General · Language · Media · Site

Biggest ideas in the universe – Sean Carroll chats concepts

[Communicating science series]

While we’re all doing stay-at-home, theoretical physicist Sean Carroll created a chat-from-home series on his YouTube channel. Usually each informal talk (so not lecture-like organization) has a followup Q&A video.

The Biggest Ideas in the Universe is a series of videos where I talk informally about some of the fundamental concepts that help us understand our natural world. Exceedingly casual, not overly polished, and meant for absolutely everybody.

The Biggest Ideas in the Universe | 10. Interactions (May 26, 2020)

Last time we figured out that when you start with a theory of noninteracting fields and quantized it, you could think of the result as a theory of noninteracting particles.

This is Idea #10, “Interactions.” Last time we dipped a toe into quantum field theory, seeing how quantizing fields leads to particles. Now we let the particles interact with each other, and see how the results are characterized by Feynman diagrams.

The Biggest Ideas in the Universe | 9. Fields (May 19, 2020)

We’ve talked about the quantum mechanics of particles, now it’s time to apply those ideas to fields. It requires a bit of effort to understand how a quantum field – which is really a wave on top of a wave, when you think about it – ends up looking like particles in the right circumstances. But it’s worth it.

This is Idea #9, “Fields.” A little bit about classical fields, but mostly concentrating on quantum field theory, and in particular on why a quantized field ends up looking like particles. This one is a bit challenging!

• The Biggest Ideas in the Universe | 8. Entanglement (May 12, 2020)
This is Idea #8, “Entanglement.” I talk about what entanglement means, how it showed up in classic work by Einstein, Schrödinger, and Bell, and the crucial role it plays in competing formulations of the foundations of quantum theory, including Many-Worlds and others.
Entanglement is one of the most important features – arguably, when you get down to it, the most important feature – of quantum mechanics, but it’s often glossed over in how we introduce the subject to students. Here we dive in, and I use this as an excuse to eventually talk about some of the different physical theories vying to be a complete formulation of quantum mechanics.”
• The Biggest Ideas in the Universe | 7. Quantum Mechanics (May 5, 2020)
This is Idea #7, “Quantum Mechanics.” We talk about the quantum recipe — the basic ingredients of wave function and Schrödinger equation, and how they are mixed together — leaving deeper interpretational issues for later.
“This installment runs down the basic ideas; we’ll leave the conceptual heavy lifting for next week.”
• The Biggest Ideas in the Universe | 6. Spacetime (Apr 28, 2020)
This is Idea #6, “Spacetime.” Which, naturally, is about the major idea underlying special relativity – that both space and time are different parts of one unified, four-dimensional spacetime. Learn the real reason why two twins age differently if one stays home and the other zips out near the speed of light.
Related posts

A photon’s frame of spacetime — no rest for the massless

Acceleration causes gravity, gravity causes acceleration

• The Biggest Ideas in the Universe | 5. Time (Apr 21, 2020)
This is Idea #5, “Time.” We talk about what time is, whether it’s “real,” and about why it seems to move in just one direction. That gets us a bit into entropy, which is a teaser for a later video in the series.
Related posts

Whence the arrow of time?

• The Biggest Ideas in the Universe | 4. Space (Apr 14, 2020)
This is Idea #4, “Space.” We talk about what is meant by three-dimensional space, why it might be three-dimensional, and why space exists at all. Why do we live in position space, rather than in momentum space, anyway? And what’s so important about “locality”?

The Biggest Ideas in the Universe | Q&A 4 – Space (Apr 19, 2020)

[My notes] Position and momentum – why we live in position space. Phase space and Hamiltonians. State of the system (initial conditions) and carry forward in time. H(x,p) = kinetic + potential. [So, it’s all about prediction.] Example: Simple harmonic oscillator. Partial derivatives.

In physics, a dimension is just a direction to go. How can that be small? “Small” means you can only go so far. (Small distances correspond to high energies.) Planck scale not be able to see at all.

Background on string theory. Particular theories. Bosons, fermions. Dimensions: 26, 10, 11 – bosonic, superstrings, supergravity (a version of gravity which is supersymmetric, with 2-branes). M-theory. Compaction of dimensions (compactification) vs. branes. Gravity cannot be confined to a brane (it’s a feature of spacetime itself). So far hypothetical (no predictions confirmed).

Locality & holography. Black holes. When gravity becomes important. Information and entropy. Locality violated? When gravity is strong, locality is an approximation. (We take locality for granted.)

Indirect observable effects of extra (invisible) dimensions? Generations of particles <-> the size and geometry of the extra dimensions in principle. The energy density of the vacuum might depend, for example, on the extra dimensions.

The Biggest Ideas in the Universe | 3. Force, Energy, and Action (Apr 7, 2020)

This is Idea #3, “Force, Energy, and Action.” Already I have backslid on my idea that every idea would be encapsulated in just one word, but these three seemed to flow together.

The Biggest Ideas in the Universe | Q&A 3 – Force, Energy, and Action (Apr 12, 2020)

The Biggest Ideas in the Universe | 2. Change (Mar 31, 2020)

This is Idea #2, “Change.” Which is a less-threatening way of saying “Calculus,” which is the mathematics of continuous change.

The Biggest Ideas in the Universe | Q&A 2 – Change (Apr 4, 2020)

The Biggest Ideas in the Universe | 1. Conservation (Mar 24, 2020)

[Correction: at 17:51 I say kinetic energy is a vector, I meant to say “scalar.” Kinetic energy has a size, but doesn’t point in a direction.]

In this installment – the very first idea we cover! – I talk about “Conservation.” The idea that a certain property, like momentum or energy or electric charge, stays the same over time. In my view, realizing that this is true – and the corollary, that the world naturally moves, rather than needing something external to keep it moving – represents the real transition between pre-modern and modern physics.

The Biggest Ideas in the Universe | Q&A 1 – Conservation (Mar 29, 2020)

Errata: at 7:23 I say “equilateral” triangle when I really just meant “right” triangle. (Also isosceles.)

The Biggest Ideas in the Universe | 0. Introduction (Mar 19, 2020)

5 thoughts on “Biggest ideas in the universe – Sean Carroll chats concepts

  1. Re Sean Carroll’s YouTube video “The Biggest Ideas in the Universe | 5. Time” (Apr 21, 2020): “We talk about what time is, whether it’s ‘real,’ and about why it seems to move in just one direction.”


    time, time, what has become of you?
    Newton looked around and saw eternity –
    was there really anything new?
    entropy was so hard to see,
    so many bits to be.

    past-present-future such a totality,
    a 4D map in general relativity.
    time travel some possibility,
    back and forth invariantly?
    the present so easy to see.

    we came from so different spaces,
    that moment we met full of memory,
    in uncertain times and real places,
    only passing so inevitably,
    the plan was so hard to see.

    i wanted to scatter the past, no block verse,
    our clocks dividing moments distinctly,
    sometimes repetitious, you spoke so terse,
    being together so practically,
    the future was so hard to see.

    did our trains pass coincidently
    as coffee swirled in your cup,
    or not so absolutely?
    micro changes just messing up
    what was so hard to see.

    if we replayed our flicks backward
    was everything just too free?
    when did we stop moving forward
    from a start in low entropy?
    our trajectory was so hard to see.

    like labeled ticks on our flicks
    becoming full of ambiguity,
    the changing scenes of our picks
    passing in time’s crazy tapestry,
    in the arrow of time will you remember me?

  2. Re Carroll’s YouTube video “The Biggest Ideas in the Universe | 6. Spacetime” (Apr 28, 2020): “This is … about the major idea underlying special relativity – that both space and time are different parts of one unified, four-dimensional spacetime.”


    we’ll circle back to an energy ride,
    but first let’s talk more classically
    about spacetime and the limit C.
    why the idea of union counts aside
    from space and time as separately.

    it’s really about relativity,
    theories special & general clearly.
    we’ll be presenting top-down Minkowski’s 4D
    (not starting with Newton’s universality)
    then get back to how everyday we see.

    gone Newton’s space and time so absolutely,
    not just adding a coordinate to three,
    when we travel from point A to B.
    a new structure mathematically,
    paradoxes go away conceptually.

    how is going A to B in some ways
    like traveling along in space 3D?
    a clock’s elapse not coordinately,
    and light cones demonstrate how spacetime plays,
    all proper time intervals will agree.
    (but order may not so easily
    if moving non-inertially)

    spacetime’s a dynamic,
    there’s no need to panic,
    when shifting into space-like,
    traveling less as time-like,
    moving straightly’s not the trick

    maybe you’ve heard the twins paradox gaveled.
    one rocketing far to Alpha Centauri,
    even prized Feynman got this incorrectly –
    the spacetime path distance that’s traveled,
    not acceleration as the key.

    so how match those cones to normality,
    where coordinate time is all we see.
    a light-year unit shifts dramatically,
    while by second closes on horizontally,
    a surface like Newton’s apparently.

    the beauty of unification arises
    full of various surprises:
    as length contracted, time stretched,
    a common origin fetched,
    energy and momentum unifying.

    but our vision is not yet complete,
    paradigm aligned classically,
    predicting where all the stuff will be.
    quantum mechanics is a strange treat,
    and merging gravity so trying.

  3. Re Carroll’s YouTube video “The Biggest Ideas in the Universe | 7. Quantum Mechanics” (May 5, 2020): “This installment runs down the basic ideas …”

    Is it possible to declutter quantum field theory (QFT) of legacy baggage? A way to better avoid classical rabbit holes?

    What I’d like to see is a “top-down” a-historial presentation starting with a lean QFT, and taking that seriously – all-permeating energy fields sans any point-particle tone. Then unpacking a wave-like mathematical formalism. Would that just get the Standard Model?

    I’d be interested in a sparse energy framework: (1) perhaps with some structural (sandwich) model like Frank Wilczek’s Grid, and (2) a relational model like Lee Smolin’s for properties like charge and mass – seeking to eliminate unnecessary intrinsic properties.

    I’d be interested in a toy model for the interplay of field and vacuum energy – a case for equal footing as energy counterparts. Space as a “roiling soup” of vibrations. Frothy fields and foamy vacuum. And how that energy is bundled, and the field dynamics when those bundles are confined, perhaps based on Grid coupling (superpositions like Don Lincoln’s analogy of adjacent tuning forks).

    I’d be interested in, for example, starting with the electron field, the photon field, up/down quark field(s), and the gluon field(s). And excitations, localized vibrations, superpositions. The positron field the same as the electron field – positrons as reverse excitations. Whatever “reverse” (or equal-and-opposite) may mean.

    I’d be interested in 3D visualizations of Fourier transforms. If I could visualize a “ripple, or excitation, or bundle-of-energy” in a field – with all the relevant properties, then maybe a “reverse” excitation would be straightforward, eh.

  4. Re Sean Carroll’s YouTube video “The Biggest Ideas in the Universe | 9. Fields” (May 19, 2020), here’re my reactions.

    It’s all about energy.

    Is there an alternative to viewing a quantum field as a superposition of an infinite number of harmonic oscillators?

    And, can we really separate the vacuum from quantum fields? If entangled to some degee? We typically abstract them as superpositions – as like layers; but for an entangled system, the wave function associates amplitudes with the entire configuration of the system, not part-by-part. (And any isolated system is only an approximation.)

    In some sense, I’d be interested in starting with an entangled interactive energy “fabric” and then how fields emerge and then how those modes get us to talking about particles as stable modes (and observables).

    If the energy density function is not merely a superposition of separable wave functions – the vacuum-energy wave function and the fermion-energy wave function, … then there’s just one amplitude. Not adding amplitudes of two wave functions.

    Energy density function = wavefunction(E-vacuum + E-field)

    Energy density function ≠ wavefunction(E-vacuum) + wavefunction(E-field)

    No infinite gradients.

    Then we predict energy density probabilities, which can be decomposed into modes which have modes themselves and which have properties of interest.

    Do Feynman diagrams, as visual tools, use vacuum energy in an ad hoc way? As an add-on perturbative device which simplifies computing interactions. As such, a successful approximation (at least in QED vs. QCD), but sidestepping entanglement of vacuum and fields?

    Does entanglement create spacetime?


    Wave function

    Hilbert space

  5. I enjoyed the wrap-up to “The Biggest Ideas in the Universe | 10. Interactions” (May 26, 2020). The “what to do about the energy density of empty space” is what intrigues me. As well as ways to do 3D visualizations of localized field interactions, like a electron vibration and “reverse” vibration (positron) annihilating.

    (transcript quote) … that’s why I say the Feynman diagram is a story. there is a true story, which is what is the amplitude for a certain reaction to happen, but the intermediate steps are calculational devices. you kind of know that has to be true because the real process that you’re looking at is the sum of all of these different diagrams and all the different diagrams have different sets of virtual particles in them. they’re not exactly the same. so Feynman actually … so anyway but that’s … that is the way of thinking about Feynman diagrams: they’re … they’re calculational devices for doing calculations in quantum field theory.

    I just don’t want you to take all of the steps along the way overly literally. okay, including the virtual particles in between.

    Now, I’ll reveal that Feynman when he was inventing this – he had an aspiration. he was hoping that he was not coming up with a set of calculational tools to do quantum field theory better. he was hoping that he was replacing quantum field theory with a good old fashioned particle theory. the theory of particles that be created or destroyed, following the rules of these Feynman diagrams. turned out not to be right.

    Again, many missteps along the way to any great idea, and it turns out this is a way of thinking about quantum field theory, not a replacement for it. part of his motivation was the cosmological constant problem. remember, we talked last time about the fact that when you have fields pervading the whole universe, these fields have energies and the energies can be added up; and if you just do it naively, the quantum mechanical contribution to the energy of empty space is infinitely big.

    And this is one of those things that physicists have known for a long time; but before around the 1980s, they didn’t worry about it that much. But some people did. so Feynman worried a little bit about it. Philip Andersson worried about it a little bit for different reasons. But they thought that this was a problem with quantum field theory. right, that it gave an infinite energy density to empty space. if you could replace the fields with particles you could get rid of that infinite energy.

    It didn’t work. so we are left with both this wonderfully accurate calculational device of Feynman diagrams and this somewhat unnatural formalism of quantum field theory. we don’t know what to do about that. we didn’t … I’m not gonna reveal what to do about that.

    We still don’t know what to do about the energy density of empty space. But we’re thinking, and it might be that we do in one way or the other have to replace quantum field theory. But in the mean time it is absolutely the best way we have of understanding nature currently available.

Comments are closed.