Albert Einstein, with his theory of special relativity, established that the speed of light is the absolute speed limit of the universe. A rocket ship attempting to accelerate to the speed of light encounters some well known effects: time dilation, mass increase, and length contraction. The closer to the speed of light it gets, the higher its mass climbs, the slower its passage of time, and the shorter its length. To actually reach the speed of light, it would need to acquire infinite mass, zero passage of time, and zero length, which would require infinite energy. (Translation: you can’t do it.)
Things are marginally more hopeful for photons, which have no mass. They always travel at the speed of light. As the unit of all electromagnetic radiation, they enable communication at the speed of light. But that’s the fastest they enable it at.
Often when these facts come up in discussions, someone raises the possibility of using quantum entanglement for communication. Entanglement, we are told, is a non-local effect. Doesn’t this, as a fair amount of science fiction implies, mean that there might be some effect we could use in the future for faster than light communication?
Unfortunately, the answer is no. This doesn’t come from a pessimistic view of the possibilities, but from an understanding of what entanglement actually is, an understanding I have to admit I’ve only recently fully come to appreciate. I first got it when reading Adam Becker’s What Is Real?, but wanted to wait to discuss it until I’d gotten some confirmation from Sean Carroll’s Something Deeply Hidden, which I’m currently reading.
That understanding is that entanglement is inherently about information. When two quantum objects interact, they become entangled with each, meaning that they’re described by an overall common wave function. But that description, for most people, isn’t very enlightening. So let’s do an analogy.
Imagine both Alice and Bob, living far away from each other, each have a subscription to the New York Times, and each of them knows about the other’s subscription. Let’s further suppose they both have very reliable and timely delivery of their paper. When Alice gets a particular issue of the Times and looks at it, she knows that Bob is getting the same issue with the same information. You could say that each copy of the same issue of the Times is entangled with every other copy, including Alice’s and Bob’s, which is to say, they share a causal history that enables information about one to provide information on the other.
So far this isn’t any big deal. Alice and Bob each know what the other is seeing, but can’t use that information in any way to communicate with each other. If Bob alters his copy of the Times, it doesn’t effect Alice’s. All it really does is break the entanglement between them, that is, erase his ability to use his paper to know what’s in Alice’s copy. (Technically since information is always conserved, it spreads the entanglement around, but let’s not get sidetracked.)
So what’s the big deal with entanglement? Well, let’s say that a very special issue of the Times comes out, a quantum version of the paper, one that is in a superposition of possible states until a reader actually looks at it. One branch of the superposition says the stock market went up yesterday, the other says it crashed. Under standard interpretations of quantum mechanics, it is meaningless to talk about what the paper actually says until someone looks at it.
But, as soon as Bob or Alice actually look at their paper, the wave function of the quantum copy collapses into a definite value. When Alice looks at her copy, she knows what Bob will see, even though Bob hasn’t looked at his yet. This is true even if Alice and Bob are separated by light years. In other words, what the paper says isn’t a definite value, until either Bob or Alice (or some other subscriber) looks at theirs, but as soon as either does, the other’s copy instantly becomes definite too, with the same values. But if both copies were in an undefined state prior to their collapses, how do those copies “know” which one to collapse to so they agree with the other?
This is the aspect of quantum theory that bothered Einstein enough to co-author a paper with Boris Podolsky and Nathan Rosen in 1935, the famous EPR paradox paper. In their view, it indicated that quantum theory could not be complete. Einstein famously called it “spooky action at a distance”. Bell’s theorem would eventually prove he and his co-authors wrong, at least if everything is happening in one consistent universe.
But just like our classical edition of the paper, there’s nothing Alice or Bob can do to their quantum copies that would allow them to communicate. Again, if Bob alters his copy, all he does is break the entanglement (technically spread it around).
Bringing this back home to particles, there’s nothing you can do with one particle of an entangled pair of particles that will control the state of the other particle. (Other then bring them back together and have them interact again.) Yes, the act of measuring the first particle causes the other to assume a definite value, but there’s no way either party can know ahead of time what those values will be. And attempting to control them, alters the particle’s state, breaking (spreading) the entanglement.
This might be frustrating, because we seem so close. But of course, that closeness is an illusion, borne of a misunderstanding of what actually happens with entanglement.
To be clear, quantum entanglement, under most interpretations of quantum mechanics, violates the spirit of special relativity. It allows communication of a sort between the entangled items, but it doesn’t violate the letter of relativity, since it’s not communication we’re able to actually do anything with.
Unless of course, I’m missing something?
SelfAwarePatterns 
Thanks. I wrote an article for better or worse – ‘Consciousness and Quantum Mechanics – Is it too early to rule out the Copenhagen classic interpretation’?
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There are many variants of Copenhagen. I think the more epistemically humble ones are fine in a cookbook sort of way. None of the currently viable interpretations have any unique evidence in their favor, so I’m intellectually agnostic on them.
I do think we can rule out consciousness though. If consciousness was the major factor, building a quantum computer would be a lot easier. The problem is keeping the qubits from decohering is the biggest implementation issue, and that decoherence happens despite no conscious entity looking at them. Extreme isolation from the environment seems to be the only way to preserve the superpositions.
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OTOH, there is a growing body of evidence that suggests nature has found a way to leverage quantum effects even in hot, wet, messy environments. (And there are quantum effects other than entanglement. I wouldn’t rule out quantum consciousness just yet.)
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I have to admit I basically rule it out. I’m willing to rule it back in, but it’ll take some evidence (widely reproducible and validated) where that’s the only plausible explanation to make me do it.
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Are you referring to “consciousness causes the collapse” or the idea that the brain might be using some aspect of QM to produce consciousness?
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You’re obviously going for a bare-bones explanation, as you left out a lot of details I know you know. (The newspaper analogy risks giving the idea of hidden variables — which Bell’s Inequality rules out. Unfortunately, almost any analogy would, since there really isn’t anything analogous to superposition in the macro world.)
I was reading a (2008) article by Scott Aaronson about quantum computing (and what it can’t do), and it contained a consideration I hadn’t realized: Yet another reason reason time travel is probably impossible is because it would enable computations that could solve problems in PSPACE. It would, in the process, trivially make P=NP. He suggests that such a limitation might be very similar in nature to the limits thermodynamics places on physics.
And, of course, FTL communication would allow a form of information time travel, which would break causality as we know it… and allow hyper-computation — also a violation of reality as we believe it.
So, yeah,… no entanglement ansibles! They would break reality! 😀
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I’m definitely going bare bones here, putting just the minimum amount to make the point. (Hopefully it’s not too minimal.) I did mention Bell’s theorem, just to make sure people know there’s more to it. But yeah, keeping a blog post on this short enough for people to read it inevitably means leaving out a lot.
That said, don’t assume I know some crucial point. I think you know a lot more about this than I do. It’s entirely possible I goofed somewhere.
That Aaronson article sounds pretty good. I might have to look it up. My take on quantum computing is that it’ll be useful for some things, but not the magic system so many people seem to assume it’ll be.
I wouldn’t put it past us to eventually figure out a way to break reality. Hopefully if we do, we’ll figure out a way to patch it back up. 😉
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“It’s entirely possible I goofed somewhere.”
Not that I could see. And the newspaper analogy does make the key point that changing your copy can’t possibly affect other copies. (Where it risks misleading is in that entangled particles don’t have the same information, as newspapers would. Entangled particles have complementary information. (If one is measured to have spin up, the other would necessarily be measured to have spin down.))
“That Aaronson article sounds pretty good. I might have to look it up.”
Scientific American, March 2008, The Limits of Quantum Computers. I don’t know if it’s available online. I’m going through my old issues (as something to do while I eat breakfast) before dumping them in the recycle. It’s kind of interesting to compare 10-15 years ago with now. A lot of the internet and personal computing device stuff is now rather quaint.
“My take on quantum computing is that it’ll be useful for some things, but not the magic system so many people seem to assume it’ll be.”
Indeed. In fact, it looks like QC won’t make P=NP — problems in NP will remain computationally challenging. There is also that quantum algorithms are part of the challenge — any problem has to be broken down such that wrong answers tend to cancel while correct answers re-enforce. So they’ll only help with certain kinds of problems.
(The bummer, in a way, is that one thing they will be good at is factoring large integers, and that’s what will break a lot of current encryption schemes that rely on the difficulty of factoring.)
“I wouldn’t put it past us to eventually figure out a way to break reality.”
Ha! I used to have a theory (or plot for a SF story) that those extremely energetic cosmological events — quasars and whatnot — were from civilizations that had discovered how to tap into, for example, vacuum energy. But they got something important just a little wrong, so when they threw the switch to test the full-sized version… 😀
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“Where it risks misleading is in that entangled particles don’t have the same information, as newspapers would.”
A better example Becker used was two billiard balls that bounced off each other. If you knew the states (position, spatial and angular momentum, etc) of the balls, but then after the bounce only had information on one of the balls, you could deduce the information about the other one. That’s closer to the truth, but was too complicated for my purpose.
“Scientific American, March 2008, The Limits of Quantum Computers. I don’t know if it’s available online.”
A Sci Am url comes up in google, but coughs up a 502 then I access it. But someone posted a PDF of it: https://www.cs.virginia.edu/~robins/The_Limits_of_Quantum_Computers.pdf
“so when they threw the switch to test the full-sized version…”
Like bugs at night reaching the zapper. Be careful what you try to achieve!
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Nah, information time travel wouldn’t break causality as we (thought we) know it, that’s already broken. Causation that remotely resembles our intuitive idea of causation only exists on the macro-level.
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“Causation that remotely resembles our intuitive idea of causation only exists on the macro-level.”
You don’t accept the causality of physics?
Time travel obviously would break causality at the macro level. There’s the canonical “killing your grandfather” example, and Aaronson talks about a computer capable of sending the result of a very long computation back in time… but once you have the answer, you turn off the computer!
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I do accept the causality of physics, if you mean causality as understood by physicists, especially those well versed in CPT symmetry, relativity, and entropy. It’s the causality “of laypersons” (meaning: as understood by them) that’s suspect.
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Okay,… I guess I don’t really understand what you’re saying about time travel then.
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I’m not saying anything about time travel, and I don’t think “information time travel” counts as time travel in a more straightforward sense.
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Then I guess your original comment went completely over my head. 😦
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Yeah sorry. It touched on my hobby-horse, the difference between intuitive causality and causal analyses that can be made to work with modern physics. As Bertrand Russell only slightly overstated, “The law of causality, I believe, like much that passes muster among philosophers, is a relic of a bygone age, surviving, like the monarchy, only because it is erroneously supposed to do no harm.”
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“It touched on my hobby-horse, the difference between intuitive causality and causal analyses that can be made to work with modern physics.”
Ah, entiendo! I’m kind of blind to “intuitive” things… they’re so often wrong. 🙂
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Help me out here. This question has bothered me for some time, and I really don’t know if it’s profoundly stupid. If mass increases as we approach the speed of light, and a body contracts, then at say 99.9% the speed of light, would a body become a black hole, or perhaps even a singularity? Does that even make sense, or am I missing something rather large here?
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That’s an excellent question. I don’t know that it wouldn’t.
Although before that point was reached, you’d likely have this issue:
http://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html
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That’s an excellent question.
Are you sure? 😉
OK, so the problems occur with larger bodies, but what about smaller bodies, like atoms? An atom has mass, accelerate it to near light speed inside a vacuum, and then… what?
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Atoms don’t have much mass. CERN uses (IIRC) lead ions sometimes and gets them going pretty fast. But still not anywhere near what’s needed for black holes.
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Also, my understanding is that very low mass black holes degrade almost instantly in a flash of Hawking radiation.
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An interesting question. But just 99.9% c is nothing. The gamma is only 22.4. But when we’re talking 99.999999% c, then it’s a bit over 7000. It curves sharply upwards from there. 🙂
We don’t see relativistic particles doing anything special, but they’re pretty light to begin with. And it would take either (or both) so much time or so much energy to get anything massive up to nearly c that it’s kind of outside our experience. Certainly nothing natural can do it.
I’m not physicist enough to know if velocity acts to prevent a fast massive object from collapsing, but it might be a factor.
We do think that if we used energy levels high enough to examine below the Planck distance, that would indeed create a black hole — thus masking whatever we wanted to study at that tiny scale. But that distance is unimaginably tiny. Blow a hydrogen atom up to the size of our galaxy, and the Planck distance would still only amount to the length of an amoeba.
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We don’t see relativistic particles doing anything special, but they’re pretty light to begin with
Why would they be excluded?
I’m sorry if these questions are dumb, but I just can’t wrap my head around it.
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It has to do with the equation governing what’s required for black hole collapse. Specifically, the Schwarzschild Radius. The equation is: R=(2GM)/(c^2)
So, for two examples, if the Sun were compressed into a ball just 3 km in radius, it would collapse into a black hole. You’d have to compress the Earth down to a ball with a radius of just 8.7 mm to get it to collapse.
That’s with mass. And since E=mc^2, it takes a huge amount more energy to do the same thing. It essentially takes the Planck Energy (1.22×10^19 GeV) to collapse something the Planck Length (1.616×10^-35 m) into a black hole.
Particles aren’t excluded. They just don’t have enough rest mass, nor momentum mass, to be close to collapse.
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I think I get it. Thanks.
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“I’m not physicist enough to know if velocity acts to prevent a fast massive object from collapsing, but it might be a factor.”
Something that just occurred to me: in the frame of the spaceship, its mass wouldn’t be that high. But the mass of a lot of stuff in the universe would be. Relativity makes my head hurt.
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That’s a very good point. It would have the energy from outsiders’ POV, but not in its own frame. It may well be velocity can’t accomplish the job.
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I was just wondering a similar point – is it rest mass, or observed mass, that determines a black hole? If it’s observed mass, then a spaceman and his ship may be ordinary matter from his point of view, but a black hole from our POV.
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The internet consensus seems to be that it wouldn’t form a black hole.
https://physics.stackexchange.com/questions/3436/if-a-mass-moves-close-to-the-speed-of-light-does-it-turn-into-a-black-hole
The answer appears to be that its own reference frame is what matters.
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“The answer appears to be that its own reference frame is what matters.”
The more I think about it, the more I think that’s exactly right. BH dynamics occur within the frame of the BH.
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I think you need to include a warning when you say that “entanglement is inherently about information.” When physicists talk “information”, they mean something different from Shannon information and also something different from cognitive science’s use. (Justifications for the two halves of that claim available on request.) The fact that the “two” “objects” are described by a common wavefunction, on the other hand, really nails the fact of entanglement.
I like the way the Everett interpretation handles the decoherence of the wavefunction in the communication between Alice and Bob. When Alice observes her particle, the interaction caused by her particle spreads into her environment at light speed, and likewise in Bob’s neighborhood. So of course by the time they can communicate about the results of their little Bell’s Theorem experiment, they will have decohered into the same sub-state, i.e. “split” into the same “world”, and so their measurements will correspond in the expected way.
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“When Alice observes her particle, the interaction caused by her particle spreads into her environment at light speed, and likewise in Bob’s neighborhood.”
How does that jibe with that the record of the results of their independent measurements always correlate? (Those records don’t change in any way.)
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When Alice meets up with Bob, the Bob-state she meets up with will perforce be the one that decohered along with this-Alice into the correlated state. For example, this Alice-state got a spin-up photon and this Bob got spin-down. Meanwhile, in the “other universe” (AKA orthogonal state), that-Alice got spin-down and that-Bob got spin-up.
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Edit: correlated stated should be correlated state.
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Maybe I don’t understand your original comment, then. How is the MWI any different from the Copenhagen one? Don’t both have the issue of “spookiness”? Why do Alice and Bob decohere together?
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Hi paultorek, the philosophical challenge I have with the Everett approach is that although it provides an explanation for how the wavefunction collapses (i.e. the wavefunction doesn’t collapse – both possible outcomes are true) but replaces this with a question of how entanglement comes about in the first place. When, in this model, does the universe split into two universes, and what is the mechanism for this instantaneous universe-wide division of “all of reality” into “all of reality – twice”?
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“When, in this model, does the universe split into two universes,”
Yeah, that’s just it. The moment Alice makes a measurement, she splits reality into two, both of which necessarily contain a Bob. (Or vice versa, if Bob measures first.)
If Bob makes his measurement just a fraction of a second later, he’ll see a result determined by Alice. But Bob could also be much further away from the particle source and thus makes his measurement much later.
But either way, once Alice makes a measurement, there have to be two Bobs.
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And of course time and simultaneity are not straightforward in relativity …
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True dat!
Although, for purposes of this experiment, we can posit that Alice and Bob are in the same reference frame. Of course, someone passing by might might see Bob make a measurement before Alice!
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OK Wyrd and Steve, these are really good questions, and I will try…
First, on decohering together. In a pseudo-equation, when the people measure their photons, the result is 1/sqrt(2) (A-photon up & Alice says “up” & B-photon down & Bob says “down”) + 1/sqrt(2) (A-photon down & Alice says “down” & B-photon up & Bob says “up”). Because Alice and Bob are huge systems in quantum terms, covering many dimensions in Hilbert space, the two sides of the + sign have already become orthogonal states. The quantum possibilities can already be considered to have decohered. “Already” in the previous sentence is taken in Alice&Bob’s common reference frame.
From there in their labs, the decohered separate states continue to interact with parts of the universe in-between the labs, but all the interesting action has already happened. But if you just apply the usual operations to the “equations” we’ve got so far (or their proper versions), to model those further interactions, you’ll wind up with Alice and Bob agreeing – twice over, once for each orthogonal state – that the correlation came out as expected.
When does the universe split?
Whenever you find it absurd to bother keeping track of the tiny interaction between two for-all-practical-purposes orthogonal states, you can declare the universe to have split. This might depend on your tastes, or what’s at stake, and you don’t have to settle on an exact moment. When does a child become an adult? It’s fuzzy, and/or conventional. Unlike the maturation of a child, though, the dimensionality of a decohering quantum state grows with astonishing rapidity.
Thus spake Wikipedia on Quantum decoherence.
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Thanks for responding, paultorek. I can’t help thinking that there’s a sleight of hand going on here, with this invoking of huge systems.
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Frank Wilczek once wrote in Physics Today (paraphrasing from memory) “as the number of dimensions approaches infinity, the probability that two vectors are orthogonal approaches one”. When I read that sentence and realized that *I knew what that meant*, I finally admitted to myself that I’m a total nerd.
Anyway, the point here is that as more and more particles interact, the number of dimensions involved (spins of different particles on different axes, for example) increases. The outcome “spin-up on photon A” becomes entangled with many other features, and so the resulting state becomes a high-dimensional vector which is likely to be orthogonal (to a good approximation) to the vector representing the other state. Orthogonal states can’t interact. Voila, decoherence.
If this handwavey verbal explanation isn’t good enough, there’s plenty of actual math in the Wiki article on quantum decoherence.
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Thanks for the explanation. I understand what you are saying here, but it brings me no closer to understanding what MWI says about when the universe splits, or even whether it does. I am not laying any of the blame for that on you!
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“[A]s the number of dimensions approaches infinity, the probability that two vectors are orthogonal approaches one”
This is exactly what “semantic vectors” leverage. A high dimensional space (500+) insures that the random vectors created for semantic notions tend to be highly orthogonal to any other vector. I agree: it’s way cool!
(FWIW, I did some simple exploration of this a while back. See the last comment on this post of mine.)
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Unless you’re claiming some kind of hidden variable scenario, I don’t understand how this explains why the results Alice and Bob get are always correlated regardless of their separation in space or the timing of their measurements.
It gets a lot more interesting when Alice and Bob measure spins on different axes. If Alice measures up-down spin, and Bob measures left-right spin, their results have zero correlation. (If they both measure up-down or left-right, results are 100% correlated.)
But if they use, zero degrees (up-down), 120 degrees, and 240 degrees (splitting the circle in thirds, that is), then their results are correlated according to the quantum probabilities (rather than the linear probabilities hidden variables would require.) It’s this aspect of Bell’s Inequality that makes it clear a measurement of an entangled particle somehow “instantly” affects what would be measured by the other particle.
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I don’t see the need for hidden variables. The photons were generated as a pair in superposition, at the central point between Alice and Bob. We can mentally trace the propagation to Alice in her lab, which happens at light speed. Then Alice does her measurement, decoheres into “two” Alices, each of which sends a different message via laser modulation to Bob(s), which again propagates at light speed. Of course when Alice decoheres, half her measure becomes mutually hidden from the other half, so I guess you could call that “hidden variables” if you wanna stretch the term.
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“I guess you could call that ‘hidden variables’ if you wanna stretch the term.”
Oh, heavens, no, I’m fine with the standard definition.
I’m not sure if we’re on the same page or not, so let me run it by you as I understand it:
Let’s say Alice and Bob are separated by a spacelike interval (forbidding instant communication). They each receive entangled photons. They simply record the polarization of those photons (both use filters oriented at 0°). According to each of them, they get a random string of yes-no results indicating whether the photon passed the filter.
Weeks later they meet, compare results, and see that their random strings match exactly.
If, on the other hand, Bob set his filter to 90°, then the two strings would have no correlation at all.
On the other other hand, if Bob set his filter to 30°, then they would see a correlation that matches according to quantum statistics — according to cos(θ)^2. (In fact, all their results correlate that way, because: cos(0°)^2 = 1.00 and cos(90°)^2 = 0.00)
The my question is that, since Alice is presumably splitting the universe on each measurement, and so is Bob, why do their measurements correlate the way they do? It seems as if, when Alice splits into Alice-1 (photon detected) and Alice-2 (photon not detected), Bob necessarily splits the exact same way.
Bringing SR into the picture (see elsewhere in thread), it’s not possible to determine if Alice caused Bob’s results, or if Bob caused Alice’s results, but that paper suggests the symmetry of the situation means both views are valid.
Thots?
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I don’t see why the 30 degree case raises any new issues, compared to the 0 degree case. The states’ amplitudes (joint states in particular) are described by the Schrödinger equation, so for a full accounting, we get 3 Alice “up” + Bob “1 oclock” for every 1 Alice “up” and Bob “7 oclock” for every 3 A “down”+B “7” for every 1 A “up” + B “7”. You could view it as a split into 8 universes for every photon, or 4 universes, two of which are thrice as “weighty” as the other two. I prefer the latter way of talking, with the caveat that any talk of “worlds” is hazardous if taken too simplistically.
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“I don’t see why the 30 degree case raises any new issues, compared to the 0 degree case.”
It doesn’t. It was more of an aside. The only thing is that with both filters at 0° to each other, or with them set at 90° relative to each other, the results are indistinguishable from “hidden variable” theories.
It’s only when the filters are set 0° < θ < 90° that the Bell Inequality is violated and we see the quantum probability versus linear probability difference.
“we get 3 Alice ‘up’ + Bob ‘1 oclock’ for every 1 Alice ‘up’ and Bob ‘7 oclock’ for every 3 A ‘down’+B ‘7’ for every 1 A ‘up’ + B ‘7’”
I’m sorry, I’m completely at sea on that one. [ERROR: Unable to parse input.]
In the “world” where Alice detects a photon, that photon is now polarized at 0° because of that detection. (It was undetermined before.) Which means, Bob, with his filter set 30° to hers, has a cos(30°)^2 = 0.75 chance of detecting that photon.
It works in reverse. If Bob detects the photon, his detection means Alice has that 0.75 chance. That’s that symmetry thing that seems to allow either to “cause” the photon’s polarization to be determined.
So, in MWI, are we up to two “worlds”? (1) Alice detects, Bob detects; (2) Alice detects, Bob doesn’t? Or, in reverse: (1) Bob does, Alice does; (2) Bob does, Alice doesn’t? Are you saying those add up to four? (They aren’t symmetrical?)
If Alice doesn’t detect a photon (in the case where she’s “first” and causes the photon’s polarization to be determined), does that mean the photon is polarized 90°? Which means Bob has a cos(60°)^2 = 0.25 chance of detecting it? Or, in reverse, if Bob doesn’t and causes “collapse” then Alice has that same 0.25 chance (since it would now be 90° to Bob and, thus, 60° to Alice)?
So there’s the same two (or four?) “worlds”… is that where you get eight?
Seems like the possibilities are: (1) Alice does, Bob does; (2) Alice does, Bob doesn’t; (3) Alice doesn’t, Bob does; (4) Alice doesn’t, Bob doesn’t. Four? (Granted with different “weights.”)
Oh, this MWI stuff makes my head hurt! I need more beer!!
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Why do they have to use filters? How about birefringent crystals instead? That way you don’t have to know “expect one photon per second” or such.
In that un-parse-able sentence I was just saying that the ratios of weights are 3:1:1:3 (in your list of possibilities; mine was ordered differently for no good reason).
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“Why do they have to use filters? How about birefringent crystals instead?”
I don’t really know, but it might have to do with that a filter provides a go/no-go situation with well-defined mathematics. Perhaps it makes the experiment with photons more like similar experiments, such as electrons, that measure the spin direction of particles.
“I was just saying that the ratios of weights are 3:1:1:3”
Ah, good, so we were on the same page.
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“(Justifications for the two halves of that claim available on request.)”
Yes, please. 🙂
I’m not sure if I’m understanding your description of how Everett handles the Alice and Bob scenario. It seems to leave open the possibility that they might have incompatible answers in some universes. I’m reading Carroll’s book, but so far I’m disappointed that he hasn’t directly addressed this issue.
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Shannon-information is symbol system dependent. So, if I send you a handwritten message and we view the symbol system as that of the English language, the fact that one of my “A”s looks a little different than another doesn’t count toward the information content of my message. (Indeed, one of the virtues of our alphabet is that it mostly allows clear distinctions between the 26 letters, robust against small hand tremors of the writer, and such.) But to a physicist that little wiggle on the “A” does count as information.
To a cognitive scientist, information is information *had* by someone. Not so to a physicist. A physical state could be inaccessible to all agents, but still count toward the information content of the universe.
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Doesn’t it depend on what’s being considered? That your “A”s look different might be of considerable import if you were forging a message from someone else.
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Totally does depend on what’s being considered. Shannon information is explicitly relative in just this way. That’s my reason for saying it’s different from physical “information”.
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As you say, Shannon is relative, so it also applies to physical information. I see Shannon as being about the limits of information, especially information transfer. Information itself, whatever kind, is what Shannon limits apply to.
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OK, I get that. So if current physical theories turn out to be wrong, physicists can say, well *that* information – information expressed in the language of that theory – isn’t the “information” I meant.
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“So if current physical theories turn out to be wrong, physicists can say, well *that* information – information expressed in the language of that theory – isn’t the “information” I meant.”
Heh, sure! As I said above, from my perspective, Shannon is about (the fidelity of) information transfer, and doesn’t really speak to the truth or falsity of that information. Put another way, the limits would apply to fiction as much as facts.
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Thanks for the clarification.
I think symbolic information and cognitive information are special cases of physical information. But only physical information is conserved. So rearranging physical information may destroy other types of information, since they’re dependent on certain forms for their efficacy, but the physical information itself endures. (Except maybe in black holes.)
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Good discussion. I don’t think you’re missing anything, but I think that the scientific community is still missing a really convincing explanation of the mechanics of entanglement, or at least I have not seen one.
BTW there is a kind of weak communication, in that if Alice reads in her copy of the NYT that the stock market went up, she also knows that Bob read it crashed. If Alice and Bob agree to take action based on what they read, Alice can obtain information about what Bob is doing (selling stocks) even though in classical relativity theory she couldn’t possibly know that.
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Hey Steve,
How are you doing? Haven’t heard much from you lately.
It seems like entanglement is intimately, well, entangled, with the core issues of quantum mechanics. Like QM overall, there doesn’t appear to be any explanation that doesn’t contain absurdities.
On weak communication, not sure if I’m understanding your point. I was thinking if it amounted to pre-arranged strategy until I got to this part: “even though in classical relativity theory she couldn’t possibly know that.” What additional communication is enabled by the quantum aspect?
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To explain better, if Alice and Bob pre-arrange a set of actions they will undertake based on what they discover when they read their entangled newspapers, then they will know something that otherwise they wouldn’t know. Specifically, if Bob states that he will sell his stocks if he reads that the market crashed, then when Alice reads her copy, she will know what Bob is doing.
Of course this is not really any different to the classical case where two copies of the newspaper are sent – one to Alice, one to Bob – and it is known in advance that they will be different. So whichever copy Alice receives, she knows that Bob receives the other copy.
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Thanks for the clarification. That matches my initial understanding.
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Hey Steve! Long time no chat.
“I think that the scientific community is still missing a really convincing explanation of the mechanics of entanglement…”
Yeah, it’s pretty much just something that seems to be true. True and “spooky.”
“BTW there is a kind of weak communication,…”
I have trouble calling that any kind of “communication” given that the results are always random to both Alice and Bob. They communicated earlier to respond to those random results in a specific way, but as you point out later, “Of course this is not really any different to the classical case where two copies of the newspaper are sent.”
You can’t even really say Alice knows what Bob will do, because maybe Bob didn’t get his newspaper, or maybe it was altered before he got it, or maybe something happened to Bob’s eyes and he can’t read anymore, or maybe he changed his mind about what to do. (“Many a slip,…” 🙂 )
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Hi Wyrd, yes, definitely spooky. “Something” is propagating instantaneously across arbitrary distances. It may not be information. It may not be a particle or a wave. But it’s something spooky. No wonder Einstein hated it.
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I was reminded recently that even wave-function collapse has that instantaneous “information” change — it’s what Einstein objected to in the first place as “spooky,” and it led to the EPR paper involving entanglement as a way to highlight it.
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This lines up with my understanding of quantum entanglement as well. But there do seem to be some professional physicists who won’t let go of the dream. So I’m not 100% convinced quantum communications are impossible; I’m only 99.9% convinced of that.
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That works. I’m 99.9% sure you should never be 100% convinced of anything. Black swans and all that.
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Not really sure why everyone is so stymied by the instantaneous change in information, information that defies the rules of special relativity. Maybe if Einstein had read and understood Kant’s ontology, he would not have been so puzzled by the instantaneous change in information that occurred as a result of the wave-function collapse. Information as information is everywhere the same, it’s just that; information.
Science and physics deals almost entirely with posteriori information, which is information that is embedded in matter and energy. Because posteriori information is embedded in matter and energy, it is constrained by the correlations which gave us the construct of special relativity. In contrast, a priori information is not constrained by special relativity, because that information is neither matter nor energy. And that is exactly the information that was observed in the infamous two-slit experiment, a priori information.
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Lee,
a priori information was observed? Doesn’t observation make it a posteriori by definition?
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The short answer is no, because a priori information is “never” observed. In the double-slit experiment, when the wave function collapses to unity in one place and zero elsewhere, nothing physical is moving from one place to the other which can be observed, it’s simply a known, or a priori. The compelling question then becomes: How can something be a known when it is never observed? That is the “spooky-ness” Einstein was referring to…
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WRT MWI: There’s a neat article in Forbes by Chad Orzel: Many Worlds, But Too Much Metaphor.
He recommends that MWI should stand for “Metaphorical Worlds Interpretation” — because this whole “multiple copies” business isn’t really real. It’s just a “bookkeeping” convention.
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The problematic aspect here is that the wavefunction of the universe has everything in complicated superposition states, but when we select out a tiny piece of it as our system of interest, we often see that system only in single states, not a superposition of multiple states. The question that’s too often un-asked, though is: What measurement would you do to demonstrate that your system is really in a superposition?
This puts the finger on the problem as I see it. The MWI theory as commonly discussed appears to contradict experiments like the double split experiment. It posits the creation ex nihilo of new universes. This article seems to be saying that’s not what happens, instead it talks of the bookkeeping trick that lets us split off pieces of the wavefunction and consider them in isolation and yet when we perform the double split experiment, we are not performing a bookkeeping trick – nature is apparently performing a trick on us, as we see only one definite state, not a superposition.
Why do we not detect the particle at both slits if both wavefunctions exist?
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“Why do we not detect the particle at both slits if both wavefunctions exist?”
I’m not quite sure I follow your point. Under MWI, if detectors exist to monitor both slits, each detects a photon, but in separate “universes” that are split due to the quantum probability of the photon being detected at either slit. (Does that answer your question, or did I miss the point?)
Sean Carroll has a new blog post that gets into this: The Notorious Delayed-Choice Quantum Eraser (I only just read it last night and will need to read it again to try to make sense of what he’s saying. I don’t really care for the tone of the post, so I have to read through that.)
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I guess this relates to paultorek’s explanation of why large (macroscopic) systems are necessarily orthogonal. As large objects, we are trapped in one state, while a subatomic particle can happily oscillate between two states.
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I am gradually coming round to this Everett approach, but I feel my understanding still contains holes. I am also thinking that we really ought to be able to solve quantum gravity already. If the universe is described by the Schrodinger equation, in which matter, energy, spacetime are mere variables and/or solutions; and if we have a set of equations (GR) that relate matter and energy to spacetime; we ought to be able to just solve the whole damn thing, or at least write down a set of equations that describe the universe without having to dream up new stuff like strings and so forth.
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It doesn’t seem like anyone has closed all the holes. Carroll certainly admits to some, such as how many branches actually happen.
On quantizing gravity, Carroll pointed out something in the book I’d never considered before. The mathematics of the wave function happen against the background of spacetime. Quantizing gravity may involve quantizing spacetime. If we quantize spacetime, we now have waves against the backdrop of other waves, and spacetime starts to look emergent from the entangling of all those waves. Entropy figures into all of this. But as I noted in my post on the book, this was the section I had the hardest time following. (It was also the most speculative, so my motivation to follow it wasn’t as strong.)
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That’s one of the disconnects between GR and QFT. The former requires no background (it is the background) while the latter assumes a background.
I’ve wondered if we should just accept that duality the way we do wave-particle duality. (I’m still holding out, against the odds, for spacetime being smooth even though energy/matter is quantized.)
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[can of worms in hand … looking for can opener … ah, here we go]
I’m pretty sure what Carroll (and MWI) is saying is that the electron, being a wave function thing, goes through both slits every time. It’s just that if something interacts with the electron after going through the slits but before it hits the screen, the wave function is changed and so the interference is borked. But note that it’s possible to preserve some interference if you confine your measurement to a single axis, in which case interference due to orthogonal axes are not borked.
*
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Worms, cool, let’s go fishing! 😀
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Essentially, what Sean Carroll has done is postulate that wave-function is the “thing-in-itself”. This is in direct contrast to the Copenhagen interpretation which postulates that matter has both wave-like and particle-like properties. Unfortunately, neither one of those interpretations has anything to do with the true nature of reality. The entire “wave-function” is a bogus intellectual construction which was derived from observing the interference patterns created by the infamous two-slit experiment.
Quantum physics has never been my area of expertise, But upon examining much of the evidence surrounding the two-slit experiment, there is another explanation for those interference patterns that nobody has even entertained. Furthermore, my understanding of the cause of those interference patterns correspond precisely with my theories. It’s all so fascinating…
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Now if only my computer and yours was entangled, I would change the settings to have the new comments start at the top.
As per the entanglement. May I suggest that the whole theory of how things are is wrong. The idea of force particles being responsible for magnetic flux and gravity etc. is wrong. if we see them as little balls of force, then there must be distance between them. If so, how does one influence the other and its direction. And the force would be outward, not attractive, so nothing would ever hold together.
So, its my hypothesis that the planets and stars are held together with filaments microscopic but millions of miles long. And every atom forms these spaghetti arms. but like light, they are mass-less. But they have some type of friction and connection to other things like other stars. IF we were to look at the universe it would be a spaghetti bowl. lines of force bunched together connecting whole areas with other areas.
So, there would be no quantum entanglement as is seen today, a type of magic, but simply a few filaments remaining directly connected from one atom or group of atoms to another and from that the information is transferred.
And for the creation of everything from gravity to Electron motive force, there is simple loops of this spaghetti that curve around and cause magnetic poles.
its most likely that these filaments are like batteries in series, and possibly these batteries are shaped like a I or a T or a H in their quantum forms but are constructed or assembled in shapes to cause the building blocks of every thing. Its very possible the whole universe is full of these and that there is No space at all but these become aligned and connected by the initiation of some grouped cluster of these shapes then forcing others to line up like a chain forming long chains crossing the galaxy. If that is the case, then someone could use a machine to create force to Isolate the I cells and create shapes and make atoms from what would be to the naked eye, empty space.
Thats something to chew on.
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Thanks for commenting!
As I understand it, gravitons, the putative force carrier for gravity, remain hypothetical. The model that definitely works for gravity is the warping of spacetime, although it doesn’t explain gravitation at the quantum level. The possibility remains that gravity might be different than the other forces.
On those other forces, I get where you’re coming from about bosons, the force carrier particles. The usual metaphor of a ball being thrown between the fermions (such as quarks and electrons) works for the repulsive version of the interaction, but not for the attractive one. I’ve seen boomerangs used as another metaphor for the attractive side. But the thing to remember is that these are metaphors, so they have limitations. It’s the mathematics which reportedly bear out the relationship.
That’s not to say that alternate models aren’t possible, but they have to meet all the observational successes of the standard model, and that turns out to be much harder than it looks. There’s a reason physicists have converged on the standard model, even though many are dissatisfied with it.
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My model of squatty H shaped fundamental quantum strings actually could easily fit into standard models. It just means that what was thought to be a force particle was actually a grouping of my squatty H’s flying free from any sort of long quantum string.
A good example of this is metal. We see from a microscopic level that it is all lined up, like someone stitching a sweater. my Squatty H patters could form the basics of atoms that then have a predisposition to line up like fractals line up.
It also may explain the Fibonacci. All I would have to do to prove all this is to deconstruct things from the complex back. It would be like taking snowflakes and photographing a billion different ones, putting them through a computer program to find common base shapes that would permit these more complex shapes.
Most likely in those cases the design of snowflakes has something to do with the trillions of force lines running through the sky and its more likely that the atoms are influenced by these forces to organize slightly different depending on the location the snow flake is being made.
My Model makes an assumption that there is likely several fundamental shapes based on one shape. So you might get 3 H’s touching in such a way to make a triangle, or them fitting close together to form and H inside an H or another would be them end for end, and another side by each. So right there we have 4 basic shapes. And from there everything could be made.
DNA happens to run like that. And form there you can make patterns which could explain every different fundamental particle that then works as a totally different animal than the next one with different charges, but if disassembled they could be come the other.
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And all that would explain quantum locking, the connection of solar systems, gravity. the strong and weak forces etc. all based on a attractive fundamental shape of a squatty H or I.
With this model I figured out how quantum levitation works and believe I could with some experiments could make it such to remove inertia from a space ship and be able to connect to these filaments through space quantum locking to them so you could hover or move through any point of space with no jet fuel. And the cost would be like running an elevator. Just don’t blow a fuse or your car would fall out of the sky.
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https://www.bing.com/videos/search?q=quantum+locking+and+levitation&pc=MOZI&ru=%2fsearch%3fq%3dquantum%2blocking%2band%2blevitation%26pc%3dMOZI%26form%3dMOZLBR&view=detail&mmscn=vwrc&mid=4843B59864F87B05EABA4843B59864F87B05EABA&FORM=WRVORC
you will like this one. Its very possible our brains have connection to very fine strands of information that goes beyond our person. IF we could quantum lock those. could we control people??
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