That's what most conventional scientists who have looked into this have decided.
Why that's the case was never made clear enough for me to understand.
There are also other scientists who claim we will be able to travel into the past, but only into an alternate time line / alternate universe. This could be either a very dangerous, or very pointless exercise. Going back in time and getting Microsoft stock before it exploded might not work out so well if in the alternate universe Bill Gates slips on a banana peel and falls down a flight of stairs soon after you buy the stock.
Or IBM accepts his proposed cash deal for the rights to DOS.