Julian Date: 24591138.16
2020-2021: XXXI
THE DAILY ASTRONOMER
Thursday, October 15, 2020
Remote Planetarium 109: General Relativity V: Questions
If matter and energy are the same, can energy affect space-time in the same way that matter does? -L.M.
Matter is a form of energy, However, unlike matter, energy lacks any inertia, or resistance to changes in its motion. As matter possesses inertia, it will warp its local space-time regions in ways that energy cannot.
A light beam's path is deflected by massive objects. However, a light beam will not distort space-time, itself.
Let's pretend that the Sun vanished from the Universe in an instant. Would Earth fall out of its orbit immediately or would it take some time? -J.S.
This question pertains to gravity waves, a topic we'll be discussing in greater detail Monday. The answer is that gravity waves travel at light speed. So, for instance, if the Sun vanished from the cosmos instantaneously, Earth would remain under its gravitational influence for the next 8.4 minutes, the amount of time light requires to travel between the Sun and Earth. Earth would then start to move along a path tangential to its previous orbit.
Is it true that in General Relativity, a massive object's gravitational influence doesn't extend infinitely? -C.W.
Yes. According to GR, a massive object's gravitational influence will not extend beyond the level of galaxy groups. On scales smaller than these groups, spacetime exhibits similar behaviors to Newtonian gravity. (From a Newtonian perspective, gravity is of infinite extent.) However, at the level of galaxy groups, the galaxies are moving apart as a consequence of cosmic expansion. Moreover, this expansion rate is increasing due to the influence of dark energy. At this level, spacetime wouldn't behave like Newtonian gravity.
A massive object's gravitational influence will not extend beyond the level of galaxy groups according to General Relativity.
Will I really age faster if I live at the top of a building than in its basement?
-J.W.
Yes, but the effect will be slight. A person at the top of a building will be farther from Earth than a person in its basement. Consequently, time will be slower for the observer lower down in the building than higher up. However, you won't experience that much more time than the subject farther down: nanoseconds per year, only....hardly noticeable.
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