Past, is something that we are used to, but do we really understand it? What is the past? Can we go there? Is it a place that sometime we will find the way to visit (time machines)? We have memory of the past and sometimes after a vivid memory experience we say “Seems like it happened yesterday”. And depending on the memory, sometimes we want to forget the past (like 9/11) and other times we want to bring it back.
But what science says about the past?
Time is something a foundamental problem of Physics. There are two theories in physics, one that describes the very large like galaxies (relativity) and one that describes the very little like electrons (quantum mechanics). These two theories cannot be combined together, they don’t agree, and this is a headache for the Physicists; for more than 70 years they try to combine the two theories in a one with no success. Lately some say that the problem is because we do not understand the notion of time. Some others went one step further claiming that time does not exist and with it the past does not exist either!
The histories of the universe,depend on what is being measured, contrary to the usual idea that the universe has an objective observer-independent history.
But you can say that all these are just theories. Not exactly; The latest years we have experiments that give results that are very strange;
delayed choice experiment
Quantum theory is known to predict phenomena that are very difficult to understand with our “classical” way of experiencing the world around us. According to quantum theory there is an innate strange characteristic of matter; it is has a nature of both a wave and a particle, but we can experience only one. This is demonstrated with the famous “double slit experiment”¹. John Archibald Wheeler after studying the double slit experiment he proposed a thought experiment were he demonstrated that the choice of an observer can change the outcome of an event already happened! (If you have the time you can read about the thought experiment here). We had to wait for 30 years until a real experiment to be possible, that showed that Wheeler was right (read about the experiment here)
In short, the experiment says that on the observation of an event we can affect choices made even billion years ago; Let’s assume that there is a photon (the particle that light consists of) that has started a journey billion years ago from a very distant star. The photon reaches finally here in in Earth, through one of a vast choice of possible paths. We do not know the path but we can define it by choosing specific instrument of measurement; by choosing whether to use the two-telescope set up or not, which are the instruments that would give us the information about which side of the universe the photon passed.
The outcome of the experiment is very hard to understand; one could say that by choosing the type of the measurement we decide about a an event that happened billions of years ago (the path that the photon had chosen), and thus we change (or determine) the past! So depending on what questions we set (type of measurement) we decide about the answers that we get back, despite if the question concerns events in the past.
This is one interpretation, but there are other also and they depend on the interpretation of quantum mechanics. Another interpretation could be that we do not change (or determine) the past but we choose a version of infinite possible ones; According to what is called the “Many-worlds interpretation” we do not change the past but we choose one universe of and infinite possibility of universes that matches out measurement’s outcome.
Either changing the past is possible, or choosing the correct universe is possible, there is one important factor for both interpretations; choice. 9 years after 9/11 let’s wish that no one in the future will choose to do something similar.
¹ We can find a lot of links for the description of the experiment but I would recommend a search in you-tube for Dr Quantum and double slit.