As Stephen Hawking points out, the best proof that time travel isn't possible is that the present hasn't been overrun by tour groups from the future.
"As Stephen Hawking points out, the best proof that time travel isn't possible is that the present hasn't been overrun by tour groups from the future."
How does he know they aren't disguised as Japanese tourists?
You and Stephen are time travelers ... from present to future. Referring to the reverse is not necessarily time travel but time reversal, which would not be sensed by those moving from present to future. In short, Stephen isn't always right in his speculations.