Gravitational Effects on Light
Most recent answer: 02/13/2011
- Farhan (age 14)
You have asked some very interesting questions!
1) Gravity does indeed affect light. All light in the presence of a gravitational source either "bends" or shifts its frequency, but unless the gravitational field is extremely strong it's difficult or impossible to detect with the naked eye. Using precise instruments, we can measure the light from a star and determine this effect, which gives us information about the star's gravitational field.
2) Stars, including our sun, are extremely massive but not massive enough to trap light in its gravitational field. That doesn't mean they do not bend light. Some stars actually bend light so much that, were they not millions and billions of lightyears away, we would definitely notice something funky going on. Neutron stars are the densest stars that we are aware of, and if you were a reasonable distance away from one you would be able to see more than half of the star at any time!
That's hopefully a better visual explanation of why you would be able to see more than half of a neutron star. From a "head on" perspective, a neutron star would look like this:
3) There are objects in the universe, however, which have a strong enough gravitational field that no light can escape from a certain region around its center! These objects are known as Black Holes.
4) Light does exert force on other objects, too! That is to say, when you shine a flashlight or a laser on a wall, the light pushes on the wall! Don't get too worried, though. The force is usually very small--powerful lasers exert a force around ~10^-9 Newtons (about the weight of a grain of sand).
I hope this answers some of your questions! You're doing some great thinking.
(published on 02/13/2011)
Follow-Up #1: Gravitational waves and LIGO detection
- Chloe Quayle (age 57)
A gravity wave, like an electromagnetic wave, has only transverse components. It does not affect distances in the direction of its travel. So if a gravity wave strikes the earth perpendicularly to the earth's surface then the north-south and east-west components will alternatively shrink and expand. The vertical component is not affected. Since the arms of the LIGO detector are aligned in the NS-EW directions they will see oscillations in their differences.
On the great question of why the scale changes of the apparatus and the light waves don't just cancel, here's a nice article: http://scitation.aip.org/content/aapt/journal/ajp/65/6/10.1119/1.18578. The basic idea is that if you had a gravitational wave affecting both the apparatus and the pre-existing light-waves in it, the effects would initially cancel, just as you suspected. In the actual set-ups, the light waves are replaced with new ones generated from the lasers, and these new waves are not stretched by the gravitational wave. So the number of wavelengths of these new light waves that fit in the apparatus does change. It turns out that in between the initial effect (light and apparatus change together, so no phase shift) and the final effect (new light waves, full phase shift) there's a gradual onset of the phase shift due to the increased travel time of the waves inside the stretched arm. That has an interesting implication. A LIGO-type detector would not work for high-freqency gravity waves, ones for which the travel time inside the apparatus is small compared to the wave period. The highest frequencies of the recently observed chirps, however, were below that limit. Mike W.
(published on 03/04/2016)
Follow-Up #2: LIGO and gravity wave detection
It's actually kind of the reverse. The wave does nothing to the relative position of a wave crest just reaching a mirror. They stay at the same place as each other. It's later crests that weren't near the mirrors that then have to travel farther in one arm than the other arm and thus don't arrive quite in phase with each other.
We found this question hard until we read this nice link: http://scitation.aip.org/content/aapt/journal/ajp/65/6/10.1119/1.18578.
(published on 04/20/2016)
Follow-Up #3: clarifying LIGO
The wave crests we were discussing were those of the light.
As I understand it, the gravitational wave affects (to lowest order) only the spatial coordinates, not the time coordinates. So one arm of the interferometer becomes shorter and the other becomes longer- and so do the light waves initially within those arms. What do we mean by changing "length" in a case where meter sticks change along with everything else? What we mean here is essentially the length of time that it would take a light wave to travel back and forth through that arm. That's what changes. The gravity wave makes it take longer for the light waves to travel along one arm than the other. So if the two light waves are initially exactly out of phase at the detector, the different travel times cause them to get a little bit in phase there. That's why the gravity wave causes some light to be detected.
(published on 04/21/2016)