5 ms·

But of course. We reside in one of them. Space ("bigger") and time are interrelated concepts (same damn thing/Minkowski vector) - if you have less time in an a

by madaxe 13y ago

But of course. We reside in one of them. Space ("bigger") and time are interrelated concepts (same damn thing/Minkowski vector) - if you have less time in an area (i.e. due to a gravity well, like Earth's) you can equally view it as more space. Ergo, the "volume" of "space" measured from within Earth's gravity well out to a fixed distance (say EML-1, for the sake of argument), differs to the same volume as measured from outside of the gravity well. There's also another way of looking at their hypothesis. If cosmic voids have less space in them (i.e. more time) (if you don't have any matter-energy in a volume, then what "keeps" time?), this would explain redshift pretty neatly in terms of the refractive index of space-time. Beats the hell out of dark matter and dark energy, both of which are fudge-factors for a fundamental facet of the nature of the universe which we Do Not Understand yet. It's all relative.

pdonis 13y ago
Space ("bigger") and time are interrelated concepts (same damn thing/Minkowski vector) - if you have less time in an area (i.e. due to a gravity well, like Earth's) you can equally view it as more space. The metric in a gravity well like Earth's is not the Minkowski metric, so you can't interpret gravitational time dilation (which is what you are referring to by "less time" in a gravity well) the same way you would interpret time dilation in special relativity. The simplest way to see this is to observe that gravitational time dilation is present even when everything is at rest relative to each other, whereas time dilation in special relativity requires relative motion. the "volume" of "space" measured from within Earth's gravity well out to a fixed distance (say EML-1, for the sake of argument), differs to the same volume as measured from outside of the gravity well. This is not correct as you state it. What is correct is that the volume you measure depends on your state of motion: the volume of space you will measure out to a certain radius from the Earth will be larger if you are at rest relative to the Earth than if you are free-falling inward towards the Earth. But two observers both in the same state of motion will measure the same volume of space out to a fixed radius from the Earth, regardless of their location relative to Earth's gravity well. If cosmic voids have less space in them This isn't consistent with their hypothesis; their hypothesis is that the voids have more space in them, not less. So you can't combine this with anything in their model to get a valid answer; you're starting with inconsistent premises. (i.e. more time) Less space does not mean more time; see above. (if you don't have any matter-energy in a volume, then what "keeps" time?) The voids don't have zero matter-energy; they just have a lot less than other parts of the universe (at least, according to our best current observations.) That said, spacetime itself can "keep time" even if there is no matter-energy present in a particular location.
- astrobe_ 13y ago
  Since you seem to have some insights on the topic -- the article points out that > Lavinto and co say that when light enters a Tardis region, it is deflected sharply by the greater curvature there. That’s not what astronomers observe at all Given that "light" is, I believe, our only way to observe the universe, how can one tell that it is not deflected? I mean if for instance the heliosphere had some unknown refractive properties, how could we discover it since we only have light to determine the position of objects outside of the solar system?
  - JonnieCache 13y ago
    I think until gravitational wave observatories start to work, we are indeed stuck with light. And maybe neutrinos and other particles, but they aren't very good for observing things due to our inability to make a "lens" for them, no?
  - yk 13y ago
    We measure spectra. So there would be no way to measure the redshift of a single wavelength, e.g. a laser, but since we have the full spectra, we can measure the relative position of spectral features and the wavelength of these features to compare them with laboratory measurements.
    - pdonis 13y ago
      we can measure the relative position of spectral features That will only tell us about redshift/blueshift; it won't tell us anything about what path the light took to get to us.
      - yk 13y ago
        Yes, but that is a rather general problem. Best you can do is to look at the so called Lyman-alpha forest, which gives you essentially the mass distribution along the path. And this gives you a good estimate for the deflection.

by madaxe 13y ago

pdonis 13y ago

Space ("bigger") and time are interrelated concepts (same damn thing/Minkowski vector) - if you have less time in an area (i.e. due to a gravity well, like Earth's) you can equally view it as more space. The metric in a gravity well like Earth's is not the Minkowski metric, so you can't interpret gravitational time dilation (which is what you are referring to by "less time" in a gravity well) the same way you would interpret time dilation in special relativity. The simplest way to see this is to observe that gravitational time dilation is present even when everything is at rest relative to each other, whereas time dilation in special relativity requires relative motion. the "volume" of "space" measured from within Earth's gravity well out to a fixed distance (say EML-1, for the sake of argument), differs to the same volume as measured from outside of the gravity well. This is not correct as you state it. What is correct is that the volume you measure depends on your state of motion: the volume of space you will measure out to a certain radius from the Earth will be larger if you are at rest relative to the Earth than if you are free-falling inward towards the Earth. But two observers both in the same state of motion will measure the same volume of space out to a fixed radius from the Earth, regardless of their location relative to Earth's gravity well. If cosmic voids have less space in them This isn't consistent with their hypothesis; their hypothesis is that the voids have more space in them, not less. So you can't combine this with anything in their model to get a valid answer; you're starting with inconsistent premises. (i.e. more time) Less space does not mean more time; see above. (if you don't have any matter-energy in a volume, then what "keeps" time?) The voids don't have zero matter-energy; they just have a lot less than other parts of the universe (at least, according to our best current observations.) That said, spacetime itself can "keep time" even if there is no matter-energy present in a particular location.

astrobe_ 13y ago
Since you seem to have some insights on the topic -- the article points out that > Lavinto and co say that when light enters a Tardis region, it is deflected sharply by the greater curvature there. That’s not what astronomers observe at all Given that "light" is, I believe, our only way to observe the universe, how can one tell that it is not deflected? I mean if for instance the heliosphere had some unknown refractive properties, how could we discover it since we only have light to determine the position of objects outside of the solar system?
- JonnieCache 13y ago
  I think until gravitational wave observatories start to work, we are indeed stuck with light. And maybe neutrinos and other particles, but they aren't very good for observing things due to our inability to make a "lens" for them, no?
- yk 13y ago
  We measure spectra. So there would be no way to measure the redshift of a single wavelength, e.g. a laser, but since we have the full spectra, we can measure the relative position of spectral features and the wavelength of these features to compare them with laboratory measurements.
  - pdonis 13y ago
    we can measure the relative position of spectral features That will only tell us about redshift/blueshift; it won't tell us anything about what path the light took to get to us.
    - yk 13y ago
      Yes, but that is a rather general problem. Best you can do is to look at the so called Lyman-alpha forest, which gives you essentially the mass distribution along the path. And this gives you a good estimate for the deflection.