## String theory, a possible "theory of everything"

What is the true nature of everything? To answer this question, we think of ways to describe it. We test these ideas so that we know what works and what needs to be removed. But the more we learn, the more weird these stories seem. String theory is an example of this. A well-known, controversial and often misunderstood story about the true nature of nature. Why did we come up with it and is it correct at all?

To understand the reality of nature we looked at things on an ever smaller scale, which caused astonishment. We discovered worlds of small animals (now known as bacteria, etc.). Wonderful worlds in the dust, zoos full of bizarre creatures, complex protein robots. It later turned out that all this consisted of moleculestructuresThese molecules were rebuilt by even smaller things: atoms.We thought we were finally there, the smallest scale of reality. Until we let them crash into each other particle accelerators like in CERN and Fermilab.We discovered things that cannot be divided into even smaller building blocks: subatomic particles (elementary particles). But now we faced a problem, these particles are so small that we could no longer (and still cannot) see them.

Denk er maar eens over na: wat is kijken eigenlijk? Om iets te zien hebben we licht nodig. Een electromagnetic wave. This wave hits an object and is reflected in your eye. This wave carries information about the object from which it is reflected so that your brain can form an image with it. So you cannot see anything without interacting with it in some way. "Dark" is therefore nothing more than an absence of photons (light particles), so nothing is reflected in your eye.

So looking is not a problem for everyday objects, but it is for elementary particles. They are so terribly small that an electromagnetic wave is too big to hit them. Visible light therefore passes through them without any interaction.

We can try to solve this problem by creating electromagnetic waves with a much smaller one wavelength, but a smaller wavelength means more energy. So when we hit a particle with a wave that contains a lot of energy, it just bounces off the particles, so we don't see anything yet.

By looking at a particle we change it. So, we cannot measure the properties of elementary particles. This fact is so important that it has a name: The Heisenberg Uncertainty Principle.It's the foundation of quantum physics.

So, what does a particle look like? We do not know yet. If we try very hard we can see a hazy sphere of influence, but not the particles themselves. We simply know they exist. But if that's true, how can we make scientific theories about it? We did something that people are very good at: we came up with a new story, a mathematical fiction.

**The story of the point particle.**

We decided that we were going to consider a particle as a point in space, so it has coordinates. *From now on we call the vertical line of a coordinate system (think of graphs) the y-axis, the horizontal line the x-axis and the depth line the z-axis. This means that, just like in our daily life, we have 3 dimensions. So we have to give the point particles a coordinate where 3 places are known. Imagine that you are meeting with someone, then you also give a coordinate in 3 dimensions (actually 4, since you often also specify a time). *Every electron (an elementary particle) is now a point with a final electrical charge and a certain mass. In this way, physicists could define the electrons and calculate all of their interactions. This is also called quantum field theory and it solves quite a few fundamental problems. The whole standardmodel is based on it, and that same standard model can explain every experiment that has ever been done, until now.

For example, some quantum properties of the electron have been tested and are so terribly accurate that there is only a margin of 0.0000000000002% uncertainty. We are almost nowhere to be more precise than that.

However, particles are not actually points at all, but describing them as if they are we get a pretty impressive and accurate picture of the universe. Not only has theoretical science advanced as a result, but a fair number of new discoveries have been made and technologies developed that we use every day.

But there is one big problem: gravity. In quantum theory, all forces of nature are transmitted by certain particles. But according to Einstein general relativitygravity is not like the other forces in the universe. If the universe is a game, the particles are the actors, but gravity is the stage. To put it a little easier, general relativity is a theory about geometry (geometry), the geometry of spacetime self. Of distances that we have to describe with absolute precision (just think of the moon landing, etc.). But in the quantum world there is no absolute unit of measurement, you cannot determine distances with certainty. So breaks our theory of gravity on this scale, we simply don't understand how gravity behaves there yet. In short: gravity and quantum mechanics don't work together.

When physicists tried to introduce gravity as a new kind of particle, the math just didn't make sense anymore, and that's a big problem. If we could marry quantum mechanics and gravity, we would have the theory of everything. Very smart people came up with a new story. They wondered: what is more complex than a point? A line or a wire (string). And so stringtheory was born

**What makes string theory so elegant?**

String theory describes the different elementary particles as different vibrations of that string. Just as the different vibrations of a violin or guitar string give you different notes, the different vibrations of these strings deliver different particles. And most important of all, gravity could be added. String theory promised all fundamental forces of nature (strong nuclear force, electromagnetism, weak nuclear force and gravity, they are, at least, as far as we know) together in the universe in one elegant mathematical model. This naturally generated a lot of enthusiasm and hype. String theory was quickly promoted to possible theory of everything. But unfortunately, there are still problems.

Much of the math in the theory doesn't work in our universe with only 3 spatial dimensions (x, y, z) and 1 time dimension (yes, as far as we know now we live in a 4th dimensional universe). String theory needs 10 dimensions to work (9 spatial and 1 time dimension). So theoretical physicists started making calculations in mathematical model universes and then trying to eliminate the other 6 dimensions and thus describe our own universe. But so far no one has succeeded and no prediction of string theory has been proven in an experiment. So, string theory has not yet revealed the secrets of the universe to us. You could now say that string theory is completely meaningless. But science is about experimentation and predictions. Physics is based on math, 2 + 2 = 4. This is true no matter how you get used to it. The math in string theory is correct, which is why string theory is still very, very useful. With string theory we can still try some questions about it quantum gravity that have occupied us for decades, such as: how do black holes work and the information paradox. String theory can point us in the right direction. And as long as theoretical scientists look at it that way, it is an important weapon to discover the true nature of the universe. It can help them unravel new aspects of the quantum world and also lead to wonderful math. Perhaps string theory is not the theory of everything, but like the point particle story, it can be an extremely useful story for learning more about our universe.

*For now we don't know the true reality of our universe, but that's okay. We continue to come up with stories and ideas so that one day we will know.*