NIST Pair of Atomic Clocks Reveal Einstein's Relativity at a Personal Scale | Hacker News Reader

NIST Pair of Atomic Clocks Reveal Einstein's Relativity at a Personal Scale

nist.gov

118 pointslelf55 comments

When I first learned about this in college it really really bothered me. (obviously it still does to some extent)

That time is not constant on a scale that lets us measure it from space vs the earth much less from upstairs to downstairs is somehow deeply unsettling. If you're traveling at 10 m/s as you go out of the gravity well you go faster and faster because time speeds up, but your velocity per unit time stays the same.

Something that is really hard for my brain to wrap around in any meaningful way.

During some radio work this became actually a thing in that if you're doing time-of-flight measurements of originating RF energy and one receiver is in a platform flying above and one is on the ground, at high time resolutions you start having to correct for difference in the rate of time. This is definitely something you have to do if you want to get really accurate time from GPS satellites. So freaky it hurts.

It didn’t bother me so much when I finally understood that the duration perceived by an observer is simply equal to the path integral of distance along their worldline.

When you think about it, you will realise there’s almost nothing else it could reasonably be.

I don't get what this means at all. Can you try to Eli5

To figure out how far a distance you traveled to get to my house, I need to know which path you took.

To figure out how long a time you traveled to get to my house, I need to know which path you took.

Our ordinary intuition is instead,

"To figure out how long a time you traveled to get to my house, I need to know the time on my clock when you left and the time on my clock when you arrived."

But that turns out to be wrong if you care about a high degree of precision or if you were traveling very fast.

Also because there is no meaning for the first value (there is no “synchronicity” between separate clocks).

Might be reasonable, but it means that comparing timestamps between two observers requires knowing the observers' entire history back to some reference timestamp no? Still a headache.

Also, if we added the speed of light to an object's speed you'd get really weird results like you'd see a car coming towards you before the car actually arrived.

Trying to stay alive in such a world would be quite a challenge.

All the cars I see coming, I see them before they arrive

Cars are not superluminal.

I meant like this: https://youtu.be/_pEiA0-r5A8

You would see the image of a car before the actual car arrived

How do you feel about it vs leap seconds vs leap smears? I really wish timestamps were tied to TAI instead of UTC time (and therefore subject to leap seconds).

I was pretty bothered just knowing that time is a real thing and not some construct.

That really depends on what you mean with real thing, I would say our understanding of time and space is heading towards them being emergent phenomena and not fundamental ones. As far as we know, there is no thing in the universe called the time that you can just measure, you can only look at the ordering of events. And then we invent a variable called time that lets us express the laws of physics reproducing the observed events in a simple way, but that is much more a convenient mathematical and everyday tool than a real thing than one usually realizes.

Time is absolutely a construct, just like the iPad I’m typing this on is a construct. I really can’t be sure it exists, but the model I have in my head that I’m tapping on a large flat surface with my fingers agrees with my observations. If in my next tap I get a painful burning sensation in my finger and I hear the fire alarm go off I may need to change my model to consider that I was mistaken and I may be touching a what appears to be a flame instead.

Isn't time "merely a man-made method of measuring the motion of the Earth around the Sun?"

> If you're traveling at 10 m/s as you go out of the gravity well you go faster and faster because time speeds up, but your velocity per unit time stays the same.

You seem to be confusing coordinate time with proper time. The time you experience as you get out of a gravity well is proper time, which, as a sibiling comment said, is something you obtain when you do the path integral of the path you travel. This time is not exactly affected by dilation. Anything traveling with you will not have its time go faster or slower. Anything that is not traveling with you, say stationary on Earth, will indeed have its passage of time shifted with respect to you.

Coordinate time is the time observed by an observer at the center of the gravity well, looking at you (not always, but generally so for most calculations. Also, observer = origin of frame of reference) and that is what is morphed and bent around. Since you're an observer as well, you also see other points experience time differently, since from your point of view, they're moving away from you.

PBS SpaceTime has a treatise on relativity. There are a dozen videos worth watching on this topic alone. The video about “the speed of causality” is the one that caused me to level up:

https://youtu.be/msVuCEs8Ydo

Here is a 12 video playlist on relativity:

https://www.youtube.com/playlist?list=PLsPUh22kYmNAmjsHke4pd...

Here is a six video series on measuring space and time:

https://www.youtube.com/playlist?list=PLsPUh22kYmNBkabv9M4fX...

Reminds me of project GREAT:

http://www.leapsecond.com/great2005/

I wish my dad threw us in the back of the minivan with three atomic clocks and drove us up a mountain just the prove general relativity.

http://leapsecond.com/great2005/tour/

Thanks for the really interesting link! Posts like this are why I read Hacker News.

I appreciate the commitment to the bit to get WA vanity plate 9192MHZ.

Sadly, WA no longer allows digits on vanity plates at all. I was hoping to get 6835 MHz, but no such luck...

Is it that digits are prohibited or that four digits followed by three letters is prohibited?

When that photo was published, WA was still running sequential plates as three digits followed by three letters; since then, they ran out, and they chose three letters followed by four letters. When that runs out, they will most likely switch to four numbers followed by three letters, so it makes sense to reject personalized plates that match the future series of sequential plates.

You can have numbers on a WA vanity plate. You can't have 4 numbers followed by three letters.

Of course the WA DOL site is a web app that doesn't support deep linking. You can get to the web app to check plate availability and rules here: https://fortress.wa.gov/dol/extdriveses/NoLogon?Link=Persona...

You may have to visit https://www.dol.wa.gov/vehicleregistration/sppersonalized.ht... first.

I wonder if we can measure gravitational waves with the articles setup? I mean gravity and time are linked afaik?

No, the effect of gravitational waves is far too small. Those have to be detected by watching for actual movement.

> over a million billion times per second

So... Over a quadrillion times per second? I don't get why people do this. Stephen Hawking did the same in his book ("X star is over a million million million million miles away"). Just give me the number straight. Breaking into millions of billions of trillions just makes it more confusing as I try to consolidate orders of magnitude.

There's a reason! It's to prevent confusion: https://en.wikipedia.org/wiki/Long_and_short_scales

Except that the phrase the OP picked up on is "million billion". So is that 10^15 or 10^18?

I suspect the reason is then that readers may not know what a quadrillion is.

After a thought experiment I was able to determine the meaning of "billion":

- It can be said that there are one thousand million (1,000,000,000)

- It can be said that there are ten thousand million (10,000,000,000)

- It can be said that there are one hundred thousand million (100,000,000,000)

But it cannot be said that there are a million million (1,000,000,000,000), it is said that there is a "billion".

- Then, there may be a million billion (1,000,000,000,000,000,000)

But there cannot be a billion billion (1,000,000,000,000,000,000,000,000), it is said that there is a "trillion"

- Then, there may be a million trillion (1,000,000,000,000,000,000,000,000,000,000)

- There may be a billion trillion (1,000,000,000,000,000,000,000,000,000,000,000,000)

But there cannot be a trillion trillion (1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000), you say "quadrillion", and so on...

Translating this with the google translator has been hell, every time I typed "one thousand million" and "million x" it translated into "billion", and every time I typed "billion", it translated into "trillion".

I don't think that's quite right

one thousand thousand (1,000 * 1,000) == one million (1,000,000)

one thousand million (1,000 * 1,000,000) == one billion (1,000,000,000)

one thousand billion (1,000 * 1,000,000,000) == one trillion (1,000,000,000,000)

Basically "one thousand ____" means "add 3 zeros"

one million million (1e6 * 1e6) == one trillion (1e12)

one million billion (1e6 * 1e9) == one quadrillion (1e15)

Basically "one million ____" means "add 6 zeros"

And... where is the error in my comment?

I just skipped the sequence: one thousand billion, ten thousand billion, one hundred thousand billion so as not to lengthen my comment and I did it by jumping to the millions directly.

Google translator makes a creeper with the "billions", read my comment in english.

I go the other way. I think if we started telling people that Jeff Bezos was worth two hundred thousand million dollars [0] more of them would say, what the fuck?, that’s disgusting.

The b in billion has become meaningless. Nobody has anything to anchor it to.

[0]: https://www.cnn.com/2020/08/27/tech/jeff-bezos-net-worth-200...

This article reminded me of true atomic wristwatches [0]. If precision gets good enough one day, you could get a synced pair and give one to a friend and compare after the years who has relativistically aged faster :)

[0] https://www.hodinkee.com/articles/richard-hoptroffs-atomic-w...

Stand-up Maths did a really good (and quite self-deprecating) video of both the Special Theory of Relativity the General Theory of Relativity and how they explain time dilation that helped my brain make sense of what it was having a hard time understanding.

https://youtu.be/Z4oy6mnkyW4

Needs a 2010 tag.

Only in your frame of reference...

nice, I laughed

I've had people incredulous at time dilation effects in relativity. I give them the example of this having been directly measured in the 70's. There, they put one clock in the lab and another on a jet and flew it around a bit.

https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim...

Also, high-resolution GPS requires SR and GR corrections to work. This is a working technology.

The real clincher is that your toes are older than your nose

There is something about weed that can slow down time, but your ability to learn is hindered too. Question is, how to slow down time and be able to learn a language overnight..

Perhaps there's a way to use this difference in time velocity at different altitudes to boil water to rotate a turbine and make electricity?

No. Conservation of energy still exists in General Relativity.

You guys are so boring. I was referring to a joke which tells that when science discovers a way to extract energy out of irregularities in spacetime manifold, the reactor would still boil water to make electricity.

Sounds a bit like the Asimov book "The Gods Themselves".

... when the clocks are stopped or lowered and placed back in the same "Frame of Reference" ?

What is even more intriguing is that this tiny deviation is responsible for you being attracted to Earth :)

In weak gravitational fields like on Earth, Gravitation is only a time distortion.

Remember how there was a thought experiment where you couldn't tell the difference between accelerating at 1G in an elevator, or being in a 1g gravity field?

If the elevator is more than a few CM taller than the clocks, now you can. Bolt one to the floor, the other to the ceiling.

If they run at the same rate, it's 1g acceleration. If there is the predicted difference, you're in a 1g field.

You can't tell the difference between 1g acceleration and a uniform gravitational field. The earths gravitational field is not uniform but radial.

ELI5 time - where would a truly uniform gravitational field exist?

Nowhere. Gravity is just inherently radial. (But if you have a very large spread it can look fairly uniform, of course. Especially if your measure of “flat” is actually curved to the same radius.)

Nowhere in the real world, but that’s okay since we are dealing with thought experiments. An infinite plane of mass is roughly what you’d need so that the contributions of each element add up to a uniform field.

Nowhere, i.e. if you are in the falling elevator the tennis balls will fall downwards if it's uniform but any real gravitational field (apart from something of infinite radius) will make them converge gradually towards eachother as they fall towards the CoG

In the pasture where I keep my spherical cows.

Why would that be? What if the acceleration happens from the top of the elevator?

Reply on news.ycombinator.com