In human history, there have been many interesting and epic feuds -- the Hatfields and McCoys, Bette Davis and Joan Crawford, or the Notorious B.I.G and Tupac. Many of us love to read in tabloids or history books about the salacious details of how the bad blood came to be.
Just like these human characters, scientific theories can also fall into disagreement, causing just as much drama in the science world.
Recently, a group of scientists claimed to have found a fatal tension between two of the scientific community's most mind-blowing theories: superstrings and dark energy. If the authors are correct, one of the two theories is in trouble.
Superstring theory is a candidate theory of everything, with the operative word being "candidate," meaning it is not yet accepted by the scientific community. It tries to explain all observed phenomena of the universe with a single principle. At its core, it predicts that the smallest building blocks of the cosmos aren't the familiar atoms and protons, neutrons, and electrons; nor are the smallest building blocks the even-smaller quarks and lepton that my colleagues and I have discovered. Instead, superstring theory suggests that the very smallest building blocks of all are tiny and vibrating "strings."
These strings can vibrate in different ways -- essentially different notes -- with each note looking like one of the known subatomic particles. Waxing slightly poetic, superstring theory explains the universe as a vast and cosmic symphony.
The other popular theory, called dark energy, is quite different. Astronomers have long known that the universe is expanding. For decades, we thought we understood that, because gravity is an attractive force, this expansion would slow over the lifetime of the universe. It was therefore a surprise when, in 1998, astronomers discovered that not only was the expansion of the universe not slowing down -- it was speeding up.
To explain this observation, astronomers added a type of energy -- called dark energy -- to Einstein's equations describing the behavior of gravity. Dark energy is an energy field that permeates the entire universe. And, because the expansion of the universe is accelerating, dark energy must exist and it must be positive. The reason we know that is simple. If the dark energy didn't exist or was negative, the expansion of the universe would be slowing down.
So, what is it about these two theories that has caused such a conflict?
In a nutshell, it's hard to make a superstring theory with positive energy and yet the accelerating expansion of the universe demands it. If one theory is completely accurate it means that a key aspect of the other is wrong. And, on the face of it, things look bad for superstring theory. This is because while dark energy is still a theory, the accelerating expansion of the universe is not. Thus, dark energy is probably true, while superstring theory still remains only a conjecture.
But there's a reason that scientists aren't rushing to media platforms to spread the news that superstring theory has been disproved.
It's because superstring theory is fiendishly complex. Aside from the prediction of subatomic vibrating strings, it also predicts that there are more dimensions of space than our familiar three. In fact, the theory predicts that there are nine in total -- 10 if you include time. You'd think that this would be a fatal flaw of the theory, but these additional dimensions are thought to be invisibly small.
Since these extra dimensions (if they exist) are smaller than our best instrumentation can detect, we don't know what their shapes are, and scientists must consider all possibilities. But there are a lot of possibilities. In fact, there are more configurations than there are atoms in a million universes just like ours. It's a crazy big number.
So, what conclusion can we draw?
With so many possible configurations, it would seem that superstring theory could predict just about anything, yet the scientists who pointed out the theories' disagreement are making the bold claim that none of these configurations result in the existence of a positive and constant energy (aka, the theory of dark energy).
And all the data recorded so far have made scientists feel relatively confident that dark energy not only exists, but is also both positive and nearly constant, making it seem likely that, if only one of these theories can be true, it's dark energy for the win. Still, it's premature to make any conclusions about the superstrings. It's possible that scientists are not right about the nature of dark energy and they are using powerful instruments like the Dark Energy Survey to refine their measurements.
The bottom line is that physicists are going to have to take this new idea seriously. It's not quite a WWE cage match, but it's going to be fun to watch these theories fight it out.