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> Comments: Published version, apart from minor editing (e.g. corrections of 'center' to 'centre') > Journal reference: Eur.J.Phys. vol. 37, 035602 (2016) If Brexit occurs, Britain will no longer be a European nation, and therefore the European Journal of Physics will no longer be obliged to accept "centre" as the standard spelling of "center", right?
> Comments: Published version, apart from minor editing (e.g. corrections of 'center' to 'centre') > Journal reference: Eur.J.Phys. vol. 37, 035602 (2016) If Brexit occurs, Britain will no longer be a European nation, and therefore the European Journal of Physics will no longer be obliged to accept "centre" as the standard spelling of "center", right?
What about the center of a black hole? Is it younger than the surface?
That is a bit more difficult to reckon. A distant observer watching their unfortunate friend falling into a black hole will see their friends clock tick slower and slower such that they will never, from the outside, be seen to cross the event horizon, the passage of time will slow towards zero. From the outside, time does not appear to pass at all inside a black hole. The unfortunate traveller, however, won't even no…
For starters, there are probably lots of black holes in our universe (and even in our galaxy). What happens if the unfortunate friend falling into the black hole is being watched from an orbit close to another black hole?
(General Relativity has the somewhat frustrating property that adding a black hole to a black hole spacetime does not obey the principle of linear superposition, so things can get quite messy if the two black holes in the paragraph above are close together. One can get even kinkier, for instance by considering a stellar-mass black hole orbiting a supermassive black hole, and kinkier still if both of them rotate with unaligned rotational axes and/or with opposite spins.)
Next, the metric expansion of space means that an isolated black hole would be more like a Schwarzschild-de Sitter solution. There we have the problem that two widely-separated observers can be highly cosmologically redshifted with respect to each other even if neither is anywhere near a black hole. One can of course also consider a collapsing universe in which two observers are blueshifted with respect to one another, and one could consider what that blueshift would do to observations by A of B if B were falling into a black hole in the collapsing universe.
Moreover, the presence of matter can make a mess of things. One can contrive an arrangement of matter near to a black hole which is sufficiently dense as to offset the gravitational time dilation of someone near the horizon.
Finally, there's kinematics: we can subject the unfortunate friend and the observer to a relative ultraboost wherein the doppler blueshift undoes the gravitational redshift.
A relatively ultraboosted Schwarzschild black hole would look a little odd to the relatively ultraboosted observer: https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultrab... Things get weirder when you add other black holes, stars, dust, and so on to the Aichelburg-Sexl picture.
> From the outside, time does not appear to pass at all inside a black hole
From outside the event horizon of a static black hole there is no way to tell, even if one uses tricks like the above to do away with the objection that in general time appears to come to an effective standstill infinitesimally outside the horizon.
However, if the infaller brings in significant mass or angular momentum, we would expect the horizon to reflect the internal configuration change, since it's the internals that generate the horizon in the first place. This is a dynamical rather than a static (up to tiny perturbation) black hole.
This is extremely fiendishly difficult, and is run into in practice in terms of trying to work out exactly what the full theory says about the final merger of two black holes. Roughly speaking, the gravitational interactions between two merging black holes is best described by a metric wherein there is an effective third (and sometimes fourth) body.
> Comments: Published version, apart from minor editing (e.g. corrections of 'center' to 'centre') > Journal reference: Eur.J.Phys. vol. 37, 035602 (2016) If Brexit occurs, Britain will no longer be a European nation, and therefore the European Journal of Physics will no longer be obliged to accept "centre" as the standard spelling of "center", right?
We will most certainly still be a European nation. Just not a member of the European Union. We are still located on the European continent. Its spelled centre.
In that sense, perhaps London remains a European city while still the capital of a not really European nation.
Earlier quoted context omitted.
General Relativity
Parent comment is making a joke about “year” as a unit measuring how many times you go around the sun. Less time has passed for the core per relativity, but is it not the same number of years? The more official definition of years defines it in terms of days, which break down further to SI’s rigorously defined second as the base unit of time. But it’s still weird to think about.
Does the same apply to the Sun? How old is the centre of the Sun compared to its surface?
There's a little bit of smoke-and-mirrors going on here. Yes, the same effect happens on/in the sun (in fact it's even more extreme because the sun's gravitational field is much stronger than earth's). But no, it's not quite correct to say that the center of the sun -- or of the earth for that matter -- is "older" than the surface because the material in both bodies moves around. If you put a clock at the center and…
> Comments: Published version, apart from minor editing (e.g. corrections of 'center' to 'centre') > Journal reference: Eur.J.Phys. vol. 37, 035602 (2016) If Brexit occurs, Britain will no longer be a European nation, and therefore the European Journal of Physics will no longer be obliged to accept "centre" as the standard spelling of "center", right?
Equating being "a European nation" with EU membership is surely a trolling.
How is it possible that the core is several years younger if the core and the surface have gone around the sun the same number of times?
Earlier quoted context omitted.
The question is slightly ill-formed. I understand why you'd ask it, but there are a couple of possible answers, none of which work directly. - No, because a black hole has nothing underneath the event horizon. (This is true from the perspective of anyone outside it, at least to a first approximation. Gravity is affected by curvature, and whatever might be in the centre cannot be the cause of the event horizon , excep…
> the extreme warping of space-time turns "inwards" into "future", making the centre of the hole a point in time, not space. Assuming an exact black hole solution to the Einstein Field Equations (EFEs), what's happening is that any observer sufficiently close to the singularity will inevitably collide with the singularity. One can then define the horizon the outer boundary at which the future light cone attached at e…
What about the center of a black hole? Is it younger than the surface?