Oct 162017
 

Today, a “multi-messenger” observation of a gravitational wave event was announced.

This is a big freaking deal. This is a Really Big Freaking Deal. For the very first time, ever, we observed an event, the merger of two neutron stars, simultaneously using both gravitational waves and electromagnetic waves, the latter including light, radio waves, UV, X-rays, gamma rays.

From http://iopscience.iop.org/article/10.3847/2041-8213/aa91c9

The significance of this observation must not be underestimated. For the first time, we have direct validation of a LIGO gravitational wave observation. It demonstrates that our interpretation of LIGO data is actually correct, as is our understanding of neutron star mergers; one of the most important astrophysical processes, as it is one of the sources of isotopes heavier than iron in the universe.

Think about it… every time you hold, say, a piece of gold in your hands, you are holding something that was forged in an astrophysical event like this one billions of years ago.

 Posted by at 2:33 pm
Sep 302017
 

Move over, Donald Trump. To heck with you, hurricane victims in Puerto Rico. See if I care about Catalonia voting for independence. Here is some real news™ from Canada instead, about a branch of the Royal Bank of Canada, which has been closed since August because a family of raccoons decided to make the ceiling of the place their new home.

Toronto bank branch closed after raccoon family moves in, damages the place.

The damage is extensive. The branch will reportedly stay closed until sometime in October.

You have to admit though that these animals are cute. Even when they are doing their best and try to look ferocious and angry.

 Posted by at 9:14 am
Sep 272017
 

Interesting forecast, courtesy of the Weather Network earlier this afternoon:

Yes, that is a snow symbol in the upper left corner. And yes, my American friends, the 29 degrees is Centigrade.

Warm snow, I guess.

(The “Accumulating snow” headline for Goose Bay is probably valid. But the upper left corner was supposed to describe current conditions here in Ottawa.)

 Posted by at 6:21 pm
Sep 272017
 

So here it is: another gravitational wave event detection by the LIGO observatories. But this time, there is a twist: a third detector, the less sensitive European VIRGO observatory, also saw this event.

This is amazing. Among other things, having three observatories see the same event is sufficient to triangulate the sky position of the event with much greater precision than before. With additional detectors coming online in the future, the era of gravitational wave astronomy has truly arrived.

 Posted by at 2:42 pm
Sep 252017
 

Today is September 25. In one of the coldest capital cities in the world. Yet this is the temperature according to the weather monitor gadget on my desktop (but also according to the thermometer on our balcony):

Yes, 3233 C. Or 9091 F for my American friends. The record for this day? A little under 30 C.

No, it does not feel like autumn at all.

On an unrelated note, yes, I do like to use desktop gadgets on Windows 10.

 Posted by at 3:38 pm
Sep 192017
 

Predatory journals have been plaguing the academic publishing world for many years, and the problem is getting worse. As a recent Nature article revealed, even experienced researchers get scammed by them sometimes. Inexperienced, researchers, especially from non-English speaking countries, are easy prey.

The rise of predatory publishing. From Wikipedia.

Take, for instance, this researcher who recently sent me his paper after it has been published in a predatory pay-to-publish open access journal. He saw the fact that his paper was accepted a validation of his ideas. In reality, his paper was badly flawed, its main conclusions based on naive mistakes that would have been pointed out by a competent referee (or even editor!) during a normal peer review process. But predatory journals are not interested in rejecting papers; they are into maximizing their revenue.

There used to be a wonderful list of predatory, maintained by Jeffrey Beall. Unfortunately, Beall decided to take down his Web site, thus depriving us of an essential resource.

In my response to the aforementioned researcher, I listed a few criteria by which a predatory publisher can be identified. I know, I know, such lists exist, but these are characteristics that I personally consider important:

  1. Open access: Obviously not all open access journals are predatory, and there are a few predatory journals that are not open access. But the vast majority are, since they (for obvious reasons) cannot build a real subscriber base, so their main or sole source of revenue is author fees.
  2. Publication fee that is often too low to cover the real costs of publishing: The publication fees charged by legitimate journals to publish papers, e.g., with open access easily run up to a thousand dollars or more. It indeed costs that much to guide a paper through the peer review process and then prepare it for publication through a proper copy editing and proofreading process.
  3. No real history to the journal: Predatory journals tend to be new, with few (if any) notable papers.
  4. Low quality papers with uncorrected English (typos, grammatical mistakes, incomprehensible sentences) from unknown authors: All it takes is one peek at papers with very bad quality English to know that the journal has no real editorial staff or policies and they publish anything so long as the fees are paid.
  5. Many papers that do not appear on arxiv.org, as having been rejected there for quality reasons: If the journal specializes in an area that is covered by arXiv, e.g., theoretical physics or astrophysics, yet the papers published by it do not appear on arXiv, that is an almost certain indicator that it is a journal preferred by cranks and crackpots, whose submissions are rightfully rejected by arXiv moderators.
  6. An unusually large number (often hundreds) of young journals from the same publisher: Predatory publishers tend to launch a very large number of journals, e.g., dozens if not hundreds of “British journal of this” or “American journal of that” or similar names designed to suggest legitimacy. (Lately, some predatory publishers even went so far as to hijack the name of obscure but distinguished journals, e.g., from Eastern Europe.)
  7. No association with any known, reputable research organization, publication house or university: Reputable, top quality journals are usually associated with a research institution. For instance, Physical Review is published by the American Physical Society; Science is published by the American Association for the Advancement of Science. A variant on this theme is when the journal is, in fact, associated with an institution but the institution itself is phony.

This list of criteria is, of course, not complete. But I am quite certain that any journal that scores high on all seven of these is, in fact, a predatory journal.

 Posted by at 9:48 am
Sep 152017
 

NASA’s Cassini spacecraft is no more.

Launched 20 years ago, Cassini arrived at Saturn in 2004 and has been studying the ringed giant ever since. Cassini also carried the Huygens probe, which executed a successful descent into the dense atmosphere of Saturn’s moon Titan, and even transmitted data from its surface.

Its fuel nearly exhausted, Cassini was steered into a trajectory that led to its intentional demise: a fiery plunge into Saturn’s atmosphere earlier this morning. As planned, the spacecraft was able to transmit observations until the very end, when its thrusters were no longer able to maintain its attitude during the descent.

Program manager Earl Maize and operations team manager Julie Webster embrace after signal loss.

I feel sad that Cassini is gone, but I should also feel elated because it has been an incredibly successful mission. I just hope I live long enough to see another probe visiting Saturn, perhaps a probe or set of probes that are designed to land on Titan, maybe even sail its hydrocarbon seas, in search of possible life on that icy world.

 Posted by at 10:11 am
Aug 222017
 

Here is a belated picture of yesterday’s solar eclipse, taken by my friend David in New York City:

His equipment is (semi-)professional but the solar filter that he used wasn’t. Still, it is a heck of a lot better than anything I was able to see (or project with a makeshift pinhole camera). I suggested to him to obtain a quality solar filter by 2024. Who knows, we may meet in Watertown to watch totality.

 Posted by at 10:39 pm
Jul 302017
 

I just came across an interesting slide.

It was part of a presentation by Bill Foster, a member of an endangered species in the United States Congress: a scientist turned politician. He gave a talk at the April meeting of the American Physical Society. This slide from his talk speaks for itself:

I don’t have data for Canada, other than a list of a grand total of 6 engineers serving in our federal House of Commons. That low number suggests that Canada’s Parliament would not be positioned too far from the U.S. Congress in this chart.

Is this a bad thing? I hesitate, because I note that totalitarian regimes tend to have many scientists among their leaders. Is it because scientists are more likely to prefer authoritarianism? Or more likely to serve autocrats? I don’t know. I do know that as a free citizen, I much prefer to be governed by a dysfunctional Congress or Parliament than by a totalitarian Politburo, regardless of the number of scientists in these bodies.

 Posted by at 11:38 am
Jul 272017
 

There is a brand new video on YouTube today, explaining the concept of the Solar Gravitational Telescope concept:

It really is very well done. Based in part on our paper with Slava Turyshev, it coherently explains how this concept would work and what the challenges are. Thank you, Jimiticus.

But the biggest challenge… this would be truly a generational effort. I am 54 this year. Assuming the project is greenlighted today and the spacecraft is ready for launch in ten years’ time… the earliest for useful data to be collected would be more than 40 years from now, when, unless I am exceptionally lucky with my health, I am either long dead already, or senile in my mid-90s.

 Posted by at 11:27 pm
Jul 132017
 

Slava Turyshev and I just published a paper in Physical Review. It is a lengthy, quite technical paper about the wave-theoretical treatment of the solar gravitational telescope.

What, you say?

Well, simple: using the Sun as a gravitational telescope to image distant objects. Like other stars, the Sun bends light, too. Measuring this bending of light was, in fact, the crucial test carried out by Eddington during the 1919 solar eclipse, validating the predictions of general relativity and elevating Albert Einstein to the status of international science superstar.

The gravitational bending of light is very weak. Two rays, passing on opposite sides of the Sun, are bent very little. So little in fact, it takes some 550 astronomical units (AU; the distance between the Earth and the Sun) for the two rays to meet. But where they do, interesting things happen.

If you were floating in space at that distance, and there was a distant planet on the exact opposite side of the Sun, light from a relatively small section of that planet would form a so-called Einstein ring around the Sun. The light amplification would be tremendous; a factor of tens of billions, if not more.

But you have to be located very precisely at the right spot to image a particular spot on the exoplanet. How precisely? Well, that’s what we set out to figure out, based in part on the existing literature on the subject. (Short answer: it’s measured in tens of centimeters or less.)

In principle, a spacecraft at this distance, moving slowly in lateral directions to scan the image plane (which is several kilometers across), can obtain a detailed map of a distant planet. It is possible, in principle, to obtain a megapixel resolution image of a planet dozens of light years from here, though image reconstruction would be a task of considerable complexity, due in part to the fact that an exoplanet is a moving, changing target with variable illumination and possibly cloud cover.

Mind you, getting to 550 AU is costly. Our most distant spacecraft to date, Voyager 1, is just under 140 AU from the Sun, and it took that spacecraft 40 years to get there. That said, it is a feasible mission concept, but we must be very certain that we understand the physics thoroughly.

This is where our paper comes in: an attempt to derive detailed results about how light waves pass on both sides of the Sun and recombine along the focal line.

The bulk of the work in this paper is Slava’s, but I was proud to help. Part of my contribution was to provide a visualization of the qualitative behavior of the wavefront (described by a hypergeometric function):

In this image, a light wave, initially a plane wave, travels from left to right and it is deflected by a gravitational source at the center. If you squint just a little, you can actually see a concentric circular pattern overlaid on top of the distorted wavefront. The deflection of the wavefront and this spherical wave perturbation are both well described by an approximation. However, that approximation breaks down specifically in the region of interest, namely the focal line:

The top left of these plots show the approximation of the deflected wavefront; the top right, the (near) circular perturbation. Notice how both appear to diverge along the focal line: the half line between the center of the image and the right-hand side. The bottom right plot shows the combination of the two approximations; it is similar to the full solution, but not identical. The difference between the full solution and this approximation is shown in the bottom left plot.

I also helped with working out evil-looking things like a series approximation of the confluent hypergeometric function using so-called Pochhammer symbols and Stirling numbers. It was fun!

To make a long story short, although it involved some frustratingly long hours at a time when I was already incredibly busy, it was fun, educational, and rewarding, as we gave birth to a 39-page monster (43 pages on the arXiv) with over 300 equations. Hopefully just one of many contributions that, eventually (dare I hope that it will happen within my lifetime?) may result in a mission that will provide us with a detailed image of a distant, life-bearing cousin of the Earth.

 Posted by at 10:56 pm
Jun 012017
 

Donald Trump, Demagogue-in-Chief of America the Greatest, now took his proud nation to new heights: America joined forces with the ever-so-enlightened, wonderful regime of Bashar al-Assad in Syria, along with Central America’s Daniel Ortega in Nicaragua, announcing that his nation will withdraw from the Paris Climate Agreement.

Americans must be so proud. Gone are the days of Obama leading from behind… instead, their orange-skinned leader is now proudly leading them behind.

To be honest, I don’t mind it too much. I always wondered just how effective the Paris agreement was going to be anyway. And it’s not like Ottawa’s climate is too hot… nor do I have any children to worry about, so why should I care if we leave behind a messed up world when my generation dies?

The only thing that bothers me about this is the, well, stubborn anti-intellectualism and outright, blatant stupidity. Not just the Deceiver-in-Chief’s, mind you. A few hours ago I witnessed a brief debate between a CNN anchor and Rand Paul about the nature and origin of the current climate change and its comparison to past climate events. Talk about the blind leading the sightless…

 Posted by at 6:32 pm
May 312017
 

I am watching the morning news and it’s all about numbers. Some good, some not so good, some really bad. Here are a few, in descending order:

  • 2018: The year when Ottawa plans to introduce a new low-income transit fare.
  • 417: The provincial highway number of the Queensway, which has been reopened after yesterday’s huge crash.
  • 175.6: The amount of rain, in mm, that Ottawa received in the month of May.
  • 80: The estimated number killed by a massive ISIS terrorist bomb in Kabul.
  • 21: The highest expected temperature of the day and, incidentally, the entire week, in Centigrade.
  • 15: The new minimum wage, in Canadian dollars, as proposed by the Ontario provincial government.
  • 7: The age of a baby, in months, who died allegedly due to her mother’s negligence in Gatineau.

I thought of turning these bullet points into a numbered list, but that would have been too confusing.

 Posted by at 8:25 am
May 182017
 

I am no photo artist, and my best camera is, well, my phone. That’s it.

Even so, a few minutes ago I felt compelled to take a couple of photographs. We are a few minutes away from sunset and a big storm just began. Then I looked out my window and I found the building across the street brighter than the sky above.

The light came from the other side of the sky. The Sun was not visible but the sky in that direction was bright enough to light things up.

Photographs (especially, photographs taken with a phone) really don’t do these sights justice. The contrasts were amazing.

 Posted by at 8:29 pm
Mar 172017
 

Recently, I answered a question on Quora on the possibility that we live in a computer simulation.

Apparently, this is a hot topic. The other day, there was an essay on it by Sabine Hossenfelder.

I agree with Sabine’s main conclusion, as well as her point that “the programmer did it” is no explanation at all: it is just a modern version of mythology.

I also share her frustration, for instance, when she reacts to the nonsense from Stephen Wolfram about a “whole civilization” “down at the Planck scale”.

Sabine makes a point that discretization of spacetime might conflict with special relativity. I wonder if the folks behind doubly special relativity might be inclined to offer a thought or two on this topic.

In any case, I have another reason why I believe we cannot possibly live in a computer simulation.

My argument hinges on an unproven conjecture: My assumption that scalable quantum computing is really not possible because of the threshold theorem. Most supporters of quantum computing believe, of course, that the threshold theorem is precisely what makes quantum computing possible: if an error-correcting quantum computer reaches a certain threshold, it can emulate an arbitrary precision quantum computer accurately.

But I think this is precisely why the threshold will never be reached. One of these days, someone will prove a beautiful theorem that no large-scale quantum computer will ever be able to operate above the threshold, hence scalable quantum computing is just not possible.

Now what does this have to do with us living in a simulation? Countless experiments show that we live in a fundamentally quantum world. Contrary to popular belief (and many misguided popularizations) it does not mean a discretization at the quantum level. What it does mean is that even otherwise discrete quantities (e.g., the two spin states of an electron) turn into continuum variables (the phase of the wavefunction).

This is precisely what makes a quantum computer powerful: like an analog computer, it can perform certain algorithms more effectively than a digital computer, because whereas a digital computer operates on the countable set of discrete digits, a quantum or analog computer operates with the uncountable infinite of states offered by continuum variables.

Of course a conventional analog computer is very inaccurate, so nobody seriously proposed that one could ever be used to factor 1000-digit numbers.

This quantum world in which we live, with its richer structure, can be simulated only inefficiently using a digital computer. If that weren’t the case, we could use a digital computer to simulate a quantum computer and get on with it. But this means that if the world is a simulation, it cannot be a simulation running on a digital computer. The computer that runs the world has to be a quantum computer.

But if quantum computers do not exist… well, then they cannot simulate the world, can they?

Two further points about this argument. First, it is purely mathematical: I am offering a mathematical line of reasoning that no quantum universe can be a simulated universe. It is not a limitation of technology, but a (presumed) mathematical truth.

Second, the counterargument has often been proposed that perhaps the simulation is set up so that we do not get to see the discrepancies caused by inefficient simulation. I.e., the programmer cheats and erases the glitches from our simulated minds. But I don’t see how that could work either. For this to work, the algorithms employed by the simulation must anticipate not only all the possible ways in which we could ascertain the true nature of the world, but also assess all consequences of altering our state of mind. I think it quickly becomes evident that this really cannot be done without, well, simulating the world correctly, which is what we were trying to avoid… so no, I do not think it is possible.

Of course if tomorrow, someone announces that they cracked the threshold theorem and full-scale, scalable quantum computing is now reality, my argument goes down the drain. But frankly, I do not expect that to happen.

 Posted by at 11:34 pm
Mar 152017
 

Chemistry is weird. Even in inorganic chemistry, there are some really strange compounds. There is, for instance, phosphotungstic acid: \({\rm H}_3{\rm P}{\rm W}_{12}{\rm O}_{40}\). Never heard of it until today.

And then there is this one (if it exists at all):

Somebody posted this on Google+. They wanted to know what its properties are. And now, so do I. There are a few tungsten compounds listed in online databases, but this is not one of them. Does it even exist? I don’t know. For some reason, I expect it to have properties not unlike those of tungsten carbide, but I could be completely off the mark.

 Posted by at 11:40 pm
Jan 312017
 

A short while ago, I turned on a computer. Like several of my other computers, this one is also configured to display a weather widget on the desktop. Here is what it showed:

If only it were true! Alas, the reason for this overly optimistic weather report had to do with the fact that the computer in question has last been turned on more than four months ago, back in September. In reality, this is what our weather is like right now:

And even that is a significant improvement over the −21°C that greeted me early in the morning.

Yup, this is Canada.

 Posted by at 8:52 pm
Jan 202017
 

Enough blogging about politics. It’s time to think about physics. Been a while since I last did that.

A Facebook post by Sabine Hossenfelder made me look at this recent paper by Josset et al. Indeed, the post inspired me to create a meme:

The paper in question contemplates the possibility that “dark energy”, i.e., the mysterious factor that leads to the observed accelerating expansion of the cosmos, is in fact due to a violation of energy conservation.

Sounds kooky, right? Except that the violation that the authors consider is a very specific one.

Take Einstein’s field equation,

$$R_{\mu\nu}-\tfrac{1}{2}Rg_{\mu\nu}+\Lambda g_{\mu\nu}=8\pi GT_{\mu\nu},$$

and subtract from it a quarter of its trace times the metric. The trace of the left-hand side is \(-R+4\Lambda\), the right-hand side is \(8\pi GT\), so we get

$$R_{\mu\nu}-\tfrac{1}{4}Rg_{\mu\nu}=8\pi G(T_{\mu\nu}-\tfrac{1}{4}Tg_{\mu\nu}).$$

Same equation? Not quite. For starters, the cosmological constant \(\Lambda\) is gone. Furthermore, this equation is manifestly trace-free: its trace is \(0=0\). This theory, which was incidentally considered already almost a century ago by Einstein, is called trace-free or unimodular gravity. It is called unimodular gravity because it can be derived from the Einstein-Hilbert Lagrangian by imposing the constraint \(\sqrt{-g}=1\), i.e., that the volume element is constant and not subject to variation.

Unimodular gravity has some interesting properties. Most notably, it no longer implies the conservation law \(\nabla_\mu T^{\mu\nu}=0\).

On the other hand, \(\nabla_\mu(R^{\mu\nu}-\tfrac{1}{2}Rg^{\mu\nu})=0\) still holds, thus the gradient of the new field equation yields

$$\nabla_\mu(\tfrac{1}{4}Rg^{\mu\nu})=8\pi G\nabla_\mu(T^{\mu\nu}-\tfrac{1}{4}Tg^{\mu\nu}).$$

So what happens if \(T_{\mu\nu}\) is conserved? Then we get

$$\nabla_\mu(\tfrac{1}{4}Rg^{\mu\nu})=-8\pi G\nabla_\mu(\tfrac{1}{4}Tg^{\mu\nu}),$$

which implies the existence of the conserved quantity \(\hat{\Lambda}=\tfrac{1}{4}(R+8\pi GT)\).

Using this quantity to eliminate \(T\) from the unimodular field equation, we obtain

$$R_{\mu\nu}-\tfrac{1}{2}Rg_{\mu\nu}+\hat{\Lambda} g_{\mu\nu}=8\pi GT_{\mu\nu}.$$

This is Einstein’s original field equation, but now \(\hat{\Lambda}\) is no longer a cosmological constant; it is now an integration constant that arises from a conservation law.

The vacuum solutions of unimodular gravity as the same as those of general relativity. But what about matter solutions? It appears that if we separately impose the conservation law \(\nabla_\mu T^{\mu\nu}\), we pretty much get back general relativity. What we gain is a different origin, or explanation, of the cosmological constant.

On the other hand, if we do not impose the conservation law for matter, things get interesting. In this case, we end up with an effective cosmological term that’s no longer constant. And it is this term that is the subject of the paper by Josset et al.

That being said, a term that is time-varying in the case of a homogeneous and isotropic universe surely acquires a dependence on spatial coordinates in a nonhomogeneous environment. In particular, the nonconservation of \(T_{\mu\nu}\) should lead to testable deviations in certain Parameterized Post-Newtonian (PPN) parameters. There are some reasonably stringent limits on these parameters (notably, the parameters \(\alpha_3\) and \(\zeta_i\) in the notation used by Clifford Will in the 1993 revision of his book, Theory and experiment in gravitational physics) and I wonder if Josset et al. might already be in violation of these limits.

 Posted by at 9:43 pm
Jan 142017
 

So here is another thing I don’t expect to see from Donald Trump: Publishing an article in the highly respected multidisciplinary journal Science.

His predecessor, the still sitting Barack Obama did just that: his article about “The irreversible momentum of clean energy” was published yesterday, January 13, 2017. In it, he makes the case that economic growth does not depend on energy-related emissions, and that combating climate change does not require accepting lower growth or a reduced standard of living.

 Posted by at 9:33 pm
Dec 242016
 

Once again, I feel compelled to use the same image and same words that I have been using for many years, to wish all my family, all my friends, indeed everyone on the good Earth a very merry Christmas: the words of the astronauts of Apollo 8.

I know, I know, it’s the same thing every year. But there really aren’t any better words. Just imagine: three human beings, for the first time in human history, far from the Earth, in orbit around another celestial body. And back on Earth, one of the most troubled years in recent history: 1968. So on Christmas Eve, with about a billion people listening—a full one quarter of the Earth’s population at the time—they greeted us Earthlings with the opening passages from the Book of Genesis, the common creation mythology of several major religions.

And then Frank Borman ended the broadcast with words that are as appropriate today as we are heading towards more troubled times as they were back then: “And from the crew of Apollo 8, we close with good night, good luck, a Merry Christmas – and God bless all of you, all of you on the good Earth.”

 Posted by at 9:10 am