3 ms·

"persistent changes in the relative position of pairs of points in space due to the passing of a gravitational wave." So the idea isn't that space itself is st

by p1mrx 5y ago

"persistent changes in the relative position of pairs of points in space due to the passing of a gravitational wave." So the idea isn't that space itself is storing information, just that a gravitational wave doesn't put everything back where it found it. Presumably if you cleared out all the stuff, there would be no memory left behind.

pdonis 5y ago
That's my understanding, yes. So the "permanently distort spacetime" part in the article's title is misleading.
- zuminator 5y ago
  What you are both saying seems incompatible not just merely the title but also with the text of the article, where is states, "Earth’s gravitational pull also tends to restore LIGO’s mirrors to their original position, erasing its memory. So even though the kinks in space-time are permanent, the changes in the mirror position — which enables us to measure the kinks — are not." In other words, the article is claiming that there are two things being affected, spacetime itself, and the objects embedded in spacetime, but only the objects are temporarily displaced, spacetime itself is permanently displaced. At least, that's my reading. But I wonder, notwithstanding the article's assertion, are objects really completely restored back to their former position in spacetime? Spatial coordinates, sure, but timewise? I don't claim to know the answer, just posing this as a question, but imagine a really powerful gravitational wave going through a large object. Wouldn't it create some kind of timewise interference pattern in the object where at the peaks of oscillation, the constituent subatomic particles would feel a stronger gravitational force and age more slowly than the particles at the troughs of oscillation? So after the wave had passed, you'd be left with an object that has areas of divergent age? I mean even with standard GR we experience aging on a gradient, in that our feet are somewhat younger than our heads thanks to the gravitational pull of the Earth. ( https://www.theatlantic.com/technology/archive/2010/09/study-your-head-is-older-than-your-feet/63539/ https://www.theatlantic.com/technology/archive/2010/09/study... )
  - pdonis 5y ago
    > What you are both saying seems incompatible not just merely the title but also with the text of the article I would not trust the text of the article. Pop science articles are often untrustworthy even on much simpler topics. Actual papers are much better sources. > are objects really completely restored back to their former position in spacetime? Of course not; that's impossible. Any talk about restoring objects to their original position can only mean position in space. (And since "position in space" depends on a particular choice of reference frame, one then needs to ask what reference frame.) > the constituent subatomic particles would feel a stronger gravitational force and age more slowly than the particles at the troughs of oscillation? The "rate of aging" does not depend on "gravitational force". In situations where such concepts as "gravitational force" and "gravitational potential" are even applicable (which will not include very strong gravitational waves in any case), the "rate of aging" depends on gravitational potential. (For example, in the vicinity of the Earth, one's altitude above the Earth affects "rate of aging".) However, one can still assess differential aging in more general situations, just not with the rule of thumb you gave. > after the wave had passed, you'd be left with an object that has areas of divergent age? This is possible, but, as noted above, it cannot be assessed with the rule of thumb you gave. You would have to know the details of the spacetime geometry of the gravitational wave (and you would have to be prepared for some pretty heavy duty numerical computations).

by p1mrx 5y ago

pdonis 5y ago

That's my understanding, yes. So the "permanently distort spacetime" part in the article's title is misleading.

zuminator 5y ago
What you are both saying seems incompatible not just merely the title but also with the text of the article, where is states, "Earth’s gravitational pull also tends to restore LIGO’s mirrors to their original position, erasing its memory. So even though the kinks in space-time are permanent, the changes in the mirror position — which enables us to measure the kinks — are not." In other words, the article is claiming that there are two things being affected, spacetime itself, and the objects embedded in spacetime, but only the objects are temporarily displaced, spacetime itself is permanently displaced. At least, that's my reading. But I wonder, notwithstanding the article's assertion, are objects really completely restored back to their former position in spacetime? Spatial coordinates, sure, but timewise? I don't claim to know the answer, just posing this as a question, but imagine a really powerful gravitational wave going through a large object. Wouldn't it create some kind of timewise interference pattern in the object where at the peaks of oscillation, the constituent subatomic particles would feel a stronger gravitational force and age more slowly than the particles at the troughs of oscillation? So after the wave had passed, you'd be left with an object that has areas of divergent age? I mean even with standard GR we experience aging on a gradient, in that our feet are somewhat younger than our heads thanks to the gravitational pull of the Earth. ( https://www.theatlantic.com/technology/archive/2010/09/study-your-head-is-older-than-your-feet/63539/ https://www.theatlantic.com/technology/archive/2010/09/study... )
- pdonis 5y ago
  > What you are both saying seems incompatible not just merely the title but also with the text of the article I would not trust the text of the article. Pop science articles are often untrustworthy even on much simpler topics. Actual papers are much better sources. > are objects really completely restored back to their former position in spacetime? Of course not; that's impossible. Any talk about restoring objects to their original position can only mean position in space. (And since "position in space" depends on a particular choice of reference frame, one then needs to ask what reference frame.) > the constituent subatomic particles would feel a stronger gravitational force and age more slowly than the particles at the troughs of oscillation? The "rate of aging" does not depend on "gravitational force". In situations where such concepts as "gravitational force" and "gravitational potential" are even applicable (which will not include very strong gravitational waves in any case), the "rate of aging" depends on gravitational potential. (For example, in the vicinity of the Earth, one's altitude above the Earth affects "rate of aging".) However, one can still assess differential aging in more general situations, just not with the rule of thumb you gave. > after the wave had passed, you'd be left with an object that has areas of divergent age? This is possible, but, as noted above, it cannot be assessed with the rule of thumb you gave. You would have to know the details of the spacetime geometry of the gravitational wave (and you would have to be prepared for some pretty heavy duty numerical computations).