The Leap Second

There was a great presentation given by Tom Van Baak on measuring time, precision, accuracy etc. at this year's FOSDEM . Abstract an video are available at https://fosdem.org/2015/schedule/event/precise_time/, he has lots of additional time-related information on his website http://leapsecond.com/.

This is compared to the ~1 mile vertical shift resolution of cesium clocks. The fun part of cesium clocks is that you throw three in the back of a minivan and take them camping! http://leapsecond.com/great2005/

If you're interested in precise time keeping, this is Time-Nuts is a great place to start (http://www.leapsecond.com/time-nuts.htm).

You have to compare two clocks. Another example of time dilation is this: take two really precise clocks, carry one up a mountain and back, and compare it to the clock that stayed at sea level. The one at sea level will have run slower and be behind compared to the other clock because it spend some time in a place where gravity was less. Here is a guy that did exactly that in 2005 on a tour using his van. He even took his kids: http://www.leapsecond.com/great2005/tour/

Yes. You can actually measure this just by the mass difference of a mountain here on Earth, with a sufficiently accurate pair of clocks. http://www.leapsecond.com/great2016a/index.htm

> taking a clock to a mountain http://www.leapsecond.com/great2005/tour/

See for example the experiment conducted by Tom Van Baak about 15 years ago: http://leapsecond.com/great2005/. He drove three Cesium clocks up Mount Rainier and returned after a week. He compared them to a clock he left at home for the journey. The graph that he shows on the page and his associated commentary is interesting. Ultimately, a clock is simply an abstract device that goes tick-tick-tick at some regular rate. Once one starts measuring the phenomenon itself---mechanics of the human heart, tidal forces on the Earth, friction in a pendulum, or the uncertainty principle in atoms---it is no longer feasible to treat it as objective time. International Atomic Time (TAI) itself consists of an ensemble of 400 atomic clocks, with the collective being more accurate (the proper word is "stable", I think?) than any of its constituents.

There are time and frequency standards that people rely on for all sorts of things. This sort of equipment can be used to calibrate the current time and frequency standards. Also for high precision physics measurements etc. Clocks aren’t really that important with 10ms resolution at best, although it can be fun playing with old time standard kit http://www.leapsecond.com/pages/atomic-bill/

Yeah, gravitational time dilation is about 1e-16 seconds per metre of elevation near the surface http://leapsecond.com/great2016a/index.htm so Boulder is about 1e-13 fast. The rotation of the earth is stable to about 1e-9 http://leapsecond.com/ten/ which is also about the same as the rate difference between UT1 and UTC (0.2 ms / 86400).

In case you haven’t read the story of taking an atomic clock on a road trip, it’s a fun read: http://leapsecond.com/great2005/tour/

I've never worked for Symmetricom but if you're interested in timekeeping be sure to check out the excellent Time-Nuts mailing list.[1] [1] http://leapsecond.com/time-nuts.htm

Thank you! Found it within first link you shared! It was wilby.me This was the time dilation post : http://www.leapsecond.com/great2005/tour/

I clicked on surprise me and landed in a fun family science project [1]. [1] http://www.leapsecond.com/great2005/tour/

Don't miss a previously-featured-on-HN article from the same site about taking an atomic clock up Mt. Rainier to check if you would actually gain billionths of a second as predicted by GR: http://www.leapsecond.com/great2005/ And pictures: http://www.leapsecond.com/great2005/tour/

Yes, the original Mt Rainier experiment in 2005 caught the attention of Stephen Hawking and I was asked to repeat the experiment for his PBS/BBC TV series in 2016. It was mid-winter up here in PNW so we used Mt Lemmon in Arizona instead: http://www.leapsecond.com/great2016a/ And in 2018 I was asked to be part of a "time travel" episode on a History channel show. We used Palomar Mountain in California: http://www.leapsecond.com/great2018a/ Each experiment was a little different; different combination of clocks, different audience, different mountain, different elevations, different latitude, etc.

Tests of gravitational time dilation are also within the reach range of enthusiastic amateurs using second-hand atomic clocks: Part 4 (starting pdf page 44) of http://web.stanford.edu/group/scpnt/pnt/PNT18/presentation_files/I08-V… (slides) Or if you prefer non-PDF: http://leapsecond.com/great2005/ http://leapsecond.com/great2016a/

Here's some handwringing from before the first rollover of the GPS epoch: http://www.leapsecond.com/notes/gpswnro.htm Using the leap second indicator didn't occur to me, but burning in the week of manufacture (or firmware compile date, as ublox apparently does) did.

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/ 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.

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/

It's rather amazing that anyone with a couple of atomic clocks can see the effects of relativity themselves: http://leapsecond.com/great2005/

You are going to love this: http://www.leapsecond.com/great2005/ The short version is that this guy hauled three cesium atomic clocks up a mountain while on vacation with his kids. They returned 23 ns older then the guy's wife (who stayed home to study for her nursing board exams). Note that time dilation isn't just caused by motion (special relativity), but also by proximity to a gravity well (general relativity).

If anyone is wondering: yes, you can own your very own atomic clock for a "reasonable" price! I encourage everyone to read [0] where a father takes his kids and a few atomic clocks up a mountain and back down. By looking at the clock drift due to changes in gravity, he was able to observe relativity! [0]: http://leapsecond.com/great2005/

Also see this experiment from the same site: http://leapsecond.com/great2005/ Quote: "So, yes, not only do we live in a time when atomic clocks are altimeters, but when relativity is child's play. It was the best extra 22 nanoseconds I've ever spent with the kids."

Sorry to correct you, but that single atomic wristwatch is totally inadequate to reliably detect the effects predicted by general relativity. You'd better use at least three (and compare their readings with the home-based hydrogen maser) for that experiment, which TVB dutifully carried out - on a weekend-trip with his kids. http://leapsecond.com/great2005/

The timing output from a $5 gps receiver isn't particularly impressive. You have to go to pretty long time intervals for a non-timing GPS to win out. Depending on your application those intervals may be thoroughly irrelevant, e.g. using the device as a frequency source rather than a clock. There is also, of course, the issue of infrastructure dependence. Particularly since wireless telephony has moved almost exclusively to GNSS time we're going to have a really bad time if kessler syndrome takes out the GNSS satellites. Edit: Here is an example adev chart for a inexpensive atomic clock vs what appears to be a pretty good timing GPS receiver: https://www.thinksrs.com/images/instr/prs10/PRS10diag2LG.gif So in that case the GPS accuracy only beats out the free running atomic clock at intervals greater than 200,000 seconds or so. Here is a collection of older timing receivers: http://www.leapsecond.com/pages/3gps/gps-adev-mdev.gif A quick look didn't turn up any cheap non-timing receivers, but my experience is they're pretty bad (I mean relative to atomic standards, of course). There are better timing receivers than the ones charted above, of course, but they are not $5. Their cost is now in the same general ballpark as surplus atomic clocks. Of course, if you have both you can sync one to the other with whatever time constant maximizes the composite performance and have the best of both. (and primary atomic clocks don't have this drift issue, but sadly the days of…

Eh, doing the math[0], it seems like you'll actually drift about 1 nanosecond every ~35 minutes if I pick a mountain near my house vs sea level. That means you'd have about 1 microsecond of difference in your clocks in about 3 weeks. For a 1 millisecond drift though, that'd take about 66 years. That's still pretty safe, but it's definitely not "you never have to worry about this˟" territory. I'm sure some others can chime in with cases where 1 microsecond clock drift (in cases expecting very accurate clocks) can cause problems. >>> g = 9.8 c = 299_792_458 >>> h = 4392 # height of Mt. Rainier >>> drift_per_second = (g*h)/(c**2) >>> print(drift_per_second) 4.789023865263744e-13 >>> nanosecond = 1.0e-9 >>> seconds_to_drift_one_ns = nanosecond / drift_per_second >>> print(seconds_to_drift_one_ns) 2088.1081993625176 >>> print(seconds_to_drift_one_ns / 60) # minutes till a 1 ns drift accumulates 34.80180332270863 >>> print((seconds_to_drift_one_ns * 1000) / 60 / 60 / 24) # days till ~1 microsecond accumulates 24.167918974103213 [0] - http://www.leapsecond.com/great2016a/#%3A~%3Atext%3DConceptually%2C%20…

> So if a photon is emitted from the sun, it's passing earth immediately? Then why does sunlight need 7 or 8 minutes to get to earth? From the photon's perspective, it passes earth immediately. From our perspective, it takes 7 or 8 minutes. > And let's say, i travel one light-year at the speed of light, that should be instantaneous, right? The odometer would show 1 light-year, my watch would should 0 seconds and some decimals. How much would I have aged by the end of the journey? You would have aged as much as your watch says you would have aged. Zero seconds. >If I travelled at one percent of speed of light, same distance, i suppose 100 years would elapse for me, how much would elapse on earth? I don't know how to do the math, but a very, very long time would have passed on earth. > And what if I left earth at 50percent of light speed , traveled 1 light-year away and did a turn and came back to earth at same speed. For me, it would be 1 year, but if I had a twin brother who was waiting on earth, would i now be a year younger than him? And how is this possible? Yes, you would be younger than him, and it's possible because that's just how relativity and time dilation work. It's even practically measurable in "real" life: http://www.leapsecond.com/great2005/tour/

Quartz clocks can be inaccurate and can vary greatly with temperature. Beyond temperature-compensated crystal oscillators (TCXO) which are designed to be reasonably stable over a prescribed temperature range, there are oven-controlled crystal oscillators (OCXO)[1], that are held at a stable temperature in a small can with a heating element controlled by something like PID control. For anything requiring affordable stable timekeeping (this includes high data rate/quantization multimeters like the HP 3458A), an OCXO is a pretty good way to go. GPS is a fine reference to start one from and with specialized hardware GPS itself can be used as a MHz clock. For everything else there are TCXOs, and if you really don't care on-die RC oscillators can be used (this is the internal oscillator on an Atmega, for example, the default until you add an external crystal). If money is no object, atomic clocks are of course also an option. Past press mentioned that Google uses GPS-referenced atomic clocks to keep their data enter clocks in sync [2]. As an amusing aside, my kitchen oven clock has a poorly-chosen TCXO, such that it is not stable over the temperature range normally experienced by an over control panel. It's a great example of not designing to the application. Whenever I bake something, time accelerates on the oven clock. It's like a cookie-making time machine. If you are interested in learning more about the hardware of crystal oscillators, this application note from HP is…

I prefer to assume he's lugging a 12U mini relay rack around with him, including a UPS and cellular fallback for networking, so he can run an Android VM and use it as his cell phone. See also, this atomic clock wristwatch: http://leapsecond.com/pages/atomic-bill/

http://www.leapsecond.com/time-nuts.htm Any time we fix an old radio that goes down to 60 Khz, I always have to see if WWVB is coming in loud enough to be heard.