Time travel in the past The past weeks I watched some episodes from "Through the Wormhole" with Morgan Freeman.
When talking about traveling back in time, they said you can only travel back up to the date when the first time travel occured.
So, if for instance time travel will be discovered in 2045, all time travelers(from any given time) won't be able to travel in time earlier than 2045.
Why is this? Can anyone explain this please? I know this is just theoretical but still I would like to know the concept behind it.
 A: I think it's worth expanding a bit on Hal's answer to try and make it a bit less technical. We denote a point in spacetime as $(t, x, y, z)$ i.e. both the position $x, y, z$ and the time $t$. In the absence of time machines we can only pass through a spacetime point once. Of course you can go back to the point in space $x, y, z$ but only at a later time so you can't get back to the point $(t, x, y, z)$. If it were possible to pass through $(t, x, y, z)$ go somewhere else then get back to $(t, x, y, z)$ your trajectory would form a loop, and we call this a closed timelike curve (or CTC). It's closed because it's a loop and timelike is a technical term that means you don't have to travel faster than light to go round the loop.
For any particular CTC there will be some earliest time that lies on the loop, so by going round the loop you can only get as far back in time as this earliest point. The point that Morgan Freeman is making is that for all the types of time machine we know about this earliest point corresponds to the creation of the time machine. So the statement is true for all the time machines we know about. I don't know if there is a general rule that says it must be true for all time machines, but I suspect not.
A: The basic idea is generated from solutions to general relativity that permit Closed Timelike Curves as first discovered by Kurt Godel. One thing to understand is that solutions to the Field Equations of General Relativity are metrics.  Godel discovered a metric called the Godel Metric which permitted CTCs.  If one is willing accept General Relativity as part of a final theory, one has to find a mechanism which eliminates CTCs in areas where we have experimental evidence that they don't occur (e.g. the normal experience of everyday life).  
A: The idea of a time machine that allows you to go back to when the machine was created is not apparently forbidden by the laws of physics (see Rennie's answer above). 
The technique is based around an accelerated worm-hole, which works as follows:


*

*You make a worm-hole, so it has two ends; S (stationary) and A (accelerated).

*In the year 2020, A gets accelerated to near-C, cruises for a bit, then slows back down to "rest". It goes in a curved path so that it comes back close to S.

*Assume that for S, this whole escapade took 10 years so S's clock shows 2030. Note that S has been in existence for 10 years, calmly waiting for A to come back.

*For A, because of time-dilation the trip took 1 year so A's clock reads 2021.

*Because of the way time connects through the wormhole, if you now enter A (in the year 2030), you will pop out of S in the year 2021.


So you now have a pipe connecting 2030 to 2021 and can jump back and forwards through time as much as you like.
Although this doesn't appear to violate any physical laws, it does create a few paradoxes:


*

*Causality paradox: You go back to 2021 and meet yourself, 9 years before you're due to get into the worm-hole. Future-you shoots Past-you. Future-you jumps back to 2030. What happens to all the experiences you had (in the original timeline) between 2021 and 2030? How did future-you jump into the worm-hole the first time anyway, since you had been shot in 2021? Has anyone seen Looper?

*Information paradox: You take your iPhone 11 from 2030 back to 2021. It doesn't work very well since there is no 6G network. You give it to Apple. They copy it without having to do the research that created it in the first place.

*Energy paradox: You unplug your fully-charged tritium niobate battery from your car in 2030 and bring it back to 2021. Once you hook it up, you have free electricity for a year. Where did this energy come from? Admittedly, 2030 has lost the 1.21 Gigajoules your battery contains but does conservation of energy really hold over time?


I truly have no idea where that leaves us...
