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Rutherford did say something like this but suggesting a barmaid instead of a six-year-old, while Hilbert suggested the first man on the street. Technically, if we assume the statement correct, then nobody has understood anything about science, ever. But I'd like to have my go at explaining Physics-y stuff as simply as I can, and, perhaps equally importantly, without too much of the "woaaah quantum mechanics is so weird, you just have to accept it like this even though it's completely non-intuitive" crap one often finds in science articles written for the laymen.

Sunday, February 14, 2016

Gravity waved! (Q&A)

If you've been online even for a moment in the last few days, you might have heard something about it. Gravitational waves have been detected! The starting point to understand what those are is general relativity, and I am planning to write a post about it one day. But, due to popular demand,[citation needed]  I will go ahead of myself and answer some questions about this exciting observation. 

Note that there are many other sources from where you can learn about gravitational waves in general, and the measurement in particular. Just a few suggestions: the American Physical Society viewpoint (the paper was published in Physical Review Letters, a scientific journal published by the APS), the New York Times take on the story, or simply this cool PhD comics video. I hope that the Q&A below will further quench the thirst for understanding of the scientific enthusiast. 

Q: So is that just another type of a wave?
A: This one is quite unique. Other types of waves, like waves in water, sound waves, or electromagnetic waves (light), are in some ways very different from one another, but they do share an important characteristic. Namely, they can be described as an oscillating modulation of something physical, be it the water particles, air pressure, or electric field intensity. Mathematically, we would write this as some amplitude that depends on position and time - A(x, t) - and the wave nature appears in the fact that there is some periodicity in the time dependence. Gravitational waves, however, are different and kinda hard to imagine properly, because it is the position and time intervals themselves that change! Mathematically, one might write the effect of a gravitational wave as something like x'(x, t), t'(x, t), which is to say that space and time themselves change when such a wave passes through. (Note: this is not the way one would typically express the waves mathematically, but I think it's a good illustration.)

Q: So, like, a wave stretching the fabric of space and time?
A: You hear that a lot these days, but I don't like the 'fabric' part of the statement. It implies that space and time are made out of something. As I've already discussed, Einstein's view was rather that there are objective, absolute events, which appear to us imbued with the qualities of a location and a moment in time. Measuring these qualities defines our subjective space-time, and the laws of relativity tell us how to communicate with observers with different subjective space-times. In any case, whether space-time is physical or a figment of our perception is a big can of philosophical worms. I would thus much rather just say 'a wave stretching space-time' and throw the 'fabric' away to keep the interpretation load to a minimum.

Q: So what did we detect?
A: General relativity explains gravity as curvature of space-time. Our Sun for example curves our space-time in such a way that if we try to follow a 'straight' path, we end up orbiting around it. This is a fairly static effect: mass implies curvature just by being somewhere. Another prediction of the theory, however, is that waves of space-time curvature can be radiated by accelerating massive objects. An extreme example of accelerating masses is a system of two black holes rotating around one another. This is what we detected. As black holes can pack a lot of mass in a tiny volume, the centrifugal acceleration in such a system is tremendous, and so is the amplitude of the emitted gravitational waves - at least as compared to any other emitters. Compared to the sensitivity of our apparata, the amplitude was still barely big enough for us to measure - and we've been trying for decades. In short, we detected two black holes that circled around one another and eventually collided 1.3 billion years ago.

Q: Ok, I know enough astronomy to know that we see something that happened 1.3 billion years ago because it happened 1.3 billion light years away from us. So did we, like, see two black holes about to collide somewhere (1.3 billion light years away), point our gravitational wave detector in that direction, and see the waves coming in? 
A: The first part of the statement is correct: gravitational waves also travel at the speed of light, so indeed the black holes were 1.3 billion light years away. The second part is wrong. For starters, we cannot 'point' our gravitational wave detector in any particular direction: it's a giant device that 'listens' for space-time disturbances coming from anywhere (do see the PhD comics video). More importantly, however, we had no other way to detect this black hole collision: we only know about it because of the LIGO measurement. In fact, even though the first-ever gravitational wave detection is what made the headlines, it is worth noting that this is also the first-ever detection of a black hole binary system, and of two black holes colliding to form one! 

Q: Well that's pretty cool.
A: Yes! A great deal of the excitement comes from the fact that we now have a whole new tool to explore the Universe with. Black holes in particular are very elusive; up until recently, we didn't even know whether they existed. It's easy for us to observe stars, since they emit so much light, but light cannot be emitted from the inside of a black hole. Thus, our observations are always based on indirect methods that typically rely on the extreme space-time bending in the vicinity of black holes. For example, light also gets bent, which results in the so-called gravitational lensing that we can observe and consequently infer the presence of a black hole. Gravitational-wave experiments will now add another dimension to our observations - and hence understanding - of the Universe.

Q: What did the collision look like?
A: To us, it looked like this:
B. P. Abbott et al. (LIGO Scientific Collaboration and the Virgo Collaboration), Phys. Rev. Lett. 116, 061102 (2016)
The top curve shows the signal that was recorded by the LIGO detector. The bottom curve shows our prediction (based on General Relativity) of what the signal from two colliding black holes would look like. The agreement between the two curves confirms the observation (careful statistical analysis was made to substantiate that claim). It is also worth noting that the same signal at the same time was detected by a similar detector located in Louisiana. This is important for us to be sure that it wasn't some fluke due to random noise. 

Q: Isn't that a bit contrived? To me, that doesn't look anything like two black holes colliding...
A: Your brain does similar inferences all the time. Everything that you perceive as 'real' is your brain's interpretation of the signals it receives from your sensors, like your eyes. 'Seeing' something is an electromagnetic signal recorded by your retina not too different from the one in the figure above. It's your brain who is responsible for your subjective feeling of 'really' seeing it. We tend to think that we are really perceiving something when the interpreting process is sub-conscious. What we are doing with gravitational waves is, we are using our brains again, but this time consciously - through Einstein's theory. This is because they just don't have the capacity to record and interpret the gravitational-wave signal on their own. 

Q: Does this change everything we thought we knew about the Universe?
A: On the contrary, it confirms once again the extremely successful theory of General Relativity. Einstein predicted the existence of gravitational waves exactly 100 years ago, and, like a prophet who is actually worth listening to, he was once again correct. Actually, General Relativity works so well when it comes to astronomically-sized objects that I don't think there were many physicists doubting the existence of gravitational waves. For me at least, their existence was obvious, I just wasn't sure if I'd live to see a detection, since they are so elusive. It's pretty cool that I did. And of course it's very important to have the experimental confirmation, as we should never over-trust our theories. All in all, this is one more beautiful testimony to the fact that science just. works. bitches. 

Q: Is this quantum gravity?
A: No, this is classical General Relativity stuff. We actually don't have a complete theory of quantum gravity due to the infamous difficulty of combining gravity with the other forces that are now explained by the Standard Model on the quantum level. A theory of quantum gravity might contain a graviton - a particle that 'carries' the gravitational interaction - but this is for now hypothetical. And since I'm not a big fan of the 'particle' notion, I'd rather think of the graviton in relation to the gravitational waves: the graviton is the quantum of a gravitational wave, i.e. the smallest indivisible amount of energy that can be radiated by such a wave. There is a perfect analogy between this and a beam of light which is composed of photons - indivisible packets of light energy. In the case of a macroscopic number of quanta, or packets, the quantum nature is lost and both light and gravity can be described by the corresponding non-quantum theories (Maxwell's equations and General Relativity, respectively).

Still, it's worth noting that the LIGO experiment also managed to set an upper bound on the mass of a graviton, provided it exists.

Q: Thanks!
A: No prob. 

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