Friday, December 19, 2014

The Physics of "Gravity" and "Interstellar"

Hanukkah is a Jewish holiday celebrating a miracle whereby oil sufficient for just one day burned for eight.  Over this past week, an entirely different miracle took place: I, a father of two young girls watched two movies, none of which contained a single princess.

I really got my nerd on: I rented "Gravity" and then went to the theater to see "Interstellar".  Both films feature a lot of physics, but they could not be more different.  "Gravity" is a ninety minute expedition featuring orbital mechanics, while general relativity weaves its way through the three hours of "Interstellar".

You are probably expecting me to dissect these two films and expose all of their scientific inaccuracies.  In truth, I enjoyed both of them immensely, but would like to address one major problem with each one.

Gravity - Clooney didn't need to die...

Spoiler alert (oops, too late, sorry).    

This movie actually gets quite a few things right.  The conservation of angular momentum of each body is well done.  When things begin to spin due to some angular impulse, they just keep on spinning.  Also, the central premise whereby a collision between satellites in space gives rise to an exponential increase in space debris is not entirely far-fetched.

I feel the need, however, to eviscerate the most important scene in the movie; anyone with a basic appreciation for orbital mechanics should pick up on it.  It's when Sandra Bullock and George Clooney's characters are trying to grab on to a broken space station.  They nearly lose the chance to do so when at the last moment, Bullock becomes tethered to the station, and Clooney becomes tethered to Bullock.

What should happen next, is the energy stored in the tether should cause them to spring back towards the station.  What actually happens is inexcusable.  For some unknown reason, steady-state tension manifests in both tethers, as though gravity were acting on the astronauts, but not the space station.  They are all in orbit: a perpetual state of falling.  There should be no tension in these tethers other than that coming from the inertia of the bodies.  The astronauts should be able to propel themselves to the space station with the slightest of tugs.

Instead, Clooney cuts himself loose in order to save Bullock.  Again, when he cuts the cord, he should continue to float in the same orbit he is in alongside everything else, but instead he flies away.  If the director wanted to find a physics scenario where one astronaut could self-sacrifice to save the other, I am sure that one consistent with the laws of physics could have been dreamed up.

I mean, the movie is called GRAVITY!  Shouldn't a NASA specialist or a graduate student look through the physics in it to make sure there are no major flaws?

Interstellar - Black holes alter space time, but that's not all they do...

This movie was just awesome in so many ways.  Since it is fairly new, I won't give anything away.  I just want to discuss the effects of general relativity near black holes.

Significant time dilation can occur due to special relativity (when moving at near light speed) and due to general relativity (when near a huge, dense mass).  This film sees much of the latter occur - to the tune of a Lorenz factor of more than 60,000 (one hour in proper time translates to seven years back on Earth).

Such a massive distortion of time can only happen in the presence of a seriously wicked gravitational field.  In such a field, your ship and everything inside it would turn to atom soup.  Let me explain.

The gravitational force is attractive in nature, and is proportional to the inverse square of distance.  If you stand on the surface of Earth, your feet are ever so slightly closer to the center of the planet than your head is.  Consequently, your feet are pulled down ever so slightly more towards the Earth than your head.  This difference is so minute (tiny fraction of a Newton) that it is indiscernible.  However, replace the Earth with a black hole, and the difference is dramatic.

In short, nothing can withstand the rip apart nature of a gravitational field that causes substantial time dilation.  But, this movie was so entertaining, that I was happy to suspend my disbelief.

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