I assume everyone reading this has heard by now of the LIGO experiment detecting gravitational waves for the first time.
The discovery is encapsulated in this very short YouTube.
It is a little confusing without explanation. I saw the video at other links without the explanation and it didn’t make sense: Why were there two chirps? The answer is simply that they play the chirp twice, so that if you want to hear it more than once, you don’t have to keep restarting the video!
But in any case, the chirp is the sound of two black holes spiraling around each other faster and faster as they get closer together, and the sound stops when they’ve combined into one black hole. That is, the frequencies you hear in the chirp are determined by the speed of the spiraling. (The whole recording does get sped up a bit so that it is in a good range for human ears to hear.) Note that these black holes were approaching each other at nearly the speed of light!
From the exact measurements of the characteristics of the chirp, scientists can determine that the two black holes were 36 and 29 times the mass of the Sun, respectively. And the resulting black hole is 62 times the mass of the Sun. These objects are pulled in by the enormous gravity so that they are remarkably small. A black hole that is 30 times the mass of the Sun will have a radius of only 55 miles. Keep in mind that the radius of the Sun is 432,474 miles!
But wait, 36 plus 29 is 65, so why did the resulting black hole only have 62 solar masses??
You might recall Einstein’s equation, E = MC^2 (pronounced E equals MC squared). Energy and mass are equivalent and can be converted from one to the other. And what happened is that 3 solar masses were converted into the energy of the gravitational waves that were detected in the experiment! How much energy is that? "For comparison, that’s about 10 trillion times the energy emitted by the sun over the course of an entire year. Or: "8.5 billion trillion trillion Hiroshima atomic bombs.”
Or to put it yet another way, "Calculations show that the peak gravitational wave energy radiated was more than the combined light energy released by all the stars in the observable universe.” !!!!!!!!!!
Note that black holes are objects we can’t see with we can’t see with electromagnetic radiation. That’s the point of black holes… their gravitational force is so strong that light waves can’t escape it. We are “seeing” these black holes with gravitational waves instead of light waves. We have never directly observed a black hole before… up until now, we’ve only been able to indirectly infer their presence. But now we can “see” and “hear” them directly. Now we have far more reason to believe they actually exist than we had before.
The next part of this write-up attempts to give some background about why this actually proves something and has pushed science forward in a significant way. If you dont care about that, or if you already know why, there’s no reason to read further!
The scientific method is to generate a theory that has implications that can be tested. Then you run the test. If the test fails, then there is no reason to give further weight to that particular theory, and you move on. But if the test succeeds, then it is extremely likely that there is something to the theory worth further exploration.
Consider Einstein’s completely theoretical ideas about general relativity. That theory generated a hypothesis: that the light of a distant star which must travel close to the Sun before reaching our eyes would be bent. If you looked at the star with a telescope at night, so that the Sun was not a factor, the star would appear to be at one location; but when you looked at that same star during the day, when its light had to brush by the Sun on its way to us, the star would appear to be at a different location. The time of a total eclipse was chosen to make the measurement because that way the starlight would be visible during the day.
Einstein’s theories enabled him to compute the exact angle by which the apparent location of the star would shift during the eclipse. That was the hypothesis: If General Relativity was a correct theory, that shift would be detectable; if it wasn’t, no shift would occur.
And lo and behold, the predicted shift did occur. The news made newspapers around the world. It’s considered to be the first experimental proof of General Relativity, and one of the greatest scientific discoveries of all time.
With regard to the LIGO experiment, there was a predicted shape of the waveform that would occur as two black holes spiraled into each other. This was exactly equivalent, from the standpoint of the scientific method, to Einstein’s prediction of the exact angle that the starlight would be deflected by the Sun’s presence. If we could detect that waveform in nature, the theory would be vindicated.
And LIGO did detect a waveform having exactly the predicted characteristics (at least within the limits of experimental error, as was also true with Einstein’s starlight prediction), and which has no other explanation that scientists can conceive of. You can see it and hear it. Its really quite amazing. It’s not like they detected all sorts of waveforms that were slightly similar to the theory, and picked one that happened to match it more exactly. They didn’t.
They detected THIS one, and it just happened to completely conform with the theory. It is simply far too unlikely to have happened by chance alone. Just as Einstein didn’t detect lots of starlight deflections, and pick one that happened to correspond to the theory. A deflection coinciding with the presence of the Sun had never been looked for before, and when it was, its angle conformed to the theory. Out of all the possibilities that could have happened, starting with the overwhelmingly great possibility that no deflection at all would be detected, just happening to conform to the predicted angle would be extremely unlikely. So much so that that possibility can be dismissed as being too unreasonable to take seriously. Same thing with LIGO.
Note that all this doesn/\'t mean that the theory is 100% complete and perfect. In the history of science, there are always further nuances to discover, or new contexts that make us understand old theories in a new way. But it DOES mean that there is something about the theory that is highly revealing of an aspect of nature; and thus is something that will provide a basis from which further discoveries can be made.