This is so cool because we used our ears (the LIGO detectors) to hear/feel the gravitational wave hit then used our eyes (electromagnetic radiation telescopes) to focus in on where we thought we felt the ripple. And what's more, machines all over the planet were involved. Just awe-inspiring.
Wonderfully summed up. Thank you. By the way I thought gravitational waves have only been a theory. How can they be detected and is there such a thing as a gravitational quantum ?
Gravitational wave detectors like LIGO (https://www.ligo.caltech.edu/page/what-is-ligo) are very precise laser range finders used as rulers. They can measure extremely small expansions and contractions of space itself.
Discovering a quantized theory of gravity is possibly _the_ major open question in physics atm. We presume there's a 'graviton' of some sort, but have not observed it yet.
If I'm remembering the analogy in the livestream correctly, if you applied the laser's precision to astronomical scales, it would be the equivalent of measuring the distance to the moon with a tolerance the width of a human hair. And in the scales they deal with, a tolerance one-tenth the diameter of a proton.
These are completely classical waves, exactly like a pressure wave from a detonation, expanding in a sphere outwards from a neutron star merger. While a pressure wave is a rapidly moving but small change in gas density, a gravitational wave is a veery rapidly moving but veeery small change in the "density" of spacetime itself.
These gravitational waves are predicted by (completely non-quantum) general relativity. Einstein predicted their existence in 1916, but thought we would never have good enough detectors to measure them.
These waves travel at the speed of light, right? How is light itself affected by gravitational waves? I guess also changed, if they're using lasers to detect them, yea?
The waves do travel at the speed of light, since that's the upper speed limit in GR.
The light is affected only because the distance it has to travel changes when spacetime compresses and expands. So the time it takes from A to B changes, but also the wavelength of the light.
What is used in Ligo etc. is interference. They shoot two perpendicular laser beams that collide in a point. Ordinarily, the lasers interfere at this point and everything is aligned so they cancel each other out almost perfectly. But when a gravitational wave changes the length in one of the arms, the interference isn't perfect anymore and you can detect the laser signal.
To a very very good approximation, a gravitational wave front hitting Earth is a flat plane. This means the detectors cannot see waves that hit the arms at close to 45°, as well as waves that hit the Earth's surface close to vertically at the detector location.
Jesus, I can't imagine the type of resolution necessary to make those detectors work!
So the indication that "gravity wave happened" is wavelength change? Freaky stuff, I really have a hard time wrapping my head around all this, even after reading layman intros.
No, it's not wavelength change, it's the number of wavelengths that fit in each "arm" of the detector. With 4 km arms and 1000 nanometer laser wavelength (actual numbers), you will have 4 000 000 000.00 wavelengths that fit in each arm. When a gravitational wave passes, one arm will have 4 000 000 000.10 wavelengths and the other is unchanges. Since we're working with interference, this parts-per-billion change is converted into a large change in light.
Mechanical analogy: there are two very long very fine pitch helical gears that are both suspended from one end and mesh perfectly at the other end. When the gravitational wave makes one gear undetectably longer, we can easily see that the gears no longer mesh.
How do they distinguish between 4 000 000 000.10 and 4 000 000 001.10 wavelengths? Do they simply count how often they cross a full-phase change and keep track of the direction of that change?
Your statement is true, but it is important to emphasise that they travel at the speed of light in vacuum, this is important because the universe is only almost empty and electro-magnetic radiation actually travels slower than the speed of light in vacuum in this medium. This means that one would expect for light to arrive a few seconds after the detection of the gravitational wave.
The statement is also only true for perturbations with respect to a fixed background metric. In principle it is possible for space-time to expand much faster than the speed of light (this is believed to have happened just after the big-bang).
These kind of waves do, but not all gravitational waves do. (You can think of the class these are as "normal", smooth waves.)
Sufficiently "foamy" ones should act like waves passing through material and cause interference based losses while sufficiently strange waves will travel faster than light, a la the hypothetical warp drive using negative mass.
It's worth mentioning that there is a very common confusion around what is meant by a theory. It isn't the same as a guess. It has to be well supported. We've had good reason to think gravitational waves existed based on the well supported math of gravity.
Last year we got the first expirmental confirmation of this.
The correct term of an idea that hasn't been tested yet is a hypothesis. A `theory' is a full featured explanation. Generally, one requires a theory to be 'accepted as true' by the majority of the scientific community.
The nomenclature here really sucks, because colloquially theory and hypothesis are essentially synonyms. However, it makes no sense to talk about the hypothesis of gravity. This discussion comes up most often around claims like "The theory of evolution is just a theory".
But the work on the gravitational wave detectors started much earlier, some fundamental calculations to support the current project were made already in the 1967, 50 years ago! That scientist (Rainer Weiss) got a half of the Nobel Prize in Physics this year for that:
Richard Feynman contributed his (valid) argument in support of the energy actually carried by the gravitational waves at the first American conference on general relativity, GR1, in 1957, 60 years ago:
Regarding the false understanding of the term "scientific theory" by the non-scientific public:
"A scientific theory is an explanation of some aspect of the natural world that has been substantiated through repeated experiments or testing [emphasis mine]. But to the average Jane or Joe, a theory is [wrongly] just an idea that lives in someone's head, rather than an explanation rooted in experiment and testing [the actual scientific meaning]."
The gravitational waves are the consequence of the General Relativity, the theory introduced in 1915 by Einstein (102 years ago), but which was also based on the centuries of the previous observations and calculations i.e. all the discoveries of the gravitation (Newton 1687, 330 years ago), electricity, magnetism, electromagnetic waves (more than 200 years ago), etc.
"Though these words [like "theory"] may be routinely misunderstood, the real problem, scientists say, is that people don't get rigorous science education in middle school and high school. As a result, the public doesn't understand how scientific explanations are formed, tested and accepted."
That I have to write all this is a kind of confirmation of that statement.
Just the same, the science of global warming is also based on the scientific observations and valid theories since 1824 (almost 200 years ago):
People misunderstand this stuff in part because of the failure to make a distinction between a theory (which is often a mathematical description of a system, though this depends on the discipline) and an interpretation (which really is an idea in someone’s head, though often one we have good reason to believe). Scientists mostly work with the former day-to-day, but the general public is mostly interested in the latter.
Usually this distinction isn’t really a big deal, but in some cases - quantum mechanics, for example - there’s a big difference between the two, with multiple interesting interpretations to consider.
When we don’t make this distinction it creates an opening for people with poor understanding (or, occasionally, bad motives) to nitpick extremely well-established theories on the basis of quibbling about interpretation. I worry about this with respect to climate change specifically: deniers come up with a million reasons that climate change may be partially natural, or that this or that industry may be unfairly maligned, but we can’t get sucked into those debates so much that we forget the big picture painted by the data and models we have.
I don't know about the quantums but to detect the waves you basically split a laser beam using mirrors and redirect each split to a detector. When the wave hits it stretches each split in the beam slightly which causes an interference pattern. That pattern, as it evolves over time as the wave hits, can be converted into sound which is that "chirp" that everyone talks about.
