The Contest Between Gravity and Quantum Physics
nautil.us
nautil.us
A spinning astronaut in a universe without stars can discern whether she or her partner is spinning, particularly if they work together a little.
The astronaut can change her moment of inertia by curling into a ball or spreading out: if the astronauts' relative rotation rate changes, she's rotating. If it doesn't, she's not.
Things are more complicated if they're both rotating, but with mental math and freshman/high-school physics, they'll be able to sort it out completely.
Even in a universe without any external reference, it's possible to place limits on your rotation rate. If a centrifugal force is palpable when extending an appendage, then you're rotating.
> The distinction between the two astronauts makes sense only when you reintroduce the rest of the universe. In the classic interpretation of general relativity, then, inertia exists only because you can measure it against the entire cosmic gravitational field.
This seems like a case of a spherical cow[1], the thought experiment is simplified down to only these two astronauts. No fabric of space around them, and so no inertia.
Perhaps it would have been less confusing if the thought experiment had used two normal people floating in nothingness? The presence of spacesuits makes it seem like they exist within some sort of reasonable environment.
The current, locally-determined, laws of physics would produce the same outcomes for our rotating astronauts with or without the presence of anything else in the universe.
The way I frame the problem is generally by imagining a gyroscope (say, a spinning bike wheel) somewhere in the universe far away from any galaxy or massive body. Relative to which frame of reference will it maintain orientation?
But imagine two astronauts in empty space and let's turn one upside down stacking them head to head, or helmet to helmet. Now spin one of them around his vertical axis [1].
The situation looks completely symmetric to both of them. Why should the relative speed of rotation change if one of them extends or retracts his limbs but not if the other one does the same thing? What breaks the symmetry of the situation in absence of any other matter in the universe that could be used as a preferred frame of reference for defining spinning versus being at rest?
[1] I am not sure if you could do that, but I tend to say no. One of the astronauts would have to try to spin the other one which should make himself spin in the opposite direction, conservation of angular momentum. Ad hoc it seems like you could establish a preferred frame of reference, a frame of zero angular momentum this way.
You're absolutely correct that I'm assuming the current understanding of how inertia works. It would be fascinating if we were to somehow perform the experiment and didn't get the usual result.
Your symmetry argument is excellent and well-stated. I have no complete counter.
To extend the argument: If just one astronaut is in an otherwise empty universe, can she tell whether or not she is spinning at 100 RPM by extending her arms to sense centrifugal force? The usual laws of physics would say that she can. A laser-gyro should work too. I think Mach would guess that she can't.
Because I can't make an empty universe, I don't know how to do the experiment, but it's interesting to ponder.
If the spinning astronaut took a yo-yo out of her pocket, she would observe it accelerates radially away from her. The stationary astronaut's yo-yo would drift freely with no acceleration.
The situation is not totally symmetric: The yo-yo behaves differently for the spinning astronaut than it does for the stationary one. That is why the article is wrong; the two situations are measurably different, not equivalent.
In newtonian mechanics, the equivalence principle says that any two inertial coordinate frames look "the same". For example, if you have two spaceships heading toward each other, the picture looks "the same" as if you pretend that one spaceship is stationary and the other is moving twice as fast, or vice versa.
This is not true only of geometric relationships, but also for the laws of physics: If you choose to view things in coordinate frame A, and apply the same laws of physics, you will still get the same predicted motion as if you had done the calculation in coordinate frame B. There is no experiment that can tell you which frame is "moving", because you always get the same answer.
With your example of two rotating bodies, it is true that the picture looks geometrically "the same" if we pick a different rotating reference frame. The problem is that the laws of physics do not look "the same" under this transformation. If I choose a coordinate frame where the spinning astronaut is stationary, and I try to predict what will happen using the laws of physics of the original frame, I will get wrong results-- the motion of the yo-yo will look "unphysical" for a stationary system. So when you change between rotating and stationary coordinate frames, the laws of physics change too.
That means I can tell whether I am in an "absolutely rotating" coordinate frame by doing tests to measure the laws of physics (i.e. are there coriolis and centrifugal forces acting on me?), without any reference to some other, external frame.
So the fundamental question is under what circumstances the picture looks "the same". In both cases, geometric relationships are equivalent, but in only one of the cases are the laws of physics also equivalent. The original article lied by saying the laws of physics are the same in the rotating case, and that is simply not true.
Does that make sense?
We can define "absolute rotation" as rotation relative to that coordinate frame. We will not have any trouble agreeing on who is rotating and who is not, because only one coordinate frame will observe perfectly inertial movement, and all the others will measure some coriolis and centrifugal forces.
See this video for better intuition: https://www.youtube.com/watch?v=_36MiCUS1ro
[1] http://physics.stackexchange.com/questions/1048/what-if-the-...
Centrifugal force in a rotating frame is an inevitable consequence of a universe in which conservation of momentum exists. Conservation of momentum is an inevitable consequence (by Noether's theorem) of a universe in which translational symmetry exists (i.e. the laws of motion are not different at some displacement dx). By all observations, all three of those properties hold for appropriate choices of frame.
You cannot suppose a universe with both translational symmetry and symmetry under rotational movement; the two are in conflict. If conservation of momentum holds in some coordinate frame, then it will not hold in coordinate frames that are rotating relatively to it.
If you suppose a universe where a "rotating" reference frame is the one where conservation of momentum holds, then I would be forced to ask "rotating relative to what, if not an inertial frame?" And the inhabitants of such a universe would still certainly find it meaningful to label that frame as "inertial" and define rotation as relative to it.
If you are asking how we tell rotating and non-rotating reference frames apart, the answer is trivial-- non-rotating reference frames obey inertia and rotating frames do not; and there is no ambiguity or disagreement between observers. If you are asking why there exists an inertial reference frame at all, the answer is that this arises from translational symmetry. If you are asking why the universe has chosen a particular coordinate frame to be "non-rotating" over any other, the question seems rather meaningless, because it does not make sense to define rotation except with respect to an inertial frame.
So I'll just say that pondering the nature of the universe, that I am just a region of low entropy, that anything I do will have no discernible effect on the universe really makes any problems I have seem like small potatoes!
Classically Physics - Everything is local. Quantum Physics - Everything is non-local
Both theories are proven to the extent anything can be proven both mathematically and experimentally.
Problem is that they contradict each other and we have no way to combine them in ways that make sense philosophically. All attempts have some sort of missing element.
Article propose yet another attempt to combine them. My uneducated guess is that somethings not going to be like we thought and we are back to two incompatible models.
No, that's not true. The classical world is an emergent property of the quantum world, and the manner in which it arises is perfectly well understood. It's just that the popular press has not gotten the memo.
It does nothing to resolve the other issues, like the quantum field's mass becoming greater than observed at the scale of the universe or other fundamental issues. Relativity and QM as we currently understand them really are at odds with each other and it's not just an interpretation thing.
Unless someone who know a lot more than I do tells me thats wrong I will go with what I have been told and read.
I am aware that many people want to demystify it or overmystify it, but the basic facts as far as I understand them are still that there are no interpretation of QM that satisfies everything and to the extent that it does you can't predict with it (like string theory)
This is not something that can be explained with the press didn't get the memo. Some very smart people have spent decades on this and still not come to a conclusion.
I have heard many such attempts to brush it off as a non-issue. I still haven't found anything satisfactory even to my un-educated understanding of it.
Depends on what you mean. The explanation of how classical reality emerges from the wave function is mathematical. Once you understand that you will understand why the apparent "contradiction" between classical locality and quantum non-locality isn't a contradiction any more than Newton's absolute space and time is a "contradiction" of Einstein's relative space and time. Classical reality is an approximation that holds under certain circumstances (systems with a large number of mutually entangled degrees of freedom), just a Newtonian mechanics is an approximation that holds under certain circumstances (low velocities and low spacetime curvature).
But that is ultimately the difference. One predicts a discrete universe the other the opposite. Both are true but can't be reconciled without introducing all sorts of issues.
This was an attempt to do a tl;dr. Some people will disagree with you that it's no big mystery but you both agree on the math and experimental data. And that shows two differen't "universes"
That is at least my understanding which admittedly is a lay mans understanding, but I have read enough and followed enough discussions to know that not everyone agrees with your memo characterization :)
No, you're wrong about that. They can be reconciled. They have been reconciled. QM is no more mysterious than relativity.
> Some people will disagree with you
I suppose. Some people deny evolution and climate change too.
I am saying that they can't be reconciled philosophically and so we have different interpretations none of them fully adequate to create a unified theory.
I am guessing that you are one of those who don't care about that part and thats fine. But to claim that there is no conflict and no disagreement and that other physicists doesn't is wrong.
The evolution and climate change remark is just arrogant and uncalled for not sure whats up with the attitude.
I know that's what you're saying. I'm saying you're wrong about that. They can be and have been reconciled both mathematically and philosophically.
> The evolution and climate change remark is just arrogant and uncalled for not sure whats up with the attitude.
It's because your argument has the same form as their arguments. You have made a bald assertion of impossibility with no supporting evidence ("They can't be reconciled philosophically", c.f. "Evolution can't explain X" for some value of X.) Moreover, you have apparently not even bothered to look at the references I provided to support my position because you have made no substantive comments regarding them. You are arguing from manifest ignorance.
> I have read enough and followed enough discussions to know that not everyone agrees with your memo characterization
Sure. You can also find extensive literature supporting the creationist and climate-change-denialist positions too. They are all demonstrably wrong, notwithstanding that many people remain in denial about it.
So in other words taking things out of context is how you came to that conclusion. Well done. Thats not the meaning of what I said. I was talking about the CURRENT state as most talk about it. I made no claims about universal impossibility simply that this is the current positions.
Which is funny enough why the Rons talk tries to debunk some of that mysticism ironically only to turn into one himself.
A quick search on Rons name show plenty of debunking I can't even be bothered as I have seen this before whether this or electrical universe theory etc.
http://motls.blogspot.com/2012/10/evading-quantum-mechanics-...
In this discussion you are the climate sceptic and the creationist not me.
"At the end, he says lots of correct things"
Well, there's a reason that the correct things are at the end and the beginning of the talk contains seemingly incorrect things. That's because the first half of the talk is a reductio-ad-absurdum argument against the Copenhagen interpretation, i.e. it deliberately assumes a false premise and reasons from there to a false conclusion (a violation of relativity) in order to show that the premise is false. And I take great pains to point this out. (Yes, I am "the Ron" as you put it.) Start at the 4:50 mark and watch for about two minutes.
So in other words. Your interpretation. Which is exactly what I asked you in the beginning.
You believe you found a way to interpret it. Great now line up with all the others who also have their interpretation.
You are the climate sceptic here you are the creationist. You know what you have to do to prove you theory and you know what awaits if you do and I am sure "the press" will get that memo.
Until then. Despite my laymans understanding of QM, I think I will stick with my original TL;DR which tries to stay away from any specific interpretation.
HN comments don't lend themselves well to long-form writing. But in this case it wasn't really necessary to go past that one small fragment to see that Motl had not in fact paid attention to the details because he's attacking a straw man.
> Your interpretation
No, not mine. Nicolas Cerf and Chris Adami's 9http://arxiv.org/pdf/quant-ph/9605002v2.pdf). And W.H. Zurek's (http://arxiv.org/abs/quant-ph/0306072). And David Mermin's (http://arxiv.org/abs/quant-ph/9609013). And Hugh Everett's (http://plato.stanford.edu/entries/qm-everett/#3, though I do put a slightly different spin on it -- no pun intended.) Arguably even John von Neuman's, and there is some evidence that Heisenberg actually understood the connection between measurement and entanglement as well.
To be clear: I am not claiming to have discovered anything new. My only contribution is a pedagogical one: a reductio-ad-absurdum argument showing how Copenhagen, if it were true, would lead to faster-than-light communication.
> You are the climate sceptic here you are the creationist
Another characteristic of creationists is that they, like you, adamantly refuse to actually look at the evidence presented to them.
Not true in the slightest. The Many-Worlds interpretation is local.
Many worlds is a way to interpret not the actual theory? Or am I wrong?
I'm not sure it is even though Wikipedia currently says that it is local. The recent experiment http://www.nytimes.com/2015/10/22/science/quantum-theory-exp...
seems to show clear non local effects. I don't think that contradicts Many-Worlds.
Much of the question comes down to an aspect that Everett, the inventor of Many-Worlds kind of hand waved over and didn't really explain. He said roughly lets assume all outcomes happen and assign a probability P to each one without really explaining where the Ps came from. In the NYT described experiment the probabilities seem to be such as to show non local effects but Everett and his theory don't really say much as to how they got that way.
And despite what the experimenters seem to believe, the experiment you linked to does not show non-local effects. That's not really the way that science and experimentation work. The experimental result rules out a particular class of physical theory - specifically any theory that's local and real, but a deep assumption of the derivation (so deep that one doesn't realize one is making it) is that every measurement has one outcome, which of course Many Worlds does not assume.
I don't think that anyone is hand waving about where probabilities come from -- they are the integral of the squared modulus of the wave function. Nothing ambiguous about the mathematics of the theory. If you're asking why this equation rather than some other equation, it's an open question, and an interesting one. But the Many Worlds Theory can perfectly well do its job as a predictor of experiment without justification of where the equation comes from. You may as well ask "Why the Schrodinger Equation rather than some other equation?". It's also worth pointing out that all interpretations of QM have the same open question because they all have some form of the Born Rule without justification.
I am under no illusion that the tediums of my life are more significant that the potential destruction of an entire anonymous solar system, but when conditioned on my own experience it seems a bit more pertinent. I'd posit that it's the recognition of this conditioning operation that let's us get a wider/better perspective on things.
Some star in an obscure galaxy went supernova, and none of us would be here if it hadn't.
Ie, even if the explosion occurred on a much faster time scale than their sensory biology, and thus wiped out their planets in an effective instant, I'd argue it made an impact even though no one was left to observe it.
If the life forms were about to achieve interstellar transport and colonise another star system, clearly there's an impact on the resultant state of that star system.
Otherwise, it would have an essentially negligible impact on anyone in a separate galaxy.
https://golem.ph.utexas.edu/~distler/blog/archives/002645.ht... (this is a comment, scroll up for the post)