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Q & A: Why expect a graviton?

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Most recent answer: 08/21/2015
If gravity is actually only the result of an objects straight path through 4 dimensional space, which is curved by a massive object near the object - why is there any discussion and search for a graviton ? Surely if, as general relativity suggests, ( and I'm not a physicist just an interested layman so I could be completely wrong) gravity is not a "force" but just appears as such because we can't contemplate 4 dimensional space and the curvature of such space - why would there be a lepton associated with it ?
- Andy (age 45)
Melbourne, Vic, Australia

There are two steps to this.

The first would be "why look for gravitational waves?" since there are major efforts to detect these. These waves are predicted by general relativity itself, so the quantum aspect isn't involved.

Why do we expect there to be gravitons, the quantized version of those waves? There are no experimental searches for them, and there's no real hope of ever directly detecting them. [I was wrong: see below*] However, if gravity was a purely classical effect, without quantum uncertainty, then in principle an observation via gravitational waves would allow quantum things to be measured beyond the limits of the uncertainty principle. That would unravel the self-consistency of the whole quantum structure. So it seems that either everything has to be quantized or nothing. We know from violations of the Bell inequalities (search this site and elsewhere) that anything like a purely classical universe is ruled out. So we think that gravity must ultimately be quantized.

p.s. The graviton is not a lepton. It's a massless spin-2 boson.

Mike W.

*The BICEP2 collaboration claimed to have seen what looks like the leftover effects of the quantum zero-point spread of gravitational waves, left as tiny "B-mode" ripples in the polarization of the cosmic microwave background radiation. It now looks like they really saw somethig more local and less interesting. If one of the more sensitive versions of this experiments works, then it will be fair to say that a real quantum gravity effect will have been seen. /Mike W.

(published on 08/04/2012)

Follow-Up #1: Has the graviton been discovered yet?

Has the graviton been discovered yet? If it has not, then how can it be detected?
- Bobby (age 16)
Austin, TX, US

Not yet, and I suspect it will be a long time before direct observation of a single graviton will occur.*   The graviton is a hypothetical quantum mechanical  mass-less particle associated with the gravitational field, just as a photon is associated with  electromagnetic fields.  Unlike the photon, which can be very energetic and detected with off-the-shelf hardware, gravitons carry extremely small amounts of energy, interact only weakly, and carry minute amounts of detectable energy. 

The one thing you can look for, however, is the presence of gravitational waves.  Indirect evidence for this has been seen in certain binary star orbital decay. 
and .

Direct searches for gravitational waves are ongoing in several laboratories around the world such as LIGO in the USA and VIRGO in Europe. 


* Lee is certainly right about detection of single gravitons, but here's a news flash about quantized gravity:

Now the BICEP2 collaboration has seen what looks like the leftover effects of the quantum zero-point spread of gravitational waves, left as tiny "B-mode" ripples in the polarization of the cosmic microwave background radiation. In other words, they've probably seen something that wouldn't be there unless gravitational waves were quantum mechanical, So in that sense, gravitons have now been seen! /Mike W.

(published on 01/04/2010)

Follow-Up #2: Can an accelerated object produce gravity waves?

given the equivalence of acceleration and gravitation, would changes in acceleration produce gravity waves and/or emit gravitons?
- Robert Kemper (age 68)
Miami, fl, USA

Hello Robert,

Good question.   Although an accelerated charged particle can produce electromagnetic wave and photons, an accelerated body cannot produce gravitational waves or gravitons.  The difference is that a photon is a spin-one object whereas a graviton is a spin-two object and you need a somewhat more complicated arrangement.    The hope of gravitational wave experiments such as LIGO in the USA and VIRGO in Italy is that they will see a pair of neutron stars that orbit each other or a spinning neutron star that has a mass quadrupole moment.   So far none have been observed but upgrades to the detectors are in progress. 

Wikipedia has a very nice article on the subject.  


(published on 08/21/2015)

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