# Can gravitational waves diffract off massive objects?

Can gravitational waves diffract off massive objects?

If so, how much mass is needed to significantly disturb the waves propagation?

It seems to be still a research question for specific situations , but the answer is yes, gravitational waves diffract. example this paper:

Emission of gravitational waves from binary systems in the galactic center and diffraction by star clusters

From the abstract:

The diffraction pattern of gravitational waves emitted by a binary system by a cluster of stars has been also analyzed. We remark that this is only a preliminary-theoretical work than can acquire more interest in view of the next-coming gravitational wave astronomy era.

in the paper:

it suffers diffractive effects while interacting with stars. Hence, by considering each star as a circular slit and applying the well known theory of wave diffraction, we can evaluate the expected diffraction patterns on the observer plane.

Other papers studying diffraction of gravitational waves here and here.

• Circular slit is an open circle enclosed by opaque medium. Is that what you also mean by slit? Or you meant a circular disc instead. In any case, for GW, is there anything opaque? Isn't opaqueness needed for diffraction to take place? Gravitational lensing can apply to GW though.
– kpv
Apr 6, 2016 at 6:17
• Please note that opaqueness (total, or partial) for GW can mean opaqueness for gravity itself. Which is not known to exist as far as I know.
– kpv
Apr 6, 2016 at 6:21
• @kpv It is not I that mean, it is the researchers who wrote the paper and use it as a mathematical analogue. The existence of masses affects the stress energy tensor. An arriving gravitational wave which distorts the stress energy tensor will distort differently where masses exist and that is what the papers study and that is diffraction. Apr 6, 2016 at 6:25
• The paper refers to Babinet principle while talking about star as slit. Babinet principle talks about opaqueness. So, it seems opaqueness is a necessity while discussing diffraction. I am not sure whether increased stress energy tensor can be considered even partially opaque for GW. Just because, opaqueness for GW may also turn out to be opaqueness for gravity itself, I would like to consider it a matter of terminology. I do agree, there will be some effect - I am not sure whether it can be called diffraction in the sense we know it. It sounds more like lensing effect to me.
– kpv
Apr 6, 2016 at 6:59

When light enters a media it gives some energy to its charges and there is a reemission that, added to the initial wave, makes the velocity change, makes the refractive index value. To stretch and expand the earth when passing, in order to move the mirrors of the Ligo interferometer, a similar phenomena may exist. And, of course when traversing other objects in cosmos.

Diffraction is bending (turning) around the corners. The turning is not possible without having slow speed at the edge, and faster speed around it.

For gravitational waves, there is no known theory that says that GW speed changes through (or in proximity of) any matter.

Therefore, no diffraction of gravitational waves.

Even if diffraction is due to reflections caused by the irregularity of the edge, still no diffraction for GW as they are not expected to reflect off anything except probably, the singularity.

Thinking more on the question - Diffraction is a phenomenon of light where it bends around corners (i.e. obstacles). There is no known normal obstacle for gravitational waves, because, they pass through everything. Therefore, the concept of diffraction itself should not apply on gravitational waves. While saying "no obstacle", I have excluded complex entities like black hole/event horizon/singularity.

• I'm not a relativist so I am hesitant to offer an answer, but I suspect this is incomplete. While the local speed of light is invariant, the coordinate speed can vary as measured at locations away from the observer, and this happens particularly in the context of massive objects. That is one way (the easiest?) to understand the Shapiro delay. Apr 6, 2016 at 3:52
• @dmckee: Even though I did not understand the link between the question, and Shapiro delay, your comment made me to re-think. I have added few more lines towards the end of the answer.
– kpv
Apr 6, 2016 at 5:50
• I think this is not correct Apr 6, 2016 at 6:10
• @annav: Per my comment under your answer, I am not certain if diffraction is the right term for the effect, so, I will not delete this answer for the time being.
– kpv
Apr 6, 2016 at 7:05
• @kpv I have linked to three papers where relativity researchers call it diffraction. Apr 6, 2016 at 8:50