May 212016

Not for the first time, I am reading a paper that discusses the dark matter paradigm and its alternatives.

Except that it doesn’t. Discuss the alternatives, that is. It discusses the one alternative every schoolchild interested in the sciences knows about (and one that, incidentally, doesn’t really work) while ignoring the rest.

This one alternative is Mordehai Milgrom’s MOND, or MOdified Newtonian Dynamics, and its generalization, TeVeS (Tensor-Vector-Scalar theory) by the late Jacob Bekenstein.

Unfortunately, too many people think that MOND is the only game in town, or that even if it isn’t, it is somehow representative of its alternatives. But it is not.

In particular, I find it tremendously annoying when people confuse MOND with Moffat’s MOG (MOdified Gravity, also MOffat Gravity). Or when similarly, they confuse TeVeS with STVG (Scalar-tensor-Vector Gravity), which is the relativistic theory behind the MOG phenomenology.

So how do they differ?

MOND is a phenomenological postulate concerning a minimum acceleration. It modifies Newton’s second law: Instead of \(F = ma\), we have \(F = m\mu(a/a_0)a\), where \(\mu(x)\) is a function that satisfies \(\mu(x)\to 1\) for \(x\gg 1\), and \(\mu(x)\to x\) for \(x\ll 1\). A good example would be \(\mu(x)=1/(1+1/x)\). The magnitude of the MOND acceleration is \(a_0={\cal O}(10^{-10})~{\rm m}/{\rm s}\).

The problem with MOND is that in this form, it violates even basic conservation laws. It is not a theory: it is just a phenomenological formula designed to explain the anomalous rotation curves of spiral galaxies.

MOND was made more respectable by Jacob Bekenstein, who constructed a relativistic field theory of gravity that approximately reproduces the MOND acceleration law in the non-relativistic limit. The theory incorporates a unit 4-vector field and a scalar field. It also has the characteristics of a bimetric theory, in that a “physical metric” is constructed from the true metric and the vector field, and this physical metric determines the behavior of ordinary matter.

In contrast, MOG is essentially a Yukawa theory of gravity in the weak field approximation, with two twists. The first twist is that in MOG, attractive gravity is stronger than Newton’s or Einstein’s; however, at a finite range, it is counteracted by a repulsive force, so the gravitational acceleration is in fact given by \(a = GM[1+\alpha-\alpha(1+\mu r)e^{-\mu r}]\), where \(\alpha\) determines the strength of attractive gravity (\(\alpha=0\) means Newtonian gravity) and \(\mu\) is the range of the vector force. (Typically, \(\alpha={\cal O}(1)\), \(\mu^{-1}={\cal O}(10)~{\rm kpc}\).) The second twist is that the strength of attractive gravity and the range of the repulsive force are both variable, i.e., dynamical (though possibly algebraically related) degrees of freedom. And unlike MOND, for which a relativistic theory was constructed after-the-fact, MOG is derived from a relativistic field theory. It, too, includes a vector field and one or two scalar fields, but the vector field is not a unit vector field, and there is no additional, “physical metric”.

In short, there is not even a superficial resemblance between the two theories. Moreover, unlike MOND, MOG has a reasonably good track record dealing with things other than galaxies: this includes globular clusters (for which MOND has to invoke the nebulous “external field effect”), cluster of galaxies (including the famous Bullet Cluster, seen by some as incontrovertible proof that dark matter exists) and cosmology (for which MOND requires something like 2 eV neutrinos to be able to fit the data.)

MOG and the acoustic power spectrum. Calculated using \(\Omega_M=0.3\), \(\Omega_b=0.035\), \(H_0=71~{\rm km}/{\rm s}/{\rm Mpc}\). Also shown are the raw Wilkinson Microwave Anisotropy Probe (WMAP) three-year data set (light blue), binned averages with horizontal and vertical error bars provided by the WMAP project (red) and data from the Boomerang experiment (green). From arXiv:1104.2957.

There are many issues with MOG, to be sure. Personally, I have never been satisfied with the way we treated the scalar field so far, and I’d really like to be able to derive a proper linearized version of the theory in which the scalar field, too, is accommodated as a first-class citizen. How MOG stands up to scrutiny in light of precision solar system data at the PPN level is also an open question.

But to see MOG completely ignored in the literature, and see MOND used essentially as a straw man supposedly representing all attempts at creating a modified gravity alternative to dark matter… that is very disheartening.

 Posted by at 5:23 pm
May 202016

CNN reports that ACARS messages received from MS804 in the minutes before its disappearance indicate a fire on board.

I was able to find online a copy of the screenshot discussed by CNN:

CNN’s talking heads are still discussing the terrorism theory, but in light of these data, it looks increasingly unlikely. It is very difficult to imagine a terrorism scenario that begins with one of the cockpit windows.

 Posted by at 8:25 pm
May 182016

In the fourth volume of the Hitchhiker’s Guide to the Galaxy “trilogy”, we learn that just before the Earth was about to be destroyed by the Vogons to make way for a new interstellar bypass, the whales left. They left behind a simple parting message: “So long and thanks for all the fish.”

Which makes me feel rather alarmed now that I am learning that hundreds of North Atlantic right whales went missing. I hope it’s not a bad sign.

 Posted by at 7:55 pm
May 182016

A question on Quora made me reminisce about old computer games that make me feel somewhat nostalgic.

I’ve been involved with computer games both as a player and in a professional capacity for a very long time.

Long before I laid my hands on a personal computer, I was an avid player of Trek on a PDP/11. This was a game written for text terminals, simulating the mission of the Starship Enterprise through Klingon-infested space:

Another game of similar vintage, which I used to play on a peer-to-peer QNX network, is Hack:

Then there was the Commodore-64. Here are two Commodore-64 games that I remember fondly. Impossible Mission:

And Jumpman:

After the Commodore-64 came the Amiga. One of the first games I played on the Amiga 500 was the absolutely surrealist Mind Walker:

Very weird game. Memorable, algorithm-generated music. Ahead of its time.

Moving on to the PC (actually, I first played these on the Atari ST), there are the classic INFOCOM games. (Yes, I am taking the liberty of classifying pure text adventure games as “video games”.) Best known perhaps is Zork:

But there was also the unforgettable apocalyptic story of Trinity:

The equally unforgettable A Mind Forever Voyaging in which you get to play a disembodied artificial intelligence:

And the hilarious Hitchhiker’s Guide to the Galaxy with its fiendishly impossible puzzles:

Still on the text game front, back in 1991 I began playing what was for me the first multiplayer online game, British Legends, aka. MUD (Multi-User Dungeon):

Meanwhile, on my PC, I was busy playing Duke Nukem, its platform versions first, eventually moving on to Duke Nukem 3D (which exists to this day in a community supported 32-bit high-resolution version, complete with NSFW imagery):

And then came Myst, the “killer app” for CD-ROMs:

Last but not least, a game that gave me tremendous amounts of joy, Lands of Lore: Guardians of Destiny (with none other than Patrick Stewart lending his voice acting skills to the CD version):

I remember all these games very fondly. And they are all still eminently playable, and very enjoyable, to this day.

 Posted by at 3:42 pm
May 112016

No, I am not using expletives.

Or rather, I’ve been using some expletives, but *#0808# is not code for one of them.

It is an actual code that I can enter into my Samsung phone to get to a service menu that allows me to re-enable USB functions that somehow got turned off.

Although it took only about 15 minutes to find this particular code, it marked the end of a rather frustrating 24 hours. Last night, as it was just about to complete installing 24 Microsoft updates, my workstation locked up. The incomplete installation of updates managed to mess up my Microsoft Office setup, and made it impossible to install some still missing updates. Which meant that I had to use System Restore to get back to a known-good state first, and then redo the updates.

As a result, much of my day was consumed (and it’s not like I slept much last night either.) And as if that wasn’t enough, my phone also suddenly decided that it didn’t want to connect to my workstation anymore… hence my need for the aforementioned code.

All is well that ends well, though, and in the end I managed to install everything. It’s just that those hours of my life that I lost, I’ll never get them back.

It also reinforced my conviction that I made the right decision when, a few days ago, I decided to invest some money and purchase parts for a new workstation and server. It’s about bleeping time… this machine served me well for over seven (!) years, and seven years in this profession is almost an eternity.

Still waiting for some of the parts though. Although I ordered everything from the same supplier,, the shipments come from at least four different locations in North America.

 Posted by at 8:47 pm
May 052016

And just when you least expect it… Russia celebrates its victories in Syria over ISIL with a class act, an amazing concert by the Mariinsky Theatre Orchestra of St. Petersburg, held at the ancient amphitheater that is at the center of Palmyra’s Roman era ruins, badly damaged (not to mention desecrated with barbaric public executions) by the Islamic State.

Russia, of course, intervened not for reasons of altruism but because American indecisiveness offered them an opportunity to prop up Assad’s regime. Nonetheless, I much rather watch an amazing concert like this than public beheadings.

And the music was, in fact, amazing. It included a piece titled Quadrille, from contemporary Russian composer Rodion Shchedrin’s opera Not Love Alone. I think I ought learn a little more about contemporary Russian opera.

The concert was carried live (of course) by RT, complete with a televised greeting by Putin. Not unlike a similar concert that the same orchestra held in South Ossetia, after Russia’s brief war with the Republic of Georgia a few years ago.

 Posted by at 11:12 pm