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You can observe it via gravitational lensing. For instance: http://scienceblogs.com/startswithabang/2011/04/20/how-gravitational-lensing-show/ http://scienceblo

by thechao 10y ago

You can observe it via gravitational lensing. For instance: http://scienceblogs.com/startswithabang/2011/04/20/how-gravitational-lensing-show/ http://scienceblogs.com/startswithabang/2011/04/20/how-gravi... .

lngnmn 10y ago
You can observe nothing but the fact that mass distorts light. Everything else is imagination.
chongli 10y ago
We can observe the effect of gravitational lensing. The claim that dark matter causes it is part of the same claim that dark matter is responsible for galactic rotation curves looking the way they do. Thus it does not provide any further support for the dark matter hypothesis.
- deleted 10y ago
  [deleted]
dogma1138 10y ago
We've seen instances already where gravitational lensing was explained through other means (very dim clusters, dust, and even camera issues), we also don't know if the space time on those scales is "smooth", if it isn't then light will not travel in a straight line which would account for the lensing effects. The spacetime curvature, mass/energy, and gravity have quite a few interesting interactions as in gravity as a force, vs gravity as a feature of spacetime, vs potential other forms of gravity. We aren't even sure at this point if a curved spacetime without mass/energy that would generate that curve, would actually produce gravity or not since we aren't able to observe this nor do we have a complete theory of gravity.
- XorNot 10y ago
  Except we have good evidence that the universe is very close to flat, space-time wise. A hypothesis that there's local unexplained curvature...Well that's really what dark matter is all about. What's curving it?
  - pharrington 10y ago
    edit: Just ignore my comment, it's wrong. (for posterity) My understanding is that spacetime being flat is akin to it being the surface of the universe's curves. Explain, expand, or correct me if that's wrong; I'm certainly no physicist.
- ajross 10y ago
  > we also don't know if the space time on those scales is "smooth", if it isn't then light will not travel in a straight line which would account for the lensing effects. Except space isn't perfectly smooth. It does curve, and that curvature is gravity. That's what general relativity is all about. If you want to postulate some way for the universe to curve absent observable mass, go ahead: you're doing nothing but restating the dark matter problem in a (minimally) different way.
  - raattgift 10y ago
    > If you want to postulate some way for the universe to curve absent observable mass The de Sitter vacuum is an exact solution to the Einstein Field Equations of General Relativity that does exactly that. Our expanding universe will very likely closely approximate de Sitter space in the extreme far future. There are many other vacuum solutions; it's normal in General Relativity to think of curvature as being a background laid down by hand (ideally approximating nature) and then perturbing that background with matter or probing it with test particles. > Except space isn't perfectly smooth. General Relativity is defined on a smooth (infinitely differentiable) manifold. This is very important! https://en.wikipedia.org/wiki/Smoothness https://en.wikipedia.org/wiki/Smoothness There can be arbitrary curvature in the manifold, however, and unfortunately with the Einstein Field Equations you can pretty easily produce manifolds with curvature singularities, and those are manifestly not smooth (and that is one driver for research aiming to extend General Relativity). But to the first order, your experience of gravitation near the surface of the Earth emerges from the very weak manifold curvature the planet and its microscopic components and their motions source. The nearest strong curvature is a couple of kiloparsecs away.

by thechao 10y ago

lngnmn 10y ago

You can observe nothing but the fact that mass distorts light. Everything else is imagination.

chongli 10y ago

We can observe the effect of gravitational lensing. The claim that dark matter causes it is part of the same claim that dark matter is responsible for galactic rotation curves looking the way they do. Thus it does not provide any further support for the dark matter hypothesis.

deleted 10y ago
[deleted]

dogma1138 10y ago

We've seen instances already where gravitational lensing was explained through other means (very dim clusters, dust, and even camera issues), we also don't know if the space time on those scales is "smooth", if it isn't then light will not travel in a straight line which would account for the lensing effects. The spacetime curvature, mass/energy, and gravity have quite a few interesting interactions as in gravity as a force, vs gravity as a feature of spacetime, vs potential other forms of gravity. We aren't even sure at this point if a curved spacetime without mass/energy that would generate that curve, would actually produce gravity or not since we aren't able to observe this nor do we have a complete theory of gravity.

XorNot 10y ago
Except we have good evidence that the universe is very close to flat, space-time wise. A hypothesis that there's local unexplained curvature...Well that's really what dark matter is all about. What's curving it?
- pharrington 10y ago
  edit: Just ignore my comment, it's wrong. (for posterity) My understanding is that spacetime being flat is akin to it being the surface of the universe's curves. Explain, expand, or correct me if that's wrong; I'm certainly no physicist.
ajross 10y ago
> we also don't know if the space time on those scales is "smooth", if it isn't then light will not travel in a straight line which would account for the lensing effects. Except space isn't perfectly smooth. It does curve, and that curvature is gravity. That's what general relativity is all about. If you want to postulate some way for the universe to curve absent observable mass, go ahead: you're doing nothing but restating the dark matter problem in a (minimally) different way.
- raattgift 10y ago
  > If you want to postulate some way for the universe to curve absent observable mass The de Sitter vacuum is an exact solution to the Einstein Field Equations of General Relativity that does exactly that. Our expanding universe will very likely closely approximate de Sitter space in the extreme far future. There are many other vacuum solutions; it's normal in General Relativity to think of curvature as being a background laid down by hand (ideally approximating nature) and then perturbing that background with matter or probing it with test particles. > Except space isn't perfectly smooth. General Relativity is defined on a smooth (infinitely differentiable) manifold. This is very important! https://en.wikipedia.org/wiki/Smoothness https://en.wikipedia.org/wiki/Smoothness There can be arbitrary curvature in the manifold, however, and unfortunately with the Einstein Field Equations you can pretty easily produce manifolds with curvature singularities, and those are manifestly not smooth (and that is one driver for research aiming to extend General Relativity). But to the first order, your experience of gravitation near the surface of the Earth emerges from the very weak manifold curvature the planet and its microscopic components and their motions source. The nearest strong curvature is a couple of kiloparsecs away.