– How do we know black holes (etc) still exist?


That is, if the closest one we can observe is 1600 light years away, are we not observing these things as they were 1600 years ago? How do we know black holes didn’t just pop up for a brief time window, and now they are gone?

In: 2

We have no reason to suspect they magically pop into or out of existence; if they did, we should have seen the effects of them doing so, even if we can’t see the black holes themselves.

Even if you can’t see the black hole, you can observe the effect of the black hole’s gravitational field on spacetime around it.

Our minds and culture are full of ideas. The best ones are those that help us successfully navigate through life. One solid principle we can use to help identify the best ideas is: does it make predictions about what we can expect to experience, and when we test those predictions, do they work out?

Einstein had an idea about gravity back in the early 1900’s. His idea is called General Relativity. This idea makes a whole lot of incredibly specific predictions about what we will see if we do certain specific experiments. Many of those experiments had not been done in Einstein’s time, but over and over and over again, the predictions General Relativity makes, turn out to be true.

General Relativity is an incredibly trustworthy idea.

One of the predictions it makes is that black holes would exist. Black holes were unknown in Einstein’s time, and the first photographs of one were captured only a few years ago.

General relativity says that black holes will not just disappear. Since general relativity has been so incredibly reliable so far, there’s no good reason to doubt that the black hole 1600 light years away is still there.

> That is, if the closest one we can observe is 1600 light years away

That’s the closest one that astronomers can observe “directly” (or at directly as you can ever observe a black hole). There is a supermassive black hole at the center of our own galaxy, but 1) there are a lot of stars and stuff between it and us, making that area hard to look at; and 2) it’s not actively feeding. It sucked up all the stuff that it could suck up millions or billions of years ago. Since black holes are, well…*black*, what astronomers want to observe is the *accretion disk* around the black hole, which is the gas and plasma and stuff that gets superheated as it rubs against itself swirling around the black hole. Our supermassive black hole (Sagittarius A*) does not have an accretion disk, so there’s not much to look at, there.

However, we *can* see the effects of that black hole. You can see in [this gif](https://cdn.vox-cdn.com/thumbor/HUPrAepWcueBotT0JPcAF-Qh7_I=/800×0/filters:no_upscale()/cdn.vox-cdn.com/uploads/chorus_asset/file/9984865/stars_orbit.gif) (found in [this article](https://www.vox.com/science-and-health/2018/1/8/16822272/black-hole-looks-like-what) how stars at the center of our galaxy are orbiting *something*. It’s marked in that gif, but in [this gif](https://cdn.vox-cdn.com/thumbor/a6tef_Av7GgpizrLek-PD0yhuZ0=/800×0/filters:no_upscale()/cdn.vox-cdn.com/uploads/chorus_asset/file/11759533/2018_07_26_11_40_29.gif) (from the same article) the “raw” images where there is nothing visible that the stars are orbiting. That something is, of course, the supermassive black hole Sagittarius A*.

So, the idea that we can’t observe black holes over time is somewhat flawed. We can’t *directly* observe a black hole ever – that’s kind of the point of a black hole. But there are plenty of ways to know that they’re there: observing the behavior of stars around them, detecting light from the accretion disk, detecting gravitational waves from black hole mergers, looking for [gravitational lensing](https://hubblesite.org/contents/media/images/2022/001/01FRKBDN5YKMM9ZMT5Q7TSN4RN)… Astronomers have detected black holes through all of these methods, and continue to observe them. Scientists at CERN **may** be able to create [microscopic black holes](https://angelsanddemons.web.cern.ch/faq/black-hole.html), although those would be extremely short-lived (fractions of a second).

Regardless, the ability to observe black holes is not required to predict their behavior. In fact, Einstein predicted the existence of black holes long before they were observed just by considering the implications of his theories of Relativity. Based on the mathematical equations, Einstein saw what could be possible. It wasn’t until decades later in 1971 that astronomers confirmed their existence. Based on those equations, Einstein made a number of predictions about their behavior. Stephen Hawking made even more predictions about black holes based on various equations of relativity and quantum mechanics. Those equations give scientists ideas about how black holes behave, and there’s really no way for a black hole to just *pop* into existence and then *pop* back out of existence. Hawking predicted that black holes could *slowly* evaporate through Hawking Radiation – losing energy and therefore mass until they eventually disappear. However, the time it takes depends on the size of the black hole. The larger the black hole is, the slower it evaporates. Stellar-mass black holes will take billions and billions of years. Supermassive black holes like Sag A* will take hundreds of billions, if not trillions of years.

How can one be sure that when you put a shoe in a shoe box that the shoe is really still in there? It’s important to remember that a black hole is a PREDICTION of general relativity (our modern theory of how gravity works), first put forth in 1916. Without that their idea simply wouldn’t exist. And then we observe them in modern times and they behave as the theory predicts. That is the situation.

One of course could ask, “How do we know that reality isn’t sprinkled with things that our laws of physics SEEM to exactly explain and model and we didn’t even know to look for until our theories predicted it but also have the additional feature that they randomly YOINK themselves out of existence to mess with us?”. The answer, I suppose, is we don’t.

The city I live in is pretty old. Due to its age, some buildings in the historical centre have slightly slanted floors. I’ve been in a house with such a floor. It’s not so that you’ll immediately notice, but put a tennis ball on the floor and it’ll roll away from you.

In other words, look at the floor and you won’t say something’s off about it. Look at the ball and you know not only will you find out that it is slanted, you will also quickly learn which side of the room is higher than its opposite side.

Same with black holes. Even if we can’t see them, we can see the effect they have on the things around them.