Doctor...whom did you say?

From the BBC:

Researchers speculate that time travel can occur within a kind of feedback loop where backwards movement is possible, but only in a way that is 'complementary' to the present.

In other words, you can pop back in time and have a look around, but you cannot do anything that will alter the present you left behind.

The new model, which uses the laws of quantum mechanics, gets rid of the famous paradox surrounding time travel.

And the article is illustrated with an obsolete British police box (heh heh).