And it’s not because the writers are all physics Ph.D.’s who have traded in equations and laboratories for a glamorous life in Hollywood. It’s because the Big Bang Theory is one of the very few shows to have a full-time science advisor: David Saltzberg, a particle physicist at UCLA. David confers with the writers, reads every script, provides complicated-looking equations for the white boards in Sheldon and Leonard’s apartment, and suggests the occasional physics joke.

And now David, encouraged by some of his well-meaning friends, is going to be explaining the science behind the show in his new blog:

• The Big Blog Theory

The show is a comedy, but the science here is completely serious — read about dark matter, quantum mechanics, monopoles, and all sorts of good stuff. I’m sure much of this was explained carefully in the original scripts, but landed on the cutting-room floor in interests of time.

The Big Bang Theory, of course, raises strong feelings among scientists. Right here at Discover, you can read both pro and anti feelings about the show. The complaints are mostly about the cheerful reliance on various stereotypes that we would just as soon see stamped out. All four of the main scientist characters are socially maladjusted guys; the one main non-scientist is a blonde woman with severe science-phobia.

I think the critique of sexism is mostly fair. In the real world, plenty of brilliant socially-maladjusted scientists are female! (To be fair, Penny represents the everyperson character to which the audience is supposed to relate; in almost every activity not related to science or technology, she is much more competent than the boys.) The critique that all these nerdy scientist characters somehow damage the image of science I find much less compelling — even though, in the real world, plenty of brilliant scientists aren’t socially maladjusted at all. It is, after all, a sitcom, not a public-service announcement; sitcoms get a lot of their mileage out of stereotypes. And as socially awkward as the scientist characters are, they are also portrayed as lovable and warm people at heart. Shows like this humanize science, and who knows what ten-year-old kid will see an episode and start thinking that physics is a career to which real people can actually aspire.

Now if we could just get across the idea that even young girls can aspire to these careers, we’d be getting someplace.

So it’s great to see the producers of ABC’s hit TV show LOST jump on the bandwagon. This fall they will be releasing the DVD collection of the fifth season, and the Blu-ray edition is going to feature a special treat: mini-”lessons” on various academic subjects related to the show. (The final season of the show begins early in 2010.) One of those subjects is time travel, and you have a pretty esteemed group of professors guiding you through this fascinating subject: Nick Warner of USC (who taught me general relativity back in the day), our old friend Clifford Johnson, and myself. Suffice it to say, I’ve seen the rough cut, and they did a good job — and we had quite a bit of fun. I was only included because having all the professors speak with British accents would have seemed a bit posh.

And along with that, they’ve just launched an associated website: LOST University. You can see what the other courses in the curriculum are going to be, including Philosophy and Foreign Languages. At the moment the website is essentially promotion for the DVD’s themselves, but I’m hoping more content will appear over time. LOST has a tradition of enhancing the show with quite elaborate online activities, in the form of alternate reality games. So hopefully this new site won’t simply be an advertisement — one of the lessons of new media is that giving away cool stuff for free makes it more likely that people will pay money for the even cooler stuff.

To be clear: the science of time travel on LOST does not necessarily obey all the rules. None of us had anything to do with the show itself, and I have no idea what the writers did in terms of seeking science advice. But understanding how the rules are broken can serve as fodder for teaching moments just as easily as seeing them obeyed. That’s life here “on the cutting edge of tomorrow.”

Docking from Mato Atom on Vimeo.

For the many international CV readers, today is the US’s Independence Day celebration, which is in large part an excuse to bar-b-que meat products, blow up fireworks, and drink beer. If you’re tuning in from abroad, you are probable sober enough to read Daniel’s upcoming post on gravitational waves. For the rest of the drunken US crew, you can probably handle the Muppets.

PS. While we’re talking beer, I must recommend the current Full Sail Limited Edition L.T.D. (Recipe No. 3), sold in bottles with the pale blue label. Seriously. Try some.

(h/t: Again with the CakeWrecks)

It’s pretty clear that Iranian security is using 1984 as a how-to guide. Spying on your family as a social good.

The situation in Iran is no laughing matter; it remains to be seen whether Ayatollah Khamenei has painted himself into a corner where further large-scale violence is inevitable. Our thoughts are with the Iranian people demanding their rights of self-government.

Jerry Zucker and his wife Janet Zucker deserve a great deal of credit for turning the idea of the Exchange into a reality. More importantly, for a twelve-year-old such as I was at the time, The Kentucky Fried Movie was a major event in modern cinema. So I was pleased to see that the title of Jerry’s post (”I’d Like to Thank the National Academy”) was the same one that I had used when I gave a talk at the NAS annual meeting. Not that either one of us should be overly proud of that particular line.

Also, he gets away with saying stuff like this:

The really great thing about these scientists is that because their brains are exactly two-and-a-half times the size of the average person’s in the movie business (although in fairness, that also includes talent agents), they are actually more creative and therefore much better at coming up with science-related ideas for movies than our so-called “creative community.” I don’t mean to offend anyone but as much as I loved Slumdog Millionaire, it’s no Viagra. Often, science gets tacked on like wallpaper in a story, but when it’s really integrated into the narrative it can take things in surprising new directions. And thanks to the Exchange and the National Academy of Sciences, research just became much more fun.

That thing about the brain sizes is what they call “creative license.” But it’s deployed in the service of making a good point! Scientists are good at coming up with ideas, and it would be great if a closer relationship between science and Hollywood helped some of those fun ideas percolate into the wider culture. (My giant brain scoffs at giving specifics about how this will actually happen.)

0. There are no paradoxes.

This is the overarching rule, to which all other rules are subservient. It’s not a statement about physics; it’s simply a statement about logic. In the actual world, true paradoxes — events requiring decidable propositions to be simultaneously true and false — do not occur. Anything that looks like it would be a paradox if it happened indicates either that it won’t happen, or our understanding of the laws of nature is incomplete. Whatever laws of nature the builder of fictional worlds decides to abide by, they must not allow for true paradoxes.

1. Traveling into the future is easy.

We travel into the future all the time, at a fixed rate: one second per second. Stick around, you’ll be in the future soon enough. You can even get there faster than usual, by decreasing the amount of time you experience elapsing with respect to the rest of the world — either by low-tech ways like freezing yourself, or by taking advantage of the laws of special relativity and zipping around near the speed of light. (Remember we’re talking about what is possible according to the laws of physics here, not what is plausible or technologically feasible.) It’s coming back that’s hard.

2. Traveling into the past is hard — but maybe not impossible.

If Isaac Newton’s absolute space and time had been the correct picture of nature, we could simply say that traveling backwards in time was impossible, and that would be the end of it. But in Einstein’s curved-spacetime universe, things are more flexible. From your own personal, subjective point of view, you always more forward in time — more technically, you move on a timelike curve through spacetime. But the large-scale curvature of spacetime caused by gravity could, conceivably, cause timelike curves to loop back on themselves — that is to say, become closed timelike curves — such that anyone traveling on such a path would meet themselves in the past. That’s what respectable, Einstein-approved time travel would really be like. Of course, there’s still the little difficulty of warping spacetime so severely that you actually create closed timelike curves; nobody knows a foolproof way of doing that, or even whether it’s possible, although ideas involving wormholes and cosmic strings and spinning universes have been bandied about.

3. Traveling through time is like traveling through space.

I’m only going to say this once: there would be no flashing lights. At least, there would only be flashing lights if you brought along some strobes, and decided to start them flashing as you traveled along your closed timelike curve. Likewise, there is no disappearance in a puff of smoke and re-appearing at some other time. Traveling through time is just like traveling through space: you move along a certain path, which (we are presuming) the universe has helpfully arranged so that your travels bring you to an earlier moment in time. But a time machine wouldn’t look like a booth with spinning wheels that dematerializes now and rematerializes some other time; it would look like a rocket ship. Or possibly a DeLorean, in the unlikely event that your closed timelike curve started right here on Earth and never left the road.

Think of it this way: imagine there were a race of super-intelligent trees, who could communicate with each other using abstract concepts but didn’t have the ability to walk. They might fantasize about moving through space, and in their fantasies “space travel” would resemble teleportation, with the adventurous tree disappearing in a puff of smoke and reappearing across the forest. But we know better; real travel from one point to another through space is a continuous process. Time travel would be like that.

4. Things that travel together, age together.

If you travel through time, and you bring along with you some clocks or other objects, all those things experience time in exactly the same way that you do. In particular, both you and the clocks march resolutely forward in time, from your own perspective. You don’t see clocks spinning wildly backwards, nor do you yourself “age” backwards, and you certainly don’t end up wearing the clothes you favored back in high school. Your personal experience of time is governed by clocks in your brain and body — the predictable beating of rhythmic pulses of chemical and biological processes. Whatever flow of time is being experienced by those processes — and thus by your conscious perception — is also being experienced by whatever accompanies you on your journey.

5. Black holes are not time machines.

Sadly, if you fell into a black hole, it would not spit you out at some other time. It wouldn’t spit you out at all — it would gobble you up and grow slightly more corpulent in the process. If the black hole were big enough, you might not even notice when you crossed the point of no return defined by the event horizon. But once you got close to the center of the hole, tidal forces would tug at you — gently at first, but eventually tearing you apart. The technical term is spaghettification. Not a recommended strategy for would-be time adventurers.

Wormholes — tunnels through spacetime, which in principle can connect widely-separated events — are a more promising alternative. Wormholes are to black holes as elevators are to deep wells filled with snakes and poisoned spikes. The problem is, unlike black holes, we don’t know whether wormholes exist, or even whether they can exist, or how to make them, or how to preserve them once they are made. Wormholes want to collapse and disappear, and keeping them open requires a form of negative energies. Nobody knows how to make negative energies, although they occasionally slap the name “exotic matter” on the concept and pretend it might exist.

6. If something happened, it happened.

What people want to do with time machines is to go into the past and change it. You can’t. The past already happened, and it can’t un-happen. You might wonder what’s to stop you from jumping in your time machine, finding your high-school self, and convincing them that they really shouldn’t go to the senior prom after all, thereby saving yourself all sorts of humiliation. But if you really did go to the prom, then that can’t happen. The simple way out, of course, is to suppose that travel into the past is simply impossible. But even if it’s not, you can’t change what already happened; every event in spacetime is characterized by certain things occurring, and those things are fixed once and for all once they happen. If you did manage to go back in time to your years in high school, something would prevent you from dissuading your younger self from doing anything other than what they actually did. Even if you tried really hard.

7. There is no meta-time.

The least realistic time-travel movie of all time might be Back to the Future. When Marty McFly changes the past (violating Rule 6), the future “instantaneously” changes. What the hell is that supposed to mean? Time measures the temporal interval between different events in spacetime, and can be quantified by clocks. There is no set of clocks outside the universe, with respect to which you can go muck around in the past and have effects propagate into the future “at the same time.” Likewise, your brain is not going to change to remember things differently, nor will any other record-keeping device such as diaries or photographs or embarrassing sex tapes. Sorry about that.

8. You can’t travel back to before the time machine was built.

Right now, at the particular place you are sitting, at the time when you are sitting there, one of two things is true: either there is a closed timelike curve passing through that point in spacetime, or there is not. And that situation will never change — no matter what clever engineers may do in the future, if they create closed timelike curves they cannot pass through events in spacetime through which closed timelike curves did not pass (corollary of Rule 6). Or in plain English: if you build a time machine where there wasn’t one before, it may be possible for future travelers to come back to that time, but nothing can help you go back to times before the machine was built.

9. Unless you go to a parallel universe.

Parallel universes — the kind we contemplate in the many-worlds interpretation of quantum mechanics (MWI) — provide potential loopholes for some of the above rules. According to the MWI, there exist different “branches” of the wave function of the universe, distinguished by different observed outcomes for the measurement of quantum events. In the celebrated Schrödinger’s cat thought experiment, there is a “universe” where the cat is alive, and one where it is dead. Some imaginative (but respectable) physicists, especially David Deutsch, have speculated that we could combine this idea with the possibility of closed timelike curves to contemplate travel into the past of a different universe. If time travel is unlikely, this idea is (unlikely)², but it’s not inherently paradoxical.

If you could travel to the past in a different branch of the wave function, then we are allowed to contemplate changing that past in a self-consistent way, because it’s no longer really “your” past. So almost all cinematic invocations of time travel — where they are constantly mucking about, changing the past in crucial ways — would have to appeal to something along these lines to make any sense. But even if you can change what you thought was the past, all of the rules of continuity and sensibility still apply — no flashing lights, no disappearing, no sudden changes in the future, no re-writing of your memories, etc.

10. And even then, your old universe is still there.

Remember Rule 0: no paradoxes. If you have reliable records of having made some unwise decisions regarding your social life in high school, then those decisions were made, and can’t be un-made. Even if you go into a different branch of the wave function, where you bestow some wisdom-of-experience on your younger self, you would only be changing the history of that universe. There is still the universe you left behind, with all of your bad decisions still intact. That’s life in the multiverse for you. It remains for future scholars to write Ph.D. theses along the lines of Utility Functions and Moral Dilemmas in an Ensemble of Multiple Interacting Universes. But it’s just a matter of time.

[* Update: Spock does not actually travel backwards in time and become Kirk's grandfather, nor lover, nor does he write Shakespeare's plays. That was a "joke." I am reliably informed that the Spoiler Patrol and Internet Rectitude Society does not appreciate "jokes."]

- One billion downloads in nine months. This is a really, really big number. For perspective, this is equivalent to every man, woman, and child in the US downloading three iPhone applications. Or one out of every seven people in the entire world downloading an application. It’s one download every time your heart beats for your whole life. It’s one application download for every hundred stars in the galaxy. Or one application every fourteen years, for the entire history of the Universe. This is a lot of downloads. In a market space that didn’t even exist a year ago. All of this has happened in only nine months (an average of more than 40 downloads a second, day and night, nonstop). Here’s a “visual” of one billion, from the wikipedia page:

- The billionth download happened to be the application Bump, something I posted about last month (and, because of April Fool’s, was considered suspect). Small world. Maybe Andy will get to shake Steve Jobs’ hand? That would be almost as exciting as my brother hanging out with Lindsay Lohan.