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They would still 'see' gravitationally active stars turning into black holes. They would infer a form of anomalous radiation from seeing the stars collapsing, a

They would still 'see' gravitationally active stars turning into black holes. They would infer a form of anomalous radiation from seeing the stars collapsing, and they would guess that there are other fields that interact with this other form of matter.

pavel_lishin 11y ago
That's assuming that there are DM stars in the same place that our stars are (which is probably a safe assumption to make, if DM stars exist at all.)
- biswaroop 11y ago
  No, I'm referring to real matter stars. DM critters would see the gravitation from real stars, and would watch them implode into black holes.
- ars 11y ago
  A particle that interacts only with gravity can not make a star.
  - pavel_lishin 11y ago
    I'm hypothesizing that dark matter can interact with itself, just as our matter can interact with itself.
    - ars 11y ago
      It's more complicated than that. Think about it from an energy point of view - if it interacts with something it makes (transforms) energy. Where does that energy go? Normal particles get hot and make blackbody radiation, i.e. photons. Dark matter can't. Does it have a temperature? Can temperature flow between the particles? Without blackbody radiation you are going to have some violations of the laws of thermodynamics. Unless dark matter has some other particle to act as an equivalent. So you are hypothesizing an entire family of dark matter particles, and a whole set of new forces. Two entire universes each with entire sets of particles, that only interact via gravity. We could have a second sun and a second earth, that overlap us and never know it, we would just think that the gravitational constant is different (because our perceived mass is higher). Sounds like a wonderful setting for a science fiction/horror story.
      - pavel_lishin 11y ago
        There was a story on HN not too long ago about a proposed Dark QCD - implying different kinds of Dark Matter, Dark Chemistry, etc., a whole "parallel universe" of Dark Matter. And yup, my friends and I have been brainstorming story ideas all morning!
      - biswaroop 11y ago
        Actually, dark matter could make photons. Anti-dark-matter, if it exists, could annihilate and produce photons and radiation. http://www.space.com/9302-dark-matter-stars-solve-cosmic-mystery.html http://www.space.com/9302-dark-matter-stars-solve-cosmic-mys...
ars 11y ago
How would they see that? The only gravity you can see is the sum total of all fields that influence you. There is no ability to see gravity directionally, and if all the gravity causes you to accelerate you don't see anything at all. Normal particles can see the influence of gravity only particles, but gravity only particles in free fall can not, the experience nothing at all.
- biswaroop 11y ago
  >> Normal particles can see the influence of gravity only particles, but gravity only particles in free fall can not. This is a surprisingly interesting question. Can a dark particle 'feel' gravity? Most physicists think that any local experiment in a freely falling lab is independent of the velocity and position of the lab in spacetime (the strong equivalence principle). Here local means confined to the lab, but also small compared to variations in the gravitational field. However, you can setup nonlocal gravitational observatories in a number of ways that preserve the equivalence principle: 1) If the DM critters have a way to do non-local measurements, like measuring tidal forces, then they could measure gravity gradients over time. If they can communicate, they can set up gravity gradient sensors in freefall around stars and infer both the spatial distribution and time evolution of gravitational masses. 2) If the DM critters have clocks, they can measure the gravitational redshift around stars and discover stars. This should work even in a freely falling lab. 3) The DM critters could see gravitational waves passing their freely falling lab and infer changes in gravitational fields. This assumes gravitational waves exist (not a bad assumption).
  - ars 11y ago
    But a gravity only particle can't do any of those things. 1: They can't measure gradients because they can not "feel" another particle except via gravity. You can only measure a gravity gradient by comparing it to another force, but they have no second force to compare to. 2: They can't see redshift because they can't see photons at all. 3: They can't see gravitational waves for the same reason as #1, to see a wave you compare two particles (distance in current experiments), but they can't do that, any comparison would be via gravity and would be affected identically, so there is nothing to compare with. Interestingly pure energy only experiences gravity (I don't means photons, I mean simply energy, that "thing" which is conserved). Which is why I personally think dark matter is simply energy, there is no extra particle. The only question is what form does this energy take.

pavel_lishin 11y ago

That's assuming that there are DM stars in the same place that our stars are (which is probably a safe assumption to make, if DM stars exist at all.)

biswaroop 11y ago
No, I'm referring to real matter stars. DM critters would see the gravitation from real stars, and would watch them implode into black holes.
ars 11y ago
A particle that interacts only with gravity can not make a star.
- pavel_lishin 11y ago
  I'm hypothesizing that dark matter can interact with itself, just as our matter can interact with itself.
  - ars 11y ago
    It's more complicated than that. Think about it from an energy point of view - if it interacts with something it makes (transforms) energy. Where does that energy go? Normal particles get hot and make blackbody radiation, i.e. photons. Dark matter can't. Does it have a temperature? Can temperature flow between the particles? Without blackbody radiation you are going to have some violations of the laws of thermodynamics. Unless dark matter has some other particle to act as an equivalent. So you are hypothesizing an entire family of dark matter particles, and a whole set of new forces. Two entire universes each with entire sets of particles, that only interact via gravity. We could have a second sun and a second earth, that overlap us and never know it, we would just think that the gravitational constant is different (because our perceived mass is higher). Sounds like a wonderful setting for a science fiction/horror story.
    - pavel_lishin 11y ago
      There was a story on HN not too long ago about a proposed Dark QCD - implying different kinds of Dark Matter, Dark Chemistry, etc., a whole "parallel universe" of Dark Matter. And yup, my friends and I have been brainstorming story ideas all morning!
    - biswaroop 11y ago
      Actually, dark matter could make photons. Anti-dark-matter, if it exists, could annihilate and produce photons and radiation. http://www.space.com/9302-dark-matter-stars-solve-cosmic-mystery.html http://www.space.com/9302-dark-matter-stars-solve-cosmic-mys...

ars 11y ago

How would they see that? The only gravity you can see is the sum total of all fields that influence you. There is no ability to see gravity directionally, and if all the gravity causes you to accelerate you don't see anything at all. Normal particles can see the influence of gravity only particles, but gravity only particles in free fall can not, the experience nothing at all.

biswaroop 11y ago
>> Normal particles can see the influence of gravity only particles, but gravity only particles in free fall can not. This is a surprisingly interesting question. Can a dark particle 'feel' gravity? Most physicists think that any local experiment in a freely falling lab is independent of the velocity and position of the lab in spacetime (the strong equivalence principle). Here local means confined to the lab, but also small compared to variations in the gravitational field. However, you can setup nonlocal gravitational observatories in a number of ways that preserve the equivalence principle: 1) If the DM critters have a way to do non-local measurements, like measuring tidal forces, then they could measure gravity gradients over time. If they can communicate, they can set up gravity gradient sensors in freefall around stars and infer both the spatial distribution and time evolution of gravitational masses. 2) If the DM critters have clocks, they can measure the gravitational redshift around stars and discover stars. This should work even in a freely falling lab. 3) The DM critters could see gravitational waves passing their freely falling lab and infer changes in gravitational fields. This assumes gravitational waves exist (not a bad assumption).
- ars 11y ago
  But a gravity only particle can't do any of those things. 1: They can't measure gradients because they can not "feel" another particle except via gravity. You can only measure a gravity gradient by comparing it to another force, but they have no second force to compare to. 2: They can't see redshift because they can't see photons at all. 3: They can't see gravitational waves for the same reason as #1, to see a wave you compare two particles (distance in current experiments), but they can't do that, any comparison would be via gravity and would be affected identically, so there is nothing to compare with. Interestingly pure energy only experiences gravity (I don't means photons, I mean simply energy, that "thing" which is conserved). Which is why I personally think dark matter is simply energy, there is no extra particle. The only question is what form does this energy take.