Timeline for Correct definition of “speed” of gravity
Current License: CC BY-SA 4.0
9 events
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| Jul 11, 2020 at 16:31 | comment | added | Jacopo Tissino | @dan: I think the container-content idea is bringing you off course. A better way to see it is to consider the spacetime manifold as the basic unit from which we can construct tensor fields which describe particles and such. One of the tensor fields we use is $g_{\mu\nu}$, the metric, which we can use to measure distances and which in GR becomes a dynamical field, changing according to the mass of things. Mathematically, the manifold can be defined without a metric. All of these tensor fields must obey certain laws (their excitations cannot exceed a speed of $c$, for example). | |
| Jul 10, 2020 at 16:38 | comment | added | athena | @Steve: I understand your point of talking of a "density" of space-time itself. For me it is the same topological confusion of between the container and the content. The two can't be "inside" the same frame of laws. To say it bluntly: highway and cars don't obey the same speed limits :). | |
| Jun 3, 2020 at 10:28 | comment | added | fqq | There's no need for any of that, a basic understanding of differential geometry is enough to solve these "problems". Anyway, this is all off topic and does not help the answers. | |
| Jun 2, 2020 at 23:27 | comment | added | Steve | I should add as well, gandalf61's reformulation of the analogy as being (effectively) a redistribution of the rubber sheet's density across its surface (rather than a stretching of its surface into an orthogonal dimension) is not much better, because it still begs the question of what it means for space itself to have a density, when density is a measure of how much of something is found "within a locality of space". Jettison the "distortion of space entirely", and just admit you're employing the concept of a Lorentzian aether but refusing to speak its name. | |
| Jun 2, 2020 at 23:14 | comment | added | Steve | @fqq, I'm not insinuating GR is wrong. I'm saying the analogy is wrong, as it simply raises more questions than it answers. It's the sort of analogy that is beloved of those who possess no critical faculties, no inclination to ask the relevant question of the analogy, which is that if the 3rd dimension is what the 2d sheet stretches into, then into what does space-time stretch? It is well-known that "distortion of space-time" is merely an interpretation of GR (along with many other philosophical oddballs in physics), and it can be jettisoned entirely without impugning the correctness of GR. | |
| Jun 2, 2020 at 22:56 | comment | added | fqq | @Steve the analogy is not exact, but GR works very well and does not need extra dimensions. Comments here are not the appropriate place to insinuate that GR is wrong. | |
| Jun 2, 2020 at 17:38 | comment | added | Steve | The problem with the rubber sheet analogy is that the stretch has to extend into another dimension which is additional to the kind of universe being described. It's easier to stop pretending that it's the fabric of space being stretched, and that it's just the things in space which are being modified. In the same way that when we use a magnet to attract a nail on a worktop, it's much easier to see the sense that the magnet is moving the nail by the presence of a magnetic field, not that space is being stretched or condensed so as to bring the nail closer without it actually moving. | |
| Jun 2, 2020 at 11:06 | comment | added | athena | Fully agreed with your better analogy. This is my way of understanding distortion of space-time itself. The rubber sheet is fully wrong since it is the result of Earth’s gravitation perpendicular to the rubber sheet. This is very bad analogy everyone should try to remove. BTW I also fully understand your proof there is no need of extra-dimension for a ”distortion” to exist. But on purpose I didn’t use the word “move” here. | |
| Jun 2, 2020 at 10:00 | history | answered | gandalf61 | CC BY-SA 4.0 |