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|I have a question about quantum entanglement that has been bugging me for years.|
My understanding is that a photon in superposition will create an interference pattern on a screen with a double slit in front of it, and if you activate detectors at the slits to determine which slit the photon passes through, the wave function will collapse and the photon will not create an interference pattern on the screen.
It is also my understanding that when you have a pair of entangled photons, they are both initially in superposition, and if you use a detector on one, the wave function collapses for both photons. So if you send them each toward a screen with a double slit in front of it, both screens will either show an interference pattern if you haven’t used a detector, or they won’t if you did. The detector can determine the spin direction or polarity of one photon, and because they are entangled, you will know the spin direction or polarity of the other instantly (due to quantum non-locality).
Now let’s say Alice and Bob are at two different points in interstellar space, 2 light years apart, and they want to communicate. Directly between them (1 light year from each), we have a photon generator that creates pairs of entangled photons, and sends the photons from each pair toward Alice and Bob. Once the photons arrive, if Alice uses a detector, she can know the spin direction or polarity of Bob’s photon (the other photon of the entangled pair).
But Alice and Bob can’t control the spin direction or polarity of their photons. The no-communication theorem shows that if Alice detects the spin direction or polarity of her photon, and Bob detects the spin direction or polarity of his photon, they will each know the spin direction/polarity of the other, but they will have no way to change the spin of the other, so no information can be transferred directly. And if they don’t use a detector, they won’t know what the spin direction or polarity is, even for their own photon.
But what if Alice and Bob don’t bother trying to determine the spin direction or polarity. Let’s say Alice and Bob both have screens with double slits in front of them, with detectors at the slits. If Alice wants to send a bit of information to Bob, she either turns on the detector at her slits (a binary 1) or she doesn’t (a binary 0). If she leaves her detector off, her photon generates an interference pattern on her screen, and because her photon is entangled with Bob’s, his photon creates an interference pattern on his screen too, so he instantly knows that Alice sent him a 0. When the next pair of photons arrive at their interferometers, if she turns on her detector, her waveform collapses, and no interference pattern is created on her screen. Because her photon was entangled with Bob’s, the waveform of his photon also collapses, and no interference pattern is created on his screen either, so Bob instantly knows that Alice sent him a 1.
In this way, Alice and Bob simply need to agree which timeslots will be used for sending bits in each direction. The generator can be setup to send a stream of entangled photons in each direction, and Alice and Bob can each send bits to each other instantly during their assigned timeslots. They only need to wait 1 year for the photons in the stream to reach them, but thereafter, communication is instantaneous.
Is there a key that I’m missing, or is FTL communication possible?
This question was asked several times through askphysics, but I was thinking whether or not to answer such a simple question. But when I pondered, I felt that there is something which I can tell the world through the answer to this post.
If you search, you can find several definitions to “Machine”
To be quite simple, A machine is any device which makes human efforts easier. It may be used to magnify human capabilities, physical or psychological. It may be partially or completely automated.
A machine is helpful to save human effort, not energy. There are even cases where the machine is not providing any mechanical gain of effort, but changes its direction so that the effort can be conveniently applied.
As per COLLIN’S DICTIONARY
In regards to the double slit experiment with detectors and firing electrons one at a time ….a lab is set up where we run 100 separate experiments where a single electron is fired through a double slit say 500 times each cycle to build a pattern. Beforehand the computer will make a random heads or tales decision as to whether or not the next cycle will be run with the detectors on or off. So statistically 50% of the time the detector will be off and no one will know beforehand we will only know after the fact by reviewing the data stored in the computer as to whether or not the next consecutive cycle was run with the detector on or off.
There’s a catch….the computer is required to make a second random heads or tales decision to entirety erase all the data from an individual experiment so statistically for half of the 100 individual experiments (or cycles) we won’t know if the detector was on or off. In only 50% of the individual runs can we say for certain if the cycle was run with the detectors on or off.
Out of 100 runs how many particle behavior patterns would you expect to see and how many wave patterns would you expect to see.
Hello,I dont know how to do this short exercise,I mean it does not make any sense.The only thing i know is that the answer is 3.3cm but i cant do the procudure finding the same result.This is the exercise:
A concave mirror makes a 3 cm image of a 6 cm object when the object is placed 10 cm from the mirror. What is the focal length of the mirror?
It really would help me if you lend me a hand on this.