The answer to the title question is “No. But also yes.”
Recently, the 2022 Nobel Prize in physics was awarded to Alain Aspect, John Clauser, and Anton Zeilinger for work related to quantum entanglement and the Bell inequality. Why it was awarded now, instead of 30 years ago, is something I’m not clear on. But that’s beside the point.
The award led to a spate of articles, videos, etc., trying to explain “spooky action at a distance” to non-experts. I’ve written before, in my “Quantum marbles” post, about how people who do this usually explain it in a way that misses the point. So, I thought a follow-up post was in order.
(Usual disclaimers apply. I’m not an expert on this topic, and I have no business writing about it.)
You should have in your mind the standard EPR experiment, perhaps featuring our standard heroes, Alice and Bob, located far away from each other, each with the ability to measure one of a pair of entangled particles. We’re discussing what they can do with this ability.
And yeah, predictably, most of these new articles have been less than great. Always getting distracted by irrelevancies about wave functions, hidden variables, whether a particle has a definite state before it is measured, and whether the laws of physics are probabilistic. And always remembering to add that, sorry, entangled particles cannot be used to usefully communicate faster than light.
But is that last claim even true? It depends on precisely how it’s worded and interpreted. If you’re going to make a claim like this, you should word it with surgical precision. Let’s start with the question:
- Can we use entangled particles to usefully communicate faster than light?
This question is ambiguous, and the answer to it lies in the region of ambiguity. The main issue is the precise definition of the word “communicate”. Here’s another question:
- Can we use entangled particles to do something useful that would be impossible if the particles weren’t entangled?
This is still slightly ambiguous. If entangled particles are, and will always be, your only means of communication, then indeed they are useless. Let’s clarify it:
- Can we use entangled particles, in conjunction with lightspeed messages, to do something useful that would be impossible if the particles weren’t entangled?
The answer to this question is indisputably, unconditionally yes. It’s been experimentally verified. It’s what that Nobel Prize was all about. And the only thing that entangled particles bring to the table is their apparent faster-than-light communication-like ability.
At this point, I recommend you stop reading my attempt to explain the power of entangled particles, and instead go watch YouTuber Udiprod’s excellent video “Gamification of Bell’s Theorem.” It’s the clearest explanation I’ve seen.
Is it really communication?
As mentioned previously, the word communication is ambiguous. Dictionary definitions tend to equate communication with sending messages (the preferred technical term is “signals”). But I don’t think we should expect a dictionary to explain all the possible things that could fall under the general concept of communication.
Sending a signal means what you think it means. If I send you a message that reads “the combination is 12345”, and upon receiving it, you can immediately try 12345 as the combination, then I’ve sent a signal.
It turns out there are forms of generalized communication that are weaker than sending a signal, yet still better than nothing. One class of weak generalized communication is a non-signaling correlation. A certain subset of non-signaling correlations is what entangled particles can do. And they do it faster than light.
A faster than light non-signaling correlation does not lead to any time travel paradoxes. It’s faster than light signals that do that.
It’s a little hard to explain non-signaling correlations, and how they could be useful. There are situations in which two cooperating partners can use entangled particles to correlate their actions, in combination with local events that only one party knows about, to make choices that will ultimately prove beneficial to them.
Notice that I’m careful not to use words like send, transmit, or receive. With the partners separated in spacetime, neither in the other’s light cone, there is no way to tell who measured their particle first. There is no way to tell who is the sender, and who is the receiver.
Personally, I think it’s reasonable to consider non-signaling correlations to be a kind of communication, but I’ll try to stick to terms like “generalized communication”.
Is it really faster than light?
It’s worth emphasizing that no benefit from entangled particles can be realized until after relevant lightspeed messages arrive. But that doesn’t mean there’s no benefit. Consider that the benefit of knowing tomorrow’s winning lottery numbers cannot be realized until after the lottery happens. But such knowledge is still beneficial, and what’s more, it must be acted upon before the lottery happens.
Don’t read too much into this analogy. You can’t use entangled particles to win a typical lottery. But it would be possible to design an interplanetary lottery with silly rules that would be hackable using entangled particles.
Technically, what’s been pretty well disproven is the idea that the universe is both LOCAL and REAL.
LOCAL means the speed of light is never exceeded, not even in deep reality.
REAL means…, well, it seems that there are different definitions of REAL. Some people seem to use it to mean that a particle has a definite state even before it is measured. But that definition just doesn’t work, because the lack of a definite state is neither necessary nor sufficient to explain away the seeming violation of LOCALITY.
What REAL should mean, roughly speaking, is the idea that we’re living in a single objectively real universe that is basically what it appears to be. We’re not living in a multiverse, or in the Matrix. We’re not all too insane to understand logic. The universe isn’t some grand conspiracy where its entire history is calculated in advance and somehow known to entangled particles when they are created. Etc.
So, there are ways to save LOCALITY, by sacrificing REALITY. And my personal philosophy might even lean in the direction of one of them.
But from the perspective of us human beings living our daily lives in our seemingly real world, I think the sensible viewpoint is that this generalized communication really is happening faster than light. Deep philosophy isn’t that great an explanation of how we can do actual useful things with what seems to be faster-than-light communication.