3 ms·

> Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative numbe

> Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. This assertion is false. GR, even without FTL, allows for closed time-like loops, which violate causality. Violations of causality are not logical contradictions like dividing by zero. (Though, of course, there are alternative mathematical systems that allow for dividing by zero. And taking the square root of a negative number is surely not impossible. You just get an Imaginary result.) It's just that a universe in which causality is violated all the time would be a very strange one, and consequently, it is reasonable to assume that if there are, for instance, closed time-like loops in the universe, they are not traversed very often.

lutusp 14y ago
> GR, even without FTL, allows for closed time-like loops, which violate causality. The theory may "allow" them, but this is not to say they actually exist in reality. http://en.wikipedia.org/wiki/Causality_(physics) http://en.wikipedia.org/wiki/Causality_(physics) Quote: "In the theory of general relativity, the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause, even if the spacetime is curved." http://hipercom.inria.fr/~jacquet/retro1/crazy-science-corner/causality.html http://hipercom.inria.fr/~jacquet/retro1/crazy-science-corne... Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible."
- nessus42 14y ago
  > Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible." And there's no evidence that they are not possible. With no evidence either way, one must remain agnostic. On the other hand, we do have evidence that closed time-like loops are possible: GR has never been observed to be wrong, and GR predicts their existence under the right circumstances. It may certainly be the case, that even though closed time-like loops are possible, they do not actually exist in our universe and never will. Paradoxical universes are ruled out by logic alone, and so the space of possible universes without closed time-like loops (or least without humans traversing them) may outnumber the space of possible universes with time-like loops in them, allowing us to make a probabilistic argument against their occurrence in our universe. My transfinite number theory is rusty, however, so YMMV.
  - lutusp 14y ago
    > And there's no evidence that they are not possible. In science, the default precept toward ideas without evidence is the null hypothesis -- they are presumed to be false. This is why Bigfoot is not accepted as a scientific theory -- until it is observed, it doesn't exist. http://www.null-hypothesis.co.uk/science/item/what_is_a_null_hypothesis/ http://www.null-hypothesis.co.uk/science/item/what_is_a_null... Quote: "In statistics, the only way of supporting your hypothesis is to refute the null hypothesis. Rather than trying to prove your idea (the alternate hypothesis) right you must show that the null hypothesis is likely to be wrong – you have to ‘refute’ or ‘nullify’ the null hypothesis." Saying "there's no counterevidence, therefore ..." is not a scientific statement, because it can be said about anything not conclusively falsified, including meadowland fairies and honest politicians. > With no evidence either way, one must remain agnostic. Not in science. It's nice to have an open mind, but not to the degree that one's brains spill onto the ground.
    - nessus42 14y ago
      This is just untrue. When presented with a hugely precise and successful theory that has never been demonstrated wrong in even the slightest degree, that is certainly evidence (not proof, but evidence) towards whatever claims that theory makes. If you don't understand this, then you don't know what the word "evidence" means.
      - lutusp 14y ago
        > This is just untrue. When presented with a hugely precise and successful theory that has never been demonstrated wrong in even the slightest degree, that is certainly evidence (not proof, but evidence) towards whatever claims that theory makes. Nonsense -- perfect nonsense that the history of science repeatedly falsifies. 1. Science is empirical -- it must be. If it's not empirical, it's not science. 2. GR succeeded with every challenge that was put before it until it was compared to quantum theory, at which point it failed. Your position is that, because it had passed any number of difficult tests until then, it should have been assumed to reflect reality in all respects. But this is not how a scientist approaches theories and experiments. Any aspect of a theory that has not been tested, is assumed to be false until there is evidence to support it. The ether theory of the 19th century was successful until it failed one crucial test. The Phlogiston theory was successful until it failed one crucial test. The Ptolemaic theory of orbital mechanics seemed correct until Galileo looked through his telescope. All scientific theories are assumed to be false until there is evidence to support them, then, after evidence supports them, they are assumed to be perpetually falsifiable by new evidence. An unfalsifiable theory is not a scientific theory. > If you don't understand this, then you don't know what the word "evidence" means. Science is not law. Learn about science. > When presented with a hugely precise and successful theory that has never been demonstrated wrong in even the slightest degree ... You need to learn about quantum theory, which falsifies GR. Start here: http://library.thinkquest.org/04apr/01330/currentphysics/conflicts.htm http://library.thinkquest.org/04apr/01330/currentphysics/con... The present enthusiasm for superstring theory is that it might -- might! -- replace both GR and quantum theory, with a more comprehensive theory that would resolve the conflict between the two earlier theories.

lutusp 14y ago

> GR, even without FTL, allows for closed time-like loops, which violate causality. The theory may "allow" them, but this is not to say they actually exist in reality. http://en.wikipedia.org/wiki/Causality_(physics) http://en.wikipedia.org/wiki/Causality_(physics) Quote: "In the theory of general relativity, the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause, even if the spacetime is curved." http://hipercom.inria.fr/~jacquet/retro1/crazy-science-corner/causality.html http://hipercom.inria.fr/~jacquet/retro1/crazy-science-corne... Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible."

nessus42 14y ago
> Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible." And there's no evidence that they are not possible. With no evidence either way, one must remain agnostic. On the other hand, we do have evidence that closed time-like loops are possible: GR has never been observed to be wrong, and GR predicts their existence under the right circumstances. It may certainly be the case, that even though closed time-like loops are possible, they do not actually exist in our universe and never will. Paradoxical universes are ruled out by logic alone, and so the space of possible universes without closed time-like loops (or least without humans traversing them) may outnumber the space of possible universes with time-like loops in them, allowing us to make a probabilistic argument against their occurrence in our universe. My transfinite number theory is rusty, however, so YMMV.
- lutusp 14y ago
  > And there's no evidence that they are not possible. In science, the default precept toward ideas without evidence is the null hypothesis -- they are presumed to be false. This is why Bigfoot is not accepted as a scientific theory -- until it is observed, it doesn't exist. http://www.null-hypothesis.co.uk/science/item/what_is_a_null_hypothesis/ http://www.null-hypothesis.co.uk/science/item/what_is_a_null... Quote: "In statistics, the only way of supporting your hypothesis is to refute the null hypothesis. Rather than trying to prove your idea (the alternate hypothesis) right you must show that the null hypothesis is likely to be wrong – you have to ‘refute’ or ‘nullify’ the null hypothesis." Saying "there's no counterevidence, therefore ..." is not a scientific statement, because it can be said about anything not conclusively falsified, including meadowland fairies and honest politicians. > With no evidence either way, one must remain agnostic. Not in science. It's nice to have an open mind, but not to the degree that one's brains spill onto the ground.
  - nessus42 14y ago
    This is just untrue. When presented with a hugely precise and successful theory that has never been demonstrated wrong in even the slightest degree, that is certainly evidence (not proof, but evidence) towards whatever claims that theory makes. If you don't understand this, then you don't know what the word "evidence" means.
    - lutusp 14y ago
      > This is just untrue. When presented with a hugely precise and successful theory that has never been demonstrated wrong in even the slightest degree, that is certainly evidence (not proof, but evidence) towards whatever claims that theory makes. Nonsense -- perfect nonsense that the history of science repeatedly falsifies. 1. Science is empirical -- it must be. If it's not empirical, it's not science. 2. GR succeeded with every challenge that was put before it until it was compared to quantum theory, at which point it failed. Your position is that, because it had passed any number of difficult tests until then, it should have been assumed to reflect reality in all respects. But this is not how a scientist approaches theories and experiments. Any aspect of a theory that has not been tested, is assumed to be false until there is evidence to support it. The ether theory of the 19th century was successful until it failed one crucial test. The Phlogiston theory was successful until it failed one crucial test. The Ptolemaic theory of orbital mechanics seemed correct until Galileo looked through his telescope. All scientific theories are assumed to be false until there is evidence to support them, then, after evidence supports them, they are assumed to be perpetually falsifiable by new evidence. An unfalsifiable theory is not a scientific theory. > If you don't understand this, then you don't know what the word "evidence" means. Science is not law. Learn about science. > When presented with a hugely precise and successful theory that has never been demonstrated wrong in even the slightest degree ... You need to learn about quantum theory, which falsifies GR. Start here: http://library.thinkquest.org/04apr/01330/currentphysics/conflicts.htm http://library.thinkquest.org/04apr/01330/currentphysics/con... The present enthusiasm for superstring theory is that it might -- might! -- replace both GR and quantum theory, with a more comprehensive theory that would resolve the conflict between the two earlier theories.