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Black holes are some of the most fascinating,  mysterious and downright strange objects in  
the universe. They have such strong  gravitational pull that anything that  
gets sucked into a black hole disappears  forever - even light cannot escape the  
clutches of a black hole. We know that  nothing ever comes out of a black hole,  
but what happens to the stuff that does fall  in? What's actually inside a black hole?
Scientists have long been fascinated with black  holes. The existence of objects with gravitational  
fields too strong for light to escape had been  theorized about by the greatest scientific minds  
since as far back as the 18th century. After  Einstein developed his revolutionary theory of  
general relativity in 1915, scientists finally had  a way to calculate and describe the phenomenon of  
a region of space from which nothing can escape.  Still, the existence of black holes remained a  
mere mathematical curiosity until the 1960s,  when Stephen Hawking popularized the concept.
Until very recently, the only way we've  been able to find black holes is by  
combing the skies for the telltale signs  of their interactions with nearby objects.  
Gasses and stars near a black hole behave  differently than elsewhere in the universe,  
affected by the black hole's gravity and giving  off a high-energy light invisible to the naked  
eye but detectable with telescopes using  special tools. Through reverberation mapping,  
scientists can measure the radiation thrown off  by a rotating disc of debris around a black hole  
and pinpoint its location, even if  they can't see the black hole itself.
Then, in 2019, scientists did something that  was previously thought to be impossible - they  
captured an image of a black hole using the  International Event Horizon Telescope. Finally,  
the world had solid, photographic proof of the  existence of black holes. Technically, what they  
captured was the black hole's shadow, since the  black hole itself does not reflect any light,  
but nevertheless, it was a major breakthrough that  would have thrilled Einstein just a century ago.
Despite our inability to get near one, or to even  see one with our own eyes, thanks to the tireless  
and groundbreaking work of mathematicians and  theoretical physicists, we have been able to  
learn quite a lot about the formation, properties  and behavior of black holes, even before we were  
able to view them with the help of technology. A  black hole is a place in space where the force of  
gravity is so strong that nothing can escape from  it - not even light. The gravitational pull is so  
strong because in a black hole, all matter has  been squeezed into an inconceivably tiny space.
Mass refers to the amount of matter or “stuff”  in an object. The smallest black holes,  
which were believed to have been created  at the time the universe began, might be  
as small as a single atom, but have the same  mass as a large mountain. Stellar black holes,  
created when the center of a large star collapses  in on itself in a supernova, can have a mass up  
to 20 times greater than our sun. The largest  ones, called supermassive black holes, have  
masses greater than a million of our suns, and  lie at the center of every galaxy in the universe.  
The supermassive black hole at the center of  our Milky Way galaxy, Sagittarius A, has as  
much mass as 4 million of our suns, all condensed  into a ball only as big as a few million Earths.
If our sun were somehow replaced by a black hole  with the exact same mass, would the Earth be in  
danger of falling into the black hole, never to  be seen again? The black hole's gravitational  
pull would be identical to the sun's, despite the  fact that it would be much smaller. The Earth and  
the rest of the solar system would continue to  orbit the black hole in the same way they orbited  
the sun - though without the heat and warmth we're  used to from the sun. Thankfully, our sun is not  
massive enough to collapse into a black hole,  so it's not something we have to worry about.
If we're able to know so much about how  black holes form and what happens around them  
without having visited or even seen one, can we  possibly have any clue what's actually inside  
a black hole? Though it would be impossible for  anyone who somehow made it into a black hole to  
report back to us, scientists do have some pretty  good ideas about what goes on inside a black hole.
The closest black hole to earth is more than  1,000 lightyears away, so unfortunately it  
doesn't look like we'll be sending any exploratory  missions there anytime soon. But, if we could,  
what would we find out? What would the first  brave explorers see inside a black hole?
Anyone brave enough to get near a black  hole wouldn't actually be able to see it  
as they approached. The objects outside the  black hole can fall into it, effectively  
impacting whatever is inside, but the black hole  cannot affect the exterior universe because its  
gravitational pull is so strong that anything  sucked inside, including light, can never escape.  
If light cannot escape, there is nothing for  our eyes to see when we look at a black hole.  
Instead, explorers approaching the  black hole would have to be on the  
lookout for the cloud of gas and debris  orbiting it to know when they're close.
As the brave explorers drew closer and closer  to the black hole, they would notice a threshold  
beyond which there is nothing but an empty, black  ... nothingness - this is the Event Horizon,  
the black hole's last frontier,  and the point of no return.
A black hole itself is a singularity - a point  of infinite density, where all matter has been  
squeezed into an infinitely tiny space  and the laws of physics cease to apply.  
The singularity is shrouded from view by the  Event Horizon, what we would consider to be the  
surface of the black hole, though it's not  a “surface” in the true sense of the word.  
The event horizon is not a real physical  barrier or a membrane, but rather,  
it's an invisible barrier at a particular  distance from the singularity, a threshold  
beyond which the gravitational pull is so  strong that nothing, not even light, can escape.
Once something - or someone - has  crossed the Event Horizon, there's no  
going back. Once they've pierced the veil, they'll  begin the long fall towards the singularity,  
the ultimate one-way trip. The fall into the black  hole wouldn't be instant - a brave explorer would  
have a few moments to enjoy the experience. The  time it takes to reach the singularity after  
crossing the event horizon depends on the size  of a black hole. In the smallest of black holes,  
like one the size of a few of our suns, it  would take less than an eyeblink to reach the  
singularity, but in a massive one millions of  times larger than the sun, the explorer would  
have a few heartbeats in which to take in their  surroundings. What they would see and experience  
in those few brief seconds would seem like  something straight out of a sci-fi movie.
In the universe outside of a black hole,  objects and people are free to move in  
any direction they choose - up, down, left,  right, backwards or forwards - but they are  
always moving forwards through time. Inside the  event horizon, though, there is no other option  
but to travel towards the singularity. Inside  a black hole, the singularity is the future,  
and all matter that falls into a black  hole must simply travel towards it.  
No matter which direction someone in a black  hole is travelling or which way they turn, the  
singularity is somehow always right in front of  them, while at the same time, always out of sight.
As the daring explorer falls forward beyond  the event horizon and is drawn towards the  
singularity, they would find that the space  around them is anything but black. In fact,  
they'd be completely enveloped in light  from the surrounding universe - light that  
crossed the event horizon and is falling  towards the black hole along with them,  
doomed never to escape. The extreme  gravitational forces inside the event  
horizon would bend and twist the light as  it gets pulled towards the singularity.
Those same gravitational forces word be  hard at work on the explorer's body, too.  
The black hole's tidal forces would exert  different amounts of gravitational pull  
on different parts of the body, squeezing it at  the midsection and pulling and stretching from  
end to end, in a process appropriately dubbed  “Spaghettification”. As the explorer's body  
stretched, infalling light would be concentrated  into a band of light wrapped around their waist,  
until both body and light lose all dimensionality.
If the heroic explorer could peel their eyes away  from their body's physics-defying acrobatics,  
they would witness another strange phenomenon  - Time Dilation. The universe they left behind  
beyond the Event Horizon would still be there,  but would appear to have sped up and be moving  
much faster than time inside of the black hole.  But to anyone looking on from the outside,  
they would see something much different.  To them, as the explorer began to approach  
the event horizon, they would have  appeared to move slower and slower,  
and it would look like they never crossed the  Event Horizon or fell into the black hole at all.
When the falling explorer finally catches a  glimpse of the singularity, they would see...well,  
nothing. Reaching the singularity would feel like  landing on the surface of an enormous, empty,  
featureless, black planet stretching from horizon  to horizon. The explorer's body, by now distorted  
beyond all dimensionality, would be crushed into  a tiny ball of infinite density, before dissolving  
irrevocably into the singularity, never to be seen  or heard from again. All that they've seen and  
all they've experienced inside of the black hole  would be dissolved with them, lost to us forever.
Those on the outside could certainly  attempt to send signals into the black hole,  
but those signals would only join the explorer's  body in the singularity. Even if someone inside  
a black hole could somehow receive the signals  and attempt to reply, the black hole's intense  
gravity would ensure that no information would  ever be able to escape the black hole. At least  
whatever dirty secrets the explorer took into  black hole with them would be safe forever...
It's truly awe-inspiring to realize that  we have only recently gotten photographic  
proof of a phenomenon that's been theorized about  for centuries and mathematically proven for 100  
years. Even more incredible is the fact  that, as much as we know about black holes,  
we have yet to see inside one. Everything we  think we know about what's actually inside a  
black hole is still purely mathematical theory  and conjecture until we can figure out how to  
get a real look inside - and live to tell the  tale. That doesn't seem likely anytime soon,  
but science continues to surprise us, so we  just might find out sooner than we think!
If this video hasn't scared  you off of space exploration,  
you'll have to check out “What It  Takes to Join Space Force”. Or,  
if you prefer to remain on terra firma,  you'll like this other video instead!
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黑洞裡面到底有什麼? (What's Actually Inside A Black Hole?)

27 分類 收藏
Summer 發佈於 2020 年 9 月 21 日
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