# Can gravity be hypothetically used to collapse the wave function?

1. Nov 5, 2015

### sahej1030

Sorry if this is a silly question, I was just womdering about it.

So instead of putting Schrodinger's cat in a box we put in a room and instead of realising poison and thus killing it we push it from from one end of the room to the other end.

We will be able to notice that the force of gravity is coming from a different direction.

Is it possible?

(English isn't my first language so I apologize if there were any grammatical errors).

2. Nov 5, 2015

### Dewgale

First of all, how were you planning on making gravity work sideways?

Second of all, by the definition of the whole problem, we aren't inside the room/box, and are unaware of what is happening inside apart from the fact that there is a cat, and something potentially deadly is happening to it. So we can't see the cat fall and either die or survive. How would changing the method of death affect things? Would we be any the wiser of whether or not the cat is dead or alive?

3. Nov 5, 2015

### sahej1030

What I meant to say was that the cat will be in superpostion being alive at both ends of the room but hypothecially the force of gravity between the cat and the observer will be weakened as the distance between them increases.

So can this difference in the force of gravity between the cat and the observer help collapsing the wavefunction as we will know the position of the cat either at one end of the room or the other?

4. Nov 5, 2015

### DrewD

If the information about the cat's position is available to the observer then the wave function "collapses". If the cat's position can be determined by the gravitational field created by the cat, then it will collapse. In practice I am pretty sure this would be nearly impossible to measure.

5. Nov 6, 2015

### sahej1030

> If the information about the cat's position is available to the observer then the wave function "collapses". If the cat's position can be determined by the gravitational field created by the cat, then it will collapse. In practice I am pretty sure this would be nearly impossible to measure.

Thanks for answering, but if the position of something can be determined by the gravitational field created by it and not by its interaction with photons which thus gives it momentum, isn't the uncertainty principle broken?

6. Nov 6, 2015

### ddd123

Interaction isn't even necessary for measurement: https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester . The uncertainty principle only gives a statistical limit to the precision of observables noncommuting to the ones you measure.

7. Nov 6, 2015

### Staff: Mentor

There are a number of misconceptions here. First collapse is not part of QM - its part of some interpretations but not of the formalism:
https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics

The other issue is alive being a macroscopic concept can't be part of a superposition - you cant have a superposition of alive and dead. In Schroedinger's cat, despite the misconceptions promulgated in pop-sci literature, the cat is never in a superposition of alive and dead. The superposition is at the radioactive source - we have a superposition of particle detected and not detected - that particle numbers can be in superposition is a concept from QFT. That observation occurs at the particle detector and everything is common-sense classical from that point on - the cat is alive or dead - period. Nobody seriously considered otherwise. The purpose of the thought experiment was to highlight a blemish in Copenhagen - namely it does not give an unambiguous way to determine what an observation is or exactly where the quantum classical boundary resides. Great strides have been made in fixing that up - but a few issues remain. Nevertheless it is generally thought to be resolved.

Getting back to your original question gravity is so weak it cant act as an observation eg how it could be a position measurement I have no idea.

Thanks
Bill

8. Nov 6, 2015

### Staff: Mentor

That's incorrect.

Your example looks like a variant of the quantum eraser. While the decoherene of an observation can in simple cases be undone it cant work without an interaction. For example the photon goes through a half silvered mirror - that's an interaction.

Thanks
Bill

9. Nov 9, 2015

### Elemental

Well actually, Penrose proposes something like this in his book,The Road to Reality. When the quantum superposition involves a large enough number of particles, through interaction with environment, that the different superpositions cause sufficiently different spacetime curvatures, the system becomes unstable and collapses into a definite macroscopic state. No conscious observer needed. This makes more sense to me than most other interpretations I've heard....

Elemental

10. Nov 9, 2015

### Elemental

And now I see from the list of similar threads at the bottom this was explored before (should have read them first). But I like Penrose's idea, just wonder if it can be credibly established.

11. Nov 10, 2015

### Mentz114

I go with Ballentine who asserts that a state vector is not a statement about one object but a statistical prediction for an ensemble of identically prepared objects. Nothing is ever actually two different things at the same time. There is no collapse and certainly no dead/alive animals.

What you've written seems to me to be weird and unjustified speculation.

12. Nov 10, 2015

### ddd123

Sorry I meant interaction with the measured object, not the measuring photon: do you still disagree with that?

Penrose's is a modified nonlinear SE, so it's falsifiable in principle.

13. Nov 10, 2015

### Staff: Mentor

A measurement is an interaction between what is being measured and what does the measuring.

Thanks
Bill

14. Nov 10, 2015

### DrChinese

This paper may be a little tangential, you be the judge. They look at the effects of gravitational photon-photon scattering (assuming there is such a thing) and how that might be different for entangled photons. Turns out there is a small difference. Observation of that might demonstrate an answer to the OP's question.

http://arxiv.org/abs/1511.01237

The effect of entanglement in gravitational photon-photon scattering
Dennis Raetzel, Martin Wilkens, Ralf Menzel
(Submitted on 4 Nov 2015)
"The differential cross section for gravitational photon-photon scattering calculated in perturbative quantum gravity is shown to be larger for polarization entangled photons than for not entangled photons."

15. Nov 10, 2015

### ddd123

But the half-silvered mirror is not what is being measured.

16. Nov 12, 2015

### Staff: Mentor

We seem to be going around in circles here.

How about you CAREFULLY explain why you think this is a measurement that involves something being measured not interacting with something doing the measuring. If you can then the modern understanding of what an observation is goes out the window and would be big, no really big, news.

Thanks
Bill

17. Nov 12, 2015

### ddd123

I would just be paraphrasing this study: Experimental realization of interaction-free measurement.

18. Nov 12, 2015

### Staff: Mentor

Ok - got it now.

As they explicitly say it is via weak measurements which is something entirely different. Since you are interrogating a region you are in fact interacting with that region and weakly measuring it. From many such measurements you can infer things about the region. There is still interaction going on.

Weak measurements SEEM to violate many rules of QM, but don't really. For example you can measure both the position and momentum of an object at the same time using weak measurements. But the accuracy of those measurement never violates the uncertainty principle.

Thanks
Bill

Last edited: Nov 12, 2015
19. Nov 13, 2015

### ddd123

Got it, thanks!