Yes,  A story about time and frequency standards.  They actually used
numbers like 10E16 in the story.  Apparently at that level your clock can
measure a change in elevation of a few centimeters because of the
relativistic effects of the reduced gravity field in just a few cm.

On Mon, Nov 3, 2014 at 6:28 AM, xaos xaos@darksmile.net wrote:

--

Chris Albertson
Redondo Beach, California

Small correction: The numbers were 10E-16.

One important concept that was discussed was this:
If the next generation clock was even more accurate
(maybe by an order or two), then no two clocks
can ever agree on the time.

Minute changes in gravity and other factors will
always make each clock completely different.

So, to that I said: WOW! Wait just a damn minute.
I got into this so I can tell time precisely. Now I'm back
to to the beginning.

I know I am exaggerating a bit here but still.

-GKH

On 11/03/2014 11:09 AM, Chris Albertson wrote:

You know, I was thinking that exact same thing as
the story went on.

The one (important) thing I got from Tom's story
was that kids might like the idea of the trip,
but the details might seem boring. Although,
I'm sure, Tom had a blast.

I was planning a similar trip from Astoria Queens, NYC
which is sea level, to Adirondack Mountains, upstate New York.

Never found enough friends to make it tho :(

-GKH

On 11/03/2014 11:40 AM, Bob Stewart wrote:

On Mon, Nov 3, 2014 at 8:17 AM, xaos xaos@darksmile.net wrote:

No I think it was "one part in 10E16" ;)  But the interesting thing was
they used numbers rather then saying something like "really super ultra
tiny".

But you are right, no two clocks will ever agree at that level because they
will experience different gravitational fields.  At this level the reason
to have a clock is no longer to tell time.  It is to measure the
gravitational field.  With an array of many clocks like these we might be
able to map the density of the interior of the earth or detect black holes
or who knows what.  I think it opens up a new area of observation.  When
ever this happens we discover things we never would have thought of.  Maybe
in 40 years these Strontium oscillators will be mass produced for $2 each.

Does anyone know how much "g" changes per cm of altitude?  I'm to lazy to
figure it out.

--

Chris Albertson
Redondo Beach, California

Why Strontium over Caesium?
Is it because it just sounds more hi-tech ? LOL

Maybe stupid question to most here, but I do
not know the answer.

-GKH

On 11/03/2014 12:59 PM, Chris Albertson wrote:

Chris Albertson writes:

I have a question about that.  If I understand correctly, recent IAU
resolutions have decoupled the definition of the SI second from the
terrestrial geoid, which is too fuzzy to be used for a definition.  Instead
the geoid potential is held fixed by (or defined by) a constant.  Potential
with respect to what exactly?  "At infinity" is all very well, but there
are local gravity sources (solar, even galactic) that would seem to
complicate any operational realization of this definition.

Sorry if this is a bit off-topic.  I'd like a simple, clear explanation for
the layman that drills down on exactly how the current definitional scheme
can be realized to arbitrary precision.  For example, assume that we must
go off-earth at some point to get a better timescale.  How fuzzy is the
solar potential ("soloid")?

Cheers,
Peter

Hi Chris,

That's correct. When it comes to frequency standards the official SI second is defined only for sea level. We know time and frequency are "bent" by speed or gravity; time is the integral of frequency; and frequency is a function of height (h) by approximately gh/c². It's that simple. But it's a very tiny effect.

Planet gravity fields decrease quadratically over large distances (1/R²) but approximately linearly near the surface. So here on Earth, with g = ~9.8 m/s² and c = ~300,000 km/s, frequency increases by about 1e-18/cm, or 1e-16/m, or 1e-13/km. This is called gravitational time dilation, or blueshift.

Now, for amateurs like us who just make things at home or buy and repair atomic clocks on eBay, numbers like 1e-18 and 1e-16 are completely out of range: that's what government labs are for. But the 1e-13 number is interesting, and approachable -- especially if you live near a tall mountain.

If you take a 1e-14 stable cesium clock up 1 km, it will run fast by about 1e-13 (in frequency) and thus it will gain about 10 ± 1 ns per day (in time, or phase) compared to a clock left down at home. These days, time differences at the nanosecond level are easily measurable -- so that's what I did with http://leapsecond.com/great2005/

Of course, NIST & USNO always have much better clocks than we do, so they can measure the effect of smaller elevation changes, over smaller time scales. Just amazing. Maybe we'll be able to buy an optical clock on eBay 20 years from now.

Note that their clocks are not (yet) portable and consequently you can make a more accurate gravitational time dilation / general relativity measurement at home by taking vintage hp 5071A cesium beam microwave clocks up a tall mountain than they can with record-setting strontium optical clocks inside a NIST building.

Essentially, if you take a clock to high altitude for a weekend you create a super-duper blueshift "microscope". Instead of unimaginably small numbers like 1e-18, I went up about 1340 meters (instead of just 1 cm) and I stayed up there about 42 hours (instead of one second). Thus my cm-second "magnification factor" was 1340 * 100 * 42 * 3600 = 20 billion! That reduces a crazy tiny number like 1e-18 to a real, tangible, measurable, fun-with-family, DIY time dilation number like 2e-8, or 20 ns.

/tvb

OK, I am going to show my ignorance now. Being in my 70th year, I forgot an awful lot of what I learned in school.

Anyway, regarding time and gravity, I certainly believe the mathematics of Einstein and others, however, I have a hard time believing that man-made instruments to measure the effects of gravity on time is valid. For example in a Cesium clock, time is a function of the transition time between two hyperfine lines of Cesium atoms. So, does gravity affect this transition time within the Cesium atoms? It may very well, but, I am not smart enough to know that. Maybe someone can help.

Also, when someone mentioned moving a very sensitive scale up in elevation and noting the difference due to gravitational effects, also seems odd to me. Seems like even in the most sensitive scales, weight is measured as the difference between the weighing platform and the body of the instrument. Here again, moving the whole assembly up in elevation it would seem to me that gravity would affect both the platform and the body, and the relative weight indicated should remain the same. What am I missing besides gray matter? Thanks - Mike

Mike B. Feher, EOZ Inc.
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960 office
908-902-3831 cell

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@febo.com] On Behalf Of Tom Van Baak
Sent: Monday, November 03, 2014 3:55 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] NPR Story I heard this morning

Hi Chris,

That's correct. When it comes to frequency standards the official SI second is defined only for sea level. We know time and frequency are "bent" by speed or gravity; time is the integral of frequency; and frequency is a function of height (h) by approximately gh/c². It's that simple. But it's a very tiny effect.

Planet gravity fields decrease quadratically over large distances (1/R²) but approximately linearly near the surface. So here on Earth, with g = ~9.8 m/s² and c = ~300,000 km/s, frequency increases by about 1e-18/cm, or 1e-16/m, or 1e-13/km. This is called gravitational time dilation, or blueshift.

Now, for amateurs like us who just make things at home or buy and repair atomic clocks on eBay, numbers like 1e-18 and 1e-16 are completely out of range: that's what government labs are for. But the 1e-13 number is interesting, and approachable -- especially if you live near a tall mountain.

If you take a 1e-14 stable cesium clock up 1 km, it will run fast by about 1e-13 (in frequency) and thus it will gain about 10 ± 1 ns per day (in time, or phase) compared to a clock left down at home. These days, time differences at the nanosecond level are easily measurable -- so that's what I did with http://leapsecond.com/great2005/

Of course, NIST & USNO always have much better clocks than we do, so they can measure the effect of smaller elevation changes, over smaller time scales. Just amazing. Maybe we'll be able to buy an optical clock on eBay 20 years from now.

Note that their clocks are not (yet) portable and consequently you can make a more accurate gravitational time dilation / general relativity measurement at home by taking vintage hp 5071A cesium beam microwave clocks up a tall mountain than they can with record-setting strontium optical clocks inside a NIST building.

Essentially, if you take a clock to high altitude for a weekend you create a super-duper blueshift "microscope". Instead of unimaginably small numbers like 1e-18, I went up about 1340 meters (instead of just 1 cm) and I stayed up there about 42 hours (instead of one second). Thus my cm-second "magnification factor" was 1340 * 100 * 42 * 3600 = 20 billion! That reduces a crazy tiny number like 1e-18 to a real, tangible, measurable, fun-with-family, DIY time dilation number like 2e-8, or 20 ns.

/tvb

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