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First atomic clock wristwatch (2003) (leapsecond.com)
201 points by dddddaviddddd 4 months ago | hide | past | favorite | 91 comments

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/

as a track marshal that routinely denies track entry (probably every single event) due to inappropriate battery mounting and wire mismanagement, these pictures terrify me.

It's a cute experiment with the kids, and i'm glad they got away without injury -- but on the other hand i've witnessed first hand what a lead battery that is floating around the cabin can do during a wreck -- and what I witnessed was, sadly, deadly for the driver.

single orientation nylon straps that are strapping heavy square objects against foam cushioned seats and sandwiching the chaos with human meat on both sides. It's a recipe for disaster regardless of what vector the force of the impact comes from -- and that's ignoring the batteries.

Please folks , when you drive a vehicle the heavy stuff gets strapped behind a bulkhead at the front or rear of the car, or atop the car with appropriate and well-tested strapping & mounting equipment.

Don't risk your life for the sake of convenience.

Do you honestly expect these people to add proper tie down points to the floor of a minivan for a one time trip?

The batteries are behind a seat row, the stuff in the seat is strapped in, the loose stuff is in the pass front foot well per OSHA guidelines (I figure most here worship at that altar). It'll be fine. They're not tracking the minivan.

I know that every time something like this is posted everyone likes to swoop in and talk about how X or Y is unsafe and you need eye protection every time you pick up a screwdriver and fall protection every time you even glance sideways at a step ladder because that's a way to score easy virtue points but it just comes across as tone deaf posturing. They aren't driving the van around like this 40hr a week for an entire career likewise it is not reasonable to expect them to implement a tie down solution that scales to that level.

It only takes one time to die.

I’m not sure why you read GP’s comment as virtue signaling. Sure, those examples you gave sound like virtue signaling. I dislike that sort of people probably more than you. Maybe it was his italicized “terrified” comment. I guess you were thinking “Seriously? This terrifies you?”

But GP is talking about something more serious. Maybe read their comment again. He literally said he saw someone die due to a similar situation.

Your OSHA comment sounds like you have a chip on your shoulder about something.

If it helps you process things better or at least put things in a different perspective, how would you feel if someone said they were _terrified_ that the kids weren’t wearing seatbelts? Would your response be slightly different?

> how would you feel if someone said they were _terrified_ that the kids weren’t wearing seatbelts? Would your response be slightly different?

That would still be an overreaction but to a lesser extent. Especially if they were talking about how dangerous it would be on a track.

Your comment just shows how stubborn you are to prove a point.

At least I gave you the benefit of the doubt and seriously reflected about both yours and GP’s viewpoints.

I wasn’t talking specifically in this article context. It was a completely made up hypothetical situation yet actually does happen in real life resulting in deaths. According to a quick search when typing in “seatbelt deaths”, Google says in 2017 that ~33% of vehicle deaths in the U.S. were or could have been prevented if seatbelts were worn. That’s tens of thousands of people, _each year_.

What if you saw a drunk driver get into a car headed for the highway? Same response? “Oh, no you’re just exaggerating, he only lives 5 blocks down the street, he’ll be ok”.

You’re complaining about one extreme where people overreact over safety. Yet, you exist at the other extreme, telling people that they’re overreacting just to fit your narrative. Terrible.

EDIT: I see that the most recent comment was from a Dylan. I thought I had been replying to “throwaway”. Are you two the same person? It’s even more funny thinking that someone was so annoyed by someone talking about safety that they had to make a throwaway to complain lmao.

Couldn't be a throwaway made for the purpose of this discussion, new accounts show up in green for a couple of weeks.

If we were only talking about something that is dangerous at say, 100mph+, then sure - just drive slowly, it will be fine.

This is just isn't - this stuff will kill you in a 30mph collision. If you saw someone put a baby on the hood and drive around like that would you also say "oh do you expect these people to secure their babies for a one time trip - relax!". Like, it's just not safe. At all.

And the fact that it's a one-off makes it worse, not better - they won't be used to the handling a van this heavy, you will try to come to a stop and discover you're missing a car length - enough to get t-boned by a truck. I don't know why people treat it as some kind of wild and completely improbable incident that just won't ever happen - of course it will! These sort of things happen all the time!

>>not reasonable to expect them to implement a tie down solution that scales to that level

It's unreasonable to not expect them to.

> they won't be used to the handling a van this heavy, you will try to come to a stop and discover you're missing a car length

200 pounds of clock in a seat, and 300 pounds of battery with a removed seat. That's on par with simply having two people sit in the middle row.

Yes, 500 pounds of gear sounds about right. I was aware that the car was slightly heavier than usual and drove accordingly. There was no hurry; it was not a race, just a leisurely weekend trip, about 2.5 hours in the car, with an average speed of maybe 35 mph.

With 3 expensive clocks, 3 priceless kids, not to mention hot coffee in hand, I accelerated and decelerated modestly. The drive from Bellevue to Paradise Lodge up on Mt Rainier is mostly rural and then follows a long slow winding mountain road to 5000 ft elevation.

If there were 4 adults (and not 2 adults and 2 kids) with that amount of extra weight in the minivan, then you would see that they would be cutting it extremely close or be over for the maximum recommended amount of weight.

Besides, it’s not just the pounds that matter. Would you rather be hit by 200 pounds of hardness/sharpness/person bouncing to-and-fro or not be hit at all during a car crash cause that 200 pounds, whether object or person, was secured?

That setup would probably be illegal in many countries today. There's a lot of improperly secured equipment in that van. Even a relatively minor crash can get some of it flying around and crushing bones or killing someone.


You haven’t read Hacker News then...

> If my bike ride is just 5 minutes why bother wearing a helmet?

It reads much easier like this.

If you only make a couple bike rides like that in your life then skipping the helmet isn't a big deal.

It only takes one time to die.

I’m assuming you’ve never met anyone that died due to a bike. So insensitive.

It's okay to take a risk occasionally.

It's a bad idea to climb a mountain or go sky diving every day, but doing it once doesn't make you a reckless person.

Doing it once and not being properly prepared does make you a reckless person. To use an exaggerated example like you did, would you go skydiving without taking a class ahead of time? Would you go without a parachute? Would you go without a backup?

Stop being facetious just to prove your point, please.

Going skydiving, with a class, with a parachute, with a backup, is still dangerous.

Climbing a mountain with good equipment and a buddy is still dangerous.

Being "properly prepared" is only relevant inside a context. You can't look at just preparation when deciding if something is too dangerous. The actual risk numbers are the important thing here.

If I'm driving 100 miles, and I do something sloppy that doubles my risk of dying in a car crash, the actual danger I face is less than the danger of skydiving just once with perfect prep. If it's also a special occasion where I'm going to get a huge amount of enjoyment out of it... the risk seems okay.

Even if we're focused just on car safety, we should be far more upset when someone buys a car without top-tier safety ratings than about some one-off trip like this.

Being able to point at some specific danger, like a battery on the floor, is just bikeshedding. What matters is the risk per day/month/year, and most of that risk is invisible.

I’m not going to read your comment since you’re clearly being facetious.

1. I'm not.

2. Do you really think that's an appropriate comment?

To be pedantic, bike helmets are uncommon in bike safe countries like mine / NL.

Cycling fatalities still make up a third of total traffic fatalities in the Netherlands. And almost 70% of those several hundreds who die in cycling accidents are over 65, people who are much more likely to lose their balance and are more sensitive to an impact, meaning a bit of protection would have gone a long way. That's just fatalities, think of all the non-fatal accidents, and how may of the 6000 people hospitalized each year just for unilateral accidents (with bollards or lamp-posts) need an MRI because they bumped their head. The numbers don't get better without adding protection because according to SWOV almost 80% of cycling accidents are unilateral or between cyclists.

But there's a cultural opposition to helmets in many countries, chief among reasons being that "helmets don't look good" and "real cyclists don't need one". If you didn't wear one your whole life, you won't start at 65.

30% of vehicle deaths are due to traumatic brain injuries. There's also a cultural opposition to wearing helmets in motor vehicles.

Is there a cultural opposition?

In my 30 years of life, I’ve never read an article, seen a TV show, watched a movie, read a book, heard a conversation, etc where the idea about wearing helmets in cars was even discussed.

I’ve never even heard a comedian joke about it.

Who is opposing this? Are you saying there’s opposition just because it’s a thing that doesn’t exist?

And yet, it turns out there have been various attempts at a motoring helmet over the years:


So that does open the question about whether suppression of this lifesaving technology has been something intentional on the part of automotive companies and enthusiasts (see: "jaywalking") or if it's the other way, that making road safety issues about only cycling helmets is an attempt to de-legitimize that mode and blame victims.

i tapped a kwall in a drift car (as one does) and the battery detached and shattered.

that was fun.

I am realizing how ordinary my childhood was.

I know! Three atomic clocks riding in the middle and the kids in the back playing on a GameCube.

Real men demand real time. Technology. Elegance. Tradition.

HP Cesium line, the man's watch.

Cheaper than most Patek Philippes while maintaining better time! Technology steadily marches forwards.

Add to that the gym membership you will need to get fit enough to carry it around... and the chiropractor bills for when you realize you weren't fit enough after all.

Warranty void if product is submerged in water over 100m deep, travels at greater than 30,000 km/s, or is taken into outer space.

If the warranty expires, is the duration based on the manufacturer's reference frame, or the watch's?

What about if approaching a super dense point where space-time warps?

We can certainly add that clause.

But might we suggest that if you do encounter such a scenario within the earth's atmosphere, that you should address that situation immediately? Otherwise there may soon be no circuit court judges available to hear your case.

Spacetime anomalies are covered only from the observer's reference point. Doh!

You should stay away from those, the damned thing is already heavy enough as it is...

There is an actual atomic clock watch on the market that is a little bit smaller:


These days it's not as complicated. Microchip has a line of "chip scale" atomic clock modules you could use to make one. Not sure what the pricing is.


I wonder if it's safe to wear a radioactive source on the wrist like that. Or is it not strong enough to matter?

Cesium atomic clocks are not radioactive, nor are they based on radioactive decay. The isotope used for cesium beam atomic clocks is natural and stable Cs-133. The clock is based on an ultra-precise energy transition near 9.192 GHz.

You may be thinking of the nasty radioactive isotope Cs-137, which is unnatural and often a byproduct of nuclear tests or accidents.

It's not unlike Carbon or Potassium; Carbon 12 is the safe stuff, the rarer Carbon 14 is radioactive. Potassium 39 is the safe one and rare Potassium 40 is radioactive. Which is why bananas are slightly radioactive:


You can make a cesium clock radioactive by placing a banana on top of it. See page 1, 36, 37 of:


What is the banana equivalent dose of working 8 hours a day in an old office building with granite stone floors, and granite wall paneling? I've read that all granite is slightly radioactive.

Roughly 0.18 to 3 mSv per annum [0]. (EU regulations limit it to 3 mSv/a maximum [0]).

One banana is 0.1 µSv [1].

The granite is therefore 0.18 mSv / 0.1 µSv = 1800 bananas.

This is compared to 4 mSv normal yearly background dose [1]. The granite increases background radiation by 0.18 / 4 * 100 = 4.5%. The yearly dose from natural potassium in the body is 0.39 mSv, so I wouldn't worry about the extra 0.18 mSv if I were you.

You're at much greater risk if you live in a house which used reinforced concrete with steel from the Ciudad Juárez accident - after that accident, 109 houses were demolished. [2] [3]

[0] https://www.nature.com/articles/jes200944

[1] https://xkcd.com/radiation/

[2] https://en.wikipedia.org/wiki/List_of_civilian_radiation_acc...

[3] https://news.ycombinator.com/item?id=24891947

> which is unnatural

There's nothing unnatural about Cs-137. Fission occurs naturally in U-235.

Cs 133 isn't radioactive. Tritium is, but at least until recently, it was still commonly used in watches for illumination. Low-level beta source, expensive but not especially hazardous.

>at least until recently

Has something changed? I know tritium sources are on the decline, but it always tickled my mind that radioactive lume was available to consumers for reasonable prices.

Haven't heard of it being used recently, except perhaps in gunsights, but a quick Google search brings up quite a few tritium-illuminated watches at moderate prices. I would've guessed that button-activated LED backlighting would have replaced it by now.

Edit: interestingly, Wikipedia says that radium was still used in watches until 1970.

Indeed it is!

It draws quite a lot of power. From what I remember, the lipo battery required for 16hrs w/ no display would be the same size as the already chunky module itself, requiring wristband integration. Better to have a quartz oven and fire up the CSAC only once per hour as a reference, still yielding one factor of 10 in precision.

All in all it's better suited for a nice pocket watch with a 10MHz out connector. Any takers at 15k$+?

Hah. Admittedly I didn't look up the power consumption. I came across these modules a few years ago randomly and seeing this article reminded me of them.

I wasn't seriously suggesting it's a practical answer by any means =)

Note that bathyshawaii.com is likely no longer owned by the maker, and it most certainly is NSFW (oh, keep scrolling).

Thanks, DanG. Author here. I happen to be reading HN right now if anyone has questions about cesium clocks or using them to demonstrate relativistic effects. This is just a DIY hobby of mine.

It's a bit melancholy now with the collapse and all, but just wanted you to know a paper copy of that image was taped up on the Clock Room wall at Arecibo Observatory when I left (somewhere in 2015).

Tempus edax rerum :(

How hard are cesium clocks to maintain / repair? I've seen some listed online for ~1k with various startup errors and always thought it'd be fun to fix one up, but I'm very much an entry level atomic clock enthusiast!

If you have experience with analog and digital electronics it's not very hard. Atomic clocks were part of Hewlett-Packard's glory days and in that era the user got full documentation, schematics, theory, trouble-shooting guides, etc. The manuals are all still available online or on eBay.

There is also the time-nuts mailing list where hundreds of us share questions and experiences with repairing and running old atomic clocks. The archive is 20 years deep so there's lots of good info on bringing dead cesium and rubidium clocks back to life:


The one problem is if the old cesium clock you buy has "run out of gas". The quantum mechanics physics experiment encapsulated inside the tube is a one-way street so once the tube runs out of cesium one has to replace it. The typical solution is to buy a few cesium clocks over time on eBay and mix parts until you have one that works well.

I'll second that... one of the things that you might encounter is a tube that has been out of service for a long time will have quite a bit of gas leaked inside, and you might have to let the ion pump run for days to get the leakage current down to an acceptable level before turning on the rest of the circuitry. My friend actually ended up using a high voltage supply one particularly troublesome tube, but in the end we were able to get it going... it just needed a lot of ion pumping.

The bandwidth of the tube is VERY narrow (on the order of a few hertz, which means you have to be quite close on the frequency of the crystal oscillator before it can lock in.

You also want to be careful when adjusting for the Zeeman frequency to pick the right peak, or you will end up with locked to the wrong transition, and your clock will drift ,although in a very precise amount of drift ;-)

It's fun, once it works. It's easier the second or third time, of course.

Have you had any encounter with astronomical interferometers?

Cesium beam atomic clocks rely on an internal supply of cesium metal contained in an oven. This supply usually lasts 5-10 years, after which the tube containing the physics package needs to be replaced. They are interesting to work on, once you get the hang of it, you can diagnose them in an hour or two.

Rubidium cell atomic clocks, on the other hand have a vapor cell which is a closed tube, and doesn't get expended. Thus Rubidium clocks have far longer service lives. You can find them surplus on Ebay for less than $200. The stability is slightly less, but still far more accurate than most electronics technicians need to do adjustments and calibrations.

[Update - Far more than you ever thought you wanted to know about atomic clocks] The main difference between an atomic clock and a quartz crystal oscillator is that in a crystal oscillator, you have a continuous signal that is generated from the circuitry and passed through the crystal, and amplified in a feedback loop. A crystal oscillator can be made with a single transistor and a few passive components.

In an atomic clock, there is a stabilized quartz oscillator, which actually keeps time, but it is then steered up or down in frequency slightly by using a harmonic of that frequency, along with a very slight amount of frequency modulation, to inject into a cavity containing the atom of interest.

In Cesium (Caesium outside the US) beam clocks, the physics package is all packed inside a large vacuum tube. Inside the tube is an oven with microscopic holes in the top containing cesium metal, the oven is heated to about 200 degrees. A small stream of individual atoms exit. Those atoms pass through a magnetic field which diverts them according to spin. The atoms then drift through a microwave chamber where there is a 9.129 Ghz signal imparted. If the frequency of the signal exactly matches the natural resonance frequency of the cesium atom, its spin will flip.

On the other side of the drift chamber, a second magnet selects only those atoms which have flipped state, all other atoms are diverted and trapped. Then the atoms encounter a hot wire with high voltage where they are ionized, them passed through another magnetic field which is a mass spectrometer, this serves to filter out impurities and contaminant that are present in the tube. There is a fine slit that allows entrance only to the cesium beams (now regardless of their spin), and they imping upon a plate in a photomultiplier.

The net result of all this physics is that if you have exactly the correct frequency, there is a DC current of a microampere or so output, too high or too low, and the output drops quickly. The line width is on the order of a few hertz.

To sample against this, the quartz oscillator is multiplied in frequency the appropriate amount using multipliers and phase locked loops, and combined with a very small amount of frequency modulation at 137 hertz.

The DC output of the tube varies with the modulation... if it increases with increasing frequency, the reference clock is too slow... if it is out of phase, the reference clock is too fast, and if you get a 274 hz second harmonic, you are right on frequency.

They take about 10-20 minutes to warm up and lock.

There are some adjustments, and it is possible to have them locked on the wrong frequency if you aren't careful.

Cesium clocks, used, are on the order of $5000


Rubidium clocks are easier to use... they are also quartz oscillators probing with FM signals, but instead of a stream of atoms, there is a lamp with one isotope of rubidium which is then passed through a chamber with microwaves and the other isotope of rubidium... if the frequency is just right, the light transmission dips by 1%, and this is used to lock the oscillator.

Rubidium standards are about 10 times less stable, but have far longer service lives, and can be had used for about $200.

Cesium beam atomic clocks are available on the surplus market, though not nearly as common or as cheap as Rubidium. The clock in the photo is model hp 5071A and I got it on eBay 20 years ago. The tube will last on the order of 7 to 20 years with continuous use, depending on which type of tube is installed.

This was one of the atomic clocks I used for the time dilation / relativity experiment linked at the bottom of the page.

I loved that story back then... with the advent of optical lattice clocks, and smaller sources, etc... have you done an update to the experiment?

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:


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:


Each experiment was a little different; different combination of clocks, different audience, different mountain, different elevations, different latitude, etc.

Just been going through your blog, and I'm in love with your Synchronome clock, which begs the question (apologies if this is answered somewhere):

Of all the clocks you own, what's your favourite and why?

You can get a rubidium time standard in an IC package.[1] It's about 50mm square, comparable to a CPU chip. Talks USB. Error is under 100ns/day. Time servers should have one.

[1] https://www.mouser.com/new/microchip/microchip-macsa5x-atomi...

Leaves car full of batteries and cables running in front of a lodge for two days. If this were 2020, every three letter agency would be surrounding the place.

I anticipated that, so I had relativity books and physics magazines with pictures of Einstein on the front seat just in case we got stopped and the officer wanted a quick lesson in the fun of measuring time dilation. It was 2005, the 50th anniversary of the first cesium clock, the 100th anniversary of the theory of relativity, so it was also a current news topic.

Actually, a three letter agency did ask me what was going on: NPS, the National Park Service! But they kindly allowed us to park in front of the Lodge for the weekend. The advice I was given by old timers was to refer to the electronics simply as accurate clocks instead of atomic clocks or cesium clocks because many people false trigger on words like cesium or atomic. You'll notice that the official name on the front panel of a 5071A is "Primary Frequency Standard".

Love it!

What's the cheapest clock you can purchase (used or new) where you can still do a similar experiment (i.e. that's accurate enough to measure dilatation over a day/week/month period of time)?

This is interesting, I would like to get an Atomic Clock (5071A Primary Frequency Standard), however I can't seem to find any in the UK (Ebay, LocalEbay or other places)

Are they a restricted item because of the radio-isotope?

Look for any vintage caesium clock made by National, Varian, Hewlett-Packard / Agilent / Symmetricom / Microsemi / Microchip, FEI, FTS / Austron / DATUM, Oscilloquartz, and maybe others. None of them are a restricted item because of a "radio-isotope". Again, cesium 133 is not a radioactive isotope. Atomic timekeeping is based on ultra-precise quantum mechanics, not random nuclear radioactive delay.

Just take your time. Some of us look for months and even years before we find one worth the eBay risk.

There is one fact to consider. I have heard that the 5071A clock is still ITAR classified. It's not because of "atomic" or "cesium", but because the internal design is so advanced that the US decided it was essentially like a military weapon.

If in doubt look for vintage models like 5060, 5061, 5062, or 4050, 4060, 4065 instead of the still current model 5071.

Please tell me more about your atomic clock. Which one did you eventually buy, how much did it cost, and how do you use it?

As a patient hobbyist collector I try to buy one of each. My goal was to learn the history and theory of each one, explore the magnificent design, measure the actual timing accuracy, read as many old books or articles about them, and in some cases meet the designers of the clocks. Atomic timekeeping, like computing, is a new enough field that a few of the old timers are still around.

For the valley-mountain gravitational time dilation experiments that I've done I use 5071A cesium clocks. Even though 25+ years old, no commercial clock works better for this portable application. That is to say, rubidium clocks and older model cesium clocks are not quite good enough to detect these tiny relativistic effects with confidence.

Others here have mentioned prices. Surplus rubidium atomic clocks can be found for as low as $100 on eBay. Surplus cesium clocks range from $1k to $10k, depending on model and condition. Factory new 5071A are close to $100k so no one I knows is crazy enough to do that. If you are very patient and lucky on eBay you will find 5071A for as low at $2k to $5k once every couple of years.

Thank you for your reply! I didn't realize you were the author of the blog until now.

Out of curiosity, do the output formats of the different models differ? With output format I mean the way that the (current) time is represented.

Not being familiar with atomic clocks, my hope and expectation is that the output is something different than a single large integer (compare unix timestamp). :)

The Cesium isotope used in atomic clocks (133Cs) is not radioactive. This seems to be a common confusion between "atomic" and "radioactive".

"When engineering doesn't involve product or design"

More like "armwatch" I'd say.

Wolverine is your brother-in-law? Dude. You better be good to her.

What are the current use cases for atomic clocks?

Adjusting clocks that aren't atomic.

Cellular base stations used to have rubidium sources disciplined to GPS as a reference frequency for synthesis. AIUI, put the expensive, precise part in the base station, have the cheap handset lock onto that. Free accuracy. Not sure if they still do it that way. That's where all the cheap used rubidium sources on eBay come from.

Also GPS (and other GNSS system) satellites themselves have atomic clocks.

Low latency time source for stuff like Spanner that appreciates times that are the same across disparate data centers?

They have applications in the database space, e.g. see Google's spanner: https://www.theverge.com/2012/11/26/3692392/google-spanner-a...

Do these "atomic" clocks that get synched by an external ER source account for the (minute) travel time of said ER source? I'm assuming they do.

how much does it weigh?

About 70 lbs (30 kg), so not difficult to lift or carry with two hands. But it took a real man (my brother-in-law, construction worker) to hold it outstretched like you see in the photo.

And all this time I felt like a total wimp for finding that photo really daunting. 5071a is lighter with the batteries out, but not that light! :)

Less than the embarrassment of being the only person on your block without one.

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