Questions on Time

Question: I have one question. What is time? And is time travel really possible? If so, can we travel to the past?
Asked by: Sunil Hosakoti
Answer: (and, that's technically three questions).

For your first question, time is one of the four dimensions we live in. It's like asking "what is length" or "what is depth." Time is an intrinsic part of the construction of our reality. Einstein unified space and time (which is now known as 'spacetime'). From there, answering the question, "What is spacetime?" would probably win you a Nobel Prize.

Time travel is possible, yes. Depending on your definition, it's possible in three ways. Forward time travel, backwards time travel, and time travel as a result of time dilation.

For time dilation, you can only 'travel' forwards. It's not time travel in the traditional sense. If I was in a spaceship, traveling around the Earth at a significant fraction of the speed of light, the clocks on my ship are moving slower in comparison to the clocks of the people on Earth. I could be in my ship for one Joshua day and come back to find 5 Earth years have passed. I'm not really 'traveling' in time; instead, time is simply moving slower for me than time for people on Earth.

Forward and backwards time travel use the same type of mechanisms (in the traditional sense that is). They both involve the warping and folding of the spacetime. Forward time travel is 'easy'. With large amounts of energy, it's theoretically possible to travel forward in time with relative ease. Backwards time travel is mathematically possible, but many of the solutions to the equations that allow us to travel backwards in time require some order of infinity (which isn't physically possible to achieve). Even though there isn't any physical reason preventing us from traveling back in time, nature seems to have safeguards set up preventing that. There are, however, some workarounds to that problem.

References, sources, and further reading can be found here: http://www.fromquarkstoquasars.com/questions-on-time/