Hi there, I'm a physicist and cosmologist at Caltech as well as an author and speaker. My research involves the origin of the universe and the multiverse, entropy and complexity, the mysteries of quantum mechanics, and the nature of dark matter and dark energy. I've written books about the Higgs Boson and about the arrow of time.

I'll be speaking at the upcoming World Science Festival in New York City (May 28 - June 1st). One of the discussions I'm part of, Measure For Measure: Quantum Physics And Reality, will be live streamed at http://www.worldsciencefestival.com/livestreams. I'll also be joining a conversation on Science and Story with Steven Pinker, Jo Marchant, Joyce Carol Oates, and E.L. Doctorow; and moderating a panel discussion about the movie Particle Fever.

Some fun videos, including recent debates:

Proof: https://twitter.com/seanmcarroll/status/471310943318577154

UPDATE: Thanks everyone! Back to reality with me now.

Comments: 282 • Responses: 88  • Date: 

seanmcarroll31 karma

Time's up everyone! Thanks for some great, thought-provoking questions. Let's do it again sometime.

arrowoftime24 karma

Hi Sean -- three questions.

  1. If I lived on a metric with two timelike dimensions, could I unscramble my eggs in a closed timelike breakfast?
  2. How do you think the new Godzilla compares to your version in 1998?
  3. U jelly of my username?

seanmcarroll39 karma

  1. You could do that and more.
  2. Don't you dare refer to Godzilla 1998 as my version.
  3. Yup.

goodevidence18 karma

As a fellow physicist (experimenter with ATLAS) also with an interest in philosophy, I'm really impressed by your efforts to push naturalism and engage philosophers. I fully agree with your materialist and reductionist arguments, supporting that we do understand physics within the everyday regime, and this should inform our metaphysics, philosophy of mind, criticism of pseudoscience, etc. I also like that you emphasize emergence as being an important concept in explaining the nested emergent ontologies in chemistry, biology, economics, etc. My question is why you seem to draw a hard line that ethics and morality cannot be analyzed in the same way. You seem to entertain the criticisms that some people say of "scientism", that there are some things (like ethics) to which the reductive program of science will not be able to explain. This strikes me as totally inconsistent with the thrust of reductionism and naturalism. I think there are reasonable explanations for why ethics emerge as a set of regularities and good strategies anytime you have groups of people. The constraints on ethics seem to be completely determined by the natural world, including the limits of resources, the needs of our physiology, the laws of probability and game theory, etc. There is no fundamental is/ought divide. "Ought" is a higher descriptive term we give to actions that bring out good situations, for which there are objective metrics, such as health and satisfaction. In this regard, I'm sympathetic with some utilitarians and what Sam Harris seems to be describing in the Moral Landscape. Why do you think emergence from natural laws can introduce new concepts like temperature, phase transitions, supply and demand, but not ethics?

seanmcarroll6 karma

Stepping in belatedly to mention that I put this quick answer to Ryan up on my blog:

Essentially, science is about describing the world, not passing judgment on it. Temperature, phase transitions, and supply and demand are all concepts that helps us understand what happens in the world. Morality is just a completely different endeavor.(Of course you can scientifically study how human beings actually behave — including what they judge to be “moral” — but that’s different than studying how they should behave.) Scientific claims can be judged by experiments, moral claims cannot.

See also:





Phaz16 karma

What do you think is the biggest misconception us laymen have of the multiverse theory?

seanmcarroll31 karma

The biggest misconception is probably that the theory is developed by scientists saying "hey, let's add more universes!" The truth is that the extra universes are predicted by equations we developed for completely separate reasons.


najibmok16 karma

We often hear physicists say that other bubble universes in the multiverse may have different laws of physics. Is there something pointing to this? Or is it just a "why not" speculation?

seanmcarroll24 karma

There is something very definite pointing to it. In theories with extra dimensions of space (e.g. string theory), there are many ways for those extra dimensions to be curled up and compactified. And each different way shows up as different "laws of physics" in the local region where it applies. It's like the difference between ice/water/vapor -- same underlying stuff, very different apparent properties.

viewerfromnowhere15 karma

Biologist Stephen Gould is famous for proclaiming that science and religion have "non-overlapping magisteria." What's your view? Do science and religion necessarily conflict?

escherbach14 karma

Is cosmic inflation unitary? (and what odds do you give on BICEP2 result holding up?)

seanmcarroll26 karma

For those following at home, "unitary" is physics-speak for "conserving information from moment to moment." And the answer is yes, inflation is unitary. Of course information might be dispersed far away, or hidden in other branches of the wave function, but that doesn't make inflation any different from other kinds of cosmological evolution.

I think BICEP2 will probably hold up -- but it's not unreasonable to be skeptical.

escherbach11 karma

Thanks, amazing to think inflation might be a (mathematically) reversible process too!

One last cheeky technical question:

In the Many-Worlds Interpretation can you get the Born Rule with exponent 2? (Or do you put the 2 in by hand?)

seanmcarroll10 karma

Yes you can. That's the subject of my upcoming paper.

bluengreen712 karma

Why are WIMPs the leading candidate for dark matter?

seanmcarroll17 karma

There is something called the "WIMP miracle," which notes that a WIMP (weakly-interacting massive particle) has just the right interaction rate (that fixed by the weak nuclear force) to give approximately the right relic abundance of particles leftover from the Big Bang. Strong interactions would let all the particles annihilate away, while weaker ones would leave too many still around today. But the WIMP paradigm is under some pressure, as we haven't found the buggers yet.

Credrus11 karma

What is the current state of dark matter research? Has anything new come up that changes our perspective or helps us understand dark matter better?

seanmcarroll13 karma

There is definitely dark matter out there. We don't know what it is, but we're trying hard to detect it. There have been provocative clues, but nothing at all definitive at the moment. See e.g.:


bobthebobd10 karma

Is there anything you know of that you feel the program Cosmos got wrong or didn't explain properly or you'd like to further elaborate on?

P.S. I won't hold it against you if you've never seen it, but that's where I get all my facts from :)

seanmcarroll15 karma

I've seen only a little bit of it. (My wife, Jennifer Ouellette, does weekly recaps for the LA Times -- look for those.) I have the impression it's been quite accurate, but there will always be nits to pick.

EonShiKeno7 karma

Love your debating. What is your core methodology in how you give your points and respond?

seanmcarroll26 karma

I just try to be as logical and clear as possible, and make a real effort to understand my opponents on their own terms, rather than setting up a straw man to oppose.

pubestash7 karma

Which interpretation of quantum mechanics do you lean towards?

seanmcarroll12 karma

Yep, many-worlds. It's the simplest and most well-defined formulation of QM. I'll have a paper coming out this week (maybe next) about it.

Afferent_Input6 karma

How do we know that universe is ~90 billion light-years wide when the big bang only happened 13.8 billion years ago?

EDIT: fix some numbers, and to add, seems like we would only be able to guess that it's about ~27.6 billion light-years wide... Also, I heard you on the Probably Science podcast. You were fantastic!

seanmcarroll10 karma

It's expanding! And it's expanding faster all the time -- the universe is accelerating. The correct statement is "starting from our position today, if you trace light cones back to the far past, and use that to define a region of space, the size of that region today is about 45 billion light-years across." Doesn't sound so surprising (though perhaps less comprehensible) when you put it that way.

Acgcbc6 karma

Hello Sir! You mention in the TM Wissen interview that it’s not always about where you (abstract ‘you’ here, not just you as in Sean Carroll) went to college, but what you do and what contributions you make that help you to be recognized by the scientific community and help put you in a better position to make even more contributions to the scientific community. I find myself (and I’m sure many others share this position too) to be under similar circumstances you, as Sean Carroll, went through in undergrad and as you describe (I paraphrased from) in the interview referenced. My question is, as an aspiring physicist (as implied, I’m only an undergrad) what exactly can I do to succeed / flourish in the community like you and other physicists have (for example, John Preskill)? I want to contribute to the scientific community, but I have observed many others lose their way on this path. I understand the need to study rigorously and to embrace passion for this sort of pursuit (and try to get on a professor’s research-team, but after multiple attempts that avenue seems to be ineffective), but any other type of advice to both me and those who read this would be, in my opinion, truly appreciated. Thank you for taking the time to do this AMA.

seanmcarroll15 karma

To be honest, making real contributions to science is hard, no matter what academic path you are on. There are many more people who want to be scientists (at least in academia) than there are jobs.

Having given that disclaimer -- if it's what you're passionate about, go for it! Study everything you can, work hard, and pay close attention to how successful scientists around you go about their work. There is no one right way to be a great scientist; everyone finds their own path.

JaminTheGray6 karma

What, if anything, could convince you that metaphysical naturalism isn't the philosophy that best explains reality?

seanmcarroll36 karma

Easy -- repeated, well-documented evidence for something that can't be explained naturally. Ghosts, angels, God coming down and scolding me. My first instinct would always be to look for a natural explanation, but given enough data and a stubborn inability to explain it without supernatural intervention, I would willingly change my mind.

aatishreddit5 karma

I'm curious if you've recently changed your mind on any firmly held beliefs or views?

seanmcarroll10 karma

Talking with Tim Maudlin over dinner, he came very close to converting me into a mathematical Platonist. But afterward I came back to my senses. Still not completely firm in my convictions, though.

EssYouEffSee5 karma

Hi Sean. If, before ElectroWeak Symmetry Breaking particles had no mass, how did the Big Bang with a singularity of infinite density happen?

seanmcarroll8 karma

Why not? "Density" here means "density of energy," not "density of mass." And massless particles can certainly have energy!

main_hoon_na4 karma

Hi! I'm an astro major, and I saw you when you gave a guest lecture at Caltech earlier this year (in our physics 1b, taught by Professor Fiona Harrison) and was super honored to meet you and get a picture and autograph.

Extrapolating from two data points (Spacetime and Geometry, and Introduction to Astrophysics), why do you like big orange textbooks so much?

seanmcarroll4 karma

Easy: (1) Spacetime and Geometry is red, and (2) I didn't write Introduction to Astrophysics.

main_hoon_na1 karma

Wow, that all-nighter is getting to me. It's definitely red-orange, though. :P

Are you related to B.W. Carroll, to make my mistake a little less painful?

seanmcarroll14 karma

Aren't all living organisms related, when you come right down to it?

nuviremus4 karma

Thank you Dr. Carroll for doing this AMA!

My question: What is your opinion of the BICEP team's decision to not revise their work regarding the discovery of B-modes in the CMB? Is this the way you think scientists should stand by their work, or should they be more willing to revise their data in the face of scrutiny they've faced the past few weeks?

By the way had the pleasure of meeting you when you came to OSU to give a public lecture on your new book. Thank you for the signing your book for me.

seanmcarroll6 karma

I think the BICEP2 team is trying to be careful. They made at least one mistake, which was to use data "scraped" from a powerpoint slide in their analysis. That opened the floodgates to people disagreeing with them on the basis of similar work. But they're trying to be careful and rigorous. Probably we won't know anything for sure until other experiments can directly measure the same part of the sky in multiple frequencies.

BeakOfTheFinch3 karma

You are my favorite advocate for naturalism. Please tell me you are writing a book on the subject. The short coverage in the last book was not enough.

seanmcarroll3 karma

Thanks! Stay tuned for new books down the road -- right now I'm busy doing physics research for a while.

DevonJWlodyga3 karma

Dr. Carroll, thank you for doing this. You inspired me to actually join reddit with this Q and A.

I worry that I don't know enough to ask this question and that it might be a ridiculous one, but here it goes.

If a black hole singularity could create a new universe as has been hypothesized in the past, could matter that continues to fall into that singularity be manifested in the hypothetical new universe as something akin to our universe's cosmological constant-like dark energy?

It's a question that has been bugging me for a few years now. Thanks for any thoughts in advance!


seanmcarroll3 karma

I think that, if a new universe is born inside a black hole, it happens at some particular moment in time. That is, it's born with whatever matter and energy it has, and doesn't continue to gain new energy. (But who knows for sure? Mostly likely, new universes aren't created inside black holes at all.)

bstruck3 karma

Dr. Carroll, have you seen WLC's reactions to your debate with him that he has published over the last few weeks? Any plans to respond to him? Thanks.

seanmcarroll5 karma

I've seen them, but no immediate plans to respond. I hope that what I said in the debate and in my follow-up blog post made my points clearly!


TheBoson3 karma

could you post the WLC reaction?

phenotype0013 karma

Does time ever end? Is it possible that after a (huge number here) Graham's number as argument to Ackermann's function years, a random quantum fluctuation makes another universe somehow? Why this kind of Boltzmann brain paradox is a problem, what if that's really the way it is?

seanmcarroll3 karma

Anything is possible. Right now I suspect time does not end, but it's certainly not something we are sure about. I used to worry about quantum fluctuations creating Boltzmann brains, but upon further thought I realized that was a mistake (at least in many reasonable scenarios).


Steuard3 karma

Hi, Sean! In discussing your recent work with Boddy and Pollack on quantum fluctuations, you made a point of emphasizing that the methods and conclusions rely on the Everett "many worlds" formulation of quantum mechanics. I was always raised to believe that the various formulations were essentially indistinguishable in their predictions for actual experiments. Can you talk about that a bit?

seanmcarroll4 karma

Hi Steuard! (I'm guessing that spelling is fairly unique, and you're the Steuard I know...)

Thinking that different formulations of QM are empirically equivalent is common, but it's not right. Things like Copenhagen aren't really coherent formulations of a well-defined physical theory. Real formulations of QM (many-worlds, dynamical collapse, hidden variables) will definitely make distinguishable predictions. We merely pointed out that they can be very important for big-universe cosmology.

leapofawesome3 karma

Is there a difference between the of topologies of spacetime and gravity near massive bodies?

seanmcarroll6 karma

Not that anyone knows of, but maybe I'm misunderstanding the question. Gravity is the curvature of spacetime. The topology of spacetime doesn't change near massive bodies, but its geometry does.

Phaz2 karma

When I first heard you speak about how The Laws Underlying The Physics of Everyday Life Are Completely Understood and how they even make a nice equation I was fascinated. I was also a bit shocked that this was the first time I heard anything along those lines and that it wasn't already common knowledge (much like the answer to the question "is light a particle or a wave?" isn't common knowledge, though many of us have heard the question)

So, my question is, how controversial is this among your peers? Does everyone who understands things at that level pretty much agree that we understand it all?

seanmcarroll6 karma

I think it's something that is clearly true, but many people haven't thought about. And when they do think about it they might not want to admit it. But I'm on a campaign to spread the word!

bobthebobd2 karma

You brought up God.

What would be a scientific experiment to prove God? (vs super advanced alien)

The being being examined is willing to be shot at, tell you future, past, destroy moon and put it back again. Would that be the proof they are God? or just a super-advanced alien?

Is there anything that can definitively scientifically prove existence of God?

seanmcarroll7 karma

I don't think "prove" is ever the right word to use when talking about science.


We collect data, and try to judge what the best theory is to account for that data. If enough data came in in favor of God, we would believe it.

Very_Little_Gravitas2 karma

Hi Sean! Big fan. As a theoretical physicist, where do you stand on the nature of mathematics in relation to the universe? Was it invented, or discovered?

seanmcarroll9 karma

I think math is discovered, in the sense that it couldn't have been any other way. But I don't think math is "out there" in some Platonic reality. I think that math is a set of principles, not a kind of "stuff."

schrodingasdawg2 karma

If I'm not mistaken, you're an advocate of MWI. Do you think MWI sounds as convincing in the Heisenberg picture as it does in the Schrodinger picture? Do you think you'd be able to make the case for MWI while sticking entirely to the Heisenberg picture in your argument?

seanmcarroll2 karma

It's picture-independent. The Heisenberg and Schrodinger pictures are completely equivalent; anything you can say in one picture can be said in the other.

felipenunesnc2 karma

Okay, so I created this account on reddit only because of this xD Hey Sean, I wanted to know more about how coding (as in computers) can be related to our physical reality. I've been wondering about how our formulas work and how they ''match'' with the logic behind software. Maybe this is a stupid question, but which methods of research could be used to go further on this subject? I guess that's all. Thank you for your time Sean.

seanmcarroll3 karma

We seem to live in a world that functions much like a computation: that is, we have some input representing the world at one moment of time, and the algorithm given by the laws of physics advances it forward. Indeed, a reversible computation, since the laws seem reversible.

Is this observation interesting/useful/provocative/profound? I honestly don't know.


Bill_Brzozowski2 karma

I'm an Astrophysics student at The University of York and my (rough) aim is to get into the field of Astronomy. It's on my mind that getting into most areas of Physics as a career is competitive and difficult; how did you go about getting the ball rolling post-University? (I'm reading 'The particle at the end of the Universe' and can't put it down!)

seanmcarroll2 karma

Fortunately, academia has a very structure system. Undergrad, grad school, postdocs, faculty jobs. It's generally up or out. Challenging at every step, but at least the procedure is well-defined. (Unlike becoming a movie star.)

agentmage20122 karma

Hello Sean!

Ever since listening to your series of lectures in "the great courses", I've been devouring everything about physics I've been able to wrap my mind around. I watched your debate about God on YouTube. You speak in ways I hope to someday: confident about the objective world around him works.

I see that you're doing a talk in new York soon, but could we on reddit get you to come back to Philly? I'd love to sit in on a talk of yours as my first live science lecture.

As a science question, can you ELI5 the following: if gravity bends/compresses spacetime, does this mean we can find a tangibility to space, or am I misunderstanding this phenomena. It would seem like large gravity sources, like black holes, would take such an ability to an extreme, and bend spacetime into a 4th physical dimension... Or something. Sorry, I lack the knowledge (currently) to describe my question better.

Thank you for inspiring my passion in the sciences again. Before this, I felt awash in useless interests.

seanmcarroll4 karma

In some sense, space is just as tangible as anything else. It's described by a field that exists at every point. It's a field that interacts weakly, so we can freely move through it, but it's a field nonetheless. Modern physics doesn't draw a very big distinction between gravity (the curvature of spacetime) and the other forces of nature.

spcms2 karma

What is the most misunderstood term in physics?

seanmcarroll6 karma

No idea. Energy, dimension, time, space, quantum...

aatishreddit1 karma

Hi Sean. I've long admired the way in which you convey complex ideas with impressive clarity and simplicity. I've noticed that science writers often take two somewhat different approached to structuring their writing: a narrative of ideas, with interesting stories sprinkled along the way, or a narrative of a story, with interesting ideas along the way. There are of course great examples of practitioners of either side, and the line between these styles are quite blurry. When writing for a popular audience, I assume you want to draw in not only people who are interested in your subject, but also perhaps people who didn't know that they were interested in your subject. Have you thought about the merits of these different approaches towards growing a wider audience - and what do you find works best?

seanmcarroll1 karma

Thanks! I've thought a lot about the different approaches, and I think they all serve different (but equally valid) purposes. There's no right way to do it; you have to think about the subject matter, the potential audience, and what you as a writer are best equipped to do. I can love a book by Roger Penrose that is all abstract ideas, then also love a book by Mary Roach that is all hilarious stories.

CrannNaBeatha1 karma

Hello Dr. Carroll, love your blog preposterousuniverse, you inspire and empower me. Thank you.

I am not a physicist (yet!), but I have an issue on my mind that you may be of some help with. I find the symmetries of the universe to be incredibly beautiful in a deeply profound sense. I was wondering if there is any known reason for the symmetry between Electrical Fields and Gravitational Fields? That is they both obey inverse square laws, is there any reason for this? I know that the gravitational field is much weaker and I know that electric fields don't have as much range due to largely cancelling each other out.

Is it perhaps because both act on space itself and the inverse square represents some sort of inertia inherent to space?

Thank you so much for your time and for entertaining my musings!

seanmcarroll1 karma

We do know why both electromagnetism and gravity obey inverse-square laws -- but the explanation is a little technical. (I give it a try in my book The Particle at the End of the Universe.) One way of saying it is that the force-carrying particle in each case (the photon and the graviton) is (1) weakly-interacting, and (2) massless. And the particles are massless because of underlying symmetries.

Probably not too clear of an explanation, but at least it's nice to know an explanation exists?

See also:


bmk6861 karma

Hello! How has understanding physics more and more throughout your life shaped your identity/self-perception and the way you view/interact with the world around you? Also, what music do you like? Thank you for this AMA!

seanmcarroll2 karma

Music: many forms. Probably jazz is my favorite.


I think physics is actually crucial to understanding our place in the universe, at least indirectly. For example, my understanding of physics is what convinces me there is no life after death. That definitely has a big role in my desire to live a fulfilled and meaningful life while it lasts!


abdullahkhalids1 karma

What are some books you would recommend people read? Particularly in the domain of physics/science and philosophy.

What books have most influenced your views on philosophy?

seanmcarroll1 karma

Too many! My favorite as a kid was George Gamow's One, Two, Three, Infinity. Today, I love books by Daniel Dennett, David Deutsch, Lisa Randall, Brian Greene, Janna Levin, David Albert...

mohamedkamal11 karma

Hello Dr. Sean!

Is there any relation between Feynman paths formulation of QM and de Broglie–Bohm interpretation " pilot-wave theory" of QM ?

Also it will be appreciated if you write a guiding post on your blog for the fresh graduated physicists to help them in their scientific life. thanks!

seanmcarroll2 karma

I'm sure there is, but I'm not an expert on Bohm etc.

I've written many "advice" posts!


OGMac1 karma

Hi Dr. Carroll! I'm a senior at Villanova and just wanted to let you know that hearing about you over the years has helped me become confident enough to study math and physics.

I took a class on GR this past semester and it was...difficult. But I shall return to it!

seanmcarroll1 karma

Keep it up!

ScissorMeSharron1 karma

Here is a thought experiment: suppose you invented a machine that could produce a steady flow of entangled particles and sent that stream through conduits pointed in opposite directions.

Let's say the machine was in Hawaii and it sent one flow of particles to a laboratory in India and the other flow to a laboratory in the UK.

Now those laboratories are set up with extremely accurate clocks and a double slit experiment into which the particles would flow.

At first both laboratories allow their particular stream of particles to produce wave interference patterns at the their respective double slits.

My question is this: if the laboratory in the UK suddenly decided to detect the slit through which each of the particles it received passed, thus collapsing their particle stream's wave function, would the wave function of the entangled stream of particles at the Indian laboratory appear to inexplicably collapse? Would the Indians simultaneously lose their wave interference pattern?

If so is this not faster than light information exchange? Could the switching back and forth between collapsing and non-collapsing wave function states of the entangled particle steams then act as a kind of faster than light morse code?

seanmcarroll1 karma

It wouldn't destroy the interference pattern. It would just put the India particles into some particular state, which they would have when they passed through the double slits, and would still lead to interference when they were observed.

Good thought experiment, though!

invent4thefuture1 karma

Hi it's wonderful to be able to talk to you! (I'm headed into Applied Physics, and as I mentioned before I'm currently waitlisted at Caltech, but otherwise I'll be at the University of Illinois.) How much of current theory in physics can be rationally concluded to hold true for all parts of the universe? Scientists each day are discovering that current models don't hold true in certain situations, so are any laws we find actually universal? Any empiricist will endlessly try to collect information until a model can be formulated that describes what is observed, and even then they will still wonder if the known universe can truly be representative of all of this universe. As a rationalist as well, it is important to recognize that all of reality cannot have laws describing it, other than that reality must have an infinite amount of information and be dynamic (which I hope you agree with). So to sum up my point and my question, will empirically-derived physics be able to tell us anything that is truly, incontrovertibly universal?

seanmcarroll1 karma

We understand a lot about the laws of physics underlying our everyday lives.


However -- it would be crazy to imagine that those same rules necessarily applied to parts of the universe we can't observe, including parts so far away that we will never observe them. Still, our job is to come up with the best comprehensive understanding we can think of, and match it to the data. We're not anywhere near done yet, so there's no real need to worry about saying incontrovertibly universal things.

kobboi1 karma

(Warning: not as advanced a question as most below)


When trying to get familiar with cosmology, I never get around the difference between "Big Bang" as a situation with matter in confined space and the expansion of space. Did matter "explode" into some pre-existing empty vessel? Or was the vessel infinitely small as well?

You are one of the few people I know who I trust to be able to say something very clear about this in five sentences.


seanmcarroll2 karma

We don't know what happened at the BB, is the correct answer. In the conventional BB model, the universe begins to exist at that moment, rather than exploding in a pre-existing space. But we don't know what model is right, so our ideas could change. These days many people (including me) take seriously the possibility that the universe is infinitely old.

arnet951 karma

In your debate with William Lane Craig, which I enjoyed greatly, you stressed the importance of there being viable models of the universe with an eternal past. I wonder why this is important, or even relevant, if these models do not actually describe our universe?

By the way, I absolutely love watching you speak. Great fan of yours :)

seanmcarroll1 karma

I think that they do describe the universe! At least, one of them has a good chance to. We don't know at the moment.

AldoFilomeno1 karma

A pair of questions, non technical and a bit outsider (that's not necessarily bad here I hope!): 1) In order to explain the general existence and stability of fundamental laws (don't you think that's puzzling?), is there any plausibility to the idea that they have emerged from a chaotic/lawless state?? (Smolin likes that, but...) 2) In the same line, does it make sense to postulate a process of "formation of symmetries" / "symmetry restoration" from an asymmetric initial state (Nielsen & co. have worked on that) before the symmetry-breaking occurred ? Thanks a lot!!

seanmcarroll1 karma

1) I don't think it's puzzling, so there's not necessarily anything to be explained.

2) People talk along those lines, but it's usually in the context of some underlying deeper laws already in place.

Irrepressible_Monkey1 karma

One part of theoretical physics which I found even more amazing/surprising/alarming than the multiverse is that of domains/realms, which I first read of in Murray Gell-Mann's book, The Quark and the Jaguar.

The idea of not just completely separate universes as in the multiverse, but potentially interacting realities with different physical characteristics was pretty mind-blowing

I am curious as to if you know of more work and discoveries which have been made in this area. And thanks for the AMA. :)

seanmcarroll2 karma

This is a big/powerful idea, the subject of a lot of research, but often under different names, especially "emergence" but also "effective field theory." Check out books by Robert Laughlin or Philip Anderson.

wjbc1 karma

Have you studied philosophy? If so, when and where? If not, why not? Do you consider yourself a philosopher?

Have you experienced any pushback or distrust from other physicists because of your involvement in popularizing physics?

seanmcarroll2 karma

I'm not a credentialed philosopher, although I did minor in philosophy as an undergrad and took a number of classes (in grad school as well). I think the boundaries between certain kinds of physics and certain kinds of philosophy are pretty blurry, and pretty close to where I do research. I'll be submitting my first "official" philosophy of science paper soon, on deriving the Born Rule in many-worlds interpretation of quantum mechanics.

There is always pushback, no matter what you do. I keep trying to work at the intersection of what is useful and what I enjoy doing, whatever that might be.

roundedge1 karma

In a recent blog post you give a description of work you recently published regarding inflation and the interpretation of quantum fluctuations:

Our approach shows that the conventional understanding of inflationary perturbations gets the right answer, although the perturbations aren’t due to “fluctuations”; they’re due to an effective measurement of the quantum state of the inflaton field when the universe reheats at the end of inflation.

If I understand correctly, the salient feature is that quantum fluctuations are not dynamical, but rather manifestations of quantum mechanical wave function collapse. And that this means only universes which will eventually possess observers will be subject to these fluctuations.

Would you consider it unreasonable to characterize this type of explanation as a departure from causal explanations of reality? That is, given that the behaviour of the system is in some sense governed by its future potential, would this not constitute a dialectical or teleological explanation of reality?


seanmcarroll2 karma

Hmm, I gave a long response to this but it seems to have disappeared. More briefly: no, nothing non-causal about it. It's just that there aren't really "fluctuations" in a stationary state until something finally "measures" it by interacting with it.

billalexcosby1 karma

Hello Sean. Is a conscious observer required for the collapse of a wave function in quantum mechanics? (My guess is that of course it's not.) IF it is not, why do popularizers of physics seem to imply it, and never correct themselves or elaborate? I think this misunderstanding is a big reason many people tie quantum mechanics with "spiritualism" and other pseudoscience ideas.

seanmcarroll2 karma

Of course it's not! All you need is for the quantum system to interact with a larger system with many degrees of freedom (moving parts). It could be a conscious observer, but it could equally well be a video camera, or a brick, or the air, or the cosmic microwave background.

sanguisbibemus1 karma

Oooh, multiverse. I remember reading about the first successful quantum computer and how it was able to spit out answers without even running the program, like it was already exploring possibilities as soon as any variables were entered. From what I've read it seems the quantum world is a "cloud" of all possibilities, multiple universes in which every possible outcome of the input equation is calculated, leaving our universe with the closest approximation based on the properties of our physical reality. Did I just make all this up or is there actually something to that?

seanmcarroll2 karma

You somewhat made that up, but it's all based on actual physics. There is an inescapable problem when we try to translate the wild implications of the equations of our best theories into ordinary English. But yes: there are potentially many worlds out there, each existing separately in the wave function of the universe!


Soycrates1 karma

Hi Sean! I formally study the philosophy of quantum mechanics, and I'd love to know,

How often do you come across misappropriations of Heisenberg's Uncertainty Principle (e.g. people using it to claim "science is subjective")? Do you think there are other aspects of QM that are interpreted in even more pseudo-scientific ways than this principle?

In general, what frustrates you as a physicist?

seanmcarroll3 karma

Actually not too often. I think misinterpreting QM in that way is more of a Seventies mistake. These days the favorite mistake is to invoke entanglement when it doesn't apply. Maybe we can blame Deepak Chopra?

RamsesThePigeon1 karma


seanmcarroll2 karma

I don't think we have any. But it's good to keep an open mind.

Hughdapu1 karma

Hi Sean, is there anyone up and coming in your field that could potentially maybe go on to be a revolutionary as Albert Einstein?

seanmcarroll2 karma

Many such people! But it depends on what you do, not how smart you are.

Harksar1 karma

Hey there! Was there anything or anyone in particular that initially inspired you to become a physicist? Thanks!

seanmcarroll1 karma

I started reading physics books in the local library when I was in fourth grade or thereabouts. Not really sure what got me going for the first time.

cisco451 karma

So what banged in the big bang?

seanmcarroll1 karma

The universe.

asmj1 karma

Are fractals the root of it all?

seanmcarroll2 karma

I doubt it.

EonShiKeno1 karma

As a theoretical physicist which current or upcoming experimental physics projects interest you the most and why?

seanmcarroll1 karma

Many. The search for gravitational waves. Search for dark matter. Hunt for new particles at the Large Hadron Collider and elsewhere. Looking for new weak forces of nature. Macroscopic manifestations of quantum mechanics. Testing the evolution of the dark energy. Probing the early universe via the cosmic microwave background. Searching for exoplanets, and for life.

seanmcarroll1 karma

Time is running out, folks, and my finger are failing me. A couple more answers and it's back to work for me!

The_Serious_Account1 karma

How did the size of the very early universe not violate the Bekenstein bound?

seanmcarroll3 karma

The entropy of the early universe was actually quite low, because it was so smooth. Then again, the right answer is really "we have no idea how to correctly describe the very early universe, since quantum gravity will be crucial."

Clever-Username7891 karma

If you had to abandon theory and do small-scale experimental physics (i.e., not LHC-esque experiments), what would you pursue?

seanmcarroll2 karma

In physics, I think quantum information and biophysics are both extremely exciting in terms of experimental frontiers.

jdcall191 karma

Hello, I was wondering what your advice would be to someone just graduating High School and wanting to go into theoretical physics? Is there anything you wish you knew when you were going into college you now know? Any advice would be greatly appreciated, thanks for doing this.

seanmcarroll5 karma

If you can, go to a school with a great physics department, especially one that has graduate classes you can take or sit in on. Talk to more advanced students and grad students, ask them about all the steps. Take many classes. Do research if you can, and certainly get to know several professors well enough that they can write you letters of recommendation. Take the initiative, and don't wait for things to come to you -- follow the current research the best you can, and before you know it you'll be doing it yourself.

pdenlees1 karma

What is your opinion on digital physics, 'it from bit' etc?

seanmcarroll3 karma

Eh. I suspect that reality is continuous, but who knows? I'd be more impressed with a specific model than with an inspirational motto.

Newt_Ron_Starr1 karma

If you had to go into a different area of science today, what would it be?

seanmcarroll3 karma

Probably complexity and emergence. Then again I can just do that anyway!


asphysl1 karma

Does the fact that we exist now, in a 7 billion person community, mean that it is very unlikely that "intelligent life" will someday exist in an interconnected civilization of say a billion earths (with 1019 people)?

seanmcarroll2 karma

I don't think so. Maybe technologically-advanced life is fairly rare, appearing on average once per galaxy per ten billion years. Then we would be at the beginning, but the future could have many more civilizations.

But honestly, we have no idea. Some epistemic humility is in order here.

jbrisendine1 karma

Hi Sean! I'm a big fan of your work and an aspiring science writer myself and I want to ask you about your view of the appropriate use of mathematics in writing for a popular audience. How can you convey the power of equations without inducing a kind of awe that dulls skepticism, or more generally how do you deal intelligently with the relationship between physics and math when talking to a wide audience? I'm especially interested in your view as a theorist who also takes seriously the need for a consistent interpretation of QM.

seanmcarroll1 karma

Depends a lot -- on your goals, and on the audience. Sometimes I skate around equations entirely; sometimes I explain them in words; sometimes I just show them and point out what every term means. It's a struggle to convey just how non-forgiving the equations are.

[deleted]1 karma


seanmcarroll1 karma

Not sure what that would mean. Our best physical theories certainly have time being just a single dimension.

navigators771 karma


seanmcarroll3 karma

Not really -- I don't even think that's right. Not sure what the complete idea is, but there are many wrong (or misleading, or cheating) ways to derive a correct result.

oooo_nooo1 karma

I know you're a supporter of the many-worlds interpretation of quantum mechanics, and you seem to be generally open to the idea of an inflationary multiverse. These two ideas are presumably not mutually exclusive, so could it be said that we might live in a multiverse of multiverses (one which transpires in physical space, and one which must be understood in a Hilbert space)?

seanmcarroll1 karma

Yes indeed. Or, just maybe, the two notions of multiverse are secretly the same.


jy301 karma

Favorite interpretation of Quantum Mechanics that isn't many worlds? Assuming you said information based, what do I have to do to get you on the information based interpretation bandwagon?

seanmcarroll2 karma

I'm quite happy with many-worlds, but I suppose some sort of hidden-variable theory (or something more dramatic that isn't even QM) would be next in line on my list. I have tried to understand Quantum Bayesianism etc., but I think they are trying to be instrumental rather than realistic about the physical world, which I have a very hard time swallowing.

jy301 karma

I knew it..you hate us. :(

seanmcarroll3 karma

Not hate! Just the affection one feels for a beloved friend who has gone horribly wrong.

pizzaface121 karma

Hi Sean, I love your books! Thanks for being a physicist who uses his speaking abilities to promote science and for debate.

You've made the argument multiple times that there is almost certainly no life after death, astrology is bogus, ghosts aren't real, etc. based on the ability to rotate Feynman diagrams, which shows that we do not interact with any new particles because we cannot create these new particles, and based on references to this Eot-Wash paper, which shows that there are almost certainly no new forces on the length and energy scale of life on Earth. However, the Eot-Wash group explored only 1/r2 interactions. Are there any other non-1/r2 interaction terms that could be included in a beyond the standard model Lagrangian that weren't searched for?

I'm not some astrology-believer trying to prove you wrong, I just don't have enough QFT under my belt to answer this question.

seanmcarroll5 karma


The limits on fifth forces are certainly not limited to 1/r2 forces. They explicitly put limits as a function of the range of which new forces can stretch.


airwoz1 karma

How has the introduction of effective field theory as a tool in cosmology affected your approach towards the large open questions in the field? In particular the initial conditions of the early universe, cosmological horizon problem, etc.

seanmcarroll2 karma

I don't think it changes the "big questions" you mention, but it certainly will be very helpful when trying to understand the distribution of large-scale structure, as well as matching inflationary models to observable parameters.


seanmcarroll5 karma

Even computer simulations exist.

donjx1 karma

In your book "From Eternity to Here" I understood you to say that conservation of energy no longer holds for an expanding universe with dark energy. Alan Guth, on the other hand, recently (in the last year or so) said that he suspects that the net energy in the universe is zero. Are these two statements compatible? I'm open to both but my level of physics is not sophisticated to work it out on my own.

seanmcarroll3 karma

Different definitions of "energy," which is unsurprising since the term is not uniquely defined in general relativity. The energy in stuff (matter, radiation, vacuum) is not conserved, but you can get a conserved quantity by adding in a term corresponding to the energy of the gravitational field itself.


asphysl1 karma

What actually creates the gravity waves during inflation? I've heard everything from the acceleration of mass, to decaying inflaton, to blown up gravity waves from pre-inflation.

seanmcarroll2 karma

None of the above. It's just that the quantum state of the graviton field during inflation is not precisely pinned at zero (like the state for any quantum field in an accelerating universe). Once inflation ends, the wave function branches, imprinting fluctuations on the field in our universe.

atnorman1 karma

Two questions.

1: What do you think about Don Ross and James Ladyman's book Everything Must Go? I assume you read it since Ross was at MNF.

2: Want to be modded over at /r/badphilosophy? We invited Snoop Dog, Bill Gates and President Obama, but none of them have responded.

seanmcarroll3 karma

  1. I think it's a very interesting project. Certainly I'm in favor of basing metaphysics on our best physics. I worry that they go to far in reducing everything to relationships.

  2. I try not to spend too much time with bad philosophy (or bad science), trying to make room for the good stuff!

matata1 karma

What do you think about recent progress in the BH information paradox? What do you think about firewall/Hawkins's apparent horizon explanation?

seanmcarroll2 karma

The firewall paradox is a tough one.



I don't think Hawking's proposal is sufficiently concrete to count as an answer, but I don't really know what the answer might be, either.

bobthebobd1 karma

What is the most likely technology that will allow people to travel beyond speed of light?

seanmcarroll9 karma

There isn't one. Sorry. (Other than "the book," since that lets us write science fiction.)

feynman17291 karma

Thank you for doing what you're doing, Mr.Carroll. If I may be so bold, your blog is one of the primary reasons that I see myself going into physics after graduating high school. For that, there are no words to thank you.

Are there any undergrad textbooks you would recommend on cosmology/astrophysics?

seanmcarroll1 karma

Thanks! I don't know undergrad astro textbooks that well, although Frank Shu's book is good for a general overview. Barbara Ryden and Andrew Liddle both have good cosmology textbooks, check them out on amazon.

dadichi1 karma

hello Prof. Carroll. Is it even remotely possible that the vacuum energy may actually be from the external source, which we may not know about and is flowing into the space time fabric and thereby causing the expansion?

seanmcarroll1 karma

When you say "even remotely possible," many things qualify. But the vacuum energy is quite happily explained as something that is right here in our universe, a constant energy at every point in space.

viewerfromnowhere1 karma

If there are multiple universes, in what way are they separate? Does the notion of "space" make sense in this context? Is it possible for two universes in the multiverse to causally interact?

seanmcarroll1 karma

There are different versions. In the quantum many worlds, they are distinct parts of an abstract mathematical "Hilbert space." In the cosmological multiverse, the other "universes" are simply very far away.

By construction, universes tend not to interact, or we wouldn't call them separate universes. But you never know...


Jmarcano1 karma

Hi Sean! Have you considered teaching a course in Coursera, Udacity or edX?

seanmcarroll2 karma

I have considered many things! But honestly, time is extremely squeezed, so it's unlikely I will teach a course online unless someone really pays me to. I do have a couple of courses with the Great Courses, with another one (on the Higgs boson) in contemplation.


AvalancheX1 karma

Thanks for the AMA Sean, Is it true that to be able to confirm Sting Theory we would need a collider the size of the universe and also if there are other universes can we still use math that we use today as a tool to find out about different laws of physics?

seanmcarroll1 karma

A collider the size of the universe seems a bit extravagant. What we know is that, if you simply take current accelerators and scale them up, you won't reach the energies at which string theory should show itself until the machines are (literally) astronomically big.

Which means -- that's not the way to test string theory. We're trying to be more clever, but the theory itself is not very well understood.

cthulhusprophet1 karma

Hey Sean, you did a brilliant job in both debates! Can we hope to see you in more debates in the near future?

seanmcarroll3 karma

Thanks! No debates lined up for the moment. After the World Science Festival and the Cheltenham Science Festival in the UK, I'll be happily staying at home and getting work done for a while.

bobthebobd1 karma

Besides looking for life, should humans try to spread wealth (of life) around? For example should we try to introduce single cell life to planets and moons within Solar system?

seanmcarroll4 karma

I wouldn't think of doing that until we understand life much better, and especially not until we are super certain that nothing life-like already exists elsewhere in the Solar System. Not in my lifetime, in other words.