that collapse is most closely analogous to whether or not the item at issue (for example, an electron going through a double slit) is measured or not.
That is a common misunderstanding of the double slit experiment, based on physicists' incomplete understanding from early on the last century.

It's not measurement that causes the collapse, it is any irreversible interaction with the environment around the particles. All measurements involve such interaction - this is how the "measurement causes collapse" idea got started - but many other interactions that we wouldn't consider "measurement" also will cause collapse.

Googling for "quantum decoherence" will get you started, but the math involved will be a lot more than you probably want to see. Instead you might try two books that are written for non-professionals and will go a long ways towards clearing up some of the old popular misunderstandings:
David Lindley: "Where does the weirdness go"
Giancarlo Ghirardi: "Sneaking a look at God's cards"

Neither is a substitute for a real textbook, but unlike a real textbook they won't require several years of math beyond calculus to get through.

dextercioby, ohwilleke and vanhees71
Thanks Nugatory! So your - "any irreversible interaction with[in] the environment around the particles" - what does that mean? Could it be gravity? Because if it could be gravity then the electrons should always be collapsed (if there was sufficient gravity to collapse them) or never be collapsed (f there was insufficient gravity to collapse them), whether or not they were measured in the double slit experiment. My original conjecture stands (I think) - how can anyone think gravity is the answer? It just makes no sense to me.

Now, let's say its not gravity, that "any irreversible interaction within the environment" is something else. Let's say that, while the double slit experiment was being carried out, one or both of the slits was moved (let's say there was a machine in the wall that could cause the slits to move within the wall. But still no measurement. Is that an irreversible interaction within the environment? Would the wave function collapse due to the movement of the slit? My guess would be that the wave function does not collapse. But how is that any less of an irreversible interaction than a measurement? The measurement obviously has less "impact" on the electrons or photons or whatever is passing through the slits (whether wave or particle).

Thanks!

And thanks for the references, I definitely don't want to get into the math haha!

Last edited:
vanhees71
HomesliceMMA said:
My guess would be that the wave function does not collapse. But how is that any less of an irreversible interaction than a measurement?
The wave function does 'collapse', at least to some extent. The slits themselves constrain the possible states of the particles, leading to a vastly different outcome than would otherwise be the case if you took the slits away and just let the particles pass through empty space before hitting the detector.

So it's not that one setup is irreversible and the other is not, or that one is a measurement and one is not, it's that moving the slits during the experiment doesn't change it in a way that would collapse the wavefunctions to the point where you know which slit they went through unless you move one so far that the particles simply can't pass through it anymore.

Well, as far as I know at least. Someone please correct me if I'm wrong, as QM is not my strong suit.

vanhees71
HomesliceMMA said:
I heard him say it in a debate on youtube
This isn't a valid reference for PF discussion. You would need to find a textbook or peer-reviewed paper.

HomesliceMMA said:
The most basic experiment in quantum physics, the double slit experiment, shows that collapse is most closely analogous to whether or not the item at issue (for example, an electron going through a double slit) is measured or not.
No, it doesn't. "Collapse" in basic QM, without adopting any specific interpretation, is not a physical process; it's a mathematical adjustment you make in your model when you know what the result of a measurement is and you need to update the model to predict the results of future measurements.

Beyond that, different QM interpretations take different positions on whether "collapse" is an actual physical process or not. Some, like Penrose, who favor interpretations that say it is, have proposed experiments to try to figure out when this process actually occurs; one such proposal that he made at one point was that collapse would occur when different terms in a superposition had gravitational fields that differed by a sufficient amount. As far as I know this proposal never led to anything useful.

Please note that discussion of what different interpretations of QM say about collapse belongs in the interpretations subforum, not this one.

PeroK, nomadreid, vanhees71 and 1 other person
HomesliceMMA said:
I definitely don't want to get into the math haha!
If you take this position, your ability to participate in threads on QM here at PF will be very limited. There is not much that can be usefully discussed about QM without at least some amount of math.

Omega0, Maarten Havinga, Nugatory and 1 other person
Drakkith said:
The wave function does 'collapse', at least to some extent. The slits themselves constrain the possible states of the particles, leading to a vastly different outcome than would otherwise be the case if you took the slits away and just let the particles pass through empty space before hitting the detector.
This isn't what the term "collapse" is taken to mean in QM discussions. With both slits open and no measuring device at either slit, the effect of the slits on the wave function does not involve decoherence and so is, at least in principle, not irreversible; and there is no macroscopically observable "result" that would require you to update the wave function in your model to take it into account.

If you put something at each slit that produces a macroscopically observable, irreversible result when a particle passes (which is what "measuring which slit the particle went through" means), then you would need to update the wave function in your model when you observed a result, and this would be "collapse" in the sense in which that term is used in basic QM (independent of any interpretation).

Drakkith
HomesliceMMA said:
"any irreversible interaction with[in] the environment around the particles" - what does that mean?
It means that decoherence has occurred.

HomesliceMMA said:
if it could be gravity then the electrons should always be collapsed (if there was sufficient gravity to collapse them) or never be collapsed (f there was insufficient gravity to collapse them)
You are misunderstanding the proposal I described earlier, which was made by Penrose (and, IIRC, by others as well). The proposal does not claim that gravity is the only thing that can cause an actual, physical collapse; they are only saying it is one thing that can. They also give more specific conditions under which this proposed "gravity induced collapse" would occur, which I described very briefly and heuristically in post #7.

Drakkith said:
moving the slits during the experiment doesn't change it in a way that would collapse the wavefunctions to the point where you know which slit they went through unless you move one so far that the particles simply can't pass through it anymore.
My intuitive guess is that this is basically correct, but I haven't actually tried to model this mathematically, and I don't know if anyone else has or if anyone has tried to test this experimentally. It would be an interesting thing to test.

Drakkith
HomesliceMMA said:
TL;DR Summary: Gravity cannot be what logically causes collapse of wave function

Why on Earth does anyone, let along Roger Penrose, think gravity might be what causes the wave function to collapse? The most basic experiment in quantum physics, the double slit experiment, shows that collapse is most closely analogous to whether or not the item at issue (for example, an electron going through a double slit) is measured or not. If its measured, it takes one path (it is collapsed), if it is not measured, it takes all possible paths (wave). This is irrespective of gravity.

Why on Earth do people think this?
I agree with you, but I think some people think like this: Collapse is a mystery, quantum gravity is a mystery, so they must be related. This, of course, is irrational, but I believe people think like that because they want to solve the mysteries by using only those concepts that they are already familiar with. They don't want to introduce additional mysterious stuff, like hidden variables, because they know little about this additional stuff so they don't know what to do with it. All this is nothing but a version of the known phenomenon that people with a hammer see everything as a nail. See also about the availability fallacy/error/bias, e.g. here: https://www.techtarget.com/whatis/d..., the availability bias,fact than is the case.

PeterDonis said:
This isn't what the term "collapse" is taken to mean in QM discussions. With both slits open and no measuring device at either slit, the effect of the slits on the wave function does not involve decoherence and so is, at least in principle, not irreversible; and there is no macroscopically observable "result" that would require you to update the wave function in your model to take it into account.
That's an important point.

E.g., take the double-slit experiment with single photons. Then what's usually discussed in this context is the observation of a double-slit interference pattern vs. its absence when enabling to detect through which slit the single photon came. Then there's a clever setup of this experiment, where it becomes clear that it's not the measurement of which-way information (which some people call "collapse of the quantum state", but I'd never use the word collapse when discussing QT at all, but that's an interpretational issue and doesn't belong in the physics-part of the quantum-mechanics forum) that "erases" the interference pattern, but it's sufficient to enable the possibility for gaining which-way information by a measurement.

The setup is simple: You use linearly polarized photons and put quarter-wave plates into the slits. If you orient both of these quarter-wave plates in the same direction, all that happens to each photon's polarization state is that it is unitarily (!!!) transformed to some elliptic-polarization state. If you orient both quarter-wave plates in an angle ##\pi/4## wrt. the polarization direction of the incoming photons both photons get in the same circular polarization state. In this setup there's no which-way information of any form imprinted on the photons, and you'll observe a double-slit interference pattern when collecting many such treated photons on a screen behind the slits.

Now orient one of the quarter-wave plates in an angle of ##+\pi/4## and the other with ##-\pi/4## relative to the polarization direction of the incoming photons. Then, depending through which slit the photon went, it's polarized in a left- or a right-circularly polarized state, i.e., for the photon behind the slit, the polarization state uniquely tells you through which slit the photon came, if this polarization state is measured, but even when not measuring the polarization of the photons behind the slits the double-slit polarization pattern when collecting many so treated photons is completely gone. This means that it's not necessary to really determine the which-way information. It's sufficient to make this determination possible to erase the double-slit interference pattern.

mattt
Penrose's idea was that black hole evaporation involves information loss, wavefunction collapse involves information gain, and that quantum gravity provides a common mechanism. This is described in his book The Emperor's New Mind. It was far from an arbitrary idea, it was a very ingenious speculation, but these days most physicists think that black hole evaporation conserves quantum information after all.

Demystifier said:
I agree with you, but I think some people think like this: Collapse is a mystery, quantum gravity is a mystery, so they must be related.
While reading this thread, I also noticed that gravity is at least irreversible (and unshieldable), a point also made by Freeman Dyson when he pointed out that quantization of gravity might not be ... "required"

Already before Penrose, Lajos Diósi proposed some explicit model where gravity induced collapse, essentially with the same parameter settings estimated by Penrose indendentently, without knowing about the existence of Diósi's model.

ohwilleke
gentzen said:
gravity is at least irreversible
I don't think so. Einstein equations are invariant under the inversion ##t\to -t##. If your argument is based on the idea that black hole is irreversible, note that its time reversed state, the white hole, is also a solution.

Last edited:
Demystifier said:
I don't think so. Einstein equation are invariant under the inversion ##t\to -t##. If your argument is based on the idea that black hole is irreversible, note that its time reversed state, the white hole, is also a solution.
The equations do not have a "##t##" in them. The choice of letters for the independent variables is not a part of them. But more importantly the reversed solutions will violate some energy conditions, so gravity as in the equations plus energy conditions is irreversible.

martinbn said:
The equations do not have a "##t##" in them.
But they have ##x^0##, don't they?
martinbn said:
But more importantly the reversed solutions will violate some energy conditions
No they will not. Even the Schwarzschild solution, which describes the static metric without a gravitational collapse, contains both the black hole and the white hole, as seen from its maximal analytic extension and the corresponding Penrose diagram.

Even Penrose himself confirms it, e.g. in the book "The Road to Reality" at page 687 where he says: "Some comments are called for, concerning this time-symmetrical dynamical determinism. First, the reader may be reassured that it is not substantially invalidated by the framework of either special or general relativity."

The reason why white holes are not physical is not because they violate energy conditions (which they don't), but because white holes require implausible initial conditions. In particular Wald (in his book on general relativity, page 155) points out that white hole requires an initial singularity and argues that "There is no reason to believe that the initial configuration of any region of our universe corresponds to these initial conditions".

Note also that one of the assumptions in the Penrose's black hole singularity theorem is the assumption that a trapped surface has formed. But the trapped surface by definition is an object which is not invariant under the time inversion. For example, wikipedia https://en.wikipedia.org/wiki/Trapped_surface says that trapped surfaces are "spacelike surfaces (topological spheres, tubes, etc.) with restricted bounds, their area tending to decrease locally along any possible future direction", where the key word is "future". So the theorem guarantees that singularities appear in the future, not in the past, because one assumes that initially an object (the trapped surface) forms that behaves in a certain way with respect to the future, not with respect to the past.

Last edited:
Penrose's hypothesis is popular, in part, because he has won a Nobel Prize for his work related to gravity, and also in part, because, if true, it provides one of the most feasible possible means by which one can observationally distinguish between classical general relativity and quantum gravity.

Some published papers discussing the idea sometimes described as the Diósi-Penrose model because the two gravity scholars came to it independently include:
Also closely related are the following papers cited in Podolsky and Lanza (2016) above:
• Kiefer, C. "Decoherence in quantum electrodynamics and quantum gravity." 46 Phys. Rev. D 1658–1670 (1992);
• Anastopoulos, C. & Hu, B. L. "A Master Equation for Gravitational Decoherence: Probing the Textures of Spacetime" 24 (2013). 1305.5231; and
• Hu, B. L. "Gravitational Decoherence, Alternative Quantum Theories and Semiclassical Gravity" 18 (2014). 1402.6584.

Last edited:
mattt, haushofer, nomadreid and 1 other person
ohwilleke said:
Penrose's hypothesis is popular, in part, because he has won a Nobel Prize for his work related to gravity
Not really, his hypothesis was more popular 20 years ago or so, before he won the Nobel Prize. In fact, at that time he was much more famous than today, two years after the Nobel prize.

gentzen and ohwilleke
Demystifier said:
Einstein equations are invariant under the inversion ##t\to -t##.
Some solutions of the EFE are invariant under time reversal, and some aren't. It is true that the solutions which aren't occur in pairs, with each being the time reverse of the other; but our universe is described by one particular solution, not a set of them. The particular solution that describes our universe is not invariant under time reversal.

Demystifier said:
the theorem guarantees that singularities appear in the future, not in the past
Not in the case of an initial singularity, such as the one in expanding FRW models. The theorem applies to those cases as well; you just reverse "future" and "past" in its statement (and similarly with other related terms) and it remains a valid theorem.

PeterDonis said:
Some solutions of the EFE are invariant under time reversal, and some aren't. It is true that the solutions which aren't occur in pairs, with each being the time reverse of the other; but our universe is described by one particular solution, not a set of them. The particular solution that describes our universe is not invariant under time reversal.
Sure, but that's not a specific property of solutions of the gravity equations. The above can be said about all physical theories, e.g. electrodynamics.

PeterDonis said:
Not in the case of an initial singularity, such as the one in expanding FRW models. The theorem applies to those cases as well; you just reverse "future" and "past" in its statement (and similarly with other related terms) and it remains a valid theorem.
Yes, but if you read carefully what I said, you will notice that I was talking about the black hole singularity theorem, not about the cosmological singularity theorem. It is mainly the black holes, not so much the expanding universe, that often create a false impression that gravity fundamentally differs future from the past. In cosmology we normally think that (in the absence of cosmological constant) the Universe in the future might start to re-collapse and end in a singularity. By contrast, in black hole physics, a scenario in which black hole turns into a white hole seems much less plausible.

Demystifier said:
you will notice that I was talking about the black hole singularity theorem, not about the cosmological singularity theorem
They're the same theorem. The theorem can be applied in either direction of time. The only difference is in whether we think an initial singularity of a given type is physically reasonable; for an FRW "Big Bang", we do, whereas for a white hole, we don't.

vanhees71
PeterDonis said:
This isn't a valid reference for PF discussion. You would need to find a textbook or peer-reviewed paper.No, it doesn't. "Collapse" in basic QM, without adopting any specific interpretation, is not a physical process; it's a mathematical adjustment you make in your model when you know what the result of a measurement is and you need to update the model to predict the results of future measurements.

Beyond that, different QM interpretations take different positions on whether "collapse" is an actual physical process or not. Some, like Penrose, who favor interpretations that say it is, have proposed experiments to try to figure out when this process actually occurs; one such proposal that he made at one point was that collapse would occur when different terms in a superposition had gravitational fields that differed by a sufficient amount. As far as I know this proposal never led to anything useful.

Please note that discussion of what different interpretations of QM say about collapse belongs in the interpretations subforum, not this one.
"
This isn't a valid reference for PF discussion. You would need to find a textbook or peer-reviewed paper."

LOL! Let me see what I can find, will get back to you stat!

"No, it doesn't. "Collapse" in basic QM, without adopting any specific interpretation, is not a physical process; it's a mathematical adjustment you make in your model when you know what the result of a measurement is and you need to update the model to predict the results of future measurements."

So wait a second. Are we into semantics again? Do you not think this is true (and please forgive my use of words, I am sure I am being imprecise):

1. You shoot a particle through a double slit;

2. You don't have a measuring device.

3. It hits (or "interacts with", whatever you want to call it) the screen behind the slits.

4. Interference is shown on the screen, despite the single "particle" shot through.

5. Add a measuring device, a single hit on the screen shows instead.

Isn't that the classic double slit experiment? And did not the measuring device turn essentially something that was behaving as a wave into something that was behaving as a point particle (again, forgive the lack of formality in my terms)? And if that happened, how is that not a physical process? To me that is absolutely physical because it caused something to that previously appeared as one thing to appear as another, at the very least.

The only thing I can even GUESS you are saying is that the measurement was really just mathematical because it just showed you where the particle ACTUALLY WAS the whole time. But wasn't that the whole "hidden information" or "hidden variables" argument/thinking that Einstein/David Bohme (or someone lol) made, but the John Bell theorem/tests refuted?

If that is not it I'm more confused than ever LOL

Thank you PeterDonis!!!

PeterDonis, there is some bug (or "feature" haha) in this forum where I cannot read your full reply without clicking on stuff, and I did not click on stuff, so I did not see your whole post before I replied - please don't get mad please if asked about something you already said. Apologies. But my question above stands (I think lol)!

And another related question on the same point - unless you think the "particle" (or object or whatever you want to call it) really was in the position it was measured to be in the whole time (i.e. hidden variables), how could the collapse not be a physical process, at the very least in part?

Another way to ask the question:

Was the item (just going to call it "item" from here on out to not get into semantics) in the measured location the whole time, or did it only become associated with that location (over any other location in its wave function) once you measured it?

If you think the former, how is that consistent with the "hidden variables" explanation of the results being apparently ruled out by tests?

If you consider the latter, how is that not a physical process? I.E., if the latter, it was a WAVE before (or at least acted like a wave), it was a POINT (or at least acted like a point) afterwards. How would that not be physical?

HomesliceMMA said:
Do you not think this is true (and please forgive my use of words, I am sure I am being imprecise):

1. You shoot a particle through a double slit;

2. You don't have a measuring device.

3. It hits (or "interacts with", whatever you want to call it) the screen behind the slits.

4. Interference is shown on the screen, despite the single "particle" shot through.

5. Add a measuring device, a single hit on the screen shows instead.
Not quite right. Probably most important is at step 5: we always get a single dot on the screen. The pattern, whether single-slit diffraction or double-slit interference, builds up over time when we repeat the experiment over and over to get more and more dots on the screen.
The probability of the dot appearing at any given point on the screen can be calculated without ever introducing the notion of collapsing the wave function. Every possible path between the source and that point on the screen contributes a number, possibly negative, to the probability of the dot appearing there; negative contributions reduce that probability. The pattern develops because we get more dots in high-probability areas and fewer in low-probability areas.
The detector makes a difference because the only possible paths are those that are consistent with what the detector has detected. Change the possible paths, we get different probabilities at each point on the screen.

PeterDonis said:
They're the same theorem.
Well, there are actually several singularity theorems. The first one was by Penrose, it was for black holes only. The second one was by Hawking, it was for cosmology only. But then some generalizations have been made which are applicable to both black holes and cosmology.

vanhees71
This may be off topic, but another way in which gravity is irreversable is the formation of black holes. Beyond some point there is nothing one can do to change the fact that there is a black hole.

Back on topic: What are the shortcomings of Penrose's propolsal?

Last edited:
gentzen
martinbn said:
This may be off topic, but another way in which gravity is irreversable is the formation of black holes.
As discussed in several posts above, formation of black holes is indeed irreversible, but it does not mean that gravity as such is irreversible. As in other deterministic physical theories, irreversibility can be traced back to the choice of initial conditions.

Demystifier said:
As discussed in several posts above, formation of black holes is indeed irreversible, but it does not mean that gravity as such is irreversible. As in other deterministic physical theories, irreversibility can be traced back to the choice of initial conditions.
Ok, show me initial conditions which lead to the formation of a white hole.

martinbn said:
Ok, show me initial conditions which lead to the formation of a white hole.
Wald in the book General Relativity discuses it at page 155. Essentially, take a late state of the black hole and in this state replace all future oriented vectors (velocities etc.) by the opposite past oriented ones. If you take this as initial state, you get a white hole.