Preposterous Universe

Monday, May 03, 2004
Life and the forces of nature

One of the most profound experiments in physics is one you may never have heard of. It's the torsion-balance experiment at the University of Washington (and others like it elsewhere in the world).

This group, led by Eric Adelberger, has recently garnered attention for testing Newton's inverse-square law of gravity down to a tenth of a millimeter. This experiment is interesting because there are good (or at least plausible) reasons to suspect that Newtonian gravity actually breaks down at around a millimeter. In particular, models with large extra dimensions of space can unify the scales of quantum gravity and particle physics if there are two extra dimensions about a millimeter in size. The Washington group has placed significant constraints on this fascinating idea.

But the profound experiment I'm referring to is the one about "testing the equivalence principle." The Equivalence Principle is Einstein's idea that you can't tell, if you are sitting in a sealed laboratory, whether your lab is on the surface of a gravitating body or accelerating through space at uniform acceleration. So if the EP is right, uncharged bodies should all fall at the same rate in a gravitational field, just as they would in an accelerating rocket.

So far, the UW experiments have not detected any violations of the EP. But if they did, you wouldn't conclude that Einstein was wrong; instead you would guess that the bodies you were using weren't really "uncharged." In other words, you would have discovered a fifth force. The best limits right now on such fifth forces are that they are less than one-trillionth the strength of gravity, if they exist at all; that's incredibly weak. The fact that there is no noticeable fifth force is one of the most profound facts of physics.

We know of four forces in nature: gravitation, electromagnetism, and the strong and weak nuclear forces. The latter two only operate over very short ranges (atomic scales and below), leaving only gravity and E&M as forces relevant to our daily lives. ("Electricity" and "magnetism" are two different manifestations of the same force.) This is the deep and astonishing fact: everything we directly see around us can be accounted for by some simple forms of matter (electrons, atomic nuclei) interacting through just those two forces. Fortunately for us, they can interact in extremely intricate ways.

Scientists like to talk about what they are currently doing research on, which by construction tends to be speculative ideas at the boundaries of our ignorance. What can easily get lost is an appreciation for how much we actually know beyond any reasonable doubt, and how little wriggle room there is. ESP, astrology, and other paranormal phenomena provide excellent examples of ideas that simply can't work. When scientists criticize these ideas, they often start talking about blind tests and repeatability and so forth. All well and good, but the fact is that these ideas have no chance of being right even before we test them directly. There is no way for the human brain to send out a signal that would read a mind or bend a spoon, nor is there any way for the planet Venus to influence your love life. Any such influence would have to be communicated by one of the forces of nature, and there are only two possibilities: gravitation and electromagnetism. In either case the size of the force would be easily detectable, and we haven't detected it.

It would be great to find a new long-range force, and there are certainly models that predict them. But even if one were found, it would be so tremendously weak that we need all of our best technology to notice its effects at all; there is no way for such a force to push around human beings (even delicate parts of their brains) in meaningful ways. This isn't to say that there's no room left for mysteries; figuring out how electrons and nuclei interact through two simple forces to create all of human culture and the rest of the visible world leaves more than enough unanswered questions for generations to come.

Ideas on culture, science, politics.
Sean Carroll

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