Even with quantum entanglement, there is no faster-than-light communication
One of the most fundamental rules of physics since it was first proposed by Albert Einstein in 1905 is that no type of information can travel faster than the speed of light in the universe.
Both massed and massless particles are needed to transmit information from one place to another, and these particles must travel either below the speed of light (for massed particles) or at the speed of light (for massless particles), as specified by the rules of relativity. It may be possible to use curved space to allow these information carriers to take "shortcuts", but they must still travel through space at or below the speed of light.
However, since the development of quantum mechanics, many people have tried to use the power of quantum entanglement to subvert this rule. Many ingenious schemes have been devised to "cheat" relativity in order to transmit information and thus achieve FTL communication. Despite admirable attempts to circumvent the rules of the universe, not only did each of these schemes fail; all of them proved to be doomed to failure.
--Even with quantum entanglement, FTL communication is still impossible in our universe.
Conceptually, quantum entanglement is a simple concept. Imagine for a moment the classical universe and the simplest of "random" experiments: the toss of a coin. If you and I both have a fair coin and flip it, we both assume that the probability of each of us getting heads and tails is 50/50. Not only should your result and my result be random, they should be independent and uncorrelated: whether you flip the coin heads or tails, the probability of me getting heads or tails should be 50/50.
However, if this is not a classical system, but a quantum system, then your coin and my coin are potentially entangled. Each of us may still have a 50/50 chance of getting heads or tails, but if you flip a coin and measure heads, you will immediately be able to statistically predict whether my coin will be heads or tails with better than 50/50 accuracy. This is the main idea of quantum entanglement: that there is a correlation between two entangled quanta; this means that if one of the quanta's quantum state is actually measured, the state of the other quanta is not immediately determined, but some probabilistic information about it can be gathered.
In quantum physics, there is a phenomenon known as quantum entanglement, where you create more than one quantum particle (each with its own separate quantum state) and something important is known in the sum of the two quantum states. It's as if there's an invisible thread connecting the two quanta (or, if two coins are entangled according to the laws of quantum mechanics, it's your coin and my coin), and when one of us takes a measurement of the coin we're holding, we can instantly know the state of the other coin in a way that's beyond the familiar "classical randomness ".
While this sounds like purely theoretical work, it has been in the realm of experimentation for decades. We create a pair of entangled quanta (photons to be specific) and then move them away from each other until they are separated by a great distance, and then we have two separate measurement instruments to tell us what the quantum state of each particle is. We make these measurements as simultaneously as possible and then compare our results together.
These experiments were so far-reaching that the research involved also won the 2022 Nobel Prize in Physics.
We found that perhaps the results of your coin and my coin (or, your photon spin and my photon spin) are correlated! Before making these crucial measurements, we had separated two photons by hundreds of kilometers and then measured their quantum states in nanoseconds. If one of the photons has a spin of +1, then the state of the other photon can be predicted with about 75% accuracy, instead of the classical 50%.
Furthermore, information about the spin of the other particle can be known in an instant, rather than waiting for another measuring instrument to send us the result of the signal, which takes about a millisecond. On the face of it, it would seem that we could know some information on the other end of an entanglement experiment, not only faster than the speed of light, but at least tens of thousands of times faster than the speed of light. However, does this mean that information is being transmitted faster than the speed of light?
By generating two entangled photons from a pre-existing system and separating them by a great distance, we can "transmit" information about the state of one photon by measuring the state of the other, even from a distance.
On the face of it, it might actually seem that information is transmitted faster than the speed of light. For example, we can try to fabricate an experiment that conforms to the following setup:
- Prepare a large number of entangled quantum particles at a (source) location.
- Teleport one set of entangled pairs a long way away (the destination) while keeping another set of entangled particles at the source.
- Have observers at the destination look for some kind of signal and force their entangled particles into either the +1 state (positive signal) or the -1 state (negative signal).
- The entangled pairs are then measured at the source and the state chosen by the destination observer is determined with more than 50/50 probability.
If this setup worked, we would really be able to know whether the observer at the far destination forced their entangled pair into the +1 or -1 state, simply by measuring the particle pair after breaking the entanglement at the far end.
This seems like an excellent setup for realizing FTL communication. All the scientists need is a fully prepared system of entangled quantum particles, an agreed system of what the various signals will mean when the measurements are made, and a predetermined time at which these critical measurements will be made. In this way, even from light years away, we can immediately learn the results of the measurements at our destination by observing the particles that we have been carrying around with us.
It's a very clever experimental scheme, but it doesn't actually pay off.
When particles make these critical measurements of the original source of entanglement and creation, something extremely disappointing is revealed: the results simply show the 50/50 chance of being in a +1 or -1 state. It is as if the behavior of a distant observer forces members of the entangled pair to be in a +1 or -1 state, but has no effect on the outcome of the experiment.
The outcome of the experiment would be exactly as expected: i.e., there would be no entanglement at all.
Where did the plan go wrong? It is at the step where we let the observer at the destination make the observation and try to encode that information into their quantum state.
We said before, "Let the observer at the destination look for some kind of signal and force their entangled particles into either a +1 state (positive signal) or a -1 state (negative signal)."
When this step is taken - forcing one of a pair of entangled particles into a particular quantum state - the action not only breaks the entanglement between the two particles, but it doesn't break the entanglement and determine what the particle's properties are; it breaks the entanglement and puts it in a new state that isn't "determined" by a fair measurement. measurement "determines" the state (+1 or -1).
In other words, the other member of the entangled pair is completely unaffected by this "forcing", and its quantum state remains random, a superposition of +1 and -1 quantum states. If we "force" one of the entangled particles to enter a specific state, we completely break the correlation between the measurements.
The only way to solve this problem is to make quantum measurements in a way that produces a specific result (note: this is not allowed by the currently known laws of physics).
If this could be done, then someone at the destination could make observations (e.g., to find out if the planet they are visiting is inhabited or not) and then measure the state of the quantum particles using some unknown process.
Unfortunately, the results of quantum measurements are inevitably random; we can't encode our preferred results into quantum measurements.
Even using quantum entanglement, it's impossible to do better than random guessing to know what's happening at the other end of an entanglement experiment.
As quantum physicist Chad Orzel writes, there's a huge difference between making a measurement (maintaining entanglement between pairs of particles) and forcing a particular outcome, which is itself a state change; and then making a measurement (without maintaining entanglement). If one wants to control, and not just measure, the state of the quantum particles, then once we have performed the state change operation, we lose knowledge of the complete state of the combined system.
Quantum entanglement can only be used to gain information about one component of a quantum system by measuring the other component, and the entanglement needs to remain constant. We cannot create information at one end of an entangled system and then somehow send it to the other end.
If identical copies of quantum states could somehow be made, then FTL communication would be possible - but that is also forbidden by the laws of physics.
There are many things we can do with the exotic physics of quantum entanglement. For example, create a quantum lock-and-key system that is virtually unbreakable using purely classical computation. But the fact is that we can't copy/clone a quantum state: because just reading the behavior of a quantum state fundamentally alters it, which is the Achilles' heel of any viable solution to achieve FTL communication using quantum entanglement.
Quantum entanglement is a rich field of research in its own right, and many of its aspects have been recognized in the 2022 Nobel Prize in Physics.
There are many subtleties to how quantum entanglement works in practice, but the key point is this: no measurement procedure can force a specific result while maintaining entanglement between particles; the inevitable randomness of the results of any quantum measurement negates this possibility.
It turns out that God is indeed playing with the dice of the universe, and that's a good thing. The fact that no information can be sent at superluminal speeds allows our universe to remain causal.
Reference link:
[1]https://bigthink.com/starts-with-a-bang/quantum-entanglement-faster-than-light/
[2]https://www.forbes.com/sites/chadorzel/2016/05/04/the-real-reasons-quantum-entanglement-doesnt-allow-faster-than-light- communication/?sh=3d67c77e3a1e
