Daily Archives: June 1, 2020

For nearly a century, theoretical physicists have been grappling with a major foundational question: how does gravity work within quantum mechanicsthe field devoted to the behavior and properties of particles smaller than atoms? The answer could open the door to a so-called theory of everything, one that explains the nature of both large objects and small particles, thereby bridging the gap between two fundamental physical theories, Einsteins general relativity and quantum mechanics.

While there are many possible explanationsstring theory arguably the best knowntheoretical physicist Francesca Vidotto offers a theory that has the advantage of focusing on the specific question of quantum gravity: how gravity works on the quantum scale. Vidotto has spent much of her life unraveling and exploring this approach, which is called loop quantum gravity.

As Vidotto and fellow theoretical physicist Carlo Rovelli put it in their book, Covariant Loop Quantum Gravity, the problem with quantum gravity is simply the fact that the current theories are not capable of describing the quantum behaviour of the gravitational field. Many of us understand gravityone of natures four fundamental forcesas the force that drives the dance of planets around other massive, celestial bodies. As children, we learn that it is what keeps our own bodies from flying off into space. Scientists once envisioned gravity as a traditional force, with the power to pull, but now, they understand it as a distortion of space-time.

But gravity does not translate easily into the mathematical language of quantum physics. It does not readily quantize. According to Vidotto, loop quantum gravity may be the key to understanding exactly how it works.

A successful theoretical framework for quantum gravity would help unravel many mysteries, from the beginning of the universe to the centers of black holes. Being able to merge general relativitywhich works well on the cosmic scalewith quantum mechanicswhich explains the workings of the miniscule scalewould provide answers that have hitherto been out of reach. We cant yet observe what happens inside a black hole, but some theoretical physicists say quantum gravity could give us access to the mouths of these phenomena, where we know gravity is so great that it lets neither light nor matter escape.

Vidotto has been pondering questions like these since at least the age of 12, when she read Stephen Hawkings A Brief History of Time. Riding her bike across the cobbled streets of her hometown in Treviso, Italy, she was struck one sunny morning with the conviction that questions about time, physics, and the universe were the kinds of questions she wanted to think about for the rest of her life. And so she has, partnering with giants in the field like Rovelli, her frequent collaborator, on theories like loop quantum gravity and spin foam.

Here, Vidotto shares thoughts on spin foam theory, her obsession with primordial quantum fluctuations, and one good way to turn kids into future scientists. She spoke to me from her new home in London, Ontario, where she teaches physics at The University of Western Ontario.

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What drew you to science, in general?

When I was a little girl, I thought I would become a ballet dancer, or something else. During my first year of school this wonderful woman, my primary school teacher, had a method of teaching science to us that is still the way in which I think science works.

I remember her taking us out into the garden on the first day of school and asking us to draw pictures of what we saw in the garden. She asked us to look at our drawings and brainstorm about questions that were arising. We had to vote for the favorite question in the class. The question we chose was, What are the rocks made of? For the rest of the year we kept on studying this question together. We were divided into small groups. We had to make hypotheses. We had to collect this information set. We had to falsify our hypotheses. And this was wonderful.

For me, the most interesting part was that it was a very democratic process, where we listened to what the other groups of children were saying. The final answer to that question was in fact a collection of all the answers that were not falsified. And thats where I saw a summary of the knowledge we had collected. So, every time I think about how to do science, what dynamics we put in place as scientists, the importance of listening to others and being free to possibly change our mindsthese are lessons I learned when I was six years old and its something I bring with me to my work today.

Sounds like a good way to get more kids interested in STEM.

I think its really important. There is a lot of attention these days to get people interested in science, but I think we really have to get children involved in this process, this very interactive, engaging process of learning so theyre interested in science as they grow older.

Why are there so few women in science? The most effective actions are when girls are really young. There are studies that show how stereotypes get ingrained when girls are just eight years old, so we have to do something before that.

Which space and time question is always top of mind?

Well, I have an obsession now that has stayed with me since I started doing my own research: primordial quantum fluctuations. [You can think of primordial quantum fluctuations as the initial conditions before galaxies, planets, the universe came to be]. Today we have an understanding that all the structures we see in our universe, galaxies, stars and so on, all originated from some small seeds in the beginning of our universe. Its a common understanding that this initial situation was of quantum origin. By running simulations where we include the presence of these fluctuations, we can recall where all the structures were that we now see.

We have a very good understanding of how big or which kind of fluctuation should be there, but we dont have a theory that really provides them from scratch, from the bottom. So the theory I work with, quantum gravity, is a very promising candidate for a theory that provides a description of the quantum properties of space-time. In particular, its a theory that is able to describe these fluctuations, the quantum fluctuations that characterize space-time in the more extreme regimes, like in the beginning of the universe.

So my obsession since the beginning of my research was to try to use this theory, try to use quantum gravity, to study these fluctuations. This has been a long journey, but we have come to some important advances using numerical techniques. Of course, there are many projects and sometimes, I have to put this question aside, but I think this is the dream that will stick with me: to see really how we can describe the beginning of our universe using these techniques.

What is the primary discovery of loop quantum gravitytheory?

Loop quantum gravity is a theory with the goal of describing the quantum properties of space-time. The main realization is the fact that, because space-time itself can be described as a geometry, geometrical quantities should appear with discrete values. So geometrical quantities are, for instance, area, volume, angle, and so on. If you try to measure these quantities at the fundamental level, you would see that they appear with only certain values and with jumps between those values. This is very similar to what happens in more familiar situations described by quantum mechanics. For example, when you study angular momentum in quantum mechanics, you discover that this property, if you try to measure it, comes only with certain values. And with jumps between them. This is the main discovery of loop quantum gravity, and everything stems from there.

Can you explain this idea ofspin foam theory, an idea youve worked on with Carlo Rovelli?

Spin foam is simply the path integral formulations of loop quantum gravity. [Path integral formulation is the mathematical theory for understanding all the possible paths a quantum particle can take to get from one point to another.]

So loop quantum gravity is about describing the quantum geometry of space-time. The discovery of Einstein is that space-time itself, the geometry itself, is not something inert; its a dynamical object. Its something that may evolve. Following this in loop quantum gravity, we want to describe how the gravitational field, the geometry, evolves, but in a quantum manner. The language in the quantum theory with which this is done is the path integral.

In quantum mechanics, for instance, you start with a particle. You see a particle at one point, and later you see the particle at another point. So you have an initial state with a particle in a given position and the final state with the particle in another position. Then you want to study what the probability is that the particle goes from one position to the other. In order to do that, you have to sum over all possible trajectories that the particle could have taken in order to go from that first position to the other. All these trajectories are the different possible paths the particle could have taken from the initial point to the final. Path integral is this idea that you have the sum of all possible paths.

Here in quantum gravity, what we do is study the probability to go from one state of the geometry to another state of the geometry. You have two states, but these two states represent a geometry, and you want to know what all the possible geometries are that interpolate from one to the other. In other words, all the possible configurations of the gravitational fields that take you from one configuration to the other.

This is spin foam. [Spin foam is a 2-dimensional model with labeled edges, vertices, and faces with the goal of describing the geometry of quantum spacetime. So named because it resembles a kind of foam.] It provides a concrete realization of how to compute this quantity, this sum of geometries. It tells you how geometry evolves, what the dynamics of the quantum gravitational field are.

There are a select few who propose a merger between the two camps: loop quantum gravity and string theory. Do you believe there is compatibility between the two?

On both sides, we are theoretical physicists, we have been trained in quantum field theory and in general relativity, and we are using these basic tools in our work. Many specific techniques used by the two communities are similar, even if this is not always recognized. If you look at how different people conduct research on loop quantum gravity, you will find a lot of different techniques. We use certain terms and if you just check in the physics literature for string theory, you will find the exact same terminology. This means that both communities are somehow using the same things, and I think we are converging on the realization that some tools, some mathematical tools, are very useful to both sides. But we have different perspectives on how to implement them.

You can think of string theory and loop quantum gravity as one emerging from the particle physics community and the other emerging from the general relativity community. One camp stresses the importance of preserving the techniques that come from particle physics, while the other community stresses the importance of preserving what we have learned from general relativity. But, if we want to write down a theory of quantum gravity, we have to converge at a certain point.

String theory doesnt really include a well-defined theory of quantum gravity. It was born to answer other questions, like unification of forces and naturalness and so on. As a by-product it was supposed to have a quantum theory of gravity in its belly. When evaluating string theory, you should be careful, because you have to be clear about what the questions are that you want to answer with that theory. With loop quantum gravity, it is easier, because loop quantum gravity is born to answer one question, that is, How do we quantize gravity? Its a bit more of a humble question with respect to the other ones. I also think that because the question is more precise, we have been most successful with respect to that particular goal.

One fundamental property of the loop quantum gravity theory is that it allows for black holes to transition into white holes, which are the opposites of black holes, spitting out matter and energy, as opposed to consuming everything.

Oh, well, yes. The fundamental property is the fact that space-time is quantized, where you have fundamental grains of space-time. But this also has a counterpart, the fact that curvature is bounded. This means that the singularities that appear in general relativity are resolved in a quantum theory of gravity. Applied to the early universe, in cosmology there is a big bounce. The same thing happens in the center of a black hole.

Instead of having a singularity in the center, we just have a region that is very, very dense. When such a high density is reached, an effective quantum force appears like a repulsion that prevents the collapse of the black hole from continuing and triggers a new expanding phase that corresponds to a white hole. The idea that you can have a transition from a black hole to a white hole was something explored in the past, in the form of the Einstein-Rosen bridge [aka wormhole]. But the idea was that you would end up being in another region of space-time.

The insight that changes the picture is the possibility to have a transition to a white hole within our universe. Imagine you have some matter that starts collapsing, gets more and more dense, and in the process turns it into the very compact object that is a black hole. Then it reaches the maximal density and also a core of minimum size. At that point it starts to expand back, its as if you were seeing a movie in rewind, and everything starts expanding in the form of an explosion. If you can have things exploding back, this can also provide a possible window to search for quantum gravity, and we can look for traces of such an explosion.

So how does loop quantum gravity theory allow for the computation of the black hole lifetime?

The kind of computation that we are doing in quantum mechanics involves computing probabilities to go from one state to another. At this stage, we are configuring states of the geometry itself, so we have a state that corresponds to classical geometry. Take for example the classical geometry of a black hole. In other states, that corresponds to another classical geometry, which is the classical description of the white hole. The two states are, basically, exactly the same except for a flip of the curvature, one the inverse of the other with respect to the curvature.

In loop quantum gravity, we can write down the transition amplitudes, i.e. probabilities to go from one state to the other. While this is a quantum process, it can be connected to the time measured by an external observer for this process to happen. This research is something that is ongoing, so what is nice is that we have a theory to write down, concretely, these probabilities. Computing these discretions, these integrals, is complicated. One possibility is to try to solve these equations numerically, which is something we are doing now.

At this time, its not clear if we can have a transition to a white hole when the black hole is macroscopicvery big. But the probability becomes very high when the black hole is smaller, as in the case of a black hole that has undergone the Hawking evaporation and become very small. In that case, you really have a situation where the probability for this black hole to become a white hole is just one. Its unavoidable. Its possible you may end up having a very small compact object of Planck size that is in a superposition. They are black and white at the same time because they behave like quantum objectsso they can be in two states at the same time.

So this will be a manifestation of what it means for space-time to be quantized. In the quantum theory, superposition is one of the main features. For quantum gravity, being in a superposition means being in the superposition of different geometries. Here, I was giving you an example where you can really have a proposition of two different geometries, a black and a white geometry, together and in the same object.

How much influence do you think that different epistemologies have on our current understanding of theoretical physics?

Philosophy has always had a strong influence in the way in which we do physics. This was true for all of the great moments in physics. Both general relativity and quantum mechanics were born in a fertile philosophical environment, influenced for instance by the philosophy of Mach, or the philosophy of Poincar. Einstein was even influenced by Schopenhauer. Those philosophers urged us to look at a world beyond the physics given by Newtonian mechanics. The twentieth century revolutions in physics were really inspired and fueled by philosophers.

Sometimes people dont have a clear idea of what philosophy means. They have some romantic idea that philosophy is about the meaning of life, something like that. Instead, philosophy is a set of tools. The same way in which physics is a set of tools to think about the world using the language of mathematics, philosophy is also a sharp set of tools to distinguish clearly what the premises are of our way of thinking. Its like a cleaning method for thinking.

I think that there is no physicist who does physics without being guided by some kind of philosophy. What I mean is that you should have a picture of what youre doing, what your methods are, what your goal is, and what you want to understand. Without a clear philosophy, you risk doing bad physics. All the great physicists, of the past and of the present, had very good training in philosophy.

Can you share the philosophy that guides your own work within theoretical physics?

My way of professing in physics is not about the unification of forces, but I do have a drive towards looking for a common language, something that provides consistent features for all the different things that we have learned in fundamental physics. For me, what is more striking is how there is a form of relationality that emerges in all the different fields of contemporary physics. From quantum mechanics to general relativity to quantum field theory to quantum gravity, we understand better and better that all of the properties that we describe for physical systems always have to be related to something else. You can think about this something else as an observer or a reference system.

We always have to specify with respect to which system we are describing a given property, even things that are supposed to be at the foundation of our understanding of the world, like things. What do we call a thing? Do we call a thing a particle? Even the answer to questions about how many particles there are, how many particles a device detects, depends on the respective system we give this property. Then you start realizing that instead of thinking about the world in terms of things, its better to think about the world in terms of relations.

My philosophical background was also informed by the feminist philosophy of sciences. Feminist philosophers have been debating how you can have a notion of objectivity while there are so many different standpoints. From each standpoint, you have access to a bit of truth, but if you want to have a full description of reality, somehow again you have to accept this plurality of standpoints and eventually find a way to put them together. I believe this gives you a powerful key to interpret the world in which we live.

In the moment in which you confront yourself with what the new physics is telling you, there are two possible ways to react to this as a physicist. You can reject what nature is telling you, hanging on to some very rigid notion of truth and objectivity; or you may embrace it and try to look for the consequences of it. For me, embracing this second possibility is much more easy, and I see this as a very rich and fruitful way of thinking.

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Loop quantum gravity: A candidate theory of quantum gravity that focuses on loops or discrete chunks organizing the geometry of quantum spacetime.

Path integral: Path integral formulation, devised by American physicist Richard P. Feynman, is a mathematical theory for understanding all the possible paths a quantum particle can take to get from one point to another.

Primordial quantum fluctuations: The initial conditions that seeded the universe and everything in it, including the galaxies, planets, and stars.

Quantum gravity: A theory, still in progress, explaining how gravity works on the quantum scale. A universally accepted theory of quantum gravity could bridge the gap between quantum physics and general relativity.

Spin foam: A 2-dimensional model with labeled edges, vertices, and faces with the goal of describing the geometry of quantum spacetime.

White hole: White holes are theoretical opposites of black holes. They spit out matter and energy, as opposed to consuming everything.

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Francesca Vidotto: The Quantum Properties of Space-Time - JSTOR Daily

Photo: Kelly Sikkema/Unsplash.

Quantum physics is strange. At least, it is strange to us, because the rules of the quantum world, which govern the way the world works at the level of atoms and subatomic particles (the behaviour of light and matter, as the renowned physicist Richard Feynman put it), are not the rules that we are familiar with the rules of what we call common sense.

The quantum rules, which were mostly established by the end of the 1920s, seem to be telling us that a cat can be both alive and dead at the same time, while a particle can be in two places at once. But to the great distress of many physicists, let alone ordinary mortals, nobody (then or since) has been able to come up with a common-sense explanation of what is going on. More thoughtful physicists have sought solace in other ways, to be sure, namely coming up with a variety of more or less desperate remedies to explain what is going on in the quantum world.

These remedies, the quanta of solace, are called interpretations. At the level of the equations, none of these interpretations is better than any other, although the interpreters and their followers will each tell you that their own favored interpretation is the one true faith, and all those who follow other faiths are heretics. On the other hand, none of the interpretations is worse than any of the others, mathematically speaking. Most probably, this means that we are missing something. One day, a glorious new description of the world may be discovered that makes all the same predictions as present-day quantum theory, but also makes sense. Well, at least we can hope.

Meanwhile, I thought I might provide an agnostic overview of one of the more colorful of the hypotheses, the many-worlds, or multiple universes, theory. For overviews of the other five leading interpretations, I point you to my book, Six Impossible Things. I think youll find that all of them are crazy, compared with common sense, and some are more crazy than others. But in this world, crazy does not necessarily mean wrong, and being more crazy does not necessarily mean more wrong.

If you have heard of the Many Worlds Interpretation (MWI), the chances are you think that it was invented by the American Hugh Everett in the mid-1950s. In a way thats true. He did come up with the idea all by himself. But he was unaware that essentially the same idea had occurred to Erwin Schrdinger half a decade earlier. Everetts version is more mathematical, Schrdingers more philosophical, but the essential point is that both of them were motivated by a wish to get rid of the idea of the collapse of the wave function, and both of them succeeded.

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As Schrdinger used to point out to anyone who would listen, there is nothing in the equations (including his famous wave equation) about collapse. That was something that Bohr bolted on to the theory to explain why we only see one outcome of an experiment a dead cat or a live cat not a mixture, a superposition of states. But because we only detect one outcome one solution to the wave function that need not mean that the alternative solutions do not exist. In a paper he published in 1952, Schrdinger pointed out the ridiculousness of expecting a quantum superposition to collapse just because we look at it. It was, he wrote, patently absurd that the wave function should be controlled in two entirely different ways, at times by the wave equation, but occasionally by direct interference of the observer, not controlled by the wave equation.

Although Schrdinger himself did not apply his idea to the famous cat, it neatly resolves that puzzle. Updating his terminology, there are two parallel universes, or worlds, in one of which the cat lives, and in one of which it dies. When the box is opened in one universe, a dead cat is revealed. In the other universe, there is a live cat. But there always were two worlds that had been identical to one another until the moment when the diabolical device determined the fate of the cat(s). There is no collapse of the wave function. Schrdinger anticipated the reaction of his colleagues in a talk he gave in Dublin, where he was then based, in 1952. After stressing that when his eponymous equation seems to describe different possibilities (they are not alternatives but all really happen simultaneously), he said:

Nearly every result [the quantum theorist] pronounces is about the probability of this or that or that happening with usually a great many alternatives. The idea that they may not be alternatives but all really happen simultaneously seems lunatic to him, just impossible. He thinks that if the laws of nature took this form for, let me say, a quarter of an hour, we should find our surroundings rapidly turning into a quagmire, or sort of a featureless jelly or plasma, all contours becoming blurred, we ourselves probably becoming jelly fish. It is strange that he should believe this. For I understand he grants that unobserved nature does behave this waynamely according to the wave equation. The aforesaid alternatives come into play only when we make an observation which need, of course, not be a scientific observation. Still it would seem that, according to the quantum theorist, nature is prevented from rapid jellification only by our perceiving or observing it it is a strange decision.

In fact, nobody responded to Schrdingers idea. It was ignored and forgotten, regarded as impossible. So Everett developed his own version of the MWI entirely independently, only for it to be almost as completely ignored. But it was Everett who introduced the idea of the Universe splitting into different versions of itself when faced with quantum choices, muddying the waters for decades.

It was Hugh Everett who introduced the idea of the Universe splitting into different versions of itself when faced with quantum choices, muddying the waters for decades.

Everett came up with the idea in 1955, when he was a PhD student at Princeton. In the original version of his idea, developed in a draft of his thesis, which was not published at the time, he compared the situation with an amoeba that splits into two daughter cells. If amoebas had brains, each daughter would remember an identical history up until the point of splitting, then have its own personal memories. In the familiar cat analogy, we have one universe, and one cat, before the diabolical device is triggered, then two universes, each with its own cat, and so on. Everetts PhD supervisor, John Wheeler, encouraged him to develop a mathematical description of his idea for his thesis, and for a paper published in the Reviews of Modern Physics in 1957, but along the way, the amoeba analogy was dropped and did not appear in print until later. But Everett did point out that since no observer would ever be aware of the existence of the other worlds, to claim that they cannot be there because we cannot see them is no more valid than claiming that the Earth cannot be orbiting around the Sun because we cannot feel the movement.

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Everett himself never promoted the idea of the MWI. Even before he completed his PhD, he had accepted the offer of a job at the Pentagon working in the Weapons Systems Evaluation Group on the application of mathematical techniques (the innocently titled game theory) to secret Cold War problems (some of his work was so secret that it is still classified) and essentially disappeared from the academic radar. It wasnt until the late 1960s that the idea gained some momentum when it was taken up and enthusiastically promoted by Bryce DeWitt, of the University of North Carolina, who wrote: every quantum transition taking place in every star, in every galaxy, in every remote corner of the universe is splitting our local world on Earth into myriad copies of itself. This became too much for Wheeler, who backtracked from his original endorsement of the MWI, and in the 1970s, said: I have reluctantly had to give up my support of that point of view in the end because I am afraid it carries too great a load of metaphysical baggage. Ironically, just at that moment, the idea was being revived and transformed through applications in cosmology and quantum computing.

Every quantum transition taking place in every star, in every galaxy, in every remote corner of the universe is splitting our local world on Earth into myriad copies of itself.

The power of the interpretation began to be appreciated even by people reluctant to endorse it fully. John Bell noted that persons of course multiply with the world, and those in any particular branch would experience only what happens in that branch, and grudgingly admitted that there might be something in it:

The many worlds interpretation seems to me an extravagant, and above all an extravagantly vague, hypothesis. I could almost dismiss it as silly. And yet It may have something distinctive to say in connection with the Einstein Podolsky Rosen puzzle, and it would be worthwhile, I think, to formulate some precise version of it to see if this is really so. And the existence of all possible worlds may make us more comfortable about the existence of our own world which seems to be in some ways a highly improbable one.

The precise version of the MWI came from David Deutsch, in Oxford, and in effect put Schrdingers version of the idea on a secure footing, although when he formulated his interpretation, Deutsch was unaware of Schrdingers version. Deutsch worked with DeWitt in the 1970s, and in 1977, he met Everett at a conference organized by DeWitt the only time Everett ever presented his ideas to a large audience. Convinced that the MWI was the right way to understand the quantum world, Deutsch became a pioneer in the field of quantum computing, not through any interest in computers as such, but because of his belief that the existence of a working quantum computer would prove the reality of the MWI.

This is where we get back to a version of Schrdingers idea. In the Everett version of the cat puzzle, there is a single cat up to the point where the device is triggered. Then the entire Universe splits in two. Similarly, as DeWitt pointed out, an electron in a distant galaxy confronted with a choice of two (or more) quantum paths causes the entire Universe, including ourselves, to split. In the DeutschSchrdinger version, there is an infinite variety of universes (a Multiverse) corresponding to all possible solutions to the quantum wave function. As far as the cat experiment is concerned, there are many identical universes in which identical experimenters construct identical diabolical devices. These universes are identical up to the point where the device is triggered. Then, in some universes the cat dies, in some it lives, and the subsequent histories are correspondingly different. But the parallel worlds can never communicate with one another. Or can they?

Deutsch argues that when two or more previously identical universes are forced by quantum processes to become distinct, as in the experiment with two holes, there is a temporary interference between the universes, which becomes suppressed as they evolve. It is this interaction that causes the observed results of those experiments. His dream is to see the construction of an intelligent quantum machine a computer that would monitor some quantum phenomenon involving interference going on within its brain. Using a rather subtle argument, Deutsch claims that an intelligent quantum computer would be able to remember the experience of temporarily existing in parallel realities. This is far from being a practical experiment. But Deutsch also has a much simpler proof of the existence of the Multiverse.

What makes a quantum computer qualitatively different from a conventional computer is that the switches inside it exist in a superposition of states. A conventional computer is built up from a collection of switches (units in electrical circuits) that can be either on or off, corresponding to the digits 1 or 0. This makes it possible to carry out calculations by manipulating strings of numbers in binary code. Each switch is known as a bit, and the more bits there are, the more powerful the computer is. Eight bits make a byte, and computer memory today is measured in terms of billions of bytes gigabytes, or Gb. Strictly speaking, since we are dealing in binary, a gigabyte is 230 bytes, but that is usually taken as read. Each switch in a quantum computer, however, is an entity that can be in a superposition of states. These are usually atoms, but you can think of them as being electrons that are either spin up or spin down. The difference is that in the superposition, they are both spin up and spin down at the same time 0 and 1. Each switch is called a qbit, pronounced cubit.

Using a rather subtle argument, Deutsch claims that an intelligent quantum computer would be able to remember the experience of temporarily existing in parallel realities.

Because of this quantum property, each qbit is equivalent to two bits. This doesnt look impressive at first sight, but it is. If you have three qbits, for example, they can be arranged in eight ways: 000, 001, 010, 011, 100, 101, 110, 111. The superposition embraces all these possibilities. So three qbits are not equivalent to six bits (2 x 3), but to eight bits (2 raised to the power of 3). The equivalent number of bits is always 2 raised to the power of the number of qbits. Just 10 qbits would be equivalent to 210 bits, actually 1,024, but usually referred to as a kilobit. Exponentials like this rapidly run away with themselves. A computer with just 300 qbits would be equivalent to a conventional computer with more bits than there are atoms in the observable Universe. How could such a computer carry out calculations? The question is more pressing since simple quantum computers, incorporating a few qbits, have already been constructed and shown to work as expected. They really are more powerful than conventional computers with the same number of bits.

Deutschs answer is that the calculation is carried out simultaneously on identical computers in each of the parallel universes corresponding to the superpositions. For a three-qbit computer, that means eight superpositions of computer scientists working on the same problem using identical computers to get an answer. It is no surprise that they should collaborate in this way, since the experimenters are identical, with identical reasons for tackling the same problem. That isnt too difficult to visualize. But when we build a 300-qbit machinewhich will surely happenwe will, if Deutsch is right, be involving a collaboration between more universes than there are atoms in our visible Universe. It is a matter of choice whether you think that is too great a load of metaphysical baggage. But if you do, you will need some other way to explain why quantum computers work.

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Most quantum computer scientists prefer not to think about these implications. But there is one group of scientists who are used to thinking of even more than six impossible things before breakfast the cosmologists. Some of them have espoused the Many Worlds Interpretation as the best way to explain the existence of the Universe itself.

Their jumping-off point is the fact, noted by Schrdinger, that there is nothing in the equations referring to a collapse of the wave function. And they do mean thewave function; just one, which describes the entire world as a superposition of states a Multiverse made up of a superposition of universes.

Some cosmologists have espoused the Many Worlds Interpretation as the best way to explain the existence of the Universe itself.

The first version of Everetts PhD thesis (later modified and shortened on the advice of Wheeler) was actually titled The Theory of the Universal Wave Function. And by universal he meant literally that, saying:

Since the universal validity of the state function description is asserted, one can regard the state functions themselves as the fundamental entities, and one can even consider the state function of the whole universe. In this sense this theory can be called the theory of the universal wave function, since all of physics is presumed to follow from this function alone.

where for the present purpose state function is another name for wave function. All of physics means everything, including us the observers in physics jargon. Cosmologists are excited by this, not because they are included in the wave function, but because this idea of a single, uncollapsed wave function is the only way in which the entire Universe can be described in quantum mechanical terms while still being compatible with the general theory of relativity. In the short version of his thesis published in 1957, Everett concluded that his formulation of quantum mechanics may therefore prove a fruitful framework for the quantization of general relativity. Although that dream has not yet been fulfilled, it has encouraged a great deal of work by cosmologists since the mid-1980s, when they latched on to the idea. But it does bring with it a lot of baggage.

The universal wave function describes the position of every particle in the Universe at a particular moment in time. But it also describes every possible location of those particles at that instant. And it also describes every possible location of every particle at any other instant of time, although the number of possibilities is restricted by the quantum graininess of space and time. Out of this myriad of possible universes, there will be many versions in which stable stars and planets, and people to live on those planets, cannot exist. But there will be at least some universes resembling our own, more or less accurately, in the way often portrayed in science fiction stories. Or, indeed, in other fiction. Deutsch has pointed out that according to the MWI, any world described in a work of fiction, provided it obeys the laws of physics, really does exist somewhere in the Multiverse. There really is, for example, a Wuthering Heights world (but not a Harry Potter world).

That isnt the end of it. The single wave function describes all possible universes at all possible times. But it doesnt say anything about changing from one state to another. Time does not flow. Sticking close to home, Everetts parameter, called a state vector, includes a description of a world in which we exist, and all the records of that worlds history, from our memories, to fossils, to light reaching us from distant galaxies, exist. There will also be another universe exactly the same except that the time step has been advanced by, say, one second (or one hour, or one year).

But there is no suggestion that any universe moves along from one time step to another. There will be a me in this second universe, described by the universal wave function, who has all the memories I have at the first instant, plus those corresponding to a further second (or hour, or year, or whatever). But it is impossible to say that these versions of me are the same person. Different time states can be ordered in terms of the events they describe, defining the difference between past and future, but they do not change from one state to another. All the states just exist. Time, in the way we are used to thinking of it, does not flow in Everetts MWI.

John Gribbin is a Visiting Fellow in Astronomy at the University of Sussex, UK and the author of In Search of Schrdingers Cat, The Universe: A Biography and Six Impossible Thingsfrom which this article is excerpted.

Thisarticlehas been republished fromThe MIT Press Reader.

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During the Independent Activities Period in 2018, senior Michelle Xu worked with the volunteer group Cross Cultural Solutions at the Ritsona refugee camp in Greece, through the Priscilla King Gray Public Service Center. I may not make a career out of public service, but I am a human being, and just like any other human being, helping the world is important to me, Xu explains. Credit: Ian MacLellan

MIT Senior Michelle Xus varied interests all involve a desire to understand the universe. I was just never particularly picky about which way to figure it out, she says.

A day in the life of Michelle Xu might include attending a quantum gravity seminar over Zoom, followed by some reading on the philosophy of time, capped off by a couple of hours of writing fiction.

If these activities seem wildly diverse, for Xu they all emerge from the same place: this desire to understand how the universe works, she says. I was just never particularly picky about which way to figure it out.

Xu is a senior majoring in physics and mathematics, with an added focus on philosophy. Her studies have centered on large questions in cosmology, including looking at the earliest days of the expanding universe through their impact on primordial black holes with Professor Alan Guth in the MIT Center for Theoretical Physics. Lately Xu has been studying high energy theory and quantum gravity under the guidance of Professor Daniel Harlow, both topics which she hopes to continue studying in graduate school at Stanford University next fall. Throughout her time in the physics department, professors Robert Jaffe, Tracy Slatyer, and David Kaiser have been strong role models and mentors as well, she says. My path in physics has been shaped and encouraged by all of these people, and without them, I wouldnt be where I am today.

During the Independent Activities Period in 2018, senior Michelle Xu worked with the volunteer group Cross Cultural Solutions at the Ritsona refugee camp in Greece, through the Priscilla King Gray Public Service Center. I may not make a career out of public service, but I am a human being, and just like any other human being, helping the world is important to me, Xu explains. Credit: Ian MacLellan

Although she was interested in physics when she first came to MIT, it was the research experience that confirmed for her that she was on the right career path. My biggest doubt was, OK, so I can do [problem sets], and I enjoy thinking about these concepts, but if I were tossed a bunch of equations and had to create something myself, could I actually do this? Xu recalls. Each summer as I worked on a different research project, I became more and more convinced that this was something I could do.

At home in Pennsylvania during the coronavirus pandemic, Xu is continuing her research with Guth and hopes to meet virtually with Harlow as well. She is staying touch with friends through social media, even starting a book club while they are scattered throughout the country. Ive been stripped of some of my usual responsibilities, like running clubs, so Im focusing more on personal interests like writing and some puzzling topics in physics and philosophy, she says.

Xus parents are scientists, and she was raised in a household where everything was approached from a scientific perspective, she says. They watched a lot of science documentaries, like Brian Greenes The Elegant Universe, that raised early questions about the nature of reality.

It was the class 24.02 (Moral Problems and the Good Life) that inspired Xu to delve deeper into philosophy as another way to probe reality. She later discovered that most of her philosophical interests lie in metaphysics and not ethics, but the problems were nevertheless interesting enough to get her hooked initially. She recalls one class discussion centered around morality and meaning in ones life, in relation to ideas like motivation and duty, that sparked an intense discussion with the classs teaching assistant. I got nerd sniped, Xu jokes. When someone poses such an interesting question or argument, you have to just drop everything to reply to it.

The TA invited her to sit in on a graduate philosophy reading group, and Xu also joined the MIT Undergraduate Philosophy Club and became a member of its executive board. She spent the spring 2019 semester at Oxford University studying philosophy and physics and in the summer participated in a weeklong summer school on mathematical philosophy for female students at Ludwig Maximilian University.

The jargon of academic philosophy can be as dense as physics terminology, Xu admits, but I think everyone could use a little philosophy in their lives. I think questions about life and the world around us can be structured in fascinating ways through the different modes of thinking in philosophy.

Thoughts about morality and responsibility came into focus for Xu during the Independent Activities Period in 2018, when she worked with the volunteer group Cross Cultural Solutions at the Ritsona refugee camp in Greece, through the Priscilla King Gray Public Service Center. People have asked her how the volunteer work fits in with her other academic interests, and she says the short answer is that it doesnt.

I may not make a career out of public service, but I am a human being, and just like any other human being, helping the world is important to me, Xu explains. Out there, I can do what any human can do do laundry or distribute food, and help people through an incredibly difficult time of their lives.

Xu shared her experiences at the refugee camp in writing, another long-time interest of hers. Inspired by the interdisciplinary science magazine Nautilus and looking for writing partners, Xu founded Chroma, MITs student-run science and humanities magazine. As editor-in-chief, she has been proud to encourage new writers, artists, and designers on campus to cross-pollinate ideas.

I think MIT is one of the few places where something like this can blossom, because everyone here is interested in the sciences in some way, she says.

Xu mostly writes fiction these days, which she calls variably OK, but hopefully improving. Last fall she took the class 21W.755 (Writing and Reading Short Stories) to sharpen her skills, because I have these things that I want to express in my writing but feel like I lack the technique to do. But especially now that Im quarantined, Im trying to write more just getting the reps in.

Writing also helps her grapple with the nature of reality in a different way, she says. To write is to build another reality. And to build something, you have to understand it.

Despite her consistent interest in the fundamental nature of reality, Xu says she does sometimes worry that perhaps she is spread across too many departments. If I want to do something significant and contribute to this world, does that mean I am lacking focus to do that correctly?

But I think you have to stay true to doing the things that pull you in, and thats the only way you can make a significant contribution to the world.

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