What is reality? What is the standard model trying to tell us? Are we living in the 11 dimensions of string theory?
The holy grail of physics is a theory of everything. It may show the origin of all forces coming from one master force, and the origin of all particles coming from one super particle. It would be the ultimate answer to every question that asks why something is the way it is.
We don’t have a theory of everything, but we do have something that comes close. This is called the standard model of particle physics. This model explains the subatomic makeup of all the atoms, explains chemistry, magnetism, nuclear physics, and many other things.
But the Standard model does not predict absolutely everything. There are 19 parameters that it does not predict. These parameters have to be calculated through measurement and put in as constants within the model.
Masses of six quarks and three of the leptons
Mass of Higgs boson and strength of the Higgs field
Mean lifetime of How 4 of the quarks decay (down, strange, charm, bottom)
Strength of electromagnetic field, strong and weak forces
A number describing the strong force interaction between quarks and gluons
There is nothing that predicts these. It is just the way our universe happens to be. There may be an underlying theory that shows that these constants could not be anything other than the numbers we see, but that doesn’t seem to be the case.
What would happen if these constants were some other number? Our universe would be anywhere from being slightly different to being completely different. This is the crux of the fine tuning argument, that these constants appear to be such that life in our particular universe can exist.
If these constants are arbitrary, then it may lend credence to the multiverse theory of reality. In other words, there could be universes where every variation of these numbers has happened.
A version of the most studied theory of everything, and the one that has the greatest number of physicists working on it, predicts such a vast number of multiverses. This is M-theory which is a theory that unites all the consistent versions of String Theory. M-theory brought all of the string theories together. It did this by asserting that strings are really one-dimensional slices of a two-dimensional membrane vibrating in 11 dimensional spacetime. Some estimates have shown that the number of multiverses predicted by the theory is 10^500 power.
This theory requires a universe with up to 11 dimensions, because the strings have to be able to vibrate in this number of dimensions in order for the theory to work mathematically. 11 dimensional mathematics is so complex that the theory has been derided by many because it’s mathematics can be manipulated to represent almost any imaginable universe. But, what if it’s true? Maybe almost any imaginable universe actually exists.
But where are the other dimensions? We only see 3 spatial dimensions, and 1 time dimension.
String theorists say that these dimensions may be compactified on a very small scale.
For example if you go down to the scales that are closer to the scale of a plank length, extra dimensions may be complex shapes, like a Calabi-Yau manifold. This is a mathematical shape that can contain all of the 6 missing dimensions. The way strings vibrate determines the particles that they represent. In string theory, the strings and their vibrations are affected by the geometry in which those strings are moving.
The issue is that the shape of those extra dimensions is not known, but if it was known, then the mathematics of String theory could tell you exactly what the vibrations would be. And this would give us a theoretical basis for the free parameters.
Can we test for these other dimensions? It turns out we can, using the Large Hadron Collider.
If after a collision of particles at extremely high energy, scientists detect less energy than was present in the two particles before the collision, then this would be an indication that the extra energy may have been transferred to these extra dimensions.
But string theory may still imply that there could be other universes where the shapes of those extra dimensions are different than what we observe. So why does our universe happen to be the way that it is? There doesn’t seem to be any reason for that.
An intriguing testable possibility is that we are part of a large multiverse. Perhaps one of a multitude of membranes of the M-theory.
It may be that we just happen to find ourselves in that one universe out of innumerable other ones where the shapes of our compactified dimensions happen to be such that life is possible. There is no other membrane where we would find ourselves.
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