After the latest post on the delayed choice experiment I’ve done some research on latest papers that comment on the experiment.Fortunately enough I found one recent paper http://arxiv.org/pdf/1007.3977 The paper tries to explain the delayed choice experiment and the confusion it may causes. The author is not of the opinion that the choice “changes” the past and he has a very good point; Special Relativity.
As you might know according to special relativity there is no preferred system of reference in the world. For example in the past that humans believed that the Earth is flat, and is the center of the world, they were assuming that our system of reference (or in simple words; our seat in the the theater of the universe) is privileged and unique. We were assuming that someone that lives on the surface of the Earth has the “best” or more accurate view of the universe. Of course Galileo first talked about the principal of relativity but until Einstein, scientists never gave up the idea of the privileged system of reference. Einstein with the theory of special relativity started from the idea that light has the same velocity for all observers which lead to the result that there is no absolute time. Time depends on the system of reference.
Going back to the delayed choice experiment and introducing relativity, then we have trouble identifying the series of the events; for some observer the event of producing the photon and measuring it can be concurrent, this observer will not experience a “changing the past” effect. According to the author;
“The (well-known) point stated in the introduction was to distinguish correlation from causation. The lesson we draw here is that this very correlation between distant measurements does not feel their relative time ordering: it does not distinguish between future and past. This implies backwards correlation but still precludes backwards causation or any other tension with relativity, effectively demystifying the delayed choice experiments. the only place where something physical happens is the place of the measurement, and the implications on conditional probabilities hold for other measurements throughout the entire space time, present and past. “
So, according to our understanding, the measurement does not affect the past but there is a statistical correlation between the measurement and the choice¹. Correlation does not prove cause and effect relationship. Let’s understand this what one example;
There was a research that stated that families that have big libraries at home, tend to have children that succeed better in life . Someone can think that libraries are the cause for children performance, but in a second thought this is a naive; Families with big libraries are most likely to belong to middle and upper classes of society. It’s obvious that upper classes children succeed better in life. The cause for the success is not the libraries but social status!
Of course what correlation can mean in the context of quantum mechanics is not easy to understand, and the example above cannot help because it implies that there is a causation relationship (social status) but is hidden from us. This does not happen in quantum mechanics, but this is another story!
¹ Please check the delayed choice experiment.
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.