View Full Version : Gravity & Light

2003-Oct-17, 03:44 PM
Does gravity bend light toward it or away from it?
Is there any indication that dark matter affects gravity and if so how?

2003-Oct-17, 05:00 PM
Gravity is the curvature of the spacetime caused by mass, and light will then follow a bent path, which would be more toward the mass. Sadly I don't know the nature of dark matter, but it must have mass, since the idea behind it is the gravitational effects we can see.

Sunil Kumar
2003-Oct-18, 12:37 AM
As per Einstein's theory the gravity bends the space time curve such that any mass including light follow the bent curve. For example, earth follows a straight path in space-time grid while orbiting sun, but it appears to have followed a elyptical orbit to us. This is because the sun's gravity has bent the space time. The effect of curvature on earth is higher than that the light, due to its mass. Have a read through Brief History of Time by Hawkings.

Sunil Kumar
2003-Oct-18, 01:46 AM
The dark matter cannot be seen by any means, but however its effect has been registered in the movements of galaxies within the galaxy-cluster. Scientists believe that the mass of dark matter dominates the universe such that more than 99% of the total mass of the universe constitutes dark matter. The suns and galaxies contain only a small fraction of mass of the universe that shines. The dark matter does not radiate any light or radiation that can be detected, that's why they are called 'dark'. There are following clues that suggest the presence of dark matter in the universe:

1) The gravitational effect of dark matter deflects the light from other galaxies like magnifying glass.

2) The mapping of movevements of galaxies within a galaxy-cluster (bubbles) indicates that all the galaxies moves at nearly identical velocity. This implies that there must be a very heavy web of dark matter around the cluster that help regulating the velocity of galaxies, otherwise all the galaxies would crunch. Indeed, the outer web of universe itself consists of dark matter. Hawkings said "searching the dark matter is like searching a black cat in a dark night".

The nature of dark matter is not known. Scientists believe they are neutrinos generated at the time of the birth of universe. They are certainly not the elements which planets and suns are composed of.

2003-Oct-18, 05:17 AM
To add to what other users have said, the fact that light bends towards large masses due to gravity (aka the curvature of space time) has helped astronomers find extrasolar planets. When a sufficiently massive planet moves in front of its parent star into our line of sight, sometimes the light from that star can be bent at just the critical amount so that the light is focussed on us like a lense (increasing the apparent magnitude of a star by a very significant amount). This can also occur with two stars, or a star and some dark matter in between etc. This technique is called gravitational microlensing.

Brian Sand
2003-Oct-18, 06:33 AM
What's up Doc?

The best analogy I have heard that explains the effects of gravity is putting a bowling ball and a golf ball on a bed. The more massive bowling ball leaves a bigger dent in the mattress. The golf ball may even start to roll toward the bowling ball. Anything that comes close to the dent(s) will surely be affected. Depending on the angle and proximity, some may roll into the side of one of the balls or curve.
I obviously didn't do the analogy any justice here. Somebody help me out?

2003-Oct-19, 03:03 AM
Does gravity affect light? Yes.
Does dark matter affect gravity? Yes.

Photons communicate the electrical and magnetic fields
Gravitons communicate the effect of gravity which appears as a field
Gluons communicate the strong force that binds the atomic nuclei together

In addition to gravitons communicating gravity it is believed that space-time is curved by these so that even massless photons are affected.

What makes dark matter hard to detect is that it does not interact with normal matter nor generate photons.

In observing the rotation rate as a function of radius in nearby galaxies it has been shown that the actual motions of stars do not follow conventional mass distribution assumptions. When "dark matter" is added the discrepancy seems to be solved.

Now we just have to find it.

I attended a lecture on one possible approach where they assumed that there was a specific amount attributed to our own solar system and its dynamics.