# Q & A: Why gravitational attraction?

Q:
When a stone etc.ACCELERATE against the center off the Earth because off gravitational attraction:1 what kind off energy transition,crossing,conveytation rise and suorce is it about?2Why does gravitation always attract and not repulse?3.In what way does the stone communicate with Earth when it accelerate against it in the gravitational field,what kind off invisible fast etc.signals?Gravitones?Radiation?Other way!What?
- Aki Tuomaala (age 45)
Finspång SWEDEN
A:
This is a toughie.   I asked our local guru this same question some years ago and he gave me some mumbo jumbo about forces that are transmitted by exchange of even-spin particles, such as the nuclear force or the gravitational force, are always attractive between particles as well as anti-particles.  Forces that are transmitted by exchange of vector particles, such as the electromagnetic force, have attractive or repulsive forces depending on un-like or like particle sign.   Beats me.  I don't understand it.   That's the way it is.
By the way, the gravitational force is thought to be transmitted via exchange of a virtual spin two particle called the graviton.  Laboratory experiments have shown that anti-neutrons are attracted by the earth's gravitational field, not repelled.  This goes the way my guru predicts.

LeeH

(published on 08/05/2009)

## Follow-Up #1: Why spin 2 for the graviton instead of spin 1?

Q:
I heard* graviton (if exists) is predicted to have spin-2 because gravity is always attractive and for gravity to be attractive in quantum field theory (QFT) it must be spin-2 particle. Is this true? if so, why a graviton must have spin-2 instead of spin-1 etc. to be always attractive in QFT? *@around 19 min BBC Radio 4 - In Our Time, The Graviton http://www.bbc.co.uk/programmes/p003k9ks
- Anonymous
A:

Hi Anything,

If the statements in ID# 14525 are true and if the graviton has spin one, then an anti-neutron would rise in the earth's gravitational field instead of falling like ordinary neutrons.   Experiments have shown that neutrons do fall in the earth's field.   It's hard to do the experiment directly on antineutrons but an equivalent experiment is under development at CERN that would test whether or not an anti-hydrogen atom would rise or fall in a gravitational field.  . It's a tough one and will take some time.  Stay tuned.

An additional reason for spin 2 for the graviton is that it must couple to the stress-energy tensor which is inherently spin 2.   See

LeeH

Just to elaborate on Lee's last point, you can think of those tensor waves as being something like a deformable medium stretching along one axis and compressing at right angles to that. The symmetry of that means that a 180° turn brings it back to the same thing. That's unlike say an EM wave, where a 180° rotation makes each field change directions.  Even spin corresponds to returning to the same thing on a 180° turn. Odd corresponds to switching sign. So you can see right away that the graviton would have to be even spin.  As for why S=2 requires purely attractive forces, our local guru has told us it's too deep for us to understand each time the question came up. /Mike W.

(published on 07/02/2014)