> Changing the properties of one particle will cause the other to instantly change too
Is this true? Doesn't that mean if we can control the spin of one particle on command, the other will instantly change according to our control?
So basically, FTL communication is possible iff controlling the spin of a particle is possible.
The important part about entangled particles is if you choose to measure two entangled particles in the same direction you will always get the opposite answer. Note that once you do such a measurement, the particles are no longer entangled and you're gonna need another timmy.
Teleporting the quantum state involves transferring the entanglement of a particle here to a particle over there. This is done by using another pair of entangled particles. It's not possible to just transmit quantum state by purely classical means.
I'll also note that the "transmitted instantly" part depends on your interpretation of quantum physics.
Ok, the way I interpret it is that entangled particles are like 2 hard boiled eggs that have collided.
It is well known that if you roll 2 hardboiled eggs into each other, only one will crack.
So basically, if I roll two hardboiled eggs into each other and then hide them in paper bags, the eggs are now "entangled" and if I separate the bags by 1 million miles, I can observe the state of my egg and know instantly what the state of the other egg is.
For example, if I peer into my bag and see my egg is not cracked, I know that the other egg is indeed cracked! However, if I change the state of my egg (crack it) then obviously nothing happens to the other egg because the eggs are no longer "entangled".
There is no "instant transmission" happening here. All the properties of the eggs were set in stone the moment the eggs collided and thus it doesn't matter how far away you move them because the "entanglement" (collision) event already happened.
This is not a very useful property of hardboiled eggs and so I fail to see how this could be useful by scaling down the eggs to photons.
The way I understand it, particles have an "absolute orientation" in space that is impossible to determine because currently we are limited to measuring the orientation relative to another orientation and getting a yes/no probabilistic outcome (also, we can only perform this measurement once because the act of measurement changes the orientation of the particle).
Nevertheless, in reality the particle has an absolute, no-guesswork spin orientation property that a theoretical godlike being running the universe simulation would be able to observe in a "read only" fashion. (We mortals have to use probability abstractions because we don't have sophisticated enough methods of getting quantum information in a read-only fashion)
So basically, when you "entangle" two particles, the act of entanglement affixes their absolute spin orientation to be polar opposites. Bell's theorem doesn't disprove that, afaik, and it accounts for the "spooky action at a distance".
Likewise an eavesdropper would have to alter one of the photon's spin in order for it to reach their eavesdropping sensor, and at that moment, it is useless. Same goes for the true recipient, because the photon passing through the eavesdropping prism changed the spin simply by reflecting it away from its original polarization, and so the "key pair" will no longer match.
Think about how networking works. If your stream of photons is treated essentially like binary (but better), then your packets will all end with a checksum of sorts. If the packet contained an error, the checksum won't match, and you resend the packet so eventually the message gets there.
It's not a very "spooky" factoid when you look at it purely mechanically. I believe this is also why we can coax out quantum behavior from water, electricity, and other things that move around easily. We can even demonstrate quantum weirdness using 3 polarized lenses like you get from the cinema.
The randomness you can envision with all this travel and bouncing and sensing adds up to a lot of noise, making it an expensive problem to solve before things get practical. Early IP networks were pretty crappy. Look at how they go now! The information is light speed. The reading of it is simultaneous. Splitting one of the protons into another pair leaves you with 3 unrelated protons that are not read at the exact same moment (a little simplified - there is math involved to calculate when is that EXACT moment).
In reality there’s some extra concepts to the quantum states because they act as though the selection of which glove is in which box doesn’t happen until we force the gloves into a definite state through opening the box. But that doesn’t change the nature of there being no information transfer.
You can do the same sort of above speed of light signaling fully classically. If you for instance consider a lighthouse with a single beam switching between two far away points. One location will know instantly the signal the other receives, but since you aren’t controlling the lighthouse moving back and forth you can’t transfer information through the signal.
>the spin you observe is effectively random, so there’s no way to know that the results match up unless you compare them with another means of communication, one that is at most the speed of light.
The term for this is called "quantum teleportation" and for each qubit that is "teleported" there are two "classically communicated" bits that are required for information to be transmitted.
When you measure particle A, you get random value 0. B is instantaneously set to 1. Or the other way around.
But you can't communicate using this, because you can't control the random value you get, thus there is no way to "modulate" the other end.
It has been proven mathematically that FTL communication of information using entanglement is impossible (under our current understanding of physics, of course). It's called a "no-go theorem":
However the way you make a measurement in qm is by causing the small subsystem to become correlated with a huge external system, so that for instance the spin of your one small particle is now 100% correlated to a thermal reservoir of maybe 10^23 other particles, such that a led lights green if the particle was spin-up and red if it was spin down.
That’s like observing your neighbor by blasting down their home with a cannon firing a continues stream of other people at them, tearing the brick walls to shreds and observing the splatter patterns to tell after the fact if your neighbors where home or not.
Now the tricky thing here is that by definition, your “observer” has to 100% correlated with the state of the system, otherwise the measuring device isn’t accurate. But the device is constraining the state of your neighbors to either “home” or “not-home” in order to be certain in the outcome. So there no room left for details like “home and playing chess” or “home cooking dinner”, and obviously those details also fairly quickly get lost once they see their walls get torn to shreds by your observer person cannon.
Now this all might make it sound like we just need more delicate measurement devices, but that’s not the case. Any device that can 100% correlate to the state of the subsystem has to force the subsystem into a definite state. So the question isn’t “can you observe a particle without changing it” it’s “can you create a device that tells you either ‘yes the particle is spin up’ or ‘yes the particle is spin down’ with 100% certainty and no other outcomes, which however does not force the system to actually be either up or down?”. This obviously is not possible.