Hacker News Reader

Comment by ars | Hacker News Reader

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.

View on HN

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.

> 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)

> 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.

Reply on news.ycombinator.com