Black hole? (dark star)

[Capiert]

What was wrong with Michell's idea?

Why isn't a very large star

 (& many of them, i.e. dark stars)

 simply the answer

 for so much invisible mass?

&

Why isn't the collision

 of 2 dark stars

 into fragments

 simply the explaination

 for so_called super novas?

 

E.g. At what distance away (from earth)

 will a specific large radius star

 become invisible to us (on earth)

 as a dark star?

(I can imagine a (distant) dark star

 would begin to become visible

 as we get near to it.

E.g. it takes time

 for gravity to slow light down to zero

 & large distance

 will give gravity that time.)

 

Why can't we (classically) calculate those (dark) star radii,

 & their distance away from earth,

 if the laws of physics hold everywhere

 (even in black holes)?

 

 

 

Edited November 18, 2017 by Capiert

[Strange]

26 minutes ago, Capiert said:

What was wrong with Mitchell's idea?

Nothing much. It was remarkably good, considering it was based on a flawed theory! 

28 minutes ago, Capiert said:

Why isn't a very large star  (& many of them, i.e. dark stars)  simply the answer  for so much invisible mass?

One possibility for dark matter is micro-black holes. The problem is that if there were enough black holes to provide the extra mass then we would probably see them (because they would travel in front of other stars and block our view of them). They would also cause gravitational lensing, which should be detectable as well. 

31 minutes ago, Capiert said:

Why isn't the collision  of 2 dark stars  into fragments  simply the explaination for so_called super novas?

We have a model based on the physics of stellar fusion that explains supernovas. Colliding black holes don't produce the same effects. Colliding neutron stars produce much larger explosions.

33 minutes ago, Capiert said:

E.g. At what distance away (from earth)  will a specific large radius star become invisible to us (on earth) as a dark star?

In principle, never. But in practice it depends on the brightness of the star and the size of the telescope. We can see stars (well, galaxies) that are nearly 13 billion light years away. 

 

[Capiert]

27 minutes ago, Strange said:

Nothing much. It was remarkably good, considering it was based on a flawed theory! 

What is the flaw?

(That was my question. What are the flaws?)

Quote

One possibility for dark matter is micro-black holes.

Not according to Michell's idea.

Micro black holes have NOTHING

 to do with a radius 500x larger than our sun!

Please stay on topic.

(But thanks for the background info, anyway.)

Quote

The problem is that if there were enough black holes to provide the extra mass then we would probably see them (because they would travel in front of other stars and block our view of them). They would also cause gravitational lensing, which should be detectable as well. 

Michell's (big) dark star will do that too.

(So that doesn't help the argument.)

Quote

We have a model based on the physics of stellar fusion that explains supernovas.

That sounds post Michell.

(I (only?) know gravity accumulates matter. Things get bigger (e.g. radii).)

I assume At Wt determines the age of isotopes, to some degree.

Hydrogen is young, uranium is very old.

Quote

Colliding black holes don't produce the same effects.

I know nothing about black holes because I've never seen 1.

Quote

Colliding neutron stars produce much larger explosions.

Are neutron stars suppose to be Michell's invisible star

 or do other alternatives exist too?

Quote

In principle, never.

That makes no sense to me.

Quote

But in practice it depends on the brightness of the star and the size of the telescope. We can see stars (well, galaxies) that are nearly 13 billion light years away. 

Do we not observe red shifts?

Do we have infrared galaxies?

Do we (not) have radio galaxies?

Isn't the (light) frequency (tendency) sinking

 into the invisible?

(Not to mention quasars, & pulsars.?)

Edited November 18, 2017 by Capiert

[Strange]

2 minutes ago, Capiert said:

What is the flaw?

Well, his idea of a dark star was based on the idea that light could not escape because the escape velocity of the star exceeded the speed of light. This does give the same radius as the event horizon of a black hole (+1 for Mitchell) but does mean that light could escape temporarily - in the same way you can throw a ball in the air at less than the Earth's escape velocity. This would cause observable effects. In the case of a black hole, the event horizon is completely inescapable.

Also, it isn't clear that light would actually be affected in the way described by Newtonian gravity (Mitchell assumed that light was made of particles that would be affected by gravity).

But it is only of historical interest as we now have a more accurate theory of how gravity works and how it affects light.

10 minutes ago, Capiert said:

That makes no sense to me.

Light never stops. It just gets fainter, so there is no limit to the distance it can travel. 

 

10 minutes ago, Capiert said:

Do we not observe red shifts? Do we have infrared galaxies? Do we (not) have radio galaxies? Isn't the (light) frequency (tendency) sinking  into the invisible?

Light gets increasing red-shifted. But we can still detect it. And other frequencies get shifted into the visible range. 

Just found this: https://en.wikipedia.org/wiki/Dark_star_(Newtonian_mechanics)

Apparently, Mitchell and Newton got gravitational redshift the wrong way round as well. And the amount of gravitational lensing.

[Outrider]

Wow what an idea for 1783! I had never heard of dark stars. Thanks for the link Strange hint, hint OP.

Certain fields, astronomy is one, many of those we remember for being wrong were geniuses in their own right.

[Capiert]

On 19 November 2017 at 12:31 AM, Strange said:

Light gets increasing red-shifted. But we can still detect it. And other frequencies get shifted into the visible range. 

How can we detect a light frequency (as light)

 that has been decreased so much (with so much (leaving) Doppler shifted speed)

 that its frequency is lower than (either) IR, radio waves, ultra sound, audio, sub acoustic (rythems).. ?

I mean the universe is infinite

 (although Einstein said it's finite),

 & the (decreasing frequency=red (Doppler) shift wrt fast speed, physics)

 tendancy is established,

 but there is still lots to discover (far beyond)

 that we don't know (about).

Edited December 9, 2017 by Capiert

[Strange]

2 hours ago, Capiert said:

How can we detect a light frequency (as light) that has been decreased so much (with so much (leaving) Doppler shifted speed) that its frequency is lower than (either) IR, radio waves, ultra sound, audio, sub acoustic (rythems).. ?

There would be a limit, I suppose, when the highest frequencies in the source have been shifted below what we can detect. But that won't happen for anything in the observable universe.

2 hours ago, Capiert said:

I mean the universe is infinite (although Einstein said it's finite), & the (decreasing frequency=red (Doppler) shift wrt fast speed, physics) tendancy is established, but there is still lots to discover (far beyond) that we don't know (about).

We can never know anything directly about what is beyond the observable universe. I suspect we can never know if the universe is actually infinite or just very large.

[Capiert]

3 hours ago, Strange said:

There would be a limit (to how low a frequency we could detect), I suppose, when the highest frequencies in the source have been shifted below what we can detect. But that won't happen for anything in the observable universe.

Why not? Please explain.

Quote

We can never know anything directly about what is (far) beyond 

in

Quote

the observable universe.

I suppose that's the catch.

Far beyond is not observable

(although still in the universe).

Quote

I suspect we can never know if the universe is actually infinite or just very large.

What do we do there?

We know the universe is very large. (=Fact.)

Infinite (universe) is an unproven assumption,

 limited to our capabilities. (=Not a fact.)

Did Einstein assume the universe is finite

 because (he ran out of (proved) facts,)

 he could not comprehend all of it?

 

With only the (remains of a tiny) average density

 (throughout the universe)

 (=outer space is not a perfect vacuum);

 & that light bends (a curved path)

 (e.g. light falls (when))

 around (=near) mass

 when travelling those immense distances;

 I guess he assumed

 a (perfect) circle (light_)path

 would eventually (be possible &) happen,

 thus limiting the universe's size

 to that radius.

 

(But average density is a random thing

 varrying with the sample size (=volume);

 & where it's taken.

Light could eventually travel a zig zag path (in 3D, not just 2D).)

 

Edited December 9, 2017 by Capiert

[Strange]

25 minutes ago, Capiert said:

Why not? Please explain.

Because the maximum red-shift within the observable universe is not enough to make things undetectable.

45 minutes ago, Capiert said:

Far beyond is not observable

Nothing beyond the observable universe is observable. (The clue is in the name!)

But it is reasonable to assume that immediately beyond the observable universe, things are about the same. Further out, who knows.

46 minutes ago, Capiert said:

Did Einstein assume the universe is finite because (he ran out of (proved) facts,) he could not comprehend all of it?

I don't know if he did assume that. Do you have a source.

He is often credited with the saying, “Only two things are infinite, the universe and human stupidity, and I’m not sure about the former” but he probably didn't say it.

[Strange]

7 hours ago, Strange said:

Because the maximum red-shift within the observable universe is not enough to make things undetectable.

Here is a good article about how the new James Webb Space Telescope will see further into the infra-red and hence see more distant galaxies: https://medium.com/starts-with-a-bang/ask-ethan-how-much-of-the-observable-universe-are-we-failing-to-see-fad5828b6189

[Capiert]

On ‎2017‎ ‎12‎ ‎09 at 3:01 PM, Strange said:

 

On ‎2017‎ ‎12‎ ‎09 at 2:11 PM, Capiert said:

Did Einstein assume the universe is finite because (he ran out of (proved) facts,) he could not comprehend all of it?

I don't know if he did assume that. Do you have a source.

 

http://www-groups.dcs.st-and.ac.uk/history/Extras/Einstein_ether.html

In this case (=of (extremely_large=) cosmic dimensions, but tiny density)

 the universe must of necessity be spatially unbounded (=flexible)

 and of finite magnitude,

 its magnitude being determined by the value of that mean density.

 

1920 GR part 3

 chapters (30,) particularly 31, (32)

 & Appendices 3 & 4.

 

Ch31 The possibility of a finite & yet unbounded universe.

 

http://www.gutenberg.org/files/30155/30155-pdf.pdf?session_id=291ace058d13f13348e18ff7fbbfcabb52aedf4c

2017_12_11_0102_Einstein’s_finite_universe_2017 12 11 0102 PS Wi.docx

[Strange]

So those say it is a possibility. From Appendix IV of that second link:

Quote

I further want to remark that the theory of expanding space, together with the empirical data of astronomy, permit no decision to be reached about the finite or infinite character of (three-dimensional) space, while the original "static" hypothesis of space yielded the closure (finiteness) of space.