I remember reading that the totalled velocity of anything through four dimensional spacetime equals the speed of light.
If you're at rest with regard to three dimensional space; you're travelling at lightspeed through time. If you start moving in three dimensional space, your speed through time slows down - such that your total velocity is always the speed of light. So, you can alter your speed through time, but you can't go into reverse (I think
).
And photons, which travel at the speed of light, don't 'experience' time.
Stephen Hawking 'proved' travelling back in time is impossible. He sent an invitation out asking time travellers from the future to attend a party - but no-one showed up
.