“Travelling to the past is difficult, but travelling to the future is easy,” writes the physicist Carlo Rovelli. This might seem obvious in that we are all travelling to the future all the time, although on reflection we are stuck in the present and never reach the future. But Rovelli is referring to it being easy to travel through centuries. This is the recipe for doing so.
First, find a black hole. That’s not difficult because there’s one in every galaxy. But be careful because “what enters a black hole does not come out again…For a black hole, the past is the outside; the future is the inside.”
Second, you need a very powerful rocket, much more powerful than any rocket that currently exists, suggesting that perhaps it’s not so easy to travel through centuries.
Third, once you have your rocket you drive it to the horizon of the black hole: the point where it is still possible to resist being pulled into the hole. The rocket will have to resist a very powerful gravitational pull, but if you can stay there for an hour when you pull away you will travel through centuries in minutes.
The explanation is simple–the stronger the gravitational pull the slower time passes. As Einstein explained, the twin who lives at the top of a mountain will age more than the twin who lives at the bottom of the mountain.
- Rovelli declares earlier in his book that “time does not exist.” What he means is that it does not exist in the way that we tend to think of it–that no matter where we are in the universe we are all immersed in the same time. Rather time is “a localised phenomenon: every object in the universe has its own time running, at a pace determined by the local gravitational field.”
- Rovelli says that travelling to the past is “difficult.” He doesn’t say “impossible”; not I think because he knows how to travel to the past but because the story of physics is of the impossible becoming possible.