According to an experiment proposed by the physicist John Wheeler in 1978 and carried out by researchers in 2007, observing a particle now can change what happened to another one – in the past.
According to the double slit experiment, if you observe which of the two slits light passes through, you force it to behave like a particle. If you don’t, and observe where it lands on a screen behind the slits, it behaves like a wave.
But, if you wait for it to pass through the slit, and then observe which way it came through, it will retroactively force it to have passed through one or the other. In other words, causality is working backwards: the present is affecting the past. Of course in the lab, this only has an effect over indescribably tiny fractions of a second.
But Wheeler suggested that light from distant stars that has bent around a gravitational well in between could be observed in the same way. This could mean that observing something now has the potential to change what happened thousands, or even millions, of years in the past.