Does it make sense to think that, if Time Travel is or ever will be possible, we would already know, because someone would have travelled back to our time or a time before ours, and it would have been documented and recounted? Or would we have dismissed it as being crazy? Or does not only Time Travel exist but the ability to wipe our memories, or perhaps the ability to be invisible or some sort of unaltering third party silent observer that goes unnoticed?