MoND denied

Almost exactly a year ago, I wrote a post about MoND -- modified Newtonian dynamics -- a new(ish) model of gravitation that purported to explain some of odd measurements of stellar and galactic motion without the necessity of dark matter.

Here's the situation.

In the 1970s and 1980s, the brilliant astronomer Vera Rubin discovered something peculiar.  The project she was working on involved mapping the speed of revolution of stars around galactic centers.  According to Newton's Law of Gravitation and Kepler's Laws of Planetary Motion (which, after all, have the same mathematical underpinning), stars farther away from the center should be moving more slowly.  This principle works fine, for example, in our own Solar System; Neptune moves more slowly than Mercury does.

This, Rubin found, turned out not to be true on larger scales.  The velocities of stars in the farther reaches of galaxies were moving just as fast as the ones closer to the center.  Nicknamed the flat rotation curve problem, it seemed like the only possible explanation was that there was more mass in the galaxy than had been detected -- something appeared to be causing the outer stars to orbit faster than Newtonian dynamics said they should.

Rubin and others called this mysterious something dark matter. 

And you probably know the amount of this stuff is significant.  If you add up all the detectable mass/energy in the universe, only 5% of it is ordinary matter.  26.8% of it is dark matter, and 68.2% is dark energy, an unrelated type of mass/energy that is thought to be responsible for the runaway expansion of the universe, and which is even less understood than dark matter is.

Dark matter interacts with regular matter via gravity, but -- as far as we can tell -- in no other way.  It seems to be completely unaffected by the other forces that act on the ordinary stuff we see on a day-to-day basis.  There have been various experiments set up to try to detect dark matter particles, but as of the time of this writing, every single one of them has come up empty-handed.  It's bizarre to think about; a substance that makes up five times more of the mass of the universe than all the regular matter put together, and thus far, we haven't the slightest idea what it's made of.

There's also the problem that the Standard Model -- the framework that accounts for all the ordinary matter particles, and how they interact -- is one of the most rigorously-tested theories in science, and performs to a level of precision that beggars belief.  And nothing in the Standard Model appears to admit of some strange extra particle(s) that might account for dark matter.

Well, along came Mordehai Milgrom, who in 1983 tried something a little like what Einstein did with respect to the luminiferous aether -- he said, "I have a new theory that eliminates the need for dark matter entirely."  It's called modified Newtonian dynamics, MoND for short, and proposes that the problem is that Newton's Law of Gravitation doesn't work for objects experiencing really low accelerations (like the stars in the outer reaches of galaxies).  Like the Theories of Relativity, it leaves the model relatively unchanged at the velocities and accelerations we encounter on a daily basis; here on the surface of the Earth, Newton still works just fine.  But what Einstein did for systems in extreme gravitational fields or high velocities, Milgrom did for systems experiencing really low accelerations -- tweaking the mathematics to make it match the observations.

And those tweaks, in one stroke, eliminated the need for some hypothetical and undetectable form of matter.

The Whirlpool Galaxy [Image licensed under the Creative Commons NASA/ESA/JPL/Hubble Heritage Team & C. Violette, M51 (2), CC BY-SA 4.0]

Well, the race was on to try to figure out which model was correct -- Newton (with dark matter) or MoND (without it).  And earlier this year, Korean astrophysicist Kyu-Hyun Chae seemed to have settled it once and for all, showing that observations of wide binaries -- pairs of stars orbiting their common centers of gravity at large distances -- matched the predictions of MoND brilliantly.

However, there were astrophysicists who immediately had objections.  Chae, they said, had not done a good job of eliminating data points that were problematic.  The difficulty is that if you're observing a binary pair from Earth, to figure out the velocities and accelerations of the stars in the pair, you have to take into account a variety of complicating factors, including:

• the speed the entire system is moving toward or away from Earth
• the eccentricity (elliptical-ness) of the orbit
• the inclination of the orbit -- how much it's tilted toward or away from us

So a group of researchers, led by astrophysicist Indranil Banik of the University of St. Andrews, has developed a technique for sifting through the data points and using the ones for which there is the best confidence in the velocity measurements (significantly, Banik's team only eliminated about twenty percent of Chae's data points).  And in their paper, which came out five days ago, they found when they do that, the agreement of the data with MoND vanishes completely.

Without the wonky data points, the measurements from wide binaries that seemed to settle the argument in favor of MoND actually agree with Newtonian dynamics...

... to a confidence of 19 σ.  To us non-scientists -- as the wonderful YouTuber Dr. Becky Smethurst explains it -- a 19 σ confidence level means there is only a one in one hundred thousand trillion trillion trillion chance that their result is a statistical fluke.  (And if you want to know more, I highly recommend watching Dr. Becky's video on the new paper, which is awesome.)

So with regards to theories of gravity, it appears that Newton is the only game in town, meaning we're stuck with dark matter.  MoND is dead in the water, so unless someone comes up with some sort of different model entirely that matches the data better than Newton does, we'll have to keep looking for this ghostly matter whose only fingerprint is its gravity.

It's an exciting time to be an astrophysicist... or just a deeply curious science nerd.

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Newton modified

Back in the 1970s and 1980s, astrophysicist Vera Rubin discovered something odd about the rates at which stars were revolving around their home galaxies; the stars in the outer reaches of the galaxy were orbiting as quickly as the ones nearer to the center.

Called the "flat rotation curve problem," this observation flies in the face of an astronomical principle that's been known since the seventeenth century, which is Kepler's Third Law.  Kepler's Third Law states that for bodies orbiting the same center of gravity, the square of the orbital period (time taken for the object to make a single orbit) is proportional to the cube of the average distance between the object and the center of gravity.  Put more simply, the farther out an orbiting object is, the slower it should be moving.  This law holds beautifully for the planets, asteroids, and comets in the Solar System.

Unfortunately, when Rubin looked at galactic rotation rates, she found that Kepler's Third Law appeared not to hold.  What it looked like was that there was a great deal more mass in the galaxy than could be seen, and that mass was spread out in some kind of invisible halo surrounding it.  That additional mass would account for the flatness of the rotation curves.

It was forthwith nicknamed dark matter.

The calculations of Rubin and others showed that the amount of dark matter was not insignificant.  Current estimates place it at around 27% of the total mass of the universe.  Only 5% is baryonic (ordinary) matter, so the matter we can't see outweighs ordinary matter by over a factor of five.  (The other 68% is the even weirder and more elusive dark energy, about which we know next to nothing.)

The problem is, every experiment designed to directly detect dark matter has resulted in zero success.  Whatever it is, it seems not to interact with ordinary matter at all other than via its gravitational pull.  These repeated failures drew rueful comparisons between dark matter and the luminiferous aether, the mysterious substance through which light waves were alleged to propagate.  The aether was proposed back in the nineteenth century because it was hard to imagine how light waves moved through a vacuum unless it had a medium -- what, exactly, was waving?  The existence of aether was conclusively disproven by the elegant Michelson-Morley experiment, which showed that unlike any other kind of wave, the speed of light waves seemed to be invariant regardless of the direction of motion of the observer.  It remained for Albert Einstein to explain how that could possibly be -- and to figure out all the strange and counterintuitive outcomes of this phenomenon, with his Special and General Theories of Relativity.

More than one modern physicist has surmised that dark matter might similarly be the result of a fundamental misunderstanding of how gravity works -- and that we are just waiting for this century's Einstein to turn physics on its head by pointing out what we've missed.

Enter Israeli physicist Mordehai Milgrom.

Milgrom is the inventor of MoND (Modified Newtonian Dynamics), a model which -- like the Theories of Relativity -- proposes that the explanation for the anomalous observations is not that there's some unseen and undetectable substance causing the effect, but that our understanding of how physics works is incomplete.  In particular, Milgrom says, there needs to be a modification to the equations of motion at very small accelerations, such as the ones experienced by stars orbiting in the outer reaches of galaxies.

With those modifications, the orbital rates make perfect sense.  No dark matter needed.

The Whirlpool Galaxy [Image licensed under the Creative Commons NASA/ESA/JPL/Hubble Heritage Team & C. Violette, M51 (2), CC BY-SA 4.0]

As with relativity -- and any other time someone has claimed to overturn a long-established paradigm -- MoND hasn't achieved anywhere near universal acclaim.  The Wikipedia article on it (linked above) states, gloomily, "no satisfactory cosmological model has been constructed from the hypothesis."  And it does lack the blindingly bright insight of Einstein's models, where taking the "problem of the seeming invariance of the speed of light" and turning it into the "axiom of the actual invariance of the speed of light" triggered a shift in our understanding that has since passed every empirical test ever designed.  Compared to Einstein's model, MoND almost seems like "Newton + an add-on," with no particularly good explanation as to why high accelerations obey Newton's laws but low ones don't.  (Of course, there's a parallel here to Einstein, as well -- at low speeds, Newton's laws are accurate, while at near-light speeds, Einsteinian effects take over.  So maybe Milgrom is on to something after all.)

After all, it's not like the other option -- dark matter -- has much going for it experimentally.

And MoND just got a significant leg up with an observation of the behavior of star clusters that was the subject of a paper in Monthly Notices of the Royal Astronomical Society last week.  In open star clusters, as new stars ignite it produces an outward push that can blow away material (including other stars), creating two "tidal tails" that precede and trail the cluster as it moves through space.  According to Newtonian dynamics (with or without dark matter), the two tails should have about the same mass.

"According to Newton's laws of gravity, it's a matter of chance in which of the tails a lost star ends up," explains Dr. Jan Pflamm-Altenburg of the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn.  "So both tails should contain about the same number of stars.  However, in our work we were able to prove for the first time that this is not true: In the clusters we studied, the front tail always contains significantly more stars nearby to the cluster than the rear tail."

This peculiar observation fits the predictions of MoND much better than it does the predictions of the Newtonian model.

"The results [of simulations using MoND] correspond surprisingly well with the observations," said Ingo Thies, co-author of last week's paper.  "However, we had to resort to relatively simple computational methods to do this.  We currently lack the mathematical tools for more detailed analyses of modified Newtonian dynamics."

So the matter is very far from settled.  What's certain is that, similar to the physicists' situation in the late nineteenth century with regards to the behavior of light, there's something significant we're missing.  Whether that's some odd form of matter that doesn't interact with anything except via gravity, or because we've got the equations for the laws of motion wrong, remains to be seen.

And of course, after that, we still have dark energy to explain.  I think the physicists are going to be busy over the next few decades, don't you?

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The swing of a pendulum

 Physicists have a serious problem.

Back in the mid-1970s, astrophysicist Vera Rubin made an interesting discovery.  She had initially been interested in quasars, but moved away from that because the subject was "too controversial" -- and landed herself in the midst of one of the biggest scientific controversies to hit the field since the discovery of the quantum nature of reality back in the 1920s and 1930s.

She was looking at the rotation rates of galaxies, and found something curious; based on what was known about gravitational interactions between massive objects, the outer fringes of every galaxy she studied were moving at the "wrong" velocity.  The outermost stars were moving far faster than the model predicted, suggesting there was some unseen mass increasing the gravitational field and whirling the edges of the galaxy around faster than the visible matter could have.

And it wasn't by a small margin, either.  Rubin's calculations suggested that there was five times the unseen stuff as there was all of the visible matter in the galaxy put together.  This was way too much to be accounted for by something like diffuse dust clouds or other agglomerations of non-luminous, but completely ordinary, matter.  Rubin nicknamed the invisible stuff dark matter, more or less as a placeholder name until the physicists could figure out what the stuff was, something most researchers figured would be accomplished in short order.

Almost fifty years later, we still are hardly any further along.  Better measurements have confirmed that there is way more dark matter than ordinary matter; Rubin's estimate was spot-on, and current data indicates that 27% of the universe's total mass is dark matter, as compared to only 5% ordinary matter.  (The other 68% is an even more mysterious thing called dark energy, about which the astrophysicists are even more completely, um, in the dark.)

Every attempt to figure out the nature of dark matter -- or even to detect it by anything else but its gravitational effects on the galactic scale -- has resulted in failure.  The leading candidate, called weakly interacting massive particles (WIMPs), has been the subject of repeated detection attempts, and every single one of them has generated "null results."

Which is science-speak for "bupkis."

At some point, you have to wonder if the scientists are going to give the whole thing up as a bad job.  The problem is, if that happens you have 95% of the universe made of stuff we can't account for, which isn't a state of affairs anyone is happy with.

So a team at the National Institute of Standards and Technology is giving dark matter one more chance to show itself, using the only way in which we're certain it interacts with ordinary matter -- gravity.

The trouble is, gravity is a really weak force.  It's only a big player in our lives because we live on a massive chunk of rock, big enough to have a significant gravitational field.  Of the four fundamental forces -- gravity, electromagnetism, and the weak and strong nuclear forces -- gravity is weaker than the next in line (electromagnetism) by a factor of 10 to the 36th power.

So gravity is 1,000,000,000,000,000,000,000,000,000,000,000,000 times weaker than the electromagnetic force that holds molecules together, generates static electricity, and toasts your bread in the morning.

How on earth could you detect something that small, when even a trace of a stray electrical field could overwhelm it by many orders of magnitude?  The NIST scientists think they have the answer: an array of over a billion tiny, incredibly sensitive pendulums, each only a millimeter long, shielded and then cooled to near absolute zero to minimize interference from other forces.

[Image licensed under the Creative Commons Ben Ostrowsky, Foucault's Pendulum, CC BY 2.0]

There are four possibilities of what could happen to the array:

• Nothing.  Then we're back to the drawing board.
• Motion of one or two pendulums only.  This is probably due to interaction with an ordinary matter particle, which would hit a pendulum and stick, causing it to swing but leaving the ones around it unaffected.
• Chaotic or random movement in a number of the pendulums.  This "noise" would most likely be caused by a fluctuation in an electric field -- i.e. the array wasn't well enough shielded.
• A coordinated "ripple" passing through the detector, setting more or less a straight line of the pendulums swinging.  This, the researchers say, would be the signal of a dark matter particle zooming through the array, and its gravitational ripple streaking across in a specific direction.

Of course, even if the best possible outcome -- option #4 -- occurs, it still doesn't tell us what dark matter is.  After all, Vera Rubin's research in the 1970s showed that it interacts gravitationally with ordinary matter (i.e., we already knew that).  But at least we'll have a demonstration that it exists, that we're not looking at something like the nineteenth century's luminiferous aether, the mysterious substance that supposedly was the medium through which light waves propagated, and was shown not to exist by the Michelson-Morley interferometer experiment (and the nature of light propagation ultimately explained by Einstein and others, decades later).

So I'll be eagerly awaiting the outcome.  Right now, the array is still in development, so it will be a while before we can expect results.  But if it generates positive results, it'll be the first conclusive demonstration that we're talking about something detectable right here on Earth, not just by its effects on distant galaxies.

Of course, that still leaves us with the other 68% unknown stuff to explain.

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Have any scientifically-minded friends who like to cook?  Or maybe, you've wondered why some recipes are so flexible, and others have to be followed to the letter?

Do I have the book for you.

In Science and Cooking: Physics Meets Food, from Homemade to Haute Cuisine, by Michael Brenner, Pia Sörensen, and David Weitz, you find out why recipes work the way they do -- and not only how altering them (such as using oil versus margarine versus butter in cookies) will affect the outcome, but what's going on that makes it happen that way.

Along the way, you get to read interviews with today's top chefs, and to find out some of their favorite recipes for you to try out in your own kitchen.  Full-color (and mouth-watering) illustrations are an added filigree, but the text by itself makes this book a must-have for anyone who enjoys cooking -- and wants to learn more about why it works the way it does.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Stretching the boundaries

Be honest, can you tell me anything about the following people?

• Annie Jump Cannon
• Jocelyn Bell Burnell
• Henrietta Swan Leavitt
• Willamina Fleming
• Maria Mitchell
• Ruby Payne-Scott
• Nancy Roman
• Vera Rubin

Okay, what about the following?

• Nikolaus Copernicus
• Johannes Kepler
• Neil DeGrasse Tyson
• Stephen Hawking
• William Herschel
• Christiaan Huygens
• Carl Sagan
• Edwin Hubble

My guess is that the typical reader recognized six or seven people on the second list, and could probably have named a major contribution for at least five of them.  I'd also wager that the average recognition for the first list is one or two -- and that most people couldn't tell you what the accomplishments were for the ones they did recognize.

Okay, I admit, it's pretty obvious what I'm driving at, here.  I'm not known for my subtlety.  And lest you think I'm deliberately comparing some chosen-to-be-minor female astronomers with a list of male Big Names, here are the major contributions for the women on the first list.

Annie Jump Cannon (1863-1941) is responsible for the current stellar classification system, in which stars are categorized by their spectral output and temperature -- an achievement that was critical for our understanding of stellar evolution.  So when you're watching Star Trek: The Next Generation and Commander Data says, "It is a typical M-class star" -- yeah, that was Annie Jump Cannon's invention.  Oh, and did I mention that she wasn't just female in a time when women were virtually prohibited from becoming scientists, but she was almost completely deaf?  Remember that when you think about the obstacles you have to overcome to reach your goals and dreams.

Jocelyn Bell Burnell (b. 1943) is an astrophysicist from Northern Ireland who was responsible for the discovery and explanation of pulsars in 1967.  Her claim that they were rapidly-rotating neutron stars was at first dismissed -- some scientists even derided the data itself, calling her discovery of the flashing star "LGM" (Little Green Men) -- and she wasn't included in the 1974 Nobel Prize awarded to scientists involved in the research that confirmed her hypothesis.  (Her other awards, though, are too numerous to list here, and she showed her typical graciousness in accepting her exclusion from the Nobel, but it pissed off a slew of influential people and opened a lot of eyes about the struggles of women in science.)

Henrietta Swan Leavitt (1868-1921) was an American astronomer who discovered a seemingly trivial fact -- that the bright/dark periodicity of a type of variable star, Cepheid variables, is directly proportional to its intrinsic brightness.  She very quickly realized that this meant Cepheids could be used as "standard candles" -- a light source with a known actual brightness -- to allow astronomers to figure out how far away stars are.  This understanding was half of the solution to the question of the age of the universe, which added to red shift proved that the universe is expanding, and ultimately led to the Big Bang theory.

Willamina Fleming (1857-1911) was a Scottish astronomer who discovered (literally) thousands of astronomical objects, including the now-famous Horsehead Nebula.  She was one of the founding members of the "Harvard Computers," a group of women who took on the task of doing mathematical calculations using data from the Harvard Observatory -- after Fleming noted that the work their male counterparts had been doing could have been bettered by her housekeeper.

Maria Mitchell (1818-1889) was an American astronomer whose accomplishments were so many and varied that I could go on for pages just about her.  She was the first female professor of astronomy at an American college (Vassar), the first female editor of a column in Scientific American, was director of Vassar's observatory for twenty years, came up with the first good explanation for sunspots, pioneered investigations into stellar composition, and discovered (among other things) a comet before it was visible to the naked eye.  She was an incredibly inspiring teacher -- twenty-five of her students went on to be listed in Who's Who.  "I cannot expect to make astronomers," she once said to her class, "but I do expect that you will invigorate your minds by the effort at healthy modes of thinking.  When we are chafed and fretted by small cares, a look at the stars will show us the littleness of our own interests."

Ruby Payne-Scott (1912-1981) was an Australian scientist who became the first female radioastronomer, who was responsible for linking the appearance of sunspots with radio bursts from the Sun and was also instrumental in developing radar for detecting enemy planes during World War II.  She was not only an astronomer but a gifted physicist and electrical engineer, and made use of all three in her research -- but opportunities for women in science were so limited that in 1963 she resigned as an astronomer and became a secondary school teacher.  But she never ceased fighting for women's voices in science, and in 2008 the Commonwealth Scientific and Industrial Research Organization began the Payne-Scott Award in her honor to support women in science, especially those returning to the research world after taking time for maternity leave.

Nancy Roman (1925-2018) was an American astronomer who was one of the first female executives at NASA, and who has been nicknamed the "Mother of Hubble" for her instrumental role in developing the Hubble Space Telescope.  She did pioneering work in the calculation of stellar velocities -- all this despite having been actively discouraged from pursuing a science career, most notably by a high school counselor when she suggested she'd like to take algebra instead of Latin.  The counselor sneered, "What kind of lady would take mathematics instead of Latin?"  Well, this lady would, and went on to be the recipient of four honorary doctorates (as well as the one she earned), received an Exceptional Scientific Achievement Medal from NASA and a fellowship with the American Association for the Advancement of Science, and was the recipient of many other awards.

Vera Rubin (1928-2016) was an American astronomer whose observation of anomalies in galactic rotation rates led to what might be the weirdest discovery in physics in the last hundred years -- "dark matter."  Her work, according to the New York Times, "usher[ed] in a Copernican-style change in astronomy," and the Carnegie Institute said after her death that the United States had "lost a national treasure."

Honestly, it's Rubin who got me thinking about all of this gender inequity, because I found out that last month the Large Synoptic Survey Telescope was renamed the Vera C. Rubin Observatory, and when I posted on social media how awesome this was, I had several people respond, "Okay, cool, but who is she?"  We like to pride ourselves on how far we've come in terms of equity, but man, we have a long way to go.  Famous straight white male scientists become household names; equally prestigious scientists who are women, LGBTQ, or people of color often become poorly-recognized footnotes.

Don't you think it's time for this to change?

The amazing Vera Rubin in 2009 [Image is in the Public Domain]

I know this is a battle we won't win overnight, but the dominance of straight white males in science has resulted in the stifling of so incredibly much talent, hope, and skill that we ought to all be working toward greater access and opportunity regardless of our own gender, skin color, or sexual orientation.  My little exercise in considering some female astronomers probably won't count for that much, but I'm hoping that it might open a few eyes, invert a few stereotypes, and stretch a few boundaries -- and whatever motion we can have in that direction is nothing but positive.

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This week's Skeptophilia book of the week is simultaneously one of the most dismal books I've ever read, and one of the funniest; Tom Phillips's wonderful Humans: A Brief History of How We Fucked It All Up.

I picked up a copy of it at the wonderful book store The Strand when I was in Manhattan last week, and finished it in three days flat (and I'm not a fast reader).  To illustrate why, here's a quick passage that'll give you a flavor of it:

Humans see patterns in the world, we can communicate this to other humans and we have the capacity to imagine futures that don't yet exist: how if we just changed this thing, then that thing would happen, and the world would be a slightly better place. 

The only trouble is... well, we're not terribly good at any of those things.  Any honest assessment of humanity's previous performance on those fronts reads like a particularly brutal annual review from a boss who hates you.  We imagine patterns where they don't exist.  Our communication skills are, uh, sometimes lacking.  And we have an extraordinarily poor track record of failing to realize that changing this thing will also lead to the other thing, and that even worse thing, and oh God no now this thing is happening how do we stop it.

Phillips's clear-eyed look at our own unfortunate history is kept from sinking under its own weight by a sparkling wit, calling our foibles into humorous focus but simultaneously sounding the call that "Okay, guys, it's time to pay attention."  Stupidity, they say, consists of doing the same thing over and over and expecting different results; Phillips's wonderful book points out how crucial that realization is -- and how we need to get up off our asses and, for god's sake, do something.

And you -- and everyone else -- should start by reading this book.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]