# 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, 2020 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, 2020 at 0:13
• @notovny I prefer a realistic as possible approach. Mar 14, 2020 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, 2020 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, 2020 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.

• The "timing problem" is actually solved by OPs idea. The reciever knows exactly when the entanglement collapse is going to happen because the package containing the entangled bit also contained a timestamp to when the sender will do his write in the entangled bit, so the reciever can read it shortly after. Nov 3, 2023 at 11:58
• @datacube This isn't physics.SE. If that's what OP wants for his story, he should go for it. But in the real-world, this simply doesn't work. Physics is an asshole, and it doesn't want us to have any real fun, including FTL communications. Even if you know when to decohere the split pair, you only get to see (the opposite of) what the other side sees. You don't get to choose which way it flips. Nov 3, 2023 at 14:05

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, 2020 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, 2020 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, 2020 at 20:12
• what I am wondering is: if quantum entanglement really was just that simple, like the split marble example, why do physics scientists make so much fuss about it. Especially about the "measuring" step. In this analogy measuring does not do anything special. Nov 3, 2023 at 12:06
• To make my marble analogy a bit more "realistic", imagine that the marbles are originally both gray (and entangled) but still in their bags. When I take my marble out, the instant I look at it, it becomes colored (either red or blue) and I instantly know that Kate's marble, presumably still in the bag has become the opposite color. This is however not that useful because Kate has no idea that the bag no longer contains a gray marble, and I can't be sure what my marble looks like while it's still in the bag. For example, if I take it out and see it's red, maybe Kate already looked at hers. Nov 3, 2023 at 14:57

I've thought a fair bit about how to explain this, and here is the best I've come up with:

1. Let's say we have a machine, that will take in a coin and split it in half so you have a "heads" half and a "tails" half. It then puts those two halves randomly in a pair of small boxes. So you put a coin in, and out comes two small boxes. Those two boxes are entangled in the sense that if you open one of them, you instantly know what the other one contains and you can only know what the other one contains if you open your box.
2. You take one box and mail the other box to me. Let's say a third party handles the actual mailing so you don't even know where my box actually is, just that I have it.
3. You open your box and find that it's the "heads" half of the coin.
4. You now know that my box, wherever it is, contains the "tails" half of the coin.

For quantum FTL communication to work via entanglement, after step 2, but before step 3, somehow you must be able to do "something" that allows you to ensure my box contains whichever half of the coin you want it to contain at that time.

This is what leads to the general consensus that communication via Quantum Entanglement is impossible because whichever half of the coin you have, there's nothing you can do to your box to ensure that I have a specific half of the coin. You can open your box and know instantly what half I have because it's the one you don't have, but that does nothing to affect my half.

So there are a few obvious ways you can do this:

1. You can modify the machine to ensure that I get a specific half, but it doesn't allow you to pick which half I have during step 3. It's not really communication.
2. If you are able to travel back in time, you could do something similar to the one above to ensure you have the box you want to have in the future, but that's it's own issue (still this is a certainly viable way to go if you can figure out how lol). You could also do this via a wormhole or teleportation, but none of these are communication via quantum entanglement but just other ways to get around the issue.

The underlying issue is when exactly is what half will be my half determined. Is it in step 1 when the coins are split and put into boxes? This would be classical physics view and in that case, communication would be 100% impossible. However, quantum mechanics suggest it might be in step 3 when you open your box. If so, then the possibility of communication via Quantum entanglement exists, but the how is a huge question mark.

IMO, I think the only way it could be done is via a tiny probability change just enough that when spread across millions or billions of atoms, it allows for detection. IE: Rather than being an exact 50% chance of it being the "heads" half, if you did something between steps 2 and 3 that made 50.01% chance of being "heads" this would work for communication if you could also do something else that made it a 50.01% chance of being "tails" instead.

Some alien conspiracy theories mention a much higher understanding of mental abilities including potential mental powers/abilities. That could lead to the kind of tiny probability adjustments I mentioned above that would allow for FTL communication via quantum entanglement.

Basically, quantum entanglement (QE) alone does not allow for communication; you need to do something else that is some kind of fundamental manipulation of reality that we don't currently understand.

Really, story-wise, you don't need QE; that's often just used in technobabble to explain why it works. Basically for your idea to work you need to go beyond QE and propose some other mechanism, and provide some additional mechanism or some unknown understanding of QE to explain away why it works when it shouldn't (likely going into additional technobabble).

Really for QE to allow for communication you basically need a device that allows you to do things far more fancy than communication, though it might first be used for communication because it's easier to do that. The device would more-less need to act like a force electron microscope that allows you to poke atoms (in order to change the spin), but the device allows you to poke atoms on the other side of the planet in real time, and what might allow you to do that? Microscopic wormholes, basically wormholes that aren't much bigger than atoms, as if they were, you could do far more than just poke atoms with them. Though at that scale perhaps nanoscopic is more accurate (and checks a "techonobabble" box).

Interestingly enough in such a case, it might be the discovery of something that breaks the no-communication theorem for QE that leads to the discovery of FTL technology.

• So you would agree that if you CAN influence the direction and know when the bits are read, you could have FTL communication? Because at that point it doesn’t matter when the quantum entanglement truly happens, so long as the bits are in the right order upon being read, and the order being picked is by what I type. Nov 3, 2023 at 16:13
• @Demigan every single answer here has correctly stated that it is not possible to communicate FTL using quantum entanglement. I think it's time to accept that.
– M S
Nov 3, 2023 at 20:24
• @Demigan Edited my post. Gist is that QE by itself does not allow for communication. Nov 4, 2023 at 3:44
• "you need to do something else that is some kind of fundamental manipulation of reality that we don't currently understand": at which point there's no need to involve QE. The theory that predicted QE explicitly rules communication out, and there's no reason to think it this is wrong. Nov 4, 2023 at 4:17
• @ChristopherJamesHuff Yeah, that's indeed the point. If you can communicate via QE, it explicitly means that something else is going on and is more useful than just communication. As I said, it's probably a suitable way to "discover" an FTL effect. There are plenty of examples in physics of such things being broken due to some kind of exception or anomaly. It typically isn't because it's wrong, but instead because it was incomplete. (I mention this purely for world-building purposes outside of a hard sci-fi setting). Nov 5, 2023 at 0:38