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The actual paper is here: https://link.aps.org/doi/10.1103/PhysRevLett.129.181301
The summary says almost nothing, so here’s what I got from the abstract. First some context: we currently don’t have any physics theory that predicts the gravitational field of a particle in a superposition. This isn’t due to a lack of ideas for theories, but in large part because the predicted effects are much too small to run experiments, for at least a few more decades.
The paper is about a simulation based one candidate theory for such a gravitational field. The interesting result is that the simulation yields a range of black hole masses predicted by the theory, even though the authors didn’t specifically plug that assumption in. This could be an argument in support of the theory. (But could also be a coincidence, or stem from some underlying math that the researchers didn’t think about, that also underlies the mass prediction, which would mean the result is a tautology.)
The article is annoyingly superficial.
It says: "This [superposition] is most commonly illustrated by Schrödinger’s cat, which can be both dead and alive simultaneously."
Schroedinger used the cat example to illustrate his view that a particle being in two mutually-exclusive states at the same time was absurd. He thought it was obvious that the cat cannot be both dead and alive simultaneously.
it's absurd because pretty
much every particle of the cat is measured and thus decohered by other particles in the cat
In the cat thought experiment, the "box" needs to be conceived of as something that totally isolates you from the cat. No "quantum influences" travel across it. Otherwise it has no point.
This is obviously impossible... hence, "thought experiment".
If you want to make the argument this makes it ultimately harder to understand rather than easier, well, I'd say the evidence overall would favor that. It seems to confuse more than it casts light on anything.
But if we did have such a box, then the cat could be alive and dead simultaneously. The dead portion of the cat superposition would consistently see the cat as dead, the living portion would consistently see the cat as alive. From within the box nothing could collapse the superposition any further. Once opened you would see only one of them. People are bothered by the macroscopic implications of this violation of our intuition of how the universe works. It is not entirely clear to me that such a violation could ever manifest in reality, though; that is one heck of a box, in a universe with gravity, neutrinos, dark matter, etc.
> In the cat thought experiment, the "box" needs to be conceived of as something that totally isolates you from the cat. No "quantum influences" travel across it.
Is that true? I’ve seen references to observed macro superpositions without such a box: https://en.m.wikipedia.org/wiki/Quantum_superposition
> A double slit experiment has been performed with molecules as large as buckyballs and functionalized oligoporphyrins with up to 2000 atoms.
> An experiment involving a superconducting quantum interference device ("SQUID") has been linked to the theme of the "cat state" thought experiment.
> A piezoelectric "tuning fork" has been constructed, which can be placed into a superposition of vibrating and non-vibrating states. The resonator comprises about 10 trillion atoms.
You're right about the experiments, but you underestimate how many orders of magnitude you are away from a cat. Not only is there the orders of magnitude in terms of atomic size, you also have to consider the orders of magnitude of time as well, since the thought experiment implicitly takes place at human time scales, where we are presumably waiting at least a couple of seconds to open the box. Adding them both together probably puts us a good 15-20 orders of magnitude away from a cat. It is difficult to imagine what box could isolate something that large that thoroughly for that period of time; it gets multiplicatively harder as you add those constraints, if not exponentially (depending on the circumstances), not merely additively.
It was not a gedanken experiment, though; it was a joke. The box is totally irrelevant, because a cat in a box is not in a box, because the cat is not isolated, because the cat is actually a compound system already. If you had a single particle in a box, okay, yes, that's valid.
The cat would not be alive and dead simultaneously regardless of the box because it's totally entangled with and decohered / observed by the other elements in the box.
A single particle, or a coherent system, can be thought of as being isolated though. Do you see what I mean
we have been able to isolate and keep coherent increasingly large systems. Isolating a cat is not beyond the realm of the theoretically possible.
it would not be a cat then.
> This is obviously impossible... hence, "thought experiment".
It seems like a four way black hole merger could possibly leave a space in the middle.
maybe you edited your post but thought experiments are not supposed to be impossible at all. they're supposed to be realistic. that's why we consider them "experiments". Einstein's photon in a moving box counting time gedanken experiment was used to illustrate very real physics
Impossible -> gedankan experiment != gedanken experiment -> impossible.
yeah you're right but the cat thing isn't a gedanken experiment because gedanken experiments need to start with rigorous statements of physical systems. the cat example is not remotely rigorous. if it were stated as such then it would be seen to be trivial
gee wilikers, you solved it!
you act like I wasn't just talking with someone here on HN who was convinced we "live in a classical world" when in reality everything is governed by the same quantum physics on every scale.
> you act like I wasn't just talking with someone here on HN who was convinced we "live in a classical world"
I didn't understand your original reply to my comment; now I understand why, I think.
Perhaps I failed to express myself clearly? I didn't say anything about quantum vs. classical worlds; I made a remark about the purpose for which Herr Schroedinger devised his cat story. His purpose was the opposite of the purpose implied by the article. He thought that the absurdity of a cat being both alive and dead demonstrated the absurdity of a particle being in two states at the same time.
That is, I was talking about Herr Schroedinger's opinions, not my own. IANAPP, but FWIW I'm convinced that quantum theory in the sense of evolution of the wave function is correct. I don't know of any convincing explanation of wave-function collapse as a result of "observation". I don't buy many-worlds, and "just shut up and solve the equations" doesn't cut it for me.
So in fact I'm convinced that we don't live in a classical world.
no, no! it was an entirely different person here almost 2 wks ago who was arguing with me about whether quantum mechanics can be understood without the math, all while making claims that showed how little he knew of the topic. he might have been trolling anyway. In my reply to you I was merely agreeing, herr denton-scratch. many worlds doesnt (afaict) posit why/how branch selection even occurs so it's sort of incomplete IMO. wavefunction collapse doesn't seem to exist either, but rather is a result of entanglement with the observer. my personal guess is something like superdeterminism in principle but I keep an open mind. I suspect that there are some basic consequences/meanings of the existence of state/information and entanglement themselves which have been too subtle to have been seen as assumptions in the past, and I am slowly working on formalizing them.
I really like the interpretation that if the cat is dead, it can't measure anything and is therefore reliably alive (quantum immortality). Of course, that doesn't change that someone else can observe the cat being dead after opening the box.
At this stage I don't think very many philosophers consider measurement as it takes meaning in quantum mechanics to have anything in particular to do with living creatures or brains or consciousness or anything.
I’ve heard black holes described as “more diameter than the circumference suggests” — to paraphrase Kip Thorne.
Is there a point where you’re at “quantum scale” around the singularity but not across the singularity? — like a quantum Einstein ring?
> I’ve heard black holes described as “more diameter than the circumference suggests” — to paraphrase Kip Thorne.
I'm not sure where Thorne said whatever you are paraphrasing, but this description does not apply to black holes. It applies to ordinary gravitating bodies like stars and planets, but unlike them, black holes do not have a well-defined "diameter". The singularity inside the hole at r = 0 is not a place in space, it's a moment of time. So there is no well-defined "spatial distance from the singularity"; that would be like having a well-defined "spatial distance from tomorrow", which makes no sense.
Seems a point in space-time, like a point in (longitude, latitude) where at latitude 90, longitude loses meaning.
Not really, no. What you are describing is a coordinate singularity; but whether a coordinate singularity has a physical meaning or is just an artifact of the coordinates depends on the situation.
In the case of the Earth, the coordinate singularity in longitude at the poles is an artifact of the coordinates: the Earth's poles are perfectly good parts of the Earth's surface and nothing physically goes wrong there. You just need to pick different coordinates if you want to cover the poles.
The case of the horizon of a black hole in Schwarzschild coordinates is similar: there is a coordinate singularity, but it's an artifact of the coordinates. You can find other coordinate charts that have no coordinate singularity at the horizon.
The black hole singularity at r = 0, however, is not an artifact of coordinates. It is a genuine physical singularity, where the spacetime curvature goes to infinity.
Thanks for the explanation of the difference.
To be fair we don't know the physical curvature goes to infinity--the math says it does.
> we don't know the physical curvature goes to infinity--the math says it does
More precisely, the math of the theory that works everywhere we can test it, when extrapolated to r = 0 inside a black hole, where we cannot directly test it, says the curvature increases without bound as r = 0 is approached. The mathematical process of extrapolating a well tested theory into a domain where we cannot directly test it is used all the time in science; there's nothing inherently questionable or unreliable about it.
The issue in this particular case is that the extrapolation gives us an answer that has a problem: it doesn't seem physically reasonable to have curvature increasing without bound in a finite time, along a finite arc length on the worldline of some observer, at which point spacetime just ends and the observer is destroyed. So most physicists believe that GR, the theory, will actually break down somewhere before this point is reached, and we will need a new theory, which most physicists believe will be a theory of quantum gravity, to tell us what actually happens in this regime.
But you're correct that we won't know for sure unless and until we have some way of testing what happens in this regime.
Are you sure that quote isn't referring to the length dilation that occurs in a gravity well? Space is non-Euclidean so you can have a sphere with a radius larger than the circumference would allow.
His quote is exactly referring to that: but my question is, are black holes curved enough that you can have a circumference at quantum scale with a diameter above that scale? — ie, at which quantum effects would happen around but not across the singularity.
What do you mean by "across the singularity"? The singularity is a single point in the center. There's no being across it really.
When the shortest path lies on some sphere around the singularity where the circumference of that sphere is “quantum scale” but chords passing near the center are not.
In the 2D example, is there some area where quantum effects happen in an annulus rather than in a disc because that central stretched region is “too big”.
> some sphere around the singularity
There is no such thing. The singularity is a moment of time, not a place in space.
The event horizon is an example of a sphere around the singularity. So is the photon sphere.
No, this is not correct. The singularity is not a place in space, so it is not possible to have a sphere around it. It makes no sense, just as having a sphere around, say, tomorrow noon makes no sense.
Perhaps instead of just saying what standard explanations are “make no sense”, you could try elaborating in any of your posts.
Saying that an event horizon or photon sphere isn’t around the singularity of a black hole doesn’t make sense, either.
What facts specifically are you trying to articulate about spacetime geometry around a black hole?
> The singularity is a single point in the center.
No, it isn't. It's a spacelike line that is to the future of every event inside the hole. It's a moment of time, not a place in space.
I'm sure that's right, but on the other hand space itself is getting "denser" nearer to the singularity. I don't know what to make of that.
> space itself is getting "denser" nearer to the singularity
Where are you getting this from? It makes no sense.
What is the gravitational force at the singularity? How does that reconcile with the maximum force?
c^4/4G
Huh? What are you saying: you don’t think there are black holes?
Edit: the page you linked is a random dude who keeps publishing on a single topic that mainstream physics has been mostly ignoring. I wouldn’t go as far as call him a crackpot, but he’s definitely in the fringes. After scanning the site I still have no idea what he’s on about, and a cursory check of published literature seems to support this is at least not an idea most people in mainstream physics would recognize.
Oh there are black holes. We even have videos [1]. But are they infinitely dense at the singularity or is there a limit?
AFAIK, most mainstream physicists assume the singularities are an artifact of the incompleteness of General Relativity, and that they won't exist in a theory of Quantum Gravity. Singularities and infinities in physics tend to indicate problems with a theory, they're the reason it's common to say that physics "breaks down" inside black holes.
I mean, look - arguing about what happens inside the even horizon is kind of fun but ultimately pointless. Just FYI, the site you linked in the parent comment does not represent anything I’d call mainstream physics.
this is better [1]
Not really, that’s just the same guy writing a PDF. It’s not cited, peer reviewed or published in a journal. It’s literally just a dude writing stuff on the internet - that’s not necessarily a problem, but there are literally tens of thousands of people with “great ideas” that marry GR and QM, and none of their theories go anywhere, or make any testable predictions.
He makes some bold claims, no predictions and doesn’t seem to get published, which strongly indicates he can’t get past peer review.
Thanks for this and for shooting it down. There is enough wild stuff on the internet that it doesn't need to be perpetuated further.
I read this maybe a couple years ago and it seemed legit, but I'm not a physics guy and didn't really research it more or notice it's just one guy. The idea of a maximum force is interesting in that it would put limits on some theories.
Our theories break down at the singularity and we don't know what happens there. That's part of why quantum gravity is such a big topic of research.
Gravity is not a force in GR; it's spacetime curvature.
I don't know where this "maximum force" thing is coming from, but it isn't from actual physics.
The force at this point is mathematically undefined except if it is 0: What could be the direction of the force ?
I would assume we are like living pixels on a computer monitor, and black holes are the boundaries of the display viewport.