UCSD prof. reexamines theory of relativity

    Thomas Murphy, a physics professor at UCSD, is one of the masterminds behind an experiment that has a chance of rewriting the theory of relativity or, at the very least, examining the workings of gravity.

    Murphy is currently heading an experiment that will send a laser pulse out to the moon to test fundamental gravity. The process is estimated to take approximately 2.5 seconds. By timing this to a trillionth of a second and dividing by the speed of light, an exact measurement of the distance to the moon should be found.

    According to Murphy, when a scientist comes up with a theory of gravity, specific predictions can be made about how the solar system works and moves.

    “If you define the initial positions and velocity of all the planets, you should be able to run the clockwork according to that theory and see how things should move from there on,” Murphy said.

    The experiment was first put in motion in 1999, when Murphy contacted Christopher Stubbs, now a Harvard professor, at the University of Washington.

    “He [had] listed off about a dozen things he was involved in, and at the very end of the list he said, ‘don’t laugh, but I’m thinking of doing this project in lunar rearranging.’ I was immediately captivated,” Murphy said.

    Since 1999, the focus has been on getting adequate funding to do the project. The experiment had received initial funding from NASA, but it was not until February that Murphy received what he calls “substantial grants” from NASA — a budget of $1.3 million over three years.

    “Now we can actually do this experiment for real,” Murphy said.

    The experiment has recently attracted increasing media attention. Though some in his field think this experiment has the potential to rewrite relativity, Murphy has his doubts.

    “In my heart, I think that we probably only have a 5- or 10-percent chance of seeing some violation of general relativity,” he said. “So my guess is, at this level of precision, general relativity will still hold up.”

    However, if the result of the experiment is that the moon’s orbit does not hold to the general theory of relativity, Murphy said, then either the experiment or the theory is wrong.

    “If we found some violation, I personally wouldn’t believe it at first, and I would look to what we did wrong in our experiment,” Murphy said. “Other people would follow on with experiments of similar or complementary natures, so we’d ultimately get to the bottom of the story.”

    The main goal of the experiment, however, is not to rewrite relativity.

    “Mainly, when you can make a measurement that precisely and that sensitively, in the shape of the lunar orbit, you start to probe a lot of physics that otherwise can’t be probed,” Murphy said. “The attempts to merge gravity and quantum mechanics predict the kinds of effects we could see in this program, so it might offer some of the first experimental evidence that those theoretical ideas for merging gravity/quantum mechanics are on the right track.”

    Despite all the science behind the experiment, Murphy said he also sees the fun in it.

    “It’s a cool project,” he said. “Even if it didn’t have this fundamental aspect of peering into the deep secrets of gravity, it’d just be fun to shoot a laser at the moon and see it come back.”

    In addition to working with Stubbs, Murphy, the principal investigator on the project, is also working with Eric Adelberger, a physics professor at the University of Washington, and UCSD students. Currently he is working with two graduate students and two undergraduate students.

    Aimee Vu, a Revelle College sophomore, is one of the undergraduate assistants on the project.

    “I’m not 100-percent sure that it will rewrite the theory of relativity, but I thought it was a cool project and wanted to work on it,” Vu said.

    Eric Mitchell, a graduate student working on the project, says that while other theories have held up well for almost 90 years, modern theorists realize they can’t hold forever, and there is potential for change.

    “The question is, where do [these theories] break down?” Mitchell said. “We’re either going to find it doesn’t hold or we’ll find it does hold. [Either way], this experiment will help others fix the theories.”

    Murphy hopes to begin the experiment in approximately six months.

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