String Theory in a Nutshell
String theory is a very famous theory which attempts to describe the nature of the universe. Despite its popularity, very few really know what it’s about, and even fewer truly understand it. I definitely don’t understand it, but I thought it would be fun to break it down and at least explain what the theory is actually saying.
Struggling to see?
In an attempt to understand the universe, throughout history, humans have always tried to look at the smallest scales that their technology has allowed. There have been lots of different theories along the way, with those like Aristotle believing matter could be cut up an infinite amount of times. But as we probed to smaller and smaller scales, we eventually got to the level of atoms. For a while, we believed that atoms were indivisible, and that they were the smallest possible piece of matter (the word atom is literally Greek for “uncuttable”). But we were wrong, as we eventually found even smaller particles which makes up these atoms. Today, we believe those subatomic particles (e.g. quarks, electrons) are the fundamental particles of the universe and the smallest form of matter.
To observe an object, photons of light travel to the object, reflect off of it, and travel back to our eyes/detecting device. But the problem with these tiny particles is that they are so small, we can’t exactly see them. The energy that photons carry when we try to look at them is large enough in comparison that it affects them. In other words, we can’t observe the system without altering it. This is the principle idea behind Heisenberg’s Uncertainty Principle, which states that we cannot measure both the position and momentum of a particle with absolute uncertainty (which is the foundation of quantum theory). But where am I going with this?
Well, what do you imagine when I say the word “particle”? Many people think of a small dot like shape, like a tiny sphere. But for the reasons I’ve just explained, no one actually knows what they look like! Instead, we effectively see a “blur” of possible positions and velocities. So, the appearance is not exact, but rather it is a distribution of probabilities. In fact, experiments show that observing an electron as a point particle allows measurements of its quantum properties that are accurate up to 0.0000000000002%! Given the macroscopic scale of the universe, we can get a very good picture using this point-like approach. However, it is clearly not perfect, and on the quantum level that makes a big difference. That’s where string theory comes in.
Gravity on a small scale
As you may be aware, there are four fundamental forces: electromagnetic, weak nuclear, strong nuclear, and gravity. All but gravity have been explained and theorised on both the macroscopic and microscopic scales. Furthermore, each force is carried by an exchange particle called a boson, and again, gravity is the only one whose boson we have not found. We currently just called it the theoretical “graviton”.
This problem stems from the fact that gravity is not really a force like the others. Einstein’s Theory of General Relativity showed us that gravity is the effect of the curvature of spacetime as a result of mass/energy being placed inside it. And so, it is a theory of geometry as much as it is a theory of a force. General Relativity describes the curving of space with absolute certainty, which appears to be fine on large scales (e.g. planets) but as we now know from quantum mechanics, nothing has absolute certainty, and this matters on the small scale. Furthermore, when the idea of a graviton was introduced to quantum theory, it led to infinities that could not be physically possible — the maths was wrong. Determined to solve this problem, and produce a theory that unites gravity with the standard model (“the theory of everything”), theoretical physicists decided that perhaps the issue is with the point-like idea of particles (in this case the graviton).
Strings
Instead of point-like particles, theorists proposed the idea that all matter is made up of tiny, vibrating strings. The idea is that a string of a certain length which oscillates at a particular frequency will give rise to the appearance of a photon, while another one of a different length and frequency plays the role of a quark, and so on. One-dimensional objects also solved the infinities problem previously mentioned. Thus, string theory quickly became a candidate for the theory of everything.
However, there are many reasons why it has not been accepted. The main one is that it requires more dimensions than we see in the physical world. You’re likely aware of the three spatial dimensions we experience in day-to-day life, but there is another dimension: time, which makes a total of four. String theory, on the other hand, requires ten. Yes, ten. There are many arguments as to why this may in fact be acceptable, with the main one being that the extra six are wrapped up together on an extremely small scale that we cannot see. Think of a cup with a straw in it. From a distance, you can only see the straw as a straight line, one dimension. As you get closer you start to see its width, a second dimension. And when you are very close, you can see its depth, the third dimension. It’s this sense of scale that some believe supports the physical possibility of string theory.
However, the theory has yet to be proven or shown experimentally, and so it can be viewed as nothing more than an idea. But that does not mean it isn’t useful. It still solves many problems that previously prevented a theory of everything, and so perhaps exploring the idea of strings may lead us to discover new ideas, which do have the capability to paint an exact picture of the universe, and truly give us a theory of everything.
Originally published at http://thephysicsfootprint.wordpress.com on January 13, 2022.
