Question: How many stars can a person see at night with the naked eye? Answer: Under ideal conditions, about 3,000 stars should be visible at night with the unaided eye, but many factors can reduce this number.

Astronomers have discovered direct evidence that most quasars came into existence during the same era, when the universe was still in its infancy. This discovery will help scientists use quasars, the most luminous objects in the sky, as tools for studying the universe back to a time when it was less than a billion years old.

Answered by Lucy Jones, Seismologist with the United States Geological Survey, and Visiting Associate in Geophysics at Caltech.

Answered by: Kip Thorne, Feynman Professor of Theoretical Physics

PASADENA—This is a new monthly feature produced by the Caltech Media Relations Office, in collaboration with Caltech's faculty, to answer commonly asked or particularly intriguing questions about science and the natural world.

Question: What would really happen if you tried to travel back in time, to before the date of your own birth, to change your parents' destiny?

Caltech biologists have found that certain proteins in the adult rat brain can strongly enhance the strength of connections between neurons for up to an hour, an effect never before demonstrated in adult brains.

Hundreds of scientific studies on the biological effects of electromagnetic fields may be invalid, researchers suggest in a letter to be published in the March 9 issue of the journal Nature.

This is a new monthly feature produced by the Caltech Media Relations Office, in collaboration with Caltech's faculty, to answer commonly asked or particularly intriguing questions about science and the natural world.

PASADENA—Caltech astronomers have counted galaxies to a limit of about 24th magnitude, the faintest ever counted in infrared light. Observing five small patches of sky with the 10-meter telescope at the W. M. Keck Observatory in Hawaii, the scientists found that the numbers of galaxies continued to rise with increasing faintness, a result that agrees well with models in which the universe is "open" and will continue to expand forever.