YouTube Video Lectures: Thinking about Quantum Gravity
By TEJINDER PAL SINGH • Jan. 21, 2019 @ 18:09 GMT

There is likely a deep connection between the study of quantum foundations on the one hand, and the much sought after quantum theory of gravity on the other. Despite the enormous success of quantum theory, there are issues in our understanding of the theory, which need addressing. These include: the nature of the quantum to classical transition, the peculiar nature of quantum non-locality, the problem of time in quantum theory, the extreme dependence of the theory on its own classical limit, and the physical meaning of the wave function. Could it be that addressing these issues requires us to reformulate / modify quantum theory, in such a way that we get rid of the theory's dependence on its own limit, and on classical space-time? If that is the case, then introducing non-classical space-time in quantum theory naturally leads us to a falsifiable quantum theory of gravity. This is the viewpoint developed in the ongoing video lecture series `Thinking about Quantum Gravity'. The lectures are addressed to those undergraduate and graduate students in physics who would like to research in quantum gravity. It is not expected that the viewer will agree with everything that is said in these lectures. Rather, it is hoped that you will find something to think about, as you develop your own thinking towards quantum gravity.



The first video is available on YouTube and every video gives the link to the next one. Your comments and criticisms will be greatly appreciated. Thank you.

--

Tejinder P. Singh

Tata Institute of Fundamental Research, Mumbai


2018: The Physics Year in Review
By ZEEYA MERALI • Dec. 31, 2018 @ 21:44 GMT

iStock/yuriz

It’s that time of year again! As 2018 comes to a close, we’re counting down the highlights (and lowlights) of the year in physics, as chosen by FQXi member and quantum physicist Ian Durham. Listen to tales of exploding labs, Nobel controversies, smashed records, foundational breakthroughs, and enterprising slime mold.

Both parts of the podcast review are now up for you to enjoy. Let us know what we missed, what we should have placed higher, and if you disagree with our picks.

With best wishes for a healthy and happy 2019 from all of us at FQXi!




Grants awarded for research on "Agency in the Physical World"
By ANTHONY AGUIRRE • Nov. 28, 2018 @ 20:02 GMT

At the end of last year, FQXi launched an exciting new partnership with the Fetzer Franklin Fund to investigate the question of how agency arises in the physical world. This far-reaching topic brings together ideas from physics, information theory, biology, artificial intelligence, neuroscience, and other fields. After a competitive selection process, we’re happy to announce the 24 projects that will be funded through our seventh Large Grant program.

The total amount given out in this round is $2M, which will be used to fund projects over the next two years. Here are some of the fascinating questions being tackled by our grantees:

1. Can consciousness be modelled mathematically?

2. Would a quantum spacetime be agent-dependent? Or would agency depend on a quantum spacetime?

3. Why do our choices seem to be able to impact the future, but not the past? How is agency related to time’s arrow?

4. Do humans have free will? And, if so, could machines have free will?

5. What would a quantum agent, itself existing in superposition, see?

We again congratulate our new grantees. We also thank everyone who applied, especially those who were invited to submit full proposals; we appreciate that these took a great deal of time and resources to prepare. Our mission at FQXi has always been to push boundaries, and to try to focus the attention and effort of the scientific community on (what we consider to be) super-interesting areas of research that for one reason or another have gone less explored than they deserve. We received many more excellent proposals than we were able to fund in this round, but we do hope to offer more grants in the future, and as always we encourage people to apply for those.

We should also mention that our collaboration with the Fetzer Franklin Fund has already supported the co-themed essay contest, “What Is Fundamental?,” which produced some outstanding submissions. You can still read and enjoy all the entries on our site.


Superhuman: Book Review and Special Podcast
By ZEEYA MERALI • Sep. 10, 2018 @ 20:48 GMT

Simon and Schuster

In the quantum realm, the act of observing something—a photon, or an electron, say—can disturb or change its properties. In a very real, physical sense, we construct reality just by looking it.

This quantum quirk came to mind while I was reading Superhuman: Life at the Extremes of Our Capacity, by evolutionary biologist Rowan Hooper. I learned that biologists can now visualise the physical form of memories in the brain—yes, you should think of the wispy, tendrilous structures that Dumbledore extracts from Harry Potter’s mind, says Hooper—and that the act of remembering degrades the accuracy of our memories each time we try to look back and recall a past event. Hooper also explains that we have evolved a flawed memory as a kind of defence mechanism that allows us to edit out the bad parts of our personal history, subconsciously reconstructing reality to make it more palatable, day by day.

By pointing out this poetic resonance between quantum physics and memory, I do not mean to suggest that we should expect to find that consciousness and intelligence are directly controlled by quantum processes in the brain (although just how these higher-level properties emerge from mindless physical laws are exactly the kinds of issues that FQXi researchers may soon be tackling as part of our Agency in the Physical World program). In fact, Hooper, who joins me on a special edition of the podcast, warns against looking for genetic building blocks for complex traits, in an overly simplistic way. But these are just some of the fascinating facts about the workings of our minds and bodies that I pulled from Hooper’s treasure trove of a book, which skilfully combines conversations with some of the most extraordinary people alive with meticulously researched ideas from the frontiers of genetics, in an effort to unpick what makes the best of us excel.

On the podcast, Hooper chats about his hunt for the intangible roots of intelligence. Is it in our genes? Or our upbringing? He’s chosen some pretty smart people to help him examine these questions: math prodigy and chess grandmaster John Nunn, (double) Booker-prize winning author of the historical novels Wolf Hall and Bring Up the Bodies, Hilary Mantel, and Nobel laureate and (appropriately enough) geneticist, Paul Nurse. Hooper quickly dismisses the “bogus” nature-v-nurture conflict often promoted in the media, stating that both undoubtedly play a role in fostering intellectual achievement. Nonetheless, the importance of genetics and innate talent is striking and, in the book, Hooper addresses whether we should feel discomfited, or maybe instead empowered, by this.

Our chat only touches on a handful of the traits that Hooper investigated for the book. Beyond intelligence and memory, Hooper met with polyglots who speak dozens of languages, round-the-world sailor Ellen MacArthur—who was apparently focused on her nautical goal from the tender age of four—and people with exceptional singing and sporting talent. If that’s not enough to make you feel inadequate, Hooper brings us lucid dreamers, who practise language and dart-throwing skills, literally in their sleep.

As well as having been a research biologist, Hooper is a seasoned writer. He is the managing editor at New Scientist, and brings some of the magazine’s trademark lightness of touch to his anecdotes. (When meeting a minimalist who has given up all but 100 material possessions in his quest for happiness, Hooper understandably ponders how many pairs of undergarments he must own. Spoiler alert: two pairs of pants.)

Hooper’s storytelling truly shines towards the end of the book, in its most moving, and most humbling, chapters. While we admire many of the people he meets in earlier chapters for the drive and focus that motivated them to pursue excellence from a young age, others had greatness thrust upon them later in life, when they met with profound misfortune. Here we meet survivors of horrific physical assault, war injuries, and diseases that left them near-death. In one of his most mind-blowing encounters, Hooper visits a woman with locked-in syndrome—almost entirely paralysed by a stroke and able to communicate only through eye movements—who earned two degrees following her paralysis, and reports being happier now than before the stroke.

It’s these personal tales of hope and resilience that stay with you long after you finish the book, inviting you to reassess life: What really makes us happy? What hidden strengths might we tap in to, at times of adversity? How can we reconstruct our realities to write the best stories for our lives—harnessing our own superhuman abilities?


Space-time from Collapse of the Wave-function
By TEJINDER PAL SINGH • Sep. 9, 2018 @ 18:40 GMT

iStock/sdominick

The world of large things such as tables, planets, stars and galaxies, is extremely different from the world of small things such as electrons, protons, atoms, and photons. The most striking difference is that a table is never found in more than one place at the same time, whereas as electron or an atom can be in many places at the same time. Why should there be this difference? After all, a table is nothing but a collection of an extremely large number of atoms. Why is it that when a lot of atoms are put together to make a large object, the property of being in more than one place is lost?

In physics, simple sounding questions sometimes have far-reaching consequences, when their answers are found. That seems to be the case here too. When we understand why a table cannot be in more than one place at the same time, we also understand where space and time come from! We have recently shown that a remarkable new physical mechanism, known as spontaneous localization, is at play, and is responsible for the emergence of space-time, and also for the aforementioned property of large objects (Tejinder P. Singh, "Space and time as a consequence of GRW quantum jumps," arXiv:1806.01297 [gr-qc] (2018), to appear in Zeitschrift fur Naturforschung A).

Quantum theory, which gives the rules for the motion of microscopic objects such as electrons, does not provide an explanation for this difference between a table and an electron. This is all the more surprising considering that the theory is extremely successful in explaining a great variety of phenomena, and is not contradicted by any experiment. The motion of large objects is described by Newtonian mechanics, which is generally assumed to be a limiting case of quantum mechanics. There is however a catch here: Newtonian mechanics can be derived as a limiting case of quantum mechanics only if we additionally assume that large objects are localized, and cannot be in more than one place simultaneously. The unexplained difference between large and small remains unexplained; it has been with us ever since the birth of quantum mechanics. It is also at the heart of the so-called quantum measurement problem, and has been immortalized by the so-called Schrodinger's cat paradox.

Could it be that apart from quantum mechanics and Newtonian mechanics, there is a new mechanics, to which the former two are approximations? This new mechanics will have a built-in mechanism such that while any object can indeed be in a superposition of the states 'here' and 'there', the superposition does not last forever. The superposition is extremely short lived if the object is made of a very large number of atoms, but it lasts for enormously long times for small systems such as electrons and individual atoms. Such a new mechanics indeed exists and is known as the theory of Spontaneous Localisation. It was proposed by Ghirardi, Rimini, Weber and Pearle in the 1980s, and is popularly also known as the GRW theory.

In quantum theory, the state of a system is described by a complex mathematical object known as its wave function. The wave function evolves according to the Schrodinger equation, which is a deterministic and linear equation. The property of an electron being here and there at the same time is described by a state whose wave-function is a linear superposition of the two wave functions: electron here, and electron there. The theory of Spontaneous Localisation (SL) says that every superposition undergoes random and spontaneous collapse to one or the other alternatives (here, or there), with collapse happening very rarely for microscopic objects, and extremely frequently for macroscopic ones. Note that SL is different both from quantum theory as well as from Newtonian mechanics. In quantum theory, which obeys the Schrodinger equation, spontaneous collapse never takes place. In Newtonian mechanics, there are no superpositions in the first place, so there is no question of collapse. SL provides a middle ground, a bridge so to say, between quantum and classical mechanics. The collapse of the wave function during a quantum measurement is a special case of spontaneous collapse, resulting from a sudden interaction between a microscopic system and the macroscopic measuring apparatus. A table stays put in one place because its wave-function repeatedly and very rapidly keeps collapsing spontaneously, thus preventing superpositions. The experimental predictions of SL differ slightly from those of quantum theory, and laboratory tests of SL have presently entered an exciting stage. If confirmed, SL will be a radical generalization of quantum theory. Apart from solving the measurement problem, it will have far reaching implications for our understanding of space-time structure, and of quantum gravity.

Space, as we usually understand it, is a classical construct. It is that which is between objects, between tables and chairs, and between planets, stars and galaxies. But if all these classical objects are staying localized because of spontaneous collapse, is it not plausible that space by itself is a consequence of collapse of the wave-function? We might want to think of space as absolute, as a given, in which objects are embedded. But this viewpoint is challenged by quantum theory. Imagine a universe consisting only of quantum mechanical objects (and having nothing classical), each of which is 'everywhere'. What physical meaning there is then, to space? Furthermore, the time parameter that appears in the Schrodinger equation is a classical parameter, being a part of classical space-time geometry. There is a consequence of the so-called Einstein hole argument that in a universe in which there are no classical objects and everything is quantum, it is not physically meaningful to assume the point structure of a space-time manifold. From the point of view of GRW theory, suppose no spontaneous collapse has yet taken place in the universe: there would then be no classical objects, nor a classical space-time. For these reasons, it becomes evident that, GRW or otherwise, there ought to exist a formulation of quantum theory without classical space-time. From this formulation, space-time and classical objects are recovered via spontaneous collapse. Only, now the mechanism is more general than the GRW model, because the setting in which collapse now takes place does not have classical time to start with. We call this Relativistic Spontaneous Localisation (RSL).

If space-time results from collapse of the wave functions of macroscopic objects, what is there, in place of space-time, prior to collapse? Ignoring gravity for now, we recall that ordinary space-time is described by the Minkowski line-element of special relativity, for the coordinates (x,y,z,t). Here, there is the elegant symmetry principle, namely that physical laws are the same for all inertial observers, and their respective coordinates are related to each other via Lorentz transformations. We propose a minimal generalization of this symmetry principle: physical laws are the same for all inertial observers, and their respective coordinates are related to each other via Lorentz transformations, but the coordinates no longer commute with each other. They become operators (equivalently matrices) (x,y,z,t), which have arbitrary commutation relations amongst them. Since time has become an operator, and is no longer a scalar parameter, it cannot be used to described evolution. Instead, evolution is described by the so-called Trace time, which is arrived at by taking the matrix trace of the operator Minkowski line-element. Matter degrees of freedom are also non-commuting; they 'live' on this operator spacetime, and the dynamics is exactly analogous to that of ordinary special relativity. We refer to this as noncommutative special relativity.

It turns out, remarkably, that in a certain thermodynamic approximation, whose details we do not discuss here, noncommutative special relativity is identical to relativistic quantum mechanics on an operator Minkowski space-time. Space-time coordinates continue to be operators, but these now commute with each other, and with the matter degrees of freedom as well. Each 'particle' is represented by a four- operator q_i^a and in 'position' representation the wave function, evolving according to a Lorentz invariant Schrodinger equation, depends on the eigenvalues x_i^a of the position four-operator, there being four such eigenvalues for every particle. This is the sought for quantum theory without classical time, and here evolution takes place in trace time. We could as well have taken this as the starting point for invoking relativistic spontaneous localization; however starting from a non-commutative special relativity underscores the underlying symmetries of the theory.

The beauty of this formulation is that the state vector lives in an extended Hilbert space, i.e. one that is endowed with the operator space-time metric. And this is the whole physical universe! There is no longer any external 3-space, nor external time, which somehow are otherwise uneasily latched on to the conventional Hilbert space of quantum mechanics.

When spontaneous localization takes place in this extended Hilbert space, every macroscopic matter degree of freedom is localized to some or the other eigenvalue of the space-time operator, as a result of spontaneous collapse of their wave-function. The full set of 'signposts' provided by these collapsed objects gives the extended Hilbert space a semblance of a classical space-time, in which macroscopic systems are embedded. In this sense collapse of the wave-function is responsible for the emergence of space-time, and we see that there is a deep connection between the problem of time in quantum theory, and the measurement problem. The microscopic degrees of freedom continue to live in the extended Hilbert space--their true home--though from the vantage point of the classical space-time produced from collapse of macroscopic objects, their dynamics appears the same as ordinary quantum mechanics, supplemented by the possibility of spontaneous collapse in space via the GRW mechanism.

How might we be sure that the idea of an extended Hilbert space, and space-time emerging because of collapse of the wave-function, is correct? There are at present two pieces of evidence for operator time and extended Hilbert space. Firstly, it helps understand the EPR paradox and peculiar nature of quantum non-locality. The physics of quantum measurement on correlated entangled particle pairs can be correctly understood only in the extended Hilbert space, because it involves collapse, and collapse takes place in operator space-time. When Alice makes a measurement on one of the particles in the pair, it instantaneously and simultaneously affects the other particle--but simultaneously in trace time. There being no notion of distance and separation in operator space-time, no influence travels from the first to the second particle. Hence, when Bob makes a measurement outside Alice's light cone, and we refer the physics to ordinary space-time, there is indeed a causality puzzle, but it is only because we have chosen the wrong frame to describe collapse. In reality, when quantum theory, operator space-time and collapse are combined, there is nothing unphysical or puzzling about quantum non-locality.

Secondly, in the extended Hilbert space, the wave-function of a particle has a non-zero amplitude to be at more than one time, for a given trace time, because time is now an operator. As a result, we predict that there will be a quantum interference in time. A particle can go through a slit now, and come back later to go through it again, and these two states (same place, different times) will interfere with each other! Interestingly, atomic physics experiments done in the past have reported results which can be interpreted sensibly only as quantum interference in time. This is a promising avenue which needs to be investigated carefully in the future, as possible evidence for relativistic spontaneous localization.

Outstanding remaining challenges in this program include generalization to quantum field theory, and incorporating gravity. Nonetheless, it appears satisfying that the tension between quantum theory and relativity can be released by generalizing to a non-commutative special relativity, and then invoking collapse of the wave-function to recover ordinary space-time. Our physical universe is a collection of wave-functions residing in the extended Hilbert space, and space-time is its emergent macroscopic limit.

(Edited on 12 September 2018 to add that in a follow-up paper I predict a new effect, quantum interference and spontaneous localisation in time. This should be testable in the lab: arXiv:1809.03441 [gr-qc] (2018).)

--

Tejinder Singh is an FQXi member and a physicist at the Tata Institute of Fundamental Research, Mumbai, India.

Reference: Space and time as a consequence of Ghirardi-Rimini-Weber quantum jumps; arXiv:1806.01297v4 [gr-qc] (2018), to appear in Zeitschrift fur Naturfortschung A. This work was supported by an FQXi mini-grant.

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