1 WHERE WE’RE GOING, WE DON’T NEED ROADS . . .

Hollywood seems to be a huge fan of veering off into no-man’s land when time travel is involved. From flux capacitors to spinning egg timers on a string they would have you believe that “All of the silly details to time travel have already been accounted for, surely then you need only believe the premise of the story!” Perhaps that is indeed the case. Maybe we should believe in the science of a flux capacitor, an ancient staff, a metal box with washers glued to it, or a giant spinning EM quantum field generator. They sound science-y don’t they? I mean, the writers must have had the decency to look into these theories before taking the leap onto the silver screen. . . right?

Well, truth be told, I believe that screen writers have a right to embellish. They are at liberty to string together all manner of fancy fallacy and wistful wording in order to suspend our disbelief for a brief time so that we may enjoy some popcorn or a hotdog and forget about the world outside.

So why bother with this essay? Well, there has been a pesky hum in the back of my mind, one that centers around this whirlpool of cinematic temporal displacement. It just can’t happen. None of it. No Marty McFly meeting his pimple-faced dad, no Hermione Granger loading up on classes, no getting kidnapped and sent back in time to be assassinated by yourself only to escape and be the very cause of your kidnapping in the first place (Phew! Winded after that one. . . ). I can already hear the cacophony of cries, “Stop ruining our fun!” you’re shouting at your screen. But guess what, it’s not me ruining your fun, it’s Thermodynamics.

Yes those immutable laws described by Einstein, (and almost every other Physicist on the planet), to be immovable, unchangeable, and indestructible. The laws of Thermodynamics have been so solidly confirmed that they are one of the only things that Physicists tend to count on. Which is a very very rare occurrence in a field that deals with the uncertainty of non-realities on a daily basis. So how do these laws affect time travel? Why do they fundamentally ruin every worm-hole, phone booth, and Delorian movie we’ve ever seen? Let’s talk about it.

2 THE FIRST RULE OF THERMODYNAMICS . . . DON’T TALK ABOUT THERMODYNAMICS

Alright, let’s not waste any time on flowery introductions. No derivations of the Sakur− Tetrode, no ODE’s or PDE’s, let’s throw all of that in the dumpster and cut right to the heart of the matter. The First Law of Thermodynamics states very generally, “The energy of a closed system is constant.” That means that if had a magic box that could contain a certain amount of energy without leaking, all of the energy in that box would always be measured as the same amount. It could transform from one state to another, it could be matter or energy (e.g. E = mc2 ), but it would always, always, contain the same amount of energy. I couldn’t add to it, or take away from it, in any way, shape, or form.

Now that we’ve got that established, I want you to think about the Universe. As far as we can tell our universe is an isolated, contained, system. It doesn’t leak energy, and it doesn’t gain energy from any outside source. (If you start messaging me about dark energy here I recommend you Google the term “Dunning-Kruger Effect”.) Stop watching Maury Povich and ponder that for a second. . . At any given moment we cannot add energy to the universe, and we can’t take energy from the universe. . . the energy is always the same. . . cool. . . so what?

2.1 POP!

How does time travel happen in most movies these days? Most of the time it’s represented as a sort of “POP!” moment. One minute the protagonist is in their time and space and then POP! they’re in the old west! Wait just a minute. . . the energy in the universe is supposed to be constant at any given moment. . .

Thanks to Einstein we know that yes, indeed, matter, momentum, and energy can be thought of as interchangeable properties in the form of E 2 = m2 c 4 +p 2 c 2 for slow moving objects going, say, 88mph. (Go brag to your non-science friends that you know the real Einstein equation now!) So mass, matter, stuff, a person, everything, is energy in one form or another. (Again, if you want to talk chakras and how we’re “all just waves of energy man”, Dunning-Kruger Effect, do it.) This energy is accounted for in the total of the universe, no matter how insignificant. A mosquito, a jet plane, or even a giraffe, is a representation of a small piece of the energy contained in our universe.

So how is it that I can take that small bit of energy, remove it, take it to another moment in the timeline of the universe, and then add it in? I CAN’T, that’s how!

2.2 THE JUG

Here is, what I hope will be, an easy to follow example. Let’s say I fill up a jug of water. That jug of water represents chemical bonds, dipole forces, dispersion forces, and all other manner and forms of energy. Now I’m going to set that jug of water on the table, and I’m going to walk away. I’m going to leave that jug undisturbed for two weeks. Then using the bracelet issued to me by the Time Agency I warp back in time one week and I set the jug of water right next to itself. Now, when I go back to my original time, there will be two jugs of water. One that has been there two weeks, and one that has been there for just a week but is actually three weeks old. I took the energy contained in that jug and I zapped it out of existence, then I added it to a time in the universe where it already existed in it’s current state and increased the amount of energy in the universe at that time. Then the paradox occurs when we reach the moment that I originally took the jug back and removed it’s energy from the universe and now there are two! We have doubled the “jug-energy” of the universe by cheating! And the universe will have none of this foolishness, the energy of the universe, at any given moment, is constant. I hope this example is clear. Let it roll around in your brain for a few days before you shout “shenanigans!” and start throwing around big words. And understand that Thermodynamics is very well tested, and has never come anywhere close to failing. (Unlike Ether, Cold Fusion, etc. . . ).

3 . . . BOGUS (*valley dude voice)

So basically Thermodynamics keeps the universe constant, homogeneous, and safe. If violations of energy conservation were allowed it is quite possible that no coherent forms of mass could have come into being. The universe would be too fluid, too changing, to chaotic, to support any form of life. We exist because of laws of Thermodynamics, and unlike man-made laws these laws are unbreakable.

There are countless violations of physical laws in 99% of the movies and television programs out there mingling amongst the masses. This is only one example that applies to various instances of the “POP!” style of time travel. I could go on and on about the multitude of disgraceful representations of misinterpreted science, but there’s a new Doctor Who episode available for download. So I will speak to you again. . . in the future.

—Brain

Producer’s note: We here at The SciFi Podcast are pleased to present the scientific musings of one of the smartest guys we know: The Brain! We’ve all been friends with Brain (aka Brian Patchett) for decades, but mostly in the context of mohawks and guitars. Now we have the pleasure of getting to read his thoughts on the real science of fiction with this “The Physics of …” series. We are very happy to post another of his ingenious Brain Blogs here. And, if you like this, there should be more in the future. We just have to figure out how to get back there first! 1.21 gigawatts, maybe?