One of the fascinating aspects of the universe is that we can only see a tiny fraction of it. It is true on a big scale as we can only see as far as our telescopes will allow; and also a reality on the tiny scale. Scientists now understand that everything we see comprises individual building blocks - small particles and subatomic particles. If you magnify an object, such as an apple, you find increasingly small building blocks enough times. First, you see living cells, but if you go down far enough, you'll see atoms and then the components of an atom: electrons, neutrons, and protons. You then arrive at the building blocks of those subatomic particles; for example, a neutron is made of 3 tiny particles called quarks1. But where does it end?
Scientists encounter a considerable problem when trying to observe tiny objects. To observe an object, you need to interact with it. We usually do this through photons (light waves). However, some subatomic particles like quarks are so small that photons will just pass around them. You may ask now: if no scientist has ever seen a quark under a microscope, how do we know they (and other subatomic particles) exist?
We know they exist because of how they influence their environment. For example, scientists conducted experiments where they collided intense beams of electrons with protons and measured the scattering pattern. The resulting pattern suggested electrons must be bouncing off other particles to create such dense patterns2.
However, scientists then took it one step further by proposing that subatomic particles are made up of another building block - tiny strings of energy. These strings of energy vibrate, much like the strings on a violin, and it’s these vibrations that determine how a particle behaves.
The String theory was born out of a desire to unify our current theories of the universe, which don't align. The theory of general relativity explains the rules for how big objects (like planets or humans like us) function, but the theory doesn't work for tiny objects, which is why we have a quantum theory. It was thought that these competing theories could unite through the complex mathematics of the String Theory. The theory proposes some wacky things like there are ten dimensions3. However, the theory doesn’t explain everything we see, and that’s why scientists have not yet run around the streets shouting “eureka!”.
You may wonder - if the String Theory doesn't comprehensively explain the nature of the universe, why do we still use it? It comes down to a few reasons.
First, the mathematics in this theory does adequately explain some of the things we see in the universe, and we can use these principles to answer questions that have long puzzled us. For example, the theory could help uncover precisely how black holes work. It's not perfect, but it's closer than other theories we've had in the past.
Second, science is about experiments and trial and error. Sometimes getting a ‘no’ to your hypothesis is more valuable than getting a ‘yes’; it may offer more information to work with.
How 'no' is valuable? Let's suppose someone comes up to you and says, "I'm going to give you three numbers. These numbers follow a rule, and you have to uncover this rule. Once you think you've figured it out, you can say these three numbers back to me, and I'll tell you whether they fit the rule". The three numbers they give you are 2, 4, and 8. Instantly, you think the rule is ‘X 2’. You say "4, 8, 16", and the person says, "Yes, that fits the rule.” However, they say that ‘X 2’ is not their rule. You start saying random sequences, like "3, 6, 9", "10, 15, 56", all of which fit the rule. You become frustrated because you're only getting a ‘yes’ and getting no closer to the answer, getting a ‘no’ will tell you much more about how the rule works. So, what's the rule? The rule is all numbers have to be in ascending order.