PDA

View Full Version : Why don't gravitons cast gravity shadows?

ianww
2010-Nov-30, 10:35 AM
Why don't gravitons cast gravity shadows? If a graviton is a particle that communicates gravity back and forth between (in the simple case) two masses, wouldn't gravity shadows form behind those masses?

If not, that must mean that a graviton can both communicate gravity to a mass and pass through that mass without losing any of its information content. But how can that be? How can the information in the graviton be communicated to a mass without the graviton losing at least some of it? Is perfect lossless communication by gravitons possible?

caveman1917
2010-Nov-30, 12:58 PM
Interesting question. I think the answer lies in the fact that forces are mediated by virtual particles, not their real counterparts. Virtual particles aren't subject to many of the constraints real particles are (such as max c, and occlusion by intermediary objects).

Let's look at this question from the EM point of view (which gives rise to an equivalent question, but QED is accepted theory while gravitons are not), EM is mediated by virtual photons.

Suppose i have a magnet at one side of an opaque piece of paper and some iron on the other side. Real photons will not be able to pass through this piece of paper, but virtual ones are (as can be seen by just doing the experiment). If we take it a bit deeper, suppose we have some massive positive charge, and put a tiny negative charge next to it. Will the negative charge cast a charge-shadow (like the moon does on an eclipse), the answer is that it won't. The virtual photons can be seen as moving straight through the negative charge. I suspect this is the same answer as the one about gravitons.

a1call
2010-Nov-30, 06:03 PM
Why don't gravitons cast gravity shadows?
There are those who hypothesize that they do:

Though, I personally don't think they would.

If a graviton is a particle that communicates gravity back and forth between (in the simple case) two masses, wouldn't gravity shadows form behind those masses?

If not, that must mean that a graviton can both communicate gravity to a mass and pass through that mass without losing any of its information content. But how can that be? How can the information in the graviton be communicated to a mass without the graviton losing at least some of it? Is perfect lossless communication by gravitons possible?

There are known examples which virtually do that. They pass through planets with hardly any interactions:

A neutrino (Italian pronunciation: [neuˈtriːno] (http://en.wikipedia.org/wiki/Wikipedia:IPA_for_Italian), meaning "small neutral one"; English pronunciation: /njuːˈtriːnoʊ/ (http://en.wikipedia.org/wiki/Wikipedia:IPA_for_English)) is an elementary particle (http://en.wikipedia.org/wiki/Elementary_particle) that usually travels close to the speed of light (http://en.wikipedia.org/wiki/Speed_of_light), is electrically neutral (http://en.wikipedia.org/wiki/Electric_charge), and is able to pass through ordinary matter (http://en.wikipedia.org/wiki/Matter) almost undisturbed.Source (http://en.wikipedia.org/wiki/Neutrino)

ETA: Perhaps an interesting analogy would be (electro)magnetic force. If I am not mistaking magnetic flux travels through an iron core better than through void/air in electronic coils. Still electromagnetic emissions cast shadows when they are (highly) not static aka light

ETA-II: Welcome to the board ianww.

thothicabob
2010-Nov-30, 06:18 PM
Umm, I could be over-simplifying this, but as I understand what a shadow 'is', it's basically an area behind an obstacle where there are fewer photons than the area around the photon; e.g. the photons are blocked by the obstacle. So far as I know, gravity doesn't behave in this way. in fact, if you're on one side of an object (your obstacle or 'shadow maker') with another dense object on the other side (your 'gravity source'), you'd be involved in the stronger gravity field created by both of those objects (discounting your own meager contribution to it), not 'blocked' from one by the other (and now I'm sure someone will pipe up with the idea of a 'negative' shadow; please, don't!).

i'm not really sure how you could even go about detecting such a shadow should one be even possible. i'm also not so sure the neutrino example would be a good comparison here - since they do pass through without interaction, there's no communication (hence no effect). Now, there could be some 'shadow' as marginally fewer neutrinos would likely be detected directly behind an object than in the area around it (assuming it was sufficienty dense so that some number would interact with the obstacle), but gravitons, if they even exist (which i'd tend to doubt anyway), likely don't behave in the same way as photons or neutrinos.

caveman1917
2010-Nov-30, 06:31 PM
i'm not really sure how you could even go about detecting such a shadow should one be even possible.

During a total lunar eclipse you could try and see wether the solar component to the moon's orbit falls away, in that case wether its distance to earth increases (too much). In principle at least.

thothicabob
2010-Nov-30, 06:44 PM
i'm not sure even that'd work, but...nice try? thing is, you'd actually have to measure whether the earth 'slips' further from the sun (in relative terms, maybe similar to what ur saying but still 'different') as the moon would 'block' some of the suns 'gravitons', but then we'd also have the moon's gravitationally attractive contribution in there to consider...and then we'd have to also double check on where the other planets were relative to the earth, moon, and sun, too, and account for their gravitational contributions (even if small, we'd not want to let some tiny, unforeseen error make us jump to some conclusion, would we? ;) ).

And in any case, from what I recall reading about the subject, the strength and wavelengths and whatnot involved with gravitons are so small/long/'hard to detect' (so hard they've not been detected yet, anyway), that it'd be tough to isolate some actual effect from any realistic margin of error, no? I'm no physicist...I just read and think a bit about the stuff...but it seems to me we're not going to detect 'gravitons' on any system so small as the earth/sun/moon system anyway, and perhaps not ever.

thothicabob
2010-Nov-30, 06:46 PM
oh, and yeah...I did catch your 'in principle, at least' qualifier. but, well....ya know! ;)

forrest noble
2010-Nov-30, 06:56 PM
Another realization I think should be that gravitons are only theoretical particles that never have been observed, like dark matter, Higgs particles, quantum foam, etc. etc. etc.

Trakar
2010-Nov-30, 07:11 PM
Why don't gravitons cast gravity shadows?

This rather presupposes the existence of several pieces of evidence that are not known to exist and which are not required to exist by current physical science understandings. We have no compelling indication that there are "gravitons" (as analogs to photons) much less can we say much about what properties they have if they do exist. Most predominant current understandings of gravity do not require their existence, though some are still trying to use a varient of them as the mechanism behind the mass-energy interaction with the fabric of space-time, but this coupling is generally attributed to the Higgs boson (which is proving nearly as elusive as any potential graviton, but is strongly and compellingly indicated by the Standard Model).

thothicabob
2010-Nov-30, 07:16 PM
Yeah...what he said! ')