Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: cowlinator on 18/12/2016 23:25:34

Title: Can you "ride" a gravitational wave?
Post by: cowlinator on 18/12/2016 23:25:34
Assume we defined a moving point in spacetime that "rides" a gravitational wave, at the speed of light.  This point would always be between a valley and peak of the amplitude of the wave. 

Would this point experience contracting space in front of it and expanding space behind it?

If so, wouldn't this point experience all of the same effects as a point within the bubble of an Alcubierre drive?
Title: Re: Can you "ride" a gravitational wave?
Post by: evan_au on 19/12/2016 11:00:36
Relativity suggests that no material object can be accelerated up to the speed of light, so no material object would be able to "ride" a gravitational wave.
Title: Re: Can you "ride" a gravitational wave?
Post by: cowlinator on 22/12/2016 01:55:08
But we're not talking about material objects, we're talking about a point in space.

If it helps, let's say it's about a photon instead.
Title: Re: Can you "ride" a gravitational wave?
Post by: evan_au on 22/12/2016 10:23:45
Quote from: cowlinator
If it helps, let's say it's about a photon instead.
A photon travels at c even without a gravitational wave. It is not clear to me how it would accelerate the photon in an ongoing manner.

However, there is one area where (bunches of) photons are thought to interact with gravitational waves - in the search for galactic core black hole mergers (ie extremely low frequency gravitational waves).

Astronomers are monitoring the pulse timing of numerous pulsars, using radio telescopes and atomic clocks. They calculate that gravitational waves from merging supermassive black holes would alternately stretch, squeeze and twist space, so the timing of pulses from distant pulsars would get alternately stretched out and then compressed (by 20ns or so), over periods of years.

By monitoring many pulsars in many directions in space, they hope to extract correlated variations in timing, perhaps enabling them to calculate the mass, separation and distance of orbiting black holes.

See: https://en.wikipedia.org/wiki/Pulsar_timing_array
Title: Re: Can you "ride" a gravitational wave?
Post by: evan_au on 23/12/2016 20:43:21
Quote
Can you "ride" a gravitational wave?
I saw a case where a gravitational wave can impart linear momentum to a physical object: When two black holes combine, the emitted gravitational waves carry considerable amount of momentum. Depending on factors like the angular momentum of the black holes, it is possible for the black holes themselves to experience a considerable asymmetrical force, imparting momentum to counteract the momentum carried away by the gravitational waves - enough to eject the combined black hole from a galaxy, in extreme cases.

And if the gravitational waves are carrying momentum away from the black hole merger, I guess it is possible that they might impart some linear momentum to nearby objects, driving them away from the site of the black hole merger.
See: https://en.wikipedia.org/wiki/Binary_black_hole#Black-hole_merger_recoil

Quote
a point within the bubble of an Alcubierre drive?
One of the goals of the Alcubierre drive is that it would allow faster-than-light travel. Gravitational waves only travel at the speed of light.

The regions of compressed and expanded space need to be at a fixed position relative to the spaceship. But gravitational waves always travel at the speed of light relative to the spacecraft, which means that you can't get "inside" the bubble and stay there.

See: https://en.wikipedia.org/wiki/Alcubierre_drive
Title: Re: Can you "ride" a gravitational wave?
Post by: yor_on on 31/12/2016 09:35:09
No, a 'gravitational wave' is 'SpaceTime', it's not a wave on a sea that you surf on, its the fabric of the universe. It 'distorts' us momentarily as we pass through it in time, as described from some other frame of reference inside. I don't see how one should/could 'ride it'.
Title: Re: Can you "ride" a gravitational wave?
Post by: Professor Mega-Mind on 04/11/2018 13:38:18
The analogy to movement by water waves ( S-waves ) does hold , and here is why : the mass moving (black holes revolving) gives up a tiny amount of kinetic energy to the G-waves .  The mass receiving the waves absorbs a tiny bit of their energy .  Each process is a dis-proportionate mirror image of the other . 
The real question here is : do grav. waves affect particles exactly the same as large objects ?
Ponder it and speak .
P.M.
Title: Re: Can you "ride" a gravitational wave?
Post by: yor_on on 04/11/2018 15:49:20
No
Title: Re: Can you "ride" a gravitational wave?
Post by: Professor Mega-Mind on 04/11/2018 16:45:50
 Brave man , yor_on !
 I'm sure you've seen small objects moved all over the place by small waves , while those same waves did nothing to the ship floating nearby .  I propose a similar effect occurs in space w/gravity waves , but the small objects are sub-atomic particles and atoms .  Ever-higher temperatures  ( motions ) means ever-higher mass (E=MC2) .  If the waves themselves have virtual mass in a manner similar to EM-waves , then you have a 2-fer for massive object DM halos !
Go for it , Gridley !
P.M.
Title: Re: Can you "ride" a gravitational wave?
Post by: yor_on on 04/11/2018 17:26:06
Still no :)
=

that is, answering the question Mr Mega Mind
Title: Re: Can you "ride" a gravitational wave?
Post by: guest46746 on 04/11/2018 17:59:18
I think that it would be better suited to propel the point behind the gravity wave's wake. If the gravity wave possessed an attraction force, it would literally pull everything behind it along. duh! lol
Title: Re: Can you "ride" a gravitational wave?
Post by: Professor Mega-Mind on 04/11/2018 19:19:49
Gravity waves travel at lightspeed , everything else cannot .  I speak of artificially placed fields anyway .
P.
Title: Re: Can you "ride" a gravitational wave?
Post by: guest46746 on 04/11/2018 21:02:09
Gravity waves travel at lightspeed , everything else cannot .  I speak of artificially placed fields anyway .

Your references to virtual fields can be applicable in this situation. I failed to see why you couldn't see it's application here! duh lol
Title: Re: Can you "ride" a gravitational wave?
Post by: guest46746 on 04/11/2018 21:06:03
Gravity waves travel at lightspeed , everything else cannot .  I speak of artificially placed fields anyway .

Your references to virtual fields can be applicable in this situation. I failed to see why you couldn't see it's application here! duh lol


edit Virtual mass for virtual fields!  lol
Title: Re: Can you "ride" a gravitational wave?
Post by: Professor Mega-Mind on 04/11/2018 21:53:27
  ....I will clarify my meaning .
Gravity waves travel at lightspeed , no MATTER can .  The artificial fields mentioned would be placed in front/back of ship to provide attraction/repulsion for propulsion.  My recommendation is that EM manipulation be pursued over gravitational manipulation .  We can do the first well , but not the second . 
Y'all got it ? Guud !
P.M.
Title: Re: Can you "ride" a gravitational wave?
Post by: yor_on on 05/11/2018 08:44:12
If you think of it cowlinator, and presume that gravity is a field existing even when its value is nul, meaning that a 'test particle' inside this field will find no forces acting upon it, then everything is part of this field. And a 'gravitational wave' would then be a 'distortion' flowing through that field. It 'flows' through SpaceTime, and it 'flows' through you.
Title: Re: Can you "ride" a gravitational wave?
Post by: Professor Mega-Mind on 05/11/2018 14:21:57
Now you see it !  The gravitational forces present in space tend to balance out .  The field is powerful , yet objects experience little or no force acting upon them .
P.
Title: Re: Can you "ride" a gravitational wave?
Post by: Colin2B on 05/11/2018 15:13:21
The analogy to movement by water waves ( S-waves ) does hold
Water waves are not S waves, they are gravity waves ie gravity is the restoring force for the water molecules. Itís worth looking at why you can ride a gravity wave to see whether the analogy with gravitational waves does hold.

Iím not aware of this description in any textbooks, but imagine a surfer on the front face of a water wave. If the wave were static the surfer would slide down the slope into the trough, but if you look ahead of the surfer you can see that the water level is rising, so the surfer is being raised up the slope at the same time as sliding down it, hence riding the wave. Itís important to remember that the water does not go forward, pushing the surfer, but only goes up and down - variation of amplitude. (Note: this is not strictly true, the water wave is a combination of longitudinal and transverse wave so the water molecules describe a circle. If you place an untethered buoy on the surface it will move up and down and forwards and backwards with no net forward motion.)

So the only reason surfers move forward is because gravity pulls them down the slope as buoyancy pushes them up. With a gravitational wave there is no external force equivalent to gravity, so it is hard to see how you might ride the wave.
As the gravitational wave passes by, any particles in the area will be displaced first one way (call it up) and then the other (call it down) perpendicular to the wavefront - bit more complex than this depending on the wave polarity because you get Ďsidewaysí movement as well for some particles. This is equivalent to the buoy on the water bobbing up and down.
Can you harness this bobbing in the same way as the surfer does? External force = ?
Title: Re: Can you "ride" a gravitational wave?
Post by: yor_on on 05/11/2018 16:41:27
a very nice answer Colin. I should have avoided 'flow' as it seems to give different thoughts depending on ones view of it.
Title: Re: Can you "ride" a gravitational wave?
Post by: Professor Mega-Mind on 05/11/2018 17:12:07
Excellent logic , Mr. C.
The answer is tidal .  Much as two stars spiraling in towards each other push an outside body further out ( Proxima ? ) , 2 BHs do it to the  nth degree .  Your argument does fold with mine ; passing G-waves are useless for propulsion , but good for stimulating particles to high temperature . 
Note-The better analogy would have been P-Waves .
....P.M.
Title: Re: Can you "ride" a gravitational wave?
Post by: Colin2B on 06/11/2018 14:48:03
I should have avoided 'flow' as it seems to give different thoughts depending on ones view of it.
Flow is ok. If you think of it as variations of gravitational field the particles would tend to flow down the gradient.

Note-The better analogy would have been P-Waves .
....P.M.
P waves are longitudinal compression similar to sound waves. Remember with gravitational waves you need to think transverse-transverse quadrupole waves

Title: Re: Can you "ride" a gravitational wave?
Post by: Professor Mega-Mind on 06/11/2018 15:45:29
I belieb ya , somebody tell Franklin!
In truth , he ain't makin' any waves !
D.