With the Constants of Physics, Everything’s Relative

Few things in this changing world are constant. Traditionally, there have been death and taxes, but modern medicine seems to be gaining on the former, and the rich have long found loopholes in the latter.

So it is with the “constants” of physics: Some are a lot more constant than others. In fact, physical constants run the gamut from firmly fixed to highly elusive; several are outright frauds.

At the American Physical Society meeting last month in Long Beach, physicists came up with new values for the gravitational constant (the strength of the force between two masses), the mass of the electron and several others too esoteric to easily explain.

What actually changed was not nature’s value for the constant but the accuracy of the physicists’ measurement. In every case but gravity, the new value reflected increasing precision. (Alas, the gravitational force between two masses is so difficult to measure that the uncertainty in its value has actually increased twelvefold during the past decade.)

But most physicists would agree that whatever the measurements say, the true value of these constants is, well, constant. They’ve probably always been the same, and they always will be.

In contrast are blatantly inconstant constants--for example, the Hubble constant, which measures the rate of the expansion of the universe. You can always find the Hubble constant hiding out at the center of controversies about the size and age of the universe. (If the universe is expanding rapidly, for example, it could have arrived at its present state fairly recently, making it younger than its oldest stars. If it’s expanding slowly, then it would have arrived where it is at a leisurely pace--plenty of time for stars to age in peace.)

Probably the only thing all astronomers agree on is that the Hubble constant does not live up to its name. The expansion of the universe has been slowing down ever since the Big Bang. In fact, astronomers think the universe expanded so quickly at the very beginning of time that it inflated exponentially--doubling its size millions of times in less than a cosmic eye blink.

*

Perhaps the most notorious constant of all is the so-called cosmological constant. This constant is repulsive in more ways than one: On one hand, it’s the energy of empty space that may be pushing the galaxies apart.

But it’s also, according to University of Chicago physicist Joshua Freiman, “the most maligned constant in the history of physics.”

Einstein dreamed it up in order to correct what he perceived as a flaw in his theory of relativity--then later called its invention his biggest blunder.

Now the cosmological constant is once again gaining popularity as a possible fix for several outstanding astronomical puzzles, but it still causes as many troubles as it cures. No one knows where it comes from or even if--for sure--it exists.

At the other extreme, Einstein is also responsible for recognizing the most constant constant in physics--the speed of light. “It’s hard to imagine how it couldn’t be a constant, because it’s so embedded in physics,” astronomer Kenneth Brecher of Boston University noted at the Long Beach meeting.

The speed of light, for example, is the constant that ties space to time. If it changed, the whole edifice of Einstein’s relativity would come tumbling down.

The speed of light is so tied to space and time, for that matter, that its value is nearly impossible to measure. The official measure of space, after all, is the meter--and the meter is defined by the distance light travels in a given span of time. So any measurement of the speed of light is by definition somewhat circular.

What can be measured precisely is light’s independence from outside influences. The speed of light is constant whether the person measuring it is running toward the source of light or away from it. And this independence can be measured quite precisely.

In fact, at the meeting, Brecher reported that new measurements using gamma ray bursts from deep space improved its precision by a factor of 100 billion.

In honor of Einstein, Brecher suggested at the meeting that light speed should be called “Einstein’s constant.”

Given all the trouble he’s had with the cosmological constant, it seems the least physics can do.