Spinoffs from Fundamental Science

I find that some people just don’t believe scientists when we point out that fundamental research has spin-off benefits for modern society.  The assumption often seems to be that it’s just a bunch of egghead esoteric researchers trying to justify their existence.  It’s a real problem when those scoffing at our evidence are congresspeople of the United States and their staffers, or other members of governmental funding agencies around the world.

So I thought I’d point out an example, reported on Bloomberg News.  It’s a good illustration of how these things often work out, and it is very rare indeed that they are discussed in the press.

Gravitational waves are usually incredibly tiny effects [typically squeezing the radius of our planet by less than the width of an atomic nucleus] that can be made only with monster black holes and neutron stars.   There’s not much hope of using them in technology.  So what good could an experiment to discover them, such as LIGO, possibly be for the rest of the world?

Well, Shell Oil seems to have found some value in it.   It’s not in the gravitational waves themselves, of course; instead, it is in the technology that has to be developed to detect something so delicate.   http://www.bloomberg.com/news/articles/2016-07-07/shell-is-using-innoseis-s-sensors-to-detect-gravitational-waves

Score another one for investment in fundamental scientific research.

 

LIGO detects a second merger of black holes

There’s additional news from LIGO (the Laser Interferometry Gravitational Observatory) about gravitational waves today. What was a giant discovery just a few months ago will soon become almost routine… but for now it is still very exciting…

LIGO got a Christmas (US) present: Dec 25th/26th 2015, two more black holes were detected coalescing 1.4 billion light years away — changing the length of LIGO’s arms by 300 parts in a trillion trillion, even less than the first merger observed in September. The black holes had 14 solar masses and 8 solar masses, and merged into a black hole with 21 solar masses, emitting 1 solar mass of energy in gravitational waves. In contrast to the September event, which was short and showed just a few orbits before the merger, in this event nearly 30 orbits over a full second are observed, making more information available to scientists about the black holes, the merger, and general relativity.  (Apparently one of the incoming black holes was spinning with at least 20% of the maximum possible rotation rate for a black hole.)

The signal is not so “bright” as the first one, so it cannot be seen by eye if you just look at the data; to find it, some clever mathematical techniques are needed. But the signal, after signal processing, is very clear. (Signal-to-noise ratio is 13; it was 24 for the September detection.) For such a clear signal to occur due to random noise is 5 standard deviations — officially a detection. The corresponding “chirp” is nowhere near so obvious, but there is a faint trace.

This gives two detections of black hole mergers over about 48 days of 2015 quality data. There’s also a third “candidate”, not so clear — signal-to-noise of just under 10. If it is really due to gravitational waves, it would be merging black holes again… midway in size between the September and December events… but it is borderline, and might just be a statistical fluke.

It is interesting that we already have two, maybe three, mergers of large black holes… and no mergers of neutron stars with black holes or with each other, which are harder to observe. It seems there really are a lot of big black holes in binary pairs out there in the universe. Incidentally, the question of whether they might form the dark matter of the universe has been raised; it’s still a long-shot idea, since there are arguments against it for black holes of this size, but seeing these merger rates one has to reconsider those arguments carefully and keep an open mind about the evidence.

Let’s remember also that advanced-LIGO is still not running at full capacity. When LIGO starts its next run, six months long starting in September, the improvements over last year’s run will probably give a 50% to 100% increase in the rate for observed mergers.   In the longer run, the possibility of one merger per week is possible.

Meanwhile, VIRGO in Italy will come on line soon too, early in 2017. Japan and India are getting into the game too over the coming years. More detectors will allow scientists to know where on the sky the merger took place, which then can allow normal telescopes to look for flashes of light (or other forms of electromagnetic radiation) that might occur simultaneously with the merger… as is expected for neutron star mergers but not widely expected for black hole mergers.  The era of gravitational wave astronomy is underway.

Giving two free lectures 6/20,27 about gravitational waves

For those of you who live in or around Berkshire County, Massachusetts, or know people who do…

Starting next week I’ll be giving two free lectures about the LIGO experiment’s discovery of gravitational waves.  The lectures will be at 1:30 pm on Mondays June 20 and 27, at Berkshire Community College in Pittsfield, MA.  The first lecture will focus on why gravitational waves were expected by scientists, and the second will be on how gravitational waves were discovered, indirectly and then directly.  No math or science background will be assumed.  (These lectures will be similar in style to the ones I gave a couple of years ago concerning the Higgs boson discovery.)

Here’s a flyer with the details:  http://berkshireolli.org/ProfessorMattStrasslerOLLILecturesFlyer.pdf

Pop went the Weasel, but Vroom goes the LHC

At the end of April, as reported hysterically in the press, the Large Hadron Collider was shut down and set back an entire week by a “fouine”, an animal famous for chewing through wires in cars, and apparently in colliders too. What a rotten little weasel! especially for its skill in managing to get the English-language press to blame the wrong species — a fouine is actually a beech marten, not a weasel, and I’m told it goes Bzzzt, not Pop. But who’s counting?

Particle physicists are counting. Last week the particle accelerator operated so well that it generated almost half as many collisions as were produced in 2015 (from July til the end of November), bringing the 2016 total to about three-fourths of 2015.

 

The key question is how many of the next few weeks will be like this past one.  We’d be happy with three out of five, even two.  If the amount of 2016 data can significantly exceed that of 2015 by July 15th, as now seems likely, a definitive answer to the question on everyone’s mind (namely, what is the bump on that plot?!? a new particle? or just a statistical fluke?) might be available at the time of the early August ICHEP conference.

So it’s looking more likely that we’re going to have an interesting August… though it’s not at all clear yet whether we’ll get great news (in which case we get no summer vacation), bad news (in which case we’ll all need a vacation), or ambiguous news (in which case we wait a few additional months for yet more news.)

The Two-Photon Excess at LHC Brightens Slightly

Back in December 2015, there was some excitement when the experiments ATLAS and CMS at the Large Hadron Collider [LHC] — especially ATLAS — reported signs of an unexpectedly large number of proton-proton collisions in which

  • two highly energetic photons [particles of light] were produced, and
  • the two photons could possibly have been produced in a decay of an unknown particle, whose mass would be about six times the mass of the Higgs particle (which ATLAS and CMS discovered in 2012.)

This suggested the possibility of an unknown particle of some type with rest mass of 750 GeV/c².  However, the excess could just be a statistical fluke, of no scientific importance and destined to vanish with more data.

The outlook for that bump on a plot at 750 GeV has gotten a tad brighter… because not only do we have ATLAS’s plot, we now have increasing evidence for a similar bump on CMS’s plot. This is thanks largely to some hard work on the part of the CMS experimenters.  Some significant improvements at CMS,

  1. improved understanding of their photon energy measurements in their 2015 data,
  2. ability to use 2015 collisions taken when their giant magnet wasn’t working — fortunately, the one type of particle whose identity and energy can be measured without a magnet is… a photon!
  3. combination of the 2015 data with their 2012 data,

have increased the significance of their observed excess by a moderate amount. Here’s the scorecard.*

  • CMS 2015 data (Dec.): excess is 2.6σ local, < 1σ global
  • CMS 2015 data (improved, Mar.) 2.9σ local, < 1σ global
  • CMS 2015+2012 data: 3.4σ local, 1.6σ global
  • ATLAS 2015 data (Dec. and Mar.): 3.6σ local, 2.0σ global to get a narrow bump [and 3.9σ local , 2.3σ global to get a somewhat wider bump, but notice this difference is quite insignificant, so narrow and wider are pretty much equally ok.]
  • ATLAS 2015+2012 data: not reported, but clearly goes up a bit more, by perhaps half a sigma?

You can read a few more details at Resonaances.

*Significance is measured in σ (“standard deviations”) and for confidence in potentially revolutionary results we typically want to see local significance approaching 5σ and global approaching 3σ in both experiments. (The “local” significance tells you how unlikely it is to see a random bump of a certain size at a particular location in the plot, while the “global” significance tells you how unlikely it is to see such a bump anywhere in the plot … obviously smaller because of the look-elsewhere effect.)

This is good news, but it doesn’t really reflect a qualitative change in the situation. It leaves us slightly more optimistic (which is much better than the alternative!) but, as noted in December, we still won’t actually know anything until we have either (a) more data to firm up the evidence for these bumps, or (b) a discovery of a completely independent clue, perhaps in existing data. Efforts for (b) are underway, and of course (a) will get going when the LHC starts again… soon!  Next news on this probably not til June at the earliest… unless we’re very lucky!

Advance Thoughts on LIGO

Scarcely a hundred years after Einstein revealed the equations for his theory of gravity (“General Relativity”) on November 25th, 1915, the world today awaits an announcement from the LIGO experiment, where the G in LIGO stands for Gravity. (The full acronym stands for “Laser Interferometer Gravitational Wave Observatory.”) As you’ve surely heard, the widely reported rumors are that at some point in the last few months, LIGO, recently upgraded to its “Advanced” version, finally observed gravitational waves — ripples in the fabric of space (more accurately, of space-time). These waves, which can make the length of LIGO shorter and longer by an incredibly tiny amount, seem to have come from the violent merger of two black holes, each with a mass [rest-mass!] dozens of times larger than the Sun. Their coalescence occurred long long ago (billions of years) in a galaxy far far away (a good fraction of the distance across the visible part of the universe), but the ripples from the event arrived at Earth just weeks ago. For a brief moment, it is rumored, they shook LIGO hard enough to be convincingly observed.

For today’s purposes, let me assume the rumors are true, and let me assume also that the result to be announced is actually correct. We’ll learn today whether the first assumption is right, but the second assumption may not be certain for some months (remember OPERA’s [NOT] faster-than-light neutrinos  and BICEP2’s [PROBABLY NOT] gravitational waves from inflation). We must always keep in mind that any extraordinary scientific result has to be scrutinized and confirmed by experts before scientists will believe it! Discovery is difficult, and a large fraction of such claims — large — fail the test of time.

What the Big News Isn’t

There will be so much press and so many blog articles about this subject that I’m just going to point out a few things that I suspect most articles will miss, especially those in the press.

Most importantly, if LIGO has indeed directly discovered gravitational waves, that’s exciting of course. But it’s by no means the most important story here.

That’s because gravitational waves were already observed indirectly, quite some time ago, in a system of two neutron stars orbiting each other. This pair of neutron stars, discovered by Joe Taylor and his graduate student Russell Hulse, is interesting because one of the neutron stars is a pulsar, an object whose rotation and strong magnetic field combine to make it a natural lighthouse, or more accurately a radiohouse, sending out pulses of radio waves that can be detected at great distances. The time between pulses shifts very slightly as the pulsar moves toward and away from Earth, so the pulsar’s motion around its companion can be carefully monitored. Its orbital period has slowly changed over the decades, and the changes are perfectly consistent with what one would expect if the system were losing energy, emitting it in the form of unseen gravitational waves at just the rate predicted by Einstein’s theory (as shown in this graph.) For their discovery, Hulse and Taylor received the 1993 Nobel Prize. By now, there are other examples of similar pairs of neutron stars, also showing the same type of energy loss in detailed accord with Einstein’s equations.

A bit more subtle (so you can skip this paragraph if you want), but also more general, is that some kind of gravitational waves are inevitable… inevitable, after you accept Einstein’s earlier (1905) equations of special relativity, in which he suggested that the speed of light is a sort of universal speed limit on everything, imposed by the structure of space-time.  Sound waves, for instance, exist because the speed of sound is finite; if it were infinite, a vibrating guitar string would make the whole atmosphere wiggle back and forth in sync with the guitar string.  Similarly, since effects of gravity must travel at a finite speed, the gravitational effects of orbiting objects must create waves. The only question is the specific properties those waves might have.

No one, therefore, should be surprised that gravitational waves exist, or that they travel at the universal speed limit, just like electromagnetic waves (including visible light, radio waves, etc.) No one should even be surprised that the waves LIGO is (perhaps) detecting have properties predicted by Einstein’s specific equations for gravity; if they were different in a dramatic way, the Hulse-Taylor neutron stars would have behaved differently than expected.

Furthermore, no one should be surprised if waves from a black hole merger have been observed by the Advanced LIGO experiment. This experiment was designed from the beginning, decades ago, so that it could hardly fail to discover gravitational waves from the coalescence of two black holes, two neutron stars, or one of each. We know these mergers happen, and the experts were very confident that Advanced LIGO could find them. The really serious questions were: (a) would Advanced LIGO work as advertised? (b) if it worked, how soon would it make its first discovery? and (c) would the discovery agree in detail with expectations from Einstein’s equations?

Big News In Scientific Technology

So the first big story is that Advanced LIGO WORKS! This experiment represents one of the greatest technological achievements in human history. Congratulations are due to the designers, builders, and operators of this experiment — and to the National Science Foundation of the United States, which is LIGO’s largest funding source. U.S. taxpayers, who on average each contributed a few cents per year over the past two-plus decades, can be proud. And because of the new engineering and technology that were required to make Advanced LIGO functional, I suspect that, over the long run, taxpayers will get a positive financial return on their investment. That’s in addition of course to a vast scientific return.

Advanced LIGO is not even in its final form; further improvements are in the works. Currently, Advanced LIGO consists of two detectors located 2000 miles (3000 kilometers) apart. Each detector consists of two “arms” a few miles (kilometers) long, oriented at right angles, and the lengths of the arms are continuously compared.  This is done using exceptionally stable lasers reflecting off exceptionally perfect mirrors, and requiring use of sophisticated tricks for mitigating all sorts of normal vibrations and even effects of quantum “jitter” from the Heisenberg uncertainty principle. With these tools, Advanced LIGO can detect when passing gravitational waves change the lengths of LIGO’s arms by … incredibly … less than one part in a billion trillion (1,000,000,000,000,000,000,000). That’s an astoundingly tiny distance: a thousand times smaller than the radius of a proton. (A proton itself is a hundred thousand times smaller, in radius, than an atom. Indeed, LIGO is measuring a distance as small as can be probed by the Large Hadron Collider — albeit with a very very tiny energy, in contrast to the collider.) By any measure, the gravitational experimenters have done something absolutely extraordinary.

Big News In Gravity

The second big story: from the gravitational waves that LIGO has perhaps seen, we would learn that the merger of two black holes occurs, to a large extent, as Einstein’s theory predicts. The success of this prediction for what the pattern of gravitational waves should be is a far more powerful test of Einstein’s equations than the mere existence of the gravitational waves!

Imagine, if you can… Two city-sized black holes, each with a mass [rest-mass!] tens of times greater than the Sun, and separated by a few tens of miles (tens of kilometers), orbit each other. They circle faster and faster, as often, in their last few seconds, as 100 times per second. They move at a speed that approaches the universal speed limit. This extreme motion creates an ever larger and increasingly rapid vibration in space-time, generating large space-time waves that rush outward into space. Finally the two black holes spiral toward each other, meet, and join together to make a single black hole, larger than the first two and spinning at an incredible rate.  It takes a short moment to settle down to its final form, emitting still more gravitational waves.

During this whole process, the total amount of energy emitted in the vibrations of space-time is a few times larger than you’d get if you could take the entire Sun and (magically) extract all of the energy stored in its rest-mass (E=mc²). This is an immense amount of energy, significantly more than emitted in a typical supernova. Indeed, LIGO’s black hole merger may perhaps be the most titanic event ever detected by humans!

This violent dance of darkness involves very strong and complicated warping of space and time. In fact, it wasn’t until 2005 or so that the full calculation of the process, including the actual moment of coalescence, was possible, using highly advanced mathematical techniques and powerful supercomputers!

By contrast, the resulting ripples we get to observe, billions of years later, are much more tame. Traveling far across the cosmos, they have spread out and weakened. Today they create extremely small and rather simple wiggles in space and time. You can learn how to calculate their properties in an advanced university textbook on Einstein’s gravity equations. Not for the faint of heart, but certainly no supercomputers required.

So gravitational waves are the (relatively) easy part. It’s the prediction of the merger’s properties that was the really big challenge, and its success would represent a remarkable achievement by gravitational theorists. And it would provide powerful new tests of whether Einstein’s equations are in any way incomplete in their description of gravity, black holes, space and time.

Big News in Astronomy

The third big story: If today’s rumor is indeed of a real discovery, we are witnessing the birth of an entirely new field of science: gravitational-wave astronomy. This type of astronomy is complementary to the many other methods we have of “looking” at the universe. What’s great about gravitational wave astronomy is that although dramatic events can occur in the universe without leaving a signal visible to the eye, and even without creating any electromagnetic waves at all, nothing violent can happen in the universe without making waves in space-time. Every object creates gravity, through the curvature of space-time, and every object feels gravity too. You can try to hide in the shadows, but there’s no hiding from gravity.

Advanced LIGO may have been rather lucky to observe a two-black-hole merger so early in its life. But we can be optimistic that the early discovery means that black hole mergers will be observed as often as several times a year even with the current version of Advanced LIGO, which will be further improved over the next few years. This in turn would imply that gravitational wave astronomy will soon be a very rich subject, with lots and lots of interesting data to come, even within 2016. We will look back on today as just the beginning.

Although the rumored discovery is of something expected — experts were pretty certain that mergers of black holes of this size happen on a fairly regular basis — gravitational wave astronomy might soon show us something completely unanticipated. Perhaps it will teach us surprising facts about the numbers or properties of black holes, neutron stars, or other massive objects. Perhaps it will help us solve some existing mysteries, such as those of gamma-ray bursts. Or perhaps it will reveal currently unsuspected cataclysmic events that may have occurred somewhere in our universe’s past.

Prizes On Order?

So it’s really not the gravitational waves themselves that we should celebrate, although I suspect that’s what the press will focus on. Scientists already knew that these waves exist, just as they were aware of the existence of atoms, neutrinos, and top quarks long before these objects were directly observed. The historic aspects of today’s announcement would be in the successful operation of Advanced LIGO, in its new way of “seeing” the universe that allows us to observe two black holes becoming one, and in the ability of Einstein’s gravitational equations to predict the complexities of such an astronomical convulsion.

Of course all of this is under the assumptions that the rumors are true, and also that LIGO’s results are confirmed by further observations. Let’s hope that any claims of discovery survive the careful and proper scrutiny to which they will now be subjected. If so, then prizes of the highest level are clearly in store, and will be doled out to quite a few people, experimenters for designing and building LIGO and theorists for predicting what black-hole mergers would look like. As always, though, the only prize that really matters is given by Nature… and the many scientists and engineers who have contributed to Advanced LIGO may have already won.

Enjoy the press conference this morning. I, ironically, will be in the most inaccessible of places: over the Atlantic Ocean.  I was invited to speak at a workshop on Large Hadron Collider physics this week, and I’ll just be flying home. I suppose I can wait 12 hours to find out the news… it’s been 44 years since LIGO was proposed…

So What Is It???

So What Is It? That’s the question one hears in all the bars and on all the street corners and on every Twitter feed and in the whispering of the wind. Everybody wants to know. That bump seen on the ATLAS and CMS two-photon plots! What… IS… it…?

ATLAS_CMS_diphoton_2015

The two-photon results from ATLAS (top) and CMS (bottom) aligned, so that the 600, 700 and 800 GeV locations (blue vertical lines) line up almost perfectly. The peaks in the two data sets are in about the same location. ATLAS’s is larger and also wider. Click here for more commentary.

Well, to be honest, probably it’s just that: a bump on a plot. But just in case it’s not — just in case it really is the sign of a new particle in Large Hadron Collider [LHC] data — let me (start to) address the question.

First: what it isn’t. It can’t just be a second Higgs particle (a heavier version of the one found in 2012) that is just appended to the known particles, with no other particles added in.   Continue reading

Is This the Beginning of the End of the Standard Model?

Was yesterday the day when a crack appeared in the Standard Model that will lead to its demise?  Maybe. It was a very interesting day, that’s for sure. [Here’s yesterday’s article on the results as they appeared.]

I find the following plot useful… it shows the results on photon pairs from ATLAS and CMS superposed for comparison.  [I take only the central events from CMS because the events that have a photon in the endcap don’t show much (there are excesses and deficits in the interesting region) and because it makes the plot too cluttered; suffice it to say that the endcap photons show nothing unusual.]  The challenge is that ATLAS uses a linear horizontal axis while CMS uses a logarithmic one, but in the interesting region of 600-800 GeV you can more or less line them up.  Notice that CMS’s bins are narrower than ATLAS’s by a factor of 2.

ATLAS_CMS_diphoton_2015

The diphoton results from ATLAS (top) and CMS (bottom) arranged so that the 600, 700 and 800 GeV locations (blue vertical lines) line up almost perfectly. (The plots do not line up away from this region!)  The data are the black dots (ignore the bottom section of CMS’s plot for now.) Notice that the obvious bumps in the two data sets appear in more or less the same place. The bump in ATLAS’s data is both higher (more statistically significant) and significantly wider.

Both plots definitely show a bump.  The two experiments have rather similar amounts of data, so we might have hoped for something more similar in the bumps, but the number of events in each bump is small and statistical flukes can play all sorts of tricks.

Of course your eye can play tricks too. A bump of a low significance with a small number of events looks much more impressive on a logarithmic plot than a bump of equal significance with a larger number of events — so beware that bias, which makes the curves to the left of the bump appear smoother and more featureless than they actually are.  [For instance, in the lower register of CMS’s plot, notice the bump around 350.]

We’re in that interesting moment when all we can say is that there might be something real and new in this data, and we have to take it very seriously.  We also have to take the statistical analyses of these bumps seriously, and they’re not as promising as these bumps look by eye.  If I hadn’t seen the statistical significances that ATLAS and CMS quoted, I’d have been more optimistic.

Also disappointing is that ATLAS’s new search is not very different from their Run 1 search of the same type, and only uses 3.2 inverse femtobarns of data, less than the 3.5 that they can use in a few other cases… and CMS uses 2.6 inverse femtobarns.  So this makes ATLAS less sensitive and CMS more sensitive than I was originally estimating… and makes it even less clear why ATLAS would be more sensitive in Run 2 to this signal than they were in Run 1, given the small amount of Run 2 data.  [One can check that if the events really have 750 GeV of energy and come from gluon collisions, the sensitivity of the Run 1 and Run 2 searches are comparable, so one should consider combining them, which would reduce the significance of the ATLAS excess. Not to combine them is to “cherry pick”.]

By the way, we heard that the excess events do not look very different from the events seen on either side of the bump; they don’t, for instance, have much higher total energy.  That means that a higher-energy process, one that produces a new particle at 750 GeV indirectly, can’t be a cause of big jump in the 13 TeV production rate relative to 8 TeV.  So one can’t hide behind this possible explanation for why a putative signal is seen brightly in Run 2 and was barely seen, if at all, in Run 1.

Of course the number of events is small and so these oddities could just be due to statistical flukes doing funny things with a real signal.  The question is whether it could just be statistical flukes doing funny things with the known background, which also has a small number of events.

And we should also, in tempering our enthusiasm, remember this plot: the diboson excess that so many were excited about this summer.  Bumps often appear, and they usually go away.  R.I.P.

ATLAS_dibosonXS

The most dramatic of the excesses in the production of two W or Z bosons from Run 1 data, as seen in ATLAS work published earlier this year. That bump excited a lot of people. But it doesn’t appear to be supported by Run 2 data. A cautionary tale.

Nevertheless, there’s nothing about this diphoton excess which makes it obvious that one should be pessimistic about it.  It’s inconclusive: depending on the statistical questions you ask (whether you combine ATLAS and CMS Run 2, whether you try to combine ATLAS Run 1 and Run 2, whether you worry about whether the resonance is wide or narrow), you can draw positive or agnostic conclusions.  It’s hard to draw entirely negative conclusions… and that’s a reason for optimism.

Six months or so from now — or less, if we can use this excess as a clue to find something more convincing within the existing data — we’ll likely say “R.I.P.” again.  Will we bury this little excess, or the Standard Model itself?