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>> Normal particles can see the influence of gravity only particles, but gravity only particles in free fall can not. This is a surprisingly interesting questi

>> 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.
- biswaroop 11y ago
  Well the idea of a DM critter implicitly assumes DM particles can interact with each other through other dark forces, but they can only interact with real matter through gravity. The thought experiment of whether you can measure directional gravity from freely falling reference frames using only gravitation is more interesting. >> You can only measure a gravity gradient by comparing it to another force This is inaccurate. A measurement of tidal forces only requires the ability to measure space and time between events/particles. >> They can't see redshift because they can't see photons at all. Gravitational redshifts affect all clocks, not just photons: it's a statement about spacetime, not literal redshifts. All pieces of matter are clocks ticking at the Compton frequency, so DM particles can be used as clocks to measure redshift. >> dark matter is simply energy, there is no extra particle I feel that 'simply energy' would be something like dark energy, causing spacetime expansion. You really do need mass for gravitational attraction, and what we measure is gravitational attraction.
  - pavel_lishin 11y ago
    > You really do need mass for gravitational attraction, and what we measure is gravitational attraction. But aren't mass and energy interchangeable? And what about this: https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics) https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics)
  - ars 11y ago
    > Well the idea of a DM critter implicitly assumes DM particles can interact with each other through other dark forces If you say that, then all bets are off. I was talking about a gravity only particle. > A measurement of tidal forces only requires the ability to measure space and time between events/particles. Except gravity only particles are not able to measure space, they have no ability to measure distance. > so DM particles can be used as clocks to measure redshift. Without some "distance" forces other than gravity, they have no ability to compare clocks. How would you use gravity to compare clocks? > You really do need mass for gravitational attraction No you don't. That's inaccurate. Gravity is caused by energy, not mass. (And obviously mass has energy.) So an object moving fast has more gravity than one moving slow.

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.

biswaroop 11y ago
Well the idea of a DM critter implicitly assumes DM particles can interact with each other through other dark forces, but they can only interact with real matter through gravity. The thought experiment of whether you can measure directional gravity from freely falling reference frames using only gravitation is more interesting. >> You can only measure a gravity gradient by comparing it to another force This is inaccurate. A measurement of tidal forces only requires the ability to measure space and time between events/particles. >> They can't see redshift because they can't see photons at all. Gravitational redshifts affect all clocks, not just photons: it's a statement about spacetime, not literal redshifts. All pieces of matter are clocks ticking at the Compton frequency, so DM particles can be used as clocks to measure redshift. >> dark matter is simply energy, there is no extra particle I feel that 'simply energy' would be something like dark energy, causing spacetime expansion. You really do need mass for gravitational attraction, and what we measure is gravitational attraction.
- pavel_lishin 11y ago
  > You really do need mass for gravitational attraction, and what we measure is gravitational attraction. But aren't mass and energy interchangeable? And what about this: https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics) https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics)
- ars 11y ago
  > Well the idea of a DM critter implicitly assumes DM particles can interact with each other through other dark forces If you say that, then all bets are off. I was talking about a gravity only particle. > A measurement of tidal forces only requires the ability to measure space and time between events/particles. Except gravity only particles are not able to measure space, they have no ability to measure distance. > so DM particles can be used as clocks to measure redshift. Without some "distance" forces other than gravity, they have no ability to compare clocks. How would you use gravity to compare clocks? > You really do need mass for gravitational attraction No you don't. That's inaccurate. Gravity is caused by energy, not mass. (And obviously mass has energy.) So an object moving fast has more gravity than one moving slow.