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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 16 July, 2008 Next Week's Question
Theory of relativity says light's energy is related to its mass
Question
Does energy have mass? Does light have mass?

Question
I like these questions because they remind me of my freshman year in college. This was 1975, and astronauts had recently placed a kind of mirror on the moon that would allow scientists on earth to aim a laser beam at the mirror and have the light reflected back to its source. The point of that was to precisely measure the distance between the earth and moon by measuring the round trip time of a laser pulse. But what most interested a group of us in college was the problem that, no matter how perfectly parallel the rays of the laser are prepared, they always spread out! This effect, called diffraction, is hardly noticeable when you play with a laser pointer in the space of a room. But during its transit between the moon and earth, the laser pulse expands from the size of the mirror, about one meter, to a few kilometers!

I can now make the connection with your questions. One of my college friends speculated that gravitational attraction might be able to oppose the spreading of a laser beam. In our physics class we had learned that light has energy and that energy, according to Einstein's E = mc2, is closely related to mass. Another thing we learned is that masses are attracted by the gravitational force, causing even a cloud of tiny dust particles in space to shrink and become more concentrated. But could the same thing happen to a laser beam?

This speculation, of a light beam collapsing by gravitational attraction, seemed far-fetched and we didn't believe it until one of us had the guts to ask Feynman. Feynman, if you haven't heard of him, was the Yoda of physics. We were all in awe of his powers at explaining things, and I still remember his answer. He told us that indeed the energy/mass of the light would be attracted to itself but that the effect would be very small unless the power of the laser was enormous.

Here is a fun way to work out the power you need in a laser beam to keep it from spreading. Since this physical effect involves both light and gravity, we might expect the power to be a combination of two constants of nature: the speed of light c, and the gravitational constant G. Only one combination, c5/G, gives a quantity of power. Try typing c5/G into Google and the search engine will convert this to more practical units: c5/G = 1052 watts! Compare this with the most powerful laser on earth today: 1015 watts.

I was, quite frankly, disappointed by these numbers. Another crazy idea from those college days, this time my own, was to focus light into a spot of such high intensity that the energy/mass of the spot would be large enough to collapse into a black hole! Unfortunately the power needed for this is the same number, 1052 watts, and the only energy source in the universe that can deliver this kind of power is a supernova.