3 ms·
It's not so much that you see a slightly different "down," but that space itself is changing such that the distance between e.g. your head and feet is (very) sl
by readams 3y ago
It's not so much that you see a slightly different "down," but that space itself is changing such that the distance between e.g. your head and feet is (very) slightly altered.
- jessriedel 3y agoThe two descriptions are equivalent. By the equivalence principle, the wave looks locally like neighboring bodies see different directions of down (and also slightly different strengths of the force of gravity in that direction).
- truculent 3y agoDumb, but earnest question: if space itself changes, how can the distance change? What is the distance a measure of, if not space itself? What's the yardstick, speed of light?
- ISL 3y agoThe yardstick is indeed the light-travel time. Gravitational waves really do change the time it takes for light to travel between two points. We use light travel-times to measure distances, thus we say that the distance between the points has changed. If it feels counterintuitive for spacetime to be changing, that's good. It is outside our human experience and perception. The strongest gravitational waves ever observed by scientists passed through everyone who was alive in 2015. None of those people noticed before the instruments registered a detection.
- truculent 3y agoThank you. If the yardstick is light-speed, is there any meaningful distinction between saying that space itself changed and that there are local perturbations to the speed of light? The thing I struggle with is that we normally think of things occupying space. If space itself gets distorted, then the size of those things should change, too. Or is that mental model a useful but ultimately incorrect way to think about the world?
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- jballanc 3y ago>None of those people noticed before the instruments registered a detection. IIRC, the reason no one noticed is that even the strongest gravitational waves are only going to "stretch" space by something less than the diameter of a hydrogen atom.
- MaxikCZ 3y agoCorrect me if I am wrong, the gravitational waves we measure have such small effect, that if we blew proton to the size of the Earth, it would get squished by distance smaller than a human hair. Edit: from wikipedia LIGO page: "(interferometers) are capable of detecting a change of less than one ten-thousandth the charge diameter of a proton" I dont know why I remember the human hair analogy, perhaps I am confusing it with something else?
- wlesieutre 3y agoYes, and light is actually how the LIGO and similar detectors can measure such small changes in distance. Light emitted from a laser is split into two beams traveling different paths, and then merged back together. When the light merges back together, if the two paths traveled took exactly the same distance (or an even multiple of the wavelength at least), then the beams add together constructively and you put back together the light from the laser. But if one path becomes longer or shorter the other, the light is out of phase with itself (peaks of the waves no longer line up with each other) and you can detect the interference between them. LIGO can detect a change in distance of less than one ten-thousandth the charge diameter of a proton. https://en.wikipedia.org/wiki/LIGO#/media/File:Gravitational_wave_observatory_principle.svg https://en.wikipedia.org/wiki/LIGO#/media/File:Gravitational...
- truculent 3y agoIncredible. Thank you