Thursday, August 25, 2011

Q: What are gravitons?

duhoc asked the following question in a comment to my post calling for reader questions:
I love cosmology and physics but understand very little about it.  What are gravitons, in simple terms? 
Gravitons can be complicated.  To that end, I want to state that I am no expert on the subject...  But maybe that's just right for a simple answer.  If my answer here doesn't do the job, let me know and I will track down a friend who researches quantum gravity or string theory to do a better job.

Gravity is one of the four fundamental forces, along with electromagnetism, strong force (which holds the nucleus of atoms together) and weak force (which is responsible for radioactive decay).  Every other fundamental force has a particle associated with it that communicates the force.  Electromagnetism has the photon, the strong force has gluons an the weak force has W and Z bosons (I know the least about these).  The only exception here is that gravity does not have a fundamental particle associated with it that has been observed.  However, it is safe to work on a theory that establishes the graviton as its communicating particle.  [Constructing theories by extending observations from similar situations to a new one that is not completely explained has proven effective before - the neutrino was a theoretical particle assumed to exist to account for the missing energy carried away in some forms of radioactivity and it was later proved to be true!]

One of the reasons that it is so difficult to directly detect the graviton is that gravity is the weakest of the fundamental forces (even though it holds the Universe together - that's because there is no negative mass and we don't know why that is either).  The weaker the force, the more effort there is needed to detect it.  For the graviton, it isn't a lack of motivation on scientists' part but a limit on technology... Just the contamination shielding for a detector the size of Jupiter, which would let us observe a gravitation once every 10 years, would be so massive that it would collapse into a back hole.

So, instead of observing the graviton directly, we can observe the effects of gravitons and gain knowledge about the its properties.  One of the most promising ways is by observing gravitational waves (and I know a lot about this!).  You may (or may not) have hear that we expect gravitational waves to travel at the speed of light.  The reason we expect it to is that we expect the graviton to be massless and massless particles cruise through the Universe at the speed of light.  But, if we detect a gravitational wave and it takes significantly longer the the time light would take to travel between the LIGO detectors and her international partners, then that would imply that the graviton has mass.  Think about that for a second...  Gravity exerts forces between masses and if the particle that communicated this force has mass itself, that can really complicate things.  There are theoretical physicists who spend their careers thinking about what if some detail of general relativity that hasn't been thoroughly verified is different than we think; what would be the consequences of that?  They know and they have more complicated theories worked out and the results of a massive graviton has been pondered by them as well.  Again, I am not a theoretical physicist but I do appreciate the complications involved.  However, one property of the Universe that has been shown time and time again is that the simple solution is usually the correct one which is why we say the we expect gravitational waves to travel at the speed of light.

There are some really interesting theories in cosmology that focus on the graviton.  For example, in string theory it is theorized that our Universe lives on a brane inside a higher dimensional bulk (where there are more dimensions than length, width, depth and time).  Here, gravitons are able to travel to other branes (universes).  If this theory is correct, then this could explain the missing mass in the Universe we call dark matter.

I like to include pictures with each of my blog post to help keep things interesting.  But, since the graviton has never been observed I haven't been able to find any strictly scientific images for this post.  However, if you would like a graviton of your very own to cuddle with, the Particle Zoo (which sells stuffed versions of elementary particles) has one you will be attracted to (ha, ha...  get it?):

I'm not endorsing or advertizing this product, I just thought it was interesting.
I hope this answers your question.  If not or if you anyone has more question, feel free to leave a comment below or ask me on Twitter @livingligo.


  1. Very nice post. Just one correction. Gravitational waves travel at speed of light only in vaccuum. In the real universe such as ours, they ( as well as photons, neutrinos) get slowed due to gravitational potential of all intervening matter along line of slight between the source and us. This delay (aka Shapiro delay) was calculated for SN 1987A and is about 5 months.
    This has also been calculated for GRB 070201 and is about 3 years.
    of course it is still negligible compared to D/c

  2. That is true and something I should have mentioned. However, the gravitational waves will travel the same speed of that light would travel, so if we see something interesting in the sky using light, it is fair to look for any accompanying gravitational waves around the same time.

    I could save face and say that I'm not incorrect since I never specified "the speed of light in a vacuum", but I did not remember to consider the Shapiro effect (and I was just lucky that the effect applied to both light and gravitational waves).

    Thank you very much for commenting about this!