Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Monday, April 3, 2023

Spookier action

One of the downsides of being a layperson rather than a scientist (and I very much consider myself to be the former, despite having been a science teacher for over three decades) is that my understanding is hampered simply because it's impossible to know all the details of research by people who are way smarter than I am.

This is worst in completely counter-intuitive disciplines like quantum physics.

That doesn't prevent me from being really interested in all this stuff.  I was just discussing quantum entanglement with a dear friend a couple of days ago (as one does), and his question was, "Could you use it to communicate information?"  On the surface, it seems like it should be possible, right?

It's not -- at least as far as our current understanding goes.  But the reason isn't obvious on first glance.  In entanglement, a pair of particles is created which can be described by a single wave function; this means that their states are correlated, and knowing the state of one of them automatically tells you the state of the other, regardless of how far apart they are.  Let's say you and I create an entangled pair that has a net spin of zero.  You take your particle to Tokyo and I take mine to Lisbon.  Then you measure yours, and find it has a spin axis pointing upward.  I know immediately that if I measure mine, it will have a spin axis pointing downward.

Graph of the wave function of a single particle [Image is in the Public Domain]

So far, it seems like, "what's so weird about that?"  It doesn't seem any more remarkable than having a matched pair of gloves each in its own sealed box, and if you open your box in Tokyo and find it's a left-handed glove, mine in Lisbon has to be a right-handed glove.  The reality of the particles is weirder -- the members of the entangled pair are neither spin-up nor spin-down until they're measured, but in a state of superposition -- existing in a field of probabilities of both states at the same time.  Only once one of them is measured does it lock in to a particular state, and that measurement is what locks in the other particle simultaneously -- something Einstein famously called "spooky action at a distance."

Okay, so why couldn't that be used for communication?  The reason is rather subtle.  Let's say you want to communicate something simple, something that can be answered "yes" or "no."  So you and I take the two particles in our entangled pair to Tokyo and Lisbon, respectively.  We agree ahead of time that once you get there, you are going to go outside to see if it's a clear day and whether you can see Mount Fuji.  If you can, you will force your particle into a spin-up state; won't that force mine into a spin-down state, thus communicating the information to me instantaneously, thousands of miles away?

The answer is no.  The reason is, you didn't just measure your particle's state, you changed it.  And this breaks the entanglement.  The moment you do anything to alter the state of your particle, it decouples it from mine, and my particle now has a 50/50 chance of being spin-up or spin-down; it's no longer affected by what happens to yours.  Every kind of information transfer known requires changing the state of the particles you're using to carry the information, and that transfer can only travel at the speed of light or slower.

So it seems like the faster-than-light "subspace communication" used in Star Trek is impossible, right?

Well... maybe.

This is where I skate out over very thin ice, because what got all this started (besides the conversation with my friend) was a paper last week in Quantum Science and Technology which -- if I'm reading it right, and I might well not be -- suggests that there might be a way around this, by sending information (1) without using particles, and (2) by having the information go directly from sender to receiver without traveling through the intervening space.

If you're thinking, "That sounds like a wormhole" -- exactly.  Hatim Salih, of the University of Bristol, says he's found a way to create a "traversable wormhole" that could transfer quantum information instantaneously.

Salih calls this even-spookier-action-at-a-distance counterportation.  "Here’s the sharp distinction," he said in a news release.  "While counterportation achieves the end goal of teleportation, namely disembodied transport, it remarkably does so without any detectable information carriers traveling across.  If counterportation is to be realized, an entirely new type of quantum computer has to be built: an exchange-free one, where communicating parties exchange no particles.  By contrast to large-scale quantum computers that promise remarkable speed-ups, which no one yet knows how to build, the promise of exchange-free quantum computers of even the smallest scale is to make seemingly impossible tasks – such as counterportation – possible, by incorporating space in a fundamental way alongside time."

"We experience a classical world which is actually built from quantum objects," said John Rarity, Salih's colleague at the University of Bristol.  "The proposed experiment can reveal this underlying quantum nature showing that entirely separate quantum particles can be correlated without ever interacting.  This correlation at a distance can then be used to transport quantum information (qbits) from one location to another without a particle having to traverse the space, creating what could be called a traversable wormhole."

Okay... that's just nifty, but... but... Einstein?  Speed of light?  How do you avoid the paradoxes that come with faster-than-light information transfer?

Maybe there's something I'm not understanding, here.  All right, to be fair, I'm sure there's a gazillion things I'm not understanding, here.  Cf. my aforementioned layperson status.  But it sure seems like if you can do this, you're talking about something that would break the cosmic speed limit for information transfer, and shake physics down to its roots.

Much as I'd love to see the world of Star Trek realized, I'm pretty certain that I'm missing something critical, and this isn't going to turn out to be what it sounds like.  There's probably some subtlety -- like the measuring-versus-changing distinction in entanglement -- that isn't apparent.

What that might be, however, escapes me.  If any physicists read this post, do enlighten me.  While I don't relish the idea of my hopes being dashed, I'm virtually certain they will be.  And as Carl Sagan so trenchantly put it, "For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring."


1 comment:

  1. I'm no expert, and even more of a layperson than you are, but this smells like cold fusion.