The Hunt For Dark Matter

Originally posted by FabianFnas
No, I meant neutrons, of some reason. Just a neutral particle that doesn't give radiation of themselves.

I am still not sure what you mean. Do you mean this Neutron:
http://en.wikipedia.org/wiki/Neutron
or do you mean 'an unknown neutral particle'?

So a 'definition' of dark matter is "some particle of a kind we don't know has a gravitational measurable effect"? So when it becomes known of what particle the dark matter consists, then it is not dark matter anymore?
Correct.

This is very interesting, very exciting indeed. But sometimes I hear scientists try to over-explain things by mention dark matter or dark energy, when there is infact a simpler, but yet unknown explanation.
The explanation, once known, will of course seem simple.
What is exciting about dark matter, is that there is so much of it. There appears to be almost 5 times as much dark matter than ordinary matter. That means we understand only one sixth of the matter in existence.
Dark energy is even bigger, making up about three quarters of all energy, keeping in mind that the other one quarter is five sixths dark matter. So over-all, we only understand less than 5% of energy in the universe.

http://en.wikipedia.org/wiki/Dark_matter

Originally posted by sasquatch672
You wouldn't expect dark matter to be uniformly distributed, would you? Light matter isn't...

I suppose what I'm on about is the possibility that early astrophysicists were correct when they theorized the ether. Space isn't empty. What we can't seeia the broth of the universe.

Just a crackpot uninformed idea.

The aether is not consistent with relativity.

Originally posted by FabianFnas
No, I meant neutrons, of some reason. Just a neutral particle that doesn't give radiation of themselves. Some of them but a relatively big mass means something in gravitational effect.

But then we also have neutrinos. Small mass times many of them means considerable gravitational effect.

So a 'definition' of dark matter is "some particle of a kind w ...[text shortened]... mention dark matter or dark energy, when there is infact a simpler, but yet unknown explanation.

Dark matter should not be compared to neutrinos, which do interact through electroweak interactions, while dark matter does not (or so weakly that we cannot see it). This is the reason why it is so hard to measure, if it exists.

Originally posted by twhitehead
We can work out the amount of mass present at the center of a galaxy and its location from the motion of stars near the center. Such mass is already attributed to black holes.
http://en.wikipedia.org/wiki/Supermassive_black_hole

There may be black holes distributed throughout the galaxy that are not being taken into account, but I do not think it is a ...[text shortened]... sible parts of galaxies. It doesn't behave like stars, so it is unlikely that it is black holes.

Is there a limit to the size of a black hole? If so, what is it and what does it suggest in this context?

Originally posted by twhitehead
We can work out the amount of mass present at the center of a galaxy and its location from the motion of stars near the center. Such mass is already attributed to black holes.
http://en.wikipedia.org/wiki/Supermassive_black_hole

There may be black holes distributed throughout the galaxy that are not being taken into account, but I do not think it is a ...[text shortened]... sible parts of galaxies. It doesn't behave like stars, so it is unlikely that it is black holes.

"We can work out the amount of mass present at the center of a galaxy and its location from the motion of stars near the center. Such mass is already attributed to black holes."

It is not that simple unless you can accurately measure the spin of the black holes. A spinning black hole drags space around with it and allows matter to orbit closer to the black hole than is possible for a non-spinning black hole.

http://www.sciencedaily.com/releases/2014/03/140305135456.htm

Originally posted by Metal Brain
Is there a limit to the size of a black hole?

I don't know.

If so, what is it and what does it suggest in this context?
I don't think it is relevant.

Originally posted by Metal Brain
It is not that simple unless you can accurately measure the spin of the black holes. A spinning black hole drags space around with it and allows matter to orbit closer to the black hole than is possible for a non-spinning black hole.

Again, this is not relevant. It doesn't affect the laws of gravity significantly enough that we would think its mass was spread throughout the galaxy when viewing at a large scale, and near the centre when observing the stars near the centre.

Originally posted by twhitehead
I am still not sure what you mean. Do you mean this Neutron:
http://en.wikipedia.org/wiki/Neutron
or do you mean 'an unknown neutral particle'?

[b]So a 'definition' of dark matter is "some particle of a kind we don't know has a gravitational measurable effect"? So when it becomes known of what particle the dark matter consists, then it is not dark ma ...[text shortened]... y understand less than 5% of energy in the universe.

http://en.wikipedia.org/wiki/Dark_matter

By Neutron I mean this Neutron: http://en.wikipedia.org/wiki/Neutron

If you have like one free neutron per cubic inch in all the intergalactic space, how can you measure its existence besides its gravitational effects?

Next question: Do we have dark matter here on Earth too? In the same proportion? If I inhale a litre of air, do I also inhale some quantity of dark matter as well? When I order a quarter pounder hamburger at McDonald, do I also eat like six quarter pounds of dark matter, and I still not gain in weight more than this quarter of a pound? Why don't we notice it here on Earth when it is like 84.5% of everything?

Originally posted by FabianFnas
If you have like one free neutron per cubic inch in all the intergalactic space, how can you measure its existence besides its gravitational effects?

I think they would be detectable by many different means.
They would behave gravitationally, the same as hydrogen gas, which is common throughout the universe. They would therefore be 'collected' on gas giants like Jupiter the same way hydrogen is - possibly more so since there are no electrical effects?

If they were in our area of space, we would see the effect as they slam into the earth.

I don't know enough astronomy/physics to tell you more, but I am fairly sure we would know.

Next question: Do we have dark matter here on Earth too? In the same proportion? If I inhale a litre of air, do I also inhale some quantity of dark matter as well?
Our current best guess for what dark matter is, is that it is WIMPS (weakly interacting massive particles) which zoom around the galaxy passing right through all objects with almost no interaction other than gravity. So it would be similar to the way neutrinos currently go right through you without you realizing. It would not be acting like gas atoms for example where when you breathe in, they enter your lungs, and are contained by your body.

When I order a quarter pounder hamburger at McDonald, do I also eat like six quarter pounds of dark matter, and I still not gain in weight more than this quarter of a pound? Why don't we notice it here on Earth when it is like 84.5% of everything?
Firstly, Dark matter is spread almost uniformly throughout the galaxy, so the percentage of it in a location containing high densities of normal matter like your hamburger, would be much much lower.
Secondly, because it is probably not interacting with the matter, you couldn't weigh it anyway.

Originally posted by twhitehead
I think they would be detectable by many different means.
They would behave gravitationally, the same as hydrogen gas, which is common throughout the universe. They would therefore be 'collected' on gas giants like Jupiter the same way hydrogen is - possibly more so since there are no electrical effects?

If they were in our area of space, we would see ...[text shortened]...
Secondly, because it is probably not interacting with the matter, you couldn't weigh it anyway.

Hmmm, interesting...

Okay, let's skip the neutron-idea.
And since it is impossible to weigh we cannot detect in on Earth.

Is it uniformely distribuated through the space, even in our vicinity, like within the solar system?
And what would then the average density be? Like one kg / litre? More? Less?

Originally posted by twhitehead
Again, this is not relevant. It doesn't affect the laws of gravity significantly enough that we would think its mass was spread throughout the galaxy when viewing at a large scale, and near the centre when observing the stars near the centre.

You said this: "We can work out the amount of mass present at the center of a galaxy and its location from the motion of stars near the center."

The spin affects how close stars orbit black holes. This is a variable that needs to be taken into account and you have not. It is very relevant. I have proved what you "work out" can be wrong because the spin variable makes those calculations relative.

Can you measure the spin of super massive black holes at the center of the galaxy?

Originally posted by FabianFnas
If you have like one free neutron per cubic inch in all the intergalactic space, how can you measure its existence besides its gravitational effects?

All of those free neutrons are bound to decay. So you'd have to observe a lot electrons, antineutrino electrons (sometimes you can also observe gamma radiation) and the appearance of a lot of positrons.

Since you don't observe that and you believe that the laws of physics are valid then you have to conclude that your hypothesis is invalid.

Originally posted by adam warlock
All of those free neutrons are bound to decay. So you'd have to observe a lot electrons, antineutrino electrons (sometimes you can also observe gamma radiation) and the appearance of a lot of positrons.

Since you don't observe that and you believe that the laws of physics are valid then you have to conclude that your hypothesis is invalid.

It wasn't even to be meant as an hypothesis, just an idea, nothing more.

So free neutrons are bound to decay. Do you know how fast? What is their half-life?

Originally posted by FabianFnas
It wasn't even to be meant as an hypothesis, just an idea, nothing more.

So free neutrons are bound to decay. Do you know how fast? What is their half-life?

It's about 10 minutes. The most common decay is:

neutron -> proton + electron + anti-electron-neutrino.

Originally posted by KazetNagorra
It's about 10 minutes. The most common decay is:

neutron -> proton + electron + anti-electron-neutrino.

And what is its half-time within a atom nucleus? Is it stable there? If so, why there and why not when it's free?

(I like this discussion, I learn a lot!)