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30th Anniversary of The Planetary Society
  

Projects: Pioneer Anomaly

What is a "New Physics"?

By Louis Friedman
10 May 2005

For nearly 30 years, as they made their ways to the edge of the solar system, the two Pioneer spacecraft were tracked with enormous accuracy -- to one part in a trillion. After scientists analyzed the tracking data with the most sophisticated statistical and computer algorithms, taking into account all the known forces that can affect spacecraft, there was still a deviation from Pioneer’s predicted motion.

This could be due to an unknown force, one that arises from a "new physics." This calls to mind the way Einstein discovered the new physics of general relativity that replaced Newtonian physics, which had described the universe so well for so long. The Pioneers may be telling us that some deep, very subtle, change is required to correct the laws of motion as we now know them.

That prospect is tantalizing, but it is more likely there isn't a "new physics" involved, and instead something is happening within physics-as-we-know-it that hasn’t been properly accounted for. There are many options: The spacecraft could be falling apart in a way we didn't account for, or we incompletely understand spacecraft behavior, or there is an error in the tracking data. Or maybe they are directing us to discover some bizarre effects of physical objects -- planets or Kuiper Belt Objects or other bodies doing strange things. Or we’ve incorrectly predicted the effects of the interstellar wind, or we need to change a variable in the supposed constants of mass and gravitation in the solar system.

If the Pioneer anomaly can be attributed to the spacecraft or the way we track it, it could refine our understanding of spacecraft and the way they fly. If the answer lies in the solar system environment, it would be a new discovery about the nature of our universe.

Occam's Razor is a principle of science that says go for the simplest explanation first. The uncovering of a new physics happens very infrequently in human understanding, and it demands the most complex explanation. The discovery that a solar system object is affecting the Pioneers in some unpredicted way would be a simpler explanation, but probably the very simplest is that something is happening on the spacecraft. Occam's Razor tells us to look at that first possibility first, and save a new physics for the last resort.

In fact, that is what the Pioneer scientists have done so far. The simple engineering explanation cannot yet be ruled out, but enough work has been done in trying all the different possibilities that even Occam's Razor allows us to cut a little way into the idea of a new physics.