"Trying to understand the way nature works involves a most terrible test of human reasoning ability. It involves subtle trickery, beautiful tightropes of logic on which one has to walk in order not to make a mistake in predicting what will happen." -Richard Feynman
If you were to send a space probe to a distant star system, gather information about it and send it back to Earth, you'd have to wait years for the information to arrive. But if you have an entangled quantum system -- say, two photons, one with spin +1 and one with spin -1 -- you could know the spin of the distant one instantly by measuring the spin of the one in your possession.
The quantum mechanical Bell test for half-integer spin particles. Image credit: Wikimedia Commons user Maksim, under a c.c.a.-s.a.-3.0 license.
Are there prospects, then, for entangling quantum particles, placing one aboard a spacecraft and sending it to a distant star, making a measurement at that distant location and then making a measurement here to know what you saw over there? It's an incredible idea to exploit quantum weirdness. While the laws of physics allow you to indeed know the properties of the other member of the pair by making a measurement here, they conspire to prevent you from transmitting information faster-than-light.
Artist’s impression of a sunset from the world Gliese 667 Cc, in a trinary star system. Image credit: ESO/L. Calçada.
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If a spacecraft had a sail and a satellite based laser shot that sail and pushed it to almost the speed of light and the craft had a communications laser pointing to another satellite, like the wire on a wire guided missile to its controller, could you see a live feed if the connection never broke even if the craft is 20 light years away?
“If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.”
In that case it seems that I haven’t understood QM, because the only ‘non-classic’ thing is that we can’t give a particle an actual axis until we measure it. What is so 'shocking' if we observe that when you measure one, that the others is its opposite?
If you could like is pointed out in the article 'force' the first one to be (+) so the other has to be (-), than wow but that's not the case, so what is all the fuss about?
QM is simply limited by our measurement equipment, that's all. If we could find a more refined way to measure particles (from the start) than a better theory than QM could be conceived. Until than the only thing we don't know is what's being emitted at the start, a random throw of the dice. Here again why should I be profoundly shocked??
@Paul Dekous #2: Sorry, you clearly don't understand quantum mechanics. Your hypothesis is what is called a "hidden variables" theory, and it has been explicitly ruled out (in any form) by experiment.
A video feed? Let's instead assume a series of craft coupled by the equivalent of fiber-optic cable (i.e., a lot). All this does is increase the path length, not change the speed limit.
So, with a delay of 20+ years (and increasing with distance), sure. It's the same with telemetry.
No. The uncertainty principle is epistemological.
@JG Bennet #1
It is not just light that is limited in speed. It is information of any kind. Information cannot travel through our universe faster than the speed of light. If a probe is 20 light years away, there is no trick that can be used to get the data gathered by that probe to us in less than 20 years. The universe won't allow it.
Take a black and white sock.. put one in a spaceship and then look at the one you have left... and you will know the color of the one on the spaceship... that's all entanglement is... you can't use that for communication... let alone for superluminal one.
Put 1000 articles about entanglement and how it's the next best thing since sliced bread.. but you still won't be able to communicate with it.
What's fascinating about entanglement is how systems get entangled in a first way.. that the system has "shared" property is not mysterious at all once you made sure that they do by entangling them in the first place.
mathematically it might seem weird.. how properties in matrices get transfered from one set to another and all the thing in hilbert space etc.. but it really boils down to 2 socks.
@Michael Kelsey #3
I'm not talking about hidden variables. I'm simply saying that according to QM, the particle doesn't have an actual axis, until you try to measure it.
If the axis of the spin was predetermined and independent of the way you tried to measure it, you could not get a violation of Bell's inequality.
@Paul Dekous #8: "Predetermined" -- that's precisely what "hidden variables" theories are about. The rest of your statement is correct.
@Michael Kelsey #9
“Predetermined” — that’s precisely what “hidden variables” theories are about.
Yes, that's why I mentioned it.
In my first comment I was talking about getting a deeper insight from the moment a particle, a wave, is created to see what goes on, not about what axis it has; because the axis is now established with the measurement.
@ Michael Kelsey #3
I would add the all important word "local", when speaking about hidden variables. Bell's Inequality deals primarily with non-locality. Hidden variables in non local theory are not disproved. Also, hidden variables in absolutely deterministic universe are also still valid. So saying "have been ruled out in any form" isn't valid IMO. Then again, haven't checked the papers on the subject for a few years... new proofs might have come up.
Imagination travels faster than light.
@Ragtag Media #12: More like 0.3 millionths of the speed of light (100 m/s). That's an upper limit, which doesn't take into account synaptic transfer speed.
@Michael Kelsey #13
Depends on how you look at it.
One can imagine John Lennon's way "Imagine there's no heaven
It's easy if you try
No hell below us
Above us only sky
Imagine all the people
Living for today"
OR One can imagine or rather conceptualize being here in the now typing on the keyboard and yet visualising one's self a million light years away on a planet in a galaxy far far away.
Imagination is the greatest gift God has given humans.
It does trump reality because it is not bound by any laws.
It still takes synaptic transfer to dig up all the memories to create the picture being imagined. Still not faster than light.
:)
@RM #14: From another perspective, you're quite right. Since brain activity takes roughly 800 ms to register in consciousness, in some sense we're all travelling backwards in time :-)
The theorizing of quantum entanglement is also based on the QM application of how the particles interact. Since you can cause them to, in a dumb down wording, clone a particle, whats to say that we cant interact with one of the particles causing instant data transmittion, given all computer data is bianary it really is just a matter how how we can, not if we can.
@Ryan Dunn #17
Information cannot travel through our universe faster than the speed of light. If a probe is 20 light years away, there is no trick that can be used to get the data gathered by that probe to us in less than 20 years. The universe won’t allow it.
Sinisa Lazarek's example in #7 is excellent. When you discover the spin of one half of an entangled pair, it reveals the spin of the other that was baked in at the time of entanglement. Absolutely no information is being transmitted faster than light by entangled particles.
In fact, according to W—ia at least, a proposal for superluminal communication is what led to the no-cloning theorem.
That's local realism, which is out.
@Narad
sorry for the confusion, my intention wasn't to say that socks behave the same way electrons or photons do. My intention was to show that i.e. spin or polarization are properties of those systems.. just as color is a property of the sock. You simply can't communicate with property alone... in any scenario.. classical or quantum.
You could communicate if you could change the property with some periodic interval. But entanglement isn't that. Entanglement is just a correlation of the property.
Now in case of QM, weather or not that property is well defined at the moment of creation of the correlated pair, or when the measurement is made is for the debate on locality and I still didn't fully decide where I stand on that.
My argument is that if general public understood that it's a correlation of properties between two objects.. like colors of socks.. and not manipulation of the i.e. colors.. then everyone realizes that it can't be used for communication.
@Sinisa #21: Narad is correct. Your description isn't quite right. The issue with entanglement is not the _existence_ of a property (like an electron is a spin-1/2 particle), but rather the value or orientation of that property in a particular instance.
Rather than writing a long post explaining this, look up "Stern-Gerlach experiment" in Wikipedia. The point of entanglement is that the "predefined condition" is that the two entangled particles have a property which adds up to a known, fixed value, but that the individual quantities (orientations) aren't fixed until you do a measurement. Consider an electron and positron (spin-1/2) which we produce from an intial spin-1 state. The two particles must have their spins aligned parallel, so they add up to 1. But we can set up our Stern-Gerlach measurement on one of them in _ANY_ orientation. Whatever we measure with it, tells us that the other particle is parallel. But we could do that measurement in any orientation (and we could even change the orientation after the particles were produced!). It's that freedom, combined with the non-classical correlations, which makes entanglement a quantum property.
I believe - but am not sure - that all this theory implies that we can never directly observe a particle or system of particles in an indeterminate state. I.e., any observation of a property must determine that property. Because if that wasn't the case, you could set up remote stations with entangled particles, with the "listening" station constantly observing its particles in an indeterminate state. The "sending" station then starts determining particle states by observation them in a specific, timed sequence, creating a Morse code signal. Since this sort of signaling should be impossible, it follows that the initial set-up - observing the particles in a way that leaves them indeterminate - is impossible.
I think perhaps only Ethan can relate to my concept of the minds imaginations ability to conceptualize faster than light concepts.
In one's mind you can go from imagining hovering over the surface of the sun in one moment to walking on Pluto the next.
That;s faster than light.
Think about it, how do you go from eating breakfast at your kitchen table to having lunch at the "Restaurant at the End of the Universe"?
It's not clear to me how this would encode a signal, since the sender can't predict the outcome of the measurement. It works if the receiver can clone the state (it can't) and the receiver either measures or doesn't measure to sent a bit. If the sender gets different results from measuring the clones, then "no measurement" has been communicated.
But I don't know what the dits and dahs (timings) could convey that very slow Morse code couldn't do in the first place.
^ "and the receiver sender either measures or doesn’t measure"
Hey, Tex.
Except you are still creating images in your mind, which takes time to construct. Everything you visualize within that scene still has to be created dependent on the direction you are looking. Even if you looked at a picture of the location you want to be in, you still have to absorb all the information of that pic. which, once again, requires time to assimilate.
You aren't encoding data in the outcome per se, you're using the timing of the other station's observations to send information. So let's pick spin just for illustration. Agreed, neither station could "decide" whether they were going to get a spin up or down result on a measurement. But the 'listening station could observe this: determination [3 seconds pass], determination [1 second passes], determination [1 second passes], determination [3 seconds pass], determination [1 second passes], determination [3 seconds pass], determination [1 second passes], determination [1 second passes], determination [3 seconds pass], determination [3 seconds pass],
determination [1 second passes], determination [1 second passes], and figure out that the sending station just said "Narad."
It doesn't convey anything a regular signal couldn't, it just conveys a normal message faster than a light beam could travel between the particles - if it worked. Since its impossible for this to work, and there's nothing hypothetically difficult about setting up the "sending" station (in fact, any time we do an experiment with entangled particles and observe one's properties, we are doing something equivalent to my 'sending' station), that must mean the configuration of the "receiving" station is a physical impossibility. Which was my point. It is impossible to set up a system such that you notice (without triggering) when an entangled, indeterminate state particle becomes determinate due to some physical process not having to do with your observational equipment. Because if that were possible, you could use it for FTL information exchange. Again, not trying to fix which quantum state you get, but by controlling and manipulating the delta t between triggering events.
Michael Kelsey, serious question:
Have any entanglement experiments been run with large numbers of parallel photon streams and error-correcting codes?
I understand the non-signalling principle, but isn't it also the case that QM allows for rare statistical violations? The example is used that it's not impossible for the book on your desk to float up in the air, spin around three times, and land on your desk, but it's so improbable that it would take longer than the lifespan of the universe to observe it even once.
That being the case, with a sufficient quantity of entangled photons, occasionally one might "signal." The problem of course is sorting the "signal" from the noise, hence error correcting codes or something similar.
This is actually testable: if it's correct, then with a setup that produces massively parallel entangled photon streams and tries to impart data to them at the "transmitting" end, what you'd observe is a very slight statistical deviation from chance in some kind of "guessing" algorithm at the "receiving" end.
Whether that could ever be scaled up to do something useful remains to be seen.
Maybe this explanation of non-signalling will help somewhat.
(Note here I am making the conventional/canonical case in favor of non-signalling and against the wild speculation in my previous comment 29.)
Alice creates two entangled photons and at first she does not know their axis of polarization. Let's call our options H for horizontally polarized, and V for vertically polarized.
She puts one photon through a polarizing filter that she can choose to orient as H or V.
When the photon encounters the polarizer, that encounter is the act of measurement. If she has set the polarizer to H, and the photon passes through to a detector, she knows she has measured an H photon. The detector registers the photon, and produces a bit: a 1, for "photon detected in the relevant interval of time."
If the photon fails to pass through the polarizing filter to her detector, the detector does not register it, so she knows she was starting with a V photon because it did not pass. The detector produces another bit: a 0, for "no photon detected during the relevant interval of time."
The same process occurs if she orients her polarizer to V. In this case, if her photon is V, it passes, and her detector produces a 1. If her photon is H, it does not pass, and her detector produces a 0.
Meanwhile at the far end of the lab, the companion entangled photon reaches Bob's workbench, where he has also set up a polarizer and a detector. He can orient his polarizer as he chooses. If the photon he receives matches the orientation of his polarizer, his detector produces a 1; if the photon does not match, his detector produces a 0.
Now you might think that Alice can send information to Bob by changing the orientation of her polarizer back and forth between H and V to represent 1s and 0s.
However this is where non-signalling comes in: Alice does not know the starting state of the photon. It's random. It's in superposition until it encounters her polarizing filter.
Here's how that looks:
Photon H, polarizer H, output 1.
Photon H, polarizer V, output 0.
Photon V, polarizer H, output 0.
Photon V, polarizer V, output 1.
For those who know basic cryptology, that's Vernam addition of a one-time random key with the bits that comprise a message. The random key is the starting state of the unmeasured photon, the cleartext (message bits) are the polarizer orientations that are controlled by Alice. The output is the ciphertext (encrypted message bits).
Now Bob at the other end is writing down the bit stream he's getting. But here's the problem:
Bob receives a 1. That 1 could have resulted from either of two conditions at Alice's end:
Photon H, polarizer H. Or, photon V, polarizer V.
Bob has no way of knowing which was actually the case, until Alice sends him her list of the 1s and 0s she observed at her end.
She can do that on foot, or by picking up the phone on her desk and calling him, or by sending him the file via the lab computer network, or by sending it to him at light speed by putting the page of bits in an overhead projector that throws the image across the room to a movie screen conveniently located next to Bob.
This situation is isomorphic to the one-time-key encryption system, where Alice has the key and the text she wishes to encrypt, and Bob receives the ciphertext (encrypted text). Bob's ciphertext "contains" the information Alice was sending, but he can't get at it until he has the key. And Alice can only send him the key at C or slower: If she tries to use entangled photon pairs to send the key, she'll only be encrypting that key with another random key, so she'll have to send that second random key to Bob at C or below.
Remember, Alice has no control over the original orientation of the photon, only over the orientation of her polarizer. The orientation of the polarizer at Alice's end is the message bit she wants to send, and the original superposed state of each photon is a random bit in the encryption key.
And the reason Bob can't decipher the bits he gets is because he does not know which set of starting conditions occurred at Alice's end (Bob's 1 could result from H + H or from V + V).
Does that make sense to anyone here who wasn't getting it earlier?
@G: All of these ideas of "signaling" with entangled systems fail for a simple reason.
Bob (the receiver) cannot know in advance what Alice is doing with her half of an entangled pair. The only way for him to know that is for Alice to tell him, which necessarily requires a conventional (i.e., STL) transmission channel.
When Bob makes his measurements, all he gets is some distribution of results, which are necessarily consistent with simple QM predictions for a particle with an unknown orientation.
It is only AFTER Bob finishes all those measurements, and he and Alice do a comparison (via a conventional STL messaging channel) that the two of them together can discover that the _joint_ distributions have a probability distribution different from what classical probabilities (or indeed, any local hidden variables theory!) would predict (this is what "violating Bell's inequality" means).
Note that you have to be able to compare the measurements at both ends in order to see this effect, which requires a conventional STL communications channel to do it.
What G described in #30 is basically how quantum encryption does work, and it again requires Alice and Bob to get their data together at some point, either before or after, by means of a conventional STL communications channel.
Hi Michael-
Re. 29, I'm looking to see if anyone's done a "monkeys and typewriters" experiment with parallel streams, as I'm thinking that they might at least get a proverbial word or two of Shakespeare;-)
Re. 30: No, that wasn't quantum encryption, that was classical one-time-key encryption. Yeeks, I hate to say this but you're mistaken on the stuff about crypto. I'm using classical one-time-key crypto as an _analogy_ to explain the non-signalling property. Quantum crypto is something else entirely as I'll explain further below.
Actual state of affairs: When Alice generates an entangled photon pair, the initial state of each photon is superposed but the polarizations of the two photons are correlated, and there's a 50/50 probability that Alice's polarizer will allow her photon to pass through. Alice knows how she's set her polarizer, but Bob does not know until Alice tells him.
That is an analogy to: A random key generator that is used to produce key stream (a long string of random bits) with which to encrypt cleartext into ciphertext using plain Vernam addition which is basically exclusive-OR (XOR).
If Alice & Bob want to use one-time key crypto, they do this: Alice gets a random bit stream. For example a piece of thorium and a Geiger counter with the output connected to a digitizer that turns the clicks into 0s and 1s. She fills up a thumb drive with the random string of 0s and 1s, and then makes a copy of the thumb drive and gives the copy to Bob (pay attention everyone, you're going to need this if Trump gets into office!;-).
Now Alice and Bob each have an identical random bit string. Alice writes a message to send to Bob. She uses an app that encrypts the message by XORing each bit of the message with the next available bit of random key. Let's say her message is 100 characters and each character is 8 bits: the message length is 800 bits, and her app uses the first 800 bits of her random key to encrypt it, and then erases those bits from the random key file so they cannot be used again.
Now Bob receives Alice's message, and his app grabs the first 800 bits from his random key file, which are identical to the 800 bits Alice used to encrypt the message. Bob's app XORs each bit of the encrypted message with a corresponding bit from the random key file, and thereby recovers Alice's original message. And of course his app wipes out those 800 bits so he can't use them again.
Now Bob replies to Alice: his message to her is let's say 30 characters x 8 bits = 240 bits. His app grabs the next 240 bits, XORs each bit with its corresponding bit from the message, to produce ciphertext. He sends that ciphertext to Alice, and his app wipes out the 240 bits from his random key file.
Alice receives Bob's message, and decrypts it using the next 240 bits from her key file. After which, her app scrubs those 240 bits from her key file so they can't be reused.
The great advantage of this is, it's absolutely unbreakable, so long as the key material is truly physically random, and it's only used once. Not even quantum cryptanalysis can break this because each possible decryption is equally probable. There is not a mathematical "aha!" point where cryptanalysis recovers a unique key by testing it against an algorithm.
The tradeoff is, this is a "Symmetric Key" system, where Alice and Bob have to meet in advance to exchange key material, for example meet at a yearly conference and exchange enough key material to let them keep emailing each other all year. Unlike "Public Key" systems (PKS generically) where they never have to meet in advance to exchange their exchange public keys.
So now we go on to quantum crypto.
Quantum cryptography does not claim to be unbreakable. However, if any attempt is made to intercept a message on its way from Alice to Bob, the interception constitutes a measurement that collapses the wavefunction of the bits in transit. When those encrypted bits arrive at their destination, the app at the receiving end recognizes that they have been measured prior to their arrival, and puts up a flag to warn you that this message was compromised.
I do not understand the physics of how that works, because any transmission hardware in between Alice and Bob will appear, to a bit string, as a means of measurement. In fact that's necessarily so, because the hardware (for example an amplifier to strengthen the signal so it can pass all the way through an undersea cable) must make a measurement of the bits in order to amplify them (or perform some other signal processing that is needed along the route). If the amplifier also happens to split the signal on the outgoing side, so it also goes to an intercept station, the bits going onward to their normal destination have no way of knowing that.
The folks working on quantum crypto have probably already run into this problem and are trying to design around it, such as by eliminating the need for any sort of signal processing along the transmission route. Or it may be that I just screwed up the description of how it works. However I'm 100% certain about the "one-time key / XOR" stuff because I designed an app based on it in the early 1980s, as an affordable means of secure crypto for the masses, that would run on cheap hardware (Commodore etc.) rather than require expensive hardware (IBM PC). Again, keep "one-time key" in mind if Trump gets elected, you're going to need it.
Quantum cryptanalysis can break any deterministic key system (one in which the key is the output of an algorithm or a non-random physical process), because it can test all possible keys simultaneously and then zero in on the one that produces intelligible recovered cleartext.
Now we go back to Alice & Bob's photons.
One-time key crypto is an analogy. The original state of the photons, before Alice measures them with her polarizer, is analogous to the random bit string used as the key stream in the one-time-key cipher. Alice choosing the orientation of her polarizer for each bit, is analogous to Alice typing a message, bit by bit. The bit string recorded by Bob at the other end of the lab, is analogous to the ciphertext generated by the one-time-key system. BUT, Bob does not yet have a copy of the key.
As it turns out, Alice does not have a direct copy of the key either, unless she extracts it by XOR-ing her detected bits, with the state of her polarizer. to produce the information about their initial state.
For example Alice's detector gets a 0, and is H. From that, Alice can tell that the initial state of her photon must have been V since it did not pass. Alice can do this for every photon she produced, thereby extracting the information about the original state of each photon. That extracted information would constitute the key to the one-time-key system, and if she gave that information to Bob, he could use it to decrypt the bits he recorded.
Alice can only send that information to Bob at c or below, because if Alice tried to send it instantaneously using the entanglement system, it would just be subject to the same uncertainties all over again.
Again, the foregoing is the consensus/canonical version of this, not my wild "with enough parallel photon streams an error might get through" speculation
Is this any more clear now?
@ Michael Kelsey
It's not every day that I manage to find someone like this who has worded it in much the same way as I was trying to (socks, no action at at a distance.. hence no non-locality, etc.),.. I can't try to put it better than that.
https://www.youtube.com/watch?v=AlIlkn3OxMI
"... people say loosely, cruedly.. wrongly... that when you measure one of the photons, that it does something to the other. But it doesn't!... "
Now, of course, we can disagree on the interpretation of qm, but am content for now that I'm not the only one with this point of view :) and that at least am not wrong in the examples given.
G=SN ??
@PJ #34: Not at all. G's comments are on target and reasonable; I misunderstood their initial question, which they clarified.
@PJ #34: ...Oh, wait! Were you perhaps proposing a cosmological variation of Newton's constant, proportional to the entropy in a given region?
No, just noting a similarity in patter from 2 different sources.
I'd put the odds at S.N.'s knowing what a one-time pad is at exactly nil. Anyway, this isn't his niche.
"@RM #14: From another perspective, you’re quite right. Since brain activity takes roughly 800 ms to register in consciousness, in some sense we’re all travelling backwards in time "
Moreover, we are only loosely bound in time.
Get someone to prick your leg with a needle. You KNOW what you did was see the needle prick you and felt the pain RIGHT THEN. Even though it took enough miliseconds to be reacted to.
Amusica and dyslexia are caused by this mechanism that reorders things in the "right" (phenomenological) order going a bit too far.
And that loose time binding is also why we humans are so musical. Music has an extended present, usually over several seconds, and harmonic music is that which has some predictability built in, whilst disharmony doesn't.
Time is odd, but mostly because we are smart enough to know about cause and effect, and were good at tricking ourselves into ignoring reality to make it so.