New class of galaxy has been discovered, made almost entirely of dark matter • Hacker News Board

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#141

Post by T0T0R0 » Sun, Aug 28, 2016, 4:48 AM UTC

evanb wrote: post #135

T0T0R0 wrote: post #122

Earlier quoted context omitted.

Yeah, yeah, I get it. And I still say that's a non-explanation with a misleading name. There's no proof of material at all, thus not matter, thus no such thing as dark matter. I'd willingly accept other names such as Dark Question Marks. Or maybe Dark Mathematical Terms Yet To Be Named . Here's a good one: D ark U nobservable N umerically C hallenged E ntities.

As I said above to another commenter, if you can explain all the observational signatures https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... in another way, you should write a paper! If you don't consider all that observational evidence reliable (which maybe you do not, based on your DUNCE name) and will only be satisfied by direct detection experiments on Earth, well, I don't know what to say to you exc…

The point being that Dark Matter doesn't even seem to interact with matter, so why call it any kind of matter.

Matter isn't matter unless collisions prove it's occupancy of space. That's pretty much why matter is considered anything at all. You can't gloss over a significant lack of collisions.

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#142

Post by acidbaseextract » Sun, Aug 28, 2016, 8:04 AM UTC

m_mueller wrote: post #139

evanb wrote: post #134

Earlier quoted context omitted.

The issue is the speed of light is so big. So, you need a LOT of energy in order to source gravity comparably to just a little bit of mass (essentially, because E=mc^2 -- or perhaps phrased more clearly in this case, m = E/c^2). Even Newtonian Gravity can bend light classically, by Galilean relativity. But, weirdly, it essentially hinges on the fact that the the mass m of the photon cancels from m*a = GmM/r^2. Of cou…

To me, the speed of light is actually really slow - meaning, if you think classically you assume c infinite (at which point, as far as I understand, relativity theory essentially behaves like classical physics). Learning that information travels way slower than the universe expands is quite unnerving. Similarly, learning that earth's fate could be determined already since hundreds of thousands of years through a hype…

"The True Nature of Matter and Mass" - https://www.youtube.com/watch?v=gSKzgpt4HBU

This whole channel will be up your alley, but this video and "The Real Meaning of E=mc^2" one directly answer your question.

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#143

Post by acidbaseextract » Sun, Aug 28, 2016, 9:04 AM UTC

m_mueller wrote: post #139

evanb wrote: post #134

Earlier quoted context omitted.

The issue is the speed of light is so big. So, you need a LOT of energy in order to source gravity comparably to just a little bit of mass (essentially, because E=mc^2 -- or perhaps phrased more clearly in this case, m = E/c^2). Even Newtonian Gravity can bend light classically, by Galilean relativity. But, weirdly, it essentially hinges on the fact that the the mass m of the photon cancels from m*a = GmM/r^2. Of cou…

To me, the speed of light is actually really slow - meaning, if you think classically you assume c infinite (at which point, as far as I understand, relativity theory essentially behaves like classical physics). Learning that information travels way slower than the universe expands is quite unnerving. Similarly, learning that earth's fate could be determined already since hundreds of thousands of years through a hype…

Watch that youtube link in my sibling comment first, but I actually wanted to take a crack at an answer. What's the change in mass, ∆m, as the star cools? We'll assume the mass of the neutron star is 1.5 solar masses.

    E = mc^2
    ∆m = ∆E/c^2

So really, what's ∆E? A hyper hand-wavy estimate:

    ∆E = Q = mc∆T (c being specific heat)

Specific heat by mass is really hard to predict, but by moles it is fairly constant, well within an order of magnitude. So we'll discuss mass in moles.

    m = neutron star moles
    m = (mass of neutron star / mass of neutron) / Avogadro's #
    m = 2.9580163e33 mol

    c = 24 J / (mol * K)

    ∆T = 1e12 K - 1e6 K

→

    ∆E = 24 * 2.9580163e33 * (1e12 - 1e6) J = 7.0991681e45 J

Substituting that back into the original, as a neutron star cools:

    ∆m = 7.0991681e45 J / c^2 = 7.89888982e28 kg

Which is like 2.6% of the mass of the original star, so a pretty solid chunk. But that number is pulled out of my ass—I am not a physicist.

But there are even more weird effects going on, due to the warping of gravity the mass of neutron stars can be up to 20% less than you'd expect based on its baryonic (neutron) constituents (questions 4 and 7):

https://www.astro.umd.edu/~miller/teaching/questions/neutron...

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#144

Post by m_mueller » Sun, Aug 28, 2016, 1:26 PM UTC

acidbaseextract wrote: post #143

m_mueller wrote: post #139

Earlier quoted context omitted.

To me, the speed of light is actually really slow - meaning, if you think classically you assume c infinite (at which point, as far as I understand, relativity theory essentially behaves like classical physics). Learning that information travels way slower than the universe expands is quite unnerving. Similarly, learning that earth's fate could be determined already since hundreds of thousands of years through a hype…

Watch that youtube link in my sibling comment first, but I actually wanted to take a crack at an answer. What's the change in mass, ∆m, as the star cools? We'll assume the mass of the neutron star is 1.5 solar masses. E = mc^2 ∆m = ∆E/c^2 So really, what's ∆E? A hyper hand-wavy estimate: ∆E = Q = mc∆T (c being specific heat) Specific heat by mass is really hard to predict, but by moles it is fairly constant, well wit…

Thank you!

About your last equation, wouldn't J/m^2/s^2 come out as kg? 7.9E2 would be very low then, no?

I wonder how much energy is stored electromagnetically and through nuclear forces though. These things are supposed to have extremely strong EM fields and I imagine every piled up nucleus like a little atomic spring that has been depressed as much as possible. Wouldn't most of the stored energy be in there?