String Theory: Testing Gravity from Extra Dimensions

By Andrew Zimmerman Jones, Daniel Robbins

The testing of gravity produces a number of ways to see if string theory predictions are true. When physicists test for gravity outside of our three dimensions, they

  • Search for a violation of the inverse square law of gravity

  • Search for certain signatures of gravity waves in the cosmic microwave background radiation (CMBR)

It may be possible that further research will result in other ways to determine the behavior of string theory or related concepts.

Testing the inverse square law

If extra dimensions are compactified in ways that string theorists have typically treated them, then there are implications for the behavior of gravity. Specifically, there might be a violation of the inverse square law of gravity, especially if gravitational force extends into these extra dimensions at small scales. Current experiments seek to test gravity to an unprecedented level, hoping to see these sorts of differences from the established law.

The behavior of gravity has been tested down to under a millimeter, so any compactified dimensions must be smaller than that. Recent models indicate that they may be as large as that, so scientists want to know if the law of gravitation breaks down around that level.

Searching for gravity waves in the CMBR

General relativity predicts that gravity moves in waves through space-time. Although string theory agrees with this prediction, in most string theory-based models of inflation, there are no observable gravity waves in the cosmic microwave background radiation (CMBR). Traditional inflation models that don’t take string theory into account do predict CMBR gravity waves.

Although the string theory landscape has predictions for scenarios where relativity breaks down, there doesn’t appear to be any mechanism in string theory for gravity waves in the CMBR, according to University of California cosmologist and string theorist Andrei Linde. (Linde made this statement in 2007 and work since then has produced some preliminary indications that string theory models of inflation may be compatible with gravity waves in the CMBR.)

At present, the evidence seems to be leaning toward there not being any gravity waves in the CMBR data. The Planck Surveyor spacecraft was successfully launched in May 2009, with even greater sensitivity than the current WMAP study. Scientists may get a more decisive take on whether these CMBR gravity waves exist at any time.