String theory has been around since the 1970s, but it was only recently that physicists began to consider its merits seriously. Why does string theory persist despite the knotty physics?
What Is String Theory?
String theory is a bit of an oddball in theoretical physics. It’s not just that it’s hard to understand; it’s also that there are many different versions of it, and they don’t all agree with each other.
But string theorists have persisted in their efforts to make sense out of this mess for more than 30 years now, and they’ve come up with some pretty interesting ideas along the way.
The basic idea behind string theory is simple enough: In quantum mechanics, particles can be thought of as tiny vibrating strings.
When two or more such strings collide, they can form new kinds of particles, like mesons or quarks.
And when you add gravity into the mix, things get even weirder. Gravity makes space-time itself vibrate, which means that particles can move through space-time instead of being stuck on fixed points.
But string theory isn’t just about what happens on a very small scale. It’s also about what happens at the big scale—the scale of galaxies and stars.
At these scales, gravity gets stronger and stronger, so if you want to describe particle interactions using Einstein’s general relativity, you need to include higher-order terms in your equations.
These higher-order terms are called “curvature corrections.” They’re complicated, though, and nobody knows how to calculate them exactly. So string theorists have had to resort to approximations.
But string theory doesn’t just give us approximate answers. It gives exact answers! For example, one version of string theory predicts that our universe should contain 10 dimensions, including time.
This dimension is curled up onto itself, forming a kind of giant cosmic string. That’s why we call it “string theory.” But string theory predicts lots of other weird things, too.
Like, for instance, black holes should emit radiation that looks like Hawking radiation. Or that the number of fundamental particles in the universe should be equal to the number of bits of information stored in the universe.
And yet, despite all of this, string theory continues to attract attention from both physicists and philosophers.
Why Do People Keep Coming Back To It?
There are several reasons. First, string theory is incredibly elegant. You might think that the math would be complicated, but it’s surprisingly easy. Second, string theory seems to work well in practice.
If you take a look at the Standard Model of particle physics, you’ll notice that it contains a lot of free parameters.
Those are numbers that are supposed to be adjusted by experimenters, but they haven’t been adjusted yet. And third, string theory explains a lot of phenomena that other theories can’t explain.
For example, string theory predicts that there should be extra spatial dimensions beyond those we can see. It also predicts that there should be additional types of particles that we can’t detect directly.
And finally, string theory predicts that the laws of nature should change depending on whether you’re inside or outside a black hole.
But the most important reason string theory has survived is that it’s still possible to make progress even if you don’t know everything about it.
There are plenty of open questions, but string theorists have found ways to address them. One approach is to use mathematical techniques called dualities.
Duality is a fancy word for symmetry. It means that certain properties of a system are related to other properties of the same system.
For example, when you put together two magnets with opposite magnetic fields, then the magnet will repel each other.
But if you reverse the direction of the magnets, then the magnets will attract each other instead.
In some cases, duality relates to different systems that seem completely unrelated. For example, the strong force
(which holds protons and neutrons together) and the weak force (which causes radioactive decay) are related by an extremely powerful duality.
So far, string theory has only explained a few of the basic forces of nature. But when string theorists started thinking about more complicated objects than pointlike particles, they realized that their theory could predict new kinds of symmetries.
One such symmetry was discovered by Michael Duff and Nathan Seiberg in the 1980s. They showed that string theory had another type of duality/symmetry:
it predicted that strings and branes were equivalent. In other words, they said that a string and a brane were the same things.
This discovery led to a revolution in string theory. Suddenly, physicists could start asking questions about what happens when you combine multiple branes into bigger branes.
And they could ask these questions without knowing anything about how many dimensions there are in space.
So now, string theory predicts that our world has six dimensions of space, plus one dimension of time.
These dimensions include three familiar ones: length, width, and height, and three others: depth, color, and charge.
But string theory also predicts that there are five additional hidden dimensions that we can’t see.
These extra dimensions are curled up very tightly, so they’re invisible. But string theory says that they exist nonetheless.
The existence of extra dimensions is exciting because it opens up a whole new way to understand the universe.
We already know that gravity works differently in higher-dimensional spaces than in lower-dimensional spaces.
This makes sense since gravity is just the curvature of spacetime. Curvature depends on the number of dimensions.
String Theory And M-Theory
And string theory shows us why this must be true. According to string theory, all of the fundamental interactions
of the standard model—including gravity—are manifestations of a single underlying interaction. That interaction is known as M-theory.
M-theory is a ten-dimensional theory, which means that it involves ten spatial dimensions. But string theory predicts that our universe is four-dimensional, so it would seem like M-theory should involve only nine dimensions.
But M-theory turns out to be much richer than that. It contains not just nine dimensions but eleven. Eleven is the number of dimensions needed to describe the full range of fundamental interactions.
M-theory also includes a fifth dimension of time. So it’s a ten-dimensional object. But string theory tells us that time is special.
Time doesn’t affect the physics of ordinary matter. If you take a particle traveling through space at high speed, then its mass stays constant.
But if you add a little bit of energy to this particle, then its mass will increase. This change isn’t due to any physical effect called “time dilation” (although it does have something to do with relativity).
Instead, it comes from an entirely different kind of effect: the fact that the particle is moving around in space.
So despite its quirks, string theory still provides a very solid explanation for many different theories. Many scientists continue to vouch for its validity and research into the theory.
It’s certainly taught us a lot. Since its discovery, it’s led to a lot more exciting hypotheses and has furthered our dimensional understanding. Only time will tell how much more String theory can explain about our world.
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