Marcelo Gleiser, Appleton Professor of Natural Philosophy at Dartmouth College, is a theoretical physicist who has worked on a diverse set of topics: cosmology, particle physics, phase transitions, condensed matter physics and biophysics. He is also a well-known author and public science communicator. A couple of months ago Marcelo suggested a guest piece for Cosmic Variance, and I’m delighted to be able to post it below. I hope you enjoy it, and I’ll encourage Marcelo to look in on the comments section and contribute there if he’d like.

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Here are some thoughts on something that has been bothering me for a while. How do we know the world is the way it is? Easy, a pragmatic person would say, just look and measure. We see a tree, a chair, a table; we hear the wind, music, people talking. We feel heat and cold against our skin. Once our brains integrate this sensorial information, we have a conception of what is real that allows us to function in the world. We know where to go, what to eat, what not to touch; we enjoy a good meal, a nice hug. But what happens when we go beyond our senses, using tools to extend our conception of reality? We don’t see galaxies with the naked eye (well, maybe Andromeda on a moonless, dry night) and much less a carbon atom. How do we know they are there, that they exist?

When Galileo showed his telescope to the Venetian senators in 1609, some refused to accept that what they saw was real. More recently, late in the 19th century, physicist and philosopher Ernst Mach refused to accept the existence of atoms, claiming they would never be seen and hence couldn’t be proven to exist. Mach and the Venetian senators were wrong. What we see through telescopes is, of course, perfectly real; we capture photons—particles of light—that a celestial body emits (or reflects, for planets and moons). If the source doesn’t emit in the visible and is so dim that we can’t capture photons between red and violet, we capture photons from radio or infrared radiation, no less real even though our eyes can’t see them. When atomic electrons jump from orbit to orbit, they also emit (or absorb) photons that can be detected by instruments or, in the case of certain transitions, by our eyes. The instruments we use in the study of natural phenomena are an extension of our senses. This amplification of reality is one of the most spectacular feats of science, allowing us to see beyond the visible. So far, so good.

The situation gets complicated when the complexity of the phenomenon forces us to filter the data, and we select to study only part of what is happening. Our brains, of course, do this all the time, what we call “focus”; otherwise, we would be flooded with such an absurd amount of sounds and images that we wouldn’t be able to do anything. When we look at a star with the naked eye or with an optical telescope, we only see part of it, what it emits in the visible. A complete view of the star would incorporate all of its emissions, in the infrared, ultraviolet, x rays, etc. This fact has a simple but, to my mind, profound consequence: our construction of reality, being necessarily filtered, is incomplete. We only know what we can measure.

In the case of elementary particle physics the situation is even more alarming. The Large Hadron Collider, for example, should start working this coming summer or early fall. In its full capacity, it should produce around 600 million collisions per second. This translates to about 700 megabytes per second of data, more than 10 petabytes (1015) per year. That’s more than a million hard drives, each with a gigabyte. To make sense of this flood of information, physicists have to filter the data, selecting events deemed “interesting.” This selection, in turn, is based on our current theories that speculate on what’s beyond the standard model of particle physics, that is, theories that speculate on stuff we don’t know is there. Although these theories are mostly pretty solid (the Higgs particle as universal giver of mass; extensions of the standard model using more than one Higgs, supersymmetry or/and more than three spatial dimensions…) they can only be confirmed through the very same experiments whose outcome they are trying to predict. Given this mechanism, there is a risk that unexpected phenomena, not predicted by any current theory and hence not included in the subset of collisions deemed interesting, will be eliminated by the data filtering process. In this case, and in a paradoxical way, the theories that we construct to amplify our view of physical reality will actually limit what we can know about nature.

Scientific progress does not always go “Boink“. Instead, it frequently goes “muttermuttermutter@!$%&*mutter”.

The thing about science is that, well, when you’re trying to figure out something new, no one can tell you the answer. There’s no hint in the back of the book, or 24-hour help line. So, if you get stuck, you may well find yourself in the scientific equivalent of a lonely mountain road with a broken axle. And no cell service. In essence, you’ve broken down in a place no one wants to go to.

But, unless you’re ready to call it quits, you somehow have to find your way out. You don’t know how long it’s going to take, or how you’re going to make it, but somehow, you’re going to find your way back to the main road. So, you keep hurling yourself at the problem, trying every MacGyver trick in your arsenal, hoping that something, anything, will work.

The thing about these periods is that while they’re about the single most frustrating and unrewarding part of the scientific process, they are also a ridiculously effective way to develop new skills. By the time you’ve tried 5 different methods of fitting a straight line to some data points, you’ve picked up a heck of a lot about statistics (or, in my case, about solving a network of partial differential equations where the time dependence of three quantities depends on the spatial variation of 1 or more of the others. Ick.).

The good news is that as you get older, you spend less and less time in such states, because you’ve got an enormous swiss army knife of tools that you carry around in your back pocket. (“Hmmm, broken axle? I recognize that particular brand of axle from Abromowitz & Stegun, and I believe I might happen to have some IDL code right here that can fix it…”) The bad news is that you can forget how much those periods, well, suck.

(Unsolicited but Related Advice for Junior Scientists: When giving talks, avoid dwelling on the Doldrums, even if you spent 90% of the project drifting around them. Focus on the science, even if it was only a small fraction of your research effort.)

Sure is quiet around here. I can’t blog much, as I’m in the final throes of book-writing. So instead, let’s have some user-generated content!

Here is a figure that I’ve drawn for use in my book.

Your mission, should you choose to accept it, is to figure out what the figure is supposed to be illustrating, and what lesson is purportedly conveyed. (Hint: that’s supposed to be an egg.) How hard can it be?

If it’s a fruitful exercise, we can repeat for other figures, similarly inscrutable.

Lots of particle physics news from the Tevatron the past two weeks, including:

• the five-sigma observation of single top quark production in both CDF and D0,
• a new measurement of the W boson mass from the D0 collaboration, and
• a new combined result from CDF and D0 on the search for the Higgs boson.

At some level the first two have a bearing on the last one, which has started to get some attention in the media, including a nice interview with Prof. Heidi Schellman from Northwestern today on Science Friday, and a piece in Scientific American to which I contributed quotes.

The observation of the production of single top quarks (rather than the easier-to-see production of top-antitop quark pairs) has been a goal of the Tevatron experiments for years. The success of this analysis demonstrates that extremely complex dissections of the data like this can be undertaken, and reveal faint signals like that of single top. The search for the Higgs boson is more difficult still.

But is the race for the Higgs boson heating up? Is there a race at all? Can the Tevatron see it before the LHC, given that the LHC has been delayed a year due to the quench incident last September?

It all hinges on the mass of the Higgs boson. In different mass ranges it decays to different final states, changing the experimental approach and changing the sensitivity of the Tevatron and LHC experiments. The new measurement of the W mass by D0 adds a bit more knowledge about what the Higgs boson mass might be, since it depends on the mass of the W boson (the carrier of the weak force, which governs nuclear beta decay) and the mass of the top quark directly.

The plot at left demonstrates where we are right now; click on it for an annotated version. I call it the “billion dollar plot” but it probably cost a lot more than that to produce, because it shows the results of decades of experimentation at the Tevatron at Fermilab, LEP at CERN, the SLC at SLAC, and other measurements. As you can see (I hope) we are in a very interesting situation: the world’s data seem to indicate that the best Higgs boson mass is deep into the territory already excluded by LEP 2 in 2000! They set a 95% confidence bound on the Higgs mass at about 114 GeV. Their limit does not extend very far beyond that mass at all; it was limited by the energy of the LEP accelerator. The mass range above 114 GeV is open experimentally, but if you take the W mass/top mass constraint seriously (in the context of the Standard Model you must) then it would certainly appear very likely that the Higgs must lie in the range 114-185 GeV, with a strong preference for the lower end.

The new Tevatron result takes a new bite out of the upper end of the range, excluding from 160-170 GeV the “sweet spot” where the Higgs can decay to two W bosons. This is in some sense “first blood” for the Tevatron: at last the two experiments can exclude a Standard Model Higgs boson somewhere it hasn’t already been excluded!

But, to my mind, the interesting end of the range is at the low end. The data favor it, and theory favors it in the sense that if nature is more complicated, and supersymmetry is manifest, then one would expect that the light Higgs boson in supersymmetry exists in the range 120-130 GeV or so. In this picture there would be heavier Higgs bosons lying in wait for either the LHC or the Tevatron, though the LHC has the edge here with higher energy.

Assuming that the improvements in the analyses continue to outpace the data, as shown in the plot below, it is not impossible that the Tevatron could start to extend the region excluded by LEP, by this summer. But discovery? A three sigma result is possible with a good deal more data, but a five-sigma discovery looks very hard, and always has.

For a gold-plated, five-sigma-significance discovery, my money is on the LHC, I have to say. But the LHC will initially see the Higgs boson decaying to two photons, and we really need to see it decaying to two quarks or two leptons to really know its nature. I think the Tevatron could do that before the LHC, measuring the decay of the Higgs boson to two b quarks, and that alone is reason enough to keep the machine running until it does, to my mind, provided there are sufficient personnel to run the detectors and analyze the data. That decision, though, is above my pay grade…

The LHC will likely be able to see the Higgs boson decaying to two tau leptons before the Tevatron can see it decaying to two b quarks. Is that a race? I view the two observations as complementary, and they both add to the scientific picture. Without the Tevatron, it will just take longer.

One last word…the Higgs boson is damned hard to see, and when the LHC turns on, a ton of other new physics may pour out first. It is a very interesting year, that’s for sure!

The results are in for the Foundational Questions Institute essay competition on “The Nature of Time,” which we discussed here. And the winners are:

First Juried Prize:

Julian Barbour on “The Nature of Time”

The jury panel admired this essay for its crystal-clear and engaging presentation of a problem in classical dynamics, namely to find a measure for duration or the size of a time interval. The paper argues lucidly, and in a historically well-informed manner, that an appropriate choice for such a measure is not to be found in Newton’s pre-existing absolute notion of time, but rather emerges, in the form of ephemeris time, from the observable motions and the assumption of energy conservation. The paper also suggests how this emergence of duration might be relevant to problems in quantum gravity.

Second Juried Prizes:

(1) Claus Kiefer on “Does Time Exist in Quantum Gravity?”

A fundamental problem in quantum gravity is that the “Wheeler-DeWitt Equation,” probably our most reliable equation of quantum gravity, does not refer to or even suggest anything like time or evolution. In this context time must emerge in the form of relations between a given system and some other system that may be considered a clock. Kiefer beautifully reviews this problem, and argues how, via quantum “decoherence,” time as described by the usual Schroedinger equation in quantum mechanics can emerge from this timeless substratum, via entanglement between physical systems within space, and the spatial metric that controls motion.

(2) Sean Carroll on “What if Time Really Exists?”

Drawing on recent developments in string theory, Carroll impressed the panel with an exciting account of how a gravitating spacetime might in fact be just a holographic approximation to a more fundamental non-gravitating theory for which “time really exists.” Contemplating the difficulties raised by strange recurrences in an everlasting universe, he argues for a strong condition on the set of allowed quantum states that would disallow such repetitions. Carroll closes by attempting to reconcile this picture with recent observations that indicate that the expansion of the universe is accelerating, with surprising results.

Tied for second is not at all bad, considering the number of interesting entries. There are more prizes, actually, as there are “community” awards as well as “juried” prizes, so check those out as well. It’s pretty amusing that the top three essays all attack, in one way or another, whether or not the subject of the competition actually exists. (I was in favor, the others were more skeptical.)

Besides the essays themselves, I very much appreciate the huge amount of work it must have been for the various judges to read through all of them and make hard decisions. Thanks to the FQXi for sponsoring the contest, and thanks to all the judges for doing a great job!

Phil has a nice post discussing the recent press release on a binary system of supermassive holes identified by Todd Boroson and (CV commenter) Tod Lauer.

Go check it out!

I have a new paper out with Sonny Mantry and Michael Ramsey-Musolf, following up on our earlier paper with Chris Stubbs. The original idea was to imagine a new long-range force that couples directly to dark matter, but not to ordinary visible matter. (A simple scalar force, which is universally attractive between any two objects, as opposed to all the messy complications of a dark electromagnetic force.) Due to the magic of quantum mechanics, such a force will couple indirectly to ordinary matter via virtual particles. So in the first paper we studied how you could use fifth-force searches in ordinary matter to look for such dark forces.

In this paper we are a little more systematic, and we follow Jo Bovy and Glennys Farrar in also considering consequences for direct dark matter detection experiments, as well as dark matter searches at colliders. Here is the (somewhat lengthy) abstract:

Implications of a Scalar Dark Force for Terrestrial Experiments
Authors: Sean M. Carroll, Sonny Mantry, Michael J. Ramsey-Musolf

Abstract: A long range Weak Equivalence Principle (WEP) violating force between Dark Matter (DM) particles, mediated by an ultralight scalar, is tightly constrained by galactic dynamics and large scale structure formation. We examine the implications of such a “dark force” for several terrestrial experiments, including Eotvos tests of the WEP, direct-detection DM searches, and collider studies. The presence of a dark force implies a non-vanishing effect in Eotvos tests that could be probed by current and future experiments depending on the DM model. For scalar singlet DM scenarios, a dark force of astrophysically relevant magnitude is ruled out in large regions of parameter space by the DM relic density and WEP constraints. WEP tests also imply constraints on the Higgs-exchange contributions to the spin-independent (SI) DM-nucleus direct detection cross-section. For WIMP scenarios, these considerations constrain Higgs-exchange contributions to the SI cross-section to be subleading compared to gauge-boson mediated contributions. In multicomponent DM scenarios, a dark force would preclude large shifts in the rate for Higgs decay to two photons associated with DM-multiplet loops that might otherwise lead to measurable deviations at the LHC or a future linear collider. The combination of observations from galactic dynamics, large scale structure formation, Eotvos experiments, DM-direct-detection experiments, and colliders can further constrain the size of new long range forces in the dark sector.

The looming problem with this whole game is that a long-range scalar force is unnatural. A scalar field should, by all rights, have a large mass, and that kind of mass drastically limits the range of the corresponding force. (That’s why the weak interactions are negligible compared to electromagnetism for everyday purposes; the W and Z bosons have a large mass, while the photon is massless.) You can keep scalar fields light by imposing symmetries, but that also tends to squelch any interesting interactions. But okay, it’s also unnatural for the Higgs boson to have a mass less than the Planck scale, or for the cosmological constant to be much less than the Planck scale. Unnatural things happen in the real world, so it’s not crazy to try to understand how they would manifest themselves.

The question is, once you’ve allowed yourself some unnaturalness, where do you stop? In this paper we’ve tried hard to minimize the number of parameters we unnaturally tuned to small values. We’ve tuned things to keep the scalar field light while not messing up the mass of the ordinary Higgs field, but tried not to tune anything else. In that case there should be mixing of the new scalar with the Higgs, and that mixing plays an important role in the phenomenology. In particular, there are implications for ground-based experiments; thus the title! It’s a long paper, but if you read one paper on the implications of a scalar dark force for terrestrial experiments this week, it should definitely be this one.

During the week of February 6, a workshop on the LHC performance was held in Chamonix, France. All of the main LHC machine folks gathered there, in one room, and discussed their strategy for the start of operations of the LHC, for all aspects of the accelerator. Reports have appeared on the blogosphere, for example here and here.

What’s new is that this afternoon at CERN, a 3 hour summary of the workshop was given in the main auditorium. And I was there. The auditorium was packed, and the audience peppered the speakers with questions. The CERN staff certainly appreciated the opportunity to hear the summaries and to ask questions. I know I did. It’s one thing to sit in California and read the slides and perhaps watch the video stream, but it’s another thing to be there in person, listen to the discourse, and to ask questions myself.

The talks ranged from safety issues, to what they learned with and without their few days of beam in 2008, to their plans for the next run. And here is the official schedule for the 2009/2010 run:

For me, the most interesting part of the talks was information on the next run:

The accelerator physicists presented the lab management with two options for the 09/10 run, depending on how many of the pressure relief valves in the arcs would be installed before the run. It’s worth noting that the full quench system will be operational in either scheme and that the pressure relief valves only serve to stem possible damage, i.e., they are not preventive. The accelerator guys were split on which plan was better. Management opted for the plan which gave beam in 2009.

The schedule is tight with no room for contingency in case of slippage.

Today, they are 1.5 weeks behind schedule, which is actually very good!

They will have a short run (few days?) with collisions at injection energy (450 GeV per beam). This is at the request of the general purpose experiments (ATLAS and CMS) in order to aid in the calibration of their detectors.

They will then run at 4 TeV per beam for a limited time (I asked specifically about this afterwards and was given various answers about the length of time at 4 TeV). Clearly, they will ramp up the beam when (and not before) they feel it is safe to do so.

Then they will run at 5 TeV per beam with the goal of collecting 200 inverse picobarns of luminosity.

To do this, they must run during the winter months December 09 – February 2010. CERN accelerators do not normally run during the winter months as the cost of electricity is 3 times higher than for the rest of the year. The additional electric bill for running the LHC during these months is $8M Euros.

It’s not clear how the lab is going to pay the additional electricity costs and the lab staff is clearly concerned about cuts, but management thinks it is manageable.

It’s not clear that the LHC will ever run at the design energy of 14 TeV. There is a problem with the number of expected magnet quenches as one tunes the beam from 6.5 to 7 TeV. Namely, it’s alarmingly high. They don’t know why yet, but are working on it. It is possible that the maximum energy the machine will ultimately reach is 13 TeV in the center of mass.

All in all, the news is good. They are expecting a reasonable set of good quality data at high energies with good discovery potential. Colliders are always slow to start up (just ask Fermilab), and the LHC will get to design parameters in good time.

The Large Hadron Collider should be lurching back to life this year, but the Tevatron at Fermilab might yet have a last hurrah. While the LHC is still fixing itself after last fall’s explosions, the Tevatron has been collecting data like mad, and hopes to continue for another couple of years. At the American Association for the Advancement of Science meeting in Chicago, Fermilab scientists said they have quite a good shot at collecting “evidence for” (although not quite “discovery of”) the Higgs boson before all is said and done.

Adam Yurkewicz at US/LHC Blogs has the scoop, and you should go there for more info. But this graph tells the basic story. It’s the probability that Fermilab will be able to find “three-sigma” evidence for the Higgs, depending on what its mass is, if the Tevatron gets to run through 2011.

Due to complicated background events, finding a particle like the Higgs isn’t just a matter of smacking together protons and antiprotons at higher and higher energies. Some possible values of the Higgs mass make it easier to find than others, since the reactions that produce it aren’t as swamped by boring known events. That’s why the Tevatron has a shot, even if LHC opens with substantially larger energies later this year. The BBC story portrays the whole thing as a race, which is fine, but to the rest of the world it’s more important to just find the darn thing than which continent gets there first. (Given that the Higgs is a boson, the smart money would seem to be on Europe.)

Last November I gave a talk at the Google outpost in Santa Monica, on dark matter and dark energy. I covered a lot of ground pretty quickly, introducing the Standard Model and the basics of the Big Bang as well as some ideas about the dark sector.

This was part of the Authors @ Google series, which features a plethora of great talks. Check out Salman Rushdie, Arianna Huffington, John Hodgman, Tyler Cowen, Anthony Bourdain, Steven Pinker, Lane Montgomery, and dozens more.

I’ve collected various YouTube videos featuring my bad self, but I honestly can’t bear to watch any of them. Can’t stand to see myself speaking (although obviously I have no issues with other people listening raptly). So if any of these are actually Rickrolls, don’t blame me.