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Maybe a physicist can tell me why I'm wrong here: We often visualize the effect of mass on spacetime by using a trampoline and a bowling ball. The outer edges

Maybe a physicist can tell me why I'm wrong here: We often visualize the effect of mass on spacetime by using a trampoline and a bowling ball. The outer edges are flat and the closer you get to the bowling ball the more spacetime is warped downward. On the outer edges, the effect is almost nonexistent and the trampoline's surface is nearly flat. What this suggests is that spacetime without mass is flat and that mass warps spacetime in a downward direction. So no mass = flat, mass = curved downward. So the effect is in the range of [0, black hole]. Obviously this is an very loose analogy but bear with me. What if spacetime acts more like a waterbed? Not only does mass warp spacetime "downward" but something (in our analogy the displaced water) pushes the rest of spacetime upward. To use a signal processing analogy, a filter will (nearly?) always affect the passband to some degree, with very high Q filters creating a knee where amplitude is increased significantly before the filter rolls off. That seems counterintuitive (at least to me) but it's nonetheless true. Using another analogy, curved spacetime creates regions of "downhill-ness". What if the space between masses get a little bit of uphill curve as well? That could explain e.g., the Pioneer anomaly and possibly at much larger scales the effect in the article.

jaddood 10y ago
Actually, considering the effects of dark matter, it seems (at least to me) to be a great candidate for what you're talking about.
paulddraper 10y ago
General relativity, which describes the effect of mass on space-time, is a very elegant theory with beautiful maths. The trampoline and ball have incidental similarity, but they're for illustration only. There's nothing that makes the waterbed theory necessarily wrong, but you'd have to posit some new maths different and more complicated than we have with GR.
- mbenjaminsmith 10y ago
  Yeah, I understand that the trampoline analogy is inadequate but I believe that we don't consider empty regions of space to have any type of "opposite" curvature or to exhibit negative gravity. I believe curvature is thought to be [0, black hole], not [-black hole, black hole]. Happy to be corrected on that, IANAP. The disparity between what GR predicts and what we observe has led to the cosmological constant / dark energy / dark matter. Of course, those aren't really things in themselves, they're just a measure of that disparity, a fudge factor. I'm suggesting that the disparity could be explained by spacetime "responding" to regions of curvature by mass with an opposite curvature. Unfortunately I'm not qualified to do anything with that idea. I just wanted to throw it out there. It's something that's been bothering me for a long time.
  - jeremyjh 10y ago
    New ideas in physics cannot start with an analogy - they need to work mathematically with all observed data first.
    - rdtsc 10y ago
      Didn't plenty of new ideas start with analogies? I can think of Feynman and Einstein who described imagining or noticing things in real life which gave rise new ideas in physics.
      - dhimes 10y ago
        Probably just a semantic difference in what they mean by "start." In this instance, the waterbed analogy would suggest some sort of negative mass-energy and a mass-energy conservation law.
      - paulddraper 10y ago
        They started by observations (e.g. light is constant speed), and using those in thought experiments (e.g. light on a train going 0.5c). They didn't start by noticing everyday phenomenon and extrapolating those observations. Similar, but different.
        paulddraper 10y ago
        ...unless you're talking about string theory. That's a close comparison to the waterbed theory.
      - mannykannot 10y ago
        You would have to explain to some extent what the analogy is, and what it means for the real world, before we could begin to discuss whether it is in any way correct, let alone useful.
    - Kroniker 10y ago
      Just to be clear- are you implying that people cannot be inspired by natural events and think of an analogy elsewhere in physics? Or are you saying that an analogy cannot be the firmament of a new theory? I would agree with the latter, but it sounds like you're stating the former, which is categorically incorrect
tylerjwilk00 10y ago
Not a physicist, but I've been quietly hoping something like this was the case for years and would usurp Dark Matter as the reason for rotational anomaly at the outer galactic edges. It makes sense that space-time is both attractive or repulsive based on the presence or absence matter. Throw in an overlapping inverse square law at the galactic boundary and perhaps this solves the anomaly by way of a steep gravity wall where attractive and repulsive forces meet. Again, not a physicists either, but I agree, it seems to make sense spatially.
zeroer 10y ago
The analogy to trampolines and bowling balls is leading you astray. There's no "pushing down", or even a downward direction to push in in general relativity.
pif 10y ago
Question: why don't you try and learn the foundations of the current theories (it's incredible how many man-centuries of high quality intellectual work you can find condensed into a book) before launching into inventing new theories out of nothing? Newton thanked the giants whose shoulders he was standing on. You are pretending you are a giant on your own. Do you sincerely believe it?
- rdtsc 10y ago
  Right but he was also hit on head by an apple according to the myth. So there place for analogies like that. Also I thought your comment seemed a bit too disparaging. GP had an idea they wanted to share. There is probably a nicer way of saying it won't work than "why don't you go to the library and learn the maths before speaking up..."
  - pif 10y ago
    > There is probably a nicer way of saying it won't work ... The question is not why it doesn't work. The real issue is why someone who worked for years in order to become expert on a certain field should spend some of his time to consider a theory coming out of absolutely nothing. There may be a nicer way to say this; please help me find it ;-)
    - scotty79 10y ago
      Because it's fun to pinpoint what exactly is wrong with some superficially plausible idea. I can uderstand that it gets very old very fast for some people but there are physicists who like educating laymen by putting themselves in their shoes and pinpointing exact spot where they believe something that actually is not true.
    - SomeStupidPoint 10y ago
      There isn't one because it doesn't need to be said. It's also not true: the theory came out of questioning what would happen if you replaced one set of equations in the model with another, and whether that might make a more interesting (or accurate) model. It's simply expressed in informal language, but the formal (and completely sensible translation) is straightforward to anyone with the necessary background. As for why experts would care, anywhere from dinner conversation (as is likely to be the case for myself; not a physics expert, but my company is, and Im curious of the answer) to it strikes a cord related to their work by giving them a new analogy, allowing them to bring more expertise to bear on the project. For my professional work, some of the biggest influences have been questions by amateurs (and the subsequent trying to address them). If it were actually such a useless idea, you'd have spent less time just refuting it than with your unnecessarily negative posts. Instead, you were negative for no clear reason (though, several uncharitable reasons might be inferred). Please refrain from making such negative posts here. They make the community worse.
deleted 10y ago
[deleted]
gpderetta 10y ago
relevant xkcd: https://xkcd.com/895/ https://xkcd.com/895/
acqq 10y ago
> could explain e.g., the Pioneer anomaly The reason for the Pioneer anomaly is considered known since 2012: https://arxiv.org/abs/1204.2507 https://arxiv.org/abs/1204.2507

jaddood 10y ago

Actually, considering the effects of dark matter, it seems (at least to me) to be a great candidate for what you're talking about.

paulddraper 10y ago

General relativity, which describes the effect of mass on space-time, is a very elegant theory with beautiful maths. The trampoline and ball have incidental similarity, but they're for illustration only. There's nothing that makes the waterbed theory necessarily wrong, but you'd have to posit some new maths different and more complicated than we have with GR.

mbenjaminsmith 10y ago
Yeah, I understand that the trampoline analogy is inadequate but I believe that we don't consider empty regions of space to have any type of "opposite" curvature or to exhibit negative gravity. I believe curvature is thought to be [0, black hole], not [-black hole, black hole]. Happy to be corrected on that, IANAP. The disparity between what GR predicts and what we observe has led to the cosmological constant / dark energy / dark matter. Of course, those aren't really things in themselves, they're just a measure of that disparity, a fudge factor. I'm suggesting that the disparity could be explained by spacetime "responding" to regions of curvature by mass with an opposite curvature. Unfortunately I'm not qualified to do anything with that idea. I just wanted to throw it out there. It's something that's been bothering me for a long time.
- jeremyjh 10y ago
  New ideas in physics cannot start with an analogy - they need to work mathematically with all observed data first.
  - rdtsc 10y ago
    Didn't plenty of new ideas start with analogies? I can think of Feynman and Einstein who described imagining or noticing things in real life which gave rise new ideas in physics.
    - dhimes 10y ago
      Probably just a semantic difference in what they mean by "start." In this instance, the waterbed analogy would suggest some sort of negative mass-energy and a mass-energy conservation law.
    - paulddraper 10y ago
      They started by observations (e.g. light is constant speed), and using those in thought experiments (e.g. light on a train going 0.5c). They didn't start by noticing everyday phenomenon and extrapolating those observations. Similar, but different.
      - paulddraper 10y ago
        ...unless you're talking about string theory. That's a close comparison to the waterbed theory.
    - mannykannot 10y ago
      You would have to explain to some extent what the analogy is, and what it means for the real world, before we could begin to discuss whether it is in any way correct, let alone useful.
  - Kroniker 10y ago
    Just to be clear- are you implying that people cannot be inspired by natural events and think of an analogy elsewhere in physics? Or are you saying that an analogy cannot be the firmament of a new theory? I would agree with the latter, but it sounds like you're stating the former, which is categorically incorrect

tylerjwilk00 10y ago

Not a physicist, but I've been quietly hoping something like this was the case for years and would usurp Dark Matter as the reason for rotational anomaly at the outer galactic edges. It makes sense that space-time is both attractive or repulsive based on the presence or absence matter. Throw in an overlapping inverse square law at the galactic boundary and perhaps this solves the anomaly by way of a steep gravity wall where attractive and repulsive forces meet. Again, not a physicists either, but I agree, it seems to make sense spatially.

zeroer 10y ago

The analogy to trampolines and bowling balls is leading you astray. There's no "pushing down", or even a downward direction to push in in general relativity.

pif 10y ago

Question: why don't you try and learn the foundations of the current theories (it's incredible how many man-centuries of high quality intellectual work you can find condensed into a book) before launching into inventing new theories out of nothing? Newton thanked the giants whose shoulders he was standing on. You are pretending you are a giant on your own. Do you sincerely believe it?

rdtsc 10y ago
Right but he was also hit on head by an apple according to the myth. So there place for analogies like that. Also I thought your comment seemed a bit too disparaging. GP had an idea they wanted to share. There is probably a nicer way of saying it won't work than "why don't you go to the library and learn the maths before speaking up..."
- pif 10y ago
  > There is probably a nicer way of saying it won't work ... The question is not why it doesn't work. The real issue is why someone who worked for years in order to become expert on a certain field should spend some of his time to consider a theory coming out of absolutely nothing. There may be a nicer way to say this; please help me find it ;-)
  - scotty79 10y ago
    Because it's fun to pinpoint what exactly is wrong with some superficially plausible idea. I can uderstand that it gets very old very fast for some people but there are physicists who like educating laymen by putting themselves in their shoes and pinpointing exact spot where they believe something that actually is not true.
  - SomeStupidPoint 10y ago
    There isn't one because it doesn't need to be said. It's also not true: the theory came out of questioning what would happen if you replaced one set of equations in the model with another, and whether that might make a more interesting (or accurate) model. It's simply expressed in informal language, but the formal (and completely sensible translation) is straightforward to anyone with the necessary background. As for why experts would care, anywhere from dinner conversation (as is likely to be the case for myself; not a physics expert, but my company is, and Im curious of the answer) to it strikes a cord related to their work by giving them a new analogy, allowing them to bring more expertise to bear on the project. For my professional work, some of the biggest influences have been questions by amateurs (and the subsequent trying to address them). If it were actually such a useless idea, you'd have spent less time just refuting it than with your unnecessarily negative posts. Instead, you were negative for no clear reason (though, several uncharitable reasons might be inferred). Please refrain from making such negative posts here. They make the community worse.

deleted 10y ago

[deleted]

gpderetta 10y ago

relevant xkcd: https://xkcd.com/895/ https://xkcd.com/895/

acqq 10y ago

> could explain e.g., the Pioneer anomaly The reason for the Pioneer anomaly is considered known since 2012: https://arxiv.org/abs/1204.2507 https://arxiv.org/abs/1204.2507