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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 grav

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 ?

epberry 9y ago
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.
jasonwatkinspdx 9y ago
Gravitational wave detectors like LIGO (https://www.ligo.caltech.edu/page/what-is-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.
- peeters 9y ago
  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.
space_fountain 9y ago
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. https://en.wikipedia.org/wiki/Scientific_theory https://en.wikipedia.org/wiki/Scientific_theory
- zyngaro 9y ago
  Theory as is not confirmed by any observation yet! Why the downvote ?
  - acqq 9y ago
    The first indirect observational confirmation of the gravitational waves was published in 1982, that is, 35 years ago: http://adsabs.harvard.edu/abs/1982ApJ...253..908T http://adsabs.harvard.edu/abs/1982ApJ...253..908T 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: https://www.nobelprize.org/nobel_prizes/physics/laureates/2017/ https://www.nobelprize.org/nobel_prizes/physics/laureates/20... 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: https://www.forbes.com/sites/startswithabang/2017/03/07/how-richard-feynman-convinced-the-naysayers-that-gravitational-waves-were-real-60-years-ago https://www.forbes.com/sites/startswithabang/2017/03/07/how-... 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]." https://www.scientificamerican.com/article/just-a-theory-7-misused-science-words/ https://www.scientificamerican.com/article/just-a-theory-7-m... The gravitation is also "a scientific theory." 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): https://en.wikipedia.org/wiki/History_of_climate_change_science https://en.wikipedia.org/wiki/History_of_climate_change_scie...
semi-extrinsic 9y ago
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.
- komali2 9y ago
  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?
  - semi-extrinsic 9y ago
    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.
    - komali2 9y ago
      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.

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 ?

epberry 9y ago

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.

jasonwatkinspdx 9y ago

Gravitational wave detectors like LIGO (https://www.ligo.caltech.edu/page/what-is-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.

peeters 9y ago
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.

space_fountain 9y ago

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. https://en.wikipedia.org/wiki/Scientific_theory https://en.wikipedia.org/wiki/Scientific_theory

zyngaro 9y ago
Theory as is not confirmed by any observation yet! Why the downvote ?
- acqq 9y ago
  The first indirect observational confirmation of the gravitational waves was published in 1982, that is, 35 years ago: http://adsabs.harvard.edu/abs/1982ApJ...253..908T http://adsabs.harvard.edu/abs/1982ApJ...253..908T 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: https://www.nobelprize.org/nobel_prizes/physics/laureates/2017/ https://www.nobelprize.org/nobel_prizes/physics/laureates/20... 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: https://www.forbes.com/sites/startswithabang/2017/03/07/how-richard-feynman-convinced-the-naysayers-that-gravitational-waves-were-real-60-years-ago https://www.forbes.com/sites/startswithabang/2017/03/07/how-... 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]." https://www.scientificamerican.com/article/just-a-theory-7-misused-science-words/ https://www.scientificamerican.com/article/just-a-theory-7-m... The gravitation is also "a scientific theory." 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): https://en.wikipedia.org/wiki/History_of_climate_change_science https://en.wikipedia.org/wiki/History_of_climate_change_scie...

semi-extrinsic 9y ago

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.

komali2 9y ago
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?
- semi-extrinsic 9y ago
  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.
  - komali2 9y ago
    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.