4 ms·

Non physicist writing here, so maybe this is dumb. What I've read about entanglement is this: do something to a particle, and a predicable thing happens to ano

Non physicist writing here, so maybe this is dumb. What I've read about entanglement is this: do something to a particle, and a predicable thing happens to another particle. So, I am dreaming of a "radio" that has no interference because instead of agitating the electromagnetic field, it manipulates one particle. On the other end there is another "radio" that measures the entangled particle and turns the movement/spin/whatever of the particle on that end into data (voice/video), and vice-versa. The problem, I guess, is the Heisenberg uncertainty principle (again, non-physicists so possibly there are workable cheats around this). So, measuring of one particle already changes it, therefore any incoming data is nullified?

Kelteseth 4y ago
I can recommend everyone this nice detailed video about FTL communications via quantum entanglement (Spoiler: It does not work). https://www.youtube.com/watch?v=BLqk7uaENAY https://www.youtube.com/watch?v=BLqk7uaENAY
- kzrdude 4y ago
  Around 11:50 it answers why it doesn't work in the way the grandparent post asked about (the part that is not about FTL or speed, just why no information is transmitted by measuring entangled spins).
Ralfp 4y ago
Speaking in programming terms, your program was called with two entangled arguments A and B. Entanglement means that if you check for value of A and its true, then B is guaranteed to be false. But if you change A to false, system becomes detangled and now both A and B are false (vs B becoming true). This is why entanglement cant be used for FTL communication.
- another_bear 4y ago
  This makes a ton of sense and has shattered my dream of an intergalactic ansible.
- k__ 4y ago
  Why does the read operation only break entanglement in 50% of the cases?
  - Ralfp 4y ago
    It doesn't. B is guaranteed to be !A and A is guaranteed to be !B. Changing either breaks the entaglement. Also because in order to see what state is particle we need to interact with it, measuring its quantum state also breaks the entaglement.
    - k__ 4y ago
      Ah, I misread it. The action that breaks the entanglement is the update of A before reading B and not the reading of A or B.
tsimionescu 4y ago
Entanglement is in fact much more vulnerable to interference than normal radio transmission - any interaction between the entangled particles and the environment quickly makes them decohere and become disentangled. This is in fact the reason why it's hard to make quantum computers - keeping a large number of particles entangled with each other is very very hard.
chriswarbo 4y ago
Entanglement is a correlation. A common example is putting clothing into boxes and moving them a light-year apart: if our box contains a left-hand glove, then we know the other box has a right-hand glove (and vice-versa). However, there's no way to create a radio using this correlation, since we can't effect whether we got the left/right glove: - If we chose it before travelling, that would (a) require measuring which is which (i.e. opening the boxes), and (b) require choosing our message ahead of time, when the boxes are together (which isn't faster-than-light communication; e.g. we could just as well write our message inside the other box; no radio needed) - If we re-arrange the fabric of our glove after measuring it, that would have no effect on the other glove (our actions would de-correlate them). The reason this glove analogy breaks down is that gloves can be explained by "local hidden variables": i.e. if we see a left-handed glove, that could have been in the box all along; nothing weird. The weirdness occurs in the correlations between different measurements. For example, if we put the clothing on our hand, we're measuring which glove it is (left or right hand); if we instead put it on our foot, we would measure which sock it is (left or right foot). Handed-ness is completely (anti)correlated: if both are measured as gloves, we always get one left-hand and one right-hand; likewise if they're both measured as socks. Handed-ness and footed-ness are uncorrelated: if one piece of clothing is measured as a glove, and the other as a sock, there's no correlation between left-hand/right-hand and left-foot/right-foot. Where it gets weird is when measuring somewhere in-between, e.g. if we measure 2/3 glove and 1/3 sock; and the other is measured 1/3 glove and 2/3 sock: their handedness/footedness is more correlated than classical physics can explain (in practice we measure the polarisation of light, which can be measured horizontally, vertically, or some angle in-between!). Experiments have ruled out "local hidden variables", like "it was always an X" (e.g. something like "the box always contained an 80/20 mixture of glove/foot, with respective left/right mixtures of 50/50 and 20/40"). Quantum physics correctly predicts those experiments (e.g. using "superpositions of left/right handedness/footedness") See e.g. https://en.wikipedia.org/wiki/CHSH_inequality https://en.wikipedia.org/wiki/CHSH_inequality
burnished 4y ago
Side note: you are not the first to dream of this, there is even a word; ansible. Wikipedia tells me that Le Guin coined it for this purpose, I personally first ran into it in the Enders Game quartet. Good question by the way, I appreciated the responses it generated.

Kelteseth 4y ago

I can recommend everyone this nice detailed video about FTL communications via quantum entanglement (Spoiler: It does not work). https://www.youtube.com/watch?v=BLqk7uaENAY https://www.youtube.com/watch?v=BLqk7uaENAY

kzrdude 4y ago
Around 11:50 it answers why it doesn't work in the way the grandparent post asked about (the part that is not about FTL or speed, just why no information is transmitted by measuring entangled spins).

Ralfp 4y ago

Speaking in programming terms, your program was called with two entangled arguments A and B. Entanglement means that if you check for value of A and its true, then B is guaranteed to be false. But if you change A to false, system becomes detangled and now both A and B are false (vs B becoming true). This is why entanglement cant be used for FTL communication.

another_bear 4y ago
This makes a ton of sense and has shattered my dream of an intergalactic ansible.
k__ 4y ago
Why does the read operation only break entanglement in 50% of the cases?
- Ralfp 4y ago
  It doesn't. B is guaranteed to be !A and A is guaranteed to be !B. Changing either breaks the entaglement. Also because in order to see what state is particle we need to interact with it, measuring its quantum state also breaks the entaglement.
  - k__ 4y ago
    Ah, I misread it. The action that breaks the entanglement is the update of A before reading B and not the reading of A or B.

tsimionescu 4y ago

Entanglement is in fact much more vulnerable to interference than normal radio transmission - any interaction between the entangled particles and the environment quickly makes them decohere and become disentangled. This is in fact the reason why it's hard to make quantum computers - keeping a large number of particles entangled with each other is very very hard.

chriswarbo 4y ago

Entanglement is a correlation. A common example is putting clothing into boxes and moving them a light-year apart: if our box contains a left-hand glove, then we know the other box has a right-hand glove (and vice-versa). However, there's no way to create a radio using this correlation, since we can't effect whether we got the left/right glove: - If we chose it before travelling, that would (a) require measuring which is which (i.e. opening the boxes), and (b) require choosing our message ahead of time, when the boxes are together (which isn't faster-than-light communication; e.g. we could just as well write our message inside the other box; no radio needed) - If we re-arrange the fabric of our glove after measuring it, that would have no effect on the other glove (our actions would de-correlate them). The reason this glove analogy breaks down is that gloves can be explained by "local hidden variables": i.e. if we see a left-handed glove, that could have been in the box all along; nothing weird. The weirdness occurs in the correlations between different measurements. For example, if we put the clothing on our hand, we're measuring which glove it is (left or right hand); if we instead put it on our foot, we would measure which sock it is (left or right foot). Handed-ness is completely (anti)correlated: if both are measured as gloves, we always get one left-hand and one right-hand; likewise if they're both measured as socks. Handed-ness and footed-ness are uncorrelated: if one piece of clothing is measured as a glove, and the other as a sock, there's no correlation between left-hand/right-hand and left-foot/right-foot. Where it gets weird is when measuring somewhere in-between, e.g. if we measure 2/3 glove and 1/3 sock; and the other is measured 1/3 glove and 2/3 sock: their handedness/footedness is more correlated than classical physics can explain (in practice we measure the polarisation of light, which can be measured horizontally, vertically, or some angle in-between!). Experiments have ruled out "local hidden variables", like "it was always an X" (e.g. something like "the box always contained an 80/20 mixture of glove/foot, with respective left/right mixtures of 50/50 and 20/40"). Quantum physics correctly predicts those experiments (e.g. using "superpositions of left/right handedness/footedness") See e.g. https://en.wikipedia.org/wiki/CHSH_inequality https://en.wikipedia.org/wiki/CHSH_inequality

burnished 4y ago

Side note: you are not the first to dream of this, there is even a word; ansible. Wikipedia tells me that Le Guin coined it for this purpose, I personally first ran into it in the Enders Game quartet. Good question by the way, I appreciated the responses it generated.