# Q & A: Gravitational effects on light

Q:
Gravity affects light, yes? Then how come the sun(or stars for that matter) gives out light? it has a very strong gravitational force. shouldn't the light be bent or something? light reaches perfectly to our eyes even if the stars were billions of lightyears away. consequently, does light(or waves) have force?
- Farhan (age 14)
Brunei
A:
Dear Fahran,

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!

(Source: Freehand!)
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:

(Source: )

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.

Sincerely,

John

(published on 02/13/2011)

## Follow-Up #1: Gravitational waves and LIGO detection

Q:
When a gravity wave affects distance, why doesn't the laser light's path distance in wavelengths become equally affected? In other words, why doesn't the LIGO experiment "cancel itself out" ? Thanks in advance
- Chloe Quayle (age 57)
Southport, UK
A:

Hello Chloe,

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.

Lee H

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

Q:
Background: Detection of gravity waves is done by syncing the interference pattern of two laser beams received at a detector -- the laser beams are reflected off two mirrors set at 90 degrees angle. Gravity waves shorten the path of one laser beam resulting in an out-of sync interference. The amount of out-of sync is then related to the strength of the gravity wave. My question is: Does the out-of sync happen momentarily as the gravity wave hits one of the mirrors, or does it happen continuously as the wave travels the (space) path between the mirror and the detector and, due to contraction of space at the gravity wave front shorten the path until the gravity wave has passed?
- Lee
A:

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.

Mike W.

(published on 04/20/2016)

## Follow-Up #3: clarifying LIGO

Q:
I checked the link; it was not clear to me -- above my pay grade! Also I could not understand your explanation "... 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..." It seems you are talking about two different waves. Are they both gw's (gravitation waves) or laser beam waves? Let me expand on your depiction of the situation (if I understand it correctly): You have a SINGLE gw crest that reaches a mirror and then travels the length of the mirror-detector arm; and you have CONTINUOUS laser beam that is traveling from the detector to the mirror and back to the detector. The gw crest shortens (or lengthens?) the space it occupies. The laser beam is passing through the same crest-space as the gw crest, and at the same speed c as gw, hence it never clears the crest-space. Well, all of these get too complicated to draw a clear picture. Could you respond possibly with several diagrams (as you did earlier in this thread) and show what happens to laser beam as it hits a moving gw crest. The way I see is that the laser beam is affected only when it travels opposite gw. The laser beam traveling in the same direction as gw stays at the same speed (c). Of course the picture is even more complicated since, more than likely, the laser beam and gw travel at some angle with respect to each other -- not along or exactly opposite, the assumptions which I made to keep things simple and to get to the gist of question. Also, even for the laser beam in the opposite direction as gw, doesn't the crest-space contraction and time dilation cancel each other's effect, i.e., slow clock measuring a short path? Here are the root questions: Does the gw-crest contract / dilate the space? How does the laser beam going through this crest get affected? How does the mass (mirror) react to the gw-crest? I know this is a grab bag of questions and possibly requires detailed explanation and may be even some relativistic computations. But then you folks may come up with some commonsensical answer.
- Anonymous
A:

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.

Mike W.

(published on 04/21/2016)