First off, why am I even talking about this? Time travel into the past isn’t real, so what’s the point?The point is that time travel and its application to archaeology/anthropology has not been proved to be impossible, so it is worth talking about the possible risks and the amazing benefits were such a technology to be invented. Being a huge physics nerd myself, I have seen a few proposed types of time travel that could affect the past to varying degrees. I will give an example of a few and talk about their pros and cons in investigating the past.
The first is Star Trek style warp-drive. There is actually a real physical model to achieve this called an Alcubierre drive that essentially shrinks space in front of your spaceship and expands space behind it. Using your warp drive spaceship, you could outrun the light-bubble of the period that you wish to study and use a powerful telescope to peer into the past. This “time travel” technique for observing the past is good because you don’t run into any causality problems–you wouldn’t be causing the past that you are trying to observe to change. However, not only would you have to build your warp-drive spaceship, but you would also need an enormous amount of engineering and resources to go into your telescope. Doing some quick calculations, I found that if you neglect glare from the sun and atmospheric effects that in order to have a resolution of 2 meters (~the height of a person) from 500 light-years away (i.e. looking 500 years into the past) you would need a telescope with a mirror that is about 4.33 million kilometers across.
If building a telescope that is 3x the diameter of the sun doesn’t sound like a good use of your time, another time travel technique that could be used to study the past is by use of wormholes. If you build a wormhole, keep one end near earth, and send the other one on a round-trip at near the speed of light, then you could send that end of the wormhole as far into the future as you want. Now, one of the downsides of this one is that it would only benefit archaeologists/anthropologists of the future. They could thank us for sending them this marvelous portal for them to traverse and study our primitive ways, but we would not be able to use it to visit our past. However, I recently found a paper that proposes a way that we could build a machine that could travel through time to a point prior to its construction, TARDIS style (not kidding…they actually named it the TARDIS! Find it at http://arxiv.org/pdf/1310.7985v1.pdf). The major problem with both of these time travel techniques is that we do not know how causal paradoxes are resolved (the whole, what would happen to you if you killed your grandfather problem). If we visited our past and tried to change it, then would it change the subject that we were studying or is there some mechanism in the universe that makes such changes impossible. One of the most popular answers for this is based on the many worlds hypothesis of quantum physics. It says that if you changed your past then you would find yourself in a new parallel universe where it acted as if that change was the way that it always had been. Either way, you would still have to be careful in testing an archaeological hypothesis because you actions in the past could affect its outcome and your results would no longer be objective.
I doubt time travel will ever happen in my lifetime, or many lifetimes to come. However, due to the scale of the projects that would make it possible and the academic and moral implications that come along with its application to the study of the human past, we should not dismiss thinking about it now so we can do the right thing if it ever does become possible.