In this we will be debating whether time travel is possible or not, this is purely theoretically based. unless the pro does what scientists have failed to do and prove that it is possible with hard evidence. I believe that time travel is impossible so I am the con, one reason I believe it is impossible is because if we had we would have meet someone who has traveled back in time. Yes this is an intentionally easy to debate so that we can get a smooth start. I will have better evidence as we continue.
I believe that it is theoretically possible for mass to travel through time.
To address your first claim, there are many reasons why the current population may not have met time-travelers. The most obvious possibility is that time-travellers are encouraged to dress for the time period, and not make their time-traveller status known. It is also possible that no one in the future cares enough to visit this current time period. Another possibility is that some mass can travel through time, but it is deemed too dangerous for human life. Overall, it is not logical to claim that something is theoretically impossible because there is no interpretable evidence of it.
The most promising method of traveling through time is through the use of cosmic strings. These are faults in the flow of the universe, hypothesized to be the result of the big bang. They have not been proven to exist, but it is definitely theoretically possible. These strings could be used to harness time dilation.
Yes it is theorized that cosmic strings could allow time travel, but the theory is that it is because moving along these strings would be the same as moving at the speed of light, it would be impossible for anything with mass to do so (assuming they exist) because the string would be shredded by the force. The reason not only life cannot move at this speed is because ANYTHING with mass would be flattened by air pressure and friction among other things. If we are talking external forces allowing us to time travel then you could also say wormholes would do the same as cosmic strings. To this I would say that for among same reasons wormholes would also fail to succeed in allowing mass to travel in time, an extra reason they would fail is due to the same as black holes all mass would additionally spaghettify all mass.
When time traveling with cosmic strings, you aren't literally on the string. The most promising idea is to fly in a figure-eight around two cosmic strings, which would not shred the strings. We could theoretically manipulate the strings to be in the ideal position for this (1). It creates a time curve. The speeds needed to benefit from this time loop would not necessarily flatten mass, especially considering this is regarding the future. We will have much more advanced technology. Regardless of the dangers involved with traveling at the speed of light (And who knows, a future civilization might redefine mass in a way that makes this possible, but probably not), you don't have to travel at the speed of light to benefit from time dilation. Cosmic strings can help us achieve time dilation more efficiently than just drifting around in space, but we do not have to be close enough to travel at the speed of light.
Strings aside traveling in the future or past especially could could theoretically erase some important event causing time to loop or even stop altogether. For example say you travel in time to the future and stop an assassination with no background on what was going on the person who was supposed to have died then stops what ever is allowing time travel (string or otherwise) not knowing this you travel back to your present time (able to to this because your string is in the past) there for not affected by the destroyed string in doing this the future that you cause is erased because your traveling back along a destroyed string not yet destroyed cause a loop therefore when reaching that point in time, time resets to the point in time you traveled back to. (This is assuming with future technology we would be able to destroy cosmic strings.)
You are not going to refute my reasoning for how time travel is physically possible with strings, then? Great.
The existence of paradoxes does not disprove the possibility of time travel. Everything in your example begins with a person traveling back in time, so I fail to see how it proves your point. If that happened, it would prove my point: time travel is possible.
Also, the multiple universe theory, which is theoretically possible, would allow loops like this to be resolved. There would be a universe for every possible outcome, so there is no need to circle in paradox. One universe exists as if the person did change time. One exists as if he didn't. They are both real.