I’ve just arrived at Fermilab for a 2-month sabbatical (gotta do my part to enhance the CV geographic distribution) and learned of a contest sponsered by the state of Illinois to chose the Seven Wonders of Illinois. It’s a gimmick, of course, to promote tourism and is a total rip-off of the ancient seven wonders of the world. Visitors to the Illinois Bureau of Tourism Seven Wonders web site can nominate their favorite wonder. Note that timing is of the essence - nominations are due by 1 March, 2007 (this Thursday!). The state has been divided into seven regions and folks are asked to pick a region in which they want to nominate a site. Online voting will then take place to pick the top sites from the nominations starting March 5. The field will be successively narrowed through the rest of the month and the Illinois tourism bureau will announce one winner for each of the seven regions on April 30.

Curious? So, ya go to the 7 wonders website and you see lots of pictures that the good folks at the Illinois tourism board consider worthy of nomination - historic courthouses, lakes, Indian burial mounds, riverboats, majestic big city buildings, but nothing - absolutely nothing - of what I consider to be the most important, and probably famous, site in Illinois: Fermilab! This is a place that is special and unique to the planet, and yet retains its roots in the prairie land of Northern Illinois. I think the good folks of the Illinois Bureau of Tourism should take note. Afterall, it is (presently) the most energetic particle accelerator on earth and has made, and has the potential to make further, fundamental discoveries of the nature of the universe. It is also honestly a wonder in itself to the human eye. The collider ring circumference is 6.28 km and can easily be viewed by an airplane heading to or from one of Chicago’s airports. The accompanying office building is a 15 story highrise, built in an A-frame shape with a spectacular multi-story atrium. To retain its natural connection to the land, the lab boasts a buffalo farm (gosh the young ones in the spring are cute!) and a prairie wilderness area.

It’s a truly spectacular site! And is more than worthy of being one of the top seven wonders of Illinois. So, there’s one day left to nominate - let’s flood’em with nominations for Fermilab! Let the folks at the Illinois Bureau of Tourism know that people from all over the world think they’ve got something truly special (which they do).

To nominate, go here. You gotta type in your address (even better if you’re out of state/country - let’em know Fermilab is famous!), and write a 250 characters maximum blurb on why you think Fermilab is a wonder. That was tough for me - I hit 250 words and hadn’t even touched all the lab’s features.

Go to it folks - let’s show’em that science is important!

Update: Alas, Fermilab did not make the first cut of sights for the first vote! I’m sure we had plenty of nominations, perhaps the committee thought it was too esoteric. C’est la vie in the science world.

Dr. Free-Ride brings to our attention Scientiae, a new blog carnival devoted to posts about women in science, engineering, technology and mathematics. Apparently something that people still like to talk about! So if you’re a blogger with a good post along those lines, go ahead and submit it. And if you’re not, feel free to submit something else good that you’ve read.

Quote of the Day: David Albert, philosopher of science at Columbia. He was interviewed for, and appeared in, What the Bleep Do We Know?, the movie that tried to convince people that quantum mechanics teaches us that we can change physical reality just by adjusting our mental state. After seeing the travesty that was the actual movie, he complained loudly and in public that his views had been grossly distorted; this quote is from one such interview.

It seems to me that what’s at issue (at the end of the day) between serious investigators of the foundations of quantum mechanics and the producers of the “what the bleep” movies is very much of a piece with what was at issue between Galileo and the Vatican, and very much of a piece with what was at issue between Darwin and the Victorians. There is a deep and perennial and profoundly human impulse to approach the world with a DEMAND, to approach the world with a PRECONDITION, that what has got to turn out to lie at THE CENTER OF THE UNIVERSE, that what has got to turn out to lie at THE FOUNDATION OF ALL BEING, is some powerful and reassuring and accessible image of OURSELVES. That’s the impulse that the What the Bleep films seem to me to flatter and to endorse and (finally) to exploit - and that, more than any of their particular factual inaccuracies - is what bothers me about them. It is precisely the business of resisting that demand, it is precisely the business of approaching the world with open and authentic wonder, and with a sharp, cold eye, and singularly intent upon the truth, that’s called science.

Read the whole thing. The use of emphases is characteristic of David’s writing style, which is also on display in his fantastic books on quantum mechanics and the arrow of time.

The only really misleading part of the above quote is choosing “the Victorians” as Darwin’s foil; things haven’t changed all that much, sadly.

While some physicists are known for their hearty support of atheism, even they can have some personal dieties. High in the physicist’s pantheon sits Richard Feynman, due not only to his obvious smarts and good work, but also to an outsized personality chonicled in a wealth of popular writings (and even a movie!). I’ve always had mixed feelings about Feynman as a cult figurehead, however. It’s nothing personal against Feynman in particular, but about the hero worship he represents. During high school or college, many aspiring physicists latch onto Feynman or Einstein or Hawking as representing all they hope to become. The problem is, the vast majority of us are just not that smart. Oh sure, we’re plenty clever, and are whizzes at figuring out the tip when the check comes due, but we’re not Feynman-Einstein-Hawking smart. We go through a phase where we hope that we are, and then reality sets in, and we either (1) deal, (2) spend the rest of our career trying to hide the fact that we’re not, or (3) drop out. It’s always bugged the crap out of me that physicists’ worship of genius conveys the simultaneous message that if you’re not F-E-H smart, then what good are you? In physics recommendation land, there is no more damning praise than saying someone is a “hard worker”.

Well, screw that. Yes, you have to be clever, but if you have good taste in problems, an ability to forge intellectual connections, an eye for untapped opportunities, drive, and yes, a willingness to work hard, you can have major impacts on the field. While my guess is that this is broadly understood to be true by those of us clever-but-not-F-E-H-smart folks who’ve survived the weeding of graduate school, postdoctoral positions, and assistant professorhood, we do a lousy job of communicating this fact to our students. I’ve always suspected that we lose talent from the field because people opt for Door #3 (drop out) when they face up to the fact that physics is frequently hard, even for very clever people. The idea that you have to be F-E-H smart to succeed gives little encouragement to continue when the going gets rough. (I have no idea if other fields have this same problem — my guess is that physicists are particularly prone to it, since we are trained early on to think that physicists are simply smarter than chemists or biologists. Those other fields are for the hard workers. We don’t put mathemeticians on this scale, because we secretly believe they’re smarter than us. Note to the biologist lynch mob: tounge is in cheek.)

Anyways, I’ve been thinking about this again in light of Po Bronson’s excellent article in New York Magazine about Carol Dweck’s research (which I read via Nordette in Blogher is coming out in a popular book Mindset: The New Psychology of Success). The article is focused on how to effectively handle praise for smart kids. The upshot (verified by a number of clever experiments), is that when you praise a kid for being smart in general, rather than for specific accomplishments or efforts, you risk paralyzing the kid with a fear of not looking smart, to the point where they will tend to shun challenges.

In follow-up interviews, Dweck discovered that those who think that innate intelligence is the key to success begin to discount the importance of effort. I am smart, the kids’ reasoning goes; I don’t need to put out effort. Expending effort becomes stigmatized—it’s public proof that you can’t cut it on your natural gifts.

Repeating her experiments, Dweck found this effect of praise on performance held true for students of every socioeconomic class. It hit both boys and girls—the very brightest girls especially (they collapsed the most following failure).

While Dweck is working primarily with preK-12 students, everything covered in the article rings true for what I’ve seen at the higher levels (both for myself, my colleagues, and students). Those of us who are fortunate enough to sail through high school often crumple when the stuff we’re allegedly good at finally becomes hard. Whether you “make it” as a physicist after that has a lot to do with how you respond at that moment. Do you take it as a sign that you’re not cut out for the game? Do you feel like a failure, and stop enjoying physics as a whole? Do you buck up and forge ahead? (Like a neutrino, you’ll probably wind up oscillating among the three mixed states for a while, before collapsing into one of them.)

I was most struck in Bronson’s article by a description of an experiment by Lisa Blackwell and Dweck on the impact on performance of how one perceives intelligence. In a science magnet school with low acheiving students, Blackwell studied 700 students, all of whom were taught a multi-session unit on study skills. One half of the group, however, also received a “special module on how intelligence is not inate”:

The teachers—who hadn’t known which students had been assigned to which workshop—could pick out the students who had been taught that intelligence can be developed. They improved their study habits and grades. In a single semester, Blackwell reversed the students’ longtime trend of decreasing math grades.

The only difference between the control group and the test group were two lessons, a total of 50 minutes spent teaching not math but a single idea: that the brain is a muscle. Giving it a harder workout makes you smarter. That alone improved their math scores.

These studies have lots of implications for higher ed in the sciences. Physics, with its strong cult of genius, is probably the canary in the coal mine.

Once again this semester, I’m teaching a ridiculously fun course - Physics 312: Relativity and Cosmology; Einstein and Beyond. As I’ve mentioned before, this course is so enjoyable because one gets to expose undergraduates with not much physics background to some of the mind-bending results of relativity, and watch them struggle with it and, usually, finally come to understand it. Great stuff.

While I have a blast with the later parts of the course - general relativity and cosmology - I have a particular soft spot for something rather close to the beginning, in the special relativity portion - the most famous equation in physics - E=mc².

So how does one go about motivating this equation for a class of students with only a little physics background, but who know some calculus? Well, perhaps the first thing to say is that, for the purposes of this course, it is much more important that they understand why an equation relating mass and energy is required, and how one might derive its form, than that they actually be able to do the detailed derivation themselves. So that is the tack that I take.

Early in the course, we review the idea that light is an electromagnetic wave. We do this by starting with Maxwell’s equations, which describe how moving and spatially varying electric and magnetic fields are related, and using them to show that, even in vacuum, if one tweaks the electric or magnetic field, then that disturbance propagates as a wave, with a given speed. We then see that the speed that arises is empirically equal to the speed of light, and hence we identify light itself with electromagnetic waves. This is a powerful idea, because students have a great deal of intuition about waves. In particular, they know that waves carry energy and momentum. So, at this point, students are pretty comfortable with the idea of light as a wave, and that light therefore carries energy and momentum.

Now, this is a great point for one of those staples of relativistic reasoning - the thought experiment. We start with one that doesn’t involve any of those worrisome relativity ideas. Think of a physicist, standing at one side of a large box, which itself is sitting on a perfectly frictionless surface (think of ice if you like). The physicist possesses a large cannon, which she is using to hurl heavy cannonballs across the box. What happens to the whole system?

Well, the box, physicist, cannon and cannonballs are a closed system, with no external forces acting on it. So one thing we know is that the center of mass of the system won’t move. Of course, that doesn’t mean that nothing will happen. As a cannonball is launched, it acquires a certain momentum, and conservation of momentum means that the box acquires the equal and opposite momentum, and sets off sliding backwards on the ice.

The next important event is that the cannonball collides with the opposite wall of the box, imparts it’s momentum to the box, and both cannonball and box come to a halt. At this point, the distribution of mass in the box is different from at the beginning (a cannonball has been transferred from one side to the other), and the position of the box has shifted. These two differences conspire in such a way that the center of mass of the system as a whole remains in the same place. All is right with the world.

Now let’s think about a second thought experiment, which is closely related to the first. All I want to make different is to replace the cannon by a powerful laser. Instead of a cannonball being propelled across the box, we’ll now think about the laser firing a pulse of light. Now, the light carries momentum, and so when the laser fires and the pulse sets off, the box will once again begin a backwards slide in order that momentum be conserved. Also once again, when the light reaches the other side and is absorbed by the opposite wall, the momentum will be transferred back to the box, which will then come to a halt. But now you see the problem. The distribution of mass in the box is the same as it was at the beginning, and no external forces have acted on the system, and yet because the box has slid backwards and no mass has been moved, the center of mass of the entire system has moved! All no longer seems right with the world.

This kind of thought experiment is what forces one to the conclusion that the idea of “center of mass” needs to be replaced by a more general concept - that of a “center of energy”. Obviously, this means that one must take into account how the distribution of energy in the system has changed, as well as the mass, when figuring out how a system should behave under no external forces. Another way to say this is that moving energy to one side of the box to the other is equivalent to moving some mass across the box - mass-energy equivalence!

This is the punch line, but one can do a little better. One can, of course, ask, when I’ve fired my laser pulse and had it absorbed on the other side, how far has the box moved? One can then ask, how much mass would I have to have moved from one side to the other in order that this movement of the box, combined with the mass movement, leave the center of mass unchanged. Equating the answer, m, to this question, with the energy, E, of the pulse, moving at the speed of light, c, yields: E=mc².

Everyone is having their fun with Conservapedia, a rightward-tilting alternative to Wikipedia that aims to ensure that future generations of conservatives grow up really dumb. A mildly-close look reveals that the major biases of Wikipedia that made this new project worth launching are (1) their insistence in using “CE” (Common Era) rather than “AD” (Anno Domini) in giving dates, and (2) the occasional Anglicized spelling. For some great examples of the way self-professed conservatives view the world, see Jon Swift, or a roundup of sciencey responses by Mark Chu-Carroll.

Here are my personal favorites, after five minutes of clicking around. Links to specific versions, as they keep changing, of course. But these look sincere, not the result of vandalism by naughty liberals!

• Atheism
  Atheism is the disbelief in the existence of any supernatural deity. This disbelief can take a number of forms, such as the assertion that deities do not exist, or the absence of any belief in any deity.

  Stalin and Richard Dawkins are prominent atheists. Dawkins wrote a book, called “The God Delusion”. Stalin is now dead, having killed millions of people in the name of Marxis-Leninism (which is predicated on atheism).

  Since atheists have no God, as a philosophical framework atheism simply provides no logical basis for any moral standard. They live their lives according to the rule that “anything goes”. In recent years, this has led to a large rise in crime[1], drug use, pre-marital sex, teenage pregnancy,[2] pedophilia[3] and bestiality.

The road from atheism to bestiality is shorter than you think!

• Stalin
  
  Josef Stalin was an atheist communist Russian dictator during World War II. He was defeated by Adolf Hitler, despite Hitler also being an atheist.

That’s the entire entry. I can’t decide which is more amusing — the amazement that one atheist could defeat another in battle, or the judgment that Hitler defeated Stalin.

• Albert Einstein
  Einstein’s work had nothing to do with the development of the atomic bomb. Nothing useful has even been built based on the theory of relativity. Only one Nobel Prize (in 1993 and not to Einstein) has ever been given that even remotely relates to the theory of relativity. Many things predicted by the theory of relativity, such as gravitons, have never been found despite much searching for them. Many observed phenomenon, such as the bending of light passing near the sun or the advance of the perihelion in the orbit of Mercury, can be also predicted by Newton’s theory.

Can’t make this stuff up.

• Anything Goes
  “Anything Goes” is the title of a 1934 musical production written by Cole Porter. Popular songs from the musical include “You’re the Top,” “I Get a Kick Out of You,” and “Anything Goes.”

  Because Porter was a homosexual, we can conclude that ‘anything goes’ was also his philosophy of life. Many atheists have adopted the song as a description of their “moral” code.

Getting the message yet?

• Sex
  1. The process by which offspring are conceived.
  2. Another term for gender.

Again, that’s the entire entry. But it says so much, don’t you think?

Would you be shocked to hear that the readership of general-circulation science magazines is overwhelmingly white, male, and middle-aged? Probably not. Of course, you might comfort yourself with the thought that lack of interest in such magazines is programmed into the DNA of women, young people, and non-Caucasians, despite evidence that the relevant genetic information is apparently evolving awfully rapidly.

Would it surprise you to learn that overtly sexualized images of women cause tangible harm to adolescents and young women? Maybe it would. Not that there’s anything wrong with sexy images of people of any gender in appropriate contexts, but in the actual context in which children grow up in our culture, the way in which these images appear enacts a vastly disproportionate toll on young girls.

Are you at all taken aback by the cover of the latest catalogue for Edmund Optics, purveyor of scientific optical equipment?

The same image appeared in ads in Physics Today. Which, by the way, is not a biker magazine.

This sales pitch has caused a bit of consternation, including a lot of conversation on the AASWomen mailing list. But it’s not just those uppity wymyn who are upset. Geoffrey Marcy of Berkeley has written to the company to complain:

Dear Mr. Radojkovic and Mr. Delfino and Mr. Dover,

As representatives of Edmund Optics, I hope you will pass the following message to the appropriate management at Edmund Optics.

I just saw the images from the Edmund Optics catalog that show a woman in a tight red skirt lounging next to some optical devices, some with the caption, “Red Hot”. I hope Robert Edmund and the board of directors of Edmund can be alerted to this problem.

As a scientist and professor at UC Berkeley I am embarrassed on behalf of the many female science students coming along. I wonder what message such images of sex objects in your ads send to bright young scientists
of both genders.

Moreover, after decades of overt discrimination against women in the physical sciences, including precluding their admission to the best universities and the denial of access to the world’s best telescopes, your ad represents a setback. It reminds us of a dark era of clear discrimination against women, a time that I’m sure Edmund Optics hopes is long gone. If so, you have made a very serious error that insults the scientific community.

As you can imagine, your ad has already generated extraordinary discussion in the scientific community, analogous to the discussion over the comments by Harvard’s president who implied that women might not have what it takes to be great scientists. In short, your company has left open the question of your equal and unbiased treatment of women in your company and in your contracts.

Sincerely,
Geoffrey Marcy
Professor of Astronomy, UC Berkeley
Elected Member, United States National Academy of Sciences

To which Bill Dover at Edmund replied, in a classic example of “not getting it”:

Hi Geoff,

Thank you for your feedback regarding the EO catalog and our recent cover. No need to be embarrassed for the many female science students coming along. Rather, encourage them to celebrate that another smart, young, and attractive female has joined the ranks of women in a technical field, which breaks the pattern of discrimination you describe. You see, the woman featured on the cover is a six-year employee of Edmund and our Trade Show Manager and Spokesperson. Over the years we’ve received numerous positive comments and she has proven herself to possess the needed technical and social ability to successfully coordinate our tradeshows that showcase our products.

The recent cover photo emphasized a new product launch by Edmund. Our Trade Show Manager coordinated the showcase of these products at Photonics West last month. Had you happened by our booth for a visit, you would have had the opportunity to meet and speak with her about our Kinematic mounts as well as receive additional technical information from two other smart, young, and attractive, female optical engineers present at the time. So that you know, this line of Kinematic Optical Mounts, Table Platforms, and Mechanical Accessories are technically situated to become the standard for optical positioning equipment in the marketplace. We are excited about the quality, features, and price of these products and know that they will be very difficult to compete with and we chose our Trade Show Manager to help commemorate their release.

Professor Geoff, please encourage ALL of your female students to join the technical, engineering, and science ranks. There are too many that fall prey to the stereotypical concepts of what a person should look like or dress like which keep them from significant contributions in our society. That said, we value the opinions of our customers and we evaluate the feedback in developing our future strategies. I appreciate the time you have taken to mention your concerns here. I hope you will take the opportunity to encourage your female students to meet our female optical engineers at Edmund Optics. I think they, and you, will be impressed with their ability to support and represent woman [sic] in engineering.

Best Regards,
Bill

As far as I can tell, he’s saying that “she” is smart (so smart that she doesn’t need a name, apparently), so it’s okay! This is America, so any talented and attractive young woman with an interest in engineering can grow up to be a Booth Babe. He forgot to mention that “Better Performance. Better Price.” is the kind of slogan that any female should be proud to be associated with!

Actually it’s not okay. We’re not going to see this any time soon:

A little parity goes a long way, though. I have a vision of the next catalog cover–it features a handsome young man, maybe in chinos or a nice pair of jeans, barefoot, shirt halfway unbuttoned, an alluring gleam in his eye. Maybe a caption like “Well Oiled Mounts.”

And even if we did, it still wouldn’t be okay. (Although it would be highly amusing.) These images don’t appear in a vacuum; as long as the way that women and men are put on display in a wider cultural context remains dramatically imbalanced, a little equal-opportunity cheesecake here and there isn’t going to fix things.

Feel free to email Bill Dover (wdover-at-edmundoptics.com) and VP of Marketing Marisa Edmund (medmund-at-edmundoptics.com) to let them know what you think. (Thanks to Chaz Shapiro for the pointer.)

One nice thing about being a scientist, or at least an academic one, is that occaisionally you get your mind blown without any drugs or anything. Someone comes along and just pulls the rug completely out from under you - a total Denial of Reality Attack - and then you are left on your own to pick up the pieces.

Today at UC Davis we had a seminar from Don Page of the University of Alberta. The title and abstract of this talk sounded like science fiction, so I reproduce it here:

Don Page, University of Alberta

Title: Is Our Universe Decaying at an Astronomical Rate?

Abstract: Unless our universe is decaying at an astronomical rate (i.e., on the present cosmological timescale of Gigayears, rather than on the quantum recurrence timescale of googolplexes), it would apparently produce an infinite number of observers per comoving volume by thermal or vacuum fluctuations (Boltzmann brains). If the number of ordinary observers per comoving volume is finite, this scenario seems to imply zero likelihood for us to be ordinary observers and minuscule likelihoods for our actual observations. Hence, our observations suggest that this scenario is incorrect and that perhaps our universe is decaying at an astronomical rate.

Boltzmann brains? WTF? Intrigued, I went. This is a well-respected, highly-cited cosmologist after all. A former student of Stephen Hawking, no less. The jargon in the abstract, though bizarre, had a certain je ne sais quoi…

The idea Don put forward is this: there’s us, the ordinary observers (OO’s) in the world, who have achieved a certain stature after billions of years of evolution in the universe, and are now capable of making quite refined (or so we think) observations of the universe. Andre Linde called OO’s “just honest folk like us.” We’ve made it as a species, man- and womankind, and we’re figuring ou the really deep things that are going on like the Big Bang, genetics, and all the rest.

Then, though, there are the BB’s in the universe: Boltzmann Brains. Random fluctuations of the fabric of spacetime itself which, most of the time, are rather insignificant puffs which evaporate immediately. But sometimes they stick around. More rarely, they are complex. Sometimes (very very rarely) they are really quite as complex as us human types. (Actually, “very very rarely” does not quite convey just how rare we are talking now.) And sometimes these vacuum quantum fluctuations attain the status of actual observers in the world. But, the rarest of them all, the BB’s, are able to (however briefly) make actual observations in the universe which are, in fact, “not erroneous” as Don Page put it.

The man was a compelling speaker, and soon I realized there was an actual intellecutal debate underway in the high end of the cosmology/high energy community as to what the role of these BB’s might be in the universe, in the very far (or maybe not so far) future. We have a certain prejudice that, well, there just aren’t so many of them out there at this stage of the game, 14 billion years after the Big Bang. We’d like to think that we have the stage at the moment, we OO’s, um, assuming there are in fact more of us out there. (Any other non-human OO’s out there, could you let us know, please that you are listening? We have a few questions for you…)

The thing is, when you start talking about very very…very rare things like Boltzmann Brains, you are talking about REALLY long times. Much longer than we’ve had on earth (and I mean 4.5 billion years) by many orders of magnitude. Numbers like 10 to the 60th years were being batted around like it was next week in this talk. By those times, all the stars and all the galaxies have gone out, and gone cold, and space has continued to expand exponentially and things are long past looking pretty bleak for the OO’s still around, who (we presume) need heat and light and at least a little energy of some sort to survive, even if we are talking about very slow machine intelligence (even slower than humans for example).

So eventually, the mere fact that there is, at these long times, just oodles of space in the universe means that the BB’s become more and more common (even if they are rare) and eventually dominate the, uh, intellectual landscape of the universe. Of course this immediately raises all sorts of questions, such as mind/matter duality, the nature of reality and consciousness and multiple consciousnesses, perceived versus objective independent reality. Not to mention whether our “universe” is the only one. Okay, I’ll stop now…

Well, at this point in the talk, being new to this and my mind already quite blown, I had trouble keeping the thread. Somehow or other Don seemed to conclude that a BB-dominated universe was absurd (though are we sure we’re not in one already?) and then posited a radically different spacetime metric, an Anti-deSitter space, which he seemed to think might contain the problem. But then he hit another question which was the title of the talk: must the universe be decaying more rapidly than we expect? I am mangling this horribly, and of course before writing this I took just a glimpse at the already voluminous amount of literature on this topic, and realized that I have a lot of reading to do, both blog and academic. So it’s best I stop and let you all go look up Botzmann Brains, as I will, and do some more reading.

Sigh. The Ultimate Fate of the Universe is of course a nice escape from our quotidian grind. But, as Lenny Susskind wrote in his inscription to us in his book The Cosmic Landscape, at a signing last fall in Davis, “Hey, things could be worse!”

(And, Lo! They were worse…)

John’s post on light-induced sonic booms has set a bad precedent of actually answering questions. (And it’s been a big hit around the internets, so our server keeps overheating.) Sensing an opportunity, commenters hungry for knowledge have chimed in to ask all sorts of perfectly good questions about cosmology. To keep things on track, let’s divert those questions to this separate thread. So this is the chance to ask all of those questions about the universe you’ve always wondered about. For example:

Q: If I plug in Hubble’s law for the velocity of a galaxy in terms of its distance (v = Hd, where H is the Hubble constant), at large enough distances the velocity will be greater than the speed of light! Doesn’t that violate relativity?

A: Yes, it would be greater than the speed of light, but no, it doesn’t violate relativity. What relativity actually says is that two objects can’t pass by each other at a relative velocity greater than the speed of light. The relative velocity of two distant objects can be whatever it wants. In fact, to be more of a stickler, the relative velocity of two distant objects is completely ill-defined in general relativity; you can only compare velocity vectors of objects at the same point. The notion of “velocity” almost makes sense in cosmology, but you have to keep in mind that it’s only an approximate concept. What’s really going on is that the space between you and the distant galaxy is expanding, which redshifts the photons traveling from there to here, and that reminds you of the Doppler shift, so you (and Professor Hubble, so you’re in good company) interpret it as a velocity. But it’s not a Doppler shift; both you and the galaxy are essentially “stationary” (although that concept is also not precisely defined), it’s just that the space between you is expanding.

In fact I already have a Cosmology FAQ that you’re encouraged to check out, and Ned Wright also has one. But feel free to ask questions here; I’m sure Mark will be happy to answer them.

A reader sent the following interesting questions:

Question I: Why doesn’t light make a sonic boom when it travels. I know its a maseless particles, but the energy in it gives it an effective mass via matter-energy equivialnce. But lets go a step forward. Why don’t messenger particles WITH mass like the W and Z boson’s make a sonic boom? They do in fact have a true mass. Or even protons in a particles acceleration traviling around Fermilab at near the speed of light make the sonic boom? Does that mean there must be a critical mass to make a sonic boom, and if so, what is it?

A true sonic boom is a shock wave. A sonic shock wave results when an object like a fast plane travels at a velocity greater than that of sound in that medium. The wave travels at an easy-to-calculate angle to the direction of motion of the object, since the object is at the leading edge of the wave creation front, and the wave emanates in a sphere from that point and spreads outward in all directions at the speed of sound. A similar effect results from a boat travelling in water: the V-shaped bow wave is in fact a shock wave.

So what about light? Well, almost. When an object like a charged particle travels through a medium (glass, or even air) in which the speed of light is less than c, the speed of light in a vacuum (300,000,000 m/s), it gives off a light shock wave. This sort of shock wave is called Cerenkov radiation, and it is VERY useful to us experimental types because it tells us we have a very fast particle going through our detectors. Now, a Z boson is electrically neutral and will not give Cerenkov radiation. A W boson has charge, and could do so in principle, but in practice its lifetime is so very short it does not travel even a microscopic distance before decaying. As for the protons circulating in the beam pipe at Fermilab, well, that’s a vacuum (and a pretty good one) so they don’t exceed the speed of light in that medium.

Light, or electromagnetic radiation in general, does not cause such a Cerenkov shock wave, but it does exhibit some other odd effects when passing through matter. For photons with wavelength roughly in the visible spectrum and shorter, you get the photoelectric effect (for which Einstein won his first Nobel Prize - it was not relativity), the Compton effect (for which, you got it, Compton won the Nobel), and for really high energy photons (gamma rays) you can get electron-positron pair production, the easiest way to make the antimatter version of electrons, and also very useful for the experimentalists. Then you also have nuclear photoabsorption, and the very odd Mossbauer effect. Happy reading!

Questions II: Why doesn’t a duck’s quack echo? The only thing I can think of is the fact that the reflecting sound waves quickly colliding negating each other, but that;s just a thought. Truth be told I have no idea why.

Who said a duck’s quack doesn’t echo? It absolutely must, just like any sound wave, off a reasonably flat surface.