# Physics question **URL:** \ **Category:** Factual Questions\ **Created:** [October 21, 2009, 6:21pm UTC](https://boards.straightdope.com/t/physics-question/514645 "2009-10-21T18:21:43Z")\ **Posts on this page:** 6\ **Page:** 3
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [October 23, 2009, 10:10pm UTC](https://boards.straightdope.com/t/physics-question/514645/41 "2009-10-23T22:10:47Z")
> [@Chronos](#): > > That’s nothing. A black hole merger could be expected to release several times the mass of the Sun in radiant energy, and yet be nearly completely undetectable to anyone even a few hundred kilometers away, since all of the energy is in gravitational waves. Well I don’t know how good a physicist **Chronos** is, but he is certainly outstanding at one-up-manship. So what would happen if you were a lot closer to this merger? Would you alternate between being very tall and very squashed? Does it weaken so quickly because it’s quadrupole radiation? What do you call the disturbance in spacetime that would be generated by an up and down movement of a large mass? I realize it wouldn’t be dipole radiation, but it seems it would create some sort of traveling disturbance. What would that be called? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 24, 2009, 7:44pm UTC](https://boards.straightdope.com/t/physics-question/514645/42 "2009-10-24T19:44:54Z")
From an up-and-down movement of a single large mass, it’s like the old joke about the dog with no legs: You can call it whatever you want, but it won’t come. A single mass moving up and down would violate conservation of momentum. If, instead, you connected two masses together and set them oscillating (so one goes down when the other goes up and vice-versa to conserve momentum) then you’ll have quadrupole (and higher order) gravitational waves. > [@](#): > > So what would happen if you were a lot closer to this merger? Would you alternate between being very tall and very squashed? Does it weaken so quickly because it’s quadrupole radiation? More or less, on what would happen to you, though the precise calculations are extremely ugly and it’s only recently that clever-enough algorithms have been devised to perform them even on the most powerful computers. And you’re also correct that the rapid fall-off is due to the waves being quadrupolar, since it’s not possible to get a monopole or dipole wave in gravity (the monopole is prohibited by conservation of mass, and the dipole is prohibited by the lack of negative mass). ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [October 24, 2009, 9:30pm UTC](https://boards.straightdope.com/t/physics-question/514645/43 "2009-10-24T21:30:16Z")
Even though it’s quadrupole, the energy of the gravitational radiation falls off as 1/r^2. (If it fell off faster, it wouldn’t be radiation.) ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [October 25, 2009, 5:35pm UTC](https://boards.straightdope.com/t/physics-question/514645/44 "2009-10-25T17:35:37Z")
> [@](#): > > A single mass moving up and down would violate conservation of momentum. Actually, I knew that. A few years ago in another forum an astronomer explained it to me. But, since physics is the realm of the frictionless surface and weightless string, I propose the following: Two almost infinitely strong people (you know what I mean) synchronize their clocks, and leave a planet in opposite directions, carrying a weightless unbreakable string attached to the planet. They accelerate and decelerate exactly the same, travel the same distance and arrive at identical extremely massive planets. Taking into considerations all relativistic effects they time their pulls to move the original planet up and down. As I said earlier I know this can’t be dipole radiation but it seems you’d get a local, oscillating, traveling. gravitational wave of some kind or another? What’s my error-the weightless string? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 25, 2009, 10:43pm UTC](https://boards.straightdope.com/t/physics-question/514645/45 "2009-10-25T22:43:10Z")
At a guess, the problem isn’t actually that the string is massless, but that you’re neglecting the tension in the string when calculating the gravitational effects. Everyone knows that mass is a source for gravity, but it’s less well-known that stress is, as well. The contribution from stress is typically negligible, but here you’re talking about sufficient tension to wiggle a planet around, which is a lot more stress than in familiar real-world situations. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [October 26, 2009, 3:12am UTC](https://boards.straightdope.com/t/physics-question/514645/46 "2009-10-26T03:12:22Z")
> [@Chronos](#): > > At a guess, the problem isn’t actually that the string is massless, but that you’re neglecting the tension in the string when calculating the gravitational effects. Everyone knows that mass is a source for gravity, but it’s less well-known that stress is, as well. The contribution from stress is typically negligible, but here you’re talking about sufficient tension to wiggle a planet around, which is a lot more stress than in familiar real-world situations. Amazing. I knew stress was a source of gravity, but I never would have thought of that in a million years. Thanks [Previous page](https://boards.straightdope.com/t/physics-question/514645.md?page=2)