In light of the recent criticisms of string theory, preeminent physicist Juan Maldacena sheds light on the promise of string theory and how it fits into our current understanding of the universe. In so doing, Maldacena also explains the ideas of holograms, dimensions, and quantum fields in terms everyone can understand.
For more featuring Juan Maldacena, join IAI LIVE: Fantasy, Faith and Physics this July 3rd. Sabine Hossenfelder, Max Tegmark, Michio Kaku, Juan Maldacena, Lisa Randall, and Mary-Jane Rubenstein debate the role of fantasy and unproven belief in modern physics. Book now.
Most people think that physics tells us that everything that exists is made out of particles, like atoms and quarks. Is that right?
Yes, indeed. The current picture of nature posits that all matter is composed of a few elementary particles. More accurately, the theory is that there are several fields that permeate spacetime. The excitations of these fields are quantized, and each elementary excitation quantum represents a particle. For example, the electromagnetic field's unit of excitation is the photon. There is another, the electron field, whose elementary excitation is the electron, and so forth. All electrons are identical because they are excitations of the same underlying field. While popular descriptions often talk about particles, our current foundational theory, the so-called “Standard Model,” relies significantly on the existence of fields.
And should we think of particles as simply very small bits of matter?
No, we should think of matter as being comprised of particles. Particles are the fundamental concept through which we describe matter. The core point is that particles are simpler than matter in general. Hence, we describe something more intricate, like matter, in terms of something simpler, namely, particles. To be more specific, as we mentioned earlier, all matter is formed from excitations of a few fields.
We think that the Standard Model is the long-distance manifestation of a deeper theory at shorter distances, but we do not yet know what that deeper theory is.
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We think that the Standard Model is the long-distance manifestation of a deeper theory at shorter distances, but we do not yet know what that deeper theory is.
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I don't believe that the idea of a dimension is abstract. It is actually very natural and has existed since the time of Euclid. Classical Greek geometry was about points, lines, and planes, which are geometric constructions of various dimensions. Adding time allows us to think in terms of "events" that happen at one time and one location in our three-dimensional space. Events are the fundamental "points" of spacetime.
What is certainly not obvious from our everyday experience is that observers moving relative to each other would perceive a different time. Even less obvious is the fact that these dimensions form a curved spacetime. This was the key insight that Einstein had when he developed general relativity.
In an interview with Closer to Truth, you mentioned that string theory posits that one string ultimately exists, and that the oscillation of that string in different, smaller dimensions generates everything from atoms to galaxies to human beings. What does this mean, exactly?
In the currently accepted "Standard Model," we recognize a few elementary fields. In string theory, the concept is that there is a single field with many components. Excitations of these fields behave like one-dimensional objects, like strings, rather than zero-dimensional particles.
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I must mention that this is only a partial view of what we think "string theory" is. The above description is appropriate in situations where these string fields are interacting sufficiently weakly.
What does it mean to say that a dimension is small?
The simplest analogy is to think of the surface of a garden hose, which is two-dimensional, but from a distance, it appears one-dimensional. If you were an ant walking on the hose's surface, you would see a two-dimensional surface. One of the dimensions is small – the direction around the hose. If the ant walks around the hose, it comes back to the starting point. However, if it walks along the length of the hose, it would not return to the starting point. Hence, we say this dimension is large.
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What we do know is that we need a consistent quantum theory that describes gravity along with the elementary particles we know. String theory is the most promising contender
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In the same way, it could be that there are very tiny, microscopic dimensions of space. They could be so minuscule that we might not be able to measure them. Why would we consider such an outrageous idea? Well, because it's not outrageous at all; it's a very natural possibility. In fact, people proposed the existence of extra dimensions soon after the discovery of general relativity. Of course, we have no experimental evidence so far for the existence of new extra dimensions. They might exist, or they might not. One could safely assume that the universe is four-dimensional all the way to the Planck scale, which is the smallest scale that can be explored according to quantum mechanics, given that we have not observed them. Another possibility is that there are small extra dimensions.
And how can one tiny string give rise to the infinite complexity of the universe?
We think that the universe we observe does not have infinite complexity. Nothing infinite has ever been observed. So, only a finite amount of complexity needs to be explained. We believe that we have an explanation for most matter (except for dark matter) in terms of the particles we already know. We think that simple initial conditions for the universe, combined with known particles, could lead to the complexity we observe. Of course, not all details are understood, and many mysteries remain, from the formation of planets to the emergence of life. However, we have no reason to believe that it couldn't work.
Our description of nature proceeds by layers; we have laws that apply at various distance scales. Biology relies on chemistry, chemistry relies on atomic physics, atomic physics relies on electrons and nuclear physics, nuclear physics relies on the Standard Model.
String theory would add a new layer. The idea is to explain the Standard Model's particles, along with gravity, in terms of a more fundamental theory. We don't know whether it is the right theory. What we do know is that we need a consistent quantum theory that describes gravity along with the elementary particles we know. String theory is the most promising contender. Maybe there are other possibilities we have not yet discovered. The other options that have been proposed don't yet have the level of consistency that string theory has. It is a question that is challenging to study experimentally due to the high energies or very short distances involved. Our hope is that by understanding the theory well enough, we could make an experimental prediction that could be checked by some observation, probably a cosmological observation. But this has not happened yet!
Following up on that, what do you make of the concept of monism, the idea that the universe is undifferentiated? Is string theory monistic?
If by monism we mean that the universe is governed by one field with many components, then yes. The question is whether there is a unified field theory that describes all of the interactions, forces, and matter present in nature. Of course, in nature, there are different objects; they are not the same, so in that sense, it is not just one thing. For example, there are two types of particles, called bosons and fermions. Some could argue that they are so fundamentally different that they cannot be part of the same thing. One could also envision a theory where there is a new type of symmetry, called supersymmetry, which relates them. So, you could argue that supersymmetry is necessary for a completely unified theory. But of course, nature has the final say on whether this is the case or not. Supersymmetry has not been discovered. Furthermore, our current Standard Model has a few different elements, so it is not monistic in the sense of having a single field. But it is monistic in the sense that it posits that the only thing that exists are quantum fields, including one associated with the geometry of spacetime.
Is there a difference between the objects a theory says exist and the laws that govern the behaviour of those objects?
The objects are defined by the laws that govern them, just like a chess piece is defined by how it moves (an old analogy due to Paul Dirac). In a relativistic quantum theory, we postulate the existence of certain fields, and then some dynamical law for their interactions. These interactions can be rather general at short distances, but there are some special types of interactions that survive at longer distances. These are the ones present in the Standard Model. We think that the Standard Model is the long-distance manifestation of a deeper theory at shorter distances, but we do not yet know what that deeper theory is.
If so, doesn’t that mean string theory can’t be monistic?
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Nothing infinite has ever been observed. So, only a finite amount of complexity needs to be explained.
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As I said above, I think that it can be monistic in the sense that only one field exists. It could also be monistic in the sense that the interactions are completely set by the structure of the theory; we think that we have no freedom to modify the interactions.
Is simplicity a guide to truth?
No, I would say that consistency is. Simplicity is preferable. As they say: the theory should be as simple as possible but not simpler! By consistency, I mean mathematical and logical consistency, as well as consistency with the laws of physics we already know, including the fundamental principles of relativity and quantum mechanics. The more general theory should reduce to the currently accepted theories in the regime that these theories have been tested.
Leonard Susskind says that “The three-dimensional world of ordinary experience––the universe filled with galaxies, stars, planets, houses, boulders, and people––is a hologram, an image of reality coded on a distant two-dimensional surface." How can that be possible, and how could we ever know it to be true? Holograms are a kind of visual illusion, so is the world of everyday experience an illusion?
Yes, we think that this is part of gravity. When we talk about an "illusion," we mean that in the description in terms of the distant surface, the description is very different. The third dimension is not present, but it emerges out of the dynamics of the two-dimensional theory. But the world we experience is still real. It is an illusion in the same sense that the surface of a table is an illusion since the table is made out of very tiny particles with empty space between them. Of course, if we hit the table with our hand, it might hurt in a very real way.
The assumption that space-time is a fundamental aspect of the universe leads to a serious problem, which is that the laws of physics break down in the centre of black holes and at the beginning of the big bang. Does this mean that space-time is a wrongheaded idea, or should we question whether there are physical laws that govern every aspect of reality?
Yes, it's very likely that spacetime is replaced by some other more fundamental concept, but we do not have a really clear and simple way to say what that is. It's one of the mysteries of quantum gravity. Understanding this mystery is the primary motivation to study quantum gravity, as it's necessary to understand it to understand the beginning of the universe.
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While popular descriptions often talk about particles, our current foundational theory, the so-called “Standard Model,” relies significantly on the existence of fields.
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Steven Weinberg claimed that the question of why there is anything at all is unanswerable. Was he right?
Maybe. It certainly does not seem to be answerable with our current ways of thinking about fundamental laws.
Might new physics shed some light on the mystery of existence?
As far as I understand, physics is about the laws of the game, but not about the ultimate reason we are playing this game.
Questions by Simon Custer, contributing editor for IAI News, the online magazine of the Institute of Art and Ideas.
For more featuring Juan Maldacena, join IAI LIVE: Fantasy, Faith and Physics this July 3rd. Sabine Hossenfelder, Max Tegmark, Michio Kaku, Juan Maldacena, Lisa Randall, and Mary-Jane Rubenstein debate the role of fantasy and unproven belief in modern physics. Book now.
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