# The Naked Scientists Forum

### Author Topic: How can it possible to detect gravitational waves?  (Read 1772 times)

#### Astrogazer

• First timers
• Posts: 5
##### How can it possible to detect gravitational waves?
« on: 12/02/2013 23:06:39 »
How can it possible to detect gravitational waves when the basis (by basis I meant space-time) of our measurement stretches in sympathy by exactly the same amount as any gravity waves there may be would stretch our basis for measurement.  Badly put, ok.  We commonly picture space-time as the rubber surface of a trampoline.  We paint on this surface lines such that from the centre to the edge is 100 units.  We check it with a laser (our laser is moving through the space-time of our rubber trampoline) and yes its 100 units.  We then pile on some bricks in the middle, we measure the distance from the edge to the middle by counting the grid lines and yes its still 100 units, we check with our laser which mirrors the rubber surface (oops I mean space-time) and yes its still 100 units.  We now bounce the bricks up and down and yes at all times there is still exactly 100 units from the edge to the centre.  If this analogy is correct, we could be buffeted 'all over the place' by gravity waves and we wouldn't feel a thing, nor measure anything either. Is this why physicists are struggling to detect gravity waves?  So what do you all think?
« Last Edit: 13/02/2013 20:13:06 by Astrogazer »

#### CliffordK

• Neilep Level Member
• Posts: 6321
• Thanked: 3 times
• Site Moderator
##### Re: How can it possible to detect gravitational waves?
« Reply #1 on: 12/02/2013 23:47:14 »
It may depend on what exactly a gravitational wave is.

Say you have a Newton's Cradle, but with just 2 balls, one on each end, suspended from a rigid frame.
Assume the wave front sequentially hits the first ball, then the second one.

When the wave front hits, the first ball gets accelerated in a slightly different direction with respect to everything else.  Then, a period of time later, the second ball gets accelerated with a similar acceleration factor.  Of course the device itself would get accelerated, but if it was rigid, it may move independently of the balls.

This could be detected by your two balls in the Newton's Cradle getting slightly further apart, then eventually returning to the same relative resting place.

There are many questions, such as how fast the wave front will be propagating, the magnitude and duration of the phenomenon, and the number of waves.

Now, say you are trying to measure the distance between two objects.  And it gets hit by a gravitational wave traveling the speed of light.  Even if you space the two objects a few light hours apart, then it may not be possible to detect the change as the recording device would always be in phase with the transmitting device.

There are also many things that can mess with our current generation crystal and atomic clocks, no doubt a gravity wave would affect them too.

Is there any way to design to detector to create torsion in the system?

Say you have 3 satellites.  A source shooting a laser at a mirror, which then reflects the light to a detector.  If you get just a little bit of torsion anywhere in the system, then one misses the target.  Could mounting everything on dumbbells be enough to create torsion?  One wouldn't even need to have a clock, as the whole device would depend on where the lasers hit the targets, and if there were any saccades, or drift.
« Last Edit: 12/02/2013 23:50:14 by CliffordK »

#### Spacetectonics

• Full Member
• Posts: 85
##### Re: How can it possible to detect gravitational waves?
« Reply #2 on: 13/02/2013 17:00:36 »
binary systems :)

#### Astrogazer

• First timers
• Posts: 5
##### Re: How can it possible to detect gravitational waves?
« Reply #3 on: 13/02/2013 20:56:46 »
What is binary systems with respects to gravitational waves?

In response to the 3 satellite set up where a laser is fired from one, hits a second then is detected (or not) by the third.  In my mind experiment I have just fired the laser and its on its way to the satellite containing the mirror.  Mid flight a wave arrives puts a kink in space-time.  Surely the laser would necessarily have to follow the kink and thus it would hit the mirror at which it was originally targeted resulting in no detection? I can't think of any setup that would detect either an angular change or a change in length where the measurement tools are confined to the same frame of reference.

#### Spacetectonics

• Full Member
• Posts: 85
##### Re: How can it possible to detect gravitational waves?
« Reply #4 on: 14/02/2013 03:35:13 »

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: How can it possible to detect gravitational waves?
« Reply #5 on: 16/02/2013 11:23:46 »
The idea is that with three lasers, constantly keeping themselves updated in distance relative each other, you get yourself a very exact measuring system. If gravitational waves exist they also will deform the space in between those lasers, redefining the time it takes for light to propagate between those lasers. So it has not to do with the lasers themselves getting deformed, but with the measurement of 'space/distance' between them.

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: How can it possible to detect gravitational waves?
« Reply #6 on: 16/02/2013 11:45:15 »
Maybe you meant that the light won't be able to reach the measuring device as it will be bent out of its original path due to that gravitational wave? Imagine a triangle of those lasers in space, the idea is that they measure 'space' in between them. A better solution should be six lasers, I think? Measuring each other, as you then cover a cube of 'space'. Take a look here The rhythm of geometry.
==

'Cube' is not the correct word here as you need 8 endpoints for that, but 6 should be sufficient I think, as you would get a 3D representation of that space. Don't remember the word describing it though :)
« Last Edit: 16/02/2013 12:02:09 by yor_on »

#### stuver

• First timers
• Posts: 1
##### Re: How can it possible to detect gravitational waves?
« Reply #7 on: 18/02/2013 07:27:30 »
This is an excellent question!

Let me introduce myself.  I am Amber Stuver and I am a scientist at the LIGO Livingston Observatory in Louisiana.  I love answering questions like this and do so on my blog Living LIGO.

I recently wrote a blog post addressing this question about how LIGO will be able to detect gravitational waves using light even though light will be stretched and compressed by a gravitational wave just like the detector itself.  The solution is not to think of the laser light as a ruler but instead think of it as a clock with each wave being a "tick".  When the light is split into parts for each of the arms, the location & timing of the light waves' peaks are in sync in each arm.  If the arms change length, the spacing of the wave peaks will change as well as the timing that they arrive back at the detector corner to recombine.  It is this change in arrival times of the wave peaks that produces the interference pattern, not the change in the wavelength of the light.

I know that this is not a very detailed explanation, but I wanted to give you a summary of the basics.  I have a more complete answer on my blog here:

Please let me know if you have any more questions, about this subject or anything else about gravitational waves!  I am more than happy to answer whatever I can!

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: How can it possible to detect gravitational waves?
« Reply #8 on: 18/02/2013 10:46:36 »
Very nice Stuver, and welcome :)

#### The Naked Scientists Forum

##### Re: How can it possible to detect gravitational waves?
« Reply #8 on: 18/02/2013 10:46:36 »