Posts Tagged ‘quantum physics’

Quantum teleportation revisit: Now with wormholes!

December 12, 2017 1 comment

Six years ago, I wrote a couple of posts on this blog musing about the physics behind quantum teleportation — first proposing a model in which quantum entanglement could resolve the philosophical condundrum of whether continuity of self could be maintained, then getting into some of the practical limitations that made quantum teleportation of macroscopic objects or people unlikely to be feasible. I recently came upon an article that offers a potential new angle, basically combining the idea of quantum teleportation with the idea of a wormhole.

The article, “Newfound Wormhole Allows Information to Escape Black Holes” by Natalie Wolchover, was published in Quanta Magazine on October 23, 2017. It’s talking about a theoretical model devised by Ping Gao, Daniel Jafferis, and Aron C. Wall, a way that a stable wormhole could exist without needing some kind of exotic matter with arbitrary and probably physically unattainable properties in order to keep it open. Normally, a wormhole’s interior “walls” would attract each other gravitationally, causing it to instantly pinch off into two black holes, unless you could line them with some kind of magic substance that generated negative energy or antigravity, like shoring up a tunnel in the dirt. That’s fine for theory and science fiction, but in practical terms it’s probably impossible.

The new model is based on a theory that’s been around in physics for a few years now, known in short as “ER = EPR” — namely, that wormholes, aka Einstein-Rosen bridges, are effectively equivalent to quantum entanglement between widely separated particles, or Einstein-Podolsky-Rosen pairs. (Podolsky, by the way, is Boris Podolsky, who lived and taught here in Cincinnati from 1935 until his death, and was the graduate advisor to my Uncle Harry. I was really impressed when I learned my uncle was only two degrees of separation from Einstein.) The EPR paradox, which Einstein nicknamed “spooky action at a distance,” is the way that two entangled particles can affect each other’s states instantaneously over any distance — although in a way that can’t be measured until a light signal is exchanged between them, so it can’t be used to send information faster than light. Anyway, it’s been theorized that there might be some sort of microscopic wormhole or the equivalent between the entangled particles, explaining their connection. Conversely, the two mouths of a wormhole of any size could be treated as entangled particles in a sense. What the authors of this new paper found was that if the mouths of a wormhole were created in a way that caused them to be quantum-entangled — for instance, if one of them were a black hole that was created out of Hawking radiation emitted from another black hole (it’s complicated), so that one was a direct outgrowth of the other on a quantum level — then the entanglement of the two black holes/mouths would create, in the words of the paper’s abstract, “a quantum matter stress tensor with negative average null energy, whose gravitational backreaction renders the Einstein-Rosen bridge traversable.” In other words, you don’t need exotic matter to shore up the wormhole interior, you just need a quantum feedback loop between the two ends.

Now, the reason for all this theoretical work isn’t actually about inventing teleportation or interstellar travel. It’s more driven by a strictly theoretical concern, the effort to explain the black hole information paradox. Conservation of energy says that the total amount of energy in a closed system can’t be increased or decreased. Information is energy, and the universe is a closed system, so the total amount of information in the universe should be constant. But if information that falls into a black hole is lost forever, then conservation is violated. So for decades, physicists (notably Stephen Hawking) have been exploring the question of whether it’s possible to get information back out of a black hole, and if so, how. This paper was an attempt to resolve that question. A traversable wormhole spinning off from a black hole provides a way for information to leave the interior of the black hole, resolving the paradox.

I only skimmed the actual paper, whose physics and math are way beyond me, but it says that this kind of entangled wormhole would only be open for a very brief time before collapsing. Still, in theory, it could be traversable at least once, which is better than previous models where the collapse was instantaneous. And if that much progress has been made, maybe there’s a way to refine the theory to keep the wormhole open longer.

There’s a catch, though. Physical law still precludes information from traveling faster than light. As with quantum teleportation, there is an instantaneous exchange of information between the two ends, but that information remains in a latent, unmeasurable state until a lightspeed signal can travel from the transmitting end to the receiving end. So a wormhole like this, if one could be created and extended over interstellar distances, would not allow instantaneous travel. A ship flying into one end of the wormhole would essentially cease to exist until the lightspeed signal could reach the other end, whereupon it would emerge at long last.

However — and this is the part that I thought of myself as an interesting possibility for fiction — this does mean that the ship would be effectively traveling at the speed of light. That in itself is a really big deal. In a physically realistic SF universe, it would take an infinite amount of energy and time to accelerate to the speed of light, and once you got fairly close to the speed of light, the hazards from oncoming space dust and blueshifted radiation would get more and more deadly. So as a rule, starships would have to stay at sublight speeds. In my original fiction I’ve posited starships hitting 80 or 90 percent of c, but even that is overly optimistic. So in a universe where starships would otherwise be limited to, say, 30 to 50 percent of lightspeed, imagine how remarkable it would be to have a wormhole transit system that would let a starship travel at exactly the speed of light. Moreover, the trip would be instantaneous from the traveler’s perspective, since they’d basically be suspended in nonexistence until the lightspeed signal arrived to “unlock” the wormhole exit. It’s not FTL, but it’s L, and that alone would be a damned useful stardrive. You could get from Earth to Alpha Centauri in just 4.3 years, and the trip would take no time at all from your perspective, except for travel time between planet and wormhole mouth. You’d be nearly 9 years younger than your peers when you got home — assuming the wormhole could be kept open or a second temporary wormhole could be generated the other way — but that’s better than being 2 or 3 decades younger. Short of FTL, it’s the most convenient, no-fuss means of interstellar travel I can think of.

Or, looked at another way, it’s a method for interstellar quantum teleportation that avoids all the scanning/transmission obstacles and impracticalities I talked about in my second 2011 post on the subject. No need to use a technological device to scan a body with a level of detail that would destroy it, then transmit a prohibitively huge amount of data that might take millennia to send in full. You just pop someone into one end of a wormhole and make sure the handshake signal is transmitted strongly enough to reach the other end. I’ve long felt that wormhole-based teleportation would be a more sensible approach than the disintegration-based kind anyway. Although we’re technically talking about black holes, so it wouldn’t be the sort of thing where you could just stand on a platform in your shirtsleeves and end up somewhere else. Also, there might be a little problem with getting torn apart by tidal stresses at either end. I’m not sure the paper addresses that.

This idea could be very useful for a hard-SF universe. My problem is that the universes I have established are a little less hard than that, though, since I tend to like working in universes with FTL travel of one sort or another. But maybe some idea will come to me for a future story. And maybe some other writer will read this and get an idea. We’re all in this together, and any worthwhile SF concept can inspire multiple very different stories.

Ars Technica interviewed me on STAR TREK transporters

September 23, 2017 5 comments

You may recall that last year, Xaq Rzetelny of the science site Ars Technica interviewed me about Star Trek temporal physics. Well, Xaq recently came across my 2011 post “On quantum teleportation and continuity of self,” and sought my input for an article tackling the same basic question for Star Trek transporters — whether or not the person who comes out of the transporter is the same one who went in. It’s a detailed and well-researched piece that also contains comments from folks like Michael Okuda and Lawrence Krauss, and you can read it here:

Is beaming down in Star Trek a death sentence?

Musings on quantum gravity

Recently I came across this article about an experiment to reconcile quantum physics with gravity, the one fundamental force that hasn’t yet been explained in quantum terms:

New Experiments to Pit Quantum Mechanics Against General Relativity

The problem with reconciling gravity (which is explained by Einstein’s General Theory of Relativity) and quantum physics is that they seem to follow incompatible laws. Quantum particles can exist in superpositions of more than one state at a time, while gravitational phenomena remain resolutely “classical,” displaying only one state. Our modern interpretation suggests that what we observe as classical physics is actually the result of the quantum states of interacting particles correlating with each other. A particle may be in multiple states at once, but everything it interacts with — including a measuring device or the human observer reading its output — becomes correlated with only one of those states, and thus the whole ensemble behaves classically. This “decoherence” effect makes it hard to detect quantum superpositions in any macroscopic ensemble, like, say, a mass large enough to have a measurable gravitational effect. Thus it’s hard to see quantum effects in gravitational interactions. As the article puts it:

At the quantum scale, rather than being “here” or “there” as balls tend to be, elementary particles have a certain probability of existing in each of the locations. These probabilities are like the peaks of a wave that often extends through space. When a photon encounters two adjacent slits on a screen, for example, it has a 50-50 chance of passing through either of them. The probability peaks associated with its two paths meet on the far side of the screen, creating interference fringes of light and dark. These fringes prove that the photon existed in a superposition of both trajectories.

But quantum superpositions are delicate. The moment a particle in a superposition interacts with the environment, it appears to collapse into a definite state of “here” or “there.” Modern theory and experiments suggest that this effect, called environmental decoherence, occurs because the superposition leaks out and envelops whatever the particle encountered. Once leaked, the superposition quickly expands to include the physicist trying to study it, or the engineer attempting to harness it to build a quantum computer. From the inside, only one of the many superimposed versions of reality is perceptible.

A single photon is easy to keep in a superposition. Massive objects like a ball on a spring, however, “become exponentially sensitive to environmental disturbances,” explained Gerard Milburn, director of the Center for Engineered Quantum Systems at the University of Queensland in Australia. “The chances of any one of their particles getting disturbed by a random kick from the environment is extremely high.”

The article is about devising an experiment to get around this and observe a superposition (potentially) in a “ball on a spring” type of apparatus. What interests me, though, is a more abstract discussion toward the end of the article.

Inspired by the possibility of experimental tests, Milburn and other theorists are expanding on Diósi and Penrose’s basic idea. In a July paper in Physical Review Letters, Blencowe derived an equation for the rate of gravitational decoherence by modeling gravity as a kind of ambient radiation. His equation contains a quantity called the Planck energy, which equals the mass of the smallest possible black hole. “When we see the Planck energy we think quantum gravity,” he said. “So it may be that this calculation is touching on elements of this undiscovered theory of quantum gravity, and if we had one, it would show us that gravity is fundamentally different than other forms of decoherence.”

Stamp is developing what he calls a “correlated path theory” of quantum gravity that pinpoints a possible mathematical mechanism for gravitational decoherence. In traditional quantum mechanics, probabilities of future outcomes are calculated by independently summing the various paths a particle can take, such as its simultaneous trajectories through both slits on a screen. Stamp found that when gravity is included in the calculations, the paths connect. “Gravity basically is the interaction that allows communication between the different paths,” he said. The correlation between paths results once more in decoherence. “No adjustable parameters,” he said. “No wiggle room. These predictions are absolutely definite.”

Now, this got me thinking. Every particle with mass interacts gravitationally with every other particle with mass, so there would be no way to completely isolate them from interacting. For that matter, gravity affects light too. So if gravity is an irreducible “background noise” that prevents stable superpositions, that would explain why quantum effects don’t seem to manifest with gravitational phenomena.

And that does sort of reconcile the two. The decoherence model, that classical states are what we get when quantum states interact and correlate with each other, basically means that classical physics is simply a subset of quantum physics, the behavior of quantum particles that are in a correlated state. So the “classical” behavior of gravity would also be a subset of quantum physics — meaning that relativistic gravity is quantum gravity already, in a manner of speaking. We just didn’t realize they were two aspects of the same overarching whole.

Now, this reminds me of another thing I heard about once, a theory that gravity didn’t really exist. It might have been the entropic gravity theory of Erik Verlinde, which states that gravity is, more or less, just a statistical artifact of particles tending toward maximum entropy. Now, what I recall reading somewhere, though I’m not finding a source for it today, is that this — or whatever similar theory I’m recalling — means that particles tend toward the most probable quantum state. And statistically speaking, for any particle in an ensemble, its most probable position is toward the center of that ensemble, i.e. the center of mass. So I had the thought that maybe what we perceive as gravity is more just some sort of probability pressure as particles tend toward their most likely states.

Now, if Stamp’s theory is right, then Verlinde’s is wrong; there must be an actual force of gravity, or rather, an interaction that correlates the paths of different particles. But it occurs to me that there may be some basis to the probabilistic view of gravity if we look at it more as a quantum correlation than an attraction. To explain my thinking, we have to bring in another idea I’ve talked about before on this blog, quantum Darwinism. The idea there is that the way decoherence works is that the various states of a quantum particle “compete” as they spread out through interaction with other particles, and it’s the more robust, stable states that prevail. Now, what I’m thinking is that as a rule, the most stable states would be the most probable ones. And again, those would tend to be the positions closest to the center of mass, or as close as feasible when competing with other particles.

So if we look at gravitation not as an attractive force per se, but as a sort of “correlational field” that promotes interaction/entanglement among quantum particles, then we can still get its attractive effect arising as a side effect of the decoherence of the correlated particles into their most probable states. Thus, gravity does exist, but its attractive effect is fundamentally a quantum phenomenon. So you have quantum gravity after all.

But how to reconcile this with the geometric view of General Relativity, that gravity is actually a manifestation of the effect that mass and energy have on the topology of spacetime? Well, that apparent topology, that spatial relationship between objects and their motions, could be seen as a manifestation of the probabilistic relationships among their position and movement states. I.e. a particle follows a certain path within a gravitational field because that’s the most probable path for it to take in the context of its correlation with other particles. Even extreme spacetime geometries like wormholes or warp fields could be explained in this way; an object could pass through a wormhole and show up in a distant part of space because the distribution of mass and energy that creates the wormhole produces a probability distribution that means the object is most likely to be somewhere else in space. Which is analogous to the quantum tunneling that results because the peak of a particle’s probability distribution shifts to the other side of a potential barrier. And for that matter, it has often been conjectured that quantum entanglement between correlated particles could be caused by microscopic wormholes linking them. Maybe it’s the other way around: wormholes are just quantum tunneling effects.

One other thought I’ve had that has a science-fictional impact: if gravitation is a “correlational quantum field” that helps the most probable state propagate out through the universe, that might argue against the Many-Worlds Interpretation of quantum decoherence. After all, gravity is kind of universal in its effect, and the correlation it creates produces what we see as classical physics, a singular state. It could be that coherent superpositions would only happen on very small, microscopic scales, and quantum Darwinism and gravitational correlation would cause a single consensus state to dominate on a larger scale. So instead of the whole macroscopic realm splitting into multiple reality-states (timelines), it could be that such splitting is only possible on the very small scale, and maybe the simmering of microscale alternate realities is what we observe as the quantum foam. It could be that the MWI is a consequence of an incomplete quantum theory that doesn’t include gravity, and once you fold in gravity as a correlating effect, it imposes a single quantum reality on the macroscopic universe.

Which would be kind of a bummer from an SF perspective, since alternate realities are useful story concepts. I’d just about come around to believing that at least some alternate realities might be stable enough to spread macroscopically, as I explained in my quantum Darwinism essay linked above. Now, I’m not so sure. The “background noise” effect of gravity might swamp any stable superpositions before they could spread macroscopically and create divergent timelines.

However, these thoughts might be applicable to future writings in my Hub universe (and as I’ve discussed before, I’ve already given up on the idea of trying to reconcile that with my other universes as alternate timelines). The Hub is a point at the center of mass of the greater galaxy — i.e. the system that includes the Milky Way proper, its satellite galaxies, and its dark-matter halo — that allows instantaneous travel to any point within that halo. I hadn’t really worked out how it did so, but maybe this quantum-gravity idea provides an answer. If gravity is quantum correlation, and all particles’ probability distributions tend toward the center of mass, then maybe the center of mass is the one point that allows quantum tunneling to the position of every other particle. Or something like that. It also provides some insight into the key McGuffin of the series, the fact that nobody can predict the relationship between Hub vectors (the angle and velocity at which the Hub is entered) and arrival destinations, meaning that finding new destinations must be a matter of trial and error. If the Hub works through quantum gravity and correlation with all the masses within the halo, then predicting vectors would require a complete, exact measurement of the quantum state of every particle within the halo, and that would be prohibitively difficult. It’s analogous to how quantum theory says that every event in the universe is already part of its wave equation, but we can’t perfectly predict the future because we’d need to know the entire equation, the behavior of every single particle, and that would take an eternity to measure. So it’s something that’s theoretically deterministic but functionally impossible to determine. The same could be true of Hub vectors.

Although… we’re only talking about one galaxy’s worth of particles, which is a tiny fraction of the whole universe. So maybe it’s not completely impossible…

Anyway, those are the musings I’ve had while lying awake in bed over the past couple of early mornings, so maybe they don’t make much sense. But I think they’re interesting.