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I don't think the bounds are tight or it can ever be. If you don't know something, you just don't know. We don't know much about negative mass or if gravitons e

I don't think the bounds are tight or it can ever be. If you don't know something, you just don't know. We don't know much about negative mass or if gravitons exist yet, two things which could make apples fall upward. There's nothing that allows us to say with absolute certainty that it isn't possible, especially on planets billions of light years away that we have no visibility into at all.

mjburgess 5y ago
We can't assign 0 probability to any position on the grounds of "humility", but "negative mass" assuming our best theories are true is probability-0. The relevant probabilities when making predictions are under the best theoretical assumptions we can make; which includes that the universe follows the same laws of physics everywhere. It isn't very interesting to observe that, absent these, no proposition has a zero probability. Eg., you dont get to claim that given the laws of physics it is possible to have a negative mass, because given no laws of physics, it is possible. When you arent given any information at all, all propositions are equally likely. Propositions like, "the universe has a maximum speed" aren't merely observational. They follow with probability-1 from fundamental universal laws of physics. If this is false, all of physics is false.
- SketchySeaBeast 5y ago
  (I realize we may in fact be saying the same thing, but I just want to express it in a different way.) > assuming our best theories are true Isn't that what the course of history teaches us, that our best theories usually aren't true? At one time spontaneous generation was our best theory on where frogs came from after it rained, and it was based upon our best observations at the time. I'm not sure why this particular point in time would be special. I'm not saying that the current laws of physics aren't a great way to model the universe as we currently observe it, but it doesn't mean they actually are true and we have to be open to that possibility, if something that doesn't fit our current models and laws appear. But not until then, mind you.
  - mjburgess 5y ago
    Yes, but I think we need to be clear here on what we mean by theory, and what information we're taking as a given. The proposition, "steel bars in this lab fall to the earth" being /false/ doesn't impact essentially any theory of physics if there are magnets in the lab. If you exclude every possible alternative, leaving gravity, then the proposition being false invalidates basically all of physics. Including the fundamental assumption that the laws of the universe are universal. So the question is what theories *require* a proposition to be true, in that, they logically entail it. When this is made clear it should be obvious that "maybe the maximum speed of the universe is different" is very very different to the claim, "maybe mars is habitable". Many lay people, entirely ignorant of the best theories we have, do not understand that the former requires extraordinary levels of evidence to take seriously; while the latter requires almost none. This type of "unknown unknown" argument often fails to make this distinction for the sake of making some fantasy as plausible as some possible reality.
- andi999 5y ago
  Well, we actually sort of know that something is different far away, since the rotation profile of galaxies do not match theoretical predictions. There are currently two different resolutions, either postulating new particles (dark matter and energy) or change of fundamental laws (MOND theories). Since we didn't manage to detect either in our solar system we know that something is different.

mjburgess 5y ago

We can't assign 0 probability to any position on the grounds of "humility", but "negative mass" assuming our best theories are true is probability-0. The relevant probabilities when making predictions are under the best theoretical assumptions we can make; which includes that the universe follows the same laws of physics everywhere. It isn't very interesting to observe that, absent these, no proposition has a zero probability. Eg., you dont get to claim that given the laws of physics it is possible to have a negative mass, because given no laws of physics, it is possible. When you arent given any information at all, all propositions are equally likely. Propositions like, "the universe has a maximum speed" aren't merely observational. They follow with probability-1 from fundamental universal laws of physics. If this is false, all of physics is false.

SketchySeaBeast 5y ago
(I realize we may in fact be saying the same thing, but I just want to express it in a different way.) > assuming our best theories are true Isn't that what the course of history teaches us, that our best theories usually aren't true? At one time spontaneous generation was our best theory on where frogs came from after it rained, and it was based upon our best observations at the time. I'm not sure why this particular point in time would be special. I'm not saying that the current laws of physics aren't a great way to model the universe as we currently observe it, but it doesn't mean they actually are true and we have to be open to that possibility, if something that doesn't fit our current models and laws appear. But not until then, mind you.
- mjburgess 5y ago
  Yes, but I think we need to be clear here on what we mean by theory, and what information we're taking as a given. The proposition, "steel bars in this lab fall to the earth" being /false/ doesn't impact essentially any theory of physics if there are magnets in the lab. If you exclude every possible alternative, leaving gravity, then the proposition being false invalidates basically all of physics. Including the fundamental assumption that the laws of the universe are universal. So the question is what theories *require* a proposition to be true, in that, they logically entail it. When this is made clear it should be obvious that "maybe the maximum speed of the universe is different" is very very different to the claim, "maybe mars is habitable". Many lay people, entirely ignorant of the best theories we have, do not understand that the former requires extraordinary levels of evidence to take seriously; while the latter requires almost none. This type of "unknown unknown" argument often fails to make this distinction for the sake of making some fantasy as plausible as some possible reality.
andi999 5y ago
Well, we actually sort of know that something is different far away, since the rotation profile of galaxies do not match theoretical predictions. There are currently two different resolutions, either postulating new particles (dark matter and energy) or change of fundamental laws (MOND theories). Since we didn't manage to detect either in our solar system we know that something is different.