Mike Cruise

University of Birmingham

Searching for the fifth dimension using gravitational waves

Gravitational Waves• The science cases for

AdLIGO, VIRGO, LISA, etc are very strong and first detections are expected in the milliHz to kilo Hz range.

• These astronomical sources are well identified, mostly observed already using other techniques and quite well understood theoretically

GW’s at Very High Frequencies

• By contrast, the case for sources at frequencies above 1MHz is far more speculative and less developed.

• There are three generic sources at frequencies above 1 MHz– The early universe– EM-GW conversion in astrophysical

plasmas– Black Hole infall in 5-D gravity

Brane Oscillations in 5D

• Seahra and Clarkson have calculated the GW emission in 5-D gravity when gravitational interactions take place between a black hole and an orbiting star

• There is the normal LF radiation from such a system, but there is also excitation of the brane separation itself

lldp e

l

mm

MRkpcMxh 2/)5(

5.021 1.011

109

This is a Source which exists!

Brane Oscillations

• Speculative Physics but a clear motivation

• Strong signals but many parameters unknown

• For sources at the Galactic Centre, there might be enough knowledge of the stellar statistics to place limits on 5D theories, or even make a detection!!.

Measuring GW’s:Laser Interferometry

Laser

Photodiode

Mass

Mass

A GW interacts with a static Magnetic Field

• De Logi and Mickelson (1977)

• • • • Cross Section for g >

Graviton

Virtual Photon( Static Magnetic Field )

Photon

3

228

c

LGBSpin states of g, B and

Higher Energy Density

Current Optical Detector

Current Plans

• The Birmingham Optical GW detector is operational now and observes the Galactic Centre with a ~15% time efficiency. Efforts are being made to raise this to about 60%

• Jodrell Bank and Birmingham are designing an interferometer working at 30GHz as well

• Future technology possibilities include internal magnetic fields in ferromagnets, etc to improve sensitivity

Conclusion

• Gravitational Wave science at high frequencies may allow tests of 5D gravity models.

• Upper limits from the current prototypes should start appearing next year

• Turning the experimental results into science will require better understanding of brane-world parameters and the astrophysics of the Galactic Centre