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3:48 AM

Q: How can I accomplish the removal of comments by another participant, on an answer I had posted, that might be considered derogatory?

About 24 hours ago, I posted an edit to my answer to a PSE question concerning the time when interstellar travel (in any form, or, at least, without specification of its form) might have begun: As stated in that edit, it replaced nearly all of the original answer, which had discussed only the ti...

bug

2 hours later…

Yashas

5:22 AM

Need advise on coming up with a name for academic publications. "First Second T. F." is causing citations to appear as "F. et al.". What should the order be "T. F. First Second" or "First T. F. Second"? "Second" the surname.

3 hours later…

glS

8:46 AM

@Yashas which citation style are you using? And with "causing citations to appear" you mean you are using latex with bibsomething I suppose?

fqq

9:31 AM

@Yashas do you mean how to write your name as author of papers? I'm sure it's been discussed a lot on academia stackexchange. Assuming western, English-speaking journals, the standard convention is FirstName1 (FirstName2) LastName.

the middle name(s) can be omitted, or only the middle initials used (not everyone has middle names in western countries anyway)

there are often mistakes for people that have multiple first or last names, or names including whitespace etc. Then it gets more complicated for everything else kalzumeus.com/2010/06/17/…

if you want your papers to be known as "second et al" then "Second" should be at the end of the name

Ishwaran

9:49 AM

Hello there. I am having a doubt. Will a body expirience time dilation by its own curvature ?

Slereah

10:00 AM

A single point will not experience it, since spacetime is always flat in its own frame of reference

But a real body isn't a single point, of course

of course the relativistic effect of a single object are too small to be measurable

Ishwaran

Anyways, it has a non-zero value ryt

Slereah

Sure

your bones will have gravitational effects on your organs and all that

moving your arms will induce tiny gravitational waves

etc etc

Ishwaran

PM 2Ring

@Ishwaran GR says that you can always cut spacetime into small chunks that are locally flat. That's just like how we can make small flat maps of the curved surface of the Earth. In a small flat region of spacetime, your clock always ticks at 1 second per second. But that clock might not be ticking at the same rate as a distant clock.

So time dilation always has to be specified as a ratio between 2 clocks. It's meaningless to ask "Is this clock experiencing time dilation?" unless you compare it to another one.

Ishwaran

10:17 AM

PM 2Ring

Gravitational time dilation is rather tiny, unless the body is very massive, and it depends on how close you are to the centre of gravity. So if you have 2 planets of identical mass, but one planet is denser & therefore smaller, the time runs a bit slower on the surface of the denser one.

Ishwaran

I guess that's what happening in the case of black hole

PM 2Ring

On Earth, the dilation on the surface compared to what it'd be without the Earth is around 69.6 nanoseconds per second, which works out to ~2.2 seconds slow per century. Wikipedia has a bunch of articles about how modern time scales handle dilation, but they are a bit technical. Eg,

Geocentric Coordinate Time

Geocentric Coordinate Time (TCG - Temps-coordonnée géocentrique) is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to precession, nutation, the Moon, and artificial satellites of the Earth. It is equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the center of the Earth: that is, a clock that performs exactly the same movements as the Earth but is outside the Earth's gravity well. It is therefore not influenced by the gravitational time dilation caused by the Earth. The TCG is the...

Slereah

Time dilation within a medium is probably something you can expect to have influence in like neutron stars I think

PM 2Ring

Oops. That should be 0.696 ns/s. Sorry

Ishwaran

10:25 AM

@Slereah What is a "medium"

Slereah

medium

Noun: medium (plural media or medias or mediums)

(plural media or mediums) The material of the surrounding environment, e.g. solid, liquid, gas, vacuum, or a specific substance such as a solvent.
(plural media or mediums) The materials or empty space through which signals, waves, or forces pass.
1626, Francis Bacon, Sylva Sylvarum: or A Naturall Historie, London: William Lee, III. Century, p. 60,[1]Whether any other Liquours, being made Mediums, cause a Diuersity of Sound from Water, it may be tried:
1642, John Denham, The Sophy, London: Thomas Walkley, Act II, Scene 1, page 12,[2]He’s old and jealous, apt for suspitions, gainst which tyrants ears
Are never clos’d. The Prince is young,

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PM 2Ring

@Ishwaran Yes. Gravitational time dilation gets extreme as you approach a black hole.

Ishwaran

I thought you were mentioning some other "medium" :P

@PM2Ring but ?

PM 2Ring

But it's a little more complicated because there's also the time dilation due to velocity, and it's usual for things near a BH to have a very high velocity.

(Sorry, I'm on my phone and my finger slipped).

Ishwaran

Another doubt, does time dilation also occurs due to rotational velocity ?

PM 2Ring

10:33 AM

Wikipedia has a nice graph on en.wikipedia.org/wiki/…

Ishwaran

@PM2Ring no problem

PM 2Ring

@Ishwaran Yes, but that's even smaller on Earth. But it's important on a young neutron star, which can spin on its axis in a fraction of a second.

Ishwaran

I Heard some black holes and magnetars spin at speed of light

close to*

PM 2Ring

I can't remember the numbers for young magnetars, but they're certainly fast. ;)

With black holes, the situation is a bit subtle because the event horizon is a mathematical surface, not a physical object. Stuff near a spinning BH has to spin, so you can say that the BH is causing the nearby space to spin. The BH spin is usually expressed in terms of a dimensionless parameter, with 0 = no spin, 1 = maximum spin, and that maximum spin is what we call "spinning at the speed of light".

From astronomy.stackexchange.com/a/20292/16685

Ishwaran

10:52 AM

@PM2Ring "nearby space to spin", does it actually spins with the spinning body ? Isn't it just inclined towards the direction of the spin ?

PM 2Ring

@Ishwaran I'm not sure what you mean. Stuff close to the spinning BH has to spin faster than stuff that's further away.

Orbits around rotating black holes get rather messy. There are a few diagrams on en.wikipedia.org/wiki/Kerr_metric

Ishwaran

11:30 AM

B. Brekke

I will be having a presentation introducing basic concepts in QM to non-physicist. At some point, I would like the audience to discuss something related to QM. However, it is challenging to come up with questions for discussion which are not just metaphysical. Does anyone have experience with this?

My presentation has no math, but I try to explain the wave nature of particles.

1 hour later…

Ishwaran

12:32 PM

@JohnRennie Is this Mains or Advanced ?

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