I’m Brian Greene, professor of physics and mathematics at Columbia University and co-founder of the World Science Festival. My latest book, UNTIL THE END OF TIME (out in paperback this week), is an exploration of the cosmos and our quest to find meaning in the face of this vast expanse. AMA!

Hi Everyone--thanks for the great questions. I have to sign off now, but feel free to ask more questions of me on twitter (@bgreene) and sign up for the World Science Festival newsletter (its free) to learn of a great many science programs we will be releasing over the coming months.



Comments: 556 • Responses: 29  • Date: 

superfuels208 karma


prhauthors191 karma

Well, I too -- like many of my colleagues -- are skeptical of the old idea of wavefunction collapse. But I am also skeptical about the many worlds approach. So, the quick answer is that I suspect we don't have the full resolution as yet. I am interested in a dark horse candidate known as the de Broglie-Bohm theory. It is not a particularly well-known or well-accepted approach. But I think it has a lot going for it.

browster155 karma

What up with muons not obeying the laws of physics?

prhauthors309 karma

It might be nothing. It might be the most important result in the past few decades. If the result holds up -- a big if -- it might mean, for example, that there is a another force of nature that we have so far overlooked.

PixelatedEntropy142 karma

Hi Dr. Green! Your books and documentaries are fascinating! Thank you for doing this! In today's social and political environment, where science denialism has reached a new high, what do you think is the best way to get evidence - based information to the people that are actively trying to ignore it, and what can science enthusiasts like myself do to promote evidence - based and critical thinking? Thank you!

prhauthors455 karma

This is an important question but in my view the answer may not be what many have in mind. Science deniers are not, generally speaking, coming to their position by a rational assessment of data and facts so providing such data and facts--however artfully/compellingly done--is not likely to have much impact. Rather, deniers often reach their position via identifying with a particular group that takes such a stance. We need to approach such people, therefore, with compassion/empathy/open-mindedness to hear why they so fully identify with this or that group. An underlying sense of distrust/getting-the-short-end-of the stick, and so on is for some the root cause and if you don't deal with the root cause you'll make no progress. Science books, science documentaries, and so on are vital and important but likely not the answer to your question.

YargmyBarge141 karma

Woah. Hey Brian, with all of the advancements that have been made recently, what is the active question that you're looking for currently? opposed to the generic, attempting to solve String Theory?

prhauthors263 karma

The possibility that quantum entanglement may provide a deeper understanding of how spacetime itself is stitched together.

relaxlu92 karma

Do you think that the muon g-2 results will eventually reach five-sigma?

And what does it mean for new physics?

With supersymmetry being one of the potential explanations for the muon g-2 results do you think that it opens the door for renewed interest in string theory?

prhauthors150 karma

Hard to say. Definitely an intriguing result. Any potential crack in the standard model is cause for interest and excitement. But the reaction must be tempered by the fact that 3 or 4 sigma results have a way of -- sometimes -- disappearing with better data.

Moggy-Man76 karma

Hi Brian, I just watched your Ted talk on whether our universe is really the only universe a few nights ago.

While I have SO many questions about the universe, in general and at all levels of complexity, particularly before and beyond it, I would like to ask how we humans can be so positively and absolutely sure about things we only have human levels of knowledge and history to build from, such as...

"Nothing can be faster than the speed of light!"

"All life must be carbon based!"

Even with using the most technical instruments and measurements, these are still based on technology humans have invented.

How can we absolutely know for a fact that these statements of absolute truth can't be countered in the future by more discoveries and knowledge?

prhauthors162 karma

We are ABSOLUTELY sure of very little. Instead, our confidence in various ideas about the universe grows as observational and experimental evidence accumulates. But however much evidence you have, say, for "nothing can be faster than the speed of light," all you need is one solid/replicable example of something exceeding light speed to invalidate the claim. As yet, for the speed of light example, there is no such example--and that's after more than a century of trying. But for other statements, such as life being carbon based, all we have in support of that is life on our planet. The excitement of finding life on other planets will be, in part, to see if life as we know it here is a good guide to life elsewhere. Conceivably, it may not be. Bottom line, scientists and people more generally, need to keep an open mind to the wonders and surprises of the cosmos.

Maybeyesmaybeno22 karma

How would you classify the transmission of information instantaneously through quantum entanglement? Is that is some way something travelling beyond the speed of light?

prhauthors85 karma

The conventional answer is that no information is transmitted through quantum entanglement. Only by slower than light transmission of the results of quantum measurements can you glean the correlations of the measurements at distant locations. And without that information the results of entangled measurements are nothing but random bits.

FluidHips62 karma

Which are the most important questions in physics and what are some ways that the layman can help make progress on these questions directly?

I know that we can help, for example, by lobbying Congress for funding, &c., but I mean in more of a sense of actually helping to solve these problems.

prhauthors92 karma

Tough question. Many if not all of the most important questions require a researcher to have substantial technical training to have a chance of making progress. Certainly, though, crowd-sourced data mining projects are one potent exception. But that may not be what you have in mind?

ChennaiSpaceCat54 karma

Hi Brian, If all of space and time expanded from a single point.. 13.8 billion years ago, shouldn't the concept of the extent of space being infinite come into question, since it hasn't really had an infinite time to expand.. however fast the rate of expansion is.. ?

prhauthors116 karma

If the full extent of space was finite at the big bang--if EVERYTHING came from a point--then you are right. Space would be finite today. But it is possible that space was infinite at the moment of the big bang. And that is the image to have in mind when thinking about space being infinite today. We physicists don't often emphasize this notion--but you are correct in your reasoning.

R34N1M47OR42 karma

Matter exists because there was an asymmetry between matter and anti-matter particles...

But I just don't get why it HAS to be that there was an asymmetry, rather than just pure chance that not all of them found their pair before inflation shot everything away.

Is it that obvious that if there were equal amounts they would have had to find their pairs? I always wondered why it's so obvious to scientists that there had to be an asymmetry in the first place rather than pure chance making it so some particles wouldn't find their pair, specially since inflation is so hilariously violent and it happened so fast. Plus, there were random fluctuations that we can see in the MBR. I'm pretty sure I just don't understand it properly because it seems weird that everybody is so sure about it.

prhauthors84 karma

It is not impossible that matter and antimatter somehow separated and thus did not fully annihilate. But if so, we would expect to find, for example, antimatter stars and galaxies. As yet, we have no data supporting that possibility. So, the more conventional approach is to imagine that matter and antimatter intermingled sufficiently fully to annihilate pairs and leave over only the excess.

Apteryx1201438 karma

If the topology of spacetime can be warped such as in the case of a wormhole, is it theoretically possible for a “bubble” of space time to form and separate (pinch off) from the rest of the universe? (Imagine a wormhole with a bulge in the middle and both openings then closed). Or is this type of curvature impossible?

prhauthors66 karma

Yes, scientists do in fact study the possibility of "baby" universes forming by a bubble of space pinching off.

Sparxmith34 karma

Thank you for giving us your time.

I have a question about black holes that has bugged me for years: Is there anything that a black hole can consume that will undo its event horizon?

More explicitly, given a black hole of an arbitrary size, is it possible to "inject" enough energy and/or matter to cause the resulting explosion to breach the black hole's event horizon? An example that's likely impossible: as two galaxies collide, a black hole and super massive star are on a direct collision course at a significant percentage of c. The star that is so massive that it's going to supernova even with the effects of the black hole syphoning off some of its mass. After the star passes the event horizon, it goes nova. Would that be sufficient energy?

Or more simply, is there a way to utilize either or both of the electroweak & strong nuclear forces to overcome the gravity well of a presumed singularity?

prhauthors58 karma

In a sense the answer is yes--there is something that black holes can consume which causes their mass to go down and hence their event horizon to shrink: black holes can consume one member of a particle/anti-particle pair that in a well-defined sense has negative energy. And by eating negative energy particles the mass/energy of the black hole decreases. This is one way of explaining the insight of Stephen Hawking showing that black holes are not completely black, that they can evaporate, that they have finite lifetimes.

ilikepotatoes2828 karma

Hello Dr. Greene,

I read your book when it first came out and it remains my favourite book to this day, so thank you for that!

My question is about free will, you may have already answered the question but I was wondering how one can be at peace with the fact that we have no free will? It's something I've thought about and would love to hear your take once again.

Thanks, Affaan

prhauthors83 karma

Thank you. Of all my books, "Until the End of Time," is my favorite too. Obviously, it is different from my other books, which focused more on explaining cutting-edge science. "Until the End of Time," is broader in its sweep and focused more on trying to make sense of it all as a human participant. And along those lines, the question of free will, and my position that we don't have traditional free will, is vital. I am at peace with this position and it is because (as in Chapter 5), I don't place value on ultimate authorship of my actions. Rather, I place value on the very fact that I can undertake actions that are simply unavailable to most other entities/objects in the physical world. Doing an AMA with a rock would be (I hope) less interesting than doing it with me. That's enough. I don't need to be the ultimate author of my actions. I am happy to cede that power to the laws of physics.

Sunservice22 karma

how can time be used as a value in scientific equations when man used it as a measurement that didnt exist before most of the things its applied to?

prhauthors48 karma

Subtle question. We find it USEFUL to invoke time in our mathematical articulation of nature's laws. Indeed, some would define physics as the study of how things change through time. But is time a human conception, an organizing principle we invented to make sense of the world? Or is time fundamentally part of reality? No one knows. I look at our theories of the cosmos as a human attempt to find coherence in reality. So, from that perspective, it is not surprising that we may make use of human-centric concepts to do so.

ClassLibToast15 karma

Do you believe we will ever be able to answer everything (why we are here, etc.), or will there always be something (say, preceding everything else) that is unexplainable?

prhauthors58 karma

I think the human mind is sufficiently creative and innovative that we will always find new questions and new mysteries to pursue. Having said that, note that the pursuit of answers to some questions will likely come to an end. For example, I do believe we will one day know the fundamental forces and fundamental ingredients of the cosmos. That will still leave open the issue of what these forces and ingredients can do -- e.g. build stars, black holes, and who knows what else.

Raptorel14 karma

There is an idea that if the Universe is closed then its total energy is zero and both space and time are emergent (one from entanglement and the other from a superposition of all possible configurations of the Universe). The Universe as a static wave function. What do you think?

prhauthors27 karma

It is certainly a nice idea, and if pressed to answer "why is there something rather than nothing?" it is the direction I often go. But note that the idea has never been developed to a point where we have a full theory, a full articulation of how the universe as we know it can emerge from nothingness. So, I would not in any way say that Leibniz's question has been resolved.

doseofsense10 karma

What are your thoughts on the recent Fermilab experiment that suggest a new force or particle? How might this impact string theory or even the Standard model?

prhauthors22 karma

If the result holds up -- again, as above, a big if -- it might mean we need to include additional forces in the standard model.

d3monic6669 karma

How do you or physicists in general deal with the concept of human death? Do you believe there is something special about our consciousness or are we just a group of atoms?

prhauthors33 karma

That's a significant theme in my latest book, "Until the End of Time." In a nutshell, consciousness is surely special -- it arises from a special arrangement of particles. But when that arrangement disintegrates, when a conscious being dies for instance, the special configuration disperses and the process of consciousness ceases. That's it, at the level of fundamental physics. Of course, at the level of human experience, death is deeply connected to meaning and purpose, motivation and inspiration. Some would say, and I explore this notion in the book, that the human desire to transcend death is the most powerful driving force shaping our species.

Ons_Ouenniche7 karma

Hey dr.Greene. First I wanted to thank you for writing your book « Until the end of time » it definitely shaped my understanding of reality in a manner that I find logical and convincing.. finally. I had a small insight While reading one of the chapters that I wanted to share with you and hear your opinions and positions about what I’m going to describe. If ‘I’ is a collection of particles in a certain arrangement, thus yielding my unique self and resulting into a certain set of behavior. If that behavior was MATH, which describes the properties of MY particles and their arrangements, So is MATH just an expression of Myself ? Can we assume that one way of solving the mystery of MATH origin can be by saying that it’s “the mother language” of the particles that make up I ? So it’s just a spontaneous act governed by this collection of particles. But the reason that we do not feel so, is what you mentioned as the higher-level story. The particles that make up our brains focus on the higher-level sensations and ignore the complexity of what’s actually happening. So we feel like it’s a discovery or an invention when it actually isn’t. 18 year old, High School student from Tunisia.

prhauthors15 karma

It is gratifying to hear that my book has affected your views. Thanks for letting me know. And your summary of what we mean by the concept of self, of the concept of "I," is right on target. We are each a collection of particles governed by the laws of physics. The particles don't "care" at all about the aggregate processes that they may be a part of. So, yes, we as humans feel the deep need to tell higher level stories--including science, art, religion, myth, and so on -- as these stories help address human concerns for value, meaning and purpose. And these stories are real and important. But they rest upon the reductionist story of fundamental physics, a story told in the language of particles and laws. The higher-level stories are stories that emerge from human understanding.

arthurpreis6 karma

Hi! Thanks for doing the AMA.

What is your current bet on the Hubble Tension dilemma? Is perhaps a systematic error? Or there is a problem with our cosmological models when measuring stuff @ CMB time? Or something else?

prhauthors16 karma

The conservative answer, with historical precedent, is that the tension will go away. However, what has no historical precedent is the precision of today's observations. If the discrepancy in the values of the Hubble parameter hold up, it could mean that cosmological history was more complex, perhaps with eras of expansion that deviate from the standard cosmological model.

Conker19855 karma

Hey Brian,

Just wanted to say hello. My wife and I watched you speak at Purdue University way back in 2014 when we first moved to the area after taking a position there. I've always enjoyed how you present incredibly complicated information and distill it down so that virtually anyone can grasp the basic concepts. My wife, going in with no prior knowledge to string theory or theoretical physics, really enjoyed the presentation as well.

Thanks for being such a great steward of science and physics. I'm still pretty terrible at math, but I'm always amazed at what can be accomplished with it.

My question is in regards to traveling across distant stars and galaxies. Are wormholes the only viable solution?

prhauthors4 karma

Many thanks. It was a while ago but I remember enjoying the trip to Perdue. Regarding deep space travel, wormholes--if they exist and if they are traversable--would certainly help. But bear in mind that with special relativity Einstein showed that you can travel ARBITRARILY far while aging arbitrarily slowly -- if you can travel arbitrarily close to the speed of light. So, if we were able to achieve such speeds, we would be able to travel to the stars.

Goljeex5 karma

If you had to bet, would you say the self is an illusion or not? Why/Why not? and how much confidence would you have in your answer?

prhauthors23 karma

An illusion in the sense that there is no independent, autonomous entity that we can point to as the "self." Instead, the "self" is a useful construct that our minds invent to help us survive. Again, we are nothing but collections of particles with an arrangement that allows us to do things that other arrangements, like rocks, can not.

burito-man4 karma

Greetings mr. Brian,

Lately I've been wondering. They say entropy can't increase. But then how could the big bang have ever taken place ?

And can you explain how the cosmic microwave background may help to explain this ?

Kind regards,

(I love your work)

prhauthors14 karma

Again, central to "Until the End of Time,' so if you want more detail you can find it there (no need to buy the book, take it out from a library). But in brief: (1) entropy CAN decrease, it is just OVERWHELMINGLY unlikely. The big bang could, in principle, have been such an unlikely drop to low entropy. (2) Entropy can go down in one region so long as it goes up in a surrounding region--and this mechanism can and does explain how orderly structures like stars and galaxies can form in a universe that in aggregate is heading toward every greater disorder.

Y_am_I_balding3 karma

I read that if the universe in infinite or if there are infinite number of pocket universes, then somewhere there would be an exact copy of each of us. Is that true?

prhauthors12 karma

Yup, that is a strange quality of a universe that is infinite in its spatial extent. The reason is simply that in any finite volume of space with finite energy there are only finitely many ways that particles can be configured. So, if space is infinite, the particle configurations must repeat -- indeed, they must repeat infinitely many times. Since you and I are just configurations of particles, copies of us must be "out" there.

Singularity3142 karma

Sir according to chaos theory if we change little in system its gives completely different and drastic result but if we see in nature like wind blows my hair and sensation of some heat from food and even more little things like apple falling from tree influence our mind and if suppose one things not happened or follow some other pattern,does its influence our mind ?? Influence like falling apple make someone scientist and is that a way in which nature talk to us, through events ?? So who i am today ( or you are) Is because of our hard work or is it nature Who forced us to become what we are today ??

prhauthors16 karma

We are indeed the product of a vast number of processes each of which could have turned out differently. So, as I explain in more detail in my latest book, our existence is extraordinarily unlikely, which to me is cause for a deep sense of gratitude.

chipsncaseo2 karma

Hi Dr. Green, loved your book taught a lot to a business major who knows nothing about science, do you think we’ll ever get to witness a boltzmann brain phenomenon?

prhauthors4 karma

No, I don't think so. We generally view Boltzmann brains as a challenge against which our cosmological theories need to prevail. We imagine our refined cosmological theory, when we have it in hand, as suppressing the possibility that we are all brains freely floating in the cosmos.

chadowmantis2 karma

Do you think you'll ever reach a point where you stop reading papers, attending conferences and following research? Leaving this alone and just doing... Life?

prhauthors7 karma

I doubt it. I don't draw a sharp distinction -- at least not now -- between "work" and "life." Maybe at some point I will...

rox_fenrir-1 karma

Hi Dr. Greene. Recently I found some interesting theories about multiverses and reality shifts ((Glitch in the matrix subreddit)

Another interesting theory says that those phenomena could be linked to the moment we started the CERN.

What's your opinion? Thank you for your time.

prhauthors17 karma

Although I write about and speak about the multiverse, I always do so -- or at least I strive to always do so -- with a highly skeptical eye. My view is that the concept of a multiverse is exceedingly speculative and so needs to be approached with a high degree of skepticism. That does not mean the notion is pseudo-science or nonsense. Rather, the possibility of a multiverse is deeply intriguing and surely useful to answer certain puzzles (such as the value of the dark energy) but until we have some concrete observational support it needs to be viewed as a mere possibility worthy of our attention but far from anything that has as yet been established.