Nano-beings

Stephen Hawking has famously argued against the likelihood of time travel by pointing out the dearth of gawping tourists from the future currently clogging up our public spaces. It's a compelling argument, but somewhat lacking in imagination.

Hawking has allowed "that Quantum theory allows for time travel on the microscopic basis", and given that any beings clever enough to have figured out how to go for a ride in the fourth dimension would probably also have made great strides with nanotechnology, they should therefore have solved the 'problem' of their biological physicality and encoded themselves onto particle-sized structures capable of time travel. Even if they didn't go to the trouble of sending their own conscious selves back into the past, they could send back millions of tiny devices that could even now be watching us!