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.
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).
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.
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.
But aren't mass and energy interchangeable? And what about this: https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics)
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.
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.
And yup, my friends and I have been brainstorming story ideas all morning!
http://www.space.com/9302-dark-matter-stars-solve-cosmic-mys...