Why can't quantum entanglement be used for speed of light communication or slower?

[jasong]

Originally Posted by xilman

It can, and it has been. Search on quantum key distribution.

I know about that, but I'm talking about coherent communication. As in being in control of what gets transmitted.

[jasong]

Originally Posted by CRGreathouse

Fundamentally, when you and a partner have a collection of entangled particles, you don't communicate information when you both measure them. Rather, you both get (in essence) the same copy of noise/static/etc. So it's as instant as you like, but it doesn't convey information, thus circumventing the ban on speed-of-light transmission. Waiting longer doesn't somehow make the particles transmit information: they'll still just show noise, like they always do. (Indeed, if you measure them without entangling them you'll get the same sort of noise.)

I was becoming psychotic right around the time I started my physics class in high school, so I didn't learn a whole lot. But the reason I said slower than light was because I know faster than light communication is theoretically impossible. I'm assuming that if deterministic commuication is possible, something will happen to slow it down. Or maybe it will need to be slowed down for a new, deterministic method to work. If one exists.

[VBCurtis]

Originally Posted by jasong

One, as far as I know, most communication involves electrons, which I believe travel at a third the speed of light.

Huh? An electron is a physical object. Why would one travel at any particular speed? That's like saying "baseballs travel at 95 MPH." They travel as fast as they're thrown/shot/fired/whatever, and the same is true of electrons.

You should also learn about the distinction between the speed electric current travels in a wire and the speed the individual electrons travel within the material.

For example: https://www.scienceabc.com/nature/wh...ectricity.html

[xilman]

Originally Posted by VBCurtis

Huh? An electron is a physical object. Why would one travel at any particular speed? That's like saying "baseballs travel at 95 MPH." They travel as fast as they're thrown/shot/fired/whatever, and the same is true of electrons.

You should also learn about the distinction between the speed electric current travels in a wire and the speed the individual electrons travel within the material.

For example: https://www.scienceabc.com/nature/wh...ectricity.html

Yup. The rest mass of an electron is roughly 0.5MeV. Before it was shut down, the LEP collider at CERN reached energies in excess of 200,000 MeV, so the relativistic mass increase factor was over 400 thousand. Exercise: convert that into a velocity with respect to the lab. The electrons within quasar jets are travelling much faster.

To be fair, neither of those are used for point-to-point communication.

[retina]

Relative to the most distant parts of the Universe that are beyond our reach we are travelling at FTL, hence the reason we can't get there. Although, perhaps I am stretching the meaning of "travelling". Many people might suggest that space moving apart is not the same as travelling (through space).

[xilman]

Originally Posted by jasong

Technically, yes, but two quick points.

One, as far as I know, most communication involves electrons, which I believe travel at a third the speed of light.

traceroute to www.mersenne.org (162.212.57.131), 30 hops max, 60 byte packets
 1  adsl (192.168.1.1)  2.210 ms  3.126 ms  3.699 ms
 2  111.red-80-58-67.staticip.rima-tde.net (80.58.67.111)  40.612 ms  41.851 ms  43.312 ms
 3  * * *
 4  121.red-81-46-0.customer.static.ccgg.telefonica.net (81.46.0.121)  73.058 ms  74.828 ms  78.087 ms
 5  117.red-80-58-96.staticip.rima-tde.net (80.58.96.117)  75.596 ms  78.166 ms  78.271 ms
 6  ae0-400-grtmadno2.net.telefonicaglobalsolutions.com (213.140.51.56)  84.640 ms  64.441 ms  64.643 ms
 7  5.53.6.64 (5.53.6.64)  65.959 ms 176.52.248.174 (176.52.248.174)  62.541 ms 5.53.6.64 (5.53.6.64)  65.891 ms
 8  176.52.248.178 (176.52.248.178)  66.971 ms  68.125 ms  69.507 ms
 9  94.142.107.37 (94.142.107.37)  70.287 ms  71.241 ms  72.988 ms
10  89.149.139.201 (89.149.139.201)  196.000 ms  197.222 ms  198.378 ms
11  ip4.gtt.net (173.205.47.98)  210.433 ms  211.312 ms  195.014 ms
12  * * *
13  Dallas-TX.r1.Public.Pwr-2xPDU-2xUPS-2N-100SLA.incero.com (144.168.34.10)  192.131 ms  192.399 ms  192.595 ms
14  * * *
15  * * *

I am communicating with you right now through a WiFi network in my house (the first in the traceroute output above). That link goes at within 0.1% of the speed of light. From there links 2-4 and possibly 5 go over metallic wires. Link between 5 and 6 is fibre from the Canaries to mainland Spain. Communications on that link travel at roughly 3/4 of the speed of light. Thereafter it's hard to interpret but the transatlantic link almost certainly goes on a fibre and I would expect the US infrastructure to use fibre in large part. How you connect to www.mersenne,org is also unknown to me but would be a surprise if it is exclusively over copper.

[CRGreathouse]

Originally Posted by xilman

Link between 5 and 6 is fibre from the Canaries to mainland Spain. Communications on that link travel at roughly 3/4 of the speed of light.

Makes sense, since the speed of light in fiber optic is about 31% slower than "the" speed of light (in a vacuum, that is).

[CRGreathouse]

Originally Posted by jasong

But the reason I said slower than light was because I know faster than light communication is theoretically impossible. I'm assuming that if deterministic commuication is possible, something will happen to slow it down.

Your current understanding is that quantum entanglement would allow communication, except that FTL isn't possible, so something happens to stop that. If the communication isn't FTL, then nothing stops that, then quantum entanglement allows communication!

This isn't how it works. There's no point at which something steps in, this just isn't what entanglement means. Reread my last post for an idea of what entanglement does mean. Or look here for a lighter take:
https://www.smbc-comics.com/comic/the-talk-3

But there are things you can do if you have the ability to communicate already. First, you could just ignore the entanglement and communicate. Bingo, lightspeed or slower communication!

Second, if you have N bits entangled and a message of length M you could use superdense coding to transmit it using only max(ceil(M/2), M - N) bits/qubits.

[xilman]

Originally Posted by CRGreathouse

This isn't how it works. There's no point at which something steps in, this just isn't what entanglement means. Reread my last post for an idea of what entanglement does mean. Or look here for a lighter take:
https://www.smbc-comics.com/comic/the-talk-3

A magnificent exposition and thanks for drawing it to my attention. Now I know of its existence I'll doubtless be pointing others in that direction.

[LaurV]

Originally Posted by CRGreathouse

Or look here for a lighter take:
https://www.smbc-comics.com/comic/the-talk-3

thanks for sharing, nice one. Not to mention that I spent half hour navigating back and forth on that site (which I didn't know). Most of the comics are junk, but few are really-really good! (of course humor is subjective, other people may think funny what we don't and viceversa).

[CRGreathouse]

Originally Posted by LaurV

thanks for sharing, nice one. Not to mention that I spent half hour navigating back and forth on that site (which I didn't know). Most of the comics are junk, but few are really-really good! (of course humor is subjective, other people may think funny what we don't and viceversa).

Alternately, navigate back and forth on
https://www.scottaaronson.com/blog/
where you will find more of the quantum computing stuff rather than more of the comics stuff. (You can also search for just quantum if you don't want his other blogging.)