From Ethan Siegel: “Could An Incompleteness In Quantum Mechanics Lead To Our Next Scientific Revolution?”

From Ethan Siegel
Apr 24, 2019

The proton’s structure, modeled along with its attendant fields, show how even though it’s made out of point-like quarks and gluons, it has a finite, substantial size which arises from the interplay of the quantum forces and fields inside it. The proton, itself, is a composite, not fundamental, quantum particle. (BROOKHAVEN NATIONAL LABORATORY)

A single thought experiment reveals a paradox. Could quantum gravity be the solution?

Sometimes, if you want to understand how nature truly works, you need to break things down to the simplest levels imaginable. The macroscopic world is composed of particles that are — if you divide them until they can be divided no more — fundamental. They experience forces that are determined by the exchange of additional particles (or the curvature of spacetime, for gravity), and react to the presence of objects around them.

At least, that’s how it seems. The closer two objects are, the greater the forces they exert on one another. If they’re too far away, the forces drop off to zero, just like your intuition tells you they should. This is called the principle of locality, and it holds true in almost every instance. But in quantum mechanics, it’s violated all the time. Locality may be nothing but a persistent illusion, and seeing through that facade may be just what physics needs.

Quantum gravity tries to combine Einstein’s general theory of relativity with quantum mechanics. Quantum corrections to classical gravity are visualized as loop diagrams, as the one shown here in white. We typically view objects that are close to one another as capable of exerting forces on one another, but that might be an illusion, too. (SLAC NATIONAL ACCELERATOR LAB)

Imagine that you had two objects located in close proximity to one another. They would attract or repel one another based on their charges and the distance between them. You might visualize this as one object generating a field that affects the other, or as two objects exchanging particles that impart either a push or a pull to one or both of them.

You’d expect, of course, that there would be a speed limit to this interaction: the speed of light. Relativity gives you no other way out, since the speed at which the particles responsible for forces propagate is limited by the speed they can travel, which can never exceed the speed of light for any particle in the Universe. It seems so straightforward, and yet the Universe is full of surprises.

An example of a light cone, the three-dimensional surface of all possible light rays arriving at and departing from a point in spacetime. The more you move through space, the less you move through time, and vice versa. Only things contained within your past light-cone can affect you today; only things contained within your future light-cone can be perceived by you in the future. (WIKIMEDIA COMMONS USER MISSMJ)

We have this notion of cause-and-effect that’s been hard-wired into us by our experience with reality. Physicists call this causality, and it’s one of the rare physics ideas that actually conforms to our intuition. Every observer in the Universe, from its own perspective, has a set of events that exist in its past and in its future.

In relativity, these are events contained within either your past light-cone (for events that can causally affect you) or your future light-cone (for events that you can causally effect). Events that can be seen, perceived, or can otherwise have an effect on an observer are known as causally-connected. Signals and physical effects, both from the past and into the future, can propagate at the speed of light, but no faster. At least, that’s what your intuitive notions about reality tell you.

Schrödinger’s cat. Inside the box, the cat will be either alive or dead, depending on whether a radioactive particle decayed or not. If the cat were a true quantum system, the cat would be neither alive nor dead, but in a superposition of both states until observed. (WIKIMEDIA COMMONS USER DHATFIELD)

But in the quantum Universe, this notion of relativistic causality isn’t as straightforward or universal as it would seem. There are many properties that a particle can have — such as its spin or polarization — that are fundamentally indeterminate until you make a measurement. Prior to observing the particle, or interacting with it in such a way that it’s forced to be in either one state or the other, it’s actually in a superposition of all possible outcomes.

Well, you can also take two quantum particles and entangle them, so that these very same quantum properties are linked between the two entangled particles. Whenever you interact with one member of the entangled pair, you not only gain information about which particular state it’s in, but also information about its entangled partner.

By creating two entangled photons from a pre-existing system and separating them by great distances, we can ‘teleport’ information about the state of one by measuring the state of the other, even from extraordinarily different locations. (MELISSA MEISTER, OF LASER PHOTONS THROUGH A BEAM SPLITTER)

This wouldn’t be so bad, except for the fact that you can set up an experiment as follows.

You can create your pair of entangled particles at a particular location in space and time.
You can transport them an arbitrarily large distance apart from one another, all while maintaining that quantum entanglement.
Finally, you can make those measurements (or force those interactions) as close to simultaneously as possible.

In every instance where you do this, you’ll find the member you measure in a particular state, and instantly “know” some information about the other entangled member.

A photon can have two types of circular polarizations, arbitrarily defined so that one is + and one is -. By devising an experiment to test correlations between the directional polarization of entangled particles, one can attempt to distinguish between certain formulations of quantum mechanics that lead to different experimental results.(DAVE3457 / WIKIMEDIA COMMONS)

What’s puzzling is that you cannot check whether this information is true or not until much later, because it takes a finite amount of time for a light signal to arrive from the other member. When the signal does arrive, it always confirms what you’d known just by measuring your member of the entangled pair: your expectation for the state of the distant particle agreed 100% with what its measurement indicated.

Only, there seems to be a problem. You “knew” information about a measurement that was taking place non-locally, which is to say that the measurement that occurred is outside of your light cone. Yet somehow, you weren’t entirely ignorant about what was going on over there. Even though no information was transmitted faster than the speed of light, this measurement describes a troubling truth about quantum physics: it is fundamentally a non-local theory.

Schematic of the third Aspect experiment testing quantum non-locality. Entangled photons from the source are sent to two fast switches that direct them to polarizing detectors. The switches change settings very rapidly, effectively changing the detector settings for the experiment while the photons are in flight. (CHAD ORZEL)

There are limits to this, of course.

It isn’t as clean as you want: measuring the state of your particle doesn’t tell us the exact state of its entangled pair, just probabilistic information about its partner.

There is still no way to send a signal faster than light; you can only use this non-locality to predict a statistical average of entangled particle properties.

And even though it has been the dream of many, from Einstein to Schrödinger to de Broglie, no one has ever come up with an improved version of quantum mechanics that tells you anything more than its original formulation.

But there are many who still dream that dream.

If two particles are entangled, they have complementary wavefunction properties, and measuring one places meaningful constraints on the properties of the other. (WIKIMEDIA COMMONS USER DAVID KORYAGIN)

One of them is Lee Smolin, who cowrote a paper [Physical Review D] way back in 2003 that showed an intriguing link between general ideas in quantum gravity and the fundamental non-locality of quantum physics. Although we don’t have a successful quantum theory of gravity, we have established a number of important properties concerning how a quantum theory of gravity will behave and still be consistent with the known Universe.

A variety of quantum interpretations and their differing assignments of a variety of properties. Despite their differences, there are no experiments known that can tell these various interpretations apart from one another, although certain interpretations, like those with local, real, deterministic hidden variables, can be ruled out. (ENGLISH WIKIPEDIA PAGE ON INTERPRETATIONS OF QUANTUM MECHANICS)

There are many reasons to be skeptical that this conjecture will hold up to further scrutiny. For one, we don’t truly understand quantum gravity at all, and anything we can say about it is extraordinarily provisional. For another, replacing the non-local behavior of quantum mechanics with the non-local behavior of quantum gravity is arguably making the problem worse, not better. And, as a third reason, there is nothing thought to be observable or testable about these non-local variables that Markopoulou and Smolin claim could explain this bizarre property of the quantum Universe.

Fortunately, we’ll have the opportunity to hear the story direct from Smolin himself and evaluate it on our own. You see, at 7 PM ET (4 PM PT) on April 17, Lee Smolin is giving a public lecture on exactly this topic at Perimeter Institute, and you can watch it right here.


I’ll be watching along with you, curious about what Smolin is calling Einstein’s Unfinished Revolution, which is the ultimate quest to supersede our two current (but mutually incompatible) descriptions of reality: General Relativity and quantum mechanics.


Best of all, I’ll be giving you my thoughts and commentary below in the form of a live-blog, beginning 10 minutes before the start of the talk. [See the full article.]

Find out where we are in the quest for quantum gravity, and what promises it may (or may not) have for revolutionizing one of the greatest counterintuitive mysteries about the quantum nature of reality!

Thanks for joining me for an interesting lecture and discussions on science, and just maybe, someday, we’ll have some interesting progress to report on this topic. Until then, you don’t have to shut up, but you still do have to calculate!

See the full article here .


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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan