Why Does the World Exist?

In Jim Holt’s enjoyable book, Why Does the World Exist? An Existential Detective Story, he recounts conversations with a wide variety of thinkers, from physicists and biologists to writers and philosophers, who have struggled with the Primordial Existential Question. You probably know my take on the issue, but Jim and I sat down at the LA Library a few weeks ago to chat about this and related issues. I think it’s safe to say we at least had a few laughs. Here’s the complete video; audio is also available as a podcast.

Jim Holt and Sean Carroll from ALOUDla on Vimeo.

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53 Responses to Why Does the World Exist?

  1. Bob Zannelli says:

    Interesting discussion. I wish Carroll and Holt could have gotten into the interesting questions brought up at the end. I thought Holt’s comments about Krauss were unfair, it seems reasonable to me to apply our best, but obviously incomplete understanding, to the question of where the universe came from. After all that’s what Carroll and Chen do in their very interesting model. Krauss does not define nothing as the quantum vacuum as Holt seems to think (he must not have read Krauss’s book) Instead the “nothing” that Krauss talks about is the “void” of third quantization, which is what you get when you apply quantum theory to space and time. Finally I think Andreas Albrecht and Lorenzo Sorbo have demonstrated that the prediction of the high probability of Boltzmann brains as compared to inflating universes is due to an incorrect way of framing the probabilities and once the calculation is done properly , the probability of inflating universes far exceed the probability of Boltzmann brains. [ hep-th/0405270v2]

  2. Dan says:

    Very interesting discussion, though I’m not a huge fan of Jim Holt. A more modest moderator might have allowed more time for answers rather than questions. But I digress. I hope there’s a way to disseminate the paper you’re writing with Alan Guth about the eternal nature of the universe. That sounds fascinating.

  3. Darren says:

    The question, “why is there something rather than nothing” is an example of how good philosophy is at asking questions, but dreadful at answering them. In this case I don’t even think it’s a good question. Why should we not ask it this way: Is nothing possible?

    Why is nothing supposed to be a default position? Of all we experience every day, all of the “something” we see around us, it seems that the default is stuff, rather than no stuff.

  4. AI says:

    To exist is defined as being a part of the World so the World exists by definition.

  5. James Cross says:


    Krauss’ book is called A Universe From Nothing. Should he have titled it A Universe From the Quantum Void?

    It amazes that physicists, many of whom are atheists, can seriously discuss Boltzmann brains.

  6. Michael says:


    Here’s another question: why no dark matter effect on the solar system?



  7. Sean Carroll says:

    Why no dark matter effect on NBA three-pointers?

  8. Lord says:

    So 2013! Have to wonder though whether we have memories of the past or of all possible pasts that could have reasonably lead us to where we are today.

  9. Michael says:

    “Why no dark matter effect on NBA three-pointers?”

    Point taken, but don’t the current models predict some impact at solar system scales (and Lebron James scales) 😉

  10. Michael says:

    Meant “and not” Lebron James scales 🙁

  11. Michael says:

    Sean, I saw the cited paper through a link from you lovely wife’s G+ page regarding this article:


    It seemed like a legitimate issue, but maybe not . . . anyway, just curious.

  12. amer says:

    Holt really needs to learn to keep his mouth shut and let his guest speak!!! Also, in the discussion there was no exploration or recognition explicitly that if all there is is a probabilistic distribution then this conversation itsef is no more than a simple distribution of matter akin to the output of Shakespeare’s monkeys. In other words there is no causality and there is no conscious meaning or comprehension. They are sitting inside one of the meaningless patterns of the quantum wave functions.

  13. Sean Carroll says:

    The predicted effects of dark matter on solar system dynamics are below what can be observed, and then when we go look for them — we don’t see them. Which adds or subtracts pretty much nothing to the status of dark matter as a theory.

  14. Peter says:


    At the beginning you’ve mentioned that perhaps space might not be fundamental. Could you elaborate on this?

  15. Meh says:

    You know my beef with the nothingness conversation/argument that seems to be common now? A state of nothing is something. “Nothing” in our universe is complete static equilibrium of the quantum field. Something comes from nothing by the addition of Time causing a change in that static state. So saying “why is there something rather than nothing” is not a question we can answer because we are restricted to the nature of our universe. Our physics are designed to describe a universe with Time because that’s the universe that we are a result of. We can tell you what “nothing” is; but that’s not the actual question philosophers want to know the answer to. The question they want to know the answer to is: “what is nothing, and where does it come from? (further) If you’re describing it as ‘something’, then doesn’t that mean that it’s no longer ‘nothing’?” And that’s why we all learned Limits in calculus, because that’s the way the multiverse works. Happy 4th of July and remember: Don’t Drink and Derive.

  16. Meh says:

    In other words, if our existence is restricted to the laws of physics, then we cannot and will never be able to answer that question because it is outside of the limits of our existence,

    Someone told me there was a brown out. A brown out being a power spike causing a temporary outage on certain electronics; like a black out. My crude joke was “A brown out? You mean when you think you have to fart, so you let it rip, but it turns out you had diarrhea?” The idea of a limit to Physics is a brown out in the physics community.

  17. Peter says:

    Thanks! Very helpful!
    (I know it’s considered a lost of bandwidth on the the internet to thank people, but it’s in my human nature to do it.)

  18. Antonio Sanchez says:

    Hi Sean,
    thanks for posting the video.

    Once again it came out the question about why the entropy was so low at the beginning.

    It seems obvious to me that if there was no space around (educated guess) prior to the big bang there was no possible grade of disorder involved (a characteristic related to a dimensional world).

  19. Bob Zannelli says:

    Antontio , Carroll rejects the idea, as far as I can tell, that Bekenstein’s entropy bound applies to the early universe which logically would have low volume phase space. He argues that there must be the same number of micro states when the universe was 1 cm is size as it now with our O region at a radius of 46 BLY because of information conservation (Unitarity) mandated by quantum theory. I love his book, Fron Eternity to Here” it’s a must read book, but on this point I don’t understand why Carroll asserts this. Isn’t the history of the Universe classical , a decoherent history? Perhaps he will respond to your question and clarify this.

  20. I’ve never seen an interview where the interviewer talks twice as much as the interviewee.

  21. SebastianS says:

    Hi Sean,
    What were you about to say at 53,31 into the video regarding Vilenkins view?

  22. Mike D says:


    I’m wondering if you wouldn’t mind (maybe in a future blog post) elaborating on your disagreement with the BGV Theorem. I read Vilenkin’s book recently and I’m not quite sure to what you were referring when you mentioned that they were starting with a set of assumptions. I know we both know of a certain theologian who likes to combine that (the BGV Theorem) with an “impossibility of an actual past infinite” argument to “prove” the universe was poofed into existence by a deity.

  23. Bob Zannelli says:

    I don’t think the reasoning involved in the BGV theorem is in much doubt, it even applies to cyclic models like the Ekpyrotic proposal. But this doesn’t really mean that the Universe ( or Multiverse if you prefer) isn’t past eternal, rather it proves that inflation can’t be past eternal. In fact several origin models produce past eternal universes without running afoul of the BGV theorem. These are the Carroll-Chen proposal, the no boundary model ( Actually discovered by Page) Linde’s tunneling model, the Aguirre-Gratton steady state inflation model and the Veneziano Gasperini pre big bang model. These are speculative of course, but any model that proposes an origin scenario absent a working quantum theory of gravity must of necessity be speculative. All of these models evade BGV by postulating time reversed histories. Based on this the BGV theorem isn’t so much an assertion that the universe can’t be past eternal , but rather that there must exist a boundary of some form for which the possibility of time reversed histories are not ruled out.

  24. Bob Zannelli says:

    Clarification ;I wrote with regard to the No Boundary Model ( Actually discovered by Page) The NBM was of course proposed by Hawking and Hartle, not Page. Hawking thought his model predicted a closed universe which reversed its arrow of time during collapse. Page pointed out to Hawking that what the NBM really predicted were time reversed histories. Hawking called this his greatest blunder, though for what’s it worth I would argue his conceding on the black hole information paradox bet , given the discovery of the Firewall problem actually falls into this category, but I digress.