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sday
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In another thread in another forum I started side tracking the thread. I'm trying to understand why communication is not possible so I am reposting as suggested some of the info over here in a more appropriate forum to continue the discussion.
I'll start with my favorite post that helped the dummy (me) get a starters grip on entanglement.
which was followed by me:
@DennisN - That was awesome! Thanks. Let me see if I have this straight then. So for them to know, they measure say 100 photons and get 70% head 30% tails. Then go measure the other sample from lab B and it should be something statistically close to 30% heads and 70% tails. Is that correct? I now see how this cannot be used for communication.
The last part I'm not understanding is wouldn't you have to measure both samples at exactly the same time to know they are entangled? I mean when I go back to Lab B to make the measurement, who's to say that the results don't come up exactly as with Lab A since the coins are rotating and would only represent the mirror at the exact moment the measurement is made...
Followed by
and a similar response by DennisN
next is my follow up question...
I'll start with my favorite post that helped the dummy (me) get a starters grip on entanglement.
DennisN said:(my apology to the original poster since this reply is somewhat off-topic, I just want to reply to sday about entanglement)
Hi sday! In addition to what mfb said, here's an analogy to describe entanglement;
Imagine two coins, each rotating fast (the coins represent photons, and the rotation represents their indefinite state, in superposition). Further imagine that you just know that they are rotating, not exactly how they are rotating.
Now you (Alice) measure one of the coins, that is by stopping its rotation, forcing it into a definite state, which can either be heads or tails. You can not decide/influence the outcome of the measurement, all you can do is measure. Let's say you measure heads.
Later someone else (Bob) measure the other coin (and the same rules apply). This coin will be measured as tails. On the other hand, if the result of the first coin would be tails, the second coin would be heads. This is entanglement.
From this, the following applies:
1. Since you can't influence any outcome (you can only measure it), you can not use this to send any classical information between Alice and Bob.
2. Imagine you do the same procedure with many pairs of coins. You can not look at the measurements of only one party (Alice or Bob) to determine if the coins were entangled. Alice will always see her measurements as completely random. Bob will always see his measurements as completely random. But if all measurements are brought together and compared pair by pair, an entanglement can be discovered.
Note 1: The description above is only an analogy, the issues are of course more detailed than this (e.g. using photons and different polarizer settings). If you are interested, I suggest you read about Bell's theorem and Bell inequalities here and/or DrChinese's page here (he's a member on PF).
Note 2: You can also search for "entanglement" in the PF Quantum Physics subforum. There are MANY threads on this topic. You can also start your own thread about it there, if you like.
Note 3: Quantum teleportation is about teleportation of states of objects, not of objects themselves. Anyway, a classical information channel is needed to exploit it.
which was followed by me:
@DennisN - That was awesome! Thanks. Let me see if I have this straight then. So for them to know, they measure say 100 photons and get 70% head 30% tails. Then go measure the other sample from lab B and it should be something statistically close to 30% heads and 70% tails. Is that correct? I now see how this cannot be used for communication.
The last part I'm not understanding is wouldn't you have to measure both samples at exactly the same time to know they are entangled? I mean when I go back to Lab B to make the measurement, who's to say that the results don't come up exactly as with Lab A since the coins are rotating and would only represent the mirror at the exact moment the measurement is made...
Followed by
mfb said:No. There is no time-dependence at the photons, it does not matter at which time you measure them.
In terms of the coin-example, one coin would have to stop as soon as you measure the other one (but you cannot see it stopping). Ok, as you can see the analogy does not work here any more.
and a similar response by DennisN
next is my follow up question...