Supersymmetry And String Theory
Supersymmetry And String Theory
The laws of physics seem to be static in our universe. This means experiments carried out anywhere in the universe will give the same result as long as you take changes in the environment into account, e.g, temperature, gravitational field strength, air pressure etc...
The laws of physics also seem to be static because when you conduct an experiment does not seem to affect its results. If you perform an experiment now, and then perform it in a million years, the results will be identical if you take changes in environment into account.
These are called symmetries of nature and in our universe everywhere, space-time seems to be treated equally.
Special relativity also tells us that all inertial reference frames have the same laws of physics and so this is also a symmetry. Observations may differ between reference frames due to changes in the environment but ultimately the same laws govern all observations. General relativity extends this symmetry to include all observers through the equivalence principle where Einstein puts all observers on equal footing. In this he claims that even accelerating observers can claim to be stationary and under the influence of a gravitational field.
This shows us how important symmetry is to Physics.
Rotational symmetry also exists in the universe. The law's of physics do not change if you rotate your experiment slightly. This must be the case since the Earth is always spinning on its axis. We can also perform one experiment in the UK and the other in Australia and experience the same laws of physics.
Spin
Spin is a property all particles have. Spin is different to angular momentum in classical mechanics because it is an intrinsic property to every particle. For example an electron has a certain spin and all electrons have this exact same spin, if the electron had a different spin then it wouldn't be an electron. |
| This is the standard model and you can see that fermions (sub atomic particles) have spin 1/2 and bosons (force carriers) have spin 1. The hypothesized gravition (the force carrier for gravity) is not included in the standard model but this is predicted to have spin 2. |
The implication for spin is that it means that there is another rotation which doesn't exist in classical physics. Does this mean there are other quantum mechanical type shifts that can give light to new symmetries?
Supersymmetry
Supersymmetry is a shift in space-time with a quantum mechanical twist. It's implication is that particles must come in pairs and each pair includes particles which have spins which differ by half a unit (superpartners). No known particles are superpartners of another known particle and so supersymmetry either does not exist or the superpartners have been found yet. |
| The Supersymmetric Standard Model |
1. Aesthetics
This point is very simple but is also very good. Why would our universe only have some symettries and not others? Wouldn't it make more sense if all symmetries are incorporated?2. Issues with the standard model
The Standard Model is extremely accurate but it is not perfect in all cases. When supersymmetry is used, all the technical issues are solved.Currently with the standard model some interactions are only consistent if some numerical parameters are slightly adjusted. If supersymmetry is used however, particles occur in pairs and some cancellations occur which leads to consistent calculations. Having to adjust parameters to get the right answer seems a bit dodgy so this makes supersymmetry seem more likely
3. Unification
Why do the four forces have different strengths?
It has been shown that the electromagnetic and weak force crystallised at 1015K and above this temperature; they are actually the same force (the electroweak interaction). This is using The Standard Model and so the next step is to add the strong force. This can be done when looking at how distance affects the strength of the forces.
A strange property of the forces is that their strengths vary with distance. The strength of the electromagnetic force increases as you get closer to the source because the field becomes more visible. The strong and weak forces become weaker as you get closer to the force however because clouds of electrons and protons between you and the source actually increase the strength of the force. When you account for quantum mechanics however and look at a distance of 10-29 cm then the forces actually become approximately equal. The energy needed to probe these distances is about 1028K.
They are only approximately equal if you do not account for supersymmetry, if you use it then the forces become exactly equal.
I think it would be a little weird and unsatisfying if 3 of the 4 forces did not converge and were actually slightly out. It seems to make more sense that supersymmetry is something the universe incorporates and that the forces can be unified.
So the next sensible question is...
Where are the superpartners?
The superpartners are over 1000x heavier than protons and so with our current technology, we can not reach the energy needed to create particles that heavy. The hope in physics was that the Large Hadron Collider would be able to find some of these heavy particles but so far it has found nothing which further suggests the particles are very heavy. (The weird particles spotted at Antarctica 2 months ago may be the first superpartners we find however!)
Superstring Theory
The original string theory developed (Bosonic String Theory) had some big problems.1. To describe all forces and matter you need fermionic patterns, however bosonic string theory only had patterns of string that have spins which are integers. This means it only included bosons (force carriers).
2. One pattern in bosonic string theory had negative mass... The tachyon. Have fun trying to think about what negative mass means.
When bosonic string theory was repaired to include fermionic patterns as well however, the strings appeared to come in pairs and later physicists discovered that the theory was naturally incorporating supersymmetry.
Therefore if string theory is correct. it is very very likely that it is supersymmetric. After reading all of this you may start getting excited as it seems like we are very close to having a complete theory of everything.
The problem then comes when you try to decide which string theory to use. There are actually 5 supersymmetric string theories: Type I, Type IIA, Type IIB, Heterotic Type 0(32) and Heterotic Type E8 x E8.
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Check Out My Previous Posts On String Theory! (Link To All String Theory Posts)
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10 Dimensions In String Theory
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