Quantum mechanics, when used in the context of time travel, has a so-called many-universe interpretation. This was
first proposed by Hugh Everett in 1957. Everett says that our reality is only one of many equally valid universes. There is a collection of universes, called a multiverse. He proposed that the universes we see are just one of an infinite number of universes, existing side by side. Each of these universes are constantly splitting, so there is a univese in which Kennedy died and one in which he lived. On in which you wash your car and one in which you don't. Every multiverse has copies of every person, structure, and atom. For every possible event, every possible outcome is said to be played out on a different universe. This interpretation of
quantum mechanics is quite controversial however, but does elicit the notion that it may be impossible to travel
backward in time to our own universe or dimension.

Consider the following patterns:



These are a result of light passing through a wall with (from top to bottom) one slit, two slits, and four slits. It is easy to explain these patterns by giving light the property of a wave. When passing through multiple slits, light waves interfere with each other to add to or cancel each other. The waves produce patterns of alternating light and dark. Einstein found that light also has the property of particles. Since light contains billions of particle, or photons, it is not hard to imagine all of these photons flying through the slits and interfering with one another just like the waves did.

But, we can eliminate the interaction of the photons with each other by sending one photon at a time through the slits. This has been done experimentally by making a beam of light so weak that only one photon at a time escaped. Putting very sensitive detectors behind the wall with slits enables us to detect where the photons hit.

After a few hours, the pattern to the left is formed.

What is found is that individual photons hit only in certain places, and never others. So, what is interfering with the photons? According to Hugh Everett, reality is much greater than just what we see in our universe. Interference is happening, but we cannot see any cause for it in our universe. Therefore, the interfering photons must be in other universes. This would mean that other universes exist and are capable of interfering with our own. This group of many universes is called a "multiverse."

The nature of the multiverse is that it is constantly splitting, which means that there are many universes very similar to ours. It is the similar ones that interact. Each time a beam of light from our universe is made, beams of light are similarly made in other universes. The photons from other universes interfere with the photons in our universe and produce the pattern that we see.

If other universes are interacting with ours, we should be able to interect act with them. Unfortunately, we have only observed interactions between universes on the subatomic level. There is a big difference between sending photons through the multiverse and sending people. To use the multivese theory for time travel we must develope ways to send people into these "other worlds" and be able to control where we send them. This task does not seem likely to happen within the next hundred years.