tag:blogger.com,1999:blog-1055612246610286525.comments2017-05-17T10:42:36.082+01:00 The Fictional AetherMark Hannamhttps://plus.google.com/100084066533915100591noreply@blogger.comBlogger331125tag:blogger.com,1999:blog-1055612246610286525.post-70362461267462578392017-02-17T22:26:28.574+00:002017-02-17T22:26:28.574+00:00Concise, poignant. Evocative, i.e., I nearly cried...Concise, poignant. Evocative, i.e., I nearly cried from your happiness of discovering, not just waves but WAVES! I am in awe. Rick Dettwyler, Ph.http://democratees.biznoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-23047531807443750452017-01-26T12:17:41.096+00:002017-01-26T12:17:41.096+00:00I was raised to believe that if you want to change...I was raised to believe that if you want to change the world, protests, rebellion, lobbying, political organising, etc., are all a complete waste of time. If you really want to change the world, writing 1000 snarky words every few weeks is the way to go. Mark Hannamhttp://www.blogger.com/profile/06011146336221492349noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-68587556737152752482017-01-25T19:05:36.629+00:002017-01-25T19:05:36.629+00:00First (and so far only) positive effect of Trump&#...First (and so far only) positive effect of Trump's presidency: It got you to start blogging again!Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-78084126418020647002017-01-19T21:00:15.790+00:002017-01-19T21:00:15.790+00:00Nevermind all that mark, what Gravitational Waves!...Nevermind all that mark, what Gravitational Waves! Penelope Rose Cowleyhttp://www.peneloperosecowley.comnoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-74889979236300341882016-08-05T20:26:53.204+01:002016-08-05T20:26:53.204+01:00And if one were to (hypothically) compare 2 interf...And if one were to (hypothically) compare 2 interferometers – one with the mirrors freely suspended, and another with the mirrors anchored – would both interferometers detect the signal - but at different levels of intensity?<br /><br />The question is whether the GW actually stretches and compresses space(time?) more in a vacuum as compared to a solid mass. Recalling Feynman’s sticky bead argument - "Feynman’s gravitational wave detector: It is simply two beads sliding freely (but with a small amount of friction) on a rigid rod. As the wave passes over the rod, atomic forces hold the length of the rod fixed, but the proper distance between the two beads oscillates. Thus, the beads rub against the rod, dissipating heat." (from Wikipedia)<br /><br />I must admit that understanding how the interferometers conceptually work is not easy but I do appreciate the explanations you have presented on this topic.<br />Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-68940067781810191792016-08-04T16:49:12.385+01:002016-08-04T16:49:12.385+01:00That's a good point, and it's often forgot...That's a good point, and it's often forgotten that both space and time are stretching. I imagine one *could* represent the distortions in time as well (a row of clocks displayed along the LIGO arm, for example), but I've never seen it done.Mark Hannamhttp://www.blogger.com/profile/06011146336221492349noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-38460217474457587792016-08-02T22:40:34.606+01:002016-08-02T22:40:34.606+01:00All the visualizations of a GW show them as a wav...All the visualizations of a GW show them as a wave moving freely floating particles in space – so this shows how the GW interacts with freely falling particles in space (not time). If GW are ripples in the fabric of spacetime – is it possible to represent the time part “visually”? http://www.einstein-online.info/spotlights/gw_waves Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-47795576826742518302016-07-11T15:40:39.727+01:002016-07-11T15:40:39.727+01:00Thank you.Thank you.greenOnYellownoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-83236283779342468212016-03-28T15:15:25.941+01:002016-03-28T15:15:25.941+01:00Polite cough: http://i.stack.imgur.com/IlSrh.jpgPolite cough: http://i.stack.imgur.com/IlSrh.jpgThe Universehttp://www.blogger.com/profile/02867342924571644602noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-21256607004981795532016-03-18T09:35:39.757+00:002016-03-18T09:35:39.757+00:00I think I almost understand this, which is great. ...I think I almost understand this, which is great. And I like your last paragraph - remember phlogiston!Hugh Joneshttp://www.blogger.com/profile/13714284003128115368noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-75864537695588293462016-03-17T16:04:44.520+00:002016-03-17T16:04:44.520+00:00Pulsar timing makes you think even harder about th...Pulsar timing makes you think even harder about this. Or it should do. Thomas Denthttp://www.blogger.com/profile/16355010444546331416noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-38845284643474529392016-03-17T15:20:44.658+00:002016-03-17T15:20:44.658+00:00I've already done the equivalent of the ants w...<a href="http://fictionalaether.blogspot.com/2015/03/the-eclipse-of-gravity.html" rel="nofollow">I've already done the equivalent</a> of the ants walking North to illustrate how curvature can lead to an apparent force. But I don't think that taking this picture and bumping it up into 4D is really what happens in GR. If I take a constant-time slice of the Schwarzschild metric, the stretching of space along a line, in comparison to the flat metric, is not naturally represented as a curved line in a 2D space -- it's just that the 1D direction has been stretched. Or am I missing something?Mark Hannamhttp://www.blogger.com/profile/06011146336221492349noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-13487675276805007902016-03-16T18:58:46.022+00:002016-03-16T18:58:46.022+00:00Well, for the effects of curvature, you can always...Well, for the effects of curvature, you can always do the two ants walking north from the equator. Starts in parallel, brought together by the curvature of the Earth. The embedding here is important though. We see a 3D space that happens to be embedded in a 4D spacetime. The ant follows a path on a 2D space embedded into a 3D space. Further, a ball has both intrinsic and extrinsic curvature. No analogy is perfect because ultimately, we are fitting mathematics to data and a good scientist should be able to accept when the mathematics fits physical measurement (with the usual caveats of overfitting, yada,yada,yada), irregardless of whether everyday life leaves you with a "physical intuition" of the result. One of the first lessons I figured out on my own in graduate school was there is absolutely no law of nature which states that human intuition is of any value.<br /><br />-JasonAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-22240765524144488042016-03-13T15:20:10.773+00:002016-03-13T15:20:10.773+00:00Tori are fun, but it's still a 2D surface embe...Tori are fun, but it's still a 2D surface embedded in a 3D space. Mark Hannamhttp://www.blogger.com/profile/06011146336221492349noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-34713157721369852692016-03-11T06:49:42.869+00:002016-03-11T06:49:42.869+00:00I don't know about an intuitive curved 3D spac...I don't know about an intuitive curved 3D space, but perhaps you could use tori to illustrate it. Start with a flat torus and a "donut" torus and explain how a 2D person living on the surface of the torus would be able to tell what kind they were in. Then extend it to 3D flat and curved tori.Chaitanya Oehmigarahttp://www.blogger.com/profile/07124499558818789776noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-32896372033767160702016-03-11T06:40:23.630+00:002016-03-11T06:40:23.630+00:00Nice article. I particularly like the "GR doe...Nice article. I particularly like the "GR doesn't tell us why it happens, it just tells us that it does" and following bit. Might just steal that (I'll credit you of course :) )<br /><br />I've been known to draw triangles on my wine glass in lipstick to try and explain curvature at parties. It's good fun! Chennihttp://www.blogger.com/profile/18353590483308864196noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-82734806975328037592016-03-10T06:03:08.925+00:002016-03-10T06:03:08.925+00:00These days I get confused about curvature, too. Is...These days I get confused about curvature, too. Is there an "everyday" example of a 3D curved space, *without* taking a 2D surface and bending (embedding) it in a 3D space? This is a real question -- I don't spend that much time these days thinking about these basic GR notions, so I really don't know. Mark Hannamhttp://www.blogger.com/profile/06011146336221492349noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-68293665470209263802016-03-10T04:50:35.655+00:002016-03-10T04:50:35.655+00:00Nice post. I tend to first focus on the effects of...Nice post. I tend to first focus on the effects of curvature and then connect curvature to energy. I've found that if people already grog curvature determining motion, which is easier to demonstrate, then they'll be much more open to energy setting the shape of space-time.<br /><br />-JasonAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-69788391363203994592016-03-08T20:49:06.919+00:002016-03-08T20:49:06.919+00:00The bending of light is not the same as diffractio...The bending of light is not the same as diffraction, because the light does not change speed -- it has not entered a "different medium", as happens when light goes from, say, glass to air. Instead, space and time are curved by the mass of the sun, and the light follows the shortest route in spacetime, which happens to be a curve. Bending of light is also predicted by Newton's theory, if you also consider light as a particle with mass, but the effect is half of that predicted by Einstein's theory. That's why the bending of light was such a triumph for Einstein's theory: his theory predicted the correct result (without knowing the answer beforehand). <br /><br />In Einstein's theory, if the Sun disappears, the change in the gravitational field will take eight minutes to reach the Earth (the light travel time). In Newton's theory, we will notice the loss of the Sun instantly. <br /><br />In Newton's theory, changes in the gravitational field do not radiate away -- they are felt instantly everywhere in the universe. That was what bothered Einstein so much that he spent 10 years coming up with an alternative.Mark Hannamhttp://www.blogger.com/profile/06011146336221492349noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-2934481004745965822016-03-08T20:15:52.634+00:002016-03-08T20:15:52.634+00:00Talk at Bristol Astro Society, 4 March 2016.
I fou...Talk at Bristol Astro Society, 4 March 2016.<br />I found your talk very interesting, it being both informative and humorous - a good combination. But I have a couple of points I would like to follow up if I may:<br />1. A member of the audience asked if the gravitational deflection of light was like difraction, and your answer was emphatically NO, echoed by others in the audience. But if light slows down in a stronger gravitational field, then light from a distant star passing close to the sun will be bent, just as light from the sun is bent when it passes through the atmosphere at sunset - the sun appears higher in the sky than it actually is, just like the stars of Eddington's measurements.<br />2. You indicate that Newton's theory had gravity act instantaneously at a distance, but so does Einstein's theory. The gravitational field of Newton is "attached" to the body, so if the body is at rest, or moving uniformly, the gravitational field will also. If another body enters the field at some distance away, it will immediately feel the effect of the gravity from the first body. Similarly with Einstein: the distorted spacetime will be be at rest or move with the body, so that again if a distant body enters the field, it too will instantly feel the effect of the warped spacetime.<br />Although Newton didn't consider gravitational waves, we can assume that, on the basis of his theory, if the gravitating body were to suddenly accelerate, the disturbance of the gravitational field will radiate away - I don't think the speed of propagation can be determined. A similar thing happens with Einstein's theory, but here the speed can be determined.<br />Thank you for your time.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-44825865290395965392016-02-25T21:02:14.682+00:002016-02-25T21:02:14.682+00:00Very interesting. That does make sense, given that...Very interesting. That does make sense, given that GR in 2D doesn't have gravitational waves at all.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-72784351132472132272016-02-25T19:43:10.325+00:002016-02-25T19:43:10.325+00:00No, the strength falls off as 1/r.No, the strength falls off as 1/r.Mark Hannamhttp://www.blogger.com/profile/06011146336221492349noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-75609781628016667542016-02-25T17:31:12.081+00:002016-02-25T17:31:12.081+00:00"Since the fractional change in the length of..."Since the fractional change in the length of the interferometer arms (on Earth) was ~10^(-18), it would still be ridiculously weak even if it were a billion times stronger."<br /><br />But wouldn't the strength go as 1/r^2, like EM waves? That would make the effect ~(10^9)^2*10^-18 i.e. 10^18*10-18 i.e. order unity at 1 LY, which sounds pretty substantial, and ~4*10^9 at 1 AU, which sounds pretty harrowing indeed. Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-76883175266269065102016-02-24T11:01:14.500+00:002016-02-24T11:01:14.500+00:00Really? If I was a good little science outreach pe...Really? If I was a good little science outreach person, I would do some calculations now. But since I'm lazy, I'll only note that the total mass of our favourite black-hole binary was 65 times the mass of our sun. A light-year is a hell of a lot more than 65 times the Earth's distance from the sun. (We are eight light-minutes from the sun, if someone wants to work out the factors.) So why would it have been such a problem?Mark Hannamhttp://www.blogger.com/profile/06011146336221492349noreply@blogger.comtag:blogger.com,1999:blog-1055612246610286525.post-62086107056679791642016-02-24T10:36:06.397+00:002016-02-24T10:36:06.397+00:00If we'd only been one light-year away I imagin...If we'd only been one light-year away I imagine we'd have bigger problems than detecting gravitational waves...Arbuthnotnoreply@blogger.com