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Anonymous: "North Korea's unilateral action of announcing it was shutting down its only..." in Whose Physics Is It...

Georgina Woodward: "Hi Heinrich I'm not convinced that prior knowledge or experience is..." in Why Time Might Not Be an...

Heinrich Luediger: "Hi Georgina, I said way or another. I'm sure you could have..." in Why Time Might Not Be an...

Joe Fisher: "Dear Board of Directors, Irrefutable evidence exists that..." in Whose Physics Is It...

Georgina Woodward: "Hellen jos, you probably only wanted to place a link, however FYI lots of..." in Does Quantum Weirdness...

Eckard Blumschein: "There was perhaps not yet a third crisis of cosmology conference after..." in The Quantum...

Jonathan Dickau: "For what it is worth... I was there! The paper by Louis Marmet cited..." in The Quantum...

Ashish Kochaar: "No words for the Quantumology. As per their figures and Dates in the May..." in Deferential Geometry

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Whose Physics Is It Anyway? Q&A with Chanda Prescod-Weinstein
Why physics and astronomy communities must take diversity issues seriously in order to do good science.

Why Time Might Not Be an Illusion
Einstein’s relativity pushes physicists towards a picture of the universe as a block, in which the past, present, and future all exist on the same footing; but maybe that shift in thinking has gone too far.

The Complexity Conundrum
Resolving the black hole firewall paradox—by calculating what a real astronaut would compute at the black hole's edge.

Quantum Dream Time
Defining a ‘quantum clock’ and a 'quantum ruler' could help those attempting to unify physics—and solve the mystery of vanishing time.

Our Place in the Multiverse
Calculating the odds that intelligent observers arise in parallel universes—and working out what they might see.

April 26, 2018

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Cosmic Dawn, Parallel Observers, and a Science Hostel in Maui: New Podcast
By ZEEYA MERALI • Mar. 21, 2018 @ 20:04 GMT

EDGES antenna, by Suzyj, Wikicommons
This month’s podcast features the exciting discovery of signs of the first stars made by astronomers using the EDGES experiment, in Western Australia (right), published in Nature, in February. It’s long been predicted that they should see such an indirect signal, which they picked up as a dip in the intensity of radiation in the cosmic microwave background (the afterglow of the big bang). But while this signal was where they thought it would be, and confirmed when they thought the first stars appeared — some 180 million years after the big bang — the detection raised new puzzles. The signal was far stronger than had been predicted. So, I spoke with cosmologist Rennan Barkana, of Tel Aviv University in Israel, who published a companion paper in the same edition of Nature, offering a possible solution: the boosted signal could be caused by an unexpected interaction with dark matter, in the early universe.

Free Podcast

Remembering Stephen Hawking; light from the first stars in the universe, with Rennan Barkana; our place in the multiverse, with Eugene Lim; & setting up a science hostel in Maui, with Garrett Lisi.


Go to full podcast

Next, reporter Sophie Hebden chatted to cosmologist Eugene Lim, of King’s College London, about what we may be able to infer about observers in parallel universes. Lim, and his colleague Richard Easther, at the University of Auckland, are examining the possibility that we live in a multiverse of neighbouring cosmoses that each have different physical laws. But how likely is it that sentient observers will arise in those regions? What are the minimal set of physical properties needed for such observers to evolve? And what might our multiversal neighbours be able to measure? Answering such questions might help explain why our universe has the peculiar rules that it does. (You can read more about Lim and Easther’s work in Sophie's article, "Our Place in the Multiverse.")

And, if you're wondering what we do when we're not podcasting, the answer, for Brendan Foster at least, is he enjoys relaxing in Maui. But on this holiday, he took some time to meet with theoretical physicist Garrett Lisi, who has opened a hostel for scientists to visit and spend time working. Listen now to hear Brendan’s verdict on whether staying in such an idyllic location can be productive for research.

Finally, we've been away for a while. In the meantime, we saw the sad passing of two giants of theoretical physics, Joe Polchinski and Stephen Hawking. The latter died after we recorded the main edition, but we've added a few words to commemorate these huge losses. Both shall be missed.
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Remembering Stephen W. Hawking (1942-2018)
By DON PAGE • Mar. 19, 2018 @ 15:46 GMT

Stephen Hawking was the greatest theoretical gravitational physicist since Einstein. Using the assumptions of Einstein's classical theory of gravity, general relativity, plus reasonable conditions on the energy density and pressure of matter, Hawking showed that the observed expansion of the universe would imply that the universe had a beginning, a big bang singularity at the beginning of time. Under similar assumptions, he showed that the surface of a black hole, called the event horizon, would have an area that cannot decrease with time but could only stay the same (if nothing fell into the black hole) or get bigger (if energy did fall into the hole).

Later, Hawking turned his attention to quantum theory, which can lead to violations of the usual reasonable conditions on the energy density and pressure of matter. He then found that actually black holes can create and emit particles, now called Hawking radiation, and shrink, presumably eventually disappearing entirely into a final burst of radiation.

This prediction of Hawking radiation is perhaps Hawking's most well known discovery, and it has almost entirely been accepted by other physicists who have studied the situation of a black hole with negligible incoming energy. However, this 1974 discovery led Hawking in 1976 to make the more radical proposal that when a black hole forms and then evaporates away, information is lost from our universe. (In more technical terms, a pure quantum state would become a mixed quantum state, with greater von Neumann entropy.)

Starting with a paper of mine in 1980 that questioned Hawking's argument for information loss and with a very small number of other papers on this in the early years, a growing crescendo of papers have appeared on black hole information. Eventually, particularly as a result of string theory arguments, the majority of papers have questioned information loss and supported the older view that information is not lost. In 2004, Hawking reversed his opinion and conceded a famous bet he had made in 1997 with John Preskill. In 2007, Hawking conceded an even earlier bet he had made with me in 1980, paying me a dollar that was actually a fake dollar with a picture of Marilyn Monroe, whom he considered to be "a model of the universe." However, there are several renowned physicists who think Hawking was originally correct and should not have conceded his bets.

Hawking also applied quantum theory to the universe and, with James Hartle of the University of California at Santa Barbara in 1983, proposed the no-boundary wave function of the universe. This was conceptually a major innovation, proposing laws of physics not only for how the universe evolves but what it was like at some initial time. The Hartle-Hawking no-boundary quantum state pictures the universe as not having a singular beginning and in some sense no beginning at any precise time at all, thereby giving a different picture than the singular beginning proved by his singularity theorems under the approximation of classical general relativity. However, one should note that it is almost certainly premature to give a precise quantum state for the universe, and indeed there are some technical problems with the Hartle-Hawking proposal. Nevertheless, it is a major conceptual advance to bring the quantum state within the laws of physics.

These are just a few of the highlights of the remarkable achievements Hawking has achieved in physics. Besides his academic work, he has done a great service in popularizing his and other advances in physics and cosmology with books such as the bestseller, A Brief History of Time. His courage in the face of enormous physical adversity has also been an inspiration to millions of people.

On a personal note, Stephen Hawking was an outstanding mentor for me, from being a co-supervisor (along with my main supervisor Kip Thorne) of my Ph.D. during his 1974-75 visit to Caltech and during my postdoctoral position under his supervision at the University of Cambridge 1976-79. He has indeed been crucial in my career, both in getting me my academic positions and in giving me plenty of ideas that I have continued to work on throughout my career. He and his family were also very close personal friends (going back to my living in his home and helping him out while being a postdoc in Cambridge). I shall miss him greatly.

Don Page is a theoretical physicist at the University of Alberta, Edmonton. He and his wife Cathy Page have also paid tribute to the late Stephen Hawking on CBC’s Calgary Explorer, Edmonton AM, and “Quirks and Quarks.”
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Stephen Hawking (1942-2018)
By ZEEYA MERALI • Mar. 14, 2018 @ 14:32 GMT

Credit: NASA
Many of you have woken to the sad news of the passing of Stephen Hawking — a towering figure in theoretical physics and cosmology, who inspired so many within science and beyond, with his intellectual insights into the nature of the universe, his wit, and his zest for the life that doctors once told him he would never have.

It’s difficult to overstate Hawking’s influence on the physics community. Roger Penrose, writing in the Guardian, and Martin Rees, in Nature, have written extremely accessible descriptions of the impact of his work. In the 1960s, with Penrose, George Ellis, and others, Hawking’s calculations helped to elucidate the mathematics of how the big bang marks the beginning of not just space, but time itself, from an infinitely small, dense “singularity.” Such singularities represent the breakdown of our best theories of physics and are also thought to lie at the heart of black holes, shrouded by an event horizon. It was believed that these event horizons act as one-way membranes that allow nothing that passes, not even light, to escape a black hole’s clutches. But, in the 1970s, Hawking’s work examining black holes with both general relativity and quantum theory, led him to propose that black holes can, in fact, slowly radiate particles, through the process of what has become known as “Hawking radiation.” This, in turn, suggests that black holes will slowly evaporate away — raising profound puzzles over the fate of information about the objects that fell in to the black hole.

This black-hole information paradox has yet to be resolved to everyone’s satisfaction; in fact, in recent years it has only become more confounding. But, as is often the way in science, posing questions can sometimes be as fruitful as offering solutions — and researchers are using this paradox, and its offshoots, as a handle to try to understand which of the two cornerstones of physics, Einstein’s general theory of relativity or quantum theory, has to give. Many FQXi members are today absorbed in projects that have their roots in Hawking’s work.

But Hawking’s influence goes deeper. As the bestselling author of A Brief History of Time, he directly inspired so many of us to study physics and cosmology, in the first place. I’ve lost count of the number of FQXi members who cite reading the book as a defining moment in their lives. I myself read natural sciences at Cambridge University, where Hawking was based and held the Lucasian Chair of Mathematics for many years. My fellow students and I never ceased to be excited to hear Hawking, or bump into him on the streets (quite literally, in the case of a friend, whom I recall returning to college one afternoon in a flustered state because she had cycled into his wheelchair). And while there might have been a danger that his celebrity status would give him an aura of unapproachability, he undercut this with his humour and the twinkle in his eyes. Hawking laced his public talks with jokes, managing to display comic timing despite speaking through a voice synthesiser. I personally loved him playfully claiming that the Spice Girls were his favourite group, for instance (or possibly that was entirely sincere, I’m not sure).

Later, as a science journalist, I was honoured that Hawking agreed to be interviewed by me for New Scientist and Nature, both about his own work, and to give his thoughts on that of others, even though providing comments clearly took a physical toll. But, of course, in addition to his scientific accomplishments, he will be remembered and admired for his commitment to engage the public and his refusal to be defined by the motor-neurone disease that struck him at just 21 years of age. That legacy will remain for generations to come.

Our thoughts at FQXi are with Hawking’s family, friends, and colleagues.
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The Quantum Hourglass—How a Quantum Time-keeper Can Replicate Continuum Temporal Events
By MILE GU • Mar. 1, 2018 @ 20:17 GMT

This post has been co-written by Mile Gu and Thomas Elliott.

Artist's impression of a quantum hourglass
In 2006, Oxford University Press set out to determine the most commonly used nouns in the English language. Topping the list was ‘time,’ closely followed in 3rd and 5th by ‘year’ and ‘day’ respectively, both delineating periods of time. This highlights how deeply embedded the concept of time is within the human psyche.

This prevalence of time is perhaps not too surprising—observing and tracking time are integral to our daily lives. Our work days are carried out according to schedules, meetings are planned with set durations. We synchronise with others by fixing times to meet, and we plan each day with the knowledge that they contain a set number of hours. To prepare for what is going to happen in the future, time-keeping is essential. Indeed, one can argue that one of the foundations modern civilization is built upon is our capacity to track time and anticipate future events.

In this context, the hourglass is distinguished for its role as one of the first accessible means of tracking time to the accuracy of seconds in medieval society. Indeed, their function is so familiar that ‘the sands of time’ has become a popular idiom, referring to the visual metaphor that the passage of time appears to flow like falling sand, steadily and irreversibly progressing from past to future.

Zoom in close enough on an hourglass, and one will see the individual grains of sand. At this level, the flow is not smooth, but inherently granular. At any moment, a finite number of grains of sand will have fallen, incrementing temporal progress in discrete packets. Time itself however appears, at any observable level, to be continuous. The hourglass analogy thus extends only so far.

This limit illustrates a pertinent observation—objects with a finite configuration space can only mimic the passage of time to finite precision. This isn’t particularly surprising: a finite configuration space can support only a finite memory. Thus, irrespective of any other physical limitations, such a system can only store the current time to a finite precision.

Indeed, this limitation is all too familiar to those in scientific fields that involve digital modelling or simulation. Theoretical models almost always assume time operates on a continuum. Whether modelling neuronal spike trains or dynamics of quantum systems, time is generally represented by some parameter t that takes on real values. Digital simulations of the resulting systems must however inevitably approximate such dynamics by discretising time.

As an illustration, consider the simulation of a particularly simple delayed Poisson process. This consists of a single system that emits only a single type of output at probabilistic points in time. The probability of an emission occurring during each infinitesimal time-interval δ is constant, with one catch: no output is ever emitted within a fixed relaxation period τ after an emission. For a device to replicate these statistics correctly, it would need to record whether it is in such a relaxation period, and if so, precisely how much time has elapsed since the last emission. Let’s call this elapsed time t. Only by storing t can the simulator know precisely much longer it must wait (i.e., the value of τ – t) before it is okay to emit the next output.

However, the variable t is real, and can take on a continuum of values. The more decimal places we wish to store about it, the more memory is required. In the limit where we want to faithfully predict the next emission with accuracy up to an arbitrary level of precision, this memory becomes unbounded. In practice, when writing computer code to perform the simulation, we would approximate t by discretising time into granular packets. For example, we could take some sufficiently small Δ and call it a day, provided we are happy to track time only to the nearest kΔ, for some integer k. A typical program for simulating such a process would have a pseudocode along the lines of:

if t is less than τ,

increment time by setting t = t + Δ


emit an output and set t=0 with probability p Δ

end repeat from start

Each iteration of the code simulates one timestep Δ. As Δ goes to zero, the output of this simulator will become statistically identical to the original delayed Poissonian process. The cost though, is that the number of values t can take scales as 1/Δ, growing to infinity as Δ goes to zero.

Thus, the more accurately that we wish to simulate the process, the more memory we need to invest. There is an ever-present trade-off between temporal precision and memory, and perfect statistical replication requires the allocation of unlimited resources. An hourglass, with a finite amount of sand, can thus never achieve exact replication.

While this trade-off is of clearly of practical relevance, it is fascinating also from a more foundational perspective. Many scientists seriously consider the possibility that we live within a simulatable reality, wherein everything in nature can be thought of as information processing. If we assume the memory capacity of this underlying computer is finite, running a program like the pseudocode above, how would it simulate processes that operate in continuous time? One may be tempted then to conclude that either we live in a computer with unlimited resources, or that continuous time exists only as a theoretical idealisation. Perhaps our universe is itself like sand in an hourglass—zoom closely enough, and everything appears granular.

While this could be a valid possibility, is there perhaps a way to avert this conundrum?

What we have not yet considered here is the quantum nature of information. The key element is that every bit of data represents some physical system with two different configurations—one which we label |0>, the other |1>. Provided the system can be isolated sufficiently well from the environment, we can also steer it into quantum mechanical degrees of freedom that possess coherence: superposition states represented by α|0>+β|1>, simultaneously |0> and |1> with specific weights dictated by α and β. The difference now is that α and β are intrinsically continuous degrees of freedom. Thus, this quantum bit—a finite physical system—contains within it a continuous parameter. Could this be leveraged to encode the continuity of time?

In our latest article, published in npj Quantum Information (4, Article number: 18 (2018)), we show that this ingredient gives us exactly what we need. Instead of using a classical hourglass where each grain of sand has either fallen or is yet to fall, we employ a quantum time-keeper where the grains of sand are in a weighted superposition of both possibilities. By deforming the weights continuously with the passage of time, we are able to prove that the delayed Poisson process could be modelled with perfect precision using finite memory.

Pragmatically, this result could immediately lead to memory savings in continuous time simulations. Numerical evidence indicates that our results apply to much more general cases, where the waiting time distribution between successive emissions is arbitrary. Such general processes, known as renewal processes, are relevant in many diverse fields of study—from modelling the firing of neurons to arrival times of buses. Thus a means to simulate such systems with less memory could have direct practical use. Similar advantages can be found when considering other continuous variables, such as position, as was shown in a companion article recently published in New Journal of Physics (2017).

The foundational consequences, however, are perhaps more exciting. Let us again entertain the scenario where we live in a simulated reality. Would the architects of this reality prefer the use of classical or quantum information? Our work shows that if they are intent on constructing a universe where time flows smoothly, then quantum mechanics may be the only feasible method. Time, should it be continuous, could well necessitate an underlying quantum reality.

Mile Gu is a physicist at Nanyang Technological University and the Centre for Quantum Technologies, Singapore. Thomas J. Elliott is a physicist at Nanyang Technological University, Singapore. Their research was supported in part by FQXi.
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Joe Polchinski (1954-2018)
By ZEEYA MERALI • Feb. 6, 2018 @ 22:29 GMT

As many of you will have heard by now, Joe Polchinski sadly passed away on Feb 2, 2018, after a long battle with cancer. For FQXi, this marks not only the loss of a brilliant mind, but a dear friend.

There have been many tributes from Joe’s colleagues celebrating his life and work. You can learn more through those links about his pioneering work on string theory, in particular, his role in the discovery of D-Branes (with Jin Dai and Rob Leigh), objects to which the ends of open strings can become tethered. (Matt Strassler has a wonderful discussion about the significance of this in relation to a description of black holes in string theory, and to the discovery of fundamental dualities between descriptions of physics using strings, black holes and gravity to those using quantum fields and particles.) But Joe’s achievements go beyond that, including his work on the string landscape (with Raphael Bousso) and more recently, articulating the blackhole firewall paradox, as the “P” in the AMPS team (with Ahmed Almheiri, Donald Marolf and James Sully). He won numerous awards, including a share of the $3-million Breakthrough Prize in 2017.nI am sure that in the coming days, many FQXi members who worked closely with Joe Polchinski will add more tributes, here and elsewhere.

Speaking as a science writer, I am sure many journalists would join me in saying that Joe was exceptionally friendly and supportive when it came to explaining his own work, and that of his colleagues. And I wanted to add one anecdote based on my many interactions with him from that perspective. The last time that I met with Joe in person was while I was researching my book. We spoke about his work on D-Branes, and the series of related discoveries by others around that time that make up the “second superstring revolution.” He was truly animated, scribbling on the blackboard and talking through how the realisation came aboutin the mid-90s. I asked him how exciting it was, back then. Was he aware of his work’s significance? Proud of this immense accomplishment?

Joe stopped, chalk in hand, and thought for good while, reaching back to how he must have felt at the time. “I’ll tell you something funny, just as a personal note,” he said. One of his sons, he explained, played roller hockey, and Joe was asked to coach the kids’ team. His first season of coaching coincided with the D-Brane discovery. “It was weird…I was much more emotionally involved in the coaching than I was in the D branes.” And it was the roller hockey with his son, he noted, that was the real highlight of his year.

Our thoughts are with Joe’s family and friends.

Joe Polchinski's: "Memories of a Theoretical Physicist"
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