geordief Posted August 6, 2016 Posted August 6, 2016 (edited) If mike would only accept the fields as the "medium" this would have been over long ago. The medium is the tedium ? (but I like that idea) I don't suppose there is a chance that spacetime can be seen as a field? Or that a field can exhibit curvature? Edited August 6, 2016 by geordief 1
Mike Smith Cosmos Posted August 6, 2016 Posted August 6, 2016 (edited) If mike would only accept the fields as the "medium" this would have been over long ago.. .. Yes but I do , Of Sorts . And I have all along , of sorts . But you guys have all been saying " There is no need of a medium" " there is no medium " I have always been happy , there is the light and electromagnetic waves of a variety of sorts AND There is a medium for IT ( the electromagnetic waves to wave in ) . I don't mind , and have never minded that the medium is made of the same sort of ' stuff ' , the wave is made of . Even if it is the same type of stuff ( not itself ) but a medium a vehicle . So if it's a field , that is sort of O.k. As long as it is not just a math coordinate and maths function that is POSITIONED IN NOTHING and FUNCTIONING ON NOTHING . I Admit I have bleeted on about the medium needing to be of ' SUBSTANCE' , but by that I was in no way insisting the substance must be material , I was meaning it must be MORE , THAN AN INSUBSTANTIAL MATHS FORMULA OR COORDINATE SYSTEN OF NO SUBSTANCE . NOTHING . That is why recently I have been worried this is not semantics , definitions . Eg . There is nothing wrong in saying " there goes a wave " " the wave is made of a heaped up bit of water " followed by another heaped up and depressed bit of water " " oh it's sort of sinusoidal in shape " oh and look it's crossing the Ocean , which is also made of a Large body of water , acting as a medium for the wave " . Great , I am happy with that . . Wave and Medium , made of same stuff. . One sitting there in all its glory , the other moving across it as a wave , with all its Energy . Now , back to the ELECTRO MAGNETIC WAVES , I am happy the wave is there , somewhere , and it's travelling across the medium , called the Electro- Magnetic field ,( not the one that's just about to cross in front of us , but a massive electro magnetic field , being in space or the Universe , as a medium , accepting incoming and outgoing EM waves . ( possibly ? Set up by Dark Energy ) Also , back to the GRAVITATIONAL WAVES . I am happy both the wave and the medium are both based around , GRAVITATIONAL MASS AND MATTER . In their case Ordinary Matter with mass and Dark Matter . ( I have suggested the GRAVITATIONAL WAVES originate from ordinary mass/ matter and to some extent gravitational wave can travel through other mass as a medium eg earthquakes P AND S WAVES, But also across space through the medium of DARK MATTER .together with some ORDINARY MATTER PARTICLES. IF EVERYTHING THAT I have mentioned , is FIELDS , then that is fine by me , I am happy with fields , what I am and have not been happy with is. NOTHING ( no medium, no nothing ) just the wave . Wave in what ? Field , something . Not nothing . O.k . So the field is the Medium . Not no medium . Not nothing ! MIKE Edited August 6, 2016 by Mike Smith Cosmos
ajb Posted August 6, 2016 Posted August 6, 2016 Electromagnetic waves originate from accelerated charges. Gravitational waves originate from accelerated masses. The problem is also one of language here - by medium one usually means a mechanical medium, something like how sound moves through the air. By a wave, we of course need something to 'wiggle' and we have been saying more-or-less from the start that we usually think of wave solutions to some field theory. For example, a gravitational waves is a little 'wiggle' in the gravitational field - which we understand to be the local geometry as described by a metric tensor. 1
Mike Smith Cosmos Posted August 6, 2016 Posted August 6, 2016 (edited) Electromagnetic waves originate from accelerated charges. Gravitational waves originate from accelerated masses. The problem is also one of language here - by medium one usually means a mechanical medium, something like how sound moves through the air. By a wave, we of course need something to 'wiggle' and we have been saying more-or-less from the start that we usually think of wave solutions to some field theory. For example, a gravitational waves is a little 'wiggle' in the gravitational field - which we understand to be the local geometry as described by a metric tensor. You had me sitting on the edge of my seat , in total agreement , until the last eight words :- Namely " the local geometry as described by a metric tensor." Now , steady up there , . This is where I have to watch , you don't take me off in a maths rapture , and you leave me for dead by the wave side ! I have to check you have not seduced me away from reality . ......... " The local geometry as described by a metric tensor." .. Is that math speak for " a wave " ? If so a wave of what ? , in what ? By what , An asteroid , A sun , a galaxy doing what ( bumping , colliding , exploding , imploding , ). 'what ' for example ? Mike Edited August 6, 2016 by Mike Smith Cosmos
ajb Posted August 6, 2016 Posted August 6, 2016 Now , steady up there , . This is where I have to watch , you don't take me off in a maths rapture , and you leave me for dead by the wave side ! I have to check you have not subdueced be , away from reality . Well, what is the problem? Do you have a real objection to the electromagnetic field as a mathematical object? You can understand this field in terms of some differential geometry - it is related to a connection on a U(1) principle bundle. Not that details matter at all, just that we do understand EM in terms of geometry. In a similar, but different way, we can understand the gravitational field to be a mathematical object that encodes all that we need. This object we understand, again in terms of differential geometry - in the standard formulation we understand the gravitational field to be a metric tensor. But we can also understand it it terms closer to EM as a connection on a principle bundle known as the frame bundle. Again, details are less important than the ideas. You will have to think about the objections you have to gravity and if these are really any different to your objections to electromagnetic theory.
Mike Smith Cosmos Posted August 6, 2016 Posted August 6, 2016 (edited) Well, what is the problem?Do you have a real objection to the electromagnetic field as a mathematical object? You can understand this field in terms of some differential geometry - it is related to a connection on a U(1) principle bundle. Not that details matter at all, just that we do understand EM in terms of geometry.In a similar, but different way, we can understand the gravitational field to be a mathematical object that encodes all that we need. This object we understand, again in terms of differential geometry - in the standard formulation we understand the gravitational field to be a metric tensor. But we can also understand it it terms closer to EM as a connection on a principle bundle known as the frame bundle.Again, details are less important than the ideas. You will have to think about the objections you have to gravity and if these are really any different to your objections to electromagnetic theory..To make this REAL ( namely reality , and my previous grave doubts) that ' one ' was using maths alone , without a fundamental reality . A ' real thing ' as opposed to a 'mathematical thing '. You speak of a field , which field ? The Field , that is out there , permeating all space and the universe . If so, where did this field come from , I am saying it came from dark energy , that's not critical , as long as it came from somewhere , where are you saying this 'all pervasive field ' comes from ? We did cover this early on , in this thread , but I think at that point , I got dismissed a bit ( not necessarily from you ) but it started this " you don't need to know , because there is No Medium " . If not dark energy , is it just there , from Big Bang or something . That is o.k. If we can agree on that, then the maths can do its geometry ,bundles, and other functions to come up with something ! As a side issue , I did raise a point about ' no actual things , crossing the medium , only packets of energy . Not sure if that makes any difference to the maths , as you probably think of photons in those terms anyway ? If we get that out of the way , as you say we can move across to Gravitational waves . Plus the other things you spoke about . Mike Edited August 6, 2016 by Mike Smith Cosmos
MigL Posted August 6, 2016 Posted August 6, 2016 So you're OK with the field being the medium. And immediately you start running off on tangents, bringing dark energy ( ? ) and dark matter particles ( ? ) into the picture. Until you realize you don't have a clear understanding of what a field is. ( a value associated with every point in space or space-time, as the case may be) And so a field isn't 'material' enough for you anymore. You need something you can touch. Something you can see. You have been told why an EM or gravitational wave cannot be a 'waving' of particles, and you won't accept the field waving. What else is there ???
Strange Posted August 6, 2016 Posted August 6, 2016 . .. Yes but I do , Of Sorts . And I have all along , of sorts . But you guys have all been saying " There is no need of a medium" " there is no medium " Because you insist this medium must be made of some sort of tangible material. This contradicts all the evidence. But if you are happy to accept the (intangible) field as the medium, then the discussion is over.
Mike Smith Cosmos Posted August 6, 2016 Posted August 6, 2016 (edited) So you're OK with the field being the medium. And immediately you start running off on tangents, bringing dark energy ( ? ) and dark matter particles ( ? ) into the picture. Until you realize you don't have a clear understanding of what a field is. ( a value associated with every point in space or space-time, as the case may be) And so a field isn't 'material' enough for you anymore. You need something you can touch. Something you can see. You have been told why an EM or gravitational wave cannot be a 'waving' of particles, and you won't accept the field waving. What else is there ??? .We can forget about the source, dark energy , I was just looking for a huge amount of energy to set up this original electro magnetic field . I am very happy with field . For some reason , this happiness was missed . I am a field man , communications and radio transmission has been my life . They will bury me, no doubt, in an electro magnetic field . It's so real to me , I have an I phone in my pocket , an I pad in my lap , a PC and modem in my cupboard , a 150 watt input transceiver up stairs , two hand held transceivers, I think about fields 10 times a day , where other men think of sex. A field is fine , but my science tells me a field is generated from somewhere . I dared to ask where this all pervasive electro magnetic field , comes from ? Is it from trillions of trillions of particles with a point charge , or some gigantic Tesla machine at either end of the universe. It's not an unreasonable question to ask . O.k. The electro magnetic field is there , I have accepted that from the start . AS LONG AS IT IS REAL . not some guess , like why don't we try that . If it is a guess , then fine , lets say it is a guess . It's as good as my guess , guessing it was dark energy . Now we accept this all pervasive field is the medium . Great . Electro magnetic medium , electro magnetic packets of some sort , travel as waves across space . Now when we are sending signals across this medium , we put more Electro magnetic fields ( photons ) into the media to ' wiggle ' it . Great it's wiggling the medium , and off they go. Electro Magnetic waves at the speed of light , through the medium . Some say actual photon particles travel at the speed of light. In nothing . What I suggest is that 'nothing ' goes anywhere ( no particles ) , just quantised packets of energy , wave across the medium , until they arrive at their destination ( or get interfered with on the way. ) . Gravity , follows a different procedure . We start at the other end. However I see no reason why the same procedure should not follow. The medium , the same stuff as the source of Gravity . GRAVITATIONAL FIELD Mike Edited August 6, 2016 by Mike Smith Cosmos
ajb Posted August 6, 2016 Posted August 6, 2016 A ' real thing ' as opposed to a 'mathematical thing '. You will have to think carefully about this. As all our interpretations of experiements and observations require a mathematical model, can you really separate the two? You speak of a field , which field ? The Field , that is out there , permeating all space and the universe . Without introduces the other forces at this stage, lets say the electromagnetic field and the gravitational field. If so, where did this field come from , I am saying it came from dark energy ... The problem is we don't know what dark energy is and so saying that it gave rise to the fields of the standard model - or just gravity and electromagnetism - is just a story. You need so show some mechanisms here. As to where the fields came from, we don't really know. What we do know is that with supersymmetry all the running couplings ('interaction strengths') of the standard model come together at a high enough energy. This suggests, in the same sort of way as electromagnetism and the weak force can be unified, all the fields of the standard model an be unified. This leaves out gravity, however maybe at or near the Planck sale a full unification is possible. We just don't really know.
disarray Posted August 6, 2016 Posted August 6, 2016 (edited) I gather that gravitational waves can extract or donate mechanical energy. I gather that the sticky bead thought experiment supports the notion (e.g., arguments by Feynman) that the waves themselves transport (transmit?) this energy. I gather that the gravitational waves produced an audible rising chirp (via the instruments) as the two black holes accelerated towards each other, with a frequency that starts in the range of around 35 Hz and rises to around 250 Hz, corresponding to wavelengths of 7,000 to 1,000 kilometers, and amplitude being around the size of an atomic nucleus. I wonder whether the notion that a gravitational wave has amplitude implies that the "waves" experience some sort of resistance. If so, would such resistance imply that gravitational waves are more than just the results of mathematical descriptions, e.g., with respect to fluctuations in spacetime? I find it interesting that Einstein and Rosen published a paper in 1936 claiming that gravitational waves were a mathematical artifact, and did not actually exist. Is a gravitational "field" more mathematical and less "physical" than, say, the field of an electron? Just as the human eye can only experience an extremely minute fraction of the EM spectrum, we nevertheless say that the part we can't experience exists, even if it is just from a mathematical standpoint. I wonder if we might say the same about gravity, viz., that it exists even though the "waves" are outside our ability to experience it with our senses. I gather that gravity doesn't cause spacetime to curve - matter/energy does. The curvature of spacetime is simple called gravity. So do we then say that gravitational waves/ripples in spacetime are caused by changes in matter/energy? Can we then in practice have spacetime without matter/energy? Edited August 6, 2016 by disarray
Mike Smith Cosmos Posted August 7, 2016 Posted August 7, 2016 I gather that gravitational waves can extract or donate mechanical energy. I gather that the sticky bead thought experiment supports the notion (e.g., arguments by Feynman) that the waves themselves transport (transmit?) this energy. I gather that the gravitational waves produced an audible rising chirp (via the instruments) as the two black holes accelerated towards each other, with a frequency that starts in the range of around 35 Hz and rises to around 250 Hz, corresponding to wavelengths of 7,000 to 1,000 kilometers, and amplitude being around the size of an atomic nucleus. I wonder whether the notion that a gravitational wave has amplitude implies that the "waves" experience some sort of resistance. If so, would such resistance imply that gravitational waves are more than just the results of mathematical descriptions, e.g., with respect to fluctuations in spacetime? I find it interesting that Einstein and Rosen published a paper in 1936 claiming that gravitational waves were a mathematical artifact, and did not actually exist. Is a gravitational "field" more mathematical and less "physical" than, say, the field of an electron? Just as the human eye can only experience an extremely minute fraction of the EM spectrum, we nevertheless say that the part we can't experience exists, even if it is just from a mathematical standpoint. I wonder if we might say the same about gravity, viz., that it exists even though the "waves" are outside our ability to experience it with our senses. I gather that gravity doesn't cause spacetime to curve - matter/energy does. The curvature of spacetime is simple called gravity. So do we then say that gravitational waves/ripples in spacetime are caused by changes in matter/energy? Can we then in practice have spacetime without matter/energy? .I must say , for someone who refers to the themselves, as a ' lay person ' , I find your comments in this matter ,very insightful, and inspirational ! I think your mulling over things ,has helped very much the flow of this discussion recently . There are clearly things , like Ronald Rumsfold said " there are still the Unknown, unknowns that remain , in that we do not , know if there are , unknowns , that remain unknown ! Or something to that effect . The things that AJB , mentions are very searching . But I have to be very careful that I am not swept away with mathmatical nomenclature, that leaves me , dizzy , and disorientated , from what is actually happening in REALITY. At this moment I am not sure how closely linked to REALITY these mathmatical models are ? I am trying to think my way through what AJB , Has said . Also I like your request for an astronaught to " shake a rug in space " . Interesting? Mike
disarray Posted August 7, 2016 Posted August 7, 2016 (edited) .Also I like your request for an astronaught to " shake a rug in space " . Interesting? I believe that the response I got was that, yes one could shake a rug in outer space. Though I can't see how it would wave (or at least no in the same way) unless there was some sort of resistance (normally, air), since amplitude is related to energy and the amount of resistance. But in terms of EM waves, I am getting the impression that the term I should sometimes be using is 'impedance' rather than resistance. (I think that one can discuss EM waves and then jump to a discussion of gravity waves, despite obvious differences.) "The impedance of free space is sqrt (mu 0/ eps 0) but it is not resistance, because it is non dissipative. An e.m. wave does not lose energy and can propagate through very long distances, even from another galaxy. The energy passes forward as the electric field, changing in time, causes the magnetic field to change in space, and vice versa, but it is not lost from the wave." https://www.researchgate.net/post/why_there_should_be_a_resistance_for_the_propagation_of_electromagnetic_waves_at_all_in_free_space The literature tends, I found, to emphasize that the actual vacuum of even outer space contains all sorts of 'things/materials' that could provide impedance to EM waves. As for gravitational waves, the better term to use perhaps would be that they disperse, while perhaps maintaining uniform wavelength and frequency. On the other hand, it seems that the term "waves" is a bit of a misnomer (and just a graphical term) with reference to gravitational waves, as "gravity does not have peaks and troughs [in the usual sense], but rather there are bands with more and less space existing in the bands [!]....'Things' are generally attracted to more space so that oscillation occurs with the frequency of a wave." As for a perfect vacuum, this general comment might be relevant to gravitational waves: "on one hand we say perfect vacuum is without material, and on the other hand [we claim that] quantum vacuum says vacuum density is very high. I am putting very strong objection towards these contradictory results and statements" In any case, one might entertain the concept that 'gravity originates from variable energy density of quantum vacuum: "Mass itself is not producing gravity, gravity is a result of lower energy density (in GR described as higher curvature of space) of local areas of quantum vacuum." vixra.org/pdf/1111.0015v2.pdf Perhaps it is relevant that "When quantifying the sensitivity of a GW detector and the loudness of a GW source, there are three commonly used quantities: the characteristic strain, the power spectral density, and the spectral energy density." www.ast.cam.ac.uk/~cjm96/noise_curve.pdf .But I have to be very careful that I am not swept away with mathmatical nomenclature, that leaves me , dizzy , and disorientated , from what is actually happening in REALITY.At this moment I am not sure how closely linked to REALITY these mathmatical models are ? As far as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality. Albert Einstein "Geometry and Experience", January 27, 1921 Edited August 7, 2016 by disarray
Strange Posted August 7, 2016 Posted August 7, 2016 I wonder whether the notion that a gravitational wave has amplitude implies that the "waves" experience some sort of resistance. Why would amplitude imply resistance? Resistance would cause amplitude to decay (as the energy in the wave was converted to heat energy in the medium). So do we then say that gravitational waves/ripples in spacetime are caused by changes in matter/energy? That sounds about right (although not just any changes). Can we then in practice have spacetime without matter/energy? In practice, no, because there is matter and energy in the universe. But in principle, you could have a universe with space-time but no matter or energy.
swansont Posted August 7, 2016 Posted August 7, 2016 I believe that the response I got was that, yes one could shake a rug in outer space. Though I can't see how it would wave (or at least no in the same way) unless there was some sort of resistance (normally, air), since amplitude is related to energy and the amount of resistance. Resistance represents dissipation, which reduces amplitude with time. It's not a requirement for oscillation. Mike's arguing for one medium. You are arguing for two.
MigL Posted August 7, 2016 Posted August 7, 2016 There is a type of resistance to the rug's wave, but not in the way you're thinking. There is no need for air or any other 'medium' for the rug to be immersed in, so it will work in space as well as on Earth. The 'resistance' is provided by the rug's inertia. In effect, the rug is the medium, and the energy of the wave is transferred to each succeeding point as the wave 'travels'.
geordief Posted August 7, 2016 Posted August 7, 2016 (edited) What happens if you bang a drum in space? Does it vibrate much longer on account of there being no atmosphere to carry away the "sound"? Edited August 7, 2016 by geordief
Mike Smith Cosmos Posted August 7, 2016 Posted August 7, 2016 (edited) What happens if you bang a drum in space? Does it vibrate much longer on account of there being no atmosphere to carry away the "sound"?. I think the next space station commission is going to be a ' hoot ' . With flapping great lengths of heavy carpet , up and down , with some gusto ! Inside or outside the module? . And Banging drums , feverishly ! Especially as the cabin will need to be evacuated of breathable air ! Unless you are planning this outside the space station ? Sounds quite fun , in addition to driving other space scientists crazy ! However if we can crack this Aether / Medium issue , it will be worth the annoyance ? Mike Edited August 7, 2016 by Mike Smith Cosmos
disarray Posted August 7, 2016 Posted August 7, 2016 (edited) Why would amplitude imply resistance? Resistance would cause amplitude to decay (as the energy in the wave was converted to heat energy in the medium). I was assuming, for the sake of argument, that spacetime was a medium: In general, "different materials also have differing degrees of springiness or elasticity. A more elastic medium will tend to offer less resistance to the force and allow a greater amplitude pulse to travel through it; being less rigid (and therefore more elastic), the same force causes a greater amplitude." http://www.physicsclassroom.com/class/waves/Lesson-2/Energy-Transport-and-the-Amplitude-of-a-Wave ["So do we then say that gravitational waves/ripples in spacetime are caused by changes in matter/energy?" -Disarray] That sounds about right (although not just any changes). But in principle, you could have a universe with space-time but no matter or energy. In practice, no, because there is matter and energy in the universe. By "principle," I presume you mean in accordance with Relativity theory....but given that things happen the way that they did and that even in the early moments of the "Big Bang" things seemed intricately connected, can we speculate or assume that our universe could have developed in such a way that there was only spacetime and no matter/energy or any forces? Furthermore, if the universe was composed of just spacetime, we presumably would not have such things as gravity (much less time dilation).....? If gravity is a key ingredient in the expansion of the universe, and gravity is also dependent upon the existence of energy/matter, and expansion is necessary for entropy and the arrow of time, it would seem to follow that we can't have a universe that consists of just spacetime. Mike's arguing for one medium. You are arguing for two. By "two," are you referring to matter/energy. If so, can we not consider that one "thing?" More likely, I suspect that you mean that I am arguing for 1) spacetime and 2) matter/energy. If so, I presume that there are other things not in these two categories, e.g., strong and weak force. ----------------------------------------- I guess my own biggest "hangup" is on the idea that "time" is as much a functioning dimension as are the 3D of space. It seems more likely to be just a human/mathematical construct that, like the term "God," is used as a filler to explain that which we don't yet understand. I am not looking for a physical-like medium per se, but rather just noting that space does seem to have qualities that make it variable, e.g., when one reads that gravitational waves correspond to bands of more space/less space and to higher and lower levels of energy. Indeed, it sounds as if space or spacetime itself can have variations in what one might refer to as "density." So, whether it be a person speeding off in a rocket at near c velocity, or measurements at various levels in a gravity well, it would seem that it might be possible that the clocks we use to measure time/motion slows down in such a way that we think that 'time' slows down, given Maxwell's claim that at the quantum level, all things (e.g., EM waves) travel at the speed of light . Presumably, this notion would require either that we don't notice that EM waves slow down given our frame of reference....This might seem credible when it comes to an earth observer vs. a space travelers impressions of time, but does not when we talk about clocks varying at various levels above sea level. This leaves the possibility that EM waves (e.g., light) can physically slow down, e.g., in terms of variations in energy levels in (bands or areas of) space......so: can we rule out the possibility of light (and other EM-related processes such as metabolism) slowing down in an effort to explain time dilation? "researchers from the University of Glasgow and Heriot-Watt University describe how they have managed to slow photons in free space for the first time. They have demonstrated that applying a mask to an optical beam to give photons a spatial structure can reduce their speed." http://www.gla.ac.uk/news/headline_388852_en.html Edited August 7, 2016 by disarray
swansont Posted August 7, 2016 Posted August 7, 2016 By "two," are you referring to matter/energy. If so, can we not consider that one "thing?" More likely, I suspect that you mean that I am arguing for 1) spacetime and 2) matter/energy. No. You are requiring that the rug be present (one medium) and the surrounding air (a second medium)
geordief Posted August 7, 2016 Posted August 7, 2016 . Banging drums , feverishly ! Especially as the cabin will need to be evacuated of breathable air ! Unless you are planning this outside the space station ? Either in or out. If outside the station we can just watch to see how long it vibrates before the vibrations subside . If inside then the skin of the station plays the role of the membrane of a drum. It seems clear to me that any atmosphere would act as a dampener to vibrations on the membrane of a drum or the skin of a spacecraft (if the crew were either having a party or playing a loud note) Just because we cannot hear a sound in space surely does not mean that we cannot set a musical (or otherwise vibrating) instrument to oscillate. I only asked because I had never heard about the exact circumstance but it does not seem a bit controversial to me. On the other hand you seem prepared to believe that a carpet shaken in space cannot be made to make a wave.. That seems an extremely unlikely outcome to me but maybe I am wrong...
Strange Posted August 7, 2016 Posted August 7, 2016 What happens if you bang a drum in space? Does it vibrate much longer on account of there being no atmosphere to carry away the "sound"? Interesting question. I assume it would vibrate for longer, but I have no idea how much energy goes into moving the air versus dissipated other ways.
disarray Posted August 8, 2016 Posted August 8, 2016 (edited) Just because we cannot hear a sound in space surely does not mean that we cannot set a musical (or otherwise vibrating) instrument to oscillate. In general, it appears that oscillation seems to imply/require elasticity (typically as a result in some disturbance in an equilibrium) . It would seem that spacetime has both oscillation (of gravitational waves) and elasticity (e.g., typical of thermodynamic materials): "We present the theory of spacetime elasticity and demonstrate that it involves traditional thermoelasticity. Assuming linear-elastic constitutive behavior and using spacetime transversely-isotropic elastic constants, we derive all principal thermodynamic relations of classical thermoelasticity...spacetime elasticity directly implies the Fourier and the Maxwell–Cattaneo laws of heat conduction. However, spacetime elasticity is richer than classical thermoelasticity, and it advocates its own equations of motion for coupled thermoelasticity"http://www.worldscientific.com/doi/abs/10.1142/S0217979212500324 How is it that we can describe spacetime as oscillating and perhaps of being elastic, yet not ascribe any physical properties to it whatsoever? In short, does not the putative elasticity of spacetime imply a degree of 'substantiality' if not 'physicality' as opposed to its being thought of as just passive and invariant "emptiness?" I return to the idea that expansion needs gravity needs energy, and would not exist in a universe in which there was spacetime but nothing else: "What can possibly make the expansion [of the universe] speed up, then? Well, general relativity says that if the vacuum has energy density, it must also have pressure! In fact, it must have a pressure equal to exactly -1 times its energy density, in units where the speed of light and Newton's gravitational constant equal 1. Positive energy density makes the expansion of the universe tend to slow down... but negative pressure makes the expansion tend to speed up.More precisely, the rate at which the expansion of the universe accelerates is proportional to - ρ - 3P But as I mentioned, for the vacuum the pressure is minus the energy density: P = -ρ. So, the rate at which the vacuum makes the expansion of the universe accelerate is proportional to 2 ρ From this, it follows that if the vacuum has positive energy density, the expansion of the universe will tend to speed up! This is what people see. And, vacuum energy is currently the most plausible explanation known for what's going on." http://math.ucr.edu/home/baez/vacuum.html But without expansion, there could not be spacetime...so energy is necessary for not only the existence of gravity, but also of spacetime itself. Edited August 8, 2016 by disarray
Mike Smith Cosmos Posted August 8, 2016 Posted August 8, 2016 (edited) I think I just saw the buckle in the track , that caused the derailments! When I took 'A' level maths at Grammar school , there were two subjects . PURE MATHS , and APPLIED MATHS . One was about ' Abstract Maths ' as formulae , sums, manipulation of number. PURE MATHS . The other about ' Real things , cars , blocks , how actual ' THINGS ' moved etc. APPLIED MATHS . I looked at the definition of TENSORS , they are Scary Sounding . But Pure . But not REAL not necessarily APPLIED on anything , just an operation . And yet they get called a MATHMATICAL OBJECT. ( glug ! ) I think this is where the buckle in the track is . Where if we don't watch it our eyes glaze over . And we can become ' derailed ! ( Non Mathmatisians, I mean ) MATHS. appears to be able to conjure up an object , out of nothing , ( say NO MEDIUM , nothing there ) , if I have got it right .? Yet it can be on something , if there IS something there ( say A MEDIUM , a Rug say , or water ) , if I have got it right ? Ref TENSORS :- https://en.m.wikipedia.org/wiki/Tensor ELECTRO MAGNETIC WAVES Even more tricky , if the thing we are looking at is a Field ( say an Electro Magnetic field ) , because at a POINT a field might just be a value at a certain position ( say so many volts and so much magnetism gauss) . We start getting maths type coordinates or geometry . And before we know where we are, we are in an ' Object' a ' Bundle ' , But we then need to ask " yes but where is the Field coming from ( a point charge , a small atomic particle , or a massive electric field set up from somewhere else ? ) . And if the charge is moving or spinning it will produce a magnetic field . Suddenly we have something real , like a medium . It's almost like Magic. It's much easier if we can think of it as a piece of carpet ( hey ! A magic carpet .. Medium ) GRAVITATIONAL WAVES So in the carpet illustration , As applied to gravitational waves , We can presume some matter, or particles is what is illustrated by 'The carpet' is the medium or Aether . The wave in the carpet is the gravitational Wave , even though it's composed of carpet material BUT thought of as moving medium . Then , let the mathematicians at it . They will 'tense ' it up as an object and 'bundle ' it up Into Maths models . And do Geographic coordinates , all over it , to express it in acceptable math form , AND express the wave and carpet in maths from one end to the other ! Somewhere in here we can say that the long series of various coordinates that can be used to identifying completely the rug with its wave in it , is a FIELD with a wave in it . I think ? Mike Edited August 8, 2016 by Mike Smith Cosmos
swansont Posted August 8, 2016 Posted August 8, 2016 On the other hand you seem prepared to believe that a carpet shaken in space cannot be made to make a wave.. That seems an extremely unlikely outcome to me but maybe I am wrong... One of the conspiracy theory objections to the moon landings is that the flag is waving in the vacuum. (The conspiracy theorists insist it requires an atmosphere, so it must have been filmed on earth.) So unless you are a part of that cabal, you can see we have empirical evidence of a fabric waving in a vacuum. But we then need to ask " yes but where is the Field coming from ( a point charge , a small atomic particle , or a massive electric field set up from somewhere else ? ) . And if the charge is moving or spinning it will produce a magnetic field . Suddenly we have something real , like a medium . It's almost like Magic. It's much easier if we can think of it as a piece of carpet ( hey ! A magic carpet .. Medium ) That would require that the field be a physical thing, rather than a mathematical construct. But that's not the real issue. An EM wave makes its own fields. It does not require that a separate field be present.
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