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Comment by woah | Hacker News Reader

How is it possible that the core is several years younger if the core and the surface have gone around the sun the same number of times?

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Time isn't universal, it doesn't tick for everybody at the same rate, in fact every particle has its own unique clock.

One thing that changes how your clock flows is being in a gravity well. We have clocks which can measure it over just a few feet of height difference. Gravity isn't so much a "force" but the bending of space and time. Clocks moving differently is the result.

> We have clocks which can measure it over just a few feet of height difference

Wait, seriously? Do you have a link??

Measuring the height of a mountain with a portable atomic clock (1000m accuracy) https://www.nature.com/articles/s41567-017-0042-3

NIST measuring speed differences < 10 m/s and height < 1 m with a pair of atomic clocks https://science.sciencemag.org/content/329/5999/1630.long

Amazing. They observe fractional frequency changes of around 4.1E-17. Thanks for sharing the links!

Couldn't see that number in the info at the link above, is that from the full article?

Yeah: "The two measurements consist of approximately 100,000 s of low-height data and 40,000 s of high-height data, and the clocks exhibit (Fig. 3) a fractional frequency change of (4.1 ± 1.6) × 10^−17."

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_f... (slides)

Or if you prefer non-PDF:

http://leapsecond.com/great2005/

http://leapsecond.com/great2016a/

Mass, gravity and general relativity. It’s a fascinating and very counter-intuitive at some moments, such as this.

Same goes for people that stay on the moon, when they go back to earth their watches are slightly different than those on earth. GPS satellites need to be synced with earth clock periodically as well for this reason.

The GPS satellites have multiple general relativity effects on their definition of time. Since they're moving so fast, the internal clocks on the satellites move slower than Earth (about 7 microseconds a day), while the decreased gravity on the satellites causes the internal clocks to speed up compared to Earth (45 microseconds a day). Net effect is a speed up of 38 microseconds.

Also, the on-board chips are pre-programmed to take into account the relativistic effects and compensate when doing their calculations.

In addition to the other excellent comments about general relativity, a "year" is only valid in expressing the time it takes to go around the Sun right NOW, not the entire existence of the Earth. The Earth's orbit has shifted throughout the lifetime of the solar system. A year circa 1000000 B.C. isn't the same as the year circa recorded history if we are considering it one revolution. so that number wouldn't represent the number of times it's gone around the Sun even excluding relativistic effects.

> The Earth's orbit has shifted throughout the lifetime of the solar system.

I'm by no means qualified to state this as fact but also I imagine that as we travel through space and hit random stuff in space (meteors, particles from stars, comet trails etc) we ever so slightly change our speed outside of the gravitational influences of other bodies, it would probably be incredibly small fractions of a second over centuries but it's something.

I wonder how much of an effect something like an extinction event asteroid on the speed of the earth both around the sun and on its rotation on its axis.

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

Now a detail that really makes things more interesting is that gravity is not constant upon the surface ( https://svs.gsfc.nasa.gov/11234 ), due to mass variations. So with that, it would be interesting to work out the age difference as a surface map. Though would need to historically adjust for tectonic shifts and other effects upon the surface mass. Of course the difference won't be as large, but still, I'd dare say that a day or so, would be more than viable upon the scale of things.

The core and the surface have gone around the Sun the same number of times, but a clock at the core would record less elapsed time for each orbit around the Sun than a clock at the surface.

General Relativity

Parent comment is making a joke about “year” as a unit measuring how many times you go around the sun. Less time has passed for the core per relativity, but is it not the same number of years?

The more official definition of years defines it in terms of days, which break down further to SI’s rigorously defined second as the base unit of time. But it’s still weird to think about.

Unless the second is defined on a Hubble flow co-moving observer, it's ill-defined enough to be ambiguous by at least 2 years out of 4 billion.

Yes, you wouldn't be able to measure it accurately, but it's nonetheless a static amount of time unrelated to the Earth's orbital period.

I'm not an astrophysicist, but I'd guess the Earth's orbital period isn't quite the same as when the planet first formed either. So using today's defined measurement of "years", the planet's age and the number of times it's been around the sun might not match up at the surface either. The impact that formed the moon must have changed our velocity a bit, right? And the sun has been losing mass (into energy via fusion) since it first formed.

That explains how it can be ...does the conversion... 75 megaseconds younger, but the whole of the earth modulo ejected material like spacecraft should be the same number of _years_ old.

I found this helpful:

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

We no longer measure time as laps around the sun.

because spacetime is curved by mass the matter in the centres of the Earth and Sun computes at a slower rate than matter at their surfaces so that from our point of view the centres of the Earth and Sun are younger because their clocks have ticked fewer times compared to their surfaces

Universe is not heliocentric

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