Too Small to See
One of the great puzzles in modern physics is that it has two theories that explain almost everything to fabulous precision. Those are General Relativity and Quantum Mechanics of course.
And those aren't 'theories' in the sense of idle speculation. Our whole electronic technology depends on the knowledge of materials provided by QM.
Consider GPS that's embedded in our phones and tablets and cars. That system uses both QM and GR to function - it uses them in parallel. They both work wonderfully. Thing is, they are incompatible. GR assumes that spacetime is smooth. QM assumes that spacetime is not smooth at the microscopic level.
This a bit of a philosophic scandal for physics. Physicists expect that since there is only one universe there must be a single explanation of it - not two complementary ones.
There is a more scientific reason why it's a problem too. There are border areas where to push further in understanding you actually have to apply both at once. If in normal affairs we can just think of GR when we think on our scale of existence, and QM at the microscale. But with a black hole you have something that certainly involves gravity, but it's so small that QM has to be taken into account too. And when that's attempted the answers that emerge are literally meaningless because the math won't deal with points.
String theory attempts solve this by being very tiny, but by having some extension. The incompatibility of QM and GR is associated with the idea that dimensionless points are a sensible concept. String theory says not. Small as they are, if strings are what reality is made of then the fact that they have a size at all, even a very tiny one, means that when you put QM and GT together you don't get a meaningless answer
Long story short - by fits and starts String Theory has emerged in the last 40 years that seems to be up to the task of merging QM and GR.
String Theory is a really interesting beast.
It is actually 5 complementary theories. They are all talking about exactly the same thing but from different perspectives.
When one of the theories leads to an absurd result just jump to another and keep going.
That's enough to make the pragmatist in me cheer:-)
But there is a fly in the ointment. String Theory is so complicated that nobody has ever used it to make a single prediction about reality.
And the strings are so small that they will never be seen.
This raises hoots of derision in some quarters.
This isn't science!!!
It's bald speculation!!!
One can only smile at how naive the hooters are.
Haven't they ever encountered Kant and his idea that we can never know things in themselves; we only encounter their appearances.
By the time we'd figured out how to handle that without going crazy we were into the time of evolution and QM and GR. There is very strong evidence for all of those but that evidence is embedded in long chains of inference.
The situation is that unseeable things can be detected because their existence is the only way to account for what we CAN see. In university I did labs where I'd do things like measure the mass of an electron or what the force of gravity was. The gravity lab was interesting. We caused a ball bearing to roll down an inclined sheet of glass over carbon paper and a sheet of graph paper leaving a nice curve. From that curve I could calculate the force of gravity to satisfying precision. But it was all completely abstract - I made tables of numbers and then did sequences of calculations that gave me an answer. The role of direct observation is crucial in science but it's role is that of grounding chains of inference.
This is very unsatisfying to those whose style of argument is to say 'Show me' but it's also a fact of life. So why should strings be any different? String Theory is sort of a theory of everything. It is already consistent with all we know and that's a very hard thing to accomplish.
But is String Theory the only way of reconciling QM and GR?
Turns out not. There is another approach called Quantum Loop Gravity which views reality as a bunch of cellular automatons.
It's a very different conception but the result is the same; an explanation of what fundamental reality is like that is consistent with everything we know.
So imagine this; we end up with two theories of everything, that each work fine as far as we know. But how would we choose between them? How do you choose between things that in principle you can't see?
What do you think?