Guest Post: Jim Kakalios on the Quantum Mechanics of Source Code

by Sean Carrollhttp://blogs.discovermagazine.com/cosmicvariance/2011/04/12/guest-post-jim-kakalios-on-the-quantum-mechanics-of-source-code/#more-6659
 
Jim Kakalios of the University of Minnesota has achieved internet
demi-fame — he has a YouTube video with over a million and a half
views. It’s on the science of Watchmen, the movie based on Alan
Moore’s graphic novel. Jim got that sweet gig because he wrote a great
book called The Science of Superheroes — what better credentials could
you ask for?
 
More recently Jim has written another book, The Amazing Story of
Quantum Mechanics. But even without superheroes in the title,
everything Jim thinks about ends up being relevant to movies before
too long. The new movie Source Code features a twist at the end that
involves — you guessed it — quantum mechanics. Jim has applied his
physicist super-powers to unraveling what it all means, and was kind
enough to share his thoughts with us in this guest post.
 
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There is an interesting discussion taking place on the internets
concerning the ending of the newly released film SOURCE CODE, that
suggests that the film concludes with a paradox. I believe that any
such paradox can be resolved – with Physics!
This entire post is one big honkin’ SPOILER, so if you want to read
about the final twist ending of a film without having seen said film –
by all means, read on, MacDuff!
 
In SOURCE CODE, Jake Gyllenhaal plays US helicopter pilot Colter
Stevens, whose consciousness is inserted into another man’s body (Sean
Fentress, a school teacher in Chicago) through a procedure that
requires a miracle exception from the laws of nature (involving
quantum mechanics and “parabolic calculus” – by the way, there is no
such thing as parabolic calculus). Thanks to some technobabble (or as
Q-Bert on Futurama would describe it – weapons grade bolognium)
Colter’s mind can only enter Sean’s body in the last eight minutes of
Sean’s life. As Sean is sitting on a city bound Chicago commuter
train, on which a bomb will explode at 7:58 AM, killing everyone
aboard, the goal is for Colter to ascertain who planted the bomb. He
cannot stop it from exploding, he is told, because that has already
happened. He cannot affect the past, but he can bring information
obtained in the past back to his present time. Learning the identity
of the bomber would enable the authorities to prevent the detonation
of a threatened second “dirty atomic” bomb is downtown Chicago.
 
While the above can be discerned from the movie trailer, what I am
going to discuss next involves the actual ending of the film, and if
you do not want this ending spoiled, you should stop reading now.
 
Colter learns that the reason his last memory is being attacked in his
helicopter in Afghanistan is that he in fact died in the crash. His
mangled body is kept artificially alive, and his brain can be
activated, and sent to inhabit the body of Sean Fentress (who happens
to be a neurological match). At the end of the film, after multiple
failed attempts, Colter manages to identify the bomber. Providing this
information to Col. Goodwin (a military officer played by Vera
Farmiga) and Prof. Rutlidge (the great Jeffrey Wright), the scientist
who designed the Source Code project, the terrorist is caught before
he can set off the second bomb, but after, of course, the first bomb
on the Chicago train explodes.
 
It is left somewhat vague as to whether Colter is going to parallel
realities, a la the Many World’s interpretation of Quantum Mechanics,
or whether he is engaging in a quantum/ neurological simulation. If
the former (which seems to be borne out by the ending) then this would
tie into notions of time travel being explored in the context of
quantum gravity. That is, if one could time travel into the past, you
need not fear any Grandfather paradox (what if you killed your
ancestor – preventing your birth, but then you would not be able to
travel back in time to ice Grandpa). Some physicists argue that time
travel is only possible via parallel realities. You do not go back in
time in your own reality, but to an alternate Earth’s past. You can
thus kill as many grandparents as you have bullets, remaining safe in
your own timeline. In any event it is assumed that the bomber is the
same person every time Colter enters the Source Code.
 
While everyone is celebrating the capture of the bomber, the first
successful trial of the Source Code project, Colter convinces Goodwin
to send him back one last time, to try to save the passengers on the
train. At the end of the eight minutes, he convinces Goodwin to
terminate his life support, allowing him to die in actuality, as the
world and his father believe happened months ago in Afghanistan.
Needless to say, he manages to stop the first bomb from exploding on
the train, hands the bomber to the authorities, and kisses his love
interest just as the eight minute mark is reached. We see Goodwin make
good on her promise and end his life support at that moment, at which
point she is arrested my the military police for acting against
Rutlidge’s instructions.
 
On the train however, Sean/Colter is still alive after the kiss. The
film implies that he goes on to live happily ever after in Sean’s
body, with Colter’s mind, while Colter’s deformed body remains at the
Nellis laboratory on life support. As the bomb never went off on the
Chicago train – there was no reason to activate Coulter and send him
into the Source Code, and the project awaits its first true trial by
fire. Thus it is indicated that we are witnessing two alternate
realities – one where Goodwin is arrested after pulling the plug on
Colter following the successful application of the Source Code, and
the other where Sean/Colter is still alive, where the Source Code
project has not been activated.
 
Among the many discussions I’ve noted on the web about the ending of
this film, I wish to address two particular issues that are being
debated by the Hive Mind. In the film’s final reality, where the bomb
does not explode on the train – does Colter’s consciousness reside in
two places at once? And, what happens to Sean Fentriss’ consciousness
in this final reality?
 
Reasonable people may reach different conclusions concerning these two
points. As I am a physics professor – I will tell you the RIGHT
answers!
 
(1) In the final reality – Colter is NOT consciousness in two places
at once. He is awake and aware in Sean’s body and at the same time his
damaged body is in the Nellis lab – IN A NON-CONSCIOUS STATE. He is
not awake and aware in the lab at Nellis, he can not initiate motion
or form an independent coherent thought. He is in essence brain dead,
kept artificially alive until there is a time and need for him to be
activated (if there is a terrorist attack).
 
Even if he is activated – this would NOT influence or affect Colter in
Sean’s body, as it would take place in Sean/Colter’s FUTURE. Remember
he was sent back to Chicago at 7:50 AM – the bomb exploded at 7:58.
Time progresses forward for both Sean/Colter and Nellis/Colter at the
same rate. This was why Goodwin and Rutlidge were upset about how many
trials it was taking – for each trial burned up a minimum of eight
minutes, and brought the second explosion closer to happening.
 
What you are doing and thinking now is not affected by what you will
be doing and thinking several hours from now. Do you know what you
will be thinking about several hours from now (ok – for the guys this
is an easy one). Nellis/Colter may not be activated for
weeks/months/years later. But even if he is – Sean/Coulter can live
his life, unaffected by what is happening in his future. There is no
paradox, for Colter in Sean’s body is only awake and conscious at one
point in time. Colter is NOT like Schrodinger’s cat, in two different
conscious states simultaneously, as they are separated in time.
 
(2) What happened to Sean’s consciousness? Here there is a potential
problem. Basically I believe Sean is dead. When Colter’s mind jumps
into his body, it over-writes Sean’s consciousness. Rutlidge probably
knows this, and ignores the ethical issues. Sean will be dead when the
bomb explodes after all, and Rutlidge believes that cannot be changed.
By sending Colter into Sean’s body, he robs Sean of the last eight
minutes of his life. As Sean is unaware that a bomb will explode,
killing him and everyone on board, he would not do anything
extraordinary in those eight minutes. Rutlidge probably believes that
it is acceptable to sacrifice the last eight minutes of one man’s life
in order to save millions of lives if they can prevent the second bomb
blast in downtown Chicago. Every time Coulter enters the Source Code
at 7:50 AM, he essentially kills Sean. Sean will die in every reality
where Colter does not enter the code, and he will also die in all N –
1 realities where he does – so this is an ethical problem of order 1/N
where N goes to infinity.
 
Alternatively, Sean may be alive in Colter’s damaged body – but there
was no suggestion that something like that was happening. Here I’m
taking the Quantum Leap analogy too literally. (There is a wonderful
tip of the hat to Quantum Leap – listen carefully to Coulter’s Dad).
 
Sorry this is so long. Never ask a professor a simple question – you
always get a lecture in reply!