String Theory
String Theory
It does this by changing our view on the elementary particles. The Standard Model of particle physics describes the elementary particles as point-like but string theory changes this to 1D bands/strings which vibrate. The strings are Planck length long, which is why they appear point-like to use when we look at them with our most powerful equipment.
String Vibrations
We know that E = γmc2 and string theory tells us that the mass of particles is determined by the energy of the vibrations of the string. This means the energy in the vibration of a string also determines a particle's response to the gravitational force because mass determines the response of a particle to gravity. This is the same for the electromagnetic, weak and strong forces. The response of particles to these are determined by the way a string vibrates.
The standard model has the problem that we have to input numbers for elementary particles and from these we can make our predictions. It doesn't actually explain why these numbers are used. For example WHY does an electron weigh 9.1 x 10-31kg? Why do protons have positive charge? These questions go on and on when using the Standard Model. In string theory only one number is needed for the theory and this is the Planck tension. This was found to be inversely proportional to the strength of a graviton (the hypothesized force carrier for the gravitational force). The Planck tension is 1039 tons and so this is why the gravitational force is so tiny on quantum scales. The very high tension also explains why the strings are so tiny as it forces them to contract to 1.6 x 10-35m.
The very high tension also means the energies of the vibration are very high. Quantum theory also tells us that we can't just have a very very very gentle vibration because vibrations come in discrete units. The minimal energy of a vibration is proportional to the tension and the total energy is proportional to the number of peaks and troughs in the strings vibration. This is the same way the minimum energy of light is proportional to the frequency and then the amplitude determines the total energy.
The total energy is proportional to the tension in the string. The tension is huge and so the energy is huge. E = mc2 and so the masses of elementary particles should be huge. The smallest masses should be at least the mass of a grain of dust (Planck mass). So why do we have many things lighter than a grain of dust?
The uncertainty principle saves the day again and tells us that string vibrations are not certain and some energy cancellations can occur between string vibrations. This leads to energies being reduced to very low and allow us to have masses much smaller than the Planck mass.
Another consequence of using strings to describe the universe is that you can have infinite vibrational patterns because you can increase the number of peaks and troughs on a vibration forever. This means there should be infinite elementary particles of increasing masses and this definitely contradicts the standard model. Well this is true and the reason why we know nothing about these particles is that the energy needed is much greater than what we can get in a lab. With current technology we can't even reach a millionth of a billionth of Planck energy. In theory if we were to reach very high energies, we would see new particles not currently included in the standard model.
Quantum Foam
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| Quantum Foam |
It's all good having a theory where everything is made up of strings but how does this actually solve the quantum foam problem?
The de Broglie wavelength equation tells us that the wavelength of a particle is h/p. The momentum of a particle is also approximately its energy. This means as we increase the momentum of an object its wavelength decreases and so we can probe shorter distances more accurately. This is the same as using high momentum light to view objects more precisely. But because the Planck length is the length of a string, strings can't probe anything less than a strings length. It is impossible to probe lengths below the size of your probe. We saw this when we discussed the uncertainty principle.
Increasing the energy of a string therefore has a limit to how accurately you can probe the universe. Eventually the energy stops increasing the resolution of your probe and instead the energy causes the string to grow. This means you can't probe the universe below the Planck length, The big problem in Physics comes on scales smaller than the Planck length and since we can't probe things on this scale, things on this scale do not exist.
If strings can't probe the things below the Planck length, then it or anything made from the strings cannot be affected by the "quantum foam". You can think of this as when we touch smooth objects, microscopically it is very grainy and discrete but because our hands are big we do not feel the graininess. Like our fingers, strings smear out the quantum foam.
It is impossible to expose the universe to the frenzy and so we can say that it does not exist. A positivist says something exists if in theory it can be probed and measured.
The reason why we had problems with quantum physics and general relativity is because we saw matter and force particles as point-like and so there was no limit to how accurately we could probe the universe. This means we were forced to consider arbitrarily short distances.
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