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  • If youve heard only one thing about black holes, it’s probably that, once inside a

  • black hole’s event horizon, nothing, not even light, can escape.

  • At which point it’s natural to wonder, if nothing can escape a black hole, how could

  • we ever observe them?

  • How do we even know they exist?

  • Well, only things inside the event horizon are stuckblack holes also gravitationally

  • pull on stuff outside their event horizons, and by looking at that stuff we can get a

  • really good sense that there’s a black hole nearby.

  • For example, lots and lots of stars orbit in pairs , but we also see stars orbiting

  • things that aren’t normal stars, but instead emit crazy amounts of x-raysand x-rays

  • in space often come from dust and gas that gets superheated while spiraling into a very

  • dense, very heavy object.

  • Anyway, by figuring out the mass and orbital characteristics of the stars whose partners

  • emit x-rays, we can determine how heavy the partners are.

  • Some parters are lightweight enough to be neutron stars , but neutron stars can only

  • get so big before they collapse in on themselvestheoretical calculations put their upper

  • size limit at around 2-3 times the mass of the sun, and the biggest ones weve observed

  • all fall inside that limit . And yet, there are plenty of stars whose orbits clearly show

  • that their x-ray-emitting partners are 5-10 times the mass of the sun, and we simply don’t

  • know anything else these could be other than black holes.

  • Sometimes you don’t even need an orbiting star at all, and just the x-rays and radio

  • waves from the hot infalling material can be used to determine the mass of a solitary

  • non-star object.

  • In some cases they turn out to be neutron stars, but in others they turn out to be way

  • too heavy, and can only be black holes.

  • There are also objects at the centers of lots of galaxies (including our own), that emit

  • lots of x-rays, radio waves and infrared radiation, but not much visible light, and we know these

  • objects are stupendously heavy because of the way that nearby stars and hot glowing

  • dust orbit them.

  • These orbits tell us the objects are both so heavy and so small they can’t possibly

  • be a star or cluster of stars or distributed clumps of other invisible matter; the only

  • thing they could be is supermassive black holes.

  • For example, in the middle of the Milky Way there’s an x-ray, radio wave and infrared-emitting

  • object calledSagittarius A*” with nearby stars orbiting it in such such small, fast

  • orbits that we know it weighs 4 million times as much as the sun!

  • And finally, weve also directly observed, on multiple occasions, gravitational waves

  • that were emitted from the inspiralling collisions of two very heavy dense objects.

  • Some of those waves have the signature of a collision between objects lightweight enough

  • to be neutron stars.

  • But other waves could only have come from collisions between objects far too heavy to

  • be anything but pairs of black holes merging to become single, bigger, black holes.

  • And in these cases, the details of the wave signatures looked exactly like what theoretical

  • black hole collision calculations predict.

  • So, in many different places throughout the universe, weve detected very dense high-mass

  • objects by their gravityeither indirectly via their affect on nearby bright stuff like

  • stars or accretion disks of gas and dust, or directly via their gravitational waves.

  • Many of these dense high-mass things are too dark to be regular stars, too compact AND

  • too dark to be clusters of stars, and too heavy to be neutron stars.

  • They exist, they behave pretty much exactly the way physics predicts black holes would

  • act, and there’s literally nothing else they could be.

  • To quote an astronomer: we havestrong confidence that black holes, or at least objects

  • that have many of the features of black holes, exist

  • In other words, if it looks like a black hole and acts like a black holewe call it a

  • black hole.

  • Thanks to NASA's James Webb Space Telescope Project at the Space Telescope Science Institute

  • for supporting this video.

  • The James Webb Space Telescope will be able to observe the most distant emissions from

  • some of the earliest supermassive black holes in primordial galaxies and hopefully help

  • us understand how black holes drive galaxy evolution and development.

  • Webb will also spot black holes via the stars, gas and dust they attract, and help us understand

  • black hole energy dynamics, including the powerful relativistic jets they can produce.

If youve heard only one thing about black holes, it’s probably that, once inside a


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B1 中級

ブラックホールの存在を知る方法 (How We Know Black Holes Exist)

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    林宜悉 に公開 2021 年 01 月 14 日