Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yor_on on 09/01/2019 12:02:50

Title: how do objects heat up in a vacuum? aka black holes
Post by: yor_on on 09/01/2019 12:02:50
Read this " http://curious.astro.cornell.edu/disclaimer/86-the-universe/black-holes-and-quasars/general-questions/436-what-type-of-energy-does-a-black-hole-have-intermediate " where it is stated that "  These objects can gain energy from the black hole, as they fall into the potential well they heat up and radiate (often in the X-ray region of the spectrum). " ?

that they do it falling from space here on Earth is due to friction, but a mass following a geodesic in a vacuum?
Why would they 'heat up'?
Tidal forces?

Can't  be the Unruh effect
Title: Re: how do objects heat up in a vacuum? aka black holes
Post by: Kryptid on 09/01/2019 16:39:35
I assume they are talking about an accretion disk, which does indeed radiate heat due to friction: https://en.wikipedia.org/wiki/Accretion_disk
Title: Re: how do objects heat up in a vacuum? aka black holes
Post by: chiralSPO on 09/01/2019 16:53:26
My understanding is that the intense heat generated in accretion disks is due to collisions between particles with drastically different relative velocities. 

EDIT: Oops! Crossed with Kryptid!
Title: Re: how do objects heat up in a vacuum? aka black holes
Post by: guest47899 on 09/01/2019 23:51:31
black holes have zero entropy. zero entropy is at absolute zero or sub absolute zero. sub absolute zero can produces extreme temperatures that exceed any positive above absolute zero temperatures.
Title: Re: how do objects heat up in a vacuum? aka black holes
Post by: Kryptid on 10/01/2019 00:55:18
Quote from: Pesq on 09/01/2019 23:51:31
black holes have zero entropy.

Black holes have finite entropy. The larger the black hole, the larger the entropy (it's linearly proportional to the surface area of the event horizon).
Title: Re: how do objects heat up in a vacuum? aka black holes
Post by: chiralSPO on 10/01/2019 02:04:44
Quote from: Pesq on 09/01/2019 23:51:31
zero entropy is at absolute zero or sub absolute zero.
Not quite. If we consider a system with fixed volume and fixed number of particles, then temperature is inversely proportional to the rate of change of entropy with respect to the rate of change of internal energy.
  T = 1/(∂S/∂U)

What this means is: iff (iff means if and only if) the temperature is positive, then entropy increases as the energy of the system increases, whereas iff the temperature is negative, then entropy decreases as the energy of the system increases. Lasers are a common example of systems in which the temperature is negative--as you put energy into the laser, it becomes more ordered, and then becomes less ordered when discharged.

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/temper2.html
Title: Re: how do objects heat up in a vacuum? aka black holes
Post by: yor_on on 10/01/2019 06:07:10
Ahh

Ok, that one ,made sense, at last :)
Thanks Kryptid and Chiral
Title: Re: how do objects heat up in a vacuum? aka black holes
Post by: Bored chemist on 10/01/2019 11:00:59
Quote from: Pesq on 09/01/2019 23:51:31
sub absolute zero.
That's a term which needs a careful definition.
Also, plenty of materials are not perfectly ordered in the crystalline state. Frozen carbon monoxide has the molecules aligned randomly for example.
So it does not have zero entropy at zero kelvin.