[thedoc (OP)]

Jordi  asked the Naked Scientists:
   
A question, black holes are black stars or holes?

About time travel to the past,i am tired of listening or see documentary about how can we travel to the past,well bad  news,if you can CONTRACT the universe,of course you can't, even if you for  example go back 100 years,well i don't know when you appear,for sure is not in this planet,maybe in the middle of the space who knows.I think there is only one time line,and doesn't go back only forward.Is the most stupid thing i ever heard.hey i am no one i only college graduate.sorry if i am condescending. i hope someone answer or say something. Bye.

by the way if i didn't wrote well is because i am Spanish, sorry.


What do you think?

[Professor Mega-Mind]

       Black Hole Time Travel
          Don't you believe it !
 Time is not a river !  It is  a form of slippage !  There is one universe extant here .  It has many properties it manifests , one of these is slippage .  The contents of the universe are not frozen , but can change ( move , or slip ) .  This is fundamental to the concepts of energy , entropy, time , & most of the science of physics .  Resistance to slippage can vary the rate of change ( or movement ) profoundly , ergo the great variability of time . One can slow one's rate of slippage to zero , but one cannot unslip or unchange . 
 In the matter of black holes ; there's somethin' movin' , and it ain't us !
 Try this :  A car-sized ovoid , far more dense than a point particle. Said " egg" having the properties of mass , spin , and charge .  This is not a wormhole !  It is the most " mattery " form  of matter in the universe !  It can even do the wrecking-ball to other super wrecking balls !
I say give it cred ,it rocks (balls?)
Ootaay !............P.M.

[Professor Mega-Mind]

...................Elucidation
The obvious question is : What support is there for this "Egg" postulation ? 
The obvious answer is : Lorentz Contraction/Einstein Gravity .
L.Con. says that as an object falls into a black hole , potential energy is converted into kinetic energy .  Profoundly greater energy content must result in profoundly smaller dimension  ( witness the relativistic spaceship paradigm ) , but it is proportional , and never zero .
Einstein Gravity is curved Space/ Time substrate , presumably inter-acting with some form of quantum pressure .  A black-hole is thereby analogous to a very deep funnel .  The deeper an object is pushed into the funnel , the more it is squeezed and compressed by it's walls .  The more massive the BH , the steeper the walls .  The steeper the walls , the further the object can be forced down it's throat . The further down the throat ,the greater the compressing forces , the smaller the object gets , but it's size never reaches zero . 
The convergence of these two paradigms reinforces the physics maxim "Matter/Energy cannot be created or destroyed...." .  The M/E that a black hole ingests does not leave the universe , it is still there , stuck in it's craw .
Alright gents , have at it !......P.M.

[Professor Mega-Mind]

................One More Note.
The contraction/compression processes indicated above , refer to the complete space/time substrate that all matter/energy is embedded upon .  Said M/E is analogous to a picture drawn upon a sponge .  When the sponge is crushed down , the picture shrinks proportionately ( even it's atoms ) .
This means that the picture always sees the sponge as the same size .
Do you see it now , Mr. Hologram ?
P.M.

[evan_au]

Are black holes black stars or holes?

This is a question about mainstream physics.

Astronomers see that when a large star (perhaps 5 or more times the mass of the Sun) reaches the end of its life and has burned all its fuel, it will explode in a supernova. This is easy to see, even when it occurs in other galaxies, because it can be as bright as the whole galaxy (for a few days).

Not nearly as visible, astronomers believe that the core of the star collapses into a compact object - a neutron star (which we can see as pulsars) or, if they are even larger, a black hole (which we have not yet seen directly, but can infer).

The black hole is not really like a normal star that burns hydrogen and emits visible light - gravity is so intense that not even light can escape. So I would avoid the term "black star".

And because no matter can accelerate faster than the speed of light, no matter that enters the black hole "event horizon" can escape. This is where the accepted term "black hole" came from - it's like a drainpipe - what goes down doesn't come out again.
See: https://en.wikipedia.org/wiki/Black_hole

About time travel to the past

This is much more speculative.

There are some theories that if you could somehow survive a trip into certain unusual black holes, you may come out somewhere else in space or time.

Other physicists & philosophers have come up with reasons why it couldn't happen (especially travel into the past). But there is no evidence for time travel at this time.

So at present, this is mostly the domain of science fiction.

if you for  example go back 100 years,well i don't know when you appear, for sure is not in this planet,maybe in the middle of the space who knows

It is true that the Earth orbits around the Sun, the Sun rotates around the center of the galaxy, and space is expanding. If you moved forwards or backwards in time by even 1 second (and stayed in the "same" place), you would be gulping vacuum (or maybe Moon rock, if you were lucky).
 
Science Fiction often plays with the ideas of time travel. It would make for a very boring show if every would-be time traveller ended up dying in a vacuum, or (if they got their arrival position and speed even slightly wrong) burning up in the atmosphere like a meteor.

[Professor Mega-Mind]

How about if they "got it right" , "went back" , and it wasn't there , because there is only one locus (moment of existence) , and it is here . 
D.

[Professor Mega-Mind]

..................Completion .
See : Are Black Holes really holes ?
........P.M.

[guest45734]

A question, black holes are black stars or holes?

There exists a recent theory about Plank stars based on Qunatum Loop Gravity, whereby all black holes have a star at the centre. Hawking radiation reduces the size of the Black hole, whilst the plank star grows inside eventually emerging out of the surface of the Blackhole like a big bang, and prevents the information loss paradox. https://en.wikipedia.org/wiki/Planck_star enjoy.

[Professor Mega-Mind]

I'll forego a wise-ass spaghetti-fication remark , and instead ask a deeply disturbing question ; Can the crushed M./E. in a black hole actually be hot , or just "frozen" in a relativistic stasis ?.........P.M.

[yor_on]

Actually it depends.
Do black holes exist? Yes we think we have observations of SMBH in the middle of our galaxy
https://en.wikipedia.org/wiki/Supermassive_black_hole

On the other tentacle there is a mathematical proof stating that due to Hawking radiation there can be no black holes anywhere. https://phys.org/news/2014-09-black-holes.html#nRlv

On the one way arrow I agree. You're perfectly correct practically.
Theoretically it's another thing.

[Professor Mega-Mind]

My observation over the years has been that whenever there is a gray zone , there emerge a host of seemingly credible theories .  Often incredible , often in conflict with each other .  Without solid observational evidence , the truth is fundamentally "Out There !" .
P.M.