Science In Real Life

Altering the Past, part 1 September 11, 2011

Discussions of time travel among physicists never fail to revolve around a rather jargon-y term: “closed timelike curves”. The exact definition of this term is not important right now. What is important is that it fixes the discussion to the question of whether or not it is possible for a chunk of ordinary matter to revisit a previous point in its own personal timeline. This is a very interesting, very speculative, and oftentimes very mathematical question. Of course, it’s not the only discussion of time travel – consider your average time-travel science fiction story. With a few notable exceptions, the same thing always happens: our heroes are flung back into the past and they must find a way to return to their own time without changing history. Sometimes they succeed, sometimes they find they were meant all along to have done something in the past, sometimes they have to undo an accidental change they already made. Save history or else!, is always the message. Frankly, I think we could all do with a little less history these days, but that’s a digression for another time.

There is a question lying at the intersection of these two realms of time travel that I found myself recently pondering. What does it mean, in the context of a physical understanding of time and space, to have altered the past? How much of a disruption is small enough to go unnoticed, and how much is enough to make sure that the planet has been conquered by giant ants when you get back to your present day?

They campaigned on a platform of free sugar cubes. What were we going to do, vote for the spiders?

To get started on this, I will have to drop some physics on you. What you’re about to see is called a Minkowski diagram, and it is the best method ever devised by man for representing the fundamental, ethereal concept of causality in a drawing simple enough to be a doodle on someone’s coffee napkin.

Pictured: Cosmic wisdom.

There’s a lot going on here. Let’s unpack it. First, we have the horizontal axis, in light years. This represents all of space. On the vertical axis is time, in years. That means that a light ray, traveling one light year per year, is a 45° line. The tricky part is that these light rays represent the limits of temporal influence. Let’s say I, sitting at Here & Now, at coordinates (0,0), want to influence something three light years away, at x = 3. Let’s call this place Deep Space Three. The fastest way to do it is to send a signal of light. That light ray will travel for three years and then arrive at Deep Space Three, at coordinates (3,3). I absolutely cannot send any information to Deep Space Three, three light years away, any sooner than that. Any events that happen at Deep Space Three before that are utterly beyond my power to alter to even the smallest extent. I cannot so much as budge a single electron. Why? Because the light ray hasn’t gotten there yet. And the light ray carries the electromagnetic force. From currently known physical laws, all influence, all information transmission, happens through interactions of one or more of the four fundamental forces. Gravity, strong, weak, and electromagnetic. That’s it. For an object to be influenced in any way is for it to interact with another object via one of those forces.

Love is not a force, no matter what Randall Munroe tells you.

But now let us turn to the past. The diagram also shows all the points, events, and information that can possibly influence me, at Here & Now. Anything that can send me a light ray exists on one of the diagonal yellow borders of the triangle labeled Past. What about the rest of the triangle? It’s largely irrelevant. With one large exception, every single interaction you have ever had and will ever have with anything else in your entire life happens by the exchange of photons, carriers of the electromagnetic force. What you think of as physical touch is the exchange of photons between the EM fields of your skin and another object. Sound waves, pressure waves in air, are transmitted when one group of air molecules pushes another. That push happens by the same EM field photon exchange. Think about everything that’s ever happened to you. Every event, every point on your version of that graph, is an electromagnetic interaction. You have lived a life built out of light rays.

Oh man, remember this day? That was hilarious.

I’m afraid we’re out of time, dear readers. Next time I will, with this background fresh in your minds, discuss the meat of the question. And, I’m going to give you some homework. Just two questions, to roll around in your minds.

1) I mentioned a large exception in the final paragraph. What is it?

2) Try to think of minimal ways to change events. What’s the smallest amount of energy required to get someone to change their mind on any kind of decision, or alter their actions in some culturally recognizable way?

Happy sciencing!