
Hypervelocity Stars Approach Speed of Light - givan
http://time.com/3602047/hypervelocity-stars/
======
throwaway_yy2Di
_" For the special case of the ejection of binary stars, described in Section
6, we employ octuple floating-point precision (~64 digits)."_

[http://arxiv.org/abs/1411.5022](http://arxiv.org/abs/1411.5022)

~~~
HCIdivision17
I don't think I've ever even _heard_ of something needing that level of
precision. That's about 18 decimal places, right?

I'm adding the paper to my read-later pile just to understand that need. (And
I don't doubt it. But man, you can describe the height of man or the plank
length in one number!)

(And my apologies if it's explained in the paper - I haven't read it yet.)

~~~
throwaway_yy2Di
No, it's 64 decimal digits ~ 213 bits of precision! Octuple precision should
have 8 * 32 = 256 bit word size. IEEE 754 single, double, and quadruple
precision FP have 32, 64, and 128-bit word sizes, with significand precisions
of 24, 53, and 113 bits.

[https://en.wikipedia.org/wiki/Floating_point#Internal_repres...](https://en.wikipedia.org/wiki/Floating_point#Internal_representation)

~~~
HCIdivision17
Even more astounding. In retrospect, the word 'digits' is pretty unambiguous,
and I shouldn't have gotten confused. But man, that is some hard-core
numerical work. I'll need to bring the paper to the top of my dinner reading
stack. The project just seems fascinating.

------
jimmcslim
Great article on Centauri Dreams speculating about what a civilisation living
on a planet orbiting such a star might experience: [http://www.centauri-
dreams.org/?p=12671](http://www.centauri-dreams.org/?p=12671)

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JumpCrisscross
> _Superfast stars would be another sort of “particles,” albeit huge, shining
> ones_

Have I misunderstood the difference between stars and neutrinos my whole life
or is the journalist getting creative?

~~~
venomsnake
I think that this strip about particles should be added to some of the
obligatory XKCD-s

[http://www.smbc-comics.com/index.php?id=3554](http://www.smbc-
comics.com/index.php?id=3554)

And yeah ... he is getting creative.

~~~
gus_massa
If you assume that electrons are made from "smashed electron paste", then the
magnetic moment of the electron is related to the angular moment of the
electron. If you divide this by the charge of the electron and some universal
constants like c an h the result is the g-factor. For the "smashed particles
paste" model you get that g=1.

(If you have a spinning rock with constant mass density and charge density,
you get g=1.)

If you use a theoretical model for fermions particles, the calculation is more
complicated but finally you get that g=2.

If you do the experiment with actual electrons, the result is
g=2.00231930436153 (yes, with a lot of decimal places). The difference is dew
to the appearance of virtual particles near the electron, and can be
calculated theoretically and experimentally, and the results agree up to the
current precision!!!

You can do the same with other particles, for example for muons
g=2.0023318414, because the corrections are slightly different.

But for protons g= 5.585694713 because they are not elementary particles, they
are composed by quarks. (They are not elemental particles, but are neither
smashed paste particles.)

More details:
[http://en.wikipedia.org/wiki/G-factor_(physics)](http://en.wikipedia.org/wiki/G-factor_\(physics\))

I think that it's a little optimistic to say that we have the definite
elementary particles, but we have hit a very strange wall.

------
dcre
> (Black hole interactions also give rise to hypervelocity stars within the
> Milky Way, but here there’s just a single black hole, and thus a lot less
> energy available.)

Pretty sure this is wrong. The abstract of the real article says that their
conclusion depends on assumptions about the eccentricities of orbits of
merging black holes, not the absolute amount of mass involved.

~~~
throwaway_yy2Di
The point isn't the absolute amount of mass, but how deep the gravitational
wells go -- how rapidly objects can orbit. It's not only about _massive_
objects; it's about _compact_ ones.

If you have only one supermassive black hole, then the 2nd and 3rd objects are
a binary star, and the interaction between _them_ is very weak. Stars can't
orbit each other faster than ~100's of km/s (~1,000's for white dwarfs):
they're limited by their low density -- they can't get very close or they'll
collide. If, instead, you have a star orbiting a _black hole_ \-- that's a
very dense object, it allows much closer approaches, much deeper into the
potential well, with orbit speeds approaching the speed of light.

Don't forget this is a _three_ body interaction. You want a star kicked out of
a black hole, with much more kinetic energy than it had going in. This kinetic
energy comes from the potential energy of some other object falling into the
black hole. How strongly the hypervelocity star gets kicked out, depends on
how strongly it can interact with the infalling object, to transfer its
energy.

(This idea is I think implicit in the paper (page 3): the "kick" velocity of a
binary object falling into a black hole (v_kick) is at most a small constant
times their mutual escape velocity (v_23)).

[http://arxiv.org/abs/1411.5022](http://arxiv.org/abs/1411.5022)

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bjornsing
> “We calculate that there should be more than a trillion stars in the
> observable universe moving at velocities of more than a tenth the speed of
> light,” says Loeb.

But wouldn't they be extremely easy to detect experimentally (through red/blue
shift a.k.a. doppler)? I'm a bit puzzled why this sounds purely theoretical...

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thisjepisje
Trillion: 10^12

Number of stars in observable universe: 10^22 to 10^24

~~~
rajacombinator
That is pretty mind blowing.

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qwerta
There was scifi novel describing something like that in 70ties.

Anyway, I seriously doubt this is possible. Escape velocity from center of
star is tiny fraction of speed of light. We might have better luck with
neutron start or black hole going at near speed of light.

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yummybear
I wonder what this would do in case of a collision with another object.

