You hear about parallel universes all the time in science fiction (see Fringe, probably a billion episodes of Star Trek, Stephen King’s Dark Tower series, etc.). But did you know that scientists take parallel universes seriously and consider them possible. There are several types of possible parallel universes. This is the fourth post in a series of five posts. The first can be found here, the second here and the third here.
Millimeters away in the multiverse
There is another type of parallel universe that Max Tegmark lumps in with the post-inflationary bubbles of Level II, but I think it deserves a bit more than the just the single-paragraph treatment he gives it. This is M-theory, often called brane theory.
M-theory derives from the current front-runner for a grand unification theory, string theory. Let’s talk about the hunt for the “grand unification theory” first. Physics has a big problem. It has two highly successful theories that describe the big and the small: Einstein’s relativity and quantum mechanics. Relativity describes how big things, like planets, stars and people, interact with each other. Quantum mechanics describes how atomic and subatomic particles interact with each other. Both theories have been proven correct time and again, but they are incompatible. So, the hunt is on for a grand unification theory, or theory of everything, to unite the two.
String theory is one attempt at this unification. There are several different flavors of it, but they all hold that all matter is made up at tiny vibrating strings when you get down to the smallest level of matter. The different frequencies at which these strings vibrate would determine what type of sub-sub-atomic particle they were. M-theory grew out of all the crazy math for string theory.
The idea of m-theory is that universes are membranes in a higher-dimensional space, just kind of floating around there. The best analogy I’ve seen is two bedsheets hanging side-by-side on laundry lines. In that analogy, each sheet would be a fully-fleshed universe. There’s a lot of neat outcomes from this theory. For example, the collision of two of these membranes could explain the weird unevenness of matter distribution in the beginning of the universe:
That is a picture of the cosmic microwave background, which represents the early universe. See how it is kind of lumpy? M-theory can account for that as a consequence of two membranes colliding. As they get close, the membranes would ripple and contact unevenly, much like the distribution of matter above.
The distance between our membrane and the next membrane might be a lot closer than you think, too. It could be only millimeters away, just in a dimension that we are unable to see, like the fourth or fifth dimension. These parallel membrane universes could vary just as much in their physics as the inflationary bubble universes do.
Now, this theory is still incomplete and needs to be taken with a big grain of salt. Further, there is no known way to test whether m-theory, let alone string theory, is correct or not. Lastly, both string theory and m-theory rely on something called supersymmetry. And the Large Hadron Collider recently showed that the most popular candidate theory for supersymmetry was wrong, which is a bit of a blow for these theories and other theories as well. They’re not dead, but they’re looking less promising by the minute.