Why scientists say we need to send clocks to the moon | Hacker News Reader

Why scientists say we need to send clocks to the moon

cnn.com

35 pointsantongribok29 comments

> On the lunar surface, a single Earth day would be roughly 56 microseconds shorter than on our home planet — a tiny number that can lead to significant inconsistencies over time.

Given that there are 86,400,000,000 microseconds in a day, we will need a leap year every 1.5 billion years to stay in sync. I think we’ll be fine.

Or more realistically, a leap second every ~49 years.

> A network of clocks on the moon could work in concert to inform the new lunar time scale, just as atomic clocks do for UTC on Earth.

Presumably some clocks would have to be on the surface of Luna facing Earth/Terra, and some clocks on the side facing away, so that the gravity well of the planet could be averaged out.

And if we're going to go through the trouble/expense of sending some gear, it might be useful to pack multiple instruments: if timekeeping is a long-term need, another long-term instrument could perhaps be a seismograph to measure for possible 'moonquakes'. Multiple clocks could mean multiple seismographs for better triangulation of events.

Idk. Is this actually needed in practice? Most RTC chips are off by several seconds per day. If you need accurate time you sync them to an outside source (a network time server or GPS). Does it matter if your oscillator is 56 microseconds off because of relativistic effects if it is off by multiple seconds because the chip is cheap?

You need some sort of network connections back to Earth anyway, can't you just spare a few NTP packets here and there and be done with it?

I haven't read the underlying white paper yet and maybe that will change my mind, but the CNN article hasn't convinced me yet that this is a problem worth worrying about.

I think the point is that if we're standardizing a lunar time, it should be done with as much precision as possible. Kind of like how we have very precise benchmarks for what our units of length are in terms of fundamental properties.

Personal computers don't care much about timing down to the nanosecond, but scientific pursuits often need that level of precision, standardizing timing with high precision would make it easier to perform experiments that depend on time correlated comparisons across the lunar surface.

It's still a weak case in my opinion, things can always be refined later.

GPS is “global” ie: Earth centric. Say you want to create LPS (s/Global/Lunar/ ~ GPS) then, yes. It matters.

> Is this actually needed in practice?

For scientific experiments of the type one might be running on the moon? Almost certainly for at least some of them.

What's the difference between correcting a lunar-bound oscillator for this new time standard versus correcting it for UTC?

> Is this actually needed in practice?

No.

The actual problem is a political one. Past missions have used the local time from where the mission was based. They want an agreed time for international missions.

This talk of ultra precise clocks being sent to the moon seems very unlikely to happen, as it isn't relevant to solving the political problem.

Then it's a lot of hot air where UTC would suffice.

"Private moon lander will carry Nokia's 4G cell network to the lunar surface this year" https://www.space.com/nokia-4g-cell-network-on-the-moon

Doesn't 4G include a time synchronization service for receivers with no GPS; and is the SI second unit (which is currently defined in terms of the cesium decay) const hardcoded in the new system?

They are talking about establishing a moon coordinate time.

https://arxiv.org/abs/2402.11150

Cell sites synchronize time with the network, like you phone does but they are talking about Earth time.

> Doesn't 4G include a time synchronization service for receivers with no GPS

Yes, but I believe accuracy is on the order of minutes.

> is the SI second unit (which is currently defined in terms of the cesium decay) const hardcoded in the new system?

Are you saying that a moon second should be defined to be slightly longer/shorter than an earth second to compensate for the relativistic difference? That wouldn't help, since relativity affects all physical processes; if you do that, all your other measures would be off too, including e.g. length (defined as the distance light travels in a given unit of time).

"Matrix Theory: Relativity Without Relative Space or Time" https://youtube.com/watch?v=B94o-P93ExU&

Does the time difference due to time dilation due to relativity change suddenly upon orbiting or landing on another planet spherical reference frame?

Not suddenly, but gradually as you enter/leave the gravitational well.

The dominating relativistic factor is the gravitational potential (ie general relativity), not the orbital velocity (special relativity), as far as I understand.

GPS compensates for both by offsetting the frequency standard of the satellites’ atomic clocks accordingly.

4G and 5G need devices to be synchronized to each other by like 1.5 microseconds or they'll start getting a lot of errors. They do a lot of time division duplexing, so if things are not well synchronized people start talking all over each other.

Now, that's just synchronized with each other, so if the Moon is off by whatever difference in time that's not a problem. It is not like that 4/5G cell is also participating in the same RF environment as cells on Earth.

Yeah, that’s what I mean: They don’t need an absolute time reference; they can just respond with whatever delay/frequency offset compensates for the one observed as they communicate.

S-CDMA did have close timing requirements between cells, as far as I know, but the GSM/3GPP family of standards never did.

TDD networks (where the same frequency is used for downlink and uplink, but with timeslots for each) do need tight timing between cells though, but it doesn't have to be absolute - just in sync with each other so one cell doesn't blast out a full power downlink while adjacent and nearby cells are quietly trying to listen for the (much quieter) uplink transmissions of their user devices.

> Seconds tick by ever so slightly faster atop a mountain than they do in the valleys of Earth.

Did they push an update with new laws of physics, or have I completely missed something?

This is because of relativity and the fact that there's more force at in the valley than the mountain.

https://en.wikipedia.org/wiki/Time_dilation

What about the fact that you are moving faster when on top of a mountain than in a valley due to the rotation of the earth and being farther from the center of rotation?

Does this also have an effect on your relative time?

Yes, it makes the equipotential surfaces of Earth's gravitational field (the surfaces on which time ticks "at the same rate") ellipsoids instead of spheres. The "geoid", which is the standard such surface that defines UTC on Earth, is the equipotential surface that averages to the Earth's sea level, and is 13 miles further from Earth's center at the equator vs. the poles.

I'm guessing it's relativity doing its job.

gravity?

Relativity: The Special and the General Theory (1916) Einstein, Albert

https://www.abebooks.com/9780517884416/Relativity-Special-Ge...

That's a 404, sad.

Gravity decreases with distance from the Earth’s center. The lower the gravitational potential (the closer the clock is to the source of gravitation), the slower time passes, speeding up as the gravitational potential increases (the clock moving away from the source of gravitation).

For an object which is accelerating time passes slower. But an object on a mountaintop isn't accelerating, because the mountain has an upward counter-force.

What am I missing here?

They did, about 108 years ago.

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