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Science and Astronomy Questions

Posted: 24 Nov 2019 05:54
by Watsisname
midtskogen wrote:
Source of the post Assuming that the Big Bang produced exactly equal amounts of matter and anti-matter, could the creation of anti-matter black holes be what shifted the balance and prevented everything from just annihilate into pure energy?

Interesting idea.  Unfortunately I don't think this would solve the problem, because if say an equal amount of matter and antimatter collapsed into black holes, then there would still be an equal amount of matter and antimatter not in black holes, which presumably would still annihilate completely.  Then we'd have either a universe full of nothing but black holes, or if those black holes were small enough to evaporate by now, a universe full of Hawking radiation.  So we'd need some mechanism for having more antimatter to collapse into black holes than matter, which only shifts the problem.  We also need this to happen over the right timescales and in such a way as to be consistent with observations.

This problem of why the universe is mostly full of matter rather than purely radiation from equal amount of matter and antimatter annihilating is called baryon asymmetry, and it's not fully understood.  There are also other types of symmetries in particle physics, which were long held to be physical laws, but then discovered to be violated under certain conditions (CP-symmetry for example).

Science and Astronomy Questions

Posted: 24 Nov 2019 07:03
by JackDole
Honestly, I find it difficult to understand these explanations, especially in English. (The video about Hawking radiation.)
Incidentally, according to the article in German Wikipedia, Hawking himself used the image of matter/antimatter particles to explain the radiation. (And I also think I read it in a book by Hawing, but I can not say for sure. It has been many years ago.)
But what I believe to have been understood is that Hawking radiation has not been proven yet, no one really knows what it is and that it's just a mathematical construct to preserve the entropy!

Science and Astronomy Questions

Posted: 24 Nov 2019 07:45
by Watsisname
JackDole wrote:
Source of the post But what I believe to have been understood is that Hawking radiation has not been proven yet, no one really knows what it is and that it's just a mathematical construct to preserve the entropy!


Energy is also just a mathematical construct so that we can write formulas and describe physical processes in terms of a conserved quantity.

That black holes have an entropy associated with their event horizons is not controversial.  We just calculated it!  That this entropy depends on the energy tells us that black holes have a temperature, and therefore they must radiate photons with a blackbody spectrum.  There's really no way around it, and Hawking made the proof rigorous.  (He did not use matter-antimatter production, but instead the propagation of modes of the vacuum fields by applying quantum field theory.  Describing this as "matter-antimatter pairs" is just a convenient but oversimplified narrative, almost as sloppy as saying the Big Bang came from nothing.) 

Is there any experimental evidence for concluding Hawking radiation exists?  Well, we do observe it in a variety of analogue black holes in the lab.  But maybe because we don't measure it directly from real astrophysical black holes, one may still be skeptical.  For any reasonably-sized black hole the radiation would be so weak that there is no hope to measure it.  However, we can make an observation with real black holes that supports it indirectly.  Specifically, we can test the other predictions of black hole thermodynamics.  Since the black hole's entropy is proportional to its horizon area, the 2nd law of thermodynamics insists that when two black holes merge together, the total horizon area cannot decrease.

This is a strict limit on how big the resulting black hole can be.  By conservation of mass, it cannot be bigger than the sum of the masses of the two original black holes.  But by law that the entropy and thus area cannot decrease, the square of the resulting mass cannot be less than the sum of the squares of the two original masses.  

Example:  If two 10 solar mass black holes merge, then the greatest possible result is 10+10=20 solar masses, but the minimum possible result is sqrt(102 + 102) = sqrt(200) = 14.14 solar masses.  Whatever difference from 20 solar masses was carried away by gravitational waves.

We can test this by looking at the gravitational waves from black hole mergers with LIGO, and indeed, the gravitational waves never carry more energy than what the laws of black hole thermodynamics allow.

(Aside: what kind of collision would result in the maximum allowed value of just adding the masses together?  That happens if the two black holes collide head-on.  Otherwise, some orbital angular momentum is shed as gravitational waves, and this reduces the total mass of the system.)

Science and Astronomy Questions

Posted: 24 Nov 2019 08:46
by midtskogen
Watsisname wrote:
Source of the post So we'd need some mechanism for having more antimatter to collapse into black holes than matter,

Yes, that was the idea. Like a tiny asymmetry in the distribution of matter and antimatter.
Watsisname wrote:
Source of the post which only shifts the problem. 

Yes, but perhaps to a context that we understand better.

Science and Astronomy Questions

Posted: 24 Nov 2019 10:04
by A-L-E-X
Watsisname wrote:
midtskogen wrote:
Source of the post Assuming that the Big Bang produced exactly equal amounts of matter and anti-matter, could the creation of anti-matter black holes be what shifted the balance and prevented everything from just annihilate into pure energy?

Interesting idea.  Unfortunately I don't think this would solve the problem, because if say an equal amount of matter and antimatter collapsed into black holes, then there would still be an equal amount of matter and antimatter not in black holes, which presumably would still annihilate completely.  Then we'd have either a universe full of nothing but black holes, or if those black holes were small enough to evaporate by now, a universe full of Hawking radiation.  So we'd need some mechanism for having more antimatter to collapse into black holes than matter, which only shifts the problem.  We also need this to happen over the right timescales and in such a way as to be consistent with observations.

This problem of why the universe is mostly full of matter rather than purely radiation from equal amount of matter and antimatter annihilating is called baryon asymmetry, and it's not fully understood.  There are also other types of symmetries in particle physics, which were long held to be physical laws, but then discovered to be violated under certain conditions (CP-symmetry for example).

I know it's just shifting the problem but it's quite possible that overall the amount of matter and antimatter universes is similar or even nearly equal and we just happen to live in a matter universe.

Science and Astronomy Questions

Posted: 24 Nov 2019 15:14
by Watsisname
A-L-E-X wrote:
Source of the post I know it's just shifting the problem but it's quite possible that overall the amount of matter and antimatter universes is similar or even nearly equal and we just happen to live in a matter universe.

The problem is that pair production must produce a lot of both matter and antimatter in this universe -- indeed within every tiny volume element of space.  So it's not easy to explain away by supposing that regions of matter and antimatter dominance are just separated by large distances after expansion, either.  There must be something in the particle physics, a type of symmetry breaking, that led to a small fraction of one being produced more than the other.  In fact there are hints that this is the case in particle accelerator experiments, which produce slightly more matter than antimatter, but again the exact nature of it remains unknown.

Science and Astronomy Questions

Posted: 24 Nov 2019 16:02
by A-L-E-X
Watsisname wrote:
A-L-E-X wrote:
Source of the post I know it's just shifting the problem but it's quite possible that overall the amount of matter and antimatter universes is similar or even nearly equal and we just happen to live in a matter universe.

The problem is that pair production must produce a lot of both matter and antimatter in this universe -- indeed within every tiny volume element of space.  So it's not easy to explain away by supposing that regions of matter and antimatter dominance are just separated by large distances after expansion, either.  There must be something in the particle physics, a type of symmetry breaking, that led to a small fraction of one being produced more than the other.  In fact there are hints that this is the case in particle accelerator experiments, which produce slightly more matter than antimatter, but again the exact nature of it remains unknown.

As far as our universe is concerned, wasn't the matter resolved awhile back because experiments showed that certain particles are produced more than their antiparticles are (I believe it was pi mesons or pions?)  So equal amounts of matter and antimatter would have been annihilated, leaving behind an excess of matter.  I dont know if this actually solves the problem, but it does give a way for it to be resolved if this was also the case in the early universe.

And if this is the case, there has to be a reason for it.  So it would mean that matter and antimatter are not quite mirror opposites of each other.....

Science and Astronomy Questions

Posted: 08 Dec 2019 17:57
by vlad01
Meteor storms, Is there some info about their predictions into the future?

I remember I used to be able to find info on various ones that were predicted but seems I get no relevant results on google anymore.

Specifically the Leonid storm, when is the next one? I recall saying they come every 33 years or so, so the next one should be due in 10 years or so but can't find any info about this.  I saw the one in 99, although there were quite many meteors it wasn't the massive storm that they predicted as least not in Australia, they actually happen again in 2001 and 2002 but at the time it was not in the media so I knew nothing about those and missed them, only finding out many years later while searching on the net.

I wonder what other outbursts there maybe in the future from the other known showers?  

Science and Astronomy Questions

Posted: 09 Dec 2019 08:50
by A-L-E-X
vlad01 wrote:
Meteor storms, Is there some info about their predictions into the future?

I remember I used to be able to find info on various ones that were predicted but seems I get no relevant results on google anymore.

Specifically the Leonid storm, when is the next one? I recall saying they come every 33 years or so, so the next one should be due in 10 years or so but can't find any info about this.  I saw the one in 99, although there were quite many meteors it wasn't the massive storm that they predicted as least not in Australia, they actually happen again in 2001 and 2002 but at the time it was not in the media so I knew nothing about those and missed them, only finding out many years later while searching on the net.

I wonder what other outbursts there maybe in the future from the other known showers?  

It was in the media here in November 2001 and I remember waking up my dad at 5 am to go outside to see it with me and it was one of the most amazing sights I have ever seen!  It was like being in a snowstorm, even so close to the city, or a very bright fireworks display!  I could see hundreds of them every minute without even looking up!  I thought the next one was in 2034?  That and the total solar eclipse in 2024 are the two astronomical displays I am most looking forward to!

Science and Astronomy Questions

Posted: 09 Dec 2019 15:54
by vlad01
It wasn't in the media here in Aus and the internet was still in it's infancy here so most including myself didn't have the internet back then. So I knew nothing of it until like 10 years later.

I heard various numbers between 2028-2035.  I can't find anything on the net about it anymore.

Science and Astronomy Questions

Posted: 19 Dec 2019 15:20
by Cantra
So lets say there is a planet in orbit inside of Mercury's orbit. Like a hypothetical Vulcan. Let's say this planet, in this alternate universe is discovered around the 1800's. At what radius would it be able to have without being detected prior? What sort of inclination would it need to not be seen with a regular pattern, and how massive would this planet be able to be at maximum?

 

Science and Astronomy Questions

Posted: 21 Dec 2019 03:04
by A-L-E-X
Watsisname wrote:
A-L-E-X wrote:
Source of the post But Wat, protons and neutrons aren't really particles are they?  They are the union of three quarks each, bound together by the strong force (gluons).  So how and why do they get treated like single masses?  Is it analogous to the sun being composed of trillions of particles and yet being treated as a single mass when computing its gravitational force?

Yes, the situations are analogous.  The Sun is a big ball of a huge number of particles, but you don't need to know which particles are where within the Sun to compute how the planets move.  All you need to know is the Sun's mass and use the fact that it is approximately spherically symmetric.  Similarly, a proton is made up of quarks, but you can compute how the electron in a hydrogen atom behaves without any reference to the quarks in that proton at all.  All you need to know is the proton's mass and charge.

Where the existence of quarks becomes important is in the physics of the nucleus and at very high energies.  If you shoot electrons into an atom with enough energy, then their de Broglie wavelengths will be short enough to interact with the quarks in observable ways.  In fact this is how the existence of quarks was first demonstrated. :)


A-L-E-X wrote:
Source of the post Also, is gravity a factor when dealing with such small particles?

On a per-particle basis, gravity is so weak compared to the other forces that we can very safely ignore it.  Let's do a simple calculation to convince ourselves of this.  What is the ratio of the strength of the gravitational force to the electrical force between two protons?

The gravitational force between them is Fg=Gmp2/r2 where of course G is the gravitational constant, mp is the proton mass, and r is the distance between them.  

The electrical force is Fe = ke2/r2, where k=1/(4πε0), e is the charge of a proton and ε0 is the electric constant.  

Their ratio is 

Image

In other words, the role of gravitation in nuclear physics is completely unmeasurable, unless you had more than 30 decimal places of precision (which we never do)!  So you can absolutely ignore it.  The only time you can't ignore it is if there are so many particles that their combined mass produces a lot of gravity, such as in extremely compact objects like neutron stars or black holes.

I was thinking of quark stars when I was reading this!  Do we have enough evidence of their existence, Wat?  I remember reading about evidence of cosmic strings and quark stars awhile back.
Then again I also read about the possibility of preons (even more elementary than quarks) and preon stars, but those didn't pan out.

Science and Astronomy Questions

Posted: 29 Jan 2020 21:24
by JackDole
Let's say two relatively large planets collide and completely destroy each other.
How many years would it take before the resulting heat radiation could no longer be measured. (Or until the heat radiation would be hidden by the heat of the star?) (Just an approximation.)
I have three examples:
Two super earths collide.
Two gas planets (such as Jupiter or Saturn) collide.
A terra planet or super earth collides with a gas planet.
And would it even be possible for them to collide and destroy each other so completely that they spread into a large asteroid and dust ring in their solar system?

Science and Astronomy Questions

Posted: 01 Feb 2020 23:51
by JackDole
JackDole wrote:
Source of the post Let's say two relatively large planets collide and completely destroy each other.
How many years would it take before the resulting heat radiation could no longer be measured. (Or until the heat radiation would be hidden by the heat of the star?) (Just an approximation.)
I have three examples:
Two super earths collide.
Two gas planets (such as Jupiter or Saturn) collide.
A terra planet or super earth collides with a gas planet.
And would it even be possible for them to collide and destroy each other so completely that they spread into a large asteroid and dust ring in their solar system?

Is my question too difficult to answer?

Science and Astronomy Questions

Posted: 02 Feb 2020 03:50
by Watsisname
JackDole wrote:
Source of the post Is my question too difficult to answer?

Yeah, this is not a question that could be answered by intuition, or even pencil and paper.  We need to know how much material is ejected with what range of velocities, the size distribution of the ejecta, how hot those fragments are, and how they cool over time while also accounting for how much of it is gravitationally consolidated into a single planet and possibly also a ring system or moons.  Then we need to integrate all the thermal radiation as a function of time, and compare with the star's radiation to see if the excess is measurable with a particular telescope over some set of wavelengths.
  
This is a job for supercomputers.  

JackDole wrote:
Source of the post And would it even be possible for them to collide and destroy each other so completely that they spread into a large asteroid and dust ring in their solar system?

Typically no.  This can sometimes happen with asteroid collisions, but when you get to planetary masses their self gravity becomes very important.  As a first approximation we could compare the kinetic energy of the collision to the gravitational binding energy of a uniform sphere with the combined mass.  For a typical planetary collision, the ratio of these two is not very different from 1, which suggests that some material may be flung into orbit around the star while the rest is consolidated into a merged planet.

However, even more material ends up in the merged planet than this calculation predicts, because it neglected that much of the energy of the impact is converted to thermal energy and radiated, rather than just kinetic energy of ejecta.