
The fastest pulsar spins at 716Hz; its equator spins at 24% the speed of light - almost
http://en.wikipedia.org/wiki/PSR_J1748-2446ad
======
joezydeco
Lemme guess, you caught this from
[http://xkcd.com/1331/](http://xkcd.com/1331/) too?

~~~
lifeisstillgood
I could not take my eyes off "One Birth" / "One Death"

Fascinating - and I think humanity is "winning"

~~~
sanoli
I couldn't take my eyes off of the fact that everytime wikipedia is edited
someone dies.

How 'bout that for the correlation=causation folks!

~~~
return0
Damnit you guys, stop editing!

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agonz253
Dropping something the mass of a tennis ball into this pulsar(assuming 2 solar
masses for it) would generate approximately the same amount of energy upon
impact as that of 12 atomic bombs like the one tested in New Mexico, and it
would reach the surface at 60% the speed of light...relativistic effects aside

~~~
saalweachter
Which is to say: the escape velocity is 60% of the speed of light! So even
starting from 24% of the speed of light at the equator you need to go 10,000x
faster than on Earth to escape.

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NAFV_P
You can see round the back of a neutron star:

[http://en.wikipedia.org/wiki/Neutron_star#Properties](http://en.wikipedia.org/wiki/Neutron_star#Properties)

~~~
jurjenh
Wow, that page led me to the fact that there _is_ such a thing as centrifugal
force... And all these years spent trying to correct people. Oh well, you
live, you learn.

[http://en.wikipedia.org/wiki/Photon_sphere](http://en.wikipedia.org/wiki/Photon_sphere)

~~~
jjoonathan
The "war on centrifugal force" is pedagogical. If you sit in an inertial
frame, all forces you intend to teach are due to interactions between objects.
This lets teachers introduce the interactions one at a time in a way that
presents minimal opportunity for the "no, that equation doesn't work here for
XYZ reasons which you won't understand until after you've taken vector
calculus" problem.

If you allow rotating reference frames then you've got to talk about
transformation rules in gory detail. Everyone understands centrifugal force,
but what about the coriolis and euler forces? Our intuition doesn't tell us
when they apply and when they don't, so we have to be systematic about it, and
that means "gory details". And guess what "being systematic about it"
involves? Moving to an inertial frame of reference and taking derivatives!

You can't be intuitively correct _or_ systematic about rotating frames without
thoroughly understanding inertial frames, so that's what you learn first,
along with strongly worded advice to stay away from non-inertial frames (e.g.
defining frame-dependent forces as "fictitious").

Anyone doing general relativity is comfortable enough with coordinate
transformations that they can understand the caveats that come with frame-
dependent forces and even take advantage of them to simplify calculations or
definitions (the process of going from a "global" to a "local" coordinate
system is highly nontrivial in GR).

That's why the Coriolis, Euler, and Centrifugal forces are called "fictitious"
while gravity isn't.

~~~
lifeisstillgood
This is what I love about HN - an anti-troll comment. Like most of us I think
a generalist knowledge confers the right to an opinion and expressing that
opinion.

Comments like this point out just how much I do not know, they sweep their
arms across a horizon of maths and hard won understanding and say "only
express that opinion _after_ you know what that means"

Anti-trolling.

It's what keeps us honest about our own limitations, and reluctant to spout
off crap.

love it. Now going to look up inertial frames and start translating.

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dfc
Linking to wikipedia pages with titles like this = Today I Learned on HN

Yet another reason I like the "don't change the title; the title is not for
editorializing" policy on HN.

~~~
Kell
So what are you proposing in this case, that the original poster had kept the
Wikipedia title? Or that the linked Wikipedia article shouldn't have been
posted at all ?

Because hn's anti editorialization policy is useless in both cases. For, in
the first case, without editorialization, the title is completely useless
except for experts on the subject. And for the second case, it has nothing to
do with title editorialization, but with content.

~~~
dfc
The policy is: "Do not change the title." What is it that you are having
trouble understanding?

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fsiefken
does this mean that time flows significantly faster on the outside of the
pulsar then on the inside, if so does this significantly impact the pulsar
dynamics?

~~~
deletes
There is not much difference. The relativistic factor is 1.030107, that is a
~3% change.

EDIT:

My amateur physics say that the gravitational well adds 9.7% to that.

I tried the same calculation for earth and it gave a sensible result: a change
of 6.95 * 10^-10.

Here is an answer from physics exchange which gets the same result:
[http://physics.stackexchange.com/questions/10089/gravitation...](http://physics.stackexchange.com/questions/10089/gravitational-
time-dilation-at-the-earths-center)

~~~
InclinedPlane
Keep in mind that differences in gravity will also affect time dilation.

~~~
gus_massa
And this correction goes in the other way.

~~~
deletes
No, time in a gravitational well runs more slowly relative to someone outside,
just as your experience time slower if you are traveling very fast.

~~~
machrider
I think the point was that time in the center of the star is more affected by
gravity than at the surface. Whereas someone on the surface (if that were
possible) would experience greater time dilation relative to the center due to
his high relative velocity. So for those two points (surface vs center), the
two time dilation effects are working opposite each other.

Edit: I may have been reading too much into it. The effects you see will
depend on which positions you're comparing, and people in this thread haven't
really specified.

~~~
deletes
I think you mixed up some things. There is no gravity or time dilation in the
center of a massive spherical object.

~~~
nealabq
If you're in a small empty room in the middle of a neutron star, you will be
floating weightless, but you're still at the bottom of a gravity well. Since
it's the well, not the acceleration that affects time, time will tick slower
for you.

(Don't try this at home.) (And if you do, don't touch the walls.)

~~~
deletes
Spacetime in the center of a perfect spherical object is the same as a point
in empty space, you can't tell them apart. Of course that only counts for the
exact point in the center. So the time will not tick slower there as to
someone far from any objects.

~~~
ubercow13
What? The lowest potential in a uniform sphere is at the centre not the
surface
([http://en.wikipedia.org/wiki/File:GravityPotential.jpg](http://en.wikipedia.org/wiki/File:GravityPotential.jpg)).
Also graviational time dilation depends on the potential not the force
(curvature) so it is irrelevant if the point at the centre is shifted in
spacetime

~~~
ubercow13
Time will still tick slower though regardless of the first part of what you
say being right

~~~
ubercow13
Your link doesn't even relate to time dilation and is also about a _hollow_
sphere, not a uniformly dense solid sphere. Moreover in a hollow sphere, time
dilation still occurs because it depends on the absolute (or rather, relative
to an outside distant observer) potential, not the gravitational field (which
is the only thing that is necessarily zero as a result of the flat spacetime
in the sphere). See for example this
[http://physics.stackexchange.com/questions/69821/a-hollow-
bl...](http://physics.stackexchange.com/questions/69821/a-hollow-black-hole)

It's like you didn't read your link

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robnite
The core of a Pulsar has never been directly observed. Their makeup is
hypothetical. Perhaps an alternative explanation for the observed behaviour
would be less astounding...
[https://www.thunderbolts.info/tpod/2004/arch/040920pulsar.ht...](https://www.thunderbolts.info/tpod/2004/arch/040920pulsar.htm)

~~~
nikster
I like that theory. Generally the problem with science is that it's turned
into a belief system almost akin to a religion by many people. Strangely, even
by very smart physicists.

They seem to overlook the fact that we don't know anything; compared to what
is out there our knowledge might approach a millionth of a millionth percent
of all knowable things; but even that is just for illustration because if you
only take into account the scale of the known universe it's going to be a
number so small as to approach 0. We know literally nothing.

It's fine to come up with grand theories of how things work - human
imagination is a wonderful thing. Just don't mistake it for facts.

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hangonhn
They also spin so regularly that you can build a clock out of their signals:

[http://en.wikipedia.org/wiki/Pulsar_clock](http://en.wikipedia.org/wiki/Pulsar_clock)

~~~
eksith
Don't we already use these as a sort of GPS for interstellar travel? I think
the Voyager plaques contained coordinates to Earth based on pulsar frequency.

~~~
hangonhn
I didn't know that. That's fascinating and "beautiful"!

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hackaflocka
Why do planets, stars and moons spin? Are there any that don't?

~~~
teamonkey
Classically, mathematically, the answer is conservation of angular momentum.
As matter falls on to the body the body needs to rotate faster to conserve the
energy of the system. A rotation happens if there is even a tiny asymmetry in
the accreting matter.

In the case of our own Moon, one side is more massive than the other and over
the course of millions of years became tidally locked so that the heavier side
always points towards the Earth.

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tsotha
This is sort of the ultimate demonstration of the spinning ice skater who
seems to be in every eighth grade science book.

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ars
Anyone feel like calculating the energy embodied in just the rotation of this
star?

~~~
raverbashing
Makes me wonder if there will be someday an electrical generator using the
rotational energy of a celestial body.

(this would need to have a reference in the main star, otherwise it would just
spin with the planet)

~~~
stcredzero
_generator using the rotational energy...would need to have a reference in the
main star_

Nope. You could launch generator pods in a hyperbolic trajectory that would
start out escaping the neutron star, but generate electricity from the
rotating magnetic field in such a way that it's captured in orbit and comes
back to its launch point.

~~~
ars
No you can't. Conservation of angular momentum doesn't allow that.

~~~
stcredzero
Yes you can. You just end up changing the angular momentum of two bodies, not
just one. When you launch the pod, you slow down the body you launched from.
The system as a whole conserves momentum and energy, but the amount of
electricity you have increases.

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angelbar
When I try to get on my head a "sun" spinning 716 times on a second I feel my
stomach sick... Lousy astronaut here :-(

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aragot
Holy cow: We can detect an object 16km large at 18,000 light-years from Earth!

