RQM - the metaphysics of it all....

[JamieSalaor]

TL;DR Summary

Is there only one universe in RQM? I'm a bit confused on how relative things are...

Hi everyone.

I've been learning about RQM, I actually had a small chat with Carlo Rovelli over email a while ago to further understand what RQM was...

This question is a little philosophical, but it's something I never got round to asking him..

In RQM it appears that at our macroscopic level there is objectivity, as due to decoherence many of the quantum disagreements are washed away.

However, though we do not notice these discrepancies they are surely still there. So, we all see a slightly different world. However, we don't all just see a different world, does that mean inhabit a different world/universe than everybody else...
That would therefore mean I am not actually seeing my friend; I am instead seeing a relative copy of him in my world... Therefore, when I interact with my friend, I do not truly interact with him. We are in different worlds. Would this mean that there are actually two versions of my friend? One in my world and the one in his world?

This is a little disconcerting... I don't know if any of you know of Amanda Gefter's book Trespassing on Einstein's Lawn but this reminds me a bit of her Cosmic Solipsism... That we are all alone in our own universes... Seems a bit too post - modern for my liking..

However.. This might just be me overthinking and misunderstanding.

I don't actually think this is what RQM actually implies, but I would like some further clarification as to why not.. I'll include a couple of things Carlo said to me below..Of course we do not see the same world: I see snow and a squirrel out of my window. Do you?
But if I we are near by and I ask what you see, I see the same as what you say. So, I see no reason to doubt that it is the same world.
If then there are some tiny differences (there are!), so be it. But this does not mean that we do not communicate and simply observe a lot of consistency.

in the macroscopic world the tiny discrepancies are washed away very rapidly and become invisible.
The "discrepancies" are of the order of the Planck constant: very small.I think that the world is objective, is real, is out there. The stones of a geologist are really out there...
Only, what they really are, is just the way those stones affect and interact their surroundings: other stones, the water that hits them, the light of the Sun hitting them.
So, a stone is described the way it affects whatever it interacts with. If it hits my skin, it hits my skin, and it manifests its properties in interacting with my skin.
So, we can describe the stone with variables that capture the way it affects my skin.
The same variables describe how it affects another stone, and so on.
However, quantum theory shows that at small scale we make a mistake that the stone "is" something else than the ensemble of these interactions, because there are tiny discrepancies between the values these variables take when they interact with one object or the other.
Subjectivity, humans, perception, have nothing to do with all that,
It is all physics.
Things are the way they affect other things. We have a clear way of accounting for these interactions, and attributing values to properties "independently from any interaction" misses crucial details that show up in quantum phenomena. It is possible for macroscopic things, but not at the quantum level of precision...

What do you guys think? Would love to hear some opinions.

Thanks!

 

[DrChinese]

Hi Jamie... some good questions. As you seem to be somewhat new to PhysicsForums, I would like to point you to the Quantum Interpretations subforum. Yes, your post is more quantum philosophy than quantum physics. I would recommend this post being moved.

https://www.physicsforums.com/forums/quantum-interpretations-and-foundations.292/

 

[PeterDonis]

I would recommend this post being moved.

Agreed. Thread has been moved.

 

[Halc]

Summary:: Is there only one universe in RQM?

From what I understand, existence is a relation under RQM, so only one universe exists relative to that which measures it. That's precisely the reason I like RQM.

In RQM it appears that at our macroscopic level there is objectivity, as due to decoherence many of the quantum disagreements are washed away.

But that's not objectivity, it's what you see. So maybe somebody measures Germany having won WWII. That's a pretty macroscopic difference, so the victor of WWII is not objective fact. The two of you are obviously not going to measure each other, so neither of you exists relative to the other.

That would therefore mean I am not actually seeing my friend; I am instead seeing a relative copy of him in my world...

This gets into identity, particularly that of your friend. The friend in another world cannot be measured by you, so is not your friend. In fact, you're listening to a past state of your friend (say he's on Titan, hours away), and you send messages to him to be received by a not-yet collapsed (relative to you) version of him in the future. Your friend now cannot be measured, being outside your past light cone, and hence is never in a collapsed state relative to you. You yourself, an extended object, cannot be said to be in a single state for the same reason.

Therefore, when I interact with my friend, I do not truly interact with him. We are in different worlds.

I wouldn't go that far. I consider the interaction to be true. You cannot be in different worlds, else no interaction would take place. There are no other worlds relative to you, so this other-world person is not your friend, but rather an abstraction.

Would this mean that there are actually two versions of my friend? One in my world and the one in his world?

No relation specified, so a meaningless question under RQM. It isn't MWI, so there are not two friends.

Of course we do not see the same world: I see snow and a squirrel out of my window. Do you?

Once I talk to you about the squirrel, the squirrel measured has long since collapsed to the same squirrel as seen by my friend. We cannot see different ones even if we took different measurements.

 

[JamieSalaor]

No relation specified, so a meaningless question under RQM. It isn't MWI, so there are not two friends.

Ok so to rephrase it. Say me and you are looking at a dog, is there multiple versions of that dog? One relative to you and one relative to me? Obviously we all share the same universe in RQM, however, that universe is ultimately relative. But governed by the same laws of physics.

I sort of think of it like this. Say I look at you and obviously I see you pretty much how anyone else sees you, however, the electrons that make up you are in a slightly different position maybe relative to someone elses observation. That sort of implies that though you are made up all the same stuff, that stuff is in a slightly different setup relative to somebody else.. Wouldnt this produce different "copies" of you in different reference frames...

Do you think I'm overthinking this? Or thinking about it wrong?
Thank you!

 

[Halc]

Ok so to rephrase it. Say me and you are looking at a dog, is there multiple versions of that dog?

No. If I see a 2nd dog, it's a different dog, not a different version of the same one.

One relative to you and one relative to me?

You and I cannot see different dogs if we can see each other. If we can't see each other, then neither of us (and our corresponding dog) exists relative to the other, so there's still only one dog.

Obviously we all share the same universe in RQM, however, that universe is ultimately relative. But governed by the same laws of physics.

Yes.

I sort of think of it like this. Say I look at you and obviously I see you pretty much how anyone else sees you, however, the electrons that make up you are in a slightly different position maybe relative to someone elses observation.

Can't be. If two people simultaneiously measure the position of an electron of mine, they'll get the same measurement. It cannot be measured in two mutually contradictory states.

Wouldnt this produce different "copies" of you in different reference frames...

There is only the one frame relative to any given thing. Again, this is not mutli-world interpretation that posits equal reality to states not measured by a given thing.

 

[JamieSalaor]

No. If I see a 2nd dog, it's a different dog, not a different version of the same one.

You and I cannot see different dogs if we can see each other. If we can't see each other, then neither of us (and our corresponding dog) exists relative to the other, so there's still only one dog.

Yes.

Can't be. If two people simultaneiously measure the position of an electron of mine, they'll get the same measurement. It cannot be measured in two mutually contradictory states.

There is only the one frame relative to any given thing. Again, this is not mutli-world interpretation that posits equal reality to states not measured by a given thing.

Ok thank you,
You've completely cleared this for me!
I'm a philosophy student not a physicist, I'm current doing some research for my dissertation on meta-physics and quantum mechanics.
Understanding this stuff qualitatively is really difficult. So thank you.

So, can I summarise to make sure I've got it.
I do not see a different version of you, we see the same version as we get the exact same measurements if we are both measuring something. Therefore I can't see a copy of someone relative to me, we truly see the same world or as Carlo put it "we see the same elephant".
We would see the same dog for the same reason.
RQM is a single world interpretation. Therefore in RQM we cannot be in different worlds... We either share a reference frame and see the same world. Or we are in different reference frames so it's then relative to that frame.

Thank you again,
Sorry if I was confusing!

 

[Halc]

I do not see a different version of you, we see the same version as we get the exact same measurements if we are both measuring something.

Rovelli would say that no system can measure itself, it being a meaningless tautology. It doesn't mean that you can't poke yourself and notice the broken leg or the lack of it, since this is one part of you measuring another. But to go to the cat in the box which (relative to the guy outside the box) is in superposition of live and dead: The cat cannot collapse its own wave function relative to the guy outside. The live cat measures its own aliveness, and the dead one as well. If there's a Wigner's friend in the box observing, the cat state collapses relative to that friend, but the friend cannot collapse himself, so relative to the guy outside the box, Wigner's friend, being unmeasured, is in superposition of having measured a live can and having measured a dead one. In this sense, Wigner's friend (or anybody) cannot have a defined state except as a relation to something else.

I say all this because of your usage of "I" above. I like to drop into 3rd person to describe such situations, so I'd word it as "Two people do not see different versions of each other" which is true in the way that you frame it above: They'll both have measured a given common system the same way, like they'd both have observed a live cat when the box is opened. They cannot have observed different cat states. Note the careful change of verb tense as well. I don't think personal identity carries into the future, thus avoiding relation of one thing with a system not yet measured.

Therefore I can't see a copy of someone relative to me, we truly see the same world or as Carlo put it "we see the same elephant".

Yes, they see the same elephant/squirrel/dog.

RQM is a single world interpretation.

I think maybe all of them are except MWI? And even then, MWI does not posit ontologically distinct universes. That's more of the popular view of it, but the paper on it makes no mention of that.

Therefore in RQM we cannot be in different worlds... We either share a reference frame and see the same world. Or we are in different reference frames so it's then relative to that frame.

OK, you can always word it as a different frame. For instance, back to the example above, Wigner puts two friends in the box with the cat. Those two friends (relative to the guy outside) are in superposition of both seeing a dead cat and both seeing a live one. There is no valid state where one friend measures live and the other measures dead. So relative to the guy outside, the two friends inside define a pair of frames, just like MWI does.

Since no system can measure (collapse) itself (because collapse of a system is a relation with something outside the system), the universe, being a closed system, cannot collapse at all, and thus does not have any kind of real state. In this sense, RQM breaks down into MWI if you pan back the observation far enough. The difference then becomes MWI positing the reality of the universal wave function, and RQM not since there is nothing relative to which it can be said to be real.

 

[PeterDonis]

Rovelli would say that no system can measure itself, it being a meaningless tautology.

But what counts as a "system"? Am I, a physical object containing some ##10^{25}## (at least) atoms, just one "system"? Or am I ##10^{25}## systems? If I'm the latter, I can obviously "measure myself", and will be doing so constantly as my atoms interact with each other. That seems like the most reasonable way to view what is going on with macroscopic objects.

 

[JamieSalaor]

OK, you can always word it as a different frame. For instance, back to the example above, Wigner puts two friends in the box with the cat. Those two friends (relative to the guy outside) are in superposition of both seeing a dead cat and both seeing a live one. There is no valid state where one friend measures live and the other measures dead. So relative to the guy outside, the two friends inside define a pair of frames, just like MWI does.

Thank you, I think I am getting my head around this now..
Final thing then. The two friends that Wigner put in the box see the same cat. That cat is either alive or dead for them. They have an answer. The cat and the two friends share a frame?

Wigner on the other hand has no idea about the state of the cat and therefore the state of his friends.
So he is in a different frame to them until he opens the box and sees the cat and his friends.
The thing is then, if let's say that cat is alive and his friends see that in the box.
What happens to the frame where the cat is dead. Or did that frame never exist?

I understand you can't measure yourself. But that cat knows before my two friends if it is alive or dead.

But anyway to conclude. Wigners two friends in the box with the cat share a reference frame. Wigner is in a different one. They are NOT in a different universe.

 

[JamieSalaor]

But what counts as a "system"? Am I, a physical object containing some ##10^{25}## (at least) atoms, just one "system"? Or am I ##10^{25}## systems? If I'm the latter, I can obviously "measure myself", and will be doing so constantly as my atoms interact with each other. That seems like the most reasonable way to view what is going on with macroscopic objects.

Yes I would have thought my particles would always be measuring me just as how the cat knows if it's alive or not (well maybe not know if it's dead)... However, when my friend measures me he measures me as I measure me... Our measurements yield the same result. We see the same world...

 

[Halc]

But what counts as a "system"?

Surely you know this. Anything can be a system, be it a particle, rock, person, galaxy, or universe. A system is anything you care to draw a circle around.

Am I, a physical object containing some ##10^{25}## (at least) atoms, just one "system"? Or am I ##10^{25}## systems? If I'm the latter, I can obviously "measure myself"

I gave an explicit example in my post about what is meant by the fact that you cannot. See the bit about Wigner's friend (or the cat) being unable to collapse the state of the system in the box relative to the observer outside the box. That's what Rovelli means by a system not being able to measure itself.

Thank you, I think I am getting my head around this now..
Final thing then. The two friends that Wigner put in the box see the same cat. That cat is either alive or dead for them. They have an answer. The cat and the two friends share a frame?

Relative to each other, the two friends share the only frame in existence. Relative to the guy outside, there are two states in superposition, each of which comprises a pair of friends that observer one unique state.

Wigner on the other hand has no idea about the state of the cat and therefore the state of his friends. So he is in a different frame to them until he opens the box and sees the cat and his friends.
The thing is then, if let's say that cat is alive and his friends see that in the box.
What happens to the frame where the cat is dead. Or did that frame never exist?

The wave function collapses relative to Wigner (I never actually called the guy outside 'Wigner' since it's not his interpretation * we're talking about, only the experiment he proposes). On that collapse, the two states in superposition (which you're calling frames) collapses into one measured state and the other state ceases to be a possibility of being measured. Neither state (frame) ever actually existed because that presumes the principle of counterfactual definiteness (PCD: that a system has a defined state in the absence of measurement), and RQM like any local interpretation (holds to principle of locality) denies PCD.

* Wigner gave a special role to human consciousness, saying only consciousness causes collapse. No other interpretation suggests any such thing, allowing a measurement to involve any interaction between systems of any sort. Even Wigner himself abandoned his interpretation due to it being driven to solipsism.

I understand you can't measure yourself. But that cat knows before my two friends if it is alive or dead.

The friends have no particular delay in observation of the poison vial being broken or not, so they'll probably know the cat's fate (the wave function will collapse relative to them) before the ill-fated cat even begins to die.

But anyway to conclude. Wigners two friends in the box with the cat share a reference frame. Wigner is in a different one. They are NOT in a different universe.

Technically, what you call a reference frame is relative to a physical event, a point in spacetime at a defined state, and all of the state of the universe that has been measured at that event (and thus contributes to the cause of that event). On a macroscopic scale, I agree with your comment.

On a quantum scale, my friend and I are defined by different events. Say we're 5 meters apart and some nucleus in my finger decays, a quantum thing. Relative to the event of my center of gravity shortly after that decay, the decay has definitely happened, but relative to my friend relative to whom that decay is outside his past light cone, I am in superposition of a finger with a decayed atom and one where it hasn't happened. Nobody notices this kind of thing since the collapse happens a couple nanoseconds later

 

[JamieSalaor]

On a quantum scale, my friend and I are defined by different events. Say we're 5 meters apart and some nucleus in my finger decays, a quantum thing. Relative to the event of my center of gravity shortly after that decay, the decay has definitely happened, but relative to my friend relative to whom that decay is outside his past light cone, I am in superposition of a finger with a decayed atom and one where it hasn't happened. Nobody notices this kind of thing since the collapse happens a couple nanoseconds later

The reason for that is because the information will need to travel at the speed of light to your friend.
So say I measure the decay of a nucleus on Earth, my friend will only actually 'see' a superposition of decayed/not decayed until the information reaches him.

Thank you again so much. This has really cleared a lot up.

 

[JamieSalaor]

Can't be. If two people simultaneiously measure the position of an electron of mine, they'll get the same measurement. It cannot be measured in two mutually contradictory states.

There is only the one frame relative to any given thing. Again, this is not mutli-world interpretation that posits equal reality to states not measured by a given thing.

The thing is I don't see how this is that much different that the copenhagen interpretation...
Things are not in defined states until measurement. A measurement takes place say on the moon I won't know about it until the information reaches me
Anything can collapse the wave function
We all see the same measurement outcomes...
Sounds a lot like how I understand copenhagen

Where does this differ to Copenhagen?
Or is this sort of a reformulation of Copenhagen?

 

[PeterDonis]

See the bit about Wigner's friend (or the cat) being unable to collapse the state of the system in the box relative to the observer outside the box. That's what Rovelli means by a system not being able to measure itself.

Ah, got it. So I can measure myself relative to myself, but not relative to you unless you and I interact.

 

[JamieSalaor]

Also. I just had a read of the Stanford Encyclopedia of Philosophy article on RQM and it says this:
"Measuring the spin projects the state on one single branch of the two, and both branches lead to consistency. Therefore, as long as we do not chase subtle interference phenomena hidden behind decoherence, RQM implies that we all ‘see the same world’."
What does subtle interference phenomena mean. Also, if we were to chase this and see discrepancies would we be able to discuss these differences?

 

[Halc]

The thing is I don't see how this is that much different that the copenhagen interpretation...
Things are not in defined states until measurement.

Both are local interpretations, so the lack of counterfactual statements is an inevitable similarity.

A measurement takes place say on the moon I won't know about it until the information reaches me

Copenhagen was originally framed as an epistemological interpretation, which means the wave function represents what we know and not what is. Most interpretations these days are ontological interpretations meaning they're descriptions of what is, not just of what is known.
So if I open the box with the cat and quick close it without looking, the state inside the box definitely collapses into dead or alive even if I never checked to see what it is. There is no superposition of those states any longer (except in said epistemological interpretation). It becomes like the 3-card Monty game where you don't know where the lady is despite it factually being one of the 3 cards.

Anything can collapse the wave function

The wave function of system X relative to external system M can only collapse by a measurement taken by M of system X. So not just anything can do that. Yes, it is a lot like Copenhagen except for the superposition being expressed as a relation. There's just system X separated from the rest of the universe by an arbitrary Heisenburg cut, which is a Copenhagen term.

Also. I just had a read of the Stanford Encyclopedia of Philosophy article on RQM and it says this:
"Measuring the spin projects the state on one single branch of the two, and both branches lead to consistency. Therefore, as long as we do not chase subtle interference phenomena hidden behind decoherence, RQM implies that we all ‘see the same world’."
What does subtle interference phenomena mean. Also, if we were to chase this and see discrepancies would we be able to discuss these differences?

OK, for one, that article calls them 'branches' rather that worlds, frames, or states. That's fine. I don't like 'worlds' because MWI sort of has copyright on it. I don't like 'frames' because it sounds like the same word as used in Einstein's relativity theory, used to reference a chosen coordinate system. 'Branches' is reasonable.

Interference phenomena refers to not actually measuring something. So I run a photon through the double slits and both observer agree on where the thing is eventually measured. But neither of us has measured which slit it has gone through, so is it still in superposition of having gone through the left one and the right one? That remains forever unmeasured, and thus there's no factual answer to it, known or otherwise. I think that's what is being referred to in that article. It was measured at point P and that's the end of the story. There's no 'how it got there' to it.

 

[JamieSalaor]

Hi again everyone
I was reading Sabine Hossenfelders blog on RQM and she came up with the following paradox

But suppose A has a dog, and he agrees with B to kill it when he measures +1. A and B separate, are out of causal contact. Both measure +1. A kills the dog.

Then he comes back into causal contact with B, and of course he takes the dog, which is nothing but a macroscopic result of a quantum measurement. But no matter what, B will always have to find that the dog is alive.

It seems pretty clear she's misunderstanding RQM.
Can anyone explain where she's going wrong?

 

[Halc]

Can anyone explain where she's going wrong?

First, the story seems wrong. If +1 is measured, the dog dies, and yet the dog is described as alive at the end, a contradiction. 'A' seems to have lied when he said what he'd do upon measuring that. Positing an impossible measurement is not a paradox, it's just wrong.

Secondly, the two are said to be out of causal contact and yet both are capable of measuring the same thing, which again seems to be a contradiction.

Third, the problem doesn't seem to invoke any interpretation at all and hence isn't a misunderstanding of RQM or any of the others. It seems to be based on empirical observations and hence is a QM exercise, not an interpretation of one.

The second problem can be solved by using entangled particles which can allow them to measure a common thing without being near each other. No mention is made of them doing that. If A measures +1, then B is going to measure -1. Both will instantly know what the other is going to report as his measurement once they done their own measurement.

So A measures +1 and kills the dog. B must therefore measure -1 (any time, including before A does his measurement) and from that know that when he meets A (and assuming A has done his measurement), A will have a dead dog.

 

[JamieSalaor]

First, the story seems wrong. If +1 is measured, the dog dies, and yet the dog is described as alive at the end, a contradiction. 'A' seems to have lied when he said what he'd do upon measuring that. Positing an impossible measurement is not a paradox, it's just wrong.

Secondly, the two are said to be out of causal contact and yet both are capable of measuring the same thing, which again seems to be a contradiction.

Third, the problem doesn't seem to invoke any interpretation at all and hence isn't a misunderstanding of RQM or any of the others. It seems to be based on empirical observations and hence is a QM exercise, not an interpretation of one.

The second problem can be solved by using entangled particles which can allow them to measure a common thing without being near each other. No mention is made of them doing that. If A measures +1, then B is going to measure -1. Both will instantly know what the other is going to report as his measurement once they done their own measurement.

So A measures +1 and kills the dog. B must therefore measure -1 (any time, including before A does his measurement) and from that know that when he meets A (and assuming A has done his measurement), A will have a dead dog.

Thank you for your reply!

My final question is after all this why is RQM often accused of solipsism?
Its obviously not..

 

[Halc]

My final question is after all this why is RQM often accused of solipsism?
Its obviously not..

Where is that stated? I think you're thinking of the Wigner interpretation, which is hardly RQM.

 

[JamieSalaor]

Where is that stated? I think you're thinking of the Wigner interpretation, which is hardly RQM.

Just other forums but people talk a lot of rubbish I reckon...
Thanks for everything!