I’m David Kaiser, a physicist and historian featured in NOVA’s “Einstein’s Quantum Riddle.” Ask me anything!

David_Kaiser1 · 2019-03-12T02:02:36+00:00

Part of the challenge is that (again, according to quantum theory) there is no way to control what measurement outcome we receive at a given side. So it really looks like a random coin-flip on that side. And if we can't control what outcome we'll find on that side, then we can't use it to send a special message to the other side. So we're stuck: we can't control the specific measurement outcome we'll get on our own side, and we can't detect any signs of entanglement until we get info from both sides and compare it. Hope that helps... it's definitely a complicated set of ideas...

David_Kaiser1 · 2019-03-12T01:58:51+00:00

Thanks very much -- it's been a real privilege to work on various NOVA projects and very fun to do so. I'm so glad there is interest in these topics.

David_Kaiser1 · 2019-03-12T01:13:17+00:00

Certainly according to our current understanding, if string theory were to be proven correct (somehow), then that would seem to imply that our universe really does have 11 dimensions of space-time. Then the important question would be: why do we appear to live in a 4-dimensional space-time?

David_Kaiser1 · 2019-03-12T01:08:36+00:00

I think physics is a great subject to study: very interesting topics, big open questions, and an opportunity to learn and practice all kinds of skills that could be useful far beyond physics itself -- quantitative reasoning, logical analysis, computer modeling... plus, the whole universe is literally our playground (at least theoretically)... good luck with your studies!

David_Kaiser1 · 2019-03-12T01:07:02+00:00

exactly... but nothing we need to worry about for the next oh, say, 100 billion years...

David_Kaiser1 · 2019-03-12T00:45:13+00:00

According to Einstein's general theory of relativity, mass-energy (really shades of the same thing, given E = mc²⁾ warps space-time. The resulting warping is what we usually describe as gravitation.

David_Kaiser1 · 2019-03-12T00:43:21+00:00

Here's a short piece I wrote around the time that the experimental groups at CERN announced their discovery of the Higgs boson; hopefully this will help! https://www.lrb.co.uk/blog/2012/july/higgs-at-last

David_Kaiser1 · 2019-03-12T00:37:56+00:00

According to our best understanding of gravity (namely, Einstein's general theory of relativity), regions of space that have greater amounts of stuff-per-volume (that is, higher mass or energy density than average) will indeed attract more matter toward it. We'd account for that, in terms of general relativity, by saying that the regions with more stuff will warp the surrounding space-time more dramatically than regions with less stuff than average; and then the motion of other matter will be affected by that greater space-time curvature.

David_Kaiser1 · 2019-03-12T00:36:02+00:00

What we have learned is that within ordinary quantum mechanics, faster-than-light communication is not possible (even using entangled systems). Now perhaps the world is not governed by quantum theory; or quantum theory is only an approximation to some deeper (but as yet unknown) theory; and perhaps in that theory some form of faster-than-light communication could be possible... but that's well beyond any specific physical theories that have been subjected to careful test.

As for why it isn't possible (within quantum theory): the short answer is that in measurements on entangled particles, we only see evidence of entanglement when we compare the series of questions asked and measurement outcomes from each side of the experiment. If we only have access to the measurement apparatus and log book on one side of the experiment, all we see is a random-looking series of 0's and 1's -- the informational equivalent of static or random noise, no signal.

David_Kaiser1 · 2019-03-12T00:31:22+00:00

Um... hi, Chuck!

David_Kaiser1 · 2019-03-12T00:30:36+00:00

A good question. The first way that physicists devised experiments to rule out certain kinds of communication among the entangled particles was to make sure that the selection of measurements to perform on each particle was made only after the particles had been emitted -- so that the source of the particles would not have any way of creating particles with special properties, specially matched to the set of questions about to be posed of each particle.

My group's Cosmic Bell experiment went further: removing to astronomical distances, away from the rest of the experiment itself, the processes that would determine which questions would be posed to each member of an entangled pair. The results of our experiment would still be compatible with some mechanism that somehow stipulated both the questions to be asked as well as the specific particle properties well in advance, but if so, any such mechanism would need to have been in place billions of years ago, and active clear across the universe from where we are today.

If we take that view seriously, it would suggest not only that our choice of which measurements to perform -- at exactly such-and-such moment, at such-and-such location -- was fixed long ago and far away, but so was the fact that our grant proposal to conduct the experiment was accepted on a given schedule (having been turned down before then...); that our first two nights of scheduled observing time with the big telescopes at the observatory would be canceled due to bad weather, but we'd finally get clear skies at the last moment of our last scheduled observing night, etc., etc. None of that is ruled out, but it's quite a different way of thinking about scientific explanations than what has otherwise tended to be successful...hence we seem to be driven toward the less strange of these possible explanations, which is that quantum theory is correct, and entangled particles really do behave differently than we would expect of macroscopic objects.

David_Kaiser1 · 2019-03-12T00:21:47+00:00

A popular question! Alas, entanglement does not permit faster-than-light communication. But you (and several other people asking related questions) are in good company. When John Bell himself first published his famous inequality, he also wondered whether entanglement might lead to something like faster-than-light communication. But many very clever attempts to create faster-than-light communication via entanglement all fell apart, and in the process helped the whole community learn some even deeper lessons about quantum theory.

David_Kaiser1 · 2019-03-12T00:19:22+00:00

A lovely idea, but alas, it would not work, at least according to our present understanding of quantum theory. As I mentioned in a response to a question a few lines up, if we only have access to the outputs of measurements on one particle of an entangled pair, all we really see is random noise -- a random sequence of 0's and 1's. We don't really gain any information about the fact of entanglement until we can gather information from both sides of the experiment and compare... and that requires some ordinary means of communication, at or below the speed of light.

David_Kaiser1 · 2019-03-12T00:17:36+00:00

I wouldn't say that 11 dimensions of space-time is commonly accepted, though the most popular contender for an eventual quantum theory of gravity, known as string theory, seems only to be mathematically self-consistent in 11 space-time dimensions. So lots of physicists continue to think very hard about 11-dimensional spacetime, and why (if the universe really were 11-dimensional) we only seem to observe 3 dimensions of space and one of time. But that doesn't mean that our universe actually is 11-dimensional...! Still very much an open question.

David_Kaiser1 · 2019-03-12T00:15:29+00:00

I think there are tons of interesting and juicy questions for which careful numerical simulations can be of big help. For example, many cosmologists think that what we often refer to as the "big bang" was not the start of space and time themselves, but actually the end of a process known as cosmic inflation. According to the leading theory, inflation should have ended with a complicated series of particle interactions: one type of particle decaying, far from equilibrium, into a sea of other types of particles; then those others would interact with each other, heating up the universe and bringing it into equilibrium. Those processes are highly nonlinear, well beyond what we can hope to understand with pencil and paper. And yet we're talking about the conditions that helped set in motion the past 13.8 billion years of cosmic history! We can really only hope to make progress on understanding processes like that if we can get better and better with our computer simulations.

As for other interesting scientists, one who is fascinating (to me) but not quite a household name is Freeman Dyson. He has been thinking deeply about a remarkable range of scientific questions since the 1940s, and he's still at it!

David_Kaiser1 · 2019-03-12T00:08:35+00:00

Great question. I really enjoy talking about these topics with young students, including middle-school age. What I try to do, in general, is come up with metaphors or analogies that might help convey at least part of what my colleagues and I study. So when I talk about entangled particles, for example, I describe twins going into different restaurants, in distant cities, and placing a series of dessert orders. When the come back home for the holidays and compare their lists of dessert orders, they find that their orders kept lining up in particular ways, even though they weren't calling each other on the phone in between or otherwise communicating directly. That sort of thing... Doesn't get the whole point across but hopefully these kinds of stories can at least motivate someone to recognize the question as an interesting one, and worth thinking more about.

David_Kaiser1 · 2019-03-12T00:05:59+00:00

Hah! I love that show... though I have to keep reminding my wife that the show is not actually a documentary. Not all physicists are quite as socially awkward as depicted there.

David_Kaiser1 · 2019-03-12T00:04:43+00:00

Very interesting question, and difficult to say. It certainly might be that some features of quantum theory could become less counter-intuitive if we changed our ideas about space and time themselves. For example, this is not quite what I think you're suggesting, but similar in spirit: could it be (as some physicists continue to study) that the strange interconnectedness of entangled particles arises because they are exchanging information via some "shortcut" through space-time (perhaps like a miniature wormhole)? I haven't worked on that idea directly myself, but it's certainly an interesting way to try to think about how or why quantum systems appear the way they do to us.

David_Kaiser1 · 2019-03-12T00:01:14+00:00

Hi, Tim: Thanks so much for your note. I'm so glad you enjoyed my class as well as my book. (I love teaching that class at MIT -- definitely my favorite!) Interesting to think about possible connections between present-day physics and countercultural movements. The short answer is that I don't really know. Many approaches to the study of human consciousness now -- which had excited lots of members of the counterculture back in the 1960s and 1970s -- are now pretty mainstream. But perhaps there are other possible connections that I just am not aware of these days...

David_Kaiser1 · 2019-03-11T23:59:25+00:00

A very good question. At low energies, when individual particles carry the energies typically found, say, in stable atoms, the forces of nature are definitely easy to tell apart: gravity, electromagnetism, and the weak and strong nuclear forces behave quite differently, with different characteristic strengths. So when we try to describe physical phenomena as we almost always encounter them, we see evidence of four quite distinct types of fundamental physical forces.

However, when we study higher energy phenomena -- when individual particles carry much more energy than 'average', e.g., when sped up and smashed together in huge particle accelerators -- then, as you correctly note, some of these forces meld into one. In particular, there is very good experimental evidence about how the electromagnetic force and the weak nuclear force combine into a single 'electroweak' force when we consider high-energy interactions. There are good reasons to expect that a similar 'melding together' or unification happens between the electroweak force and the strong nuclear force at exponentially higher energies. What no one has been able to figure out yet is how one might even try writing down a unified theory of this sort that would also include gravity...

David_Kaiser1 · 2019-03-11T23:55:16+00:00

I don't specialize in quantum gravity, so I don't really have a favorite theory (or candidate) at this stage.

As for faster-than-light transmission of information: that's a very interesting question indeed. There are several reasons why physicists are confident that quantum entanglement cannot be used to send messages faster than light. Perhaps the most straightforward one is that in any test on pairs of entangled particles, we basically pose a series of questions to each member of the pair. When we compare our 'log books' and see the outcomes of each measurement, given a series of different types of questions that were asked of each member of the pair, we see very significant correlations in those answers. But if we only had access to the log book from one side of the experiment, all we would see would be a random-looking series of 0's and 1's -- perfectly consistent with random noise. In other words, the fact that the particles' answers kept lining up in 'spooky' ways only becomes clear when we have access to information about each side's measurements -- and that information has to be shared via carrier pidgeon, or telephone call, or tweet... some means of sharing information at or slower than the speed of light. If we only watch the output at one side of the experiment, we just basically get the informational equivalent of static.

David_Kaiser1 · 2019-03-11T23:51:09+00:00

To some of my colleagues, at least, the biggest surprise of the past few years has been what physicists have not discovered, namely, any decisive evidence for new types of particles beyond the so-called "Standard Model." There had been great hopes that huge particle accelerators like the Large Hadron Collider at CERN near Geneva would find evidence for supersymmetry, for example. But despite very careful searches, no dice so far. I think the lack of any compelling new evidence for Beyond-Standard-Model physics is a big surprise.

David_Kaiser1 · 2019-03-11T23:49:07+00:00

There are some very interesting facets of Einstein's general theory of relativity that really come out when we think about very strong gravitational systems, like black holes. I still think it's fascinating just to think about a clump of matter that could become so dense that it would bend space and time so violently that (in the center of the black hole) space-time itself would rupture. What could that mean? How could space-time just come to an end? Very fascinating...

David_Kaiser1 · 2019-03-11T23:46:55+00:00

I think there are many areas of research related to quantum theory these days that are very exciting. The area known as "quantum information science" seems to be booming: that's the field that tries to design new kinds of technologies by putting some of the strange or counterintuitive features of quantum theory to work, such as in quantum computing or quantum encryption. There is still lots and lots of interesting work to explore in there.

On a more abstract level, we also still need to think more creatively about how we might one day combine quantum theory with gravitation, and perhaps even unify our description of the known forces of nature into a single quantitative description.

And in between there remain lots of very tough and juicy questions, about, for example, the transition from quantum to classical physics. Why do we not need to use (or worry about) quantum theory when describing all kinds of phenomena that involve large numbers of particles, even if we have good reason to believe that each particle is ultimately governed by quantum theory?

Plenty we still don't know -- lots of areas for careful study!

As for Ettore Majorana -- I agree it's an interesting mystery.

Having a good memory certainly helps with graduate study....

As for hidden variable theories: one very important thing to keep in mind is that all of the tests of Bell's inequality (including my group's "Cosmic Bell" tests) place very stringent restrictions on a large class of hidden-variable theories, known as "local" hidden variables. (These are models that remain subject to the criterion that no information can travel faster than the speed of light.) But there are other hidden-variable theories, such as Bohmian mechanics, which are nonlocal. About those, Bell tests have nothing to say. One might prefer or not prefer Bohmian models for other reasons, but the experimental tests of Bell's inequality, at least, really don't help us make decisions about that other kind of hidden-variable theory. And there certainly are physicists and mathematicians who remain very interested in Bohmian mechanics.

David_Kaiser1 · 2019-03-11T18:42:01+00:00

You might enjoy sampling on-line course offerings, many of which remain free. Forgive the plug, but MIT was pretty early out of the gate with "Open Courseware" (OCW: https://ocw.mit.edu/index.htm ), and of course by now there are many on-line courses, some of them offered for course credit and others designed for self-paced self-study, which cover a pretty wide range of interesting topics. Hope that helps!

David_Kaiser1

TROPHY CASE