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• Hey, Vsauce, Michael here, and today we are going to go inside a black hole. It's not

• going to be comfortable, but it will be pretty fun. Now, first thing's first: mathematically

• speaking, anything could become a black hole if you were to compress it into a small enough

• space. That's right, you, me, this camera- everything in the unvierse has what is known

• as a "Schwarzchild radius": a tiny, tiny amount of space that, were you to collapse the entire

• mass of the object into, it's density would be so great that its gravitational pull would

• be so great that not even light could escape from it. You would have a black hole.

• If you were to compress Mount Everest into something smaller than a nanometer, you would

• have a black hole. And if you were to compress the entire Earth down to the size of a peanut,

• you would have a black hole.

• But, fortunately for us, there is no known way to compress Everest or Earth in that fashion.

• But a star, many, many, many times larger than our own sun, has a much larger Schwartzchild

• radius, and when it runs out of fuel and can no longer keep itself hot enough, it collapses

• to a single, infinitesimally-small point known as a "singularity."

• It's density will be infinite, and, so, it's gravitational pull will be so strong that

• nothing can escape- not even light.

• But enough about ways black holes form- let's jump into one. First question: what would

• it look like from the outside? Well, we know that gravitational fields bend space and time.

• Stars behind our sun will actually appear to be in slightly different locations from

• Earth because the sun's gravitational field bends the light coming from those stars.

• When it comes to the gravitational fields of larger objects, like entire galaxies or,

• for that matter, a black hole, the effect is even nuttier. Light coming from object's

• behind them is significantly distorted, producing smears and smudges.

• As seen from Earth, the blue galaxy behind this red galaxy is completely distorted, like

• a fun house mirror. So, rather than appearing as it really should, it looks to us like a ring-

• a smudge all the way around the red galaxy.

• This is known as "gravitational lensing." Now, take a look at this simulation of a black

• hole with a galaxy millions of lightyears behind it. The galaxy's really not in danger

• of the black hole's "suck," but the light coming off of that galaxy certainly is. Watch

• as the galaxy passes behind the black hole and its light is contorted, twisted, and

• distorted.

• Now here's a really fun demonstration:

• What if the Earth were to orbit around a black hole? Looking from the outside, the Earth

• would look normal at first, but as soon as it passed behind the black hole, the black

• hole's gravitational field would warp the light reflecting off the Earth, producing

• this.

• For the sake of simplicity, let's jump into a simple black hole- one that doesn't have

• a charge and isn't moving. And, also, isn't already sucking up a bunch of matter- so it's

• just there on its own.

• As we approach, the distortion of the sky grows greater and greater. A larger and larger

• portion of our field of view looking forward into the black hole will be filled with darkness.

• At this point, where half of our field of view has been swallowed up in darkness, we

• have reached the "Photon Sphere."

• At this point, light is not going to necessarily get sucked into the black hole, but it doesn't

• necessarily leave it either. Instead, at this magical point in space, light, photons, can

• actually orbit the black hole.

• If you were to stop here for a moment and look to the side, you could theoretically

• travel all the way around the sphere of the black hole, right back to your face.

• A gravitational field not only warps space, it also warps time. Now, for most intensive

• purposes here on Earth, we never have to worry about that. But, near a black hole, gravity

• would be so strong that an observer standing, watching you jump into the hole, would see

• something quite strange. They wouldn't see you get sucked quickly into the hole- instead,

• they would see your approach become slower, and slower, and slower, until you reached

• a point known as the event-horizon.

• This is a point in space where, once crossed, there's no going back. It is at that point

• that light can no longer escape. And, so, to a person watching you fall into the hole,

• that would be where your journey ended. You would seem almost frozen in space, the light

• coming off your body becoming increasingly red-shifted until you simply faded into nothingness.

• They would never see you cross the event-horizon.

• But for you, of course, everything would seem fine and dandy. You would continue passed

• that horizon to your now, inevitable, death. As you continue to approach the black hole's

• singularity, your view of the entire universe would get compressed into a smaller, and smaller

• point in space behind you.

• If the black hole we're jumping into was large enough, things actually might be quite comfortable

• at that event horizon. We'll know that we're never going to escape and that our lives are

• pretty much over, but it might take us hours to actually reach a point where things started

• to hurt.

• Why would they hurt? Well, the closer you get to the singularity, the more significant the

• difference in gravitational pull is across space. And, so, parts of me that are closer

• to the singularity would be pulled more strongly than parts that were facing away, and my entire

• body would be stretched toward the singularity. The effect would be so incredible, scientists

• don't usually call it stretching, they call it "Spaghettification."

• Once you reach this point, you would be dead. Your molecules would be violently ripped and

• stretched apart, and when they got to the singularity, well, we don't really know what

• would happen. Perhaps they would completely disappear in violation of all the laws of

• physics, or, maybe, they would reappear elsewhere in the universe. It is believed that a moving,

• or spinning, black hole might actually create what is known as a "wormhole," a way of transitioning

• across space faster than light. Not in any way that violates the laws of science, but

• in a way that takes advantage of the universe's dimensions.

• For instance, if I wanted to get from this point to this point, I'd have to travel the

• distance. But, theoretically, a wormhole would do something really crazy. For instance, this.

• Now, the two points are right next to each other and I can travel between them almost

• instantaneously.

• But, again, this is all theoretical. Luckily, we do have a possible way of analyzing black

• holes right here on Earth. Enter the "Dumbhole."

• Just as a black hole does not permit light to escape, a Dumbhole is an acoustic black

• hole. It won't allow sound to escape. It doesn't have to be nearly as powerful, and scientists

• have been able to create Dumbhole's in laboratories using special fluids traveling at the speed

• of sound.

• A lot of progress still needs to be made in the world of acoustic black holes, but, we

• may be able to learn an amazing amount of information about how black hole's work by

• looking at how sound is treated in a Dumbhole.

• Now here's another good question: What would it look like to travel at the speed of light,

• say, toward the sun? Well, surprisingly, you wouldn't just see the sun immediately rush

• up toward you. No, no, no. In fact, initially, it would look almost as if the sun were receding

• away from you. Why? Because your field of view would vastly increase in size. You would

• be able to see stuff almost behind you. And here's why:

• As you sit there, not moving yet, looking at the sun, there's light coming from stuff

• behind you. But, if you travel the speed of light, you will actually reach that light

• coming from things behind you. As you reached light speed, your field of view would expand

• like this, concentrating the stuff in the middle.

• But where are you in the universe? Or, here's a better question: Where is the center of

• the universe? Well, this might sound crazy, but, it's everywhere. This is known as the

• "Cosmological Principle." No matter where you are in the universe, everything else will

• seem to be moving away from you, expanding, at the same rate.

• The universe is expanding, but not like a balloon getting bigger with all the people

• inside it. Instead, it's as if we are the surface of a balloon. If you were to put a

• bunch of dots on a balloon and then blow it up, all the dots would move away from each

• other at the same rate. And, on the surface of the balloon, there is no center.

• Take a look at these two layers. They are exactly similar, except the top layer represents

• a 5% expansion of the bottom layer.

• Let's say that you live on one of these dots, and you want to measure where everything is

• moving away from. Well, watch what happens when I line up a dot in the past and the present:

• Boom. It looks like the center of the expansion. I can do this with any dot. As soon as I choose

• a dot to be the frame of reference, it immediately becomes the center of the expansion.

• So, while dying in a black hole would be lonely, and scary, and morbid, when you look up into

• parents and friends tell you, you really, scientifically, are the center of the universe.

• Finally, what if our universe was a googolplex meters across? It is nowhere near that large.

• But, if it was, it would be so voluminous that, statistically, it would be nearly impossible

• for there not to be an exact copy of you somewhere else out there in the universe. To see why,

• I highly suggest that you click right there and check out Brady Haran's new channel "Numberphile."

• It's part of the YouTube original channel's, and I've worked with these guys before. They're

• amazing, they're my favorite kind of geeks. So, check out that video, watch their other

• stuff, and if you like math, I highly suggest that you subscribe.

• And, as always, thanks for watching.

Hey, Vsauce, Michael here, and today we are going to go inside a black hole. It's not

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# Travel INSIDE a Black Hole

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陳俊安 posted on 2013/08/29
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