Physics in a Flash


I saw the first episode of the new CW show “The Flash” last night. Like most shows on the network, it was mindless fun. It has the CW stamp of lightweight actors, cardboard characters, bubblegum philosophy and idiotic science, but it was still enjoyable. The show, of course, centers around the eponymous superhero who can move at speeds impossible for the mundane human, and how he uses this new power to right wrongs and fight evil. It’s very formulaic; one could guess within the first few minutes who fit where into the grand scheme of things: who’s evil, who’s good, who the love interest is, who will eventually betray whom, and so on. But beyond the surface entertainment, the “science” of the show provides even more fun.

I’m a physicist, and like to think about physics in this world and in the magical world of TV land. The Flash provides a good venue for some entertaining ideas: first of all, how fast does the Flash move? It’s a bit hard to tell just by watching the show, but the writers/directors provided an indirect clue when they showed him moving against a graphic of city streets as seen from above. He seemed to move about 10 blocks in about a second. In New York, 10 city blocks is 1 mile, so 1 mile per second seems a pretty reasonable guess; in metric terms, about 1,600 meters per second. This is about 1,000 times faster than the average walking speed of an adult mundane. Hmmm… Well, if he’s going from 0 to 1600 in about a tenth of a second flat, he’s got an acceleration of nearly 1600 g’s! (I’m using the fact that acceleration is change in velocity over change in time, and that 1 g is about 10 meters per second per second in metric units.) He’s got to be made of stern stuff, as even experienced jet pilots will black out at 9 g’s, and get pretty dead if they’re exposed to accelerations of better than 20 g’s for any length of time. Another way to put it is that this acceleration is something like falling from the stratosphere and hitting the ground suddenly. Oww…

This leads to some other things: if he suddenly sped up and launched himself off of a ramp into the air, he could go a distance of about 160 mile before hitting the ground again! (I’m ignoring air resistance here, and the curvature of the Earth, and Coriolis force, and a bunch of other things as well. However, the show itself is hardly realistic, so neener, neener.) What if we don’t ignore air resistance? The force due to drag of the air as he runs around is something like 800 times his own weight; think about lying on a matress with 800 people sitting on top of you. This is what he feels whenever he runs around! The power which it takes to get him to run around like this is something like 10 million times the average metabolic rate of a normal human being, or the total power used in all of the daily activities by all of the residents of a large town!

The moral: thinking about Physics makes shows like this much more fun.

A quick note:  One thing which I had forgotten (shame on me!) is that if my estimate is correct, he’s running at several times the speed of sound.  My estimates for the forces acting on him are low.  I was assuming that he wasn’t compressing the air ahead of him to any significant extent.  Running this fast,  he’s going to generate a shock wave because the compression of the air in front of him can’t get out of the way fast enough.  Someone watching him as he runs by will see him first, then hear the noise of his passage, as he is outrunning the sound waves he generates…  It’s like watching a supersonic jet pass overhead.


Matters of Gravity


I was just interviewed on the “Jason Rantz” show on KIRO radio (a Seattle station.)  The interview was much more relaxed than most of them have been – Jason was interested in what I was saying, and seemed a lot more relaxed than most of the other radio talk show hosts I’ve been interviewed by.  The interview will be about ten minutes long, although we talked for about twice as long.  Once there’s a podcast I’ll put a link up for it.

We mostly talked about movies, including the recent movie Gravity.  One thing in the discussion which got me thinking was that Jason was surprised that the chain reaction of satellite destruction which drives the main plot is a real idea.  This got me thinking – most people don’t know about it, so it’s a good idea for a blog.  Three points before I start:

1)    There are spoilers for the movie in this blog, so readers beware!

2)    Neal Degrasse Tyson mentioned this in a tweet about the movie a while ago but I want to treat this in more depth; and

3)    I did like the movie despite what I say below.

As far as the third point goes, I like movies even if they get the science wrong.  I repeat the main point of this blog: thinking about the science adds depth to the movies and enjoyment of them, not detract, at least as far as I’m concerned.  If I had to grade the space science in Gravity, I’d probably give it a B-, which is pretty good as far as movies go.  Here, of course, the gold standard for accuracy in depictions of space travel is 2001: A Space Odyssey, made in 1968.  The depictions of how objects move in space are both highly accurate and beautifully depicted, as one would expect from the pairing of Stanley Kubrick and Arthur C. Clarke.  (If you watch the movie, turn it off once you get to the final 20 minutes and imagine your own ending.)

Gravity wasn’t as good, but it did a better job than a lot of others.  There were no banked turns in space, for example.  I found the final scene hard to believe, and there were a few other points which bothered me.  For example, the parachute seemed to billow when Sandra Bullock was maneuvering the Russian space capsule, which wouldn’t happen in the absence of air.  Also, I think (from a short stint at NASA where I consulted on a project related to it) that fires don’t burn the way they were depicted in the capsule scene.  Fires get fresh oxygen through convection, which doesn’t happen in microgravity situations like on the shuttle or in any free-fall orbit. There’s a Youtube video  which shows how a candle flame extinguishes itself in free fall when dropped down a long shaft – this simulates the same physics as a spacecraft in orbit around a planet.  (The theory behind this is originally due to Albert Einstein!)  However, the movie overall was entertaining and pretty intelligent, even if I was able to guess who was going to die and in which order within five minutes.  (The attractive heroine survives, the senior astronaut played by the big-time movie star dies heroically midway through the movie, and the hapless “red shirt” is killed off asap.  Told you there were spoilers!)

The incident driving the plot of the movie is a chain reaction of satellite destruction.  The Russians deliberately destroy an old satellite (presumably so that it won’t endanger other satellites in similar orbits), but this leads to the debris from that satellite destroying others, leading to that debris destroying others, etc.  That is, the Russians create the very situation they were trying to avoid – the planned destruction of one satellite leading to the unplanned destruction of many others.  The consequences include the downing of GPS navigation on Earth and space debris at high velocities coming in to destroy the shuttle the characters are working from.

OK:  two points.  One, the chain reaction is in fact a real concern.  This is known as the “Kessler syndrome” after the space scientist, Donald Kessler, who first predicted it in 1978.  Two, if it does happen, it will not be anything like in the movie.

Issue one:  Low-Earth orbit (LEO), some 160 to 2,000 kilometers above the planet, is filled with debris from older missions.  According to the Union of Concerned Scientists, about half of the thousand or more currently active satellites are in LEO, and there are tens of  thousands of centimeter size or larger fragments of debris from launches or older, inactive satellites.  The potential is that if there is enough material populating these orbits, chain reactions like the one in the movie may start.  Space is big – really big – but there’s a lot of stuff in that particular region of space.  Given enough time, collisions happen.  The collisions take place at very high speed – collisions will take place at relative speeds of something like the orbital speed at that altitude, or 15,000 miles per hour.  It’ll be higher or lower depending on the exact orbital parameters, but whatever it is will be high enough to completely destroy the two colliding objects.

The tricky part is that anything placed into orbit tends to stay there.  It’s not like on Earth, where gravity will cause the debris to fall out of the sky – when two planes collide, the little bits from the collision don’t race off at high speeds to become hazards to other planes for years or centuries to come.  In space, even in a low-Earth orbit, there is barely any atmosphere to cause friction to make the debris fall back down into the atmosphere and burn up there.  The idea behind the Kessler syndrome is that eventually there will be enough junk that the detritus caused by one collision will lead to more, with an ever expanding circle of destruction which will eventually take out most of the satellites at that particular orbital radius.   The mathematics is almost exactly the same as a chain reaction in an above-critical nuclear bomb core:  in that, one neutron’s fission creates more than one neutron, which leads to more fission processes: one leads to two, two create four, four, eight, for example, and eventually there are enough to destroy a city.  It’s harder to do the calculation for space debris, but according to a 2010 paper by Donald Kessler, several regions above 500 km have the potential for a runaway chain reaction.

So that part of the movie reflects real concerns.  However, for dramatic effect, the movie showed the debris ripping through the shuttle, causing mayhem, killing off one of the astronauts, and playing havoc with Earth’s communications satellites and GPS in very short order.  Not going to happen.

First off, most of the satellites are in much higher orbits than LEO.  Much, much higher.  Geosynchronous satellites for satellite TV and communications are at about 35,000 km up.  The GPS satellites, supposedly taken out by the cascade, are up at about 20,000 km.  It takes a whole lot of energy to move a particle from LEO into one of those orbits – most of the particles created by the cascade won’t have enough energy, and the laws of probability dicate that a hit by the small number of the ones which have enough energy is enormously improbable.  So our communications networks are safe.

The bigger problem with the movie is that this is a very slow process.  We think of chain reactions as being fast because of devices like the atomic bomb.  However, space is so sparcely populated that even in a chain reaction situation, the average time between collisions will be months, maybe even years.  The point of the Kessler syndrome isn’t that it’s fast but that one collision will lead to more than one collision afterwards.  The chain reaction wouldn’t have posed a threat to the astronauts up there unless they got incredibly unlucky.  There also wouldn’t have been thousands of these particles just happening to be going in the same direction – the collision would have led to them being spread out in all directions.  (This is not quite true, as the particles would tend to be travelling along the trajectory of the center of mass of the two colliding particles, but it’s close enough.)

I repeat: Gravity was a fun movie.  It’s a good example of the impersonal isolated man or woman against nature film.  If I wasn’t completely bowled over by it, it still made an impact (if you’ll pardon the pun.)


Why “Star Trek: Into Darkness” Isn’t Good Science Fiction, part 1


The hallmark of good science fiction isn’t necessarily good science.  If it were completely scientifically accurate, it wouldn’t be science fiction; it would be a NOVA special.  I would claim that the hallmark of good science fiction is self-consistency.  You break the laws of Physics? OK, but you still need to play fair with your readers or your watchers.  That is, if you hypothesize some grand new technology or scientific breakthrough, don’t ignore the implications when it’s inconvenient…

By this criterion, the second of the Star Trek reboot movies, Star Trek: Into Darkness, fails utterly.  Here are only a few of the problems; I’m not even going to discuss issues involving acting and the plot. Be aware that BIG spoilers follow.

The transwarp, part 1:  In an early scene, Sherlock Hol… I mean, Smau… I mean, Khan/John Harrison, “transwarps” from Earth to the main planet of the Klingon Empire.  The transwarp was Scotty’s invention from the first movie, a combination (somehow) of the transporter with the Warp Drive.  Well, stars are many light years, meaning at least trillions of miles apart.  Even if the Klingon empire is located in the next-nearest star system, it’s a distance of about 65 trillion miles.  He has to transwarp there with an accuracy of a few feet in order to avoid falling off the big cliff he appears on top of.  This is an accuracy of about 1 part in 50 quadrillion (a quadrillion is a thousand trillion.)  To put this into context, if we knew the Earth’s diameter to that accuracy, we would know it to the size of one atom…

The transwarp drive, part 2:  OK, let’s assume we have this phenomenal accuracy.  A large part of the plot is the Federation’s anxiety over going to war with the Klingons. If you have the transwarp and the Klingons don’t, why worry?  Just transwarp an antimatter bomb on them if they make too much trouble.  (If they have transwarp as well, then everyone is in trouble…)  More things suggest themselves: with transwarp, why have starships?  Transwarp is faster, really accurate, and portable.  Maybe it’s not cheap, but starships aren’t either.  (My book estimates antimatter production costs starting at billions of dollars…)

These are only a few of the problems with the movie – stay tuned for more!

“Tea, Earl Gray, Hot”


Sometimes the best things in life are the simplest ones.    Perhaps my favorite holiday gift ever was an electric kettle, a device whose only purpose in life is to boil water — but boil it efficiently, in a fraction of the time it would take for a kettle on the stove, and for a fraction of the energy, too.  It’s simplicity itself — it has a coil which a current runs through.  The coil gets hot, heats water in a chamber sitting above it, and voila!  Boiling water.  By my estimates, the electricity costs are about a tenth to a fifth of a  cent for every cup of tea I brew.

The 23rd-century designers of the USS Enterprise seem to have lost this technology.  To get a cup of tea, Captain Jean-Luc Picard stands next to a little box in his room, says “Tea, Earl Gray, hot”, and a cup of tea is beamed in.  It seems to be an offshoot of transporter technology: you’re either beaming a cup made before from somewhere else, or assembling it whole from “pure energy” (whatever that means.)  Either way, it seems to be a damn-fool way to make a cuppa.

E=mc squared, right?  Each kilogram of matter takes 90,000 trillion joules of energy to create.  The water in a cup of tea has a mass of about one-third of a kilo, so this is 30,000 trillion joules.  But no technology is perfect: if the replicator is only 99.9% efficient, we are wasting 30 trillion joules into heat – enough to heat 100 million kilograms of water  for tea…  Just why are we doing it this way, again?

Playing the Game


Hi!  I’m a Physics professor at St. Mary’s College of Maryland, and a lifetime science fiction fan.  I just wrote a book called Wizards, Aliens and Starships: Physics and Math in Fantasy and Science Fiction (Princeton University Press,  I like playing the science fiction game: looking at the science in works of science fiction, and trying to decide if they’re reasonable or not.  This blog will feature some things which didn’t make it into the book, or look deeper into things that did.  There will be  math here, mostly  arithmetic, but some algebra as well.