
Scientists use an atomic clock to measure the height of a mountain - montrose
http://www.latimes.com/science/sciencenow/la-sci-sn-atomic-clock-gravity-20180212-story.html
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_rpd
The innovation here is that they made the atomic clock portable enough to be
taken to the top of a mountain, so that they could use the difference in clock
speed to calculate the height by general relativity.

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programmarchy
A portable vacuum chamber. Pretty cool. So unlike a pressure altimeter, this
could also work in space. And unlike a radar altimeter, you don't need a
direct line of sight to the surface.

Although seems like the measurement could also be distorted by large masses
like the moon...

~~~
mikeash
Unfortunately, you still need some communication with the outside world. To
the local observer, the clock will run at the same rate no matter where you
are. Only distant clocks will change.

~~~
7952
I wonder if you could measure the difference between two clocks on the same
craft at a distance from one another. The strength of gravity follows the
inverse square law so presumably the difference in time between the two points
will change also.

~~~
mikeash
That should work, and give you the local gravitational gradient, which would
correspond to the altitude. You might want four arranged in a tetrahedron so
you don’t need to orient them.

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prewett
The article said that this was the first time atomic clocks were taken out of
the lab to do this experiment, but definitely not the first time:

[http://www.leapsecond.com/great2005/tour/](http://www.leapsecond.com/great2005/tour/)

My favorite part is where he notes that with kids everything has to be exactly
equal: "three kids, three clocks, three sodas." My somewhat fuzzy arguments
with my brothers make me think he is a wise man.

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westurner
Quantum_clock#More_accurate_experimental_clocks:
[https://en.wikipedia.org/wiki/Quantum_clock#More_accurate_ex...](https://en.wikipedia.org/wiki/Quantum_clock#More_accurate_experimental_clocks)

> In 2015 JILA evaluated the absolute frequency uncertainty of their latest
> strontium-87 optical lattice clock at 2.1 × 10−18, which corresponds to a
> measurable gravitational time dilation for an elevation change of 2 cm (0.79
> in) on planet Earth that according to JILA/NIST Fellow Jun Ye is "getting
> really close to being useful for relativistic geodesy".

AFAIU, this type of geodesy isn't possible with 'normal' time structs. Are
nanoseconds enough?

"[Python-Dev] PEP 564: Add new time functions with nanosecond resolution"
[https://mail.python.org/pipermail/python-
dev/2017-October/14...](https://mail.python.org/pipermail/python-
dev/2017-October/149944.html)

~~~
inteleng
The thread you linked makes a good point: there isn't really any reason to
care about the actual time, only the relative time. These sorts of clocks just
use 256- or 512-bit counters; it's not like they're having overflow issues.

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sebtoast
I am confused, I always thought that moving faster makes time go slower but
the article says this:

"For example, a clock on top of a tall mountain — far from the center of the
Earth — will move a tiny bit faster than a clock at the base of that mountain,
where the gravity is stronger. It's not a mechanical error. Time itself
actually passes faster at the top of the mountain."

From what I can understand from the Wikipedia article is that time does go
slower the faster you move: "Special relativity indicates that, for an
observer in an inertial frame of reference, a clock that is moving relative to
him will be measured to tick slower than a clock that is at rest in his frame
of reference."

I did a little search and half the sites I saw said one thing and the other
half said the opposite.

Would anyone mind explaining like I'm five? Or maybe share a trustworthy
sources I could read on the subject?

Thanks.

~~~
ctdonath
[paraphrasing]

Acceleration is a kind of speed. The more you're accelerating in a given
direction, the faster you're going - even if there's something stopping you.
Gravity makes you accelerate - even if there's ground stopping you from moving
"down". So the higher the gravity, the higher the acceleration, so the higher
your (instantaneous) speed, so time goes slower for you relative to other
things not moving/accelerating as much.

If you're really close to a black hole, with LOTS of gravity, time will pass
really slowly for you relative to most everything else - even if something is
preventing you from actually falling in.

At the top of a mountain, gravity is lower. You're farther away from most of
the Earth. Less gravity (however minute) means less acceleration means less
(instantaneous) speed means time traveling relatively faster (or, well, not as
slow as further downhill).

In space, far far away from everything, and so long as you're kinda averaging
a speed of 0 relative to everything else, time ticks by the fastest (relative
to most everything else).

So if you build two clocks that are practically perfectly in sync, and you
move one to the bottom of a mountain, and one to the top, and then bring them
back together, you can measure the tiny difference between them caused by one
(the one from the top of the mountain) experiencing time faster than the other
(from the bottom).

~~~
liberte82
How much slower is time at the surface of the sun?

~~~
mdturnerphys
" . . . a clock on the surface of the sun will accumulate around 66.4 fewer
seconds in one year [than would a distant observer's clock]."

[https://en.wikipedia.org/wiki/Gravitational_time_dilation](https://en.wikipedia.org/wiki/Gravitational_time_dilation)

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timb07
A similar experiment (but to only detect the effect of altitude, not to
measure it) was done about 12 years ago, described here:
[http://www.leapsecond.com/great2005/](http://www.leapsecond.com/great2005/)

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btilly
I wouldn't think this would be exceptional as a way to measure altitude.

However it is exceptional as a way to measure gravity differences arising from
stuff inside of the Earth. See [https://academo.org/demos/gravity-
map/](https://academo.org/demos/gravity-map/) for a picture of how that varies
over the Earth.

(Though you wouldn't be measuring the strength of gravitational pull directly.
You would be measuring the gravitational potential. The strength of gravity is
the gradient of the potential field.)

~~~
marknadal
Why is the Indonesia area so red on that map?

I get that other red spots have tall mountains... but why Indonesia?

~~~
wahern

      Continental crust is less dense than marine crust. So just
      because the Himalayas are tall, they also displace denser
      mantle below, so they don’t contribute much more to
      gravitational pull.
    
      The Andes and Indonesia are strong subduction zones. Besides
      the crust on the surface, there’s another layer of (dense
      oceanic) crust below adding to the pull.
    
      -- Darren #44 comment at http://blogs.discovermagazine.com/badastronomy/2011/03/31/the-earths-lumpy-gravity/
    

See, also, the following paper, which catalogs how the angle of subduction
effects the gravimetric profile.

    
    
      Harabaglia, Paolo & Doglioni, Carlo. (1998). Topography and
      gravity across subduction zones. Geophysical Research
      Letters - GEOPHYS RES LETT. 25. 703-706. 10.1029/98GL00137.

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tahw
How the hell did they measure clock speed if the passage of time itself is
different?

~~~
phyzome
"Dear tahw, what time and date is it over there? Please write back. I shall
write again next month. Sincerely, phyzome"

...

"Dear tahw, over the past year, I have noticed something interesting in our
correspondences -- more hours have passed on your end than mine, on average. I
believe our clocks are ticking at different rate. Yours, phyzome"

(except measuring number of ticks, as communicated over fiber optics, and
possibly with other tricks to cancel out communication lag)

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_adamb
"It didn't work as nicely as we hoped, but we learned a lot and it's a start,"
Lisdat said. "Sometimes you just have to begin, and then you can figure out
how to improve."

Good sentiment to live/work by.

~~~
cr0sh
> Good sentiment to live/work by.

Definitely. I've found in my personal life that just getting started on
something can be the hardest part about any given task (whether something
physical, or implementing an idea as code or something like that). But once
you get rolling on it, things tend to come along easier.

You might not reach the end, things might fail (sometimes spectacularly), or
things will change midstream and you might not end up where you had planned.
Regardless, though, the journey can end up being better than the original
goal.

...you just have to begin.

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nyc111
Is it possible that they measured the distance not height to the reference
clock? It takes time for the signal to travel to the reference clock. But I’m
not sure about the exact geometry.

~~~
ChuckMcM
I would guess that one would send a clock signal (think of it as a square wave
at a very precise frequency) and measure the phase shift between the two
clocks. (also very precisely)

