Space Emerging from Quantum Mechanics

The other day I was amused to find a quote from Einstein, in 1936, about how hard it would be to quantize gravity: “like an attempt to breathe in empty space.” Eight decades later, I think we can still agree that it’s hard.

So here is a possibility worth considering: rather than quantizing gravity, maybe we should try to gravitize quantum mechanics. Or, more accurately but less evocatively, “find gravity inside quantum mechanics.” Rather than starting with some essentially classical view of gravity and “quantizing” it, we might imagine starting with a quantum view of reality from the start, and find the ordinary three-dimensional space in which we live somehow emerging from quantum information. That’s the project that ChunJun (Charles) Cao, Spyridon (Spiros) Michalakis, and I take a few tentative steps toward in a new paper.

We human beings, even those who have been studying quantum mechanics for a long time, still think in terms of a classical concepts. Positions, momenta, particles, fields, space itself. Quantum mechanics tells a different story. The quantum state of the universe is not a collection of things distributed through space, but something called a wave function. The wave function gives us a way of calculating the outcomes of measurements: whenever we measure an observable quantity like the position or momentum or spin of a particle, the wave function has a value for every possible outcome, and the probability of obtaining that outcome is given by the wave function squared. Indeed, that’s typically how we construct wave functions in practice. Start with some classical-sounding notion like “the position of a particle” or “the amplitude of a field,” and to each possible value we attach a complex number. That complex number, squared, gives us the probability of observing the system with that observed value.

Mathematically, wave functions are elements of a mathematical structure called Hilbert space. That means they are vectors — we can add quantum states together (the origin of superpositions in quantum mechanics) and calculate the angle (“dot product”) between them. (We’re skipping over some technicalities here, especially regarding complex numbers — see e.g. The Theoretical Minimum for more.) The word “space” in “Hilbert space” doesn’t mean the good old three-dimensional space we walk through every day, or even the four-dimensional spacetime of relativity. It’s just math-speak for “a collection of things,” in this case “possible quantum states of the universe.”

Hilbert space is quite an abstract thing, which can seem at times pretty removed from the tangible phenomena of our everyday lives. This leads some people to wonder whether we need to supplement ordinary quantum mechanics by additional new variables, or alternatively to imagine that wave functions reflect our knowledge of the world, rather than being representations of reality. For purposes of this post I’ll take the straightforward view that quantum mechanics says that the real world is best described by a wave function, an element of Hilbert space, evolving through time. (Of course time could be emergent too … something for another day.)

Here’s the thing: we can construct a Hilbert space by starting with a classical idea like “all possible positions of a particle” and attaching a complex number to each value, obtaining a wave function. All the conceivable wave functions of that form constitute the Hilbert space we’re interested in. But we don’t have to do it that way. As Einstein might have said, God doesn’t do it that way. Once we make wave functions by quantizing some classical system, we have states that live in Hilbert space. At this point it essentially doesn’t matter where we came from; now we’re in Hilbert space and we’ve left our classical starting point behind. Indeed, it’s well-known that very different classical theories lead to the same theory when we quantize them, and likewise some quantum theories don’t have classical predecessors at all.

The real world simply is quantum-mechanical from the start; it’s not a quantization of some classical system. The universe is described by an element of Hilbert space. All of our usual classical notions should be derived from that, not the other way around. Even space itself. We think of the space through which we move as one of the most basic and irreducible constituents of the real world, but it might be better thought of as an approximate notion that emerges at large distances and low energies. Continue reading

Posted in arxiv, Science | 37 Comments

Father of the Big Bang

Georges Lemaître died fifty years ago today, on 20 June 1966. If anyone deserves the title “Father of the Big Bang,” it would be him. Both because he investigated and popularized the Big Bang model, and because he was an actual Father, in the sense of being a Roman Catholic priest. (Which presumably excludes him from being an actual small-f father, but okay.)

John Farrell, author of a biography of Lemaître, has put together a nice video commemoration: “The Greatest Scientist You’ve Never Heard Of.” I of course have heard of him, but I agree that Lemaître isn’t as famous as he deserves.

The Greatest Scientist You've Never Heard Of from Farrellmedia on Vimeo.

Posted in Science | 150 Comments

The Big Picture: The Talk

I’m giving the last lecture on my mini-tour for The Big Picture tonight at the Natural History Museum here in Los Angeles. If you can’t make it, here’s a decent substitute: video of the talk I gave last week at Google headquarters in Mountain View.

I don’t think I’ve quite worked out all the kinks in this talk, but you get the general idea. My biggest regret was that I didn’t have the time to trace the flow of free energy from the Sun to photosynthesis to ATP to muscle contractions. It’s a great demonstration of how biological organisms are maintained through the creation of entropy.

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Give the People What They Want

And what they want, apparently, is 470-page treatises on the scientific and philosophical underpinnings of naturalism. To appear soon in the Newspaper of Record:

NYT

Happy also to see great science books like Lab Girl and Seven Brief Lessons on Physics make the NYT best-seller list. See? Science isn’t so scary at all.

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Big Picture Part Six: Caring

One of a series of quick posts on the six sections of my book The Big PictureCosmos, Understanding, Essence, Complexity, Thinking, Caring.

Chapters in Part Six, Caring:

  • 45. Three Billion Heartbeats
  • 46. What Is and What Ought to Be
  • 47. Rules and Consequences
  • 48. Constructing Goodness
  • 49. Listening to the World
  • 50. Existential Therapy

In this final section of the book, we take a step back to look at the journey we’ve taken, and ask what it implies for how we should think about our lives. I intentionally kept it short, because I don’t think poetic naturalism has many prescriptive advice to give along these lines. Resisting the temptation to give out a list of “Ten Naturalist Commandments,” I instead offer a list of “Ten Considerations,” things we can keep in mind while we decide for ourselves how we want to live.

A good poetic naturalist should resist the temptation to hand out commandments. “Give someone a fish,” the saying goes, “and you feed them for a day. Teach them to fish, and you feed them for a lifetime.” When it comes to how to lead our lives, poetic naturalism has no fish to give us. It doesn’t even really teach us how to fish. It’s more like poetic naturalism helps us figure out that there are things called “fish,” and perhaps investigate the various possible ways to go about catching them, if that were something we were inclined to do. It’s up to us what strategy we want to take, and what to do with our fish once we’ve caught them.

There are nevertheless some things worth saying, because there are a lot of untrue beliefs to which we all tend to cling from time to time. Many (most?) naturalists have trouble letting go of the existence of objective moral truths, even if they claim to accept the idea that the natural world is all that exists. But you can’t derive ought from is, so an honest naturalist will admit that our ethical principles are constructed rather than derived from nature. (In particular, I borrow the idea of “Humean constructivism” from philosopher Sharon Street.) Fortunately, we’re not blank slates, or computers happily idling away; we have aspirations, desires, preferences, and cares. More than enough raw material to construct workable notions of right and wrong, no less valuable for being ultimately subjective.

Of course there are also incorrect beliefs on the religious or non-naturalist side of the ledger, from the existence of divinely-approved ways of being to the promise of judgment and eternal reward for good behavior. Naturalists accept that life is going to come to an end — this life is not a dress rehearsal for something greater, it’s the only performance we get to give. The average person can expect a lifespan of about three billion heartbeats. That’s a goodly number, but far from limitless. We should make the most of each of our heartbeats.

paris-catacombs-heart

The finitude of life doesn’t imply that it’s meaningless, any more than obeying the laws of physics implies that we can’t find purpose and joy within the natural world. The absence of a God to tell us why we’re here and hand down rules about what is and is not okay doesn’t leave us adrift — it puts the responsibility for constructing meaningful lives back where it always was, in our own hands.

Here’s a story one could imagine telling about the nature of the world. The universe is a miracle. It was created by God as a unique act of love. The splendor of the cosmos, spanning billions of years and countless stars, culminated in the appearance of human beings here on Earth — conscious, aware creatures, unions of soul and body, capable of appreciating and returning God’s love. Our mortal lives are part of a larger span of existence, in which we will continue to participate after our deaths.

It’s an attractive story. You can see why someone would believe it, and work to reconcile it with what science has taught us about the nature of reality. But the evidence points elsewhere.

Here’s a different story. The universe is not a miracle. It simply is, unguided and unsustained, manifesting the patterns of nature with scrupulous regularity. Over billions of years it has evolved naturally, from a state of low entropy toward increasing complexity, and it will eventually wind down to a featureless equilibrium condition. We are the miracle, we human beings. Not a break-the-laws-of-physics kind of miracle; a miracle in that it is wondrous and amazing how such complex, aware, creative, caring creatures could have arisen in perfect accordance with those laws. Our lives are finite, unpredictable, and immeasurably precious. Our emergence has brought meaning and mattering into the world.

That’s a pretty darn good story, too. Demanding in its own way, it may not give us everything we want, but it fits comfortably with everything science has taught us about nature. It bequeaths to us the responsibility and opportunity to make life into what we would have it be.

I do hope people enjoy the book. As I said earlier, I don’t presume to be offering many final answers here. I do think that the basic precepts of naturalism provide a framework for thinking about the world that, given our current state of knowledge, is overwhelmingly likely to be true. But the hard work of understanding the details of how that world works, and how we should shape our lives within it, is something we humans as a species have really only just begun to tackle in a serious way. May our journey of discovery be enlivened by frequent surprises!

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Big Picture Part Five: Thinking

One of a series of quick posts on the six sections of my book The Big PictureCosmos, Understanding, Essence, Complexity, Thinking, Caring.

Chapters in Part Five, Thinking:

  • 37. Crawling Into Consciousness
  • 38. The Babbling Brain
  • 39. What Thinks?
  • 40. The Hard Problem
  • 41. Zombies and Stories
  • 42. Are Photons Conscious?
  • 43. What Acts on What?
  • 44. Freedom to Choose

Even many people who willingly describe themselves as naturalists — who agree that there is only the natural world, obeying laws of physics — are brought up short by the nature of consciousness, or the mind-body problem. David Chalmers famously distinguished between the “Easy Problems” of consciousness, which include functional and operational questions like “How does seeing an object relate to our mental image of that object?”, and the “Hard Problem.” The Hard Problem is the nature of qualia, the subjective experiences associated with conscious events. “Seeing red” is part of the Easy Problem, “experiencing the redness of red” is part of the Hard Problem. No matter how well we might someday understand the connectivity of neurons or the laws of physics governing the particles and forces of which our brains are made, how can collections of such cells or particles ever be said to have an experience of “what it is like” to feel something?

These questions have been debated to death, and I don’t have anything especially novel to contribute to discussions of how the brain works. What I can do is suggest that (1) the emergence of concepts like “thinking” and “experiencing” and “consciousness” as useful ways of talking about macroscopic collections of matter should be no more surprising than the emergence of concepts like “temperature” and “pressure”; and (2) our understanding of those underlying laws of physics is so incredibly solid and well-established that there should be an enormous presumption against modifying them in some important way just to account for a phenomenon (consciousness) which is admittedly one of the most subtle and complex things we’ve ever encountered in the world.

My suspicion is that the Hard Problem won’t be “solved,” it will just gradually fade away as we understand more and more about how the brain actually does work. I love this image of the magnetic fields generated in my brain as neurons squirt out charged particles, evidence of thoughts careening around my gray matter. (Taken by an MEG machine in David Poeppel’s lab at NYU.) It’s not evidence of anything surprising — not even the most devoted mind-body dualist is reluctant to admit that things happen in the brain while you are thinking — but it’s a vivid illustration of how closely our mental processes are associated with the particles and forces of elementary physics.

my-brain

The divide between those who doubt that physical concepts can account for subjective experience and those who are think it can is difficult to bridge precisely because of the word “subjective” — there are no external, measurable quantities we can point to that might help resolve the issue. In the book I highlight this gap by imagining a dialogue between someone who believes in the existence of distinct mental properties (M) and a poetic naturalist (P) who thinks that such properties are a way of talking about physical reality:

M: I grant you that, when I am feeling some particular sensation, it is inevitably accompanied by some particular thing happening in my brain — a “neural correlate of consciousness.” What I deny is that one of my subjective experiences simply is such an occurrence in my brain. There’s more to it than that. I also have a feeling of what it is like to have that experience.

P: What I’m suggesting is that the statement “I have a feeling…” is simply a way of talking about those signals appearing in your brain. There is one way of talking that speaks a vocabulary of neurons and synapses and so forth, and another way that speaks of people and their experiences. And there is a map between these ways: when the neurons do a certain thing, the person feels a certain way. And that’s all there is.

M: Except that it’s manifestly not all there is! Because if it were, I wouldn’t have any conscious experiences at all. Atoms don’t have experiences. You can give a functional explanation of what’s going on, which will correctly account for how I actually behave, but such an explanation will always leave out the subjective aspect.

P: Why? I’m not “leaving out” the subjective aspect, I’m suggesting that all of this talk of our inner experiences is a very useful way of bundling up the collective behavior of a complex collection of atoms. Individual atoms don’t have experiences, but macroscopic agglomerations of them might very well, without invoking any additional ingredients.

M: No they won’t. No matter how many non-feeling atoms you pile together, they will never start having experiences.

P: Yes they will.

M: No they won’t.

P: Yes they will.

I imagine that close analogues of this conversation have happened countless times, and are likely to continue for a while into the future.

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Big Picture Part Four: Complexity

One of a series of quick posts on the six sections of my book The Big PictureCosmos, Understanding, Essence, Complexity, Thinking, Caring.

Chapters in Part Four, Complexity:

  • 28. The Universe in a Cup of Coffee
  • 29. Light and Life
  • 30. Funneling Energy
  • 31. Spontaneous Organization
  • 32. The Origin and Purpose of Life
  • 33. Evolution’s Bootstraps
  • 34. Searching Through the Landscape
  • 35. Emergent Purpose
  • 36. Are We the Point?

One of the most annoying arguments a scientist can hear is that “evolution (or the origin of life) violates the Second Law of Thermodynamics.” The idea is basically that the Second Law says things become more disorganized over time, but the appearance of life represents increased organization, so what do you have to say about that, Dr. Smarty-Pants?

This is a very bad argument, since the Second Law only says that entropy increases in closed systems, not open ones. (Otherwise refrigerators would be impossible, since the entropy of a can of Diet Coke goes down when you cool it.) The Earth’s biosphere is obviously an open system — we get low-entropy photons from the Sun, and radiate high-entropy photons back to the universe — so there is manifestly no contradiction between the Second Law and the appearance of complex structures.

As right and true as that response is, it doesn’t quite address the question of why complex structures actually do come into being. Sure, they can come into being without violating the Second Law, but that doesn’t quite explain why they actually do. In Complexity, the fourth part of The Big Picture, I talk about why it’s very natural for such a thing to happen. This covers the evolution of complexity in general, as well as specific questions about the origin of life and Darwinian natural selection. When it comes to abiogenesis, there’s a lot we don’t know, but good reason to be optimistic about near-term progress.

In 2000, Gretchen Früh-Green, on a ship in the mid-Atlantic Ocean as part of an expedition led by marine geologist Deborah Kelley, stumbled across a collection of ghostly white towers in the video feed from a robotic camera near the ocean floor deep below. Fortunately they had with them a submersible vessel named Alvin, and Kelley set out to explore the structure up close. Further investigation showed that it was just the kind of alkaline vent formation that Russell had anticipated. Two thousand miles east of South Carolina, not far from the Mid-Atlantic Ridge, the Lost City hydrothermal vent field is at least 30,000 years old, and may be just the first known example of a very common type of geological formation. There’s a lot we don’t know about the ocean floor.

Lost City

The chemistry in vents like those at Lost City is rich, and driven by the sort of gradients that could reasonably prefigure life’s metabolic pathways. Reactions familiar from laboratory experiments have been able to produce a number of amino acids, sugars, and other compounds that are needed to ultimately assemble RNA. In the minds of the metabolism-first contingent, the power source provided by disequilibria must come first; the chemistry leading to life will eventually piggyback upon it.

Albert Szent-Györgyi, a Hungarian physiologist who won the Nobel Prize in 1937 for the discovery of Vitamin C, once offered the opinion that “Life is nothing but an electron looking for a place to rest.” That’s a good summary of the metabolism-first view. There is free energy locked up in certain chemical configurations, and life is one way it can be released. One compelling aspect of the picture is that it’s not simply working backwards from “we know there’s life, how did it start?” Instead, its suggesting that life is the solution to a problem: “we have some free energy, how do we liberate it?”

Planetary scientists have speculated that hydrothermal vents similar to Lost City, might be abundant on Jupiter’s moon Europa or Saturn’s moon Enceladus. Future exploration of the Solar System might be able to put this picture to a different kind of test.

A tricky part of this discussion is figuring out when it’s okay to say that a certain naturally-evolved organism or characteristic has a “purpose.” Evolution itself has no purpose, but according to poetic naturalism it’s perfectly okay to ascribe purposes to specific things or processes, as long as that kind of description actually provides a useful way of talking about the higher-level emergent behavior.

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Big Picture Part Three: Essence

One of a series of quick posts on the six sections of my book The Big PictureCosmos, Understanding, Essence, Complexity, Thinking, Caring.

Chapters in Part Three, Essence:

  • 19. How Much We Know
  • 20. The Quantum Realm
  • 21. Interpreting Quantum Mechanics
  • 22. The Core Theory
  • 23. The Stuff of Which We Are Made
  • 24. The Effective Theory of the Everyday World
  • 25. Why Does the Universe Exist?
  • 26. Body and Soul
  • 27. Death Is the End

In Part Three we get our hands dirty diving into some of the central features of how our world actually works: quantum mechanics, field theory, and the Core Theory describing the actual particles and forces that make up the visible universe. The discussion of the basics of quantum mechanics itself is quite brief, and I mention the Many-Worlds formulation only to emphasize that there’s nothing about QM that implies we need to be idealist, anti-realist, or non-determinist. (Those options are open, of course — but they’re not forced on us by what we know about quantum mechanics.)

More directly relevant to this discussion are the ideas of effective field theory and crossing symmetry that let us conclude the laws of physics underlying everyday life are completely known. (I used to say “…completely understood,” but too many people chose to quibble about whether we “really understand” them rather than grasping the point, so I’ve switched to “known.”) (No, I don’t think it will really help either.) In early drafts I went on a bit too long about all the quarks and gluons and so forth, since personally I think that stuff is endlessly fascinating. But it dragged down the pace a bit, so now I have an Appendix in which I give the full Core Theory equation and explain — tersely but accurately! — every single term that appears in it.

In the body of the text I concentrate more on explaining what the claim actually says and why it has a chance of being true. For example, why it doesn’t matter for everyday purposes that we don’t yet understand quantum gravity.

Physicists divide our theoretical understanding of these particles and forces into two grand theories: the Standard Model of Particle Physics, which includes everything we’ve been talking about except for gravity, and general relativity, Einstein’s theory of gravity as the curvature of spacetime. We lack a full “quantum theory of gravity” — a model that is based on the principles of quantum mechanics, and matches onto general relativity when things become classical-looking. Superstring theory is one very promising candidate for such a model, but right now we just don’t know how to talk about situations where gravity is very strong, like near the Big Bang or inside a black hole, in quantum-mechanical terms. Figuring out how to do so is one of the greatest challenges currently occupying the minds of theoretical physicists around the world.

But we don’t live inside a black hole, and the Big Bang was quite a few years ago. We live in a world where gravity is relatively weak. And as long as the force is weak, quantum field theory has no trouble whatsoever describing how gravity works. That’s why we’re confident in the existence of gravitons; they are an inescapable consequence of the basic features of general relativity and quantum field theory, even if we lack a complete theory of quantum gravity. The domain of applicability of our present understanding of quantum gravity includes everything we experience in our everyday lives.

There is, therefore, no reason to keep the Standard Model and general relativity completely separate from each other. As far as the physics of the stuff you see in front of you right now is concerned, it is all very well described by one big quantum field theory. Nobel Laureate Frank Wilczek has dubbed it the Core Theory. It’s the quantum field theory of the quarks, electrons, neutrinos, all the families of fermions, electromagnetism, gravity, the nuclear forces, and the Higgs. In the Appendix we lay it out in a bit more detail. The Core Theory is not the most elegant concoction that has ever been dreamed up in the mind of a physicist, but it’s been spectacularly successful at accounting for every experiment ever performed in a laboratory here on Earth. (At least as of mid-2015 — we should always be ready for the next surprise.)

Princess Elisabeth of BohemiaOne of my favorite chapters in the book is 26, Body and Soul, where I relate the story of Princess Elisabeth of Bohemia and René Descartes. And how, you may ask, does quantum field theory relate to an epistolary conversation carried out in the seventeenth century? Descartes, of course, was famously a champion of mind/body dualism. Elisabeth challenged him on this, asking how something (the immaterial soul) that had no location or extent in space could possibly influence something (the physical body) that manifestly did. The updated version of Elisabeth’s challenge is to ask, “How could an immaterial soul possibly affect the evolution of the particles and fields in the Core Theory? How should that gloriously precise and well-tested equation be modified?”

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Big Picture Part Two: Understanding

One of a series of quick posts on the six sections of my book The Big PictureCosmos, Understanding, Essence, Complexity, Thinking, Caring.

Chapters in Part Two, Understanding:

  • 9. Learning About the World
  • 10. Updating Our Knowledge
  • 11. Is It Okay to Doubt Everything?
  • 12. Reality Emerges
  • 13. What Exists, and What Is Illusion?
  • 14. Planets of Belief
  • 15. Accepting Uncertainty
  • 16. What Can We Know About the Universe Without Looking at It?
  • 17. Who Am I?
  • 18. Abducting God

If, as a naturalist, you want to seriously engage with people who might not agree with you already, you’re going to have to talk a bit about epistemology — how we know what we know. It’s common for non-naturalists to level the accusation (perhaps sincerely felt) that naturalists simply assume their conclusion, and use methodologies that are not sensitive to the possibility of something outside the natural world.

I don’t agree, so in The Big Picture I spend a decent amount of time talking about how we actually go about the task of understanding the world. In particular, science doesn’t presume naturalism, it concludes that it’s the best explanation for the world we experience. Provisionally, of course — science never “proves” anything in the logical sense, so we should always be open to changing our minds in the face of new evidence. I talk a good deal (maybe too much) about Bayesian reasoning and how to update our beliefs.

As an extremely simple — but usefully illustrative — example of this philosophy in action, I concocted a straw-man example of theory choice:

Simplicity is sometimes easy to gauge, sometimes it is less so. Consider three competing theories. One says that the motion of planets and moons in the Solar System is governed, at least to a pretty good approximation, by Isaac Newton’s theories of gravity and motion. Another says that Newtonian physics doesn’t apply at all, and that instead every celestial body has an angel assigned to it, and these angels guide the planets and moons in their motions through space, along paths that just coincidentally match those that Newton would have predicted.

Most of us would probably think that the first theory is simpler than the second — you get the same predictions out, without needing to invoke vaguely-defined angelic entities. But the third theory is that Newtonian gravity is responsible for the motions of everything in the Solar System except for the Moon, which is guided by an angel, and that angel simply chooses to follow the trajectory that would have been predicted by Newton. It is fairly uncontroversial to say that, whatever your opinion about the first two theories, the third theory is certainly less simple than either of them. It involves all of the machinery of both, without any discernible difference in empirical predictions. We are therefore justified in assigning it a very low prior credence. (This example seems frivolous, but analogous moves become common when we start talking about the progress of biological evolution or the nature of consciousness.)

Some people don’t like the Bayesian emphasis on priors, because they seem subjective rather than objective. And that’s right — they are. It can’t be helped; we have to start somewhere. On the other hand, ideally the likelihoods of making certain observations can be objectively determined. If you have a certain theory about the world, and that theory is precise and well-defined, you can say with confidence what the chances are of observing various bits of data under the assumption that your theory is correct. In realistic circumstances, we are often stuck trying to evaluate theories that aren’t so rigorously defined in the first place. (“Consciousness transcends the physical” is a legitimate proposition, but it’s not sufficiently precise to make quantitative predictions.) Nevertheless, it’s our job to try to make our propositions as well-defined as possible, to the point where we can use them to objectively establish the likelihoods of different observations.

Everyone’s entitled to their own priors, but not to their own likelihoods.

Sadly, as much as we might aspire to be, we humans are not always completely rational. One concept I talk about is that of a “planet of belief” — rather than grounding our beliefs on an unshakeable foundation, we assemble a collection of beliefs that hold together under a mutual epistemological attraction. That’s not a mistake, it’s the best we can do. The secret is not to grow so attached to our planets that we equip them with impregnable defense systems, so that they can never be altered no matter what new things we learn.

Planets of Belief

Here we see some representative components of three plausible planets of belief. Can you figure out what kind of person each might belong to?

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Big Picture Part One: Cosmos

One of a series of quick posts on the six sections of my book The Big Picture — Cosmos, Understanding, Essence, Complexity, Thinking, Caring.

Chapters in Part One, Cosmos:

  • 1. The Fundamental Nature of Reality
  • 2. Poetic Naturalism
  • 3. The World Moves By Itself
  • 4. What Determines What Will Happen Next?
  • 5. Reasons Why
  • 6. Our Universe
  • 7. Time’s Arrow
  • 8. Memories and Causes

It wasn’t easy to settle on how to organize all the material in The Big Picture. Ultimately I decided to start with the universe — a bit of cosmology, yes, but also some basic features of how the universe physically operates. Features that played a crucial role in the transition from an ancient view of the world, which still lingers on in our informal “manifest image” of how things work, to our modern scientific view.

Ibn-SinaOne important step in that transition was the seemingly-innocuous realization that momentum is conserved. In Aristotle’s physics, to keep something moving you had to keep pushing it. That’s a very sensible thing to believe, since it’s absolutely true in our everyday experience. It took many centuries of thinking by very smart people (including Persian polymath Ibn Sina, right) to realize that things tend to move by themselves, and are only slowed down by external forces such as friction. That’s important for physics, of course, but there is a deeper implication for our picture of what kinds of things we need to invoke to explain the universe. As I put it in the book:

Aristotle’s argument for an unmoved mover rests on his idea that motions require causes. Once we know about conservation of momentum, that idea loses its steam. We can quibble over the details — I have no doubt Aristotle would have been able to come up with an ingenious way of accounting for objects on frictionless surfaces moving at constant velocity. What matters is that the new physics of Galileo and his friends implied an entirely new ontology, a deep shift in how we thought about the nature of reality. “Causes” didn’t have the central role that they once did. The universe doesn’t need a push; it can just keep going.

It’s hard to over-emphasize the importance of this shift. Of course, even today, we talk about causes and effects all the time. But if you open the contemporary equivalent of Aristotle’s Physics — a textbook on quantum field theory, for example — words like that are nowhere to be found. We sometimes talk about causes, but they’re no longer part of our best fundamental ontology.

What we’re seeing is a manifestation of the layered nature of our descriptions of reality. At the deepest level we currently know about, the basic notions are things like “spacetime,” “quantum fields,” “equations of motions,” and “interactions.” No causes, whether material, formal, efficient, or final. But there are levels on top of that, where the vocabulary changes. Indeed, it’s possible to recover pieces of Aristotle’s physics quantitatively, as limits of Newtonian mechanics in an appropriate regime, where dissipation and friction are central. (Coffee cups do come to a stop, after all.) In the same way, it’s possible to understand why it’s so useful to refer to causes and effects in our everyday experience, even if they’re not present in the underlying equations. There are many different useful stories we have to tell about reality to get along in the world.

The idea that “cause and effect” isn’t fundamental to the workings of the universe hasn’t spread as widely as it should have, despite the efforts of smart people such as Bertrand Russell. In this first section of the book I sketch how we moved from a picture of the universe animated by causes and reasons to one that obeys patterns, without the need for anything to cause it or sustain it. Of course the idea of causality is still crucial to our everyday lives, so I talk a bit about how cause-and-effect relations are emergent phenomena in a macroscopic world with a pronounced arrow of time.

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