Four forces control the entire universe? This is a pretty astonishing claim. In fact, putting aside microscopic processes happening inside atoms, everything we see can be accounted for in terms of particles interacting through just gravity and electromagnetism. From the orbits of the planets to the flexing of your muscles, every movement in the macroscopic world arises from the interplay of these two aspects of nature.
At least, that’s the current picture. If that’s not right, all bets are off. How can we be so sure there aren’t other forces that we just haven’t yet been clever enough to find?
The answer is, we can look for them. We know where to look, and indeed we have looked. Other forces are not out there, at least not to any significant extent. Any new force we might someday discover must be so impotent over everyday distances that there’s no way it can affect the macroscopic world. If it could, we would already have found it. And yet a few researchers continue the search, since even an extremely feeble new force would be of enormous theoretical importance.
If you want to invent a new force of nature, you have to specify three things: which particles feel the force, how strong it is, and the range over which it interacts. Once you’ve fixed these properties, you know everything important about your hypothetical force, and you can set about tracking it down. For example, gravity impacts absolutely everything, and its range is infinite. Gravity grows weaker as you move away from a planet or star, but it never fades completely. Gravity is actually a very weak force compared with the others, but because it interacts with everything, it builds up when you have a really massive object. That’s why gravity is the most important force over astronomical distances.
Electromagnetism also has an infinite range, and it is much stronger than gravity. But it acts only on electrically charged particles; neutral particles like neutrons or neutrinos (the names aren’t accidental) are unaffected. What’s more, electrical charges can be positive (like a proton’s) or negative (like an electron’s). Like charges repel each other, while opposite charges attract. Even though electromagnetism is stronger than gravity, it is less important to stars and galaxies because they are made of equal numbers of positive and negative charges, leaving us with a net force of zero. On the extremely small scale, however, the push and pull of electrons and protons is what controls chemical reactions, including every compound and every process in your body.
The nuclear forces are short-range only, so we can ignore them in the macroscopic world. Most physicists expect that there are lots of other undiscovered short-range forces out there; we’re looking for them at particle accelerators. But for everyday-life purposes, what we care about are long-range forces.
Theorists like me have been proposing ideas for new long-range forces, and experimentalists have been looking for them, for quite a while now. Our favorite experimental tool toward that end is a deceptively simple device called a torsion balance: two objects of differing composition at opposite ends of a rod suspended by a wire. Any force that acts differently on the two objects will twist the wire. Torsion balances have a noble history. They were used starting in the 1880s by Hungarian physicist Loránd Eötvös to show that gravity acted equally on objects made of different materials—in other words, that there was no evidence for any new long-range forces.
After Eötvös, many people assumed the question was settled. That changed in 1986 when Ephraim Fischbach of Purdue University reanalyzed the original experiment and claimed there was evidence of a new force lurking in Eötvös’s results. Subsequent investigations did not support the claim, but the excitement got physicists thinking—and more often than not, new thinking leads to new experimental efforts.
The most accurate modern version of Eötvös’s experiment is being performed by Eric Adelberger and his team at the University of Washington in Seattle, who cheekily call themselves the “Eöt-Wash group.” They have perfected a variety of clever, ultrasensitive torsion balance experiments and have spent more than 25 years looking for any twisting that would indicate the presence of new forces operating over long distances.
So far, nothing. If they gave out Nobel Prizes for null results, these guys would be near the top of the list. If there are new forces, then, they are either too weak or too short-range to be relevant to our macroscopic world. Physicists still hope something will show up, perhaps at powerful particle accelerators, because discovering new forces would mean we’d have to develop completely new theories. But if we find them, these hidden forces will leave no imprint on the motions of atoms, molecules, or larger objects such as ourselves.
The bad news is, no tractor beams. If we want to build an apparatus that exerts influence over large distances, we are limited to using gravity and electromagnetism. Even if that’s an established fact, though, it raises as many questions as it answers. Why just those two forces? Why do they interact the way they do? How do they relate to the possibly hidden forces at
shorter distances? We can marvel at how well we understand certain aspects of nature, while never forgetting how very far we have left to go.
Sean Carroll is a theoretical physicist at Caltech focusing on inflation and the arrow of time. His blog, Cosmic Variance, appears at http://blogs.discovermagazine.com/cosmicvariance