4 ms·

After 100 years, scientists are finally closing in on Einstein’s ripples

8 points by pavornyoh 11y ago

delinka 11y ago
I wonder of someone can help me with a misunderstanding. If "spacetime" is the medium which conducts the matter and energy we (and our stars and planets) are made of, how can we ever know that it's expanding or contracting? If we look at it like Flatland, how could a Flatlander know that the sheet he lives on is actually rippled? He simply traverses his "space" unaware that he's bobbing about in a third dimension. Also in this case, you'd never be aware of being spaghettified[1] by the changing of space. So the laser light has to travel via space and if space contracts, it still has to cover the same amount of space (although, from an external extra-dimensional perspective, it might 'slow down' in a contracted space), and would thus not produce these interference patterns. If, however, our energy and matter are not 'conducted' by space, then I suppose I can begin to comprehend why such an experiment might work. 1 - https://en.wikipedia.org/wiki/Spaghettification https://en.wikipedia.org/wiki/Spaghettification
- T-A 11y ago
  > If we look at it like Flatland, how could a Flatlander know that the sheet he lives on is actually rippled? He builds a device which emits flat-balls (all right, circles) traveling at a known constant speed. Then he measures the time it takes a flat-ball to reach a target and bounce back to him (they are made of flat-rubber). And he discovers that there are fluctuations in the travel time, well above what he'd expect from known error sources. Being a flat-Einstein, he then deduces that Flatland is rippled in a third dimension, and the flat-balls are taking longer to traverse a rippled surface than a flat surface, because /\/\/\/\/\/\is longer than ------------ when you have to move along the surface of the ripples rather than go through them.
  - delinka 11y ago
    I feel you've done little but directly translate the LIGO experiment into Flatlandian physics. That doesn't answer the question. Use a sheet of paper as your Flatland. Draw two points, measure the distance. Crinkle up the paper. There's still the same 'distance' of paper between the two points. Should the points be able to travel, they have to traverse the same number of paper molecules whether the paper is crinkled or not. No one living in this Flatland can measure the crinkles in this space because they live inside this space. They would need to step outside their own space with some ability to measure variations in a third dimension. If the physics of a universe necessitates movement from one point to the next (let's suppose these points are Planck length apart, call them 'Planck-points'), if particles must be translated from Planck-point to Planck-point and can't skip anything in between, how are we to ever measure that Planck length expanded or contracted? Surely if it "takes longer" to traverse an expanded Planck length, then it hasn't expanded, but there now exists more space - more Planck-points. If gravity wells stretch and contract _matter_ (rather than space), then this fits my mental model. If they actually stretch _space_, within which matter exist and along which matter traverses, I struggle to see how we could ever detect space deformations without stepping outside our own space.
    - T-A 11y ago
      > Use a sheet of paper as your Flatland. Draw two points, measure the distance. Crinkle up the paper. There's still the same 'distance' of paper between the two points. Because the paper is not elastic. Spacetime is; it stretches. So a better analogy would be the rubber sheet with masses rolling around on it, and deforming it, which is often invoked in popular introductions to spacetime curvature. Like all analogies, it has its limitations, but it does capture the essence of the classic Wheeler quote: "Mass tells space-time how to curve, and space-time tells mass how to move." > If gravity wells stretch and contract _matter_ (rather than space), then this fits my mental model. Gravity stretches and contracts space AND matter, the latter to the extent that it can overcome the chemical bonds (and other forces) trying to keep matter together (gravity is weak, so that takes a lot of mass). We see the stretching of matter all the time as tides. Moons torn apart by it when they get too close to their planets (the Roche limit) is a more dramatic example. Spagettification of matter falling into a black hole an even more dramatic one. You can think of gravitational waves as an oscillating, self-sustaining tide traveling at the speed of light. By the way, there is a thread of thought which takes the analogy of spacetime with an elastic solid quite seriously: http://arxiv.org/abs/gr-qc/0408051 http://arxiv.org/abs/gr-qc/0408051

8 points by pavornyoh 11y ago

I wonder of someone can help me with a misunderstanding. If "spacetime" is the medium which conducts the matter and energy we (and our stars and planets) are made of, how can we ever know that it's expanding or contracting? If we look at it like Flatland, how could a Flatlander know that the sheet he lives on is actually rippled? He simply traverses his "space" unaware that he's bobbing about in a third dimension. Also in this case, you'd never be aware of being spaghettified[1] by the changing of space. So the laser light has to travel via space and if space contracts, it still has to cover the same amount of space (although, from an external extra-dimensional perspective, it might 'slow down' in a contracted space), and would thus not produce these interference patterns. If, however, our energy and matter are not 'conducted' by space, then I suppose I can begin to comprehend why such an experiment might work. 1 - https://en.wikipedia.org/wiki/Spaghettification https://en.wikipedia.org/wiki/Spaghettification

T-A 11y ago
> If we look at it like Flatland, how could a Flatlander know that the sheet he lives on is actually rippled? He builds a device which emits flat-balls (all right, circles) traveling at a known constant speed. Then he measures the time it takes a flat-ball to reach a target and bounce back to him (they are made of flat-rubber). And he discovers that there are fluctuations in the travel time, well above what he'd expect from known error sources. Being a flat-Einstein, he then deduces that Flatland is rippled in a third dimension, and the flat-balls are taking longer to traverse a rippled surface than a flat surface, because /\/\/\/\/\/\is longer than ------------ when you have to move along the surface of the ripples rather than go through them.
- delinka 11y ago
  I feel you've done little but directly translate the LIGO experiment into Flatlandian physics. That doesn't answer the question. Use a sheet of paper as your Flatland. Draw two points, measure the distance. Crinkle up the paper. There's still the same 'distance' of paper between the two points. Should the points be able to travel, they have to traverse the same number of paper molecules whether the paper is crinkled or not. No one living in this Flatland can measure the crinkles in this space because they live inside this space. They would need to step outside their own space with some ability to measure variations in a third dimension. If the physics of a universe necessitates movement from one point to the next (let's suppose these points are Planck length apart, call them 'Planck-points'), if particles must be translated from Planck-point to Planck-point and can't skip anything in between, how are we to ever measure that Planck length expanded or contracted? Surely if it "takes longer" to traverse an expanded Planck length, then it hasn't expanded, but there now exists more space - more Planck-points. If gravity wells stretch and contract _matter_ (rather than space), then this fits my mental model. If they actually stretch _space_, within which matter exist and along which matter traverses, I struggle to see how we could ever detect space deformations without stepping outside our own space.
  - T-A 11y ago
    > Use a sheet of paper as your Flatland. Draw two points, measure the distance. Crinkle up the paper. There's still the same 'distance' of paper between the two points. Because the paper is not elastic. Spacetime is; it stretches. So a better analogy would be the rubber sheet with masses rolling around on it, and deforming it, which is often invoked in popular introductions to spacetime curvature. Like all analogies, it has its limitations, but it does capture the essence of the classic Wheeler quote: "Mass tells space-time how to curve, and space-time tells mass how to move." > If gravity wells stretch and contract _matter_ (rather than space), then this fits my mental model. Gravity stretches and contracts space AND matter, the latter to the extent that it can overcome the chemical bonds (and other forces) trying to keep matter together (gravity is weak, so that takes a lot of mass). We see the stretching of matter all the time as tides. Moons torn apart by it when they get too close to their planets (the Roche limit) is a more dramatic example. Spagettification of matter falling into a black hole an even more dramatic one. You can think of gravitational waves as an oscillating, self-sustaining tide traveling at the speed of light. By the way, there is a thread of thought which takes the analogy of spacetime with an elastic solid quite seriously: http://arxiv.org/abs/gr-qc/0408051 http://arxiv.org/abs/gr-qc/0408051