# How can we travel to the past?

Category: Physics
Published: February 16, 2013

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

Mainstream science dictates that time travel to the past is impossible. The basic problem with time travel to the past is conservation of energy. For example, pretend I made a time travel portal that sends objects back three seconds in time to the exact same location, and I mount this portal a few inches above a trampoline. I turn the portal on and one second later drop a charged battery into the portal. It falls, appears three seconds earlier, before the portal was turned on, and bounces off the trampoline to a point above the portal. Three seconds later, there I am dropping the original battery and next to it is the battery that time traveled and bounced. The portal turns on and both batteries now fall through the portal. The cycle repeats itself automatically without human intervention. All I had to do was turn on the portal and drop the first battery. Two batteries become four, which become eight, and so on. A businessman might say, "Great! Free batteries. Let's sell them and make a fortune!" But the point is that once I flick on the portal and drop the first battery, the rest is entirely automatic. This means there would be no way to stop it. Keep in mind that this chain reaction is not happening at progressively later points in time. It is all happening in the same three-second loop of time. That one battery would instantly become an infinite number of batteries. They would fill the entire universe, crush us to death, and explode with all of this runaway energy.

The law of local conservation of energy is universal, and it's a good thing. It's what keeps our universe from exploding in an instant into an infinite fireball of energy. It's also what rules out time travel to the past. If the presence of a human in our scenario bothers you, we could do without him. We could just as easily have an acorn fall from a tree through a three-second time-travel portal created by a cosmic ray and bounce off the trampoline without any humans around. The explosion of the universe would still occur. The fact that the universe is still around is ample evidence that time travel to the past is impossible. Note that we have not used any ethical or social arguments involving killing your own grandfather. Time travel to the past is forbidden by simple physics even in the absence of humans. While movies about time travel to the past can be creative and thought-provoking, they are simply wrong. There are many speculative theories about time travel to the past, some made by respected physicists, but mainstream physics does not allow it.

On the other hand, time travel to the future is allowed and indeed has been measured experimentally. Relativity dictates that the rate of the forward march of time is dependent on both the speed of your reference frame as well as the strength of the gravitational field in your frame. We can travel into the future by either spending time in a vehicle traveling very fast relative to earth or by spending time in a very strong gravitational field. In both cases, time travel does not allow skipping to some future time. All reference frames must experience all seconds in the day. Rather, time just goes slower in such reference frames. If your local time runs fast on account of high speed or gravity, you can experience an hour while earth experiences two. Such time travel is verified experimentally every day by equipment that deal with precise time measurements, such as GPS satellites. Unfortunately, the time dilation effect is very weak. The highest speeds and strongest gravity that men can currently attain only transports them milliseconds into the future.