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this post was submitted on 02 Feb 2024
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What about quantum entanglement sending a signal faster than light?
(I’m just some schmo who watched an extra credit history series on quantum computing, so there’s every chance in the world that I don’t have it right. )
While the entanglement "signal" is near instantaneous, for various reasons no meaningful information can be deciphered faster than C.
Assuming our quantum theory, while not complete, is not wrong. We will not be able to engineer our way around this limit. A lot of funky shit becomes possible if you can break causality even with "just" information.
I thought the reason quantum theory is so controversial is because it does break causality. Like, currently we can’t decipher it, but is that supposed to be a permanent state- that quantum information is indecipherable until it would no longer transmit information faster than light?
You can transmit something, but it has a noise added to it. To decode it, you need to send the readings to the other end, via normal means. Basically, the receiver can tell, in hindsight, that a message was sent, but only once its other half has been received via normal means. The best you can do is get a timestamp of when the message was sent, as well as a message channel that is impossible to intercept.
The problem comes when QM meets relativity. With instant communication, you can send information into its own past. E.g. A and B are 2 planets. C is a ship, passing planet B at relativistic speeds. Planet A sends a message to B, over the FTL link. B then sends it to C, over a normal link. C, finally sends it back to A over FTL. Due to the 'tilt' of C's light cone, the "now" of A-C is behind the "now" of A-B. This allows for paradoxical situations. The maths of Relativity implies that you can't form a closed time loop like this. Such behaviours tend to imply some deeper rule, even if we haven't found its cause yet.
Quantum mechanics has a lot of strangeness. It also seems to play fast, but not loose with causality. E.g. objects can move backwards in time, but still obey causality. Others can be smeared over time space, but still collapse to a causality obeying state. Etc
I so wish we could experiment with this to see where it actually breaks down
That's one of the things we are looking for with particle accelerators, like CERN.
Quantum Mechanics is ridiculously accurate, within its domain. However, it doesn't predict, or allow for General Relativity.
GM is ridiculously accurate, within its domain, but doesn't allow for quantum mechanics.
Therefore we know both must be wrong (or at least incomplete).
Unfortunately the overlap is when gravitational forces become significant on quantum scales. There's 4 ways to study this.
We can pack a ridiculous amount of energy into a tiny space, in a controlled manner. This is the best method. We also can't do it.
We can pack a ridiculous amount of energy into a tiny space, in an uncontrolled, brute force manner. We can then hope to get lucky, or do it enough to beat the odds. This is what particle accelerators like CERN do. We can't control what hits when accurately, but we can do enough collisions that 1 in a trillion is useful, then sift through the data looking for it.
We can use tricks to 'stretch' the quantum realm. This method is limited, but interesting. Gravity wave detectors effectively do this. They can use a laser to create an effect quantum object measured in meters or more.
We can look for places where quantum gravity is dominant, and see what happens. This is what things like the web space telescope are good for. We can look closely at black holes, and neutron stars, and see what they do to space time. Unfortunately, we are also stuck with whatever the universe happens to have done.
In short, the problem is being chipped at. It's painfully slow, and buried in ever more complex maths, but it's being done. I would love to see this "solved" in my lifetime. It's unlikely, but could happen.
Quantum theory is only "controversial" to the general public, mainly because we haven't found a way to explain in simple terms things like superposition, entanglement, quantum tunneling. Quantum theory is spectacularly successful, though incomplete.
Even the "simple" stuff like the uncertainty principle takes a detailed understanding to properly grasp why there are pairs of properties that are inherently linked, and that information about one dictates how much you can know about the other. e.g. position/momentum and energy/time.
Well there’s my problem- that stuff does seem easy, so I’m probably skipping the work to understand it somewhere.
I might be wrong, but iirc quantum theory just straight up doesn't give a shit about causality. Where everything else requires the cause to be observable before effect (something travelling faster than light would result in effect being potentially observed before cause), quantum theory says, "why does the universe give a fuck whether or not we can see it? If it happened, it happened, regardless of whether or not we observed cause before or after effect."
Basically as far as we can tell there there is no information traveling at FTL speed so it just works? All information that is traveling is just as fast as c or slower.
"Certain phenomena in quantum mechanics, such as quantum entanglement, might give the superficial impression of allowing communication of information faster than light. According to the no-communication theorem these phenomena do not allow true communication; they only let two observers in different locations see the same system simultaneously, without any way of controlling what either sees." link
"In physics, the no-communication theorem or no-signaling principle is a no-go theorem from quantum information theory which states that, during measurement of an entangled quantum state, it is not possible for one observer, by making a measurement of a subsystem of the total state, to communicate information to another observer." link
Thank you for this, by the way. I was thinking of the two entangled electrons as communicating with each other, rather than people communicating with each other through the entangled electrons, which I think makes a difference, because it doesn’t rely on interpretation, but obviously we can’t measure how or if electrons “communicate.” Is it correct that one of the limitations is in interpretation or am I reading this wrong?
Well, yes. We don't know if the measurement we take is the result of a wave form collapse (we caused it) or the result of someone else having measured it, which would giving us the oposite value that they measured. We can't tell if someone "sent" information or if it was the random result and we have no way to chose what value we (or the other end) gets when we collapse it.
This isn't easy to explain over text so I'd recommend watching this video, specifically chapter "How to exploit?" as the visuals make it easier to understand.
Here is an alternative Piped link(s):
this video
Piped is a privacy-respecting open-source alternative frontend to YouTube.
I'm open-source; check me out at GitHub.