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+--- Thread: Gravitational Waves Should Permanently Distort Space-Time [NOT!] (/thread-13214.html)
Gravitational Waves Should Permanently Distort Space-Time [NOT!] - Kornee - Nov 16, 2022
https://www.quantamagazine.org/gravitational-waves-should-permanently-distort-space-time-20211208/
Gravitational Waves Should Permanently Distort Space-Time
"The “gravitational memory effect” predicts that a passing gravitational wave should forever alter the structure of space-time. Physicists have linked the phenomenon to fundamental cosmic symmetries and a potential solution to the black hole information paradox."
I knew that headline and summary had to be utter nonsense, but YouTube vids on it either disagreed among themselves or were full of GR arcane math lingo to wade through. Wikipedia proved to be the best, but still only as a brief summary:
https://en.wikipedia.org/wiki/Gravitational_memory_effect
"Gravitational memory effects, also known as gravitational-wave memory effects are predicted persistent changes in the relative position of pairs of masses in space due to the passing of a gravitational wave....
Detection
The effect should, in theory, be detectable by recording changes in the distance between pairs of free-falling objects in spacetime before and after the passage of gravitational waves...."
So not at all a 'permanent distortion of spacetime', but simply a bit surprising residual displacement - i.e. effect of GW jigglings do not exactly cancel out after the wave has completely passed through.
Presumably a reflection of the highly unsymmetric generation of GWs owing to final stage inspiral of BH-BH, NS-NS etc. binary pairs. And 'memory effect' persistence strictly applies only to free-falling test masses very far from any perturbing effects of net gravitational attraction from other masses. Hence the test masses themselves must be sufficiently small and well distanced from each other.
RE: Gravitational Waves Should Permanently Distort Space-Time [NOT!] - confused2 - Nov 17, 2022
(Nov 16, 2022 11:59 AM)Kornee Wrote: https://www.quantamagazine.org/gravitational-waves-should-permanently-distort-space-time-20211208/Is the ground shifting from 'permanent change in spacetime' to permanent change in stuff embedded in spacetime?
Gravitational Waves Should Permanently Distort Space-Time
"The “gravitational memory effect” predicts that a passing gravitational wave should forever alter the structure of space-time. Physicists have linked the phenomenon to fundamental cosmic symmetries and a potential solution to the black hole information paradox."
I knew that headline and summary had to be utter nonsense, but YouTube vids on it either disagreed among themselves or were full of GR arcane math lingo to wade through. Wikipedia proved to be the best, but still only as a brief summary:
https://en.wikipedia.org/wiki/Gravitational_memory_effect
"Gravitational memory effects, also known as gravitational-wave memory effects are predicted persistent changes in the relative position of pairs of masses in space due to the passing of a gravitational wave....
Detection
The effect should, in theory, be detectable by recording changes in the distance between pairs of free-falling objects in spacetime before and after the passage of gravitational waves...."
So not at all a 'permanent distortion of spacetime', but simply a bit surprising residual displacement - i.e. effect of GW jigglings do not exactly cancel out after the wave has completely passed through.
Presumably a reflection of the highly unsymmetric generation of GWs owing to final stage inspiral of BH-BH, NS-NS etc. binary pairs. And 'memory effect' persistence strictly applies only to free-falling test masses very far from any perturbing effects of net gravitational attraction from other masses. Hence the test masses themselves must be sufficiently small and well distanced from each other.
Assuming the latter..
I'd be tempted to tackle it obliquely.
Taking care to separate near from far field effects (we only want the far field) ..
If you were in the path of a humungous gravity wave would you survive?
If death occurs then effect probably persistent otherwise transient.
My model for a wave generator (elec eng) involves a positive/negative mass dipole which is a bit unphysical...
I'd go (50-50) with a huge short wavelength GW wave (far field) being fatal in a permanent (memorable) way.
RE: Gravitational Waves Should Permanently Distort Space-Time [NOT!] - Kornee - Nov 17, 2022
(Nov 17, 2022 12:34 AM)confused2 Wrote: Is the ground shifting from 'permanent change in spacetime' to permanent change in stuff embedded in spacetime?The sole permanent effect on space itself of a passing GW is trivially the reduced gravitational net mass of the original source. Over typically billions of light years distance, absolutely imperceptible. The same situation for say the passing EM radiation & neutrino flux from a star that goes supernova.
Assuming the latter..
I'd be tempted to tackle it obliquely.
Taking care to separate near from far field effects (we only want the far field) ..
If you were in the path of a humungous gravity wave would you survive?
If death occurs then effect probably persistent otherwise transient.
My model for a wave generator (elec eng) involves a positive/negative mass dipole which is a bit unphysical...
I'd go (50-50) with a huge short wavelength GW wave (far field) being fatal in a permanent (memorable) way.
What surprises a little for the 'gravitational memory effect' is the net transverse displacements left behind - not due to any absurd idea of permanent change in spatial shear, but an inherent unevenness in the oscillatory induced motions. Claiming space has a structure that can be permanently altered by a passing GW is like claiming throwing a stone into a pond will leave behind permanent surface ripples. No.
Regarding potential for being damaged by intense GWs, see first reply here:
https://www.quora.com/How-close-to-earth-should-a-gravitational-waves-event-happen-to-be-felt-by-humans
and here:
https://www.quora.com/Theoretical-Physics-Is-gravitational-wave-harmful-If-so-under-what-conditions-it-could-cause-death-when-it-passes-a-human-body
PS - In standard GR there can be no dipole GWs. The lowest allowed is quadrupole moment waves - which is evidently all that has been observed so far.
An alternate vector theory of gravity has claimed it can better explain the results of GW data, but still has quadrupole source waves:
https://arxiv.org/abs/1804.03520
Last time I corresponded with Hilborn - maybe two years ago, they were maintaining their stance.