Following up on my story about switching from a computer-projected talk to a blackboard talk at the last minute, Chad Orzel hashes out the pros and cons of blackboard vs. computer vs. overhead transparencies. (I talk about “computer” presentations rather than “PowerPoint” because I actually use StarOffice, which has a better equation editor and also is free.) Michael Nielsen and Doron Zeilberger give their takes as well.

This is the kind of thing scientists talk about when they’re not uncovering the mysteries of the universe. Computer presentations have become the standard in many fields, although there is a substantial wailing about the attendant impersonality (and often incomprehensibility) of the result. Edward Tufte has written a celebrated anti-PowerPoint screed, even holding the conventions of that particular medium partially responsible for the Challenger disaster (apparently NASA engineers gave a PowerPoint presentation on the problems during the flight that served to camouflage rather than highlight the potential dangers). As Chad points out, it’s one thing to stand at the blackboard and talk about theoretical ideas involving equations and some simplistic figures, but very different if you are trying to present data. Personally I will use the computer if I’m giving a colloquium or conference talk, and prefer the blackboard if I’m giving a more specialized seminar. This particular conference was small and specialized enough that using the blackboard made sense. It certainly slows down the presentation, which is almost always good. In principle a sufficiently talented speaker can go at the right pace and be perfectly understandable while using slides, but in practice the chalk tends to force you to go at a reasonable pace and leave out superfluous details that you’re tempted to include on your slides.

What scientists will never understand is why folks in the humanities will literally read their papers — just stand up there, manuscript in hand (or on podium), and read each word verbatim, even if everyone in the audience has a transcript right in front of them. What is the point of that? The first time I saw it I was baffled, and I still haven’t quite figured it out. I tried it myself when I gave a talk at a humanities conference, but to be honest I just couldn’t do it — I kept extemporizing, so much so that none of my sentences appeared just as they were on the page.

It’s good to be open-minded about the practices of different disciplines, and for a long time I excused this weird habit by figuring that humanities talks had to be extremely precise with their language, with every word chosen carefully after hours of late-night concentration. But I’ve come to believe that there’s really no excuse. The quality of presentation of a talk that is directly read off the manuscript will just never be as good as one that is given from notes or an outline. Whether or not the precision of the writing seems to be of utmost importance, it’s not as if the listeners are going to remember the talk word-for-word, so I think an engaging presentation of the general ideas will always be more effective than a stodgy reading of perfect precision. Can we scientists (who, more often than not, give awful talks, but for different reasons) somehow persuade our less quantitative colleagues to free themselves from the prison of the printed page?

Back from sunny, cicada-ridden Baltimore and the conference mentioned previously. It was a small, fun conference; the idea was to keep it simple and informal, so that people could spend time talking to each other and perhaps even get some work done. My talk was scheduled at the very end, so I didn’t get any work done, but split my time between socializing and making an electronic version of the talk (which I had given previously, but only on the blackboard at seminars). In the end it was for naught, as I complained about having to make the electronic version and received a chorus of requests to just go ahead and give the blackboard talk, which I did. It went pretty well, so we’ll see what kind of reaction the paper gets.

The participants were all theorists working on particle physics, string theory, and cosmology — overlapping fields with a lot of activity and connections these days. Some of the memorable talks:

• Gia Dvali talked about modified gravity in cosmology (as I blogged about recently).
• Angela Olinto gave a nice review of ultra-high-energy cosmic rays and neutrino experiments.
• Shamit Kachru talked about compactifying extra dimensions in string theory and the string-theory “landscape” of possible vacua.
• Alex Szalay gave an interesting talk on the huge data sets appearing in modern astrophysics — we’re moving from terabytes to petabytes, leading to files that would take years just to search using ordinary methods.
• Ann Nelson and Neil Weiner both gave talks on interactions in the dark sector (undeterred by my skepticism).
• Eva Silverstein talked about inflation from nontrivial kinetic terms for string-theory moduli, including potentially observable signals (!) of non-Gaussianity in the cosmic microwave background.
• Jacob Bekenstein talked about modified gravity as a replacement for dark matter.
• Nima Arkani-Hamed talked about “ghost” fields as dark energy.
• Marc Kamionkowski talked about dark matter that is neutral, but couples to electromagnetism through a dipole moment.
• Hitoshi Murayama talked about the minimal possible model that is consistent with all of our current data.
• Even the summary talk, by Joe Lykken, was fantastic.

You get the idea, I hope: lots of interesting ideas, bumping up against new constraints from experiments, bringing together fields that weren’t talking to each other just a little while ago (e.g., when I was in grad school). It’s an exciting time.

I’ll be away for the next few days, attending this conference at Johns Hopkins. I’ve been trying to curtail my excessive traveling, but this workshop seems to have a lot of interesting speakers; I’ll report back if any startling new ideas emerge. I’ll be giving my soon-to-be-famous talk on how inflation explains the low entropy of the early universe. If only I could find time to write the actual paper, we could put Boltzmann’s ghost to rest.

We’re on a quarter system here at Chicago, which means we don’t start until October (good) but we keep going until the beginning of June (bad). Which is to say, I have just handed out the take-home final exam for my Spacetime and Black Holes class, an undergraduate introduction to general relativity. It has been a great class, full of curious and enthusiastic students (at least two of which, Maire and Colin, have blogs; feel free to let me know if anyone else does).

Let me see, I’ve been alive for thirty-seven years, thirty-two of which have involved spending much of my time in an educational institution of one sort or another, from nursery school to being a professor. And still, I have to admit, I love the rhythms of the school year, from the fresh fall days when the campus comes back to life with arriving students, to the slogging twilight of the winter quarter, to the cusp of summer when another year has been successfully negotiated. Why would anyone want to leave to go to the real world? But that, of course, is the downside: so many people at any school are there just temporarily, whether it’s students or postdocs or, when things don’t turn out as we hope, assistant professors. Some of the UofC students at Crescat Sententia are graduating seniors, and are grumbling wistfully (sounds impossible, but it’s true) about the impending end of their days here. It’s equally bittersweet to be on the other side, as students who you’ve seen working and growing in an impossibly short time prepare to take off for their next set of challenges.

So, congratulations to all the graduating seniors, not to mention graduate students about to get their Ph.D.’s and postdocs moving on to other jobs. For the students, be sure to enjoy the ceremony, which can seem anticlimactic if you don’t take time to reflect on what you’ve accomplished. It’s a big deal.

Looks like the idea of sending robots to repair Hubble is gaining some steam. They wouldn’t be able to do everything you could do with a servicing mission, but they could install new instruments and gyros to keep the telescope running for quite a while longer. (Or not — if it’s deemed unfeasible, they might use a robot just to prepare the satellite for re-entry into the atmosphere.) It will be up to the engineers to build robots that can reliably do the job, which isn’t easy; but my bet is that it can be done. This challenge has actually energized a lot of folks at NASA, and fixing the Space Telescope is a goal everyone can agree on.

Chicago is a fantastic city in many ways; at some point I should do a series of posts on why this is the greatest city in the world to live in. One reason, believe it or not, is geography. In some respects, it’s not good to live on a large plain in the middle of a large continent; there are no mountains nearby to go climbing, and with no nearby oceans the weather can get pretty dramatic. (I once read that there are only three metropolitan areas with greater than five million people in which the temperatures regularly reached over 100 degrees F in the summer and below zero in the winter: Beijing, Moscow, and Chicago.) But there is an important benefit as well: it’s much harder to sneak up on an inland city with a nuclear weapon than it would be if we were on one of the coasts.

To be sure, Chicago is only about the fourth-ranked U.S. target that one would choose for a dramatic blowing-up; New York, Los Angeles, and Washington D.C. have to be ahead of us on the list. That, coupled with the difficulty of smuggling a nuclear weapon all the way into the interior, makes it seem relatively safe here. If I lived in one of those three coastal cities, I wouldn’t be nearly as sanguine. It’s one of those things we don’t like to talk about, but the chances of a terrorist group cobbling together the technology and raw materials for making a bomb have to be appreciable, given the half-hearted efforts that have been made to quarantine both resources and know-how thus far. (Not only have we gone quite easy on people who are known to share nuclear secrets, but our violations of the test-ban treaty and plans to build “small” tactical nuclear weapons have created a climate in which other countries do not feel encouraged to give up their own nuclear programs. Not to mention the fact that successfully building a bomb would be excellent proof against getting invaded.)

Mutually Assured Destruction, shaky as it was as a defensive doctrine under the best of circumstances, is nearly useless against terrorist organizations. There’s no way of guaranteeing we would even be able to pinpoint the true culprits, nor to counterattack if we could. If terrorists somehow get the bomb, they’re very likely to use it.

So what are the chances of a nuclear bomb being detonated in a U.S. city sometime in the next fifty years? One percent? Ten percent? These seem like reasonable numbers to me. What to do about it, I’m less sure.

Of course, risk analysis is notoriously difficult, and people tend to do a terrible job even when it’s easy. How many people would evacuate L.A. if scientists could guarantee that there was a 20% chance of a devastating earthquake (millions dead, city in ruins) in the next twenty years? I suspect not many; when the danger is so diffuse, it’s hard to take the tangible steps necessary to avoid it.

It was good to hear that Kerry is putting nuclear proliferation high on his list of foreign-policy priorities. (Even if he did choose to wear an old Monday Night Football blazer while doing so. Doesn’t he have wardrobe consultants on staff?) I don’t know how effective we can be, but doing everything conceivable to prevent a nuclear weapon from exploding in one of our cities seems like an easy priority to agree on.

I should tie up some loose ends (read: “potentially misleading intemperate statements”) in my post below about the anti-Big-Bang petition.

First, an actual physics point: Does Einstein’s general relativity really say that energy is not conserved? You will be unsurprised to hear that the answer depends on what you mean by “energy.” and what you mean by “conserved.” Before general relativity came along, when spacetime was thought of as a fixed, static background on which all the rest of physics played itself out, the answer was unambiguous; at any moment in time, there was a number we could compute (for any closed system) called the “energy,” and that number would be the same as at any other moment in time. (One way to derive this statement is as a consequence of the word “static”; time-translation invariance implies energy conservation.) Most often, we were lucky enough that the energy came in the form of an energy density defined at each point in space, which we could add up over the entire system to get the total energy.

GR changes the rules of the game. Spacetime is a dynamical object whose geometry responds to the presence of matter fields. We can now ask two separate questions: Is energy conserved for the matter fields in a given spacetime background? and Is the total energy of the universe, including matter and gravity (as manifested in spacetime curvature) conserved?

The answer to each question still depends on what you mean. Consider matter evolving in some background spacetime (so we ignore the possible energy of the gravitational field, whatever that may be). There is now no number we can calculate for a closed system that corresponds to “energy” and is conserved. This shouldn’t be a surprise, since we have violated time-translation invariance; the background geometry could be expanding or contracting, for example. In cosmology, there is no “total energy of the universe” which is supposed to be conserved — that’s why Lerner’s statement was so silly. On the other hand, there is a rule for “covariant” conservation of the local energy-momentum tensor (for experts, it’s D_aT^ab=0). This rule can be thought of as telling us exactly how the energy changes in response to changes in the background geometry, and it is what replaces the flat-spacetime notion of energy conservation. So the number of rules is the same in flat or curved spacetime; it’s not as if anything goes. But we can’t, once again, define a conserved total energy in any reasonable way.

So we should just include the energy of the gravitational field, obviously, right? The problem is there’s no good way to do that. If we blindly follow the rules for calculating the energy and apply them to general relativity, we find that they don’t give us an energy density at each point in space, but rather a boundary contribution defined solely at infinity. In other words, there is no local definition of energy density in general relativity. In the weak-field limit we can come up with good approximate notions of a gravitational energy density, and these are useful e.g. when calculating the energy lost through gravitational radiation in orbiting bodies. So perhaps we could give up on locality and stick with just a global definition. Given appropriate boundary conditions (typically that spacetime is flat at infinity) this makes sense, and we can define different notions of energy (the ADM energy, the Bondi energy) appropriate to different circumstances. But in cosmology the universe is not flat at infinity, so these circumstances don’t apply — there is generally, once again, no such thing as the conserved total energy. (In a closed universe there is — and it’s always exactly zero, for all the good it does us.)

The situation is thus a little ambiguous; whether energy is conserved in GR depends on the situation you are talking about, and what you would qualify as energy conservation. Don’t get me wrong: nobody who understands what’s going on has any disagreement about the equations or their solutions, it’s just that there are different words we can reasonably apply to them. This is actually a good example of what Thomas Kuhn talked about in The Structure of Scientific Revolutions, where he discusses how words mean different things before and after a paradigm shift. The notion of “energy” is very useful, but its status in GR is different than it is in flat-spacetime physics. The one thing we can all agree on is that background energy density that remains constant as the universe expands, and whose integral over space therefore grows, is perfectly consistent with everything we know about physics.

The other thing I wanted to revisit is my defaming remark that Big-Bang opponents aren’t very smart. Peter Woit points out a counterexample: Irving Segal, a well-known mathematical physicist who developed “chronometric cosmology” as an alternative to the Big Bang. Of course there are other counterexamples, notable among whom we should mention Sir Fred Hoyle, who did extremely important work in stellar nucleosynthesis, and later became well-known as a supporter of the Steady State model. (It was Hoyle who actually coined the term “Big Bang,” as a derogatory term to belittle the model we now know to be correct.) (Update: Another unfair slander! See the comments.)

I shouldn’t have given such a blanket indictment of the intellectual prowess of the anti-Bang folks. For all I know, Eric Lerner is a grandmaster at chess, a gourmet cook, and a crossword-puzzle wizard. What I should have simply said is that the criticisms leveled by these folks at the Big Bang are just not very smart. I can certainly imagine intelligent and reasonable arguments given against almost any scientific position; but the anti-Banging is generally done from a position of deep philosophical conviction, which tends to result in rather weak argumentation. Segal, for example, had a theory in which the apparent velocity of a distant galaxy should be proportional to its distance squared (rather than simply the distance, as in the conventional theory). He would insist that modern statistical techniques reinforced his result; typically, these techniques would involve tossing out the data that manifestly disagreed with his theory. He was extremely bright in some ways, but about this he was blind.

The point really is that the anti-Big-Bang crowd are not visionary mavericks being unfairly undermined by a narrow-minded scientific establishment; they are just crackpots. The difference can be quite subtle and even subjective, but in this case it’s pretty clear.

I think of myself as realistic and even cynical, but reality keeps surpassing my lowest expectations. The Poor Man points out something I didn’t believe until I checked for myself: the front page of the official George W. Bush campaign website features (on May 30, anyway) not a single image of George W. Bush. It does have four pictures of John Kerry, though. Lead with your strengths, I suppose.

PZ Myers of Pharyngula fame has pointed me to an online petition that was apparently first published in New Scientist. No, it’s not complaining about the Bush administration making a travesty of science (although David Appell points to one of those, too); it’s about the terrible dominance of the Big Bang model.

The complaints are not new. The Big Bang just rubs some people the wrong way, and they won’t believe in it no matter how many successes it accumulates. Some of the disbelief stems from religious conviction, but in other cases it seems to be a particular kind of philosophical outlook. Most of the skeptics, of course, have their own favorite alternatives. The most popular is undoubtedly the Steady-State model (or one of its increasingly twisted modern incarnations), but there is also something called the “plasma cosmology”, championed by the late Nobel Laureate Hannes Alfven. (His Nobel was for plasma physics, not cosmology; and the fact that he was Swedish didn’t hurt.) If you want to know in detail why the various alternatives are wrong, Ned Wright tells you.

Here is the kind of thing the petition says:

What is more, the big bang theory can boast of no quantitative predictions that have subsequently been validated by observation. The successes claimed by the theory’s supporters consist of its ability to retrospectively fit observations with a steadily increasing array of adjustable parameters, just as the old Earth-centered cosmology of Ptolemy needed layer upon layer of epicycles.

Really? How about acoustic peaks in the power spectrum of temperature fluctuations in the cosmic microwave background? And the polarization signal, and its spectrum? And the baryon density as deduced from light-element abundances agreeing with that deduced from the CMB? And baryon fluctuations in the power spectrum of large-scale structure? And the transition from acceleration to deceleration in the Hubble diagram of high-redshift supernovae? And the relativistic time delay in supernova light curves? These are just the very quantitative predictions that have come true in the last few years; the Big Bang has had a long history of many observational successes. (This is a very incomplete list; usually one doesn’t pay much attention to straightforward tests of the Big Bang framework, since they are taken for granted.)

But here is the important issue, again from the petition:

Whereas Richard Feynman could say that “science is the culture of doubt”, in cosmology today doubt and dissent are not tolerated, and young scientists learn to remain silent if they have something negative to say about the standard big bang model. Those who doubt the big bang fear that saying so will cost them their funding.

Something actually interesting is being raised here: at what point does a scientific theory become so well-established that it’s no longer worth listening to alternatives?

There’s no easy answer. Scientific theories are never “proven” correct; they simply gather increasing evidence in their favor, until consideration of alternatives becomes a waste of time. Even then, they are typically only considered correct in some domain. Einstein’s general relativity, for example, works very well in a certain regime, but that doesn’t stop us from considering alternatives that may be relevant outside that regime.

So, shouldn’t we devote a certain fraction of our scientific resources, or our high-school and secondary curricula, to considering alternatives to the Big Bang, or for that matter Darwinian evolution? No. Simply because resources are finite, and we have to use them the best we can. It is conceivable in principle that the basics of the Big Bang model (an expanding universe that was much hotter and denser in the past) are somehow wrong, but the chances are so infinitesimally small that it’s just not worth the bother. If individual researchers would like to pursue a non-Big-Bang line, they are welcome to do so; that’s what tenure is for, to allow people to work out ideas that others think are a waste of time. But the community is under no obligation to spend its money supporting them. And yes, young people who disbelieve in the Big Bang are unlikely to get invited to speak at major conferences, or get permanent jobs at research universities. Likewise astrophysicists who believe in astrology, or medical doctors who use leeches to fight cancer. Just because scientific claims are never proven with metaphysical certainty doesn’t mean we can’t ever reach a conclusion and move on.

And to be sure, the alternatives to the Big Bang are just silly. Usually I try to keep my intellectual disagreements on the level of reasoned debate, rather than labeling people I disagree with as “dumb” (that I reserve for the President); but in this case I have to make an exception. They just aren’t, for the most part, very smart. Consider this quote by Eric Lerner, petition signatory and author of The Big Bang Never Happened:

No Conservation of Energy

The hypothetical dark energy field violates one of the best-tested laws of physics–the conservation of energy and matter, since the field produces energy at a titanic rate out of nothingness. To toss aside this basic conservation law in order to preserve the Big Bang theory is something that would never be acceptable in any other field of physics.

Actually, there is a field of physics in which energy is not conserved: it’s called general relativity. In an expanding universe, as we have known for many decades, the total energy is not conserved. Nothing fancy to do with dark energy — the same thing is true for ordinary radiation. Every photon loses energy by redshifting as the universe expands, while the total number of photons remains conserved, so the total energy decreases. An effect which has, of course, been observed.

Just because a person doesn’t understand general relativity doesn’t mean they are dumb, by any means. But if your professional activity consists of combating a cosmological model that is based on GR, you shouldn’t open your mouth without understanding at least the basics. So if I get to decide whether to allocate money or jobs to one of the bright graduate students working on some of the many fruitful issues raised by the Big Bang cosmology, or divert it to a crackpot who claims that the Big Bang has no empirical successes, it’s an easy choice. Not censorship, just sensible allocation of resources in a finite world.

Will Baude at Crescat Sententia asks two profound questions that we at Preposterous are happy to answer.

First: What is the appropriate honorific for a professor at the University of Chicago? There’s a story one sometimes hears to the effect that everyone (students, faculty, presumably researchers) at the UofC refers to each other by Mr/Ms, in sort of a charming reverse-snobbery. (As Brian at That’s News to Me points out, the story is promulgated through the UofC student guide.) We’re all supposed to be a community of scholars or some such thing. But is it really true?

The existence of this story makes things more awkward than they should be, if anything; the transition from Dr/Professor to first names as students get to know professors better is ambiguous and difficult enough, and throwing the possibility of Mr/Ms in there muddles things beyond hope. But we can look at the data. A quick perusal of emails from students in my current undergrad class reveals about a 50/50 split between “Professor Carroll” and a complete absence of name (just “Hi” or some such thing). No “Mr. Carroll”‘s in evidence. But perhaps email is slightly more formal than face-to-face? I recall at least one student last quarter using “Mr.” Not that I care; students are welcome to call me by Sean, or Professor, or Dr. I would think that the rules should be close to what they are outside the academic environment; if you are meeting someone for the first time, the relevant title seems appropriate, and once you get to know them better you can use the first name. Note to students: not every professor feels this way, and some quite like being called “Professor.” And there’s no easy way to tell.

To be honest, I’m not always clear on what I should call other professors. In particular, if I am sending email to someone in my field, whose work I am familiar with but whom I’ve never met in person nor corresponded with previously, should I call them Dr/Professor or just use their first name, as would be common if we were introduced at a conference? For no especially good reason, I tend to jump right in with the first name if the person is actually in my field, but use an honorific for someone in another discipline. Presumably I feel as if we physicists are a band of brothers and sisters, all in this together and somehow all friends even if we haven’t actually been introduced. Whereas the more abstract ties of academia aren’t quite enough to allow for such assumed intimacy. I think this compromise is not unusual, actually, although I don’t have any real data.

(As I was writing this I noticed an update. Seems like the Professors are taking over.)

Will’s second question: Should an omnipotent God be omnicontracting (able to make any promise, but not to ever break those promises) or omnibreaching (able to do anything at any time, even break past promises)? That one is much easier. The concept of an omnipotent God is incoherent. There is no sensible way to define what is meant by “omnipotent.” That’s okay, because there doesn’t exist anything resembling an omnipotent God, so the logical impossibility of the concept shouldn’t bother anyone.

Never let it be said that we don’t tackle the important issues.