I’m the Germeshausen professor of the history of science in MIT's Program in Science, Technology, and Society, and also a physics professor in MIT's Department of Physics. I completed an undergraduate degree in physics at Dartmouth College and PhDs in physics and the history of science at Harvard University.

I’ve helped to design and conduct novel experiments to test the foundations of quantum theory, including the recent “Cosmic Bell” experiment on quantum entanglement that was featured in the NOVA episode. I also study the physics of the very early universe, trying to understand the Big Bang and how our present-day universe might have evolved from very different conditions.

My historical research focuses on the interplay between science, politics, and culture, especially as these have unfolded over the past century.

Currently, I’m working on two books about gravity: a physics textbook on gravitation and cosmology co-authored with Alan Guth, and a historical study of Einstein's general relativity over the course of the twentieth century. My other books include Drawing Theories Apart: The Dispersion of Feynman Diagrams in Postwar Physics (University of Chicago Press, 2005), which traces how Richard Feynman's idiosyncratic approach to quantum physics entered the mainstream, and How the Hippies Saved Physics: Science, Counterculture, and the Quantum Revival (W. W. Norton, 2011), which charts the early history of Bell's theorem and quantum entanglement, and was named "Book of the Year" by Physics World magazine. In 2010, I was elected as a Fellow of the American Physical Society. My work has also been recognized with the Pfizer Prize for best book in the field (2007) and the Davis Prize for best book aimed at a general audience (2013) from the History of Science Society, and the LeRoy Apker Award for best undergraduate physics student from the American Physical Society (1993). In 2012, I was named a MacVicar Faculty Fellow, which is MIT's highest honor for excellence in undergraduate teaching. That same year, I received the Frank E. Perkins Award for excellence in mentoring graduate students.

I enjoy writing about physics and the history of science for broad audiences, in venues ranging from the New York Times and the New Yorker magazine to Scientific American and the Huffington Post. I also do my best to describe complicated topics in accessible ways on National Public Radio, BBC Radio, and NOVA television programs, such as “NOVA Wonders: What’s the Universe Made Of?” Most recently, my group’s work was featured in NOVA’s “Einstein’s Quantum Riddle,” which premiered on Jan. 9, 2019, and explored how my colleagues and I grabbed light from across the universe to put quantum entanglement to the ultimate test.

Ask me anything about my research in physics and the history of science.

Comments: 146 • Responses: 44  • Date: 

allisoneck20 karma

It's common knowledge that the effects of quantum mechanics and entanglement can only be seen on very small scales. Is there definitive consensus on this idea? As a historian of science, do you think maybe part of that assumption has to do with the fact that Einstein, Bohr, Heisenberg, etc. felt so existentially torn between what is intuitive to the human brain and what is theoretically possible / beyond our comprehension?

David_Kaiser127 karma

In principle, the effects of quantum entanglement should be observable across arbitrary distances, if the experiments can be set up with sufficient care. And in fact colleagues have demonstrated entanglement across some pretty long distances: about 100 miles in some beautiful experiments from a few years ago (e.g., https://arxiv.org/abs/0811.3129 , https://arxiv.org/abs/1403.0009 ); and now well over one thousand miles (https://arxiv.org/abs/1707.01339), using entangled particles emitted from an orbiting satellite. So I think we now have some real-world demonstrations that quantum entanglement need not only be observable across short distances.

Nonetheless, I do think it's interesting that some of the first physicists to think carefully about entanglement were doing so long before they could have imagined the kinds of instruments and technologies with which researchers can conduct these experiments today. It's interesting to wonder about how our own imaginations today might be similarly limited!

theumbrellaman7 karma

I think to a lay public it is helpful to clarify that, even in such long range examples of entanglement, the things being entangled still tend to be small!

In principle there is no reason why an arbitrarily large object cannot be put in an entangled state. But as a practical matter the effects of interaction with the environment (so-called "decoherence") seriously limit our ability to construct such systems in real life.

David_Kaiser112 karma

Yes, thanks -- that's a very good clarification! The *distances* across which entanglement can be measured can be very very large, but the *objects* that are entangled continue to be very, very small. Indeed, it's an on-going challenge among physicists to see just how large an object can be, and still show tell-tale signs of quantum effects like entanglement. There was a very cool recent experiment that got much bigger than single particles, but still much much smaller than the size of everyday objects: https://arxiv.org/abs/1806.10615 .

609venezia11 karma

Thank you for being here! 🙏🏻

My historical research focuses on the interplay between science, politics, and culture, especially as these have unfolded over the past century.

It feels like politics and culture may be starting to strangle science, and certainly that they are failing to fully support science and take full advantage of what science offers

Given your historical perspective, does that ring true? If so, do you see a way forward--maybe a similar way that people were able to escape something like our current conditions in the past?

David_Kaiser116 karma

That's an interesting question. On the one hand, historical studies suggest that scientific research has never been entirely outside of politics. Aristotle had to navigate difficult political and patronage relationships in his day, and so did Galileo, let alone more recent researchers. So I don't think we should be looking back for some golden age when science was entirely isolated from the politics of the society in which it was embedded.

Nonetheless, we can ask whether the pursuit of scientific research was affected more or less strongly by political forces in other times, or in ways that made certain kinds of research particularly difficult, and whether those past examples might offer any lessons for today. From that perspective, I think a shift to short-term expectations for specific "deliverables" attached to various grants, and volatile funding levels year-by-year make it more difficult to plan for and support certain types of ambitious research projects than might have seemed feasible a few decades ago.

quantizedself7 karma

Do you feel that the late-19th/early-20th centuries were a special time in physics, in that great leaps of intuition (i.e, Einstein) could have a much more profound impact than would be possible today?

I've often wondered why we haven't had another Einstein, and I think it's because we've learned so much that such leaps are much more difficult for a single person's intellect. Do you think that the next big leaps in physics will come from AI working in tandem with humans?

David_Kaiser19 karma

Interesting question. One thing that has definitely changed since Einstein's day is the exponential growth in the number of scientists (including, e.g., physicists), and -- not surprisingly -- the need to specialize in topics since so much research is going on across so many areas within physics. Back in Einstein's day, a very talented researcher could aim to keep up to date on many interesting topics, and even to contribute across multiple fields of study. But today the volume of research output is so enormous, and increasing so quickly, that most people need to narrow their sights to focus on a few specific topics. I don't know if that effect is enough to explain what does feel like a slower pace for major changes across a big field like physics these days, compared to 100 years ago, but I bet it plays some role.

I don't really know what to expect from AI moving into the future. Some of my colleagues in the field have been able to use machine-learning techniques very creatively to help with (e.g.) pattern detection within large amounts of experimental data. I'm not sure that's quite the same thing as having AI algorithms really contribute to generating brand-new hypotheses or asking questions in a new way, but perhaps the kind of tandem / partnership that you mention really will lead to some fruitful new directions.

thewildthrnberry6 karma

Hey, David! Did the “Quantum Bell Experiment,” in your opinion, definitively prove that quantum entanglement is real? Or is the phenomenon still up for debate?

David_Kaiser17 karma

Good question! In general there has been really compelling evidence in favor of quantum entanglement for some time. But the community has focused on a series of so-called "loopholes" which -- at least in principle -- would have enabled a skeptic to account for the results of these experiments *without* concluding that entanglement is real.

A few years ago, several very impressive experiments managed to close a pair of these loopholes at the same time (https://arxiv.org/abs/1508.05949 , https://arxiv.org/abs/1511.03190 , https://arxiv.org/abs/1511.03189 , https://arxiv.org/abs/1611.04604 ); these experiments used different types of entangled particles than each other, and different types of instruments to perform measurements on them, which definitely helped strengthen the evidence in favor of quantum entanglement in a big way.

But even those impressive experiments had not fully addressed a different "loophole," which our own "Cosmic Bell" experiments (https://arxiv.org/abs/1611.06985 , https://arxiv.org/abs/1808.05966 ), as well as other cool recent experiments (like this one: https://arxiv.org/abs/1805.04431), set out to address. Yet a real stickler could rightly note that our "Cosmic Bell" experiments did not address one of the previous "loopholes," which had been addressed in those experiments from 2015-16.

So someone who *really* wanted an Einstein-like universe to prevail could hold out a sliver of hope that perhaps the universe obeys laws more like Einstein expected -- different than quantum mechanics -- and that for some reason one type of loophole could account for the results of one batch of experiments, whereas a different loophole could account for a different batch of experiments... But the wiggle-room left for such a determined skeptic is shrinking pretty fast!

holyshitimhomeless6 karma

Do you think there was anything before the big bang?

David_Kaiser19 karma

Good question! Partly it depends on what we mean by "the big bang." Many cosmologists (including myself) these days reserve the term "big bang" to refer to a specific set of conditions for our universe, which should have described our universe at very early times -- but which need not correspond to the very beginning of time (or of our universe) itself. So if we take "the big bang" to refer to the conditions in what is commonly known as "the standard big bang model" -- which is to say, a universe at a very high temperature and density, with most matter behaving like radiation -- then there very likely *was* an era within our own universe before the big bang. The leading theoretical contender is a theory known as cosmic inflation, which aims to describe processes even earlier in our universe that would have *resulted* in the conditions associated with the standard big bang picture. (Inflation also predicts a series of other features that should be observable in our universe today, including the subtle pattern of tiny unevenness in the cosmic microwave background radiation, which has now been measured to remarkable accuracy.)

There are even more speculative ideas that suggest there could have been any number of other phases within our universe, even prior to inflation -- and hence likewise before the "big bang." In some models, our universe might have been cycling through a series (perhaps an infinite series) of previous expansions and bounces, though those models remain less well developed than the now-standard cosmic inflation description.

Part of the challenge in determining what (if anything) might have occurred before the big bang is that -- if cosmic inflation really did occur within our universe -- empirical evidence for phases prior to the inflationary one would have been pushed very far beyond what we can ever hope to observe or measure today. So evidence of possible phases or eras that could have been even earlier might simply not be accessible to direct observation, though we are able to compare predictions for certain phenomena that could have occurred *during* inflation (as well as soon after it) with various types of measurements.

So the question remains fascinating, and very much open.

shitty-cat4 karma

Have you ever done DMT if so, what did you gather from your trip and did it help you in regards to your work?

David_Kaiser14 karma

Nope -- no insights to share on that one!

NewInSphere4 karma

How do you take care of your physical and mental health? I imagine your line of work is mentally exhausting.
Also, what are your thoughts on a breakthrough in our understanding of gravity coming anytime soon?

David_Kaiser18 karma

Trying to keep up with research and teaching can definitely be pretty dizzying (I mean for many, many people in the field these days, certainly not just me). I'm very lucky to have a supportive family and also a great circle of students, colleagues, etc. We can share in the excitement of things but also try to help each other out along the way. Not to be underestimated...

As for gravity: that's a tough one. On the one hand, we have such a beautiful, elegant, and well-tested theory of gravitation, Einstein's general theory of relativity, that seems to describe phenomena successfully all the way from distances of about a millimeter up to tens of billions of light years. On the other hand, physicists still have not found a way to describe gravitation in terms of quantum theory, even though every other fundamental (physical) force that we know about in nature now can be described in quantum-mechanical terms. So we have very strong reasons to think that our present, very successful understanding of gravity is a sort of place-holder, even though we don't yet have any conclusive evidence of what the ultimate theory of gravity might entail.

My personal hope (and it's just a hope) is that upcoming improvements in high-precision tests of genuinely cosmological phenomena might shed some light into next steps for understanding gravitation.

mauxfaux4 karma

If gravity is indeed a curvature of spacetime, and not a force, what is the purpose of the theoretical graviton?

David_Kaiser14 karma

I think you've put your finger on part of why it has remained so stubbornly difficult to develop a thorough-going quantum-mechanical description of gravity, akin to our (very successful!) description of the other physical forces. For the other physical forces -- electromagnetism, as well as the strong and weak nuclear forces -- we quantify the behavior of those forces in terms of the exchange of tiny force-carrying particles (such as the photon, an individual particle of light and the force-carrying particle responsible for the force of electromagnetism). But when we try to describe gravitation in similar terms -- as the exchange of tiny, hypothetical particles known as "gravitons" -- our usual approach breaks down. So we still don't have a fully worked out, quantitative theory of gravity at the quantum level, which would presumably involve the exchange of gravitons.

If we did have such a quantum theory of gravity, presumably the large-scale curvature of spacetime -- which is the way that Einstein's general theory of relativity describes the phenomena of gravitation -- would emerge as some sort of effect of graviton exchange. Other physical forces -- especially the nuclear forces -- have some pretty complicated mathematical structure that is analogous to how physicists describe space-time curvature, so it's not impossible to think that graviton exchange might be compatible with a description in terms of space-time curvature. But we still don't really have a clear answer in hand for the case of gravitation.

iamjacksliver663 karma

In all your research into Einstein what were a couple wow that's cool things you found out?

David_Kaiser112 karma

I think Einstein's biography continues to be really fascinating. One thing that becomes clear (which shouldn't really be surprising) is that he was, after all, a person -- a real-live person, with faults and shortcomings and blindspots like any other person, as well as (of course) extraordinary talents and insights. He himself noted later in his life that he had not always treated people as fairly as he (later) wished he had; he admitted to a close friend that he had difficulty forming close relationships and envied friends who had such happier family lives, etc.

His relationship with his first wife, Mileva Maric, ended particularly poorly. A few years before they got divorced, they were barely on speaking terms. Einstein was so confident that he would win the Nobel Prize (which had a large cash component) that he put into his divorce negotiations a promise to hand over the prize money to his ex-wife -- even though he hadn't won the prize yet!

Whereas his personal life was (even by his own, later evaluation) not always what he hoped it would be, he also was at times very brave, speaking out in favor of political causes that were quite unpopular at the time, standing up for positions he believed in.

cizzlewizzle2 karma

With regards to gravitational waves, are these things we are constantly being bombarded with like neutrinos, or more like a shock-wave that passes through, therefore we must wait for events like merging black holes or neutron stars? And given the precision required by LIGO for such detection, do you know how they isolated ambient disturbances? From the sounds of it, even nearby footsteps would register. Thanks!

David_Kaiser16 karma

We can think of gravitational waves as analogous to the shock waves you refer to: they really do arise from events or processes in outer space (such as the collision and merger of large objects like neutron stars or black holes). On the other hand, the universe is so large and there are so many of these kinds of objects out there, that we should expect (in principle) that these sorts of waves should be reaching Earth all the time -- it's just that most of the waves that reach us are from so far away, and have been traveling such enormous distances, that the height (or amplitude) of the waves that reach Earth is very, very small. (There is another source of gravitational waves -- different from mergers or collisions of huge star-like objects -- which might also be reaching the Earth all the time, with a nearly constant height from all directions in space. These would be a remnant disturbance from very near the time of the Big Bang itself. Several teams are still trying to detect these primordial gravitational waves.)

As for how LIGO and related experiments can perform measurements of such ultra-tiny disturbances: part of the process involves *comparing* signals at widely separated detectors. That way if some local disturbance (a truck driving by, seismic activity in one location, etc.) sets one detector off, the researchers can check to see if that disturbance was correlated with any signal at the far-away detector. Genuine gravitational waves travel through space, and hence they should set one detector 'ringing' and then, a little while later, set the other detector 'ringing,' whereas local disturbances should not have any matching or correlated signal in the distant detector. So that search strategy definitely helps to rule out some of the local disturbances.

H2OSeeker2 karma

First, please excuse my limited knowledge because it it difficult to wrap my brain around this hypothesis: Is possible to have a parallel universe? Also, what direction do you believe educators should be encouraging and engaging students in K-12 in sciences, STEM????Does ones lack of scientific knowledge among a culture affect ones ability to perhaps understand climate change or it it their political mind reasoning with their beliefs for it or against it?

David_Kaiser19 karma

There are two types of "parallel universes" that theoretical physicists these days sometimes think about: a cosmological context in which our own universe might just be one "bubble" within a sea (maybe an infinite sea!) known as the 'multiverse'; and a quantum-mechanical context, often referred to as the "many-worlds" hypothesis (or the Everett interpretation of quantum theory) which suggests that every time certain types of measurements are performed on quantum objects, the entire universe itself splits into copies, in one of which physicists measured one of the possible outcomes on that object, and in the other of which they measured the opposite outcome. Both ideas are interesting and some physicists are motivated to think about them because of difficult questions about the laws of nature at a very deep, fundamental level -- but it's also important to note that there's no empirical evidence (as yet) that either of these sets of ideas really describes our own world.

As for science education and thinking about the future: my own guess is that focusing on expanding access to quality education for more and more people would have a positive impact on society, over and above trying to manage the selection of subjects (STEM or otherwise) that all those students get to explore. There are so many interesting, difficult, cool questions out there that getting more and more kids excited to learn and ask questions seems like a really important goal.

Phugginay2 karma

You say you are writing books on gravity... My admittedly tiny experience (mostly watching shows like NOVA) seems to indicate that there is no such thing as gravity and that what we experience as an attractive or pulling force is actually a pushing force that comes from the warping of space-time. This, I thought, was one of Einstein's greatest breakthroughs... am I utterly wrong to think of gravity in this way?

David_Kaiser15 karma

Fair point: according to Einstein's general theory of relativity, the phenomena we associate with gravity -- like the planets orbiting the sun, or entire galaxies coalescing and even colliding with other galaxies -- arise from the warping or curvature of space and time. So there is certainly something we can call "gravity" -- that is, physical effects that are different from, say, electromagnetic forces or nuclear forces and the like -- and we describe these gravitational phenomena in terms of warping space-time.

Thorium2302 karma

Hi, Dave, fancy seeing you on reddit! You're so famous.

1st Q: When will you realize that we are all in a plutonium atom? The only black holes are....

2nd Q: Who was your favorite college roommate, and would you rather rhyme his name with "luck" or "duck"?

My best to the fam!

David_Kaiser11 karma

Um... hi, Chuck!

_Sunny--1 karma

Why do we consider there to be four fundamental forces (gravity, EM, weak force, strong force) even though Steven Weinberg proved that the EM and weak force combined into the Electroweak force at certain energies because they come from the same symmetry?

David_Kaiser11 karma

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...

myworkreddit1231 karma

Hello: 1. What is your favorite theory of quantum gravity so far? 2. Do you foresee practicable faster than light (FTL) travel or something like it (i.e. wormholes we can fit a car thru) every happening? 3. In a related question, how is quantum entanglement not the FTL transmission of info? I've read/heard many times it doesn't count as FTL, but honestly have never been able to understand how it's not based on those explanations.

Thanks!

David_Kaiser11 karma

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.

farfaraway1 karma

What is the most surprising thing that physicists have discovered in the last few years?

David_Kaiser12 karma

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.

bonkedmynose1 karma

what gravity fact most fascinates you? did you learn anything while writing this latest textbook?

David_Kaiser12 karma

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...

ANeedForUsername1 karma

Thank you for the opportunity to ask you some questions!

In your opinion, which field of quantum looks the most promising to you in the near future?

What do you think happened to Ettore Majorana?

How important is having a good memory in postgraduate programmes?

With all the limitations placed by locality, realism, contextuality, etc, will we be able to find a hidden variable theory? Are there people working to find one?

David_Kaiser11 karma

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.

MooseOnTheLoose411 karma

I am a graduating high school student considering majoring in physics (and a couple of other things), and although this question is beneath you, i was wondering what are the benefits and downfalls of pursuing a degree and career in physics?

David_Kaiser12 karma

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!

sleal1 karma

Hey there, physicist turned engineer here. I stopped pursuing physics and focused on engineering in school because I was scared of ending up in the purgatory of academia. Although I do enjoy my work as an engineer I still thirst for physics. I do not think going back to grad school for physics is in the cards for me. How do I stay sharp in that department, research, etc.?

David_Kaiser11 karma

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!

shayknbake1 karma

Why are physicists so quick to believe that dark energy and dark matter exist rather than rethinking general relativity? Isn't it a much smaller leap of faith to think that at the galactic scale and beyond, general relativity does not act the way we think it should or could be superceded by another theory of gravity (just like how Newtonian gravity worked to a certain extent but had to be rethought completely to explain more advanced phenomena)?

It seems ridiculous to me that many scientists think something like 95% of energy in the universe is something that we have never seen and potentially could never see rather than try to adjust a theory, which, granted has worked for 100 years, but needs adjusting, especially considering the fact that it bumps up against quantum physics as well.

David_Kaiser13 karma

That's a good question, and one that (rightly) comes up fairly often. I think the reason that so many of us believe that general relativity (GR) remains a viable framework within which to consider the phenomena associated with dark matter and dark energy is because efforts to *change* GR to address these specific phenomena have always -- at least so far -- led to other conclusions that are at odds with a long list of robust empirical measurements. I think of it like an elaborate jigsaw puzzle. When people study modifications of GR to try to make sense of example A or example B, in every known case so far, they might wind up giving an adequate account of those specific cases, but then can no longer account for a *huge* number of other examples, which GR does a *great* job of describing (in quantitative detail). So I don't think it's the case that most astrophysicists are sticking with GR out of stubbornness (or at least not *only* out of stubbornness), but rather because, at least so far, efforts to formulate alternatives have wound up doing *much* worse at accounting for the full range of phenomena for which GR has already been so successful.

That being said, many astrophysicists and cosmologists really do remain open to trying to find some self-consistent modification of GR, and are actively looking for empirical measurements that might reveal limitations of GR itself. (A colleague of mine just posted a detailed review article, as but one recent example: https://arxiv.org/abs/1902.10503 .)

fireballs6191 karma

How do you think Einstein would have responded to Bell experiments that verified Bell’s theorem?

What is your personal opinion on realism vis a vis quantum mechanics?

At what level is your upcoming textbook with Professor Guth written at, and what are you hoping to accomplish with it?

Thanks for doing this AMA, your line of work is very interesting.

David_Kaiser13 karma

I sometimes wonder how Einstein would respond to the raft of recent Bell tests. Wouldn't it be amazing to be able to ask him (or other members of that early generation) directly?! We know that Einstein himself was deeply interested in experiments, back in his own day, and he sometimes worked directly with experimentalists to design new experiments. So I bet his first questions would be very smart questions about experimental design, instrumentation, and all that.

I really don't have a favorite interpretation for quantum theory. I have some good friends who are especially enthusiastic about the many-worlds (or Everett) interpretation, and I always enjoy hearing them make the case. But personally I'm still pretty agnostic. What I find most interesting, to be honest, is that the *question* is now treated as significant, and worthy of serious attention from very skilled researchers. The topic wasn't always treated so seriously, so in that sociological sense I think we (collectively) have made great progress, even if we still don't have a single winning contender for these hard questions.

My book with Alan Guth on early universe cosmology will be aimed at advanced undergraduates -- few pre-requisites. For example, we will not assume that students have studied either general relativity or quantum field theory before. We will expect some familiarity with (non-relativistic) quantum mechanics and classical electromagnetism, and our goal is to try to write the book in a self-contained way, including with a few optional appendices, so that students can dig into some really interesting material about cosmology without having to wait several more years before completing tons of specific coursework. That's a fun challenge for us as textbook authors, but Alan, in particular, has taught a very successful undergraduate course on this material for years, so he has tons of great experience in trying to find a good level to introduce a lot of material to students at this stage.

axsenion1 karma

My question is a little off topic, but I hope it doesn't bother you too much.

I'm currently an undergrad at the end of my freshman year majoring in physics, but I'm not sure what I want to do with my physics studies after I graduate. I'm not super interested in research as of right now, I'm more interested in the applied physics route, perhaps working for a tech company or something similar in product development (HP, LG, etc.). Is this valid? Are there many jobs in physics besides research? Should I try to focus my studies on something more specific in physics?

Thank you very much for your time and for hosting this!

David_Kaiser15 karma

One of the things I really enjoy about teaching physics is that it provides all of us (including undergraduates) an opportunity to study a wide range of cool things, and also develop a good set of tools with which to tackle all kinds of questions, well beyond physics itself. I think the practice of learning how to pose questions sufficiently clearly to enable a quantitative analysis is itself a great skill; and of course there are opportunities to hone more specific skills that have wide use, such as certain computing modeling skills, simulations, etc. A lot of those skills can also be learned in other fields, to be sure, but I do think that physics offers a very cool area in which to learn specific tools that themselves can be applied quite broadly. Hope that helps! and good luck with your studies.

KukeSlywalker1 karma

Do experiments that violate Bell's Inequality imply that the universe as we see it is actually deterministic?

David_Kaiser13 karma

Experiments that violate Bell's inequality -- even while addressing at least some of the well-known "loopholes" -- indicate that some *combination* of otherwise very reasonable-sounding assumptions about how the world works can't be correct. So the outcome of Bell tests need not imply that the universe is actually deterministic -- by which I think you have in mind the idea that all the phenomena we ever observe, and even our "choice" of measurements to perform, were all determined by prior conditions, perhaps going all the way back to some initial time for the universe as a whole. (This view is sometimes called "superdeterminism.") With some colleagues I recently wrote a pretty long analysis to show that even models of the sort that we think Einstein would have preferred could, in principle, mimic the predictions from quantum theory for the outcome of Bell tests, *without* having to go whole-hog to the assumption of "superdeterminism" (https://arxiv.org/abs/1809.01307 ).

It could also be that the world, deep down, really is as probabilistic (and hence non-deterministic) as quantum mechanics suggests, but some other reasonable-sounding principle is just wrong when it comes to describing our universe. Maybe we are making too simple an assumption about the structure of space and time, and hence the "locality" of the usual arguments -- that local causes should only have local effects -- is actually what is being violated.

Bell tests certainly don't force us to that conclusion, either, but they *do* force us to consider that a combination of several quite plausible-sounding ideas can't all be true of our world. And I think it's pretty amazing that experimental tests can help guide us to conclusions of this sort!

isgrad1 karma

If the universe has taken ~13.8 billion years to expand to its current size, will it take equally as long for it to contract? Do we even think it will contract back down?

I've also recently heard an explanation of the multiverse theory saying that the "parallel universes" we suspect are actually just regions of space that are moving away from us faster than the speed of light, thus we can never hope to observe them. What are your thoughts on this?

David_Kaiser13 karma

From what we can presently measure about the rate at which the universe is expanding -- as well as the rate at which it is *speeding up* (accelerating) in its expansion -- it looks very much like the universe will never contract. If the mechanism that is causing its accelerated expansion today really is a "cosmological constant," which means a genuinely constant amount of energy associated with empty space, then our universe will continue to expand forever, and never recollapse. Of course, it's possible that the mechanism that is driving the accelerated expansion today is *not* truly constant, and hence could change its characteristics in the future, but so far, at least, all the measurements are consistent with the simple "cosmological constant" hypothesis.

As for multiverse: you're right that physicists often describe several different types of scenarios with similar terminology. One case is just as you describe: space is stretching so quickly that there will be regions of space futher away from us than we could ever receive any signals or information. But there could *also* be other scenarios (again, hypothetical!) in which our entire universe is embedded within some larger 'container' space, in which there could be many (perhaps infinte) independent universes. Each of these scenarios are often described in terms of a "multiverse," though they aren't really the same idea.

Jao_R2 karma

so the death of the universe will be cold and lonely?

David_Kaiser11 karma

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

cashealy1 karma

What is the Higgs boson?

David_Kaiser12 karma

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

Xithulus1 karma

Speaking of gravity, I saw a thing online a few years back that showed the rocky mountains having a slight increase in natural gravity compared to the plains of Nebraska. If that is true, is gravity say..matter being attracted to more matter? the more dense it is, the more the increase in gravity? seeing as how there is "nothing" in space with seemingly no gravity yet an asteroid has a some pull, that would seem to be the case. If this is a thing people know now, I wasn't aware and oops.

David_Kaiser11 karma

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.

JamesonsOnTheRocks1 karma

How did physicists rule out the possibility that entangled particles don't actually communicate with each other? Isn't it possible that the act of entanglement seeds each particle with the results of all possible measurements for the life of the entanglement? Doesn't this seem more probably than the idea that two particles can communicate instantaneously over vast differences?

David_Kaiser11 karma

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.

Jao_R1 karma

From the little understanding I gained from reading through an article about Einstein's Quantum Riddle, the theory that the universe is probabilistic rather than deterministic kind of validates free will, don't you think?

I also suspect that teleportation (through gateways) will be the way we beat the speed of light and the way interstellar travel will be achieved.

One practical application of quantum entanglement though is instantaneous communications - something like the Ansible. If 2 subatomic particles can mirror each other simultaneously accross cosmic distances, imagine what we could do if we could use that to devise communication that could beat the speed of light. Do you think we're close to developing that kind of technology?

David_Kaiser11 karma

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.

Jao_R1 karma

Has this been definitely ruled out? What is/were the reason/s why this is not possible?

David_Kaiser11 karma

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.

Jao_R1 karma

Your explanation went over my head. I still don't see how that rules out FTL communication. If one device contains one of the entangled particle, and the other device contains the other, wouldn't there be no need to compare outcomes from each side, since we already know that each will just be a mirror of each other? And the only thing we need to do is manipulate the entangled particle on one side? Unless the entanglement doesn't always produce a mirroring effect?!?

David_Kaiser11 karma

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...

VenaMedia1 karma

Will we one day be able to control our remote space crafts in real time using quantum entanglement? If we change the state of the entangled electrons in a binary code, couldn't we use that to eliminate the time it takes a regular communication to travel at the speed of light to the vehicle?

David_Kaiser11 karma

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.

dragoncat_TVSB1 karma

So...is 11 dimension commonly accepted?

David_Kaiser12 karma

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.

dragoncat_TVSB1 karma

Plus, I am such a big fan of NOVA and your work! Thank you for bringing physics closer to us avg IQ folks!

David_Kaiser11 karma

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.

dragoncat_TVSB1 karma

Thanks! Can you prove string theory right, or wrong? Is it ok to say, If string theory is right, then 11-dimensional universe has to be right?

David_Kaiser12 karma

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?

tsealess1 karma

Hope I'm not too late! I'm a physics undergrad myself, and I've recently found an interest in computational physics, computer simulations, numerical resolutions... What do you think that field will help us with in the future?

And also, everyone knows about Einstein and Curie, but who's an interesting (either historically or due to their personality or work) scientist who isn't so well known among the general public?

David_Kaiser12 karma

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!

CaptainMopsy1 karma

How would you explain your work in a way that a 10-year-old would understand?

David_Kaiser11 karma

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.

Iwasthey1 karma

Is Amy Farrah Fowler as pretty in real life? And, how does God factor into your factoring?

David_Kaiser11 karma

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.

DanMuffy1 karma

I hope this isn’t a waste of your time, but Is it possible that the reason we do not know both position and time of quantum particles is because of the information located in another dimension we cannot measure? I realize that the wave equations suggest that interfering at the quantum level will leave too much uncertainty, but to rephrase, could the uncertainty arise due to dimensional properties and not necessarily disturbing the “values” by observing the experiment?

David_Kaiser12 karma

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.

Inri1371 karma

Hi Professor Kaiser! Taking your class was one of my favorite experiences at MIT, and How the Hippies Saved Physics is one of my favorite books in the genre of history of science. In that vein, can I ask if you have any interesting or notable perspectives on the relationship between contemporary physics/science and contemporary counterculture?

Cheers! Tim (VIII '11)

David_Kaiser11 karma

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...

IX0YE1 karma

Is there anything else beside gravity that can bend space and time?

David_Kaiser11 karma

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.

Gutterals-1 karma

when are we finally gonna leak out of the universe and fuck shit up??

David_Kaiser15 karma

Unlikely that we could leak out of our own universe. But there are interesting questions about whether our entire universe might just be one "bubble" within a much larger "multiverse" -- and if so, what might happen if such bubble universes ever collided? All hypothetical, but interesting to think about...