MadSci Network: Physics

Re: Gravity

Area: Physics
Posted By: Tom Cull, Grad student Physics, Washington U
Date: Fri May 10 23:24:13 1996

Gravity has defied quantum (particle) explanation.  Theory abounds on

the subject of quantum gravity.  The best cases for detecting quantum

gravity effects are gravity waves.  Gravity waves occur when black holes

or stars revolve around each other in a binary system.  General relativity 

says that must be true.  These waves will travel through space, and if lucky

could pass through the earth.  Gravity waves will causes contraction in the

direction of propagation and expansion in the tranverse direction.  This

effect is opposite of tidal gravity.  Gravity waves travel at the speed

of light.

Anyway, detecting gravity waves is hard.  Gravity wave detection devices

rely on detecting minute contractions or differences in perpendicular 

distances.  One such example is a large metal cylinder with piezoelectric

sensors on it.  If a gravity wave comes along, the sensors will give an

electrical signal.  Unfortunately, the signal will be tiny for several

reasons.  One, the source will far away, and the effect will fall off as

distance squared.  Two, the magnitude of the effect depends on the 

gravitational constant and Planck's constant.  And three, human machines

can only be so accurate.  Gravity wave detection is at the very limit of 

our capacity to detect (if it exists).

The theoretical particle of gravity is a graviton.  For it to be a long 

ranged force it must be massless like a photon.  I believe it should have a 

spin of 2 (a photon is spin 1) for angular momentum.  The strong and weak nuclear

forces are understood to be short ranged because the mediating force particle has

mass.  The bigger the mass, the shorter the range.  

Gravity or general relativity does not fit well into a quantum mechanical

framework.  It must for its particle nature to be fully understood.  If it

did, then a Grand Unifying Theory (GUT) could be established, and we would

claim to understand the four forces of the universe (gravity, strong, weak, 

and electromagnetic).  

Gravity does seem to go on forever.  Much of what you have learned about

electromagnetic waves can be applied to gravitation.  Things like Gauss' Law

have gravity equivalent forms.  However, to solve this problem a pleasant marriage

between quantum mechanics and general relativity must be forged.  And it hasn't

been completed yet.

Dig a little deeper than this and you will find tons of "easy" literature.  

Go a lot deeper, and you wake up one day a general relativity grad student.


Tom Cull

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