I get that heavy gravitational pulls would make time differ but what I don’t get is if someone would age differently to this further away?! If I was to get a near a black hole then return to Earth how and why would people be older than I am. Wouldn’t I be exactly the same but the time around me whilst around the black hole would have slowed but not my aging?
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1$\begingroup$ While you're near the BH time will seem normal to you, ticking away at one second per second. But time runs slower near the BH compared to clocks back on Earth. $\endgroup$– PM 2RingCommented Jul 12, 2019 at 15:31
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3 Answers
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If the "time around" things (and people) was slowing down while the things themselves still age in the same way, then what would the idea that "time slows down" even mean?
Actually the only way it makes sense to say that time slows down is precisely when two objects depart then meet again and it appears that both objects did not experience the same lapse of time during their separation. Then one could say that time slowed down for one of the objects, but it is not a very appropriate manner to express what happens, because it misses the relative aspect. For any specific person or object on its own, time never slows down at all.
Maybe the most important point is that time is not "around": it is not a container or an element with a spatial extent. It is an abstraction allowing to compare the evolution (aging) of systems. Because in our experience when we compare evolutions such as the ways clocks tick the positions and speeds of the clocks do not matter, we have built a mental image of time as something that flows uniformly in the background, so to say. But physics tells us otherwise, and in fact speeds (and positions in a gravitational context) do matter if we want to make proper and accurate comparisons.
answered Jul 12, 2019 at 15:40
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There are two main cases:
SR time dilation, time elapses differently for two observers, because of their relative speed
GR time dilation, time elapses differently for two observers, because they are in a different gravitational zone
https://en.wikipedia.org/wiki/Time_dilation
Now in your case, you are talking about GR time dilation. If you are near a black hole, the black hole has a lot of stress-energy (not mass contrary to popular belief), and the difference between the stress-energy of the black hole and Earth (Earth has much less stress-energy), causes clocks at the black hole to tick slower, relative to clocks on Earth.
Now since time passes slower near the BH, relative to Earth, if you spend time near the BH, you age slower, relative to people on Earth.
answered Jul 12, 2019 at 17:21
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$\begingroup$ While otherwise your answer is correct +1, this particular statement is not: “the black hole has a lot of stress-energy (not mass contrary to popular belief)”. It actually is the opposite. When a star collapses, it has a lot of stress-energy that curves spacetime and causes the collapse. However, once the collapse is complete and a black hole is formed, all stress-energy disappears, but the mass remains due to energy conservation. So a black hole has mass, but a zero stress-energy. This fact is not commonly well understood and may require some pondering. $\endgroup$ Commented Nov 15 at 3:19
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$\begingroup$ A simple intuitive parallel is trying to bend a flexible plastic rod into a circle like a hula-hoop. To bend it more and more you need to gradually increase the force you are applying. However, once you connect the opposite ends together, the curved rod keeps itself curved without you needing to apply any force at all. Similarly, once the horizon of a black hole is formed, it cannot be “unformed”. The curved spacetime keeps itself curved and all stress-energy vanishes. $\endgroup$ Commented Nov 15 at 3:29
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$\begingroup$ The reason stress-energy vanishes is the behavior of the star matter falling to the horizon. The speed of the fall approaches the speed of light, so the matter gains a lot of kinetic energy at the expense of losing mass (in remote coordinates). The total energy remains the same while each particle loses mass and gains kinetic energy. The final peace of this puzzle is that stress-energy of such particles is zero. Very roughly speaking, stress energy is energy minus momentum and momentum of relativistic particles equals their energy, so stress-energy is zero. It is lost in the fall due to speed. $\endgroup$ Commented Nov 15 at 3:51
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$\begingroup$ The total mass of the black hole equals its total energy per $E=mc^2$ where $E$ is the total sum of kinetic energy of all falling particles. This energy comes from the mass these particles had before the fall. So the mass of the black hole equals the mass of the original star matter before it collapsed. As you can see, the total mass of the black hole is the same as of the collapsing star (less any losses of matter or radiation of energy), but the stress-energy of the black hole disappears once the horizon is formed and the falling matter speeds up toward the speed of light at the horizon. $\endgroup$ Commented Nov 15 at 4:05
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$\begingroup$ Finally note that the stress-energy in exact analytical solutions (like Kerr or Schwarzschild) is zero. So a real star collapsing to a real astrophysical black hole becomes indistinguishable from the idealized Kerr solution, because both fulfill the “vacuum” condition, which is not the absence of matter, as many wrongly think, but zero stress-energy. I’ll delete my comments once you’ve read them to keep the comments area clean. $\endgroup$ Commented Nov 15 at 4:23
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Time passes slower around massive objects due to relativity. You are a physical coalescence of matter and energy into "being". Your thoughts and awareness are simply electrochemical reactions unfolding throughout your nervous system over time. Time dilation around a black hole will effect all matter (whether that matter is a part of an egocentric observer or a humble rock). The electrical impulses in your brain will slow RELATIVE to those of someone on Earth, along with all other bodily processes including aging (note, due to your slowed down physiology you would still be conscious but your thoughts and feelings occur slower RELATIVE to people on earth, even though you experience them in the same way). When time slows, your body (and the environment around it) slow down! If I get two T.V.'s and play the same movie on both, one at half speed, the other at full speed, if I pause the movies at any point it will appear that the first has not been on as long as the second (even though they have both been playing for the same period of "time").Time dilation is an analogous situation; your life is no more privileged than that movie. You're just a character model in a 3D movie; when time slows down around you, your movie slows down, and so do you.
