Ira Flatow on Science: Gravity, Galileo and Newton
ALEX CHADWICK, host:
This is DAY TO DAY. I'm Alex Chadwick.
You would think something like gravity would get settled hundreds of years after Galileo and Newton, but it's not settled. The only thing is that scientists now are using very modern technology to further investigate the subject: special reflectors left on the moon by Apollo astronauts. Here with more is Ira Flatow, host of "Science Friday," regular Thursday guest on DAY TO DAY.
Ira, welcome back.
And it's pretty neat to think that things left on the moon by astronauts 35 years ago still useful, still operating.
IRA FLATOW (Host, "Science Friday"): Yeah, they are. It is kind of cool. And the stuff we're talking about are these little mirrors left on the moon, as you say, by the Apollo astronauts. And they were left there so that laser beams sent from Earth here can be reflected back to Earth, and this sort of ping-ponging of laser beams can very accurately judge the distance to the moon from the Earth and the relative speed that the moon is moving. You know, it's like pointing a radar gun at the moon and judging how fast it's moving relative to the Earth, the same principle.
CHADWICK: But, Ira, what has that got to do with gravity? How do you derive information about gravity from that?
FLATOW: Well, this is--as you say, this is an effort to try to pin down the effects of gravity and whether the ideas put forth by Galileo and Newton are really still true. Scientists are always--you know, there are never-ending battles to try to collect as much data as possible. And it goes back to Galileo's experiment, you know that one in 1604, published in 1604. He was dropping all these cannon balls off the Leaning Tower of Pisa. I know you weren't around for that.
CHADWICK: I wasn't. But it's always seemed amazing to me because you think, `Oh, right. The heavier one falls first...'
CHADWICK: Galileo figures out, `Well, no, that's not true,' and derives from that the principle of universality of gravity.
FLATOW: That's right. And then along comes Newton in 1687, and he helps out by publishing an explanation for this equivalency principle. Then you got Einstein about a hundred years ago; he does the same thing, and they've all now confirmed this principle. But it's not quite enough for modern-day scientists.
CHADWICK: They're retesting this with the moon and the Earth?
FLATOW: That's exactly what they're doing. And they're doing it by bouncing these lasers off the moon, and they have done it and they have tested the equivalency principle. And guess what? Surprise to nobody, it really works. The reason they're using the moon and the Earth is because the Earth is so much bigger than the moon, they're like two giant, you know, cannon balls, one much bigger than the other, and they measure how much and how fast they are being attracted by gravity to the sun. Are they falling toward the sun at the same rate together? And, of course, they have found that they are falling toward the sun at the same rate, so the principle is proven.
But just as importantly, these are very accurate measurements and they also verify the value of Newton's gravitational constant--that is actually the acceleration of gravity--because some theories have hinted that this constant may change over time, and that would mean that Einstein's theory of relativity would not be correct; it would need to be adjusted. But this measurement now that they have made, using--bouncing the lasers off the moon--well, so far this indicates that all is well in relativity land.
CHADWICK: Ira Flatow, host of "Science Friday," regular Thursday contributor to DAY TO DAY.
Ira, thank you for that lesson and reassurance.
FLATOW: You're very welcome, Alex.
(Soundbite of music)
CHADWICK: DAY TO DAY returns in a moment. I'm Alex Chadwick.
NPR transcripts are created on a rush deadline by a contractor for NPR, and accuracy and availability may vary. This text may not be in its final form and may be updated or revised in the future. Please be aware that the authoritative record of NPR’s programming is the audio.