# Fermilab

## Inquiring Minds

 Questions About Physics

Graviton Question

I am a sophomore in high school writing a reaserch paper for my honors chemistry class. While gathering information from the local libraries one book mentioned a theoretical boson called a graviton that would be responsible for the force of gravity. Could such a particle exist or is there any evidence that it might exist? If so, are there any efforts under way to try to discover it? I haven't been able to find very much information about this subject, or maybe im just looking in the wronge places. If I am conntacting the wrong person about this could you please forward it to someone who has information about this subject.

Thanks,
Joey Adams

Joey,

So far only limits have been placed on the existence of the graviton. It is conjectured to be a particle with spin = 2, a so-called tensor particle. You probably have seen particles are arranged primarily by their flavor, mass, and spin. The spin is a variable that arises when you treat the particle reactions in quantum mechanics.

Unfortunately, for this kind of stuff, the local town library probably will not quite cut it. You might try getting to a university library if there is one around although you would have to read books there or make copies. There is some information on the web but like everything else it is scattered around. Although Fermilab is not a central repository for this information, you can try our astrophysics web page. There are places such as the following whose main function is to collect information like this:

```pdg.lbl.gov  ==  Particle Data Group tabulates all particles that have been
confirmed.  From their bi-yearly publication of new particles
I list here (1996) a typical graviton mass limit:
Physical Revew D54 1 July, 1996

Limit (eV)       Researcher's-Name (1st one on paper)   Comment
===============================================================

< 7 x 10^-28 x h^-1   HARE      1973    Galaxy data

h = 100 kilometer/second/Megaparsec = Hubble constant

```

Of course the graviton is asssumed to be the force carrier of the gravitational field so like photons (the carrier of the electromagnetic field), it is probably massless.

Some references are:

Taylor et al. 1993 Nature (magazine) 355-132
Damour et al. 1991 APJ (Astrophysics Journal) 366-501
Goldhaber et al. 1974 Physical Review D9-119
VanDam et al. Nuclear Physics B22 397

You could try searching through Article indexes of Nature, Scientific American, and Physics Today since they have articles which you could be understand but would have plenty of information on why the graviton should exist.

Basically, Einstein's theory of gravity says that gravity is not really a force but space-time is really curved around high mass objects like stars or better black holes. So particles around these feel like they are on a steep hill and roll inward towards it. Einstein's theory has been proven true by looking at the orbit of Mercury which is a lowmass object very close to a huge high mass object (the Sun). The deviation from regular old Newtonian gravity is small but it is observable. Other recent tests include gravitational lensing where a high mass object close to the solar system (usually something dark) will bend the light from a distant star in such a way as can be detected.

Now when you try to subject gravity to quantum mechanics, no one has successfully (to my knowledge) merged the two into a theory that works. String theory is an exception but is based on completely different paradigms. If you do try to make a quantum theory of gravity though, the graviton pops out as the force carrier. An interesting thing is that all the other force carriers (photon, gluons, intermediate vector bosons) are all vectors (spin = 1).

Another resource is online preprints database:

http://www.slac.stanford.edu/FIND/hep

where you can enter:

find title graviton

and get a list of papers (some online in postscript). For example these look promising:

14) ELEMENTARY THEORY OF GRAVITON PRODUCTION PROCESSES. By R.J. Gould (UC, San Diego). 1995. Published in Annals Phys.238:83-128,1995

25) GRAVITON CREATION IN AN EXPANDING UNIVERSE. By R.G. Moorhouse (Glasgow U.), A.B. Henriques, L.E. Mendes (Lisbon, IST). GUTPA-93-12-1-REV, Jan 1994. 12pp. Revised version. Published in Phys.Rev.D50:2600-2604,1994

These are published in journals which are widely read at universities but you won't find them at the local library.

The last thing to do is possibly use the local library to get the books writen by famous physicists like Einstein, Penrose, Hawking (good one), Feynman, etc. They usually explain things well and touch on a variety of subjects. A Brief HIstory of Time (Hawking) is excellent since his specialty is gravity.

Anyway, good luck.

Glenn Blanford
blanford@fnal.gov
Fermilab Public Affairs

Joey,

Also another things researchers have looked for is gravitational waves. Large mass objects such as black holes may emit waves. These are not neccessarily gravitons just as light was not neccessarily seen as photons until high energy light such as X-rays were seen. (ie The X-rays act like small size particles sometimes more than they do as waves).

There are various researchers looking for gravity waves. One of them is Riley Newman at the University of California at Irvine Physics and Astronomy Department (professor). You could email him (get address from UCI web page) since he could really help you out better than I.

There is also a large scale facility named LIGO which is an interferometer for gravitational waves which recently went "online".

Just to end off, a true understanding of gravity theory is a lot farther off than a theory of particle physics. Hence, there are plenty of other "models" or theories which are similar (or different) from one that demands a graviton for explaining gravity at least in the context of quantum mechanics.

Glenn Blanford
Fermilab Public Affairs

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 last modified 4/24/1997   physicsquestions@fnal.gov

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