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Don't take physical law for granted

In studying the cosmos, scientists assume their physical laws are universal. They'd be in a mess if the gravitation that holds galaxies together were different from that which guided the fall of Newton's apple.

Yet their assumption is based more on faith that on evidence. That's why some of them try to test it from time to time. So far, all such studies, including one reported this summer, have only strengthened the faith.

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This latest test -- carried out by Allan Tubbs and Arthur M. Wolfe of the University of Pittsburgh -- involves light and radio waves from four distant objects called quasars. These are members of a class of objects -- the quasistellar radio sources (quasars) -- which are very compact yet very powerful sources of radiation. They offer scientists an opportunity to study basic physical processes taking place in remote parts of the universe.

In this case, these are the atomic processes by which elements emit or absorb light and radio waves of specific frequencies, thereby creating the so-called spectral lines which characterize different elements.

The quasars are moving away from us as part of the general expansion of the universe. Because of this, the lines in their spectra have lower frequencies (that is, they appear to be redder) than comparable spectral lines observed in laboratories on Earth. This is the so-called red shift. If the physical laws involved are different in the different parts of the universe where the quasars reside, this should show up in studies of the red-shifted spectra.

Tubbs and Wolfe now report they can find no such differences. They have analyzed the radio and optical spectra of the quasars in ways that enable them to estimate the magnitudes of certain natural constants compared with the magnitudes of these constants as known from experiments on Earth. The constants , which govern the physical processes involved, include such things as the ratio of the mass of an electron to that of a proton or the "fine structure constant" which determines the detailed structure of a spectral feature. Since the constants seem to be the same for the quasars and for Earth, the physical laws into which they enter must also be the same.

This strengthens the findings of an earlier study of a single quasar reported four years ago by Wolfe and by Robert Brown and Morton Roberts of the US National Radio Astronomy Observatory. That study also showed that the quasar physics appears to be the same as terrestrial physics.

While tests such as these do not prove the universality of physical law in an absolute sense, they do give physicists something more than blind faith to go on. This makes little difference in most day-to-day research. But when it comes to constructing a comprehensive scheme for the cosmos, using knowledge that often depends on the interpretation of sparce data from remote places, physicists have learned that they cannot take the foundations of their science for granted.