Moontanman Posted June 16, 2016 Posted June 16, 2016 I keep hearing gravity waves described as ripples in space time. Does this suggest that space time is a substance like the discredited notion of the aether? It seems to me that if space time can be said to ripple then a preferred reference frame is suggested by this "ripple" I know I must be off base here but how am I mistaken?
MigL Posted June 16, 2016 Posted June 16, 2016 The 'ripples' manifest themselves the same way that the 'curving' or 'warping' of space-time does. The co-ordinate system of the model describing these massive, energetic events predicts a 'ripple' spreading out from the event. We can finally detect these 'ripples' and they seem to agree with the predictions of the model ( that's HOW they were found ). That model is GR and it also suggests there is/are no preferred frames. I don't think the aether was ever exactly discredited. It just doesn't add anything, or explain anything further, to the model. It just adds unnecessary complexity, and is superfluous or redundant.
Velocity_Boy Posted June 16, 2016 Posted June 16, 2016 The 'ripples' manifest themselves the same way that the 'curving' or 'warping' of space-time does. The co-ordinate system of the model describing these massive, energetic events predicts a 'ripple' spreading out from the event. We can finally detect these 'ripples' and they seem to agree with the predictions of the model ( that's HOW they were found ). That model is GR and it also suggests there is/are no preferred frames. I don't think the aether was ever exactly discredited. It just doesn't add anything, or explain anything further, to the model. It just adds unnecessary complexity, and is superfluous or redundant. Uh, yeah, sorry to be the bearer of bad news but the Ether hypothesis (never was a full-blown theory!) was effectively debunked and disproved over 120 years ago with the Mickelson-Morley Experiment. Here's a bit more on that..............http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/mmhist.html
MigL Posted June 16, 2016 Posted June 16, 2016 Re-read the link you yourself posted... "One of the reasons for this position is that there is no direct experimental evidence for the existence of the ether - everything can be explained without it, hence the Occam's razor approach." ( fourth paragraph of the second article - The Mysterious Ether ) If you're gonna disagree with me, at least make it a challenge, and don't provide me with the information to defeat your argument.
disarray Posted June 16, 2016 Posted June 16, 2016 (edited) Yes, Moontanan...I'm just an idle philosopher, but I have recently read that there is no space"outside the universe into which the universe is expanding. I also read recently that the universe could not have come out of a complete "nothingness" since there would be no quantum laws, fields, or whatever that are needed for the universe to "spontaneously" come out of in the first place. Physicists talk about time/space as being curved, and then qualify that statement by saying that the word "curved" is only a metaphor-like verb that helps us to visualize what's going on. An illustration that is then offered is that of balls of various sizes, representing object with mass, sink into the fabric of space/time (thereby curving it) much like a bowling ball and a basketball would sink (to different depths) into the surface of a trampoline. In short, it seems that because space(/time) indeed has all these properties and qualities, (e.g., ripples, responsive to different quantities of mass, etc.), it is not just empty 'nothingness'. Though some things in relativity make some sense to me (e.g., space/time dilation in twin paradox), I wonder whether there is just some sort of shift in the substance of space/time (a space shift much like a red shift) that is consistent with Einsteins mathematical descriptions. As an aside, gravity is sometimes said to such that a satellite is really moving in a straight line around the earth, but it only seems curved because the space around the earth is curved. Surely we know for a fact that the earth is sphere-like, so that the satellites parallel path around the earth must be likewise shaped (into a near circular path) as well. Or am I missing something? Edited June 16, 2016 by disarray
Markus Hanke Posted June 16, 2016 Posted June 16, 2016 As an aside, gravity is sometimes said to such that a satellite is really moving in a straight line around the earth The meaning of "straight" in this context is different than just being "straight in space". What it means is that no forces act on the satellite anywhere along its trajectory - you can easily see this by placing an accelerometer on board, which will read zero at all times, so there are no forces in its rest frame. In other words, the satellite is in free fall at all times. If you plot its world line in spacetime ( not just its trajectory in space ! ), you find that between any two events you choose, this world line is such that it traces out the longest proper time - this is called the "principle of extremal ageing", and in that sense the world lines of test particles in free fall constitute the most direct and straight connection between events in spacetime, and such world lines are called geodesics. This does not imply that the test particle's spatial trajectory ( = the orbit of the satellite ) is a straight line - it implies only that, given initial conditions, it is the trajectory that allows the satellite to trace out the most proper time as recorded by its own clocks. Always remember that we are in spacetime, not just in space.
Strange Posted June 16, 2016 Posted June 16, 2016 I keep hearing gravity waves described as ripples in space time. Does this suggest that space time is a substance like the discredited notion of the aether? It seems to me that if space time can be said to ripple then a preferred reference frame is suggested by this "ripple" I know I must be off base here but how am I mistaken? Because there is no substance rippling, just the geometry of the distances between things. You don't think of length as being a substance, so why should changing length be a substance?
disarray Posted June 16, 2016 Posted June 16, 2016 I always think of learning as often being trying to put together the pieces of a jigsaw puzzle when you don't have the picture on the box, so that you fit together a few pieces here and there that seem to make sense, but don't really get the big picture until you have gotten further up the "learning curve" (if I can mix metaphors a bit). I have been trying to get a basic grasp on Relativity, but have not had any real "aha" moments yet. Not being mathematically minded, this makes a little sense to me, and I think you for the explanation.
Strange Posted June 16, 2016 Posted June 16, 2016 Physicists talk about time/space as being curved, and then qualify that statement by saying that the word "curved" is only a metaphor-like verb that helps us to visualize what's going on. An illustration that is then offered is that of balls of various sizes, representing object with mass, sink into the fabric of space/time (thereby curving it) much like a bowling ball and a basketball would sink (to different depths) into the surface of a trampoline. So you get the idea it is a metaphor. Good. In short, it seems that because space(/time) indeed has all these properties and qualities, (e.g., ripples, responsive to different quantities of mass, etc.), it is not just empty 'nothingness'. But then you take it literally. Not so good.
disarray Posted June 16, 2016 Posted June 16, 2016 (edited) Strange: Soooo, we can't get beyond the metaphors, be they interestingly concrete or boringly abstract? (I have read that string theory is beautifully elaborate and explains a lot of things well, but others think that it is just a big numerical model that happens to work in some ways, but is perhaps no better than a lot of other mathematical models, and could just as well be tossed into the waste basket.) So far I get the idea of concrete metaphors (e.g., stories that provide sights, sounds, etc.) not being confused with the real thing, but I find it harder to think of mathematical models in the same way with reference to relativity and quantum theory. I guess I am used to being able to visually see how one can apply geometry to surveying or use it in architecture, but am not sure at just what point one should cease to think of an equation, for example, as anything to that can be visualized. I read recently that some scientists think that there is nothing concrete in the universe at all, despite, as I recall, Victor Stenger's claim that there are irreducible particles (e.g., in his book, "God and the Atom"), and that ultimately everything is just qualities and/or mathematics. In any case, I wonder whether it is misleading for scientists to use words like space and time at all, if they are really using these terms in ways that are quite different from the way the average person thinks of them in everyday life. Perhaps they could say "Shpace" and "Thime" to slightly distinguish the terms, much as we use x' and y'. Similarly, we might speak of "shpin" to remind beginners to the field that we are not talking about spin in the usual sense....just a half-serious suggestion. (In any case, I appreciate those experts who take time and make the effort to explain things to online beginners myself) Edited June 16, 2016 by disarray
Strange Posted June 16, 2016 Posted June 16, 2016 Strange: Soooo, we can't get beyond the metaphors, be they interestingly concrete or boringly abstract? I certainly can't follow the mathematics of GR, but I have learnt enough to know that "space curving" or "space expanding" are just images or metaphors to describe the maths. The important thing is to remember that and not start trying to extrapolate from those analogies (or as some people do, try to falsify the science based on those analogies). Geometry, even before GR, was just a way of describing (mathematically) the relationships between points in space. What GR added was the knowledge that these relationships are (a) not fixed and (b) not based on traditional straight-line Euclidean geometry. In any case, I wonder whether it is misleading for scientists to use words like space and time at all, if they are really using these terms in ways that are quite different from the way the average person thinks of them in everyday life. I don't think they are used significantly differently.Roughly, "space" is just the distance between objects and "time" is the separation between events (although "event" does have a slightly more specific meaning in GR). In other words, space is what rulers measure and time is what clocks measure. Of course, there are philosophical discussions to be had about the nature of time and space, why we perceive time as only going forwards, etc. But they are independent of the physics. (And, "space" can also be used to mean the stuff that is out there beyond the atmosphere: gas, dust, virtual particles, stars, planets, etc.) 1
disarray Posted June 16, 2016 Posted June 16, 2016 (edited) Strange....Yes, that was what I gathered might be the case, but thanks for stating it. As long as we are talking about the relationship between sign and signified, please bear with me as I make a couple of more general points in this regard: I am gathering that one might make a list of those things in which the representative model is like the thing that it is trying to replicate and those that are quite different (aka signs similarity to the signified OR shadows on the wall of Plato's allegorical cave similarity to the objects seen in the light of day). We might have some sort of scale to rate the appropriateness of model to modeled. Of course there has to be some parameters as to what you are looking for. A painting of the Mona Lisa visually looks from the waist up could be mistaken by one of his students as the real woman at a distance of 100, but apart from that, the painting is not alive, is pretty much 2 dimensional, cannot carry on a conversation, etc. Indeed, we go to the movies and can put our entire mind into a "state of belief" by tricking our 5 senses into thinking that what we see on the screen is the "real thing." Onomatopoetic words, as another example, would be higher on the scale that other words....saying the word "buzz" mimics the sound of a bee more than does the word hippopotamus. As another example, a candle would be higher up on the scale when trying to represent a star to someone who had never seen one, than, say, a log. I could list similar examples with respect to smell, taste, movement, touch, etc. I guess the "rub" comes, if I may allude to Hamlet, when we cannot perceive reality with our senses, e.g. in trying to describe a quark, or dark matter, etc. Indeed, we scoff at Viking who thought that thunder was caused when Thor threw his hammer, preferring our own modern day explanation for a thunder waves speed as V = square root of k over p I would suggest that on my verisimilude scale of modeling that the image of thor’s hammer shattering the air with its vibrations might give a person who has never heard thunder a more representational “image,” in terms of what we experience with our sense of hearing, of what thunder is (or is like) than the mathematician’s symbols on a chalkboard. But there sometimes a limit to how close our models get to the actual thing:Da vinci’s picture can only look so much like the Mona Lisa and Dante’s poems only describe Beatrice so well, though at some point there seems to be diminishing returns beyond which further improvement of the picture of the poem does not give you much more significant information as to what the woman was like in person, as a flesh and blood talking human. Ditto for describing light as a wave and a particle….it is neither and we can’t get much better than to say that, beyond that, we don’t know how else to describe it. The concept of gravity is so 'foreign' to anything else that we experience with our senses that the best we can do is show that, for example, there is a tradeoff between distance and time when we have a set ceiling for the speed of light. So, I am guessing that, apart from mathematical/geometric descriptions, we cannot really experience what the concepts of time dilation and length contraction 'mean' in any other way. Perhaps my 'verisimilitude scale' is only of any use when it comes to estimating the degree to which one sensation (e.g., a candle) is 'like' another (i.e., a star), and not for abstract things such as space/time (which, apart from being a measurement) is not something that we can directly experience, especially when we talk about the curvature of space or the dilation of time, though we do experience such things as time and gravity indirectly, e.g.,, we feel time going more quickly when we are bored and we feel what its like when we drop a bowling ball on our foot. Because we can see both a star and a candle, we can make the call that the candle is more like the appearance of a star than is a log. But since we can't experience gravity with any of our senses, we cannot make the call as to whether one model of it is any closer to the real thing than another model. In short, we can comment about the truth of representational models such as a candle or a picture, but not the truth of models about abstract things such as time and gravity, anymore than we can about models of the background field of a multiverse, or some putative God, or what honesty is "like." If it waddles and quacks like a duck, chances are it is a duck, but if you've never seen a graviton, what can you point to and say, "It looks like that" or "It tastes like that" ? Edited June 16, 2016 by disarray
swansont Posted June 16, 2016 Posted June 16, 2016 I don't think the aether was ever exactly discredited. It just doesn't add anything, or explain anything further, to the model. It just adds unnecessary complexity, and is superfluous or redundant. Aether proponents couldn't come up with independent ways to successfully test the idea — everything was ad-hoc. We knew we couldn't be at rest with the aether, because of stellar aberration measurements. So when M-M tried to measure our speed and got essentially zero, the aether model failed. All subsequent efforts (e.g. partial entrainment) had to compete with relativity, and that where Occam comes into play, at least in part.
Strange Posted June 16, 2016 Posted June 16, 2016 I am gathering that one might make a list of those things in which the representative model is like the thing that it is trying to replicate and those that are quite different (aka signs similarity to the signified OR shadows on the wall of Plato's allegorical cave similarity to the objects seen in the light of day). As we have no access to the signified ("reality") other than through our models, and don't even know if it exists, such discussions are outside of the domain of science. We observe things, make models, test the models and make better models. It is purely a matter of belief whether those models pertain to anything real or not. Apparently this question is of great interest to some people. (There is a thread devoted to this subject on another science forum which has been the most active thread for about 2 years but, as far as I can tell, the same people are still just saying the same things!)
swansont Posted June 16, 2016 Posted June 16, 2016 I certainly can't follow the mathematics of GR, but I have learnt enough to know that "space curving" or "space expanding" are just images or metaphors to describe the maths. The important thing is to remember that and not start trying to extrapolate from those analogies (or as some people do, try to falsify the science based on those analogies). Geometry, even before GR, was just a way of describing (mathematically) the relationships between points in space. What GR added was the knowledge that these relationships are (a) not fixed and (b) not based on traditional straight-line Euclidean geometry. I hesitate to call geometry a metaphor, but I can see why "curved space" is tougher to latch onto for people who haven't studied physics. If you progress far enough you will be introduced to some other simple coordinate systems — cylindrical and spherical — and solve problems using them. So even if the details of GR are more advanced than that, and the curves are different, the concept of having a coordinate system that is not Euclidean is quite familiar.
Strange Posted June 16, 2016 Posted June 16, 2016 I hesitate to call geometry a metaphor, but I can see why "curved space" is tougher to latch onto for people who haven't studied physics. If you progress far enough you will be introduced to some other simple coordinate systems — cylindrical and spherical — and solve problems using them. So even if the details of GR are more advanced than that, and the curves are different, the concept of having a coordinate system that is not Euclidean is quite familiar. I think that is what I was trying to say! I don't think the geometry is a metaphor; that is what the theory is (and what reality is, if you will). The metaphor comes in when this is visualised as a rubber sheet or some "thing" called space being curved.
disarray Posted June 16, 2016 Posted June 16, 2016 (edited) @strange, you say that "I don't think the geometry is a metaphor; that is what the theory is (and what reality is, if you will). The metaphor comes in when this is visualised as a rubber sheet or some "thing" called space being curved." Saying that "what reality is" suggests to me that you think, as even Godel did, that numbers have some sort of Platonic/noumenal/real existence, and that it is only when we try to visualize what the numbers look like in a diagram that we move further away from that reality (even though such illustrations are greatly useful as a teaching tool). But I suspect that the majority of mathematicians and physicists would suggest that numbers just provide a symbolic tool that has its uses, and that different types of mathematics work for different "realities" under investigation, e.g., Newtonian for some things and quantum for others, Euclidean geometry for some and non-Euclidean for others. For example, Einstein remarking that numbers and number-based laws are not reality: “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.” But yes, this is getting into the area of philosophy and is probably of less interest to scientists who are more focused on the application of knowledge. I have no problem with the "shut up and measure" approach that some quantum physicists advocate, when it comes to asking what is really beyond what we can experience. My question was really an academic one in which I was asking whether it might be possible to make a set of criteria for determining when or whether a scientific model ceased to be representational (e.g., a 5th grade problem about figuring out the area of a football field) and became more counter-intuitive and/or heuristic, (e.g., Bohr's suggestion that we not assume that an electron has a definite pre-measurement position that can be identified by some set of coordinates, and thus can not construct a 'realistic' representational model of where it might be at other than to give some numeric probability). Edited June 16, 2016 by disarray
Strange Posted June 16, 2016 Posted June 16, 2016 @strange, you say that "I don't think the geometry is a metaphor; that is what the theory is (and what reality is, if you will). The metaphor comes in when this is visualised as a rubber sheet or some "thing" called space being curved." Saying that "what reality is" suggests to me that you think, as even Godel did, that numbers have some sort of Platonic/noumenal/real existence, and that it is only when we try to visualize what the numbers look like in a diagram that we move further away from that reality (even though such illustrations are greatly useful as a teaching tool). I added "if you will" to indicate that some interpret that geometry as describing reality, while others just think of it as a model that works. I don't really have an opinion. My question was really an academic one in which I was asking whether it might be possible to make a set of criteria for determining when or whether a scientific model ceased to be representational (e.g., a 5th grade problem about figuring out the area of a football field) and became more counter-intuitive and/or heuristic, (e.g., Bohr's suggestion that we not assume that an electron has a definite pre-measurement position that can be identified by some set of coordinates, and thus can not construct a 'realistic' representational model of where it might be at other than to give some numeric probability). I don't think so. It is an arbitrary preference as to where you want to draw the line between models and reality, with idealists at one end and naive realists at the other.
MigL Posted June 16, 2016 Posted June 16, 2016 The only reason for the aether was to explain what EM actually 'waves'. All known waves were energetic deformations of a medium and so EMR was thought to need a medium to 'wave'. The properties of this medium would have added immeasurable complexity, and would make it undetectable. Relativity had none of this complexity ( ha ! ) and didn't require a medium for EM. So the aether became redundant.
Markus Hanke Posted June 16, 2016 Posted June 16, 2016 But since we can't experience gravity with any of our senses That depends what you mean by "experience". For example, take three test particles which are spaced radially, and let them freely fall towards a central mass, like so : As time goes by, they all approach the central body, but they also increase the distance between them. Why ? Not because there are any forces acting on them ( remember there is no acceleration, so a=0, and hence F=ma=0 on all these particles ), but because they are in a spacetime that is not flat, so as they age into the future ( they can't do anything else ! ), their spatial position will change. That's curvature right there for you - it is not just some abstract, theoretical concept, but something quite tangible, and it can be easily observed. The "engine" that drives all dynamics here is the fact that everything ages into the future, and, because time and space are intrinsically linked, spatial positions and distances change as that happens. This of course also works if you arrange the particles horizontally instead of radially : While the mathematical description might be quite involved ( I plead no contest in that regard ), the actual meaning of the equations is really quite simple, and anyone can grasp it - they are just descriptions of what happens to test particles. 2
Strange Posted June 16, 2016 Posted June 16, 2016 (edited) John Baez provides a good overview of the equations and their meaning, which includes more examples like the above. So, even if you can't follow the maths (me!) you should be able to get something from it: http://math.ucr.edu/home/baez/einstein/ Edited June 16, 2016 by Strange
Velocity_Boy Posted June 16, 2016 Posted June 16, 2016 Re-read the link you yourself posted... "One of the reasons for this position is that there is no direct experimental evidence for the existence of the ether - everything can be explained without it, hence the Occam's razor approach." ( fourth paragraph of the second article - The Mysterious Ether ) If you're gonna disagree with me, at least make it a challenge, and don't provide me with the information to defeat your argument. When Clerk Maxwell wrote to D.P. Todd of the U.S. Nautical Almanac Office in Washington in 1879, he inquired about the possibility of measuring the velocity of the solar system through the ether by observing the eclipses of Jupiter's moons. Roemer had used measurements of the eclipse times to obtain a number for the speed of light. Maxwell concluded that the effects he sought were too small to measure - but that assertion came to the attention of a young naval instructor named A. A. Michelson who had just been transferred to that office. In 1878, Michelson had made an excellent measurement of the speed of light at the age of 25, and he thought the detection of motion through the ether might be measurable. Michelson proceeded to invent a new instrument with accuracy far exceeding that which had been attained to that date, and that instrument is now universally called the Michelson interferometer. In trying to measure the speed of the Earth through the supposed "ether", you could depend upon one component of that velocity being known - the velocity of the Earth around the sun, about 30 km/s. Using a wavelength of about 600 nm, there should be a shift of about 0.04 fringes as the spectrometer was rotated 360°. Though small, this was well within Michelson's capability. Michelson, and everyone else, was surprised that there was no shift. Michelson's terse description of the experiment: "The interpretation of these results is that there is no displacement of the interference bands. ... The result of the hypothesis of a stationary ether is thus shown to be incorrect." (A. A. Michelson, Am. J. Sci, 122, 120 (1881))
MigL Posted June 16, 2016 Posted June 16, 2016 (edited) Congratulations Velocity_Boy. You posted evidence that the aether is not STATIONARY. How exactly does that 'de-bunk' the aether model ? ( sorry Moontanman, none of this answers your question, does it ? ) Edited June 16, 2016 by MigL
Strange Posted June 16, 2016 Posted June 16, 2016 Congratulations Velocity_Boy. You posted evidence that the aether is not STATIONARY. How exactly does that 'de-bunk' the aether model ? Because other experiments show it can't be moving with he Earth either. So: If it exists, it needs to have physically implausible properties It isn't required by any theory It can't be stationary It can't be moving with the Earth It can't be detected So what was the point, again?
MigL Posted June 17, 2016 Posted June 17, 2016 And that's exactly what I've said all along, Strange. It may be just semantics, but... Those 'physically implausible properties' would make it extremely hard ( if not impossible ) to detect, much less 'de-bunk' and 'disprove', as another member claims. And IF it had those properties The M-M experiment would not have found a fringe shift no matter how accurate it was made. But the model seems to work just fine without a medium for EM waves so it is redundant ( not disproven ).
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