tag:blogger.com,1999:blog-22973357.post809427622987608606..comments2020-12-02T10:40:31.851-05:00Comments on Sabine Hossenfelder: Backreaction: Everything you need to know about gravitational wavesSabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger74125tag:blogger.com,1999:blog-22973357.post-22798467742887869162016-02-15T19:39:33.438-05:002016-02-15T19:39:33.438-05:00Dan Hoak said: "From this we find 36 + 29 --&...Dan Hoak said: "From this we find 36 + 29 --> 62, leaving 3 solar masses left over."<br />When I learned special relativity, we were repeatedly warned that rest masses are not additive, particularly for objects moving very rapidly relative to each other. So this is not right as it stands [and it is not what the LIGO people are really saying, though one could be forgiven for thinking Rastus Odinga Odingahttps://www.blogger.com/profile/09615544434035028500noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-1688821095351044502016-02-15T12:48:08.013-05:002016-02-15T12:48:08.013-05:00Thank you Dan Hoak, for your clear comment.
A ques...Thank you Dan Hoak, for your clear comment.<br />A question (for you and Sabine) :so it is established that the difference is 3 solar masses, expelled in the form of gravitational waves, as energy. But could it be that the merger caused the waves without having the black holes lost any mass ? Meaning that the resultng BH still has 65 solar masses, indicating that the caused energy waves has a Koenraad Van Spaendonckhttps://www.blogger.com/profile/15090279727324831109noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-11448040408902581912016-02-15T09:50:05.691-05:002016-02-15T09:50:05.691-05:00Tobias,
Please read the previous comments, this q...Tobias,<br /><br />Please read the previous comments, this question has now been asked several times. It does stretch the laser's wave-length, but this does not lead to a net zero effect. For a more detailed reply, have a look at the references Shantanu mentioned<br /><br />http://arxiv.org/abs/gr-qc/0702079<br />http://arxiv.org/abs/gr-qc/0511083Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-70478776312250642382016-02-15T09:41:16.407-05:002016-02-15T09:41:16.407-05:00hi sabine, why does the gravitational wave not str...hi sabine, why does the gravitational wave not stretch the laser photon wavelength as happens with the universal expansion of spacetime? t kosubhttps://www.blogger.com/profile/13956052025472802645noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-10024451738757461652016-02-15T08:31:12.233-05:002016-02-15T08:31:12.233-05:00LucyM, Willem-Jan,
Gravitational waves are small ...LucyM, Willem-Jan,<br /><br />Gravitational waves are small perturbations in the background that is space-time. The background is still there. The perturbations and the background can behave differently. The speed of the perturbations is bounded by the speed of light, the expansion of the background is an entirely different thing. <br /><br />You can think of the relation between the two like theSabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-79565848806102792232016-02-15T08:19:39.524-05:002016-02-15T08:19:39.524-05:00Isn't the difference that in cosmic expansion ...Isn't the difference that in cosmic expansion new space is created between any two points in space, so there is no stretching in this case, where as gravitational waves are indeed a propagating sequence of expansions and contractions?Matatanhttps://www.blogger.com/profile/02002162452634026282noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-88429539799394764872016-02-15T06:53:28.220-05:002016-02-15T06:53:28.220-05:00-w.
I have a longer post on this upcoming on For...-w. <br /><br />I have a longer post on this upcoming on Forbes tomorrow :o)Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-51084447780393928032016-02-15T06:52:13.829-05:002016-02-15T06:52:13.829-05:00Dan - thanks, I had totally lost track of this. Dan - thanks, I had totally lost track of this. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-20597261583193715702016-02-15T05:25:34.489-05:002016-02-15T05:25:34.489-05:00Very seldom, but still I have found gravitational ...Very seldom, but still I have found gravitational waves referred to as gravitational radiation. In my - limited - understanding that synonym doesn't hold at this point: gravitational waves are distortions of space time that have been detected by LIGO, confirming theory. Gravitons though have not been detected and therefore are still theoretical (and not within general relativity either of -w.https://www.blogger.com/profile/16342060829077787054noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-47947355441634291422016-02-15T04:24:32.626-05:002016-02-15T04:24:32.626-05:00Dear Noa (question about radiated energy and BH ma...Dear Noa (question about radiated energy and BH masses):<br /><br />The masses of the two black holes in the system are estimated using the first, "inspiral" portion of the waveform. The frequency evolution of this portion of the waveform, as the BHs orbit each other and move closer together, is determined by their individual masses.<br /><br />The mass of the final black hole is Dan Hoakhttps://www.blogger.com/profile/16491891046845316059noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-52673649295873105652016-02-14T19:00:47.666-05:002016-02-14T19:00:47.666-05:00Sabine says "Please see the references Shanta...Sabine says "Please see the references Shantanu mentions above, they explain it in much detail. I don't know how 'bootstrapping a restoring force' is of any use. There's an equation, it has solutions, these solutions describe waves. What else do you need?" <br /><br />I thought his question about the speed of light was interesting. I mean, another complication in the Anonymoushttps://www.blogger.com/profile/17852247942652368610noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-9520422686449095172016-02-14T18:56:40.099-05:002016-02-14T18:56:40.099-05:00If all mass generates gravity waves, and there'...If all mass generates gravity waves, and there's lots of black hole mergers, then surely spacetime is like the ocean. How does the waves of two merging black holes make it all the way here without being cut by waves from other mergers?Anonymoushttps://www.blogger.com/profile/17852247942652368610noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-1725612586045135042016-02-14T03:31:37.384-05:002016-02-14T03:31:37.384-05:00Arun. This is very small (<10^-5). The limit co...Arun. This is very small (<10^-5). The limit comes from BBN.<br />Check out<br />http://arxiv.org/abs/astro-ph/0608606 (for one paper on this)<br />ShantanuShantanuhttps://www.blogger.com/profile/16322812456382858228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-60999957032843943992016-02-14T03:22:20.031-05:002016-02-14T03:22:20.031-05:00Unknown:
Because the LIGO arm is not a photon be...Unknown: <br /><br />Because the LIGO arm is not a photon beam. Please see the references Shantanu mentions above, they explain it in much detail. I don't know how 'bootstrapping a restoring force' is of any use. There's an equation, it has solutions, these solutions describe waves. What else do you need? The speed is the same because all massless particles (or excitations) have Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-70631072402702658462016-02-13T23:39:25.414-05:002016-02-13T23:39:25.414-05:00What is the upper limit of the contribution to the...What is the upper limit of the contribution to the average density of the universe from the energy of gravitational waves? <br />Thanks in advance!Arunhttps://www.blogger.com/profile/03451666670728177970noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-38869523828806387522016-02-13T22:20:47.653-05:002016-02-13T22:20:47.653-05:00Layman questions -- all waves need something to pl...Layman questions -- all waves need something to play the role of a restoring force. As I understand it, even EM waves do it in a cheating kind of way with perpendicular electric and magnetic waves bootstrapping each other according to Maxwell's equations. What does it for gravitational waves? I studied a bit of GR at undergraduate level, so I've heard of the stress-energy tensor. Can it pshttps://www.blogger.com/profile/00467177087822511037noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-85771244433533845382016-02-13T16:29:21.053-05:002016-02-13T16:29:21.053-05:00This is probably me being stupid, but what I think...This is probably me being stupid, but what I think Unknown is saying is that if the cosmological red-shift is caused by the stretching of photon wavelengths due to the stretching of space, then there should be no net measurable interference effect, regardless of the timing between the arms -since the number of waves in a given amount of space remains constant both in expansion and contraction. JimVhttps://www.blogger.com/profile/10198704789965278981noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-47130208292105876312016-02-13T14:39:26.277-05:002016-02-13T14:39:26.277-05:00Thank you for the link Bee. I've read Scott...Thank you for the link Bee. I've read Scott's post now & the comments beneath. MMichael Fisherhttps://www.blogger.com/profile/14278512837240256168noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-75826962112597104542016-02-13T13:18:18.823-05:002016-02-13T13:18:18.823-05:00Unknown, your question is discussed in
http://arx...Unknown, your question is discussed in <br />http://arxiv.org/abs/gr-qc/0702079<br />and <br />http://arxiv.org/abs/gr-qc/0511083<br />and references thereinShantanuhttps://www.blogger.com/profile/16322812456382858228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-77075158647964007322016-02-13T12:56:10.141-05:002016-02-13T12:56:10.141-05:00Unknown,
The point is that the two arms are not e...Unknown,<br /><br />The point is that the two arms are not expanded and compressed at the same time. The interferometer measures the way two orthogonal directions behave differently. A gravitational wave is not an isotropic expansion/compression. <a href="https://upload.wikimedia.org/wikipedia/commons/b/b8/GravitationalWave_CrossPolarization.gif" rel="nofollow">See eg this gif</a>.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-89540024531575972572016-02-13T11:25:40.979-05:002016-02-13T11:25:40.979-05:00Ok, so a gravitational wave expands & compress...Ok, so a gravitational wave expands & compresses the length of one of the LIGO arms, say X %.<br />So why isn't the photon wavelength expanded & compressed by exactly the same X%, thereby resulting in a null signal?<br /><br />Expansion of space creates cosmological red shift: photon wavelength increases.<br />The gravitational wave also expands space momentarily -- why doesn't Unknownhttps://www.blogger.com/profile/14136922143569965933noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-53488982241827212712016-02-13T11:07:47.597-05:002016-02-13T11:07:47.597-05:00Btw in case any journalist is reading this, I am s...Btw in case any journalist is reading this, I am surprised there is no interview of Russell Hulse or Joe Taylor on this discovery.<br />After all it was their discovery of PSR1913B+16 and indirect evidence for GW which helped the case of building LIGO<br />shantanuShantanuhttps://www.blogger.com/profile/16322812456382858228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-69713286145228532052016-02-13T02:37:56.620-05:002016-02-13T02:37:56.620-05:00Michael,
Regarding your question how close one wo...Michael,<br /><br />Regarding your question how close one would have to be to the black hole to actually feel the deformation, <a href="http://www.scottaaronson.com/blog/?p=2651" rel="nofollow">Scott has an estimate here</a>. Roughly speaking, what I was guessing above was about right. Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-84070594321356263902016-02-12T19:03:52.934-05:002016-02-12T19:03:52.934-05:00Uncle Al,
The measured quantity ("strain"...Uncle Al,<br />The measured quantity ("strain") is amplitude, rather than intensity. So in 3D it falls off as 1/r, rather than 1/r^2. This is a key difference between GW and EM astronomy. This is why it is compared to sound rather than vision, as we get coherent phase information, and measure the amplitude rather than intensity. By improving sensitivity by a factor of x, the volume of Anonymoushttps://www.blogger.com/profile/16625971686320772947noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-1694111311714751702016-02-12T10:38:48.801-05:002016-02-12T10:38:48.801-05:00In every inspiral depiction I've seen there a ...In every inspiral depiction I've seen there a 2-D flat emission like surface ripples on water. Is it 2-D, 1/r decay with distance, and favored directions of amplitude or 3-D, 1/r^2 decay with distance, and isotropic distribution of amplitude?<br /><br />This establishes black holes exist as such. Certainly external to their event horizons, black holes are described by relativity. The ring Uncle Alhttps://www.blogger.com/profile/05056804084187606211noreply@blogger.com