I am Brian Greene, Theoretical Physicist & author of "Until the End of Time: Mind, Matter, and Our Search for Meaning in an Evolving Universe" AMA!

Hello Brian! I just watched your appearance on Joe Rogan’s podcast, and it blew me away. You’re one of the very few people who can eloquently describe the most difficult of subjects for many of us to understand. Thank you for doing this!

How long do you think before we can definitively say we’ve found the answer to quantum gravity?

Many thanks. Joe Rogan is a great interviewer.

Regarding quantum gravity: It COULD be that we have the answer right now. It could be that string theory IS the answer. It COULD be that loop quantum gravity is the answer. The challenge is testing any theory of quantum gravity. These theories come into their own at extremely high energies, extremely short distances, well beyond the reach of today's technology. So, hopefully we will find an indirect test of these theories that we can undertake--until then, they will remain hypothetical.

When explaining something complicated to the general public, how do you draw the line between using an analogy and explaining how stuff really works ?

That, really, is the art of writing general-level books. Indeed, authors draw the line between analogy and "the real thing" in different places. I strive to avoid ever turning my descriptions into cartoons of the real science. Sometimes, this makes sections of my books challenging for the general reader--but by summarizing and giving the reader the option to skip the hardest parts, I try to strike a balance, one that allows me to appeal to the more "hardcore" science enthusiast and also those who are not looking for as much detail.

Dr. Greene, what do you think will be humanity’s next big achievement with a comparable magnitude to that of the internet?

It is possible that within a reasonable time frame we may understand the very ingredients of space and time. Much as matter is composed of molecules and atoms, space and time may themselves be composed of finer entities. There are proposals now being developed that may in the not too distant future identify those entities.

What do you think is the current condition of the education system specific to nurturing budding physicists? In terms of the coursework being apt to modern research, the room for exploration and original thought, etc. How far are we from the ideal state (assuming the current research doesn't evolve too rapidly) ? Would you have some suggestions for this 3rd year undergraduate?

Thank you (Long time fan)

Don't get me started. Certainly at the lower grades, my experience is that the educational system is far too focused on assessment/grades. Wonder and excitement should drive learning, not fear of bad grades.

What words of advice would you give to a highschool student wanting to become a physicist?

Very simple: Learn the BASICS of physics and mathematics inside out. You can read about and be inspired by work at the cutting edge. But if you don't learn the basics you will never reach your potential to contribute to our understanding. I encounter many kids who want to jump over the "old" stuff and learn only about research at the frontier. That is a huge mistake. Take the time now to build a solid foundation.

I get this feeling a lot—that I know or understand or feel something—but do not yet know of the words to convey what is in me. Do you ever feel this way in your work? Of course, I do not know 99.9% of what you do, including the reasoning or the math behind all your work, but do you ever think you know something about string theory or inflation or whatever else, but do not know the words—or math—to explain it yet? You think you have some deep insight into the universe, maybe through your intuition or just based off your previous understanding of a subject, but you can’t “prove” it?

Absolutely. Language is a powerful tool for communicating ideas. But language is limited in scope. Language constrains the kinds of ideas we can articulate and share. I often find myself saying "I wish there was a word for X" where X is some idea I can sense or feel but I don't know any way of describing in words.

Do you think that the question of "consciousness" is ever possible to be explained via physics in the same way we describe other phenomenon? What other kind of reasoning or understanding could there be?

I do. I hold to a so-called physicalist perspective that imagines consciousness/mind not requiring any "spark" or "force" or "substance" that stands outside the very same types of ingredients that make up my desk or coffee cup. Some recoil at this possibility, thinking it diminishes mind. I have a different view. I consider it WONDROUS that ordinary particles governed by the usual forces of nature, when configured appropriately, can think and feel and explore. To me, that is the point--matter and laws accomplish spectacular things.

What do you think are the biggest questions about the universe right now? Are there any limitations to what humans are capable of knowing?

From my perspective, the biggest questions are (a) the nature of spacetime and (b) the nature of consciousness. Regarding limits to human knowledge--there could well be things we are unable to understand. Take dogs. They're smart creatures but they seemingly don't know much about quantum mechanics. They, seemingly, don't have the brain power. Perhaps, then, we humans too don't have the brain power to grasp certain deep truths about reality.

Just finished your podcast with Rogan. You were fantastic on the show!

Just curious, did you try Joe's sensory deprivation tank after the show? How was the experience?

We didn't do the tank. Instead, Joe put me on a number of pieces of equipment designed to decompress the spinal column. Helped a lot with my back problems. (I assume he will be sending me a steep medical bill for the expert consult.)

What are your thoughts on the simulation hypothesis? Do you see it as a realistic possibility we are in an ancestor simulation?

The simulation hypothesis is a useful thought experiment, but it is not one that I take seriously as an explanation of reality. That doesn't mean it is wrong, of course. But without any real evidence for it, I am not drawn to consider it a real explanation for the world. I do find it useful, though, to imagine a world that has a creator--the SIMULATOR--and yet need not be supernatural at all. The SIMULATOR might be a kid in a futuristic garage firing up a supercomputer running universe simulations.

Even if we eventually discover credible evidence, how do we know that that evidence itself is not simulated?

Absolutely right. Within the simulation hypothesis you need to be skeptical of everything, including all evidence and all reasoning that supposedly illuminates whether or not you are in a simulation.

I'm a physics undergrad major and have never done higher-level physics (although do have a biomedical engineering PhD). I hard really great things about your books. Only problem is there seem to be too many of them. In what order would you recommend to read them?

The books are all fairly independent.

For those interested in the big questions (origin of universe, origin of life, origin of mind, nature of free will, role of language and storytelling, myth-making and religion...and how it will all end), read the new one--Until the End of Time: Mind, Matter and Our Search for Meaning in an Evolving Universe. Personally, I would read this one first.

For those interested in unification: The Elegant Universe

For those interested in space and time: The Fabric of the Cosmos

For those interested in the multiverse: The Hidden Reality

Hello Dr. Greene! I have two questions for you. As someone who takes great inspiration from you, I was wondering who your early inspirations were and why? Secondly, I’d like to know if you have thought about making your own podcast? You are an incredibly knowledgeable and fantastic speaker who I know tons of people would love to tune in to.

I have thought now and then of having my own podcast, likely in collaboration with World Science Festival, as that event brings hundreds of great thinkers together in NY and in Australia each year. Stay tuned...

Hello Dr.Greene! My question is: what do we really mean by free will? Is this connected to conciousness? And how free will and conciousness are going to evolve if the human species survives in the far far future! Or can the human brain figure out what's human consciousness? Or does it need something way more smarter than that to figure what's really happening? Thank you :)

Different people mean different things by "free will." If you take it to mean my ability to be the ultimate author of my actions, to transcend the control of physical law, then I think that variety of free will does not exist. We are all collections of particles fully governed by physical law. However, there are "nearby" definitions of free will that are compatible with the ironclad rule of physics. For example, if by free will you mean the SENSATION of free will, that of course is real and unassailable.

Apologies for the plug, but I take up this question in some detail in Chapter 5 of my new book, Until the End of Time. I refer you there for a far more complete answer (you can likely find the book in your local library).

Hi Brian. Do you think anyone can get good at mathematics. Do you think you were born with a gift for it?

I think anyone can become proficient in math. But, from experience, not everyone has the talent to master math to the degree of becoming deeply creative with it.

Hi Brian, I remember from a few years ago watching one of your videos on YouTube explaining some of the ideas in one of your previous books. I'm not sure if I remember this correctly, but in one section where you talked about time dilation I remember hearing that by moving fast enough, things around you will appear to slow down. The part that I'm not sure if I remember correctly is when I heard that if you move at a modest speed, then at far distances you can observe time dilation. Can you clarify this for me? Thanks.

According to Einstein's special relativity, if you are observing a clock that is in motion, your measurements will reveal that the clock ticks of time more slowly than your own clock. This happens even at slow speeds, but the effects are generally too small to notice. That's why we don't experience such time dilation in everyday life. But highly accurate equipment can measure time dilation, even at relatively slow speeds. And the results agree with Einstein's predictions.

Is there anything you could ‘discover’ that you would not believe because it doesn’t align with your current worldview of the Universe? Example, Einstein and QM—Is there something that just would never make sense to you, so you would choose to ignore it?

Also, saw you in Charlottesville on Friday for your book tour; beautifully presented!

Many thanks...glad you caught the talk in Charlottesville.

Like all good scientists, I am open to any insight that comes with experiment/observational support. If that insight requires that we modify our current mathematical understanding of the world, that is fine. In fact, it would be more than fine. It would be enormously exciting. We live for conceptual revolutions. So, the notion that we scientists would resist some insight that is replicable and comes with solid evidence is quite counter to how most of us think.

I have two questions. How long does it take you to finish writing a book and what is one of your favorite physics concepts?

Books take me 2 years or so to write.

My favorite physics concepts: time dilation and quantum entanglement.

What is your opinion on the movie Interstellar?

I like the movie a lot, but did have some trouble understanding exactly what happened at the very end.

Hey Brian, what mathematics is necessary to study String theory?

The math for string theory involves all the basics--differential equations, linear algebra, differential geometry and so on. But also more specialized fields like algebraic topology and algebraic geometry.

How is a robot that senses its water level is low and goes down the hall to the kitchen to get a drink of water until satisfied different than a human that does the same thing?

Conceptually, don't both contain attention and an inner movie of their environment that they use to act on their own behalf to navigate their space to acquire resources to survive? Don't both have a feeling of thirst and satiation? Wouldn't this be machine consciousness?

I have no problem in principle with consciousness arising in a digital being. That consciousness happens in our gloppy, wet brains may well be only one example of how consciousness emerges.

What does it mean for a mind to exist?

I suspect that mind is nothing but matter that is arranged and moved in highly specific and exquisitely organized patterns. Many disagree with this, suggesting that mind transcends matter. While that could be true, my intuition is that matter, the laws of physics, and great organization is all you need to yield mind.

Hi Dr.Brian, How do black holes shape the structure of the universe? Also, how are they going to determine the evolution of the universe? Thanks!

Well, black holes are the arena in which we theorists love to play because they push our theories of gravity and quantum mechanics to the limit, to the breaking point, and that's when we are forced to develop deeper insights. Of course, as you indicate, black holes play a vital role in the physical universe--and as I describe in my new book, they may well dominate the cosmic landscape in the far, far future. But they also dominate the theoretician's landscape as well.

What color is your toothbrush?

When I buy it or after 6 months of use?

What is your opinion about the multiverse?

I consider the multiverse sufficiently well motivated to be worthy of concerted mathematically investigation. The multiverse should be within the physicists toolkit. But I will not consider the multiverse to be real until there is observational/experimental data to support it (or, perhaps, a slam dunk mathematical argument showing that a single universe model will always be inconsistent--a possibility I consider unlikely but who knows?).

Dr. Greene, first off I have to say I love all your work. Your books, your television productions, and the World Science Festival!

My question is: are there plans to make the World Science Festival truly a global event by bringing it to countries outside of the US? I live in germany, and while I'm totally capable of watching the videos in english, I would love nothing more than to be able to attend an event, be it held in german or in english!

Many thanks.

Yes, World Science Festival is spreading beyond New York. Already we have World Science Festival Australia in Brisbane each year--a festival just as large and robust as the New York version. And we are in conversation with other cities to bring the festival there too.

Hi there Dr.Brian Greene, this is Akshat! My question to you is that as dark energy is getting stronger and stronger it will one day shred apart everything from atoms to Giant galaxies, even the fabric of space itself will tear apart, so is this going to effect the geometry of the universe? Thank you for imparting your knowledge and wisdom upon me, I'm 18 with dreams of becoming an Astrophysicist :)

You are referring to what scientists often call THE BIG RIP. If the dark energy gets stronger over time, it will create an ever more powerful repulsion driving everything apart--even driving electrons away from the nucleus of atoms, causing matter itself to explode. Most scientists consider this possibility unlikely but it is allowed by the mathematical laws of physics.

This has been puzzling me lot, why there is a speed limit and why it’s the speed of light.. and why nothing in the multiverse is able to travel faster than speed of light or Universal speed limit ? Why does this speed limit exist at all and what caused it to exist ?

The basic if rough idea is that as a massive body's speed approaches the speed of light its mass grows ever larger. And therefore it takes an ever larger push to make it go faster still. As we close in on the speed of light, the mass soars toward infinity and so you would need an infinite push to make it go faster still. With no infinite pushes available, nothing can be sped up to light speed, or beyond.

My brother is convinced that Time and Gravity are the same force rather than a cause and effect relationship. Is there any conceivable logic to this theory? If not, could you give me a one-liner to disprove this and save me from him? 🙈

Depends exactly what your brother has in mind. But, for example: Earth's gravity can in fact be described in terms of the warping of time near Earth's surface. That is one less of Einstein's general relativity.

Hello Dr. Greene, listened to your latest book via Audible. Definitely some great nuggets in there. I have two questions. 1. Can a particle tunnel through an avoided crossing? 2. How do you reconcile natural selection and determinism? In natural selection the environment chooses the features an organism retains provided those features promote fitness and the choice between features hasn't been predetermined. In determinism, all processes are caused by initial conditions (from the big bang). In a purely deterministic universe, isn't what we call natural selection actually the illusion of natural selection similar to the illusion of free will? The environment makes no free choice between traits; the traits that persist do so because initial conditions determined that they would so. Am I missing something?

Natural selection is indeed nothing but physical laws acting out on material particles. However, the "higher level" description of natural selection is more useful than the reductionist account at gaining insight into the origin of species and indeed even the origin of life. This does not make natural selection an "illusion" but rather a parallel account that must be compatible with the reductionist account, one that focuses on a higher level of structure.

Indeed, in Until the End of Time I stress that the deepest insights emerge from blending a whole collection of "nested" stories: the physicist's reductionist account, the chemist's account in terms of more complex atoms/molecules, the biologist's account in terms of cells and life; the neuroscientist's and psychologist's account of self-awareness, and the humanist account in terms of the activities self-aware minds carry out from storytelling to religion to creative expression. You need ALL of these stories to have a full account of reality.

What are some of your favorite restaurants and dishes around NYC?

Hangawi's--crispy mushrooms. Not to be beat.

I loved The Fabric of the Cosmos and was planning to re-read it. What would you say is out of date? (or should I just get the new book)

Not much in Fabric is really out of date. We have discovered new things but I don't think anything in the book has been overturned.

The new book is different. It doesn't just focus explaining cutting edge science but tells a far larger and far richer story: The whole universe, beginning to end. And how we humans try to make sense of it all.

Do you have any plans to teach more courses via World Science U?

Yes...but they take a lot of time to create. I still want to teach quantum mechanics on World Science U. Have you taken my Special Relativity course on World Science U? Has both the mathematics and a highly visual treatment--that's the combination I consider most effective.

Hello! Thank you for doing this AMA. I am striving to be a physicist myself. I enjoyed reading your book The Hidden Reality, so I wanted to ask, what is your favorite multiverse theory and why?

Favorite is probably not exactly the right word. "Most likely" to be physically relevant is a description I can address. And for that, I would say the "inflationary multiverse" which imagines that our big bang may not have been unique. There may have been many big bangs giving rise to many universes. There is reasonable mathematical justification for considering this possibility seriously.

Hi Dr. Greene! What are your thoughts about life outside our planet, and in other solar systems? Additionally, do you think humans will leave Earth for another potentially habitable planet outside the solar system?

I don't have any particular insight here. The number of planets is so enormous that it would seem there has to be life somewhere out there. But it could be that the conditions necessary for life to emerge are so special that even with that number of planets life only arose once. Finding one other example of life in the cosmos would thus be a vital and impactful discovery.