Wikipedia:Reference desk/Archives/Science/2018 November 12

= November 12 =

Physics : Time
I wanted to know how could time possibly slow down with speed if it does will it affect biological growth of a human.Navjot1200 (talk) 11:39, 12 November 2018 (UTC) - PAGE ]]) 16:21, 13 November 2018 (UTC)
 * Time dilation depends on the relative speed of the observer. Your own time does not slow down from your own perspective, and everything happens exactly the same regardless of your speed (if gravity/acceleration is the same). It doesn't even make sense to ask how fast you are moving unless it is measured relative to something else. According to relativity there does not exist "fixed points" in the universe or an "absolute speed" compared to such points. You age normally but if you move very fast relative to observers then it looks to them like you are aging slower. However, if you are moving fast enough for time dilation to be significant for an observer on Earth then you are probably on a spacecraft where the gravity/acceleration may not be the same as on Earth, and that does affect biological processes. This may be avoided by a rotating spacecraft where your acceleration due to the rotation corresponds to the gravity of Earth. Gravity and acceleration are indistinguishable in the theory of relativity. PrimeHunter (talk) 12:45, 12 November 2018 (UTC)
 * Just to nitpick, while gravity and linear acceleration are indistinguishable, using a rotating spacecraft to create pseudogravity is distinguishable from Earth's gravity or from a spacecraft accelerating in a straight line. On a rotating spacecraft there would be an acceleration gradient between your head and your feet; since your feet are rotating faster, they would experience more pseudogravity. If you ran in such a spacecraft, running with the rotation direction would make you feel heavier, while running against the rotation would make you feel lighter. You would also experience Coriolis forces, so if you jumped in the air or suddenly stood up, you would feel a force pushing you backwards compared to the direction of rotation. --Ahecht ([[User talk:Ahecht|TALK
 * How is that scenario distinguishable from a spacecraft in orbit rather than in straight-line free-fall?-- Jayron 32 16:53, 13 November 2018 (UTC)
 * Both are free fall and thus are hard to tell apart, but you can still tell that you are in orbit because of the tidal forces tend to make things closer to the planet than the center of mass slowly fall and things farther from the planet than the center of mass slowly rise. Or sometimes not so slowly. See Space elevator. --Guy Macon (talk) 08:53, 14 November 2018 (UTC)
 * In free fall towards the centre of the Earth will have a quick end to time for you when you impact the surface. If there was no surface and a black hole is there instead, there will be even more serious effects. Graeme Bartlett (talk) 06:09, 15 November 2018 (UTC)
 * Depends whether you'd rather die by a splat or a slurp. ←Baseball Bugs What's up, Doc? carrots→ 16:45, 15 November 2018 (UTC)
 * "It isn't the falling that hurts you. It's the landing". Is there any particular reason for assuming that the free fall is towards the the earth? --Guy Macon (talk) 14:55, 15 November 2018 (UTC)
 * Or the Moon, or Mars, or wherever. ←Baseball Bugs What's up, Doc? carrots→ 16:44, 15 November 2018 (UTC)