Quantum Mechanics May Solve Time Travel Paradox

Since people don’t suddenly disappear because a changing of the past has prevented their births, either time travel is impossible, or something prevents any movement in the past from changing the present. According to a new quantum model, time travel can occur within a type of feedback loop that only allows backward movement in a way that is complementary to the present. Therefore, you can travel back in time as an observer, but you cannot do anything to change the present you left behind.

According to Einstein’s general theory of relativity, space-time can curve back on itself, theoretically allowing time travelers to meet younger versions of themselves. However, a team of physicists from Austria and the United States now says that, according to the laws of quantum mechanics, this can only happen if there are physical constraints which protect the present from changes in the past.

Probabilities govern quantum behavior. Before something is observed, there are a number of different possibilities about its state. However, once its state is measured, there is only one possibility. This means that once you know the present, you cannot change the present. If you know that your grandfather is alive today, there is no possibility that you killed him in the past.

According to Professor Dan Greenberger, of the City University of New York, quantum mechanics distinguishes between what might happen and what did happen. If you aren’t 100 percent sure a former schoolteacher is alive now; if there is, for example, only a 90 percent chance that he is still alive, then there is a possibility of you going back in time and killing him. However, if you know with 100 percent certainty that he is alive now, you cannot kill him in the past. Therefore, since you know that your grandfather was alive at the time he conceived your parent, you cannot go back in time and kill him before that happened.