OverJohn
OverJohn t1_j1jegp4 wrote
Reply to comment by komor555 in Are people in the international space station experiencing time faster than us? by [deleted]
If the ISS was at a geostationary orbit time would appear to travel faster on the ISS from Earth, but once you take its motion into account you find that time appears to travel slower on the ISS from Earth.
OverJohn t1_j1jc87k wrote
Reply to comment by dave200204 in Are people in the international space station experiencing time faster than us? by [deleted]
I don't think it could be that much because you would have to spend over 10 years on ISS in order for you to "lose" 0.18 seconds.
OverJohn t1_iw9fnyd wrote
Reply to is the change from high mass object to blackhole sudden or does it happen through a process? by RevolutionaryAd4161
A black hole is defined by its event horizon which starts from what is called a crease set, which is a set of events*, which are isolated in the sense that no messages can be sent between any of the events in the set.
A heuristic way of thinking about it is that a bunch of "mini-black holes" will form instantaneously at a set of points that are isolated from each other, each "mini black hole" expanding outwards from that point before merging together. In relativity whether two isolated events happen at the same time is subjective, so it is a moot question as to whether they form at the same time. For a collapsing star, effectively the merging happens very quickly and in the very early lifetime of the black hole.
Though this describes the formation of the event horizon, it is not correct to think of the singularity also forming at the crease set. All we can say about the singularity really is it is contained within the event horizon.
*An event is a point in spacetime, i.e. something with an exact location in both space and time.
https://www.worldscientific.com/doi/10.1142/S0218271811019220
OverJohn t1_j2swp18 wrote
Reply to If an object is placed at the end of of a VERY long bar that is connected to an axel, Could rotating the bar close to the axel make the object at the other move extremely fast (like light speed)? by Swade060504
Rigidity and relativity is more complicated than you might think.
Firstly, relativity constrains the speed of sound in an object. This means for example you cannot have a rigid pole where pushing one end causes the other end to move instantaneously.
Secondly, relativity constrains rigidity even further. We could conspire to apply forces across an object at the same time in its instantaneous rest frame such that the object never experiences deformation in its rest frame. When we do this the object is called "Born rigid" (after Max Born). However it turns out it is impossible to for an object to be Born rigid under all types of motion. The motion in your son's hypothetical experiment would be an example of a type of motion where it is impossible to even maintain Born rigidity.