# Semi-FTL quantum communications

I'm trying to come up with idea's that have some plausibility of sending information FTL.

So we can entangle particles together. When you measure them you influence them. If you measure one up the other becomes down. If you measure one left the other becomes right.

Lets assume that for whatever reason, someone has found a way to keep an entanglement going for 1 hour (I'll up this for my fluff story but that just complicates matters for the question). We have two computers, one on earth and one on a sattelite at half an hour lightspeed away. Each computer has a quantum segment in a hamsterball configurarion that is aligned compared to the sun (or galactic center or whatever, as long as they can change themselves to always be in more or less the same orientation its fine). We entangle some bits in the quantum segment with light and send this to the sattelite that can capture these and store them in the quantum segment (or any copies made or whatever). The quantum segment can for convenience send 100 megabite worth of entangled particles per second without running out of bits to entangle. Bits that disentangle can be re-used allowing a constant stream of entangled bits to be send.

the problem with this method is always that you can use these bits just once, and they need to travel at lightspeed to the destination so you cant send messages faster than light. But maybe you still can by cheating a bit?

Imagine this: up/down is a 0. Left/right is a 1. Diagonal means the bit hasnt been used. Now you send out a packet of bits containing non-entangled information and entangled "blanko" information that hasnt been influenced yet. The non-entangled information contains a timestamp when the packet was made and when the entangled information will be measured to give it a meaning. The non-engltangled bits could also carry other non-timesensitive information. The time it will be given meaning is set, for example at 50 minutes after it was made so no disentanglement will happen before it can be read.

The sattelite receives these packets half an hour after they were send and sees how much time has elapsed and other info in the non-entangled information. Then after the remaining 20 minutes +50 milliseconds have elapsed it reads the quantum information (taking all types of relativity into account when measuring time like we do with our current sattelites).

Now comes the cheating bit. Instead of sending packets with data to the sattelite ("how are you doing", "change course by this much" etc) you write those into the quantum bits that are at the 50 minute mark by influencing their orientation. This creates a string of zero's and one's and the unused bits are set to the front/back configuration to indicate they arent used. Then a few milliseconds later this information is read by the sattelite who cant influence the directions of the bits anymore. Immediately after reading them the send entangled particles are discarded to make space for the next string of entangled bits.

While the particles take time to get there, you've effectively send information faster than light compared to when the instruction within the entangled bits were set (this idea is somewhat similar to how alcubierre drives can use sequenced highways to overcome the "cant send negative matter FTL" problem).

Because I'm an optimist my question is: what is wrong with this setup?

• "You write those into the quantum bits that are at the 50 minute mark by influencing their orientation": this will break the entanglement. You cannot set the state of one of a pair of entangled particles to a known value without breaking the entanglement. As soon as you have "written" something into the entagled particles kept on Earth, they become disentangled with their siblings on the satellite. You may be confused about what is meant when physicists say that quantum entanglement cannot be used to transfer information; it's not an experimental result, it is a mathematical theorem. Mar 13 '20 at 20:45
• Unless you are absolutely going to pin your plot on exactly how your FTL communication works, and en exploration of what changes to physics you made to make it work, I'd probably recommend avoiding trying to pin it to real physics concepts. The well-worn genre convention "FTL communication works in this setting" is just lying there. Take it and run. Mar 14 '20 at 0:13
• @notovny I prefer a realistic as possible approach. Mar 14 '20 at 18:58

The whole "moving the entangled particles around slowly" thing is a red herring - even if you can magically teleport your entangled particles, you still can't transmit information faster than the speed of light using their entanglement.

The way that entangled particles work is not that you set one end to one state, and the other end gets the state. The way they work is that if you measure one and find that it is in a particular state, then you know that the other one is also in that state. There's no way to force it to take on any particular state without breaking the entanglement.

Source

• Reading the article and your answer, you are talking about physically turning the entangled particles for effect right? I (was trying to) talk about using the measurement to set the direction of spin of the particle. A forced vertical measurement always has a vertical up/down spin measurement while a forced horizontal always has a horizontal up/down spin measurement etc. Since it is the direction and not the up/down spin I try to use I was hoping it could work. Mar 13 '20 at 21:01
• @Demigan No, we're talking about spin states. You cannot "force a vertical measurement" without breaking entanglement. Further, spin does not have a "vertical" or "horizontal" nature. Mar 13 '20 at 21:20

It does not work this way. While it's true that measuring one half of an entangled pair that you cause the other to "flip"... the other side can't see that. The only way for them to do so is to measure it from their end. So they can't tell if they're receiving or sending. This also rules out more complicated schemes... like if you send two pair-halves.

They know which is the receive, which is for transmit. They still can't read the "receiver" without disentanglement. So if they read it early, they've prevented messages from ever arriving... and they won't do better if they wait (which defeats the purpose of FTL communication) because you have no control over which way it flips.

Even sending 1 bit of information is impossible here. We can't answer a yes or no question with it. We can't pre-arrange and say "when you see the muon flip over you know it's time!" because the act of seeing would flip that muon over even if the signal hasn't been sent yet.

There's a more complicated explanation, and it's called the No Cloning Theorem, but it's pretty absolute to the best of our knowledge.

Also, keep in mind that in most cases when physicists use terms like "up/down", they tend not to mean these things literally. These describe qualities that rarely have anything to do with macroscopically visible effects.

Sorry if this sounds harsh, but you have a fundamental misunderstanding about how quantum entanglement works. Let me try to explain:

I have a bag, a red marble, and a blue marble. I put the marbles into the bag. Now, without looking, I reach into the bag and pick a marble at random. Keeping the marble firmly clasped in my hand, so that I can't see it, I hand the bag off to my friend, Kate. Kate then reaches into the bag, grabs the remaining marble and keeps its color hidden too.

Effectively, the marbles in our hands are now "entangled". Neither of us knows which color of marble we have and the marbles in both our hands are effectively in a "superposition" of being red or blue. Now, Kate and I can go our separate ways, never looking at the marble's color, and even if she decides to go to the far side of the known universe, the marbles would still be "entangled".

This "entanglement" is broken when one of us looks at the marble, and it happens instantly. If I look at my marble and see that it is red, then I know instantly that Kate has the blue marble. Likewise for Kate, if she peeks at her marble and finds it blue, then she knows that I have the red marble, no mater how far apart we are.

While this example is greatly simplified, it is an example of the no-communication theorem. If you want to do FTL in a "realistic" way, I recommend something like wormholes or going into "Clarke-tech" territory with things like "subspace".

• I will so steal this elementary analogy! Plus one. Mar 14 '20 at 14:50
• I understand what you mean, but it does not answer my question. What you describe is the spin up/spin down characteristic, which is the exact thing I didnt use. All sources I found used a measuring device and if you measured an entangled particle it will always be in line with the measurement, either spin up or spin down (in line or in reverse inside it). If you turn the measurement device 90 degrees the particle will spin in that direction, and since no source I previously had mentioned it I assumed that the twin would then also be in this position. This is not the case I just found out. Mar 14 '20 at 18:57
• @Demigan, yes, to be clear, the mistaken assumption was that rotating a spin (i.e. with a magnetic field) would cause its far-away entangled spin to also rotate. This is not the case. In Dragongeek's marble analogy, that would be like adding a translucent yellow paint to your marble without looking at it. After measuring, you would either see green, (meaning that it used to be blue, and therefore that Kate's marble is red) or orange (meaning that Kate's marble is blue). Nothing local that you do to your marble can affect Kate's. Hence, no FTL information transfer. Mar 14 '20 at 20:12