Expert: Philip Stahl - 12/18/2007

Question
Hello Philip,

First of all, I realise AllExperts doesn't provide help with projects or homework questions (I'm an expert here as well, though on a completely unrelated subject), but this shouldn't be considered such a thing. I'd like to ask for your opinion as an expert about this subject. I have read the answer you wrote on a question about gravitation, which struck my attention as I picked it as a subject for my graduation project.

The deal was to come up with an experiment, theoretical or not, and write a good hypothesis, etc, etc. Now I've done all of that stuff and it got me my grade, but my teacher didn't really know what to think of it. He passed me because I did understand the concept of an experiment, but had his questionmarks.

I did a theoretical astrophysical experiment, defining if a graviton partical (I know, it's half-theoretical on it's own) has a velocity, like light.

So that was my hypothesis: A graviton partical has a definable velocity.

The experiment with which I intended to prove this theory might be a bit sci-fi for now, thus making it theoretical, but I believe it would be practically possible in due time.

Experiment:
- Search for a dormant black hole, the smaller the better.
- Set up a device on a reasonable distance from it that can measure gravitational pull.
- Knock an object of a considerable mass, a large asteroid or maybe a small moon, out of its orbit so that it collides
headon with the dormant black hole
- Measure the time it takes before the device registers the increase in gravitational pull
- v=s/t

Of course, even if it is a viable experiment, what the v actually is we won't know for quite some time, but I guess a theory is good too. I'd really like to know how you think about such a thing, did I miss something?

I look forward to your reply.

Pleasant holidays and a happy 2008,

Floris Houvast/ChronoDensetsu

Answer
Hello,

First of all, you are jumping the gun and “putting the cart before the horse” in your proposal that a HYPOTHETICAL particle possesses a velocity that is measurable. In other words, I dispute that you have a valid hypothesis at all.

Second, one does not commence with experiments (technically speaking) and then forge hypotheses. One first articulates a hypothesis THEN designs a suitable experiment to test it.

Far better to first back up, and hypothesize about attributes that may have prime significance: such as linearity –non-linearity, or the helicity.

For refresher: a non-scalar and massless particle (which the graviton is hypothesized to be) possesses two possible states of spin (S) – which can be right-handed, orleft-handed in respect of its direction of motion.

If you take your right hand and curl the fingers, the thumb points upwards and this is the direction of motion. For left-handed systems the rubric runs in the reverse (or inverse) sense, and the motion is downwards. These are referred to as the positive and negative helicity of the particle respectively,

Now, gravitons are hypothesized to be a particle of spin-2, so its two helicity states will be S = +2 and S = -2.

Here is the opportunity to forge a REAL hypothesis, by putting forth a possible reason for the divergence of spin between the two mediating particles: the photon (spin 1) and the hypothetical graviton (spin 2). WHY the difference? Propose a guess (hypothesis) to account for it. (A place to start in the hypothesis is to examine the wave nature of each).

Now, note that the experimental apparatus will take into account the differences you spell out in respect of the differing wave properties (say gravitational waves vs. electromagnetic).

A good place to start to get some ideas, is to look at early ways of detecting EM waves and compare these with early methods of detecting gravitational waves (you can look up Joseph Weber, who did lots of work on this).

Now – think of your own experimental approach.

Another hint: EM waves possess both electric (E) and magnetic (B) vectors which designate a particular geometry in respect of each other. (For example, in terms of right or left handed polarization, or circular polarization). How might gravitational waves be displayed and what vectors peculiar to it might be in evidence or displayed?

Another benefit of the above approach as opposed to your proposal: gravitational wave research has far more potential and promise of detecting gravitons than using black holes, however near, and trying to get a velocity measure. The reason is that a distinct, discriminatory detection of individual gravitons is impossible, because of the vanishingly small cross-section of interaction of gravitons with matter. This also implies it is essentially impossible to find 's' in your measurement, and hence find v.

In addition, I am aware of no such things as "dormant black holes". A black hole is defined a certain way in terms of the tidal forces it exerts, as well as preventing the escape of even light. No black holes are "dormant". Massive holes - such as at the center of the Milky Way- are by now means "dormant" - this is a misuse and abuse of the word.(Which means "temporarily inactive" - but there is NO evidence that supports this for bhs. If they were "inactive" they could not be black holes!)

This also being the case, you will not have any viable or working chronometer you can place in a black hole's vicinity (or proximity) to measure 't'. All timing devices will be affected by the bh and you've not given any details that might provide "shielding" to prevent that.

Your proposed experiment also skips over other vast details, for example – how to locate an appropriate hole (Kerr or other), how to haul an asteroid (or moonlet) there and so on. All of this lack of detail makes it implausible.

Bottom line: testing any hypothesis to do with gravitons makes more sense if one goes the WAVE route, as opposed to the particle route.

Resources -links on this:

http://imagine.gsfc.nasa.gov/docs/features/topics/gwaves/gwaves.html

http://www.gothosenterprises.com/gravitational_waves/

http://www.ligo.caltech.edu/LIGO_web/about/factsheet.html

And these point the way to hints for gravitational wave experiments, designs as the more profitable approach and more grounded in reality. If indeed we are talking of realistically doing these experiments one day and not just permanent science fiction.

Hope this helps!

(Note: Had you come to me before you submitted your work, I’d have advised you against it – and that it was a non-starter. I'd have proposed what I am offering here. Having already submitted your work, there is little more I can do than what I would have done - since I still am convinced it is an unproductive route to take. Sorry!)