A clock that can detect tiny shifts in the flow of time itself
npr.org
npr.org
Right now, on the top of Mount Everest, time is passing just a little bit faster than it is in Death Valley. That's because speed at which time passes depends on the strength of gravity. Einstein himself discovered this dependence as part of his theory of relativity, and it is a very real effect.
"Just to be clear, the clock in Death Valley appears to run slowly from the point of view of the eagle-eyed observer standing on top of Everest.
To a person in Death Valley, the clock appears to tick at precisely the rate it would appear to tick if both person and clock were on top of Everest."
But that's not communication. It's like slicing a coin down the middle without looking at it, hiding each half in a box, and giving one of the boxes to someone going to the moon. A week later, you both open your box, and you know exactly which side of the coin the other person has - but that's not communication.
(Someone smarter than me, please correct whatever mistakes I've made, and explain this in a better way.)
http://en.wikipedia.org/wiki/Spin_(physics)
"Although the direction of its spin can be changed, an elementary particle cannot be made to spin faster or slower."
Can you expand on the last part about statistical correlation?
Suppose you and I both get a coin and we flip them. You then turn over your hand and reveal heads. If the two coins are "entangled", there is now a greater than 50% probability that my coin will be tails, which would not be the case if the coins weren't entangled.
Quantum entanglement works essentially the same way. "Flipping the coin" is equivalent to observing some state of the particle, like spin. In both cases, entanglement is evidenced by the correlation between the states, nothing more.
Now, imagine we had entangled coins and we were in separate rooms. We flip our coins and you observe yours, noting a heads. In my room, I can't know what you did or didn't observe. So when I observe my coin and see tails, I don't realize it's a consequence of your observation. From each of our individual perspectives the coin flip looks perfectly random. This is why information transfer isn't possible through quantum entanglement.
Very interesting, thanks a lot to everyone who responded!
Isn't that unknowable? And aren't both situations (fixed vs undetermined) effectively the same from an observer's point of view?
Relativity of course makes a hash of that.
Still it's as close as you'll get with a classical analogy, and if you only have access to a fixed basis is entirely correct. Expanding beyond the classical explanation is only truly necessary when you start measuring in different bases.
Thinking about this makes it clear how communication is impossible. You will always get both answers - for different values of "you".
I'm probably mistaken, because if this were possible, then you could use it as a 1-bit communication. "If you detect that I've opened my box, then blow up the moon. Otherwise, do nothing."
Time to re-study quantum mechanics... It's strange how easy it is to forget almost everything just by not thinking about it for two years. MIT OCW had some pretty great online courses on the subject.
The answer is no, and so the analogy doesn't break down. You can't observe the other person except by speed-of-light information transfer.
If you ran two two-slit experiments on entangled particle pairs, shouldn't you be able to detect if your counterpart was observing theirs?
How would you know without observing the particle yourself? In which case, how do you know if it collapsed because of your observation or the other person's?
Not an expert by any means, but from what I'm told you can't use it to transmit information, so that's how I think of it.
Has a nice analogy, that being two radio listeners do know what each other are hearing which is cool, but can't communicate anything between them.
I would have to think about the effect of creating a one time pad.
It's kind of like when you go to a doctor and have tests done for a disease if the results are bad, positive, they call you and if the results are negative no call.
At some point a lab tech checks the sample and if it's negative for the disease they know at that moment but you know by the lack of information at some point in time; i.e. no phone call or letter.
The negative results are more like the quantum communication due to the lack of interaction than the positive results. Mine may not be not the best analogy.
(That should also make it clear why you can't use entanglement to communicate instantaneously: the other side must get the same results whether they measure before or after you've sent your message. So if entanglement allowed communication instead of just esoteric types of coordination, you could use it to send messages backwards in time.)
I would imagine magnetizing a mechanical watch would really mess things up, so at a "just under really messed up" threshold it would probably have the claimed effect.
You may be thinking of YIG garnet oscillators which are very dependent on magnetic field but they haven't been common tech in maybe 30 years. Even used on ebay they cost more than modern new microwave VCOs which is annoying. I have a box of old YIG osc in my basement which I actually used for ham radio experiments before modern solid state VCOs took over.
Also, placing a small variable capacitor in series with the quartz is an easy way to tune it, if it's too fast. I actually have a wall clock that looks nifty but is too fast, and I'm gonna tune it by trial and error, inserting small caps in series with the crystal.
Also, gravity: https://www.youtube.com/watch?v=zILwgQhjC_Q
And death valleys ticks slower. He's at sea level, which is higher than Death Valley.
This seems a bit sensationalist. The new clocks will be more accurate than any we have now, for existing applications. Those who develop new applications that actually use the increased precision will just need to be more careful about which frames of reference they use. Those who built GPS already solved some version of these problems.
It's not that timekeeping gets meaningless- it just means that there is a threshold where the variability of unsynchronized clocks disrupts certain highly precise problems.
If two clocks are at different heights, they can compute the differential in their rates and agree on a time unit. Sure, it might be 1 quadrillion ticks in on one clock and 1 quadrillion and 1 ticks on the other, but it's still a 'second' so long as both sides see it as simultaneous.
With all this uncertainty, all you could do, after getting two clocks to approximately the same altitude, would be to say that clock A is currently running a factor of X faster than clock B. What you wouldn't know is how much of the delta was due to altitude, how much due to unequal density underneath, and other such factors, compared to how much was due to the internal differences in the clocks themselves that you were trying to factor out by using multiple clocks.
As a geologist, I really wish that were possible, but what you're describing is just far too small of an effect to be detectable from the surface. Gravitational acceleration follows an inverse square law. Any signal from flow in the outer core is completely swamped by the far larger (at the surface) changes from things like local hydrological conditions, ongoing human activity, etc.
On the bright side, though, we can already measure changes in the acceleration of gravity incredibly precisely without directly using relativistic effects. Even from orbit, things like regional flooding and changes in the mass of ice sheets can be detected purely from changes in acceleration of gravity. Have a look at some of the work that came out of the GRACE mission (also see the ESA's GOCE satellite): http://www.jpl.nasa.gov/news/news.php?release=2004-224
So even if this difference is undetectable for geological purposes it could very well screw up really accurate time measurements. It all depends on whether or not those effects compound or cancel out, in the case of gravity they will likely only compound.
I'm having a lot of fun being wrong today!
EDIT: Aaaand that's from 2004. Which means I really wasn't paying attention while in college. Considering my roommate went to JPL to work on a sister project, I feel a bit of shame just learning about it now.
More pragmatically, we'll probably agree on a standard centralized time (i.e. International Atomic Time) and everyone will translate between their own locally perturbed time and the standardized time when synchronizing.
In a sense, this is not a property of the clocks, but rather just the way nature is. We can't keep these clocks in sync at various places around the world, because the thing they measure is not in sync at various places around the world. But, we've known that for about one hundred years - what's the big deal?
nullc is right, this is a bad article.
Sure, the more precise your oscillator is the more you need to worry about other effects. Basic cesium beam clocks are precise enough the altitude based corrections for relativity are required to achieve full accuracy. Each km of altitude is about 10ns/day of slew, ... I can measure this myself with equipment I have at home (I have an unusual home).
So sure, when you start making optical clocks with accuracy in the 1e-18 land then external effects may well be much harder to correct for, e.g. tides have an effect at the 1e-17 level ... but such a device could still keep time better than prior techniques. The "They just may not be able to tell us the time" is rubbish hyperbole that just serves to confuse readers who don't already know the subject well.
http://amasci.com/elect/charge1.html
It's not exactly 100% current (pun intended) but it makes the point nicely.
No one cares about your intellectual superiority, or the fact this might be a special interest for you. This article wasn't written for you, you already know and understand its content. There is literally nothing you could have got from it.
I on the other hand enjoyed the article. This isn't a field I'm familiar, so in general my knowledge was increased. That some concepts are misrepresented and dumbed down is obvious, but this is far out weighed by the general knowledge that was imparted.
Writing is all about picking an audience and conveying it effectively to the audience. It's fine to point out inaccuracy or build on the finer points, but don't be aggressive about it and don't insult the article because it offended your superior intellect.
I didn't go on and on in detail about systemic and random effects, linking to NIST tech reports on all the corrections they have to do on primary standards (http://tf.boulder.nist.gov/general/pdf/1846.pdf?origin=publi... on the redshift error at NISTs boulder facilities; and http://tf.boulder.nist.gov/general/pdf/2704.pdf on the sources of error in the F2 primary reference, see section 3.2 on relativistic effects), or pointing out people's amateur time keeping experiments where they demonstrate relativistic influence on decades old hardware ( http://leapsecond.com/ptti2006/tvb-project-great-ptti-ppt.pd... (this presentation is long and a ton of fun)), etc. or all the other bits of trash I could have pulled out to demonstrate knowing something here... because that wasn't my goal, the only point I was trying to make is that the article was likely to make many readers _less_ knowledgeable about the subject.
(But I will give those links now, because this argument is boring and time is neat!)
Perhaps you have enough background that you were not thrown off by the seeming claim that improved accuracy somehow makes these experimental references _less useful_, and you already know that relativistic effects aren't unique to optical lattice clocks and already must be compensated for, but I am sure that this is not universally the case.
What I get from the article isn't nothing... Potentially I get community around me which is less informed than they started and all that entails. Perhaps that's compensated for the fun they had reading about an interesting subject? or the additional learning they do after? I don't know, but I think the article could have been just as enjoyable without the bogus mystique that makes it misleading.
But if pointing out an article was, in my opinion, potentially misleading makes me everything that is wrong about Hacker News, I'll wear that proudly.
I, for one, think it's more likely the case that crappy shock headlines like "end time as we know it", constructed drama, and false freshness are a bigger drag on HN (and wider society) than any of my posts are likely to be... but to each his own.
Your tone and delivery is a bit offputting.
And the thing that I didn't see you comment on that I found particularly fascinating was that the increased resolution of these clocks is such that you can discern differences in relativistic effect between floor and wall. That changes how I think of time.
(My favorite headline that I read sometime around 1965 is "Dating Events in the Vicinity of a Leap Second.)
Wrt relativistic effect, the first NIST paper I cited shows that they needed altitude uncertainty less than 1m to avoid redshift from dominating the clock's error. The second shows that for the improved F2 reference redshift is one of the largest sources of uncertainty (and the corrected part is orders of magnitude larger than the other listed systemic errors). It's really cool, I agree. I'm happy to have people share in enjoying that, but sad about whatever causes the press to always have to present things as new and categorically different than what came before.
There are a number of really cool things they didn't mention: For example, these optical lattice clocks that they're talking about are solid state-- involving mostly only lasers and vacuum cells. Unlike cesium based atomic clocks, they may have reasonable prospects of being mass produced inexpensively in the future, and efforts to do this are being funded by DARPA (useful for many military applications, like jamming systems and anti-jamming, navigation, and various sensing applications). So unlike the state of the art atomic references these things may someday show up in very inexpensive equipment, and allow for some fun science experiments, improvements to reliable distributed systems, long baseline amateur radio astronomy, etc.
Or, Tom Van Baak measuring gravitational redshift with a minivan and some old HP cesium beam clocks. okay, not "floor to wall", but if you haven't read his presentation on it, you should, it's a load of fun and IMO, more accessible in that it's not talking about technology that exists in a rats nest of cables on an optical table in a single lab, but just old junk you can find surplus. :)
This article is poor scientific journalism. "New clock may end time as we know it." Seriously? How is that remotely true? There is nothing in this article that justifies that headline. If anything, this new clock affirms our understanding of the nature of time, pointing out the minute changes that we expect to see but have not had instruments sensitive enough to measure until now.
But that desire to pin down the elusive ticking of the clock may soon be the undoing of time as we know it: The next generation of clocks will not tell time in a way that most people understand.
Undoing of time as we know it? Will the existence of this clock somehow magically make all other clocks in the world turn incomprehensible? No, the truth is that most of society won't even be aware of this "undoing of time as we know it."
And on the subject of telling time in a way most people don't understand, that is already true of the cesium clocks they mention. We already have to deal with relativistic drift, of which most people are ignorant.
But this new clock has run into a big problem: This thing we call time doesn't tick at the same rate everywhere in the universe. Or even on our planet.
This line makes it out like we didn't know about relativity. Also, I hardly think more precision is a problem. If anything it gives us power to do things we never have done before. Technologies like GPS wouldn't work without the current generation of clocks. I'm excited about the new possibilities that an even more precise clock will open. It's not a problem.
They just may not be able to tell us the time.
What?!! This is incredibly misleading. The new clock will be just as able to tell time as all of the clocks we've had up until this point.
I could go on, but nullc is right, this article is crap. It is dripping with sensationalism.
You already mention GPS later in your comment, so it isn't that necessary, but I want to emphasize anyway: we are not only "dealing with", we are "making use of" it. Things we use every day would be impossible, if we wouldn't have precise enough clock to measure all that stuff already.
In turn, there were wrong, imprecise and misleading statements in that article, which does make it bad to some degree. It's not the worst of it's kind, but still pretty bad. Actually I would consider it harmful, to write something that is easy to pick up for uneducated people, that gives them impression that they understand it while giving them wrong impression about the subject, that is, being misleading. I'm not sure I could appreciate that even if it was impossible to write easy-to-understand useful quasi-scientific articles/books, but as I've seen them I must conclude it's not impossible, so that makes people writing misleading stuff with attractive headlines even more guilty.
Your comment in it's turn contains nothing, but claiming somebodies comment is "worst" based only on that being not populistic enough. I would say it makes your comment "the worst kind of comment and what is wrong with Hacker News".
> No one cares about your intellectual superiority, or the fact this might be a special interest for you.
Comments like this may often offer a fair bit of factually correct information and interesting insights. Opposing views, strong opinions, critical thinking—all this seemed more like a “feature” of HN rather than a “bug” to me.
(Sorry, I may have accidentally downvoted you; this was not intended.)
The GPS satellites already had to correct for the passage of time being different for them relative to the surface of the planet. Decades ago.
Does anyone remember the name of that article that describes various clock synchronization techniques? I think it went something like "more than you ever wanted to know about computer time" and was based on an earlier similar title about another subject.
38us/day slip is huge. That's 6 miles of GPS positional error. So GPS was designed with an correction and adjustment scheme to deal with this. So it's been a practical problem for decades.
Do you know about any specific painful effect that was revealed only after general relativity was tried to be applied to GPS satellites in particular?
We have already had clocks for decades whose perturbation by relativistic effects matters. Every GPS receiver (commonly found in now inexpensive technology in use by millions of consumers) makes relativistic corrections to the time base received from satellite signals, without which the positioning would be hopelessly inaccurate.
Why would this clock "end time as we know it", when millions of users of GPS navigation still have a naive view of time.
Special relativity covers time dilation due to relative velocity, general relativity covers time dilation due to gravity.
Wikipedia says: "Time dilation is caused by differences in either gravity or relative velocity. Both factors are at play in the case of ISS astronauts (and are actually opposing one another)."
Why wouldn't the same apply to the top of Mt Everest. Sure, unlike the ISS, Everest is actually attached to the Earth, but still it is traveling a longer distance than the oceans over the same period, and therefore would be moving faster in relative terms AFAICS.
EDIT: I guess compared to a rotating reference frame this isn't true? Clearly this isn't my area of expertise.
Popping that into MS Mathematics using the Lorentz equation, we see that gamma now is 0.2 billionths above unity. Which is super small, but definitely detectable. In contrast, 1.5mph doesn't yield anything - I'm guessing (1.5/670616629)^2 is nearing the floating point epsilon (I guess MS Math is using floats?) [1].
[0] http://www.wolframalpha.com/input/?i=orbital+velocity+of+ISS
[1] http://en.wikipedia.org/wiki/Machine_epsilon
EDIT: Right, as noted elsewhere, this assumes Everest is at the equator. Which Wikipedia tells me is not a great approximation for where Nepal is. So assume that Nepal ended up in the wrong place for a little while, and the arguement holds. Otherwise, we've got a bit more calculating to do.
What's the right answer if someone makes this counterargument: in the inertial reference frame of the Earth the Earth is not rotating, so neither the sea nor Mt. Everest are moving at all, so there is no relative velocity difference and no time dilation from velocity difference.
Btw, I'm not sure about the phrase "I'm guessing (1.5/670616629)^2 is nearing the floating point epsilon" (unlike physics, I do have a fair amount of expertise in floating point formats). The way you put it suggests that the "machine epsilon" represents the smallest increments that floating point can represent, and therefore suggests that (1.5/670616629)^2 represented as float will underflow to zero or be so inaccurate as to be meaningless (sorry if I'm misreading you).
I find the definition of "machine epsilon" given on that page somewhat confusing. I think it's more intuitive to think of floating point error in terms of percentages. That page says the "machine epsilon" for float is ~1e-7; an equivalent and IMO more intuitive way to say it is that float is accurate to ~0.00001%.
The range of float goes far smaller than 1e-7; FLT_MIN is ~1e-38, and that's not even considering subnormal numbers. So float can very easily represent the results of (1.5/670616629)^2 ≈ 5e-18, and like any other representable float this is accurate to no worse than ~0.00001%, which is pretty decent.
As for the counterargument: I don't think that works. And that's taking into account the harrowing liberties I'm willing to assume for the sake of a physics argument. In the inertial reference frame of the Earth, the Earth is rotating - as in, there's rotational inertia in that frame. This has measureable effects on stuff, from time dilation from velocity (very small) to time dilation from gravity warping spacetime (noticeably larger, but still small).
Put another way, here's a thought experiment. The mantel of the planet is molten rock, and can be treated as a viscous fluid. If the planet were spinning, the fluid would bulge out at the latitudes where the planet is spinning fastest (centrifugal force stuff). Otherwise the planet would be a sphere. This is directly measurable, and - in fact - the planet's a sphere. Mostly. It bulges out a bit at the equator [0].
So if you're careful, you'll note it's never fair to say you're in a non-rotating inertial frame on earth. But in practice it almost never matters. Unless you're doing something crazy like measuring femtillionths of a second with one of the most sensitive devices we can build - then we start being a bit wrong. Or you're just trying to be accurate with your GPS satellites (which are way higher and faster than a mountain top).
I rant a bit about this, as this sort of counterargument comes up a lot. I think it's due to the completely unreasonable mismatch of scales people are used to. Feynman ranted a bit on it about QM, and we're running into it here with relativity [1]. Here we're trying to talk about something reasonably, and the levels of precision are completely unreasonable: we're talking about a couple mile's difference over the span of thousands of miles to have an effect on the order of a few (bi|tri)llionths of a second; how do you keep such a scale in mind? It's like the silly analogies of hitting a baseball in NYC and nailing a bumble bee in San Fransico for precision. By the same token, we're hurtling through space, whipping around the sun and being wobbled so hard by the moon the ocean sloshes over our beachfronts. And that seems perfectly normal, even though it directly implies enormous forces at work.
/rant (and sorry for that - I find this sort of thing facinating)
TL;DR: I think that counterargument is simply wrong. But completely understandably so.
However, the effect of gravity is much more significant (if it weren't, the mountaintop would be flying out into space due to centripetal force).
Since acceleration is change in velocity, it's impossible to have absolute acceleration without absolute velocity. You can only say that it's accelerating compared to X.
There's definitely a constant vector difference between you and Everest's peak, and that's likely what you're thinking of. But Everest itself is under a greater strain to maintain that vector compared to you. And that's actually a measurable difference (where measurable is on scales of stupefying precision not normally used in everyday life).
And as another noted, it really should maintain that relative vector - if it didn't, the earth wouldn't be in steady-state and you'd have a changing position vector between you and it, and that'd imply one of you was moving... hopefully it's you.
(Let's ignore all the techtonic complications, as that really ruins the simplicity of the argument :)
EDIT: As noted elsewhere in the thread, Everest isn't on the equator, and - if I may be bold enough to presume - you likely aren't either. So it's not clear if you or Everest is on the outside of the spinny-go-round in the analogy.
For the purposes of wondering if velocity or gravitational effects on spacetime are the dominant factors, this has no effect. If you're actually interested in the effective relative centrifugal forces between you and Everest, then it's damn near everything.
It's actually due to acceleration, whether from gravity or from motion. And, given earth's rotation, points on the surface accelerate (except for the poles).
But, as I pointed out elsewhere in this discussion, it's distance from the axis that determines acceleration from rotation, not distance from the center of the earth. (Distance from the center does determine gravity, though).
And so is a mere 0.15% increase in height. But compare the gravitational acceleration at these two heights:
[2] Everest = 9.76322 m/s^2
[3] Sea Level = 9.831 m/s^2
Sea level is 0.69% stronger. That's somewhat more significant, and given that gravitational energy follows the inverse-square law, it makes some sense that it would amplify differences more than the linear effects of speed.
[0] https://www.google.com/search?q=radius+of+earth
[1] https://www.google.com/search?q=height+of+everest
[2] http://www.wolframalpha.com/input/?i=gravitational+accelerat...
[3] http://www.wolframalpha.com/input/?i=gravitational+accelerat...
[4] http://www.wolframalpha.com/input/?i=lorentz+factor+at+1.5+m...
[5] http://www.wolframalpha.com/input/?i=rotational+velocity+of+...
EDIT: forgot the reference to rotational velocity (and spelling). Note that the point is scale here. Gravity is n^2 versus velocity's n. It'll have a stronger effect in most cases (and any where speed overcomes, well, we call them relativistic speeds, and they'll often be some appreciable fraction of c, like 1% or more).
DOUBLE EDIT: 'AnimalMuppet makes a good point that the entire estimate for Everest's rotational velocity is flat out wrong. So I've changed my premise to match my initial incredibly incorrect assumption.
If I were to attempt to be correct about the problem, we'd see that Everest is cos(27.9881 degrees [10]) = 0.8837 [11] as fast as the equator, which sorta blasts out any height changes by long shot. (Note I'm assuming the earth to be a perfect, frictionless sphere here, and definitely not a oblate spheroid.)
The point stands, but now I'm interested in what kind of cones we could make where relative heights and latitudes yield identical velocities. Prolly as useful a question as most XKCD What If? notes :)
http://www.leapsecond.com/time-nuts.htm
I hang out more with the volt nuts. (how bout that LTZ1000A voltage ref, eh? got three in my basement, because two aren't very useful LOL). I do not remember who begat who but there is probably commentary in the archives of both, if you go back far enough.
How do they do these differences? Surely they would need a second, equally accurate clock to compare it with?
You would have to have 2 of these clocks, and compare them.
But probably all they did is just calculate the expected difference.
What is this supposed to mean?
We can measure time with non-human instruments and time is at the core of physics. How can it be a human construct?
Sure, hours, minutes, days, etc. are human constructs based on movements of the earth, but time itself?
Put an array of these in a box. Then computationally map the changes in gravity from each clock. Bingo presto, you've just created a 3-D model of the mass in the local area.
We've been able to measure gravitational acceleration very precisely for over a century now.
An 80-year-old LaCoste & Romberg gravimeter will do the job quite nicely, though it's slow to use and you need to know the elevation you're taking the measurement at very precisely. (Interestingly, the ones made before ~1950 are more precise than the ones made in the 60's to 90's. It's basically a very well made and well calibrated spring. When they switched to mass-producing them, the quality fell.)
Now you have a second problem, though... Regardless of whether you've measured things through gravitation acceleration or time dilation, going from the measured effect back to an actual mass distribution is a non-unique inverse problem. There are an infinite number of equally correct solutions (and a larger infinite number of incorrect ones). You can make a pretty good guess at what the mass distribution by applying reasonable a-priori constraints, but there's no single unique way to get the mass distribution from the gravitational effect of the mass.
If there's any application for pure gravitational sensing that can resolve the position of a higher-density mass to a few centimeters over distances of perhaps a meter or that can make simple statements about a meter-scale mass distribution, please drop me an email. We've tabled the project because we don't know of a single use for it, academic or commercial. Device cost would be in the low hundreds of thousands of dollars and require careful operation. We've thought hard about this, but haven't ever found an application for which some other sensor wouldn't be far more appropriate. X-rays, neutrons, resistivity, clever weighing, optical techniques, microwaves, touch probes, three-year-olds, lemurs, optical imaging, you name it, it's probably cheaper, better, and faster.
Generally speaking, though, accuracy of the sensor is rarely the limiting factor in gravity surveys.
For land-surveys, it's precisely knowing your elevation, correcting for the "unwanted" mass distribution around you, etc. For mobile surveys, it's correcting for the acceleration of whatever vessel the instrument is on. Any ideas you might have for improving the state of the art for the mobile case would probably be _very_ marketable.
On a separate note, though, the inverse problem, while fundementally very non-unique, is still solvable for many practical problems (e.g. we know the range of density of the materials involved and we can make a reasonable starting guess for the distribution of mass). Regardless, you're usually interested in distinguishing between a few scenarios that can be easily forward-modeled.
A lot of the imaging market appears to come from security/defense applications, either in portal-monitoring or for IED detection. There are defense contractors working on both. The former is easily spoofed (put your uranium pit in a styrofoam sphere in a truck full of grain, done), and the latter is hard to do at speed in a rugged environment.
I've spent a lot of time trying to figure out how to do gravitational imaging on the sub-meter scale, and while it does work, it's hard to get sufficient image resolution to be useful for anything other than a party trick.
If our other science weren't more interesting, I'd be doing it for fun alone. Burning 3-6 months on the project to assess feasibility was as far as I wanted to go without a clear exit strategy.
I wonder, however, if the problems you are describing are things that would work themselves out over time as the equipment improves? The precision of these clocks sound like they're many orders of magnitude better than the old gravity sensors. I think. Perhaps all we'er waiting on is some kind of crazy technological magic over the next 50 years that would involve miniaturization, improved accuracy, and an array of a million or so. (All of which I just made up)
To do it with a pair of clocks, there's a long way to go, as they measure differences in depth in a gravitational potential. If you change your distance from Earth's center by a meter, your gravitational potential will change by 9.8 m^2/s^2. If somebody heavy (100 kg) and spherical (I'm a physicist) stands a meter away from you, your potential will change by 7 x 10^-9 m^2/s^2. In short, the clocks need to get ten million (they can see a 1 cm height change) times better to sense a nearby person. Clever trickery with an ensemble of clocks will make that easier, but not by 10^7.
Not impossible, just hard. The fact that the time/frequency teams have encountered gravity, in particular through the gravitational potential, has just made their lives quite a bit harder. They now need to know a lot about the relative locations of other clocks and the mass distribution within the earth to make substantial strides forward. It's an incredible feat to have gotten to where they are, and a major challenge for the future.
This is the same type of stuff they are dealing with in the gravitational wave experiments like LIGO and VIRGO. They have amazing sensitivity but it is still lost in the noise. They don't claim to be able to make measurements to accuracy which is lower than the noise levels. And I don't see why these clock people should either.
edit: By which I mean that this clock could be precise to 10^-16 but be off by 10^-14 due to a mis-calibration.
from spacetime import spacetime, spacetimedelta
then_and_there = spacetime.here_and_now() + spacetimedelta(years=1, x=5, y=3, z=-2)
(edit: formatting is hard)
Say we have two people, Bob and Jane. Bob and Jane blink at similar rates when they are standing next to one another. Both Bob and Jane have the accurate watches described in this article.
Bob then takes a trip to space traveling near the speed of light. Right when he takes off a laser initiates both stopwatches (one on bob's wrist and one on jane's). At some time (...), from an equidistant point to both Bob and Jane a light originates and then stops both Bob's and Jane's wrist watches. Btw, both Bob and Jane have been blinking the whole time their watches have been running.
The questions:
Upon stopping the watches:
1. Do Bob and Jane see a different amount of time passed on their stopwatches? (most of us are not in contention about this one)
2. Have Bob and Jane blinked the same number of times?
For comparison, Planck time = 5.39106×10^-44 sec
We're pretty far from measuring time precisely.
Time base correctors are also cool: the first generation ones were rack sized and used core-memory. I think they were an enabling technology for on-location news.
You can buy your own: rubidium standards sell for less than $300 on ebay.
"but what time really is, is a question that I can't answer for you."
O'Brian almost correctly answers that right off the bat:
"My own personal opinion is that time is a human construct"
Time is a measurement, nothing more. It's not magic, it's not difficult to understand, it's not separate from reality.
It'd be like calculating how long it takes you to walk around your coffee table, and then being confused by what it means or what that measurement consists of.
The fact that time passes at different rates at different locations is already account for.
My Android phone is very frequently 30 seconds or more off from my friends' iOS phones. I don't even...
In theory.
In practice, I'll bet it stops keeping time before I do.
Check out the Long Now's 10,000 year clock project. http://longnow.org/clock/
Velocity is a function of distance and time. How can time have a velocity? Couldn't you also say the distances in the clock stretched or shrunk by the lorentz factor?
We need to fund this!
Is it loosing or losing? Are both correct?
Or it could just be the auto-correct (now in desktop apps as well, not just mobile).
[1] I hate 's for plurals, but how else would you write "esses".