Insight | Is spacetime fundamental, or does it emerge from something deeper?

The Short Version

Four people who start from very different places all doubt that space and time are the bottom layer of reality. The cognitive scientist Donald Hoffman argues that our best physical theories show spacetime losing its meaning at the Planck scale [1]. The physicist Andrew Strominger says that space can be understood as a hologram projected from a lower-dimensional boundary, and that time "ultimately" has to be emergent too [3]. The writer Annaka Harris reports that many physicists she has interviewed think space and time are emergent [2]. Sean Carroll explains the holographic principle that drives much of this thinking, while warning that it is speculative and that nothing in it settles the question of time [4]. They agree far less about what lies underneath. Hoffman says consciousness, Harris gives that slightly better than even odds, and the two physicists point to quantum theory and to mathematics that nobody has finished [1][2][3][4]. Carroll also rejects the step from saying spacetime is derived to calling it an illusion [4].

Table of contents

What They Said

Donald Hoffman: the theory names its own limit

Hoffman's argument about spacetime is separate from his better-known argument about perception, and it rests on physics that he says is not in dispute. General relativity and quantum theory, he points out, "both assume space and time are fundamental." [1] Yet used together, they tell us precisely where that assumption fails. At what physicists call the Planck scale, about 10 to the minus 33 centimeters and 10 to the minus 43 seconds, "space time ceases to have any operational meaning." [1]

He explains why. To look at smaller and smaller regions you need shorter wavelengths, which means more energy. Einstein tells us that energy is the same thing as mass. Concentrate enough of it in a small enough region and, in Hoffman's words, "space time collapses into a black hole." [1] The thing you were trying to observe is destroyed, and adding more energy only makes the black hole bigger [1]. There is no way to probe below that scale, so the notion of distance or duration below it has nothing to refer to.

Hoffman treats this as a credit to the theories and not a defect. Every scientific theory starts with assumptions it cannot itself explain, which is why he says, "So no scientific theory is a theory of everything" [1]. A good theory gives you the mathematics to explore its scope. "But a great theory gives you the mathematical tools to find its limits" [1]. He grants that for most purposes nothing changes: "space time is a great framework for a lot of physics," and most physicists never need to think about the Planck scale [1]. The ones who do are high-energy theorists, and he says they are already working outside spacetime.

His example is a family of mathematical objects called positive geometries, found since around 2013, which sit outside spacetime and precisely encode how particles interact within it [1]. He mentions a 10 million euro European Research Council initiative in the area [1]. One of these structures, the amplituhedron, has a parameter that takes the value four for our spacetime of three dimensions of space and one of time, but in the model the parameter can be as large as you like [1]. To Hoffman that suggests our spacetime is one of the simpler possibilities, in his image "one of the more trivial headsets that's available." [1] What makes this science and not speculation, he says, is that structures found outside spacetime can be tested by what they predict inside it, because all our data comes from inside [1].

From here Hoffman goes much further than the physicists he cites. He argues from evolutionary game theory that "the probability that any organism has ever been shaped to see any aspect of reality as it is, is zero." [1] Our senses, on this view, are like a virtual reality headset built for survival, and spacetime is the display, not the machine [1]. What is the machine? Consciousness, he says, and he is building a mathematical model that aims to show precisely how spacetime and quantum theory arise from it [1]. He puts the whole position in a sentence: "The brain does not create your consciousness. Consciousness creates the brain." [1] He acknowledges that most of his colleagues in consciousness research are physicalists who assume the reverse, and says they respect him while regretting that he left a conventional career [1].

Andrew Strominger: space is the hologram

Strominger, a theoretical physicist at Harvard, arrives at emergent space through black holes. Work by John Wheeler, Jacob Bekenstein and Stephen Hawking showed that quantum mechanics and gravity cannot be consistent unless a black hole stores an amount of information proportional to its area and not its volume [3]. Generalized, that becomes the holographic principle. Strominger introduces it with a joke at his own expense: just before the interview he had been writing a paper whose first sentence referred to "the as yet imprecisely defined holographic principle." [3] "So nobody knows exactly what it is," he says. "But roughly speaking," it says that "all the information that is in some volume of space time can be stored on the boundary of that region." [3]

He and Cumrun Vafa made this concrete for certain black holes in string theory. They found an equivalent description in which the information is stored on what he calls a holographic plate at the black hole's surface, and the region near the horizon arises as a projection from it [3]. Fridman tries to restate it, and Strominger supplies the phrase: "The bulk, the space time is the hologram." [3] He compares it to an ordinary hologram, in which a flat plate contains all the information and shining light through it produces an image that looks three-dimensional [3]. In his framing, Einstein discovered the hologram and he and Vafa discovered the plate [3].

This gives him a precise meaning for the phrase "emergent space". Asked whether time is fundamental or emergent, he answers, "I think ultimately it has to be emergent." [3] Then he reviews what emergence already means for space: "you have a holographic plate and you shine some light that's moving in space and it produces an image which contains an extra spatial dimension and time just goes along for the ride." [3] A dimension of space appears in the image that was not in the plate. The goal now, he says, is to find examples where the dimension that emerges is time, so that a system with time is described by one that has only space dimensions [3]. There are concrete ideas, he says, but they are not widely accepted [3]. Asked how to picture space and time on this view, he says, "They're both illusions." [3]

Strominger is careful about how much has actually been shown. The principle has been made precise for black holes and for a kind of universe called anti-de Sitter space. It has not been made precise for flat space, or for the expanding de Sitter space that astronomers say we live in, and he calls doing so one of the huge problems in physics [3]. His reason for thinking time cannot be fundamental comes from the origin of the universe. A big bang seems to mean that before it there was nothing, and "You can't have a theory in which something falls from nothing." [3] He says string theory has said almost nothing compelling about the Big Bang [3].

His general attitude explains why none of this troubles him. "My view is that there's no exact equation. Everything is an approximation," he says [3]. General relativity predicts singularities where it stops applying, and he reads that as a pointer to something deeper: "Edges are wonderful because that keeps us in business," [3].

Sean Carroll: holography, with the brakes on

Carroll, a theoretical physicist at Johns Hopkins, tells the history of how spacetime became a thing in the first place. Hermann Minkowski merged space and time into a single spacetime in 1907. Einstein at first dismissed that as over-formalizing, then saw that if spacetime is a thing it can have geometry, which led to the insight that "gravity is the curvature of space time." [4] Carroll's admiration for the result is complete. He says "the equation is much smarter than Albert Einstein" because it predicted the Big Bang, gravitational waves and black holes [4].

He then explains the holographic principle from the beginning. Hawking showed that black holes have a finite entropy. "the belief is, and now we're already getting quite speculative," Carroll says, that this is the largest amount of entropy you can have in a region [4]. The surprise is that the maximum scales with the area of the event horizon and not with the volume inside [4]. Gerard 't Hooft and Leonard Susskind proposed that information in quantum gravity is not held as separate bits at every point in space. It is holographic: all the information in a black hole "can be thought of as living on the event horizon rather than in the interior of the Black hole." [4]

He says the early version was a little hand-wavy, and that the idea came into its own with Juan Maldacena in the 1990s. Maldacena's correspondence relates two theories: one in a certain number of dimensions without gravity, and another with one more dimension and with gravity. The first casts a hologram into the larger space to make it look as if there is gravity [4]. "And that's holography with the vengeance," Carroll says, and it is an enormous source of interest among theoretical physicists [4].

He points out the tension this creates with the rest of physics. Our best theory of fundamental physics, quantum field theory, "is entirely local": something can happen at every point in space, and you add up all the points, which is not holographic at all [4]. It allows far more states than holography permits. Ordinary objects never notice, because they are nowhere near the limit; as far as quantum field theory is concerned, "you and I are mostly empty space." [4] Carroll and a postdoc, Oliver Friedrich, tried to turn the mismatch into a test. If quantum field theory's states overlap very slightly, a neutrino crossing the universe could gradually dissolve into others, so the highest-energy neutrinos from far away should be missing. They compared the prediction with data from the IceCube detector in Antarctica, and the data run out right where their predicted cutoff would be, so the idea is neither confirmed nor ruled out [4].

Carroll is the most cautious of the four about what follows. Asked what happens to time in the holographic principle, he says, "As far as we know, nothing dramatic happens. We're not anywhere close to being confident that we know what's going on here yet." [4] There are good unanswered questions about whether time is fundamental or emergent, whether it has something to do with quantum entanglement, whether it really exists at all. "But there's nothing specifically about holography that would make us change our opinions about time, whatever they happen to be." [4] On the Big Bang he is maximally uncertain whether there was a first moment, and offers a link between the two questions: a first moment of time would fit hand in glove with time being emergent, since "If time is fundamental, then it tends to go forever because it's fundamental." [4]

He is also the one who objects to Hoffman by name. Asked to make the case for Hoffman's view that spacetime is an illusion, Carroll declines the word. A good prediction points to something true about reality whatever we later decide the fundamental ingredients are. "To call it an illusion is just wrong, I think." [4] We might not yet have the most comprehensive way of stating it, he says, but it is still true [4]. He accepts what philosophers call structural realism, the idea that successful predictions track something real even when our deepest description changes [4].

Annaka Harris: a real structure that does not look like what we see

Harris, author of a book on consciousness, describes her work as breaking intuitions, and she treats the solidity of space and time as one more intuition that may have to go [2]. She brings two things to this discussion. The first is a report from the field: many physicists she has interviewed, including Sean Carroll, Lee Smolin, Brian Greene and Carlo Rovelli, think space and time are emergent [2].

The second is a middle position on what that means. She has recorded three conversations with Hoffman and says they may hold an almost identical view if consciousness turns out to be fundamental, though she dislikes some of his terminology [2]. Her own way of putting it is that our perceptions map onto a real structure, just not one that looks like what we experience, and she offers the holographic principle as an example of how that could be [2]. She compares human perception to living inside a snow globe, with each broken intuition a brief look outside [2]. In an article on time she explores the idea that we perceive one moment at a time, like a flashlight moving over a larger structure [2].

On what the deeper layer is, she is openly undecided. Science's default has been that consciousness arises at some point in complex processing. She now puts the odds at "51, 49" in favor of consciousness being fundamental, and stresses that she is not convinced, only that the possibility is understudied [2].

Where They Agree, and Where They Don't

They agree that spacetime is probably not the last word. Hoffman says so from the Planck-scale argument [1]. Strominger says space emerges holographically and time must too [3]. Harris reports the same view from a range of physicists [2]. Carroll presents holography as a serious and productive idea [4].

They agree on why physics is being pushed this way. In every account the pressure comes from combining quantum theory with gravity, and black holes are where the two collide. Hoffman's argument ends in a black hole [1]. Strominger's and Carroll's begin with the entropy of one [3][4].

They disagree about what is underneath. Hoffman says consciousness, and expects his field to shift once someone shows how spacetime emerges from it [1]. Harris thinks that slightly more likely than not [2]. Strominger's deeper layer is the holographic plate, a physical system in fewer dimensions [3]. Carroll's position is that quantum mechanics is more fundamental than general relativity, and he says that, as far as he knows, there is no empirical reason to think quantum mechanics is anything less than 100% correct [4]. He is a physicalist who is critical of the idea that consciousness pervades matter [4]. So the agreement that spacetime is derived hides a disagreement about almost everything else.

They disagree about the word "illusion". Hoffman uses the headset image and says we have never seen reality [1]. Strominger, asked for intuition, says space and time are both illusions [3]. Carroll says that calling spacetime an illusion is just wrong, and that the predictions made with it remain true [4]. Harris sides with a real underlying structure that our perceptions map onto [2]. The difference is partly verbal. A hologram's image is produced by something that does not resemble it, and the image still obeys rules you can rely on. Hoffman himself calls spacetime a great framework [1]. But the choice of word signals what each wants a reader to conclude: Hoffman wants you to stop trusting the display, and Carroll wants you to keep trusting what it predicts.

They disagree about how much is established. Hoffman says physicists have known for a long time that spacetime falls apart at the Planck scale, and presents physics beyond spacetime as under way [1]. Strominger says nobody knows exactly what the holographic principle is and that it has not been formulated for a universe like ours [3]. Carroll says we are nowhere close to confident [4].

They are at different stages on time. Strominger thinks time must be emergent and admits the ideas for showing it are not widely accepted [3]. Carroll says holography gives no specific reason to change one's view of time [4]. Hoffman lists a Planck time alongside the Planck length and treats both together [1].

Why It Works

The sources give several lines of reasoning, and it helps to keep them apart.

The Planck-scale argument. Finer resolution needs more energy, energy is mass, and enough of it in a small region makes a black hole. So there is a smallest scale below which distance and duration cannot be given operational meaning [1].

The area law. The maximum information or entropy in a region grows with the area of its boundary and not its volume. If volume were fundamental, you would expect capacity to grow with volume [3][4].

A worked example. Maldacena's correspondence shows a theory without gravity in fewer dimensions that is equivalent to a theory with gravity in one more. A dimension of space, and gravity with it, appears on one side and not on the other [4]. Strominger and Vafa's black hole calculation is a second case in which the plate and the hologram can both be written down [3].

Singularities. General relativity predicts places where it stops applying, at the centers of black holes and at the beginning of the universe. Strominger takes that as the theory announcing that it is an approximation [3]. Carroll links the question of a first moment to the question of whether time is emergent [4].

Structures outside spacetime that predict what happens inside it. Hoffman's positive geometries are his example of mathematics that does not presuppose spacetime and can still be checked against experiments done in it [1].

The perception argument. Hoffman's evolutionary result, and the point Harris cites from the neuroscientist Anil Seth that perception is a "controlled hallucination," suggest that the way space and time appear to us is a construction of the brain [1][2]. This is a different kind of argument from the physical ones. It concerns our experience of space and time, while the physics concerns space and time themselves, and Harris's snow globe is a reminder that a constructed view can still be a view of something [2].

Where It Applies, and Where It Doesn't

It applies at extremes. The Planck scale, black hole horizons and the first moments of the universe are where spacetime's status is in doubt [1][3][4].

It does not change everyday physics. Hoffman says most physicists need not worry about the Planck scale [1]. Carroll says ordinary matter is nowhere near the holographic limit, and that general relativity's predictions stand [4]. Emergent does not mean wrong. Carroll's own view is that there are many valid ways of talking about the natural world at different levels, and that things described at higher levels, like tables, are as real as anything [4].

It has not been shown for our universe. The precise holographic results are for black holes and for anti-de Sitter space, not for the kind of space we appear to live in [3].

It has not been tested. Carroll's neutrino comparison came out neither for nor against [4]. Strominger does not expect a yes-or-no experimental test of string theory in the foreseeable future [3].

It does not support the consciousness claim on its own. That spacetime may be derived is a view several physicists hold. That it is derived from consciousness is Hoffman's proposal, shared tentatively by Harris and rejected by Carroll [1][2][4]. The first does not imply the second.

What To Do With It

1. Separate the three claims. That spacetime is not fundamental, that it is an illusion, and that consciousness is fundamental are three different statements with different levels of support in these sources. Many physicists hold the first; Carroll rejects the second; the third is a minority view [1][2][4].

2. Ask what scale is being discussed. At human scales, and far below them, spacetime remains the working framework [1][4].

3. Use the hologram correctly. The picture is a flat plate that holds all the information and a higher-dimensional image projected from it. Strominger's wording is that the spacetime is the hologram [3].

4. Keep the area-not-volume fact in mind. It is the cleanest single reason physicists suspect that space as we picture it is not basic [3][4].

5. Treat breakdowns as clues. Hoffman's test of a great theory is that it shows you its limits, and Strominger welcomes the edges for the same reason [1][3].

6. Watch for tests. Carroll's attempt with high-energy neutrinos shows what trying to check holography against data looks like, and why it is hard [4].

7. Keep trusting what works. A theory can be superseded at the foundations and remain right in its predictions. That is Carroll's point about why "illusion" is the wrong word [4].

8. Read further. Harris recommends George Musser's Spooky Action at a Distance for accessible explanations [2].

Sources

1. Donald Hoffman on Why We've Never Seen Reality: Evolution, Spacetime and Consciousness · The School of Greatness
2. Annaka Harris on Free Will, Consciousness and the Nature of Reality · Lex Fridman Podcast
3. Andrew Strominger on Black Holes, String Theory, the Holographic Principle, and Soft Hair · Lex Fridman Podcast
4. Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · Lex Fridman Podcast