• Question: Parallel Universe and being at 2 places at onece seems science fiction and "star wars" stuff. Are there any evidences behind these claims?

    Asked by usman100 to Chris, Dave, David, Fiona, Jack on 22 Jun 2013.
    • Photo: David Freeborn

      David Freeborn answered on 22 Jun 2013:

      Hi usman100,

      You’re right, these claims do sound a lot like science fiction. Actually, I think that’s a big problem with how physics is communicated to the general public at the moment. There’s a lot of focus on science-fiction sounding claims like this, and I think sometimes this obscures the actual real physics that lies behind them.

      There’s almost no evidence for parallel universes right now. At the LHC we are looking for traces of extra dimensions: we expect the fields of some particles to be slightly weaker because part of the field will be lost to these extra dimensions. If we discover that it at least indicates other Universes could exist *in principleMATOMO_URL And there are clues from Cosmology that other Universes might have “bruised” our own Universe, but it’s very speculative right now (http://io9.com/5714803/does-our-universe-show-bruises-where-it-collided-with-other-universes).

      The ideas that there might be other Universes comes from a number of physical theories, the main ones being String Theory and the Many Worlds interpretation of Quantum Mechanics.

      But the real principles of String Theory don’t really suggest the parallel Universes would be quite like science fiction suggests. String Theory suggests there are extra dimensions of the Universe, and these dimensions would probably contain billions and billions of other Universes, possibly with slightly different laws of physics. Just because there are so many Universes, a small number would be almost the same as our Universe, just by chance, a “parallel universe” of sorts.

      But almost every one of the billions of Universes would probably be “dead” Universes, that collapse back in on themselves almost instantly, or expand out so quickly that they are almost empty. That’s because the conditions of a Universe that allow stars and galaxies and any complex structures to form are very, very specific. If the Cosmological Constant of our Universe were even 0.1% different, the Universe would not have any stars, galaxies or life at all.

    • Photo: Chris Mansell

      Chris Mansell answered on 23 Jun 2013:

      There is some evidence for things being in two places at once. It is not direct evidence (i.e. no-one has ever seen something in two places at once). The indirect evidence comes from a great number of experiments that we only know how to successfully describe in one way. This way of describing (or modelling) the experiments was developed in the 1920s and is known as quantum mechanics. We say that the description is successful if we can use it to work out how the outcomes of the experiment will change when we make changes to the experiment. In the following, I will explain how the experiments that we only know how to describe with quantum mechanics give us indirect evidence for things being able to be in two places at once.

      Like stories, experiments have a beginning, middle and end. The beginning is when we set up the experiment and then press go. The end of the experiment is when we make some measurements. We don’t really know what happens in the middle of the experiment. The only way to tell what happened in the middle is to take a look at our best description (or theory). This description usually contains lots of maths. This is normally fine but when it comes to quantum mechanics, things get a bit hard to believe.

      Let me give an example. An experiment can begin with firing a series of particles, one at a time, at a wall with two holes in it. The end of the experiment involves measuring where the particles have hit a detector on the other side of the screen. The positions where the particles have hit the detector can only successfully be described by quantum mechanics. When we take a look at what quantum mechanics tells us about the middle of the experiment, we see some maths that seems to tell us that each particle went through both holes at once. (I can say more about this maths if you would like. Just ask if you think it would be helpful.)

      Physicists find this by strange. They have double-checked the experiments and the mathematical equations so many times that they feel confident that they equations are correct. The thing they worry about is whether they have “read” (or interpreted) the meaning of their equations correctly. This is where physicists start disagreeing. They write articles about interpretations of quantum mechanics. They have conferences about the foundations of quantum mechanics. They think it is so weird that it needs further investigation.

      It’s a difficult subject and after almost 100 years, it still feels odd. Some physicists are putting the ability of particles to do several things at once (such as be in several places at once) to good use. They are trying to build “quantum computers” that – in a very loose way of speaking – have tremendous parallel processing ability.

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