Is there a prediction for the frequency of a gravity wave?

wolram

Gold Member
is it known how much gravitational force is needed to deflect a photon from its true path?
is there a prediction for the frequency of a gravity wave?
apart from the graviton are there other in vogue thories for the propogation of gravity?
can gravity be deflected?
does the speed of gravity change when traveling different mediums?
when light is deflected around the gravity of a planet,does the light follow the contours of the planet
a lot of questions, but searching the net has drawn a blank so any info on relevant sites would be appreciated......

Last edited by a moderator:
Related Astronomy and Astrophysics News on Phys.org

marcus

Gold Member
Dearly Missed
Re: gravity

Originally posted by wolram
is it known how much gravitational force is needed to deflect a photon from its true path?
......
when light is deflected around the gravity of a planet,does the light follow the contours of the planet
a lot of questions, but searching the net has drawn a blank so any info on relevant sites would be appreciated......
When light passes a pointmass it is bent by an angle
which (expressed in radians) can be calculated from the mass M
and the distance R of closest approach.

The formula for the angle of deflection is

4GM/c^2 divided by R

For the sun, GM/c^2 is a bit less than a mile (around 1.5 km).

So 4GM/c^2 is something under 4 miles (around 6 km)

So if a ray of light passes within 600,000 km of the center of the sun, then it gets bent by an angle of E-5 radians----ten microradians.

This is just 6 km divided by 600,000 km.

BTW this 4GM/c^2 length is interesting, it is the diameter of an equal mass generic (non-rotating) black hole. So a black hole with same mass as the sun would have diameter of 6 km. That is, twice the Schw. radius, the diameter of the event horizon, which from an outsider's viewpoint is a good index of the diameter of the thing.

marcus

Gold Member
Dearly Missed
webreference for that light-bending formula

http://www.livingreviews.org/Articles/Volume1/1998-12wamb/node2.html

This webpage is about the formula 4GM/c^2 divided by R

the light-bending formula is historical, in 1913 Einstein came up with the first version which had a 2 in place of the 4, and in 1914 some people went to a place in Russia to observe an eclipse and check it but war broke out and they were prevented

then in 1916 Einstein completed Gen Rel and got the right formula for light-bending, with the 4, and as soon as the war was over in 1919 another team, led by Eddington, observed an eclipse and checked it to within a 20 percent margin of error.

this was big news. it was the second confirmation of Gen Rel and made a big splash in the media

BTW you say "how much gravitational force" but the usual
idea is that the light is not feeling any force but is just going along a ("straight" = geodesic) line. So I should imagine that anything that goes in straight lines at lightspeed would be deflected the same way as light, following the curvature in the neighborhood of a massive object. So a planar gravity wave, if it's propagating at the same speed as light, would be deflected the same amount. Maybe someone else wants to correct or confirm this.

LURCH

The frequency of the gravity waves would depend entirely upon the phenomenon that produced them. One of the Current "Great Challenges" facing modern computational science is the prediction of what types of gravity waves (wave signatures) would be produced by different cosmic events. I seem to recall reading that there is a global team led by a doctor Suen (sp?) to predict these wave signatures, so that LIGO can look for them.

As I understand it, then main events for which they are trying to predict wave signatures are the collision of black holes, and the final spiral of a neutron star into a black hole.

Of course, I read that article about a year ago (I think), and these wave signatures may already be known now.

As for gravity waves being slowed by the medium through which they pass, it is currently theorized that no medium impedes their progress. This is one of the great proposed advantages to gravity wave astronomy; a source of radiation that cannot be "blocked" by intervening dust or gas or other interference.

Loren Booda

Good writing, marcus. Is it the event horizon or the Schwartzschild radius where photons may be bent into a shell around a black hole? (I believe that shell marks a symmetry between the spacetimes within and without.)

That gravity propagates at the speed of light was only just recently demonstrated.

"Branes" were hypothesized to conduct gravity directly (short-circuit dimensions) compared to the actual spacetime travel of light waves. (Conflict with previous mentioned measurement?)

marcus

Gold Member
Dearly Missed
Originally posted by Loren Booda
...Is it the event horizon or the Schwartzschild radius where photons may be bent into a shell around a black hole...

The radius of the "photon sphere" is 3GM/c^2.

In theory it is at the latter distance that light can circle round the black hole. Nice thought, light in orbit.

I think these formulas for the radius of the event horizon and the photon sphere apply only in the "vanilla" case of a non-rotating black hole. I'll see what there is on the web about it, but there is probably someone here who knows and can speak with more assurance.

wolram

Gold Member
gravity

thanks for interesting replies, im realy into gravity, but its hard to come up with info that you guys find easily ,where do you get it from? who is or are the fore most publishers on gravity related papers

marcus

Gold Member
Dearly Missed
Re: gravity

Originally posted by wolram
thanks for interesting replies, im realy into gravity, but its hard to come up with info that you guys find easily ,where do you get it from? ...
Ned Wright's cosmology site at UCLA

John Baez physics site at UCR (UC Riverside)

Ned Wright's site is incredible. I had better get you the url

http://www.astro.ucla.edu/~wright/cosmolog.htm

nature.com/nsu/

Nature magazine has "Science News Update"
with latest research of all kinds and a keyword SEARCH box.

Also a kind of strange site I just came across on google

This is Andrew Hamilton's homepage.
He likes pictures, gravity, color-vision, relativity, computer animations. Oxford maths undergrad, University of Virginia
astrophysics, a special talent for simpatico explanation
(or is that merely my impression?)

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving