Aug 032015

Here is one of the most mind-boggling animation sequences that I have ever seen:

This image depicts V838 Monocerotis, a red variable star that underwent a major outburst back in 2002.

Why do I consider this animation mind-boggling? Because despite all appearances, it is not an expanding shell of dust or gas.

Rather, it is echoes of the flash of light, reflected by dust situated behind the star, reaching our eyes several years after the original explosion.

In other words, this image represents direct, visual evidence of the finite speed of light.

The only comparable thing that I can think of is this video, created a few years ago using tricky picosecond photography, of a laser pulse traveling in a bottle. However, unlike that video, the images of V838 Monocerotis required no trickery, only a telescope.

And light echoes are more than mere curiosities: they actually make it possible to study past events. Most notably, a faint light echo of a supernova that was observed nearly half a millennium ago, in 1572, was detected in 2008.

 Posted by at 5:35 pm
Sep 202013

Grote_Antenna_WheatonThe world’s first parabolic radio telescope was, astonishingly, built in someone’s back yard.

I am reading about the radio telescope of American amateur radio enthusiast and amateur astronomer Grote Reber.

In 1937, Reber built a 9-meter parabolic reflector in his family’s back yard.

Reber was the first to make a systematic survey of the radio sky, not only confirming Jansky’s earlier, pioneering discovery of radio waves from the Milky Way but also discovering radio sources such as Cygnus X-1 and Cassiopeia A.


For nearly a decade, Reber was the only person in the world doing radio astronomy.

Reber had a long life. He spent his final years in Tasmania, one of the few places on Earth where occasionally, very low frequency radio waves penetrate the ionosphere and are detectable by a ground-based antenna.

 Posted by at 2:55 pm
Oct 062012

Wow. If these plots are to be believed, Voyager 1 may have reached the heliopause at last:

This is, well, not exactly unexpected but still breathtaking.

The discovery of the heliopause was one of the “holy grail” science objectives of the extended “interstellar” mission of the twin Voyager spacecraft. If confirmed, it means that Voyager 1 is the first man-made object to have entered the interstellar medium, traveling through a region in the outer solar system that is no longer dominated by charged particles from the solar wind. (Gravitationally, this is still very much our Sun’s domain; there are comets out there with elliptical orbits that extend to many thousands of astronomical units.)

Not bad for a spacecraft that was launched over 35 years ago and flew by Saturn just a few months into the presidency of Ronald Reagan. Its twin finished its flyby of Neptune when the Berlin Wall was still standing. And they are both still alive and well. Voyager 1 is more than 120 astronomical units from the Sun these days. It takes about 17 hours for its radio signal to reach the Earth. If all goes well, it has sufficient electrical power to operate its on-board instruments for another decade or so.

 Posted by at 12:40 pm
Sep 212012

I am reading about a new boson.

No, not the (presumed) Higgs boson with a mass of about 126 GeV.

I am reading about a lightweight boson, with a mass of only about 38 MeV, supposedly found at the onetime pride of Soviet science, the Dubna accelerator.

Now Dubna may not have the raw power of the LHC, but the good folks at Dubna are no fools. So if they announce what appears to be a 5-sigma result, one can’t just not pay attention.

The PHOTON-2 setup. S1 and S2 are scintillation counters. From arXiv:1208.3829.

But a 38 MeV boson? That’s not light, that’s almost featherweight. It’s only about 75 times the mass of the electron, for crying out loud. Less than 4% of the weight of the proton.

The discovery of such a lightweight boson would be truly momentous. It would certainly turn the Standard Model upside down. Whether it is a new elementary particle or some kind of bound state, it is not something that can be fit easily (if at all) within the confines of the Standard Model.

Which is one reason why many are skeptical. This discover is, after all, not unlike that of the presumed Higgs boson, is really just the discovery of a small bump on top of a powerful background of essentially random noise. The statistical significance (or lack thereof) of the bump depends fundamentally on our understanding and accurate modeling of that background.

And it is on the modeling of the background that this recent Dubna announcement has been most severely criticized.

Indeed, in his blog Tommaso Dorigo makes a very strong point of this; he also suggests that the authors’ decision to include far too many decimal digits in error terms is a disturbing sign. Who in his right mind writes 38.4935 ± 1.02639 as opposed to, say, 38.49 ± 1.03?

To this criticism, I would like to offer my own. I am strongly disturbed by the notion of a statistical analysis described by an expression of the type model = data − background. What we should be modeling is not data minus some theoretical background, but the data, period. So the right thing to do is to create a revised model that also includes the background and fit that to the data: model’ = model + background = data. When we do things this way, it is quite possible that the fits are a lot less tight than anticipated, and the apparent statistical significance of a result just vanishes. This is a point I raised a while back in a completely different context: in a paper with John Moffat about the statistical analysis of host vs. satellite galaxies in a large galactic sample.

 Posted by at 7:58 pm
Jun 062012

Yesterday, Venus transited the Sun. It won’t happen again for more than a century.

I had paper “welder’s glasses” courtesy of Sky News. Looking through them, I did indeed see a tiny black speck on the disk of the Sun. However, it was nowhere as impressive as the pictures taken through professional telescopes.

These live pictures were streamed to us courtesy of NASA. One planned broadcast from Alice Springs, Australia, was briefly interrupted. At first, it was thought that a road worker cutting an optical cable was the culprit, but later it turned out to be a case of misconfigured hardware. Or could it be that they were trying to fix a problem with an “intellectual property address”, a wording that appeared on several Australian news sites today? (Note to editors: if you don’t understand the text, don’t be over-eager replacing acronyms with what you think they stand for.)

I also tried to take pictures myself, holding my set of paper welder’s glasses in front of my (decidedly non-professional) cameras. Surprisingly, it was with my cell phone that I was able to take the best picture, but it did not even come close in resolution to what would have been required to see Venus.

The lesson? I think I’ll leave astrophotography to the professionals. Or, at least, to expert amateurs. Unfortunately, I am neither.

That said, I remain utterly fascinated by the experience of staring at a sphere of gas, close to a million and a half kilometers wide, containing 2 nonillion (2,000,000,000,000,000,000,000,000,000,000) kilograms of mostly hydrogen gas, burning roughly 580 billion kilograms of it every second in the form of nuclear fusion deep in its core, releasing photons amounting to about 4.3 billion kilograms of energy… and most of these photons remain trapped for a very long time, producing extreme pressures (so that the interior of the Sun is dominated by this ultrarelativistic photon gas) that prevent the Sun from collapsing upon itself, which will indeed be its fate when it can no longer sustain hydrogen fusion in its core a few billion years from now. And then, this huge orb is briefly occulted by a tiny black speck, the shadow of a world as big as our own… just a tiny black dot, too small for my handheld cameras to see.

I sometimes try to use a human-scale analogy when trying to explain to friends just how mind-bogglingly big the solar system is. Imagine a beach ball that is a meter wide. Now suppose you stand about a hundred meters away from it, like the length of a large sports field. Okay… now imagine that that beach ball is so bleeping hot, even at this distance its heat is burning your face. That’s how hot the Sun is.

Now hold up a large pea, about a centimeter in size. That’s the Earth. Another pea, roughly halfway between you and the beach ball would be Venus.

A peppercorn, some thirty centimeters or so from your Earth pea… that’s the Moon. Incidentally, if you hold that peppercorn up, at about thirty centimeters from your eye it is just large enough to obscure the beach ball in the distance, producing a solar eclipse.

Now let’s go a little further. Some half a kilometer from the beach ball you see a large-ish orange… Jupiter. Twice as far, you see a smaller orange with a ribbon around it; that’s Saturn. Pluto would be another peppercorn, more than three kilometers away.

But your beach ball’s influence does not end there. There will be specks of dust in orbit around it as far out as several hundred kilometers, maybe more. So where would the next beach ball be, representing the nearest star? Well, here’s the problem… the surface of the Earth is just not large enough, because the next beach ball would be more than 20,000 kilometers away.

To represent other stars, not to mention the whole of the Milky Way, we would once again need astronomical distance scales. If a star like our Sun was a one meter wide beach ball, the Milky Way of beech balls would be larger than the orbit of the Earth around the Sun. And the nearest full-size galaxy, Andromeda, would need to be located in distant parts of the solar system, far beyond the orbits of planets.

The only way we could reduce galaxies and groups of galaxies to a scale that humans can comprehend is by making stars and planets microscopic. So whereas the size of the solar system can perhaps be grasped by my beach ball and pea analogy, it is simply impossible to imagine simultaneously just how large the Milky Way is, not to mention the entire visible universe.

Or, as Douglas Adams wrote in The Hitchhiker’s Guide to the Galaxy: “Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”

 Posted by at 10:25 am
May 152012

I am really disappointed to learn this morning that the world will not come to an end December this year. According to a new discovery, the Mayan calendar may have had at least 17 baktuns, not 13 as previously believed, so we are good for something like another two millennia.

Just as I was getting ready to sell my house and all my earthly possessions…

 Posted by at 8:11 am
May 102012

Astronomy is supposed to be a relatively safe profession. I suppose observational astronomers may occasionally injure themselves when working on a telescope, but it’s probably rare. For them to become murder victims is even more unlikely.

So why would a Japanese astronomer, working in Chile on the Atacama Large Millimeter Array, be murdered on the street just outside his apartment?

 Posted by at 4:39 pm
Feb 222012

Why exactly do we believe that stars and more importantly, gas in the outer regions of spiral galaxies move in circular orbits? This assumption lies at the heart of the infamous galaxy rotation curve problem, as the circular orbital velocity for a spiral galaxy (whose visible mass is concentrated in the central bulge) should be proportional to the inverse square root of the distance from the center; instead, observed rotation curves are “flat”, meaning that the velocity remains approximately the same at various distances from the center.

So why do we assume that stars and gas move in circular orbits? Well, it turns out that one key bit of evidence is in a 32-year old paper that was published by two Indian physicists: Radhakrishnan and Sarma (A&A 85, 1980) made observations of hydrogen gas in the direction of the center of the Milky Way, and found that the bulk of gas between the solar system and the central bulge has no appreciable radial velocity.

However, more recent observations may be contradicting this result. Just two years ago, the Radial Velocity Experiment (RAVE) survey (Siebert et al, MNRAS 412, 2010) found, using a sample of several hundred thousand relatively nearby stars, that a significant radial velocity exists, putting into question the simple model that assumes that circular orbits dominate.

 Posted by at 10:03 pm
Feb 162012

I always find these numbers astonishing.

The solar constant, the amount of energy received by a 1 square meter surface at 1 astronomical unit (AU) from the Sun is roughly s = 1.37 kW/m2. Given that 1 AU is approximately 150 million kilometers, or r = 1.5 × 1011 m, the surface area of a 1 AU sphere surrounding the Sun would be A = 4πr2 = 2.8 × 1023 m2. Multiplied by the solar constant, we get P = sA = 3.9 × 1026 W, or the energy E = sA = 3.9 × 1026 J every second. Using Einstein’s infamous mass-energy formula E = mc2, where c = 3 × 108 m/s, we can easily calculate how much mass is converted into energy: m = E/c2 = 4.3 × 109 kg. Close to four and a half million tons.

The dominant fusion process in the Sun is the proton-proton chain reaction, in which approximately 0.7% of the total mass of hydrogen is converted into energy. Thus 4.3 million tons of pure energy is equivalent to over 600 millon tons of hydrogen fuel burned every second. (For comparison, the largest ever nuclear device, the Soviet Tsar Bomba, burned no more than a few hundred kilograms of hydrogen to produce a 50 megaton explosion.)

Fortunately, there is plenty where that came from. The total mass of the Sun is 2 × 1030 kg, so if the Sun was made entirely of hydrogen, it could burn for 100 billion years before running out of fuel. Now the Sun is not made entirely of hydrogen, and the fusion reaction slows down and eventually stops long before all the hydrogen is consumed, but we still have a few billion years of useful life left in our middle-aged star. A much bigger (pun intended) problem is that as our Sun ages, it will grow in size; in a mere billion years, the Earth may well become uninhabitable as a result, with the oceans boiling away. I wonder if it’s too early to start worrying about it just yet.

 Posted by at 12:24 pm
Jan 242012

When I write about things like precision orbit determination, I often have to discuss the difference between ephemeris time (ET) and coordinated universal time (UTC). ET is a “clean” time scale: it is essentially the time coordinate of an inertial coordinate frame that is attached to the barycenter of the solar system. On the other hand, UTC is “messy”: it is the time kept by noninertial clocks sitting here on the surface of the Earth. But the fact that terrestrial clocks sit inside the Earth’s gravity well and are subject to acceleration is only part of the picture. There are also those blasted leap seconds. It is because of leap seconds that terrestrial atomic time (TAI) and UTC differ.

Leap seconds arise because we insist on using an inherently wobbly planet as our time standard. The Earth wobbles, sometimes unpredictably (for instance, after a major earthquake) and we mess with our clocks. Quite pointlessly, as a matter of fact. And now, we missed another chance to get rid of this abomination: the International Telecommunication Union failed to achieve consensus, and any decision is postponed until 2015.

For the curious, an approximate formula to convert between TAI and ET is given by ET – TAI = 32.184 + 1.657×10–3 sin E, where E = M + 0.01671 sin M, M = 6.239996 + 1.99096871×10–7 t and t is the time in seconds since J2000 (that is, noon, January 1, 2000, TAI). To convert TAI to UTC, additional leap seconds must be added: 10 seconds for all dates prior to 1972, and then additional leap seconds depending on the date. Most inelegant.

Speaking of leap this and that, I think it’s also high time to get rid of daylight savings time. Its benefits are dubious at best, and I find the practice unnecessarily disruptive.

 Posted by at 12:23 pm
Sep 252011

Maybe I’ve been watching too much Doctor Who lately.

Many of my friends asked me about the faster-than-light neutrino announcement from CERN. I must say I am skeptical. One reason why I am skeptical is that no faster-than-light effect was observed in the case of supernova 1987A, which exploded in the Large Magellanic Cloud some 170,000 light years from here. Had there been such an effect of the magnitude supposedly observed at CERN, neutrinos from this supernova would have arrived years before visible light, but that was not the case. Yes, there are ways to explain away this (the neutrinos in question have rather different energy levels) but these explanations are not necessarily very convincing.

Another reason, however, is that faster-than-light neutrinos would be eminently usable in a technological sense; if it is possible to emit and observe them, it is almost trivial to build a machine that sends a signal in a closed timelike loop, effectively allowing us to send information from the future to the present. In other words, future me should be able to send present me a signal, preferably with the blueprints for the time machine of course (why do all that hard work if I can get the blueprints from future me for free?) So, I said, if faster-than-light neutrinos exist, then future me should contact present me in three…, two…, one…, now! Hmmm… no contact. No faster-than-light neutrinos, then.

But that’s when I suddenly remembered an uncanny occurrence that happened to me just hours earlier, yesterday morning. We ran out of bread, and we were also out of the little mandarin or clementine oranges that I like to have with my breakfast. So I took a walk, visiting our favorite Portuguese bakery on Nelson street, with a detour to the nearby Loblaws supermarket. On my way, I walked across a small parking lot, where I suddenly spotted something: a mandarin orange on the ground. I picked it up… it seemed fresh and completely undamaged. Precisely what I was going out for. Was it just a coincidence? Or perhaps future me was trying to send a subtle signal to present me about the feasibility of time machines?

If it’s the latter, maybe future me watched too much Doctor Who, too. Next time, just send those blueprints.

 Posted by at 12:43 pm
Sep 032011

I am reading an article in Science about the efforts of people like planetary scientist David Morrison to allay fears concerning a prophesied collision between the Earth and the mythical planet Nibiru. Apparently, some folks are taking this pseudoscientific hogwash so seriously, they are even contemplating suicide. Good people like Morrison are trying to talk sense into them.

Perhaps they shouldn’t. Here is my message: go ahead, kill yourself. That means that for the rest of us, 2013 will be a happier year, because fewer idiots will roam the Earth.

But just to demonstrate that I am not all arrogant and cruel, here’s another option: you can always choose to come to your senses before December 21, 2012, realize that stuff in Hollywood movies should not be confused with real life, and go on living.

 Posted by at 2:28 pm
Jul 192011

RadioAstron, aka Spectrum-R, is in orbit. If it successfully opens its 10-meter dish antenna a few days from now, it will join the list of great space-based telescopes. It also signals that Russia is still a strong space-capable nation, doing much more than cheaply ferrying foreign astronauts to the International Space Station, filling the gap left behind by the retirement of the Shuttle program.

Tomorrow will be the 42nd anniversary of Armstrong’s “One small step”. How many years do we need to wait for the next small step taken by a human, be it in the dust of the Moon, the red rocks of Mars, or the cold surface of an asteroid?

 Posted by at 5:33 pm
May 162011

Here’s one good reason to quit your day job: spend a year or so lugging a set of six astrophotography cameras across two continents, to take some 37,000 exposures of the night sky and stitch them together into an amazing all-sky picture called The Photopic Sky Survey. The scientific value of this picture may be negligible, but it is beautiful to look at, and I am sorely tempted to buy a high-quality print to hang somewhere on my wall. It is one of those pictures that give a true sensation of depth: you can see how nearby stars slowly dissolve into a diffuse, milky background, you can see how dust lanes obscure the Milky Way behind, you can see how the light of distant galaxies filters through. I wonder how a picture like this would have influenced The Great Debate.

 Posted by at 12:49 pm
Jan 222010

Here’s a nice tennis ball, photographed from both sides:

It’s a big one, mind you, almost a thousand miles across. It’s Saturn’s moon Iapetus, famous because one side of it is significantly brighter than the other. The explanation, however, is more mundane than that offered in the book version of Clarke’s 2001: A Space Odyssey; the most recent hypothesis is that the discoloration is due to the thermal migration of ice.

 Posted by at 7:44 pm
Aug 252009

I’m reading the autobiography of Fred Hoyle, and I’ve been perusing Wikipedia for background, in particular, reading about the Jodrell Bank radio telescope and its founder, Sir Bernard Lovell.

This is how I came across a news item from earlier this year, according to which Lovell recently revealed that back in 1963, he has been targeted by Soviet assassins during a visit to the Soviet Union.

This sounds improbable except… even in recent years, Russian intelligence agents/agencies have been using novel methods in assassination attempts (e.g., radioactive polonium in the case of Litvinenko). Further, the rationale Lovell gives is quite plausible: back in 1963, when satellite-based early warning systems were not yet available, something like Jodrell Bank may very well have served either as an over-the-horizon radar or perhaps using the Moon as a reflector.

Lovell promises to reveal more posthumously. What can I say? Our curiosity can wait. I wish him many more happy and healthy years.

 Posted by at 3:23 am
Jul 082009

The premier Internet physics and astronomy preprint archive, ArXiv, seems to be having some serious problems tonight. I used the catchup interface to check for new papers, only to find messages like this:

Problem displaying entry for arXiv:0907.1079

Apparently all new papers are unavailable, and many older papers, too… I checked briefly and found papers dating back to last October that appear to have vanished. Including some half a dozen or so papers of my own.

I sure hope they keep backups!

 Posted by at 3:00 am
Jun 092009

The reason why I am concerning myself with more Maxima examples for relativity is that I am learning some subtle things about Brans-Dicke theory and the Parameterized Post-Newtonian (PPN) formalism.

Brans-Dicke theory is perhaps the simplest modification of general relativity. Instead of the gravitational constant, G, the theory has a scalar field φ, and the theory’s Lagrangian now reads

L = [φR − ω∂μφ∂μφ/φ] / 16π.

Here, R is the curvature scalar and ω is an unspecified constant of the theory.

The resulting field equations are just like Einstein’s, except for two things. First, the field equations for the metric now have additional terms containing derivatives of φ; second, there is a new field equation for the scalar field φ that basically says that the d’Alembertian of φ is proportional to the trace of the stress-energy tensor.

Clever people tell you that Brans-Dicke theory is practically excluded by solar system data, as it would only work for insanely high values of ω. They demonstrate this by building approximate solutions for the theory using the PPN formalism, and find that one of the PPN parameters, γ, will have the value of γ = (1 + ω) / (2 + ω); on the other hand, observations by the Cassini spacecraft restrict γ to |γ − 1| < 2.3 × 10−5, so |ω| must be at least 40,000.

Now here’s the puzzling bit: if you solve Brans-Dicke theory in a vacuum, you find that the celebrated Schwarzschild solution of general relativity still applies:  keeping φ constant, you just get back this common solution which is known to fit solar system data well, and which has, most importantly, γ = 1 and the value of ω doesn’t matter.

So which is it? Is it γ = 1 or is it γ = (1 + ω) / (2 + ω)? Something is amiss here.

This dilemma can be resolved once you realize that whereas general relativity has a unique spherically symmetric, static vacuum solution, this is not the case for Brans-Dicke theory. This theory has an infinite family of spherically symmetric, static vacuum solutions. Indeed, I think you could actually use the value of γ to parameterize this solution space. However, once you allow some matter into that vacuum, no matter how little, you are locked in to a specific solution, for which γ = (1 + ω) / (2 + ω). In other words, the only vacuum solution that is consistent with the notion of taking the limit of a matter solution by gradually removing matter is NOT the Schwarzschild solution of general relativity, but another, incompatible solution.

This has extremely important implications for our work on MOG. So far, we have obtained a vacuum solution that appears consistent with observations on scales from the solar system to cosmology. However, a recent paper by Deng et al. challenges this work by suggesting that the MOG PPN parameter γ is not 1 and hence, the theory runs into the same trouble as Brans-Dicke theory in the solar system. Is this true? Did we pick a vacuum solution that happens to be inconsistent with matter solutions? This is what I am trying to investigate.

 Posted by at 12:45 pm
Jan 072009

Here’s an article worthy of a bookmark:

It offers a way to produce a chart in Microsoft Excel much like this one:

Filled XY area chart

Filled XY area chart

This chart is from something I’m working on, an attempt to test gravitational theories against galaxy survey data.

The link above also comes with a warning: the discussed technique doesn’t work with Excel 2007, due to a (presumably unintentional) change in Excel’s handling of certain complex charts. A pity, but it is also a good example why I am trying to maintain my immunity against chronic upgrade-itis. Two decades ago upgrades were important because they fixed severe bugs and offered serious usability improvements. But today? Why on Earth would I want to upgrade to Office 2007 when Office 2003 does everything I need and more, just so that I can re-learn its user interface? Or make Microsoft richer?

 Posted by at 3:51 pm