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[Kirian]

So on the subject of GPS, we all know that clocks in orbit around the Earth will run faster (apparently the calculations show about 38 microseconds) compared to clocks on the surface of the earth.  This is due to relativistic gravitational effects (and from the satellites traveling quickly).  Apparently there is some disagreement on whether this time discrepancy is actually relevant and accounted for.  E.g., http://www.alternativephysics.org/book/GPSmythology.htm.  Doing random searches about GPS and relativistic effects, I ran across a number of things, including arguments with people that apparently don't believe Einstein.

Anyone know much about this?

Yeah, there are actually people out there who don't believe in relativity, much as there are people who don't believe in evolution or that the Earth is an oblate spheroid.  None of these people are actual scientists.

Actual scientists understand that relativity is still an incomplete theory, much in the same way that Newton's Laws were incomplete.  Oddly though you don't see many people arguing against Newton's Laws.

TLDR version:  Those people are morons, and yes GPS engineers account for relativity.

[theory]

[initial discussion on relativity ninja'd by Kirian]

On a related but amusing note, your smartphone has two clocks inside of it: the system clock and the GPS clock.  The two are exactly and deliberately 16 seconds out of sync.

[Ozle]

[initial discussion on relativity ninja'd by Kirian]

On a related but amusing note, your smartphone has two clocks inside of it: the system clock and the GPS clock.  The two are exactly and deliberately 16 seconds out of sync.

oi! Get your own fact of the day thread!

Oh go on then, you can share.....why are they 16 seconds out on purpose?

[soulnet]

[initial discussion on relativity ninja'd by Kirian]

On a related but amusing note, your smartphone has two clocks inside of it: the system clock and the GPS clock.  The two are exactly and deliberately 16 seconds out of sync.

Is a clock really cheaper than a circuit that adds 16?

[Witherweaver]

So on the subject of GPS, we all know that clocks in orbit around the Earth will run faster (apparently the calculations show about 38 microseconds) compared to clocks on the surface of the earth.  This is due to relativistic gravitational effects (and from the satellites traveling quickly).  Apparently there is some disagreement on whether this time discrepancy is actually relevant and accounted for.  E.g., http://www.alternativephysics.org/book/GPSmythology.htm.  Doing random searches about GPS and relativistic effects, I ran across a number of things, including arguments with people that apparently don't believe Einstein.

Anyone know much about this?

Yeah, there are actually people out there who don't believe in relativity, much as there are people who don't believe in evolution or that the Earth is an oblate spheroid.  None of these people are actual scientists.

Actual scientists understand that relativity is still an incomplete theory, much in the same way that Newton's Laws were incomplete.  Oddly though you don't see many people arguing against Newton's Laws.

TLDR version:  Those people are morons, and yes GPS engineers account for relativity.

So I guess I should have looked at the actual URL and the home pages of such sites. 

[Witherweaver]

Damnit, i hate it when people hijack threads and take them off into different non-relevant directions!

This was completely a relativistic direction!  I don't know what you're complaining about!

[theory]

Most clocks use UTC time, which has leap seconds, designed to correct for the fact that the Earth is very inaccurate in its rotation.  GPS time was synchronized with UTC in 1980 but has not subsequently added leap seconds or otherwise corrected for Earth's rotation.  It is likely that the UTC and GPS will further diverge in the future.

[soulnet]

Most clocks use UTC time, which has leap seconds, designed to correct for the fact that the Earth is very inaccurate in its rotation.  GPS time was synchronized with UTC in 1980 but has not subsequently added leap seconds or otherwise corrected for Earth's rotation.  It is likely that the UTC and GPS will further diverge in the future.

Ok, so when you say 16 seconds away it actually means that they are now 16 seconds away, not forever 16 seconds away? I misinterpreted that.

[Ozle]

Most clocks use UTC time, which has leap seconds, designed to correct for the fact that the Earth is very inaccurate in its rotation.  GPS time was synchronized with UTC in 1980 but has not subsequently added leap seconds or otherwise corrected for Earth's rotation.  It is likely that the UTC and GPS will further diverge in the future.

Ok, so when you say 16 seconds away it actually means that they are now 16 seconds away, not forever 16 seconds away? I misinterpreted that.

yeah, this is what I assumed as well.

Makes much more sense now. Thanks.

Will this still be the case when I move my clock forward so I can cheat at Candy Crush though?

[SirPeebles]

...to correct for the fact that the Earth is very inaccurate in its rotation.

Indeed.  Earth had better get its act together are start accurately matching our simple models.

Also, Earth needs to work out more.  It is bulging around the waist.

[soulnet]

Also, Earth needs to work out more.  It is bulging around the waist.

Work out more? Are you out of your mind? It is already burning carbs and sweating like there is no tomorrow! If it keeps it up like this, there won't be.

[florrat]

Most clocks use UTC time, which has leap seconds, designed to correct for the fact that the Earth is very inaccurate in its rotation. 

I'm not sure if I'm correct, but what I've heard was that leap seconds have nothing to do with inaccuracy. Leap seconds happen because the Earth's rotation is slowing down (every day is a little longer than the previous one, about 60 nanoseconds shorter). This slowing down happens because the Moon causes a lot of water to move around the planet every day (known as the tides), which causes a lot of friction. This will lower the angular momentum of the earth slowly, causing days to become slightly longer.

So the days are now on average a little longer than 24*60*60 seconds, which means that we have to add a second every couple of years. Since days become longer and longer (but of course this will be in the order of milliseconds) we have to have leap seconds more frequently in the future.

[Witherweaver]

Also, Earth needs to work out more.  It is bulging around the waist.

Work out more? Are you out of your mind? It is already burning carbs and sweating like there is no tomorrow! If it keeps it up like this, there won't be.

Maybe, but it's slowing down.  Why, I remember just a few hundred years ago it was spinning a few microseconds faster.  It's been riding on its coat tails for millennia.  Plus it garbles up calories from the sun. 

[Witherweaver]

Most clocks use UTC time, which has leap seconds, designed to correct for the fact that the Earth is very inaccurate in its rotation. 

I'm not sure if I'm correct, but what I've heard was that leap seconds have nothing to do with inaccuracy. Leap seconds happen because the Earth's rotation is slowing down (every day is a little longer than the previous one, about 60 nanoseconds shorter). This slowing down happens because the Moon causes a lot of water to move around the planet every day (known as the tides), which causes a lot of friction. This will lower the angular momentum of the earth slowly, causing days to become slightly longer.

So the days are now on average a little longer than 24*60*60 seconds, which means that we have to add a second every couple of years. Since days become longer and longer (but of course this will be in the order of milliseconds) we have to have leap seconds more frequently in the future.

It's only slowing down on average.  The rotational speed actually fluctuates, and certain events (earthquakes) can make a difference.  I think that's what he meant by inaccurate in its rotation.

http://en.wikipedia.org/wiki/Earth's_rotation#Changes_in_rotation

[theory]

Most clocks use UTC time, which has leap seconds, designed to correct for the fact that the Earth is very inaccurate in its rotation. 

I'm not sure if I'm correct, but what I've heard was that leap seconds have nothing to do with inaccuracy. Leap seconds happen because the Earth's rotation is slowing down (every day is a little longer than the previous one, about 60 nanoseconds shorter). This slowing down happens because the Moon causes a lot of water to move around the planet every day (known as the tides), which causes a lot of friction. This will lower the angular momentum of the earth slowly, causing days to become slightly longer.

So the days are now on average a little longer than 24*60*60 seconds, which means that we have to add a second every couple of years. Since days become longer and longer (but of course this will be in the order of milliseconds) we have to have leap seconds more frequently in the future.

It's only slowing down on average.  The rotational speed actually fluctuates, and certain events (earthquakes) can make a difference.  I think that's what he meant by inaccurate in its rotation.

http://en.wikipedia.org/wiki/Earth's_rotation#Changes_in_rotation

Correct.  On average it is slowing down, but it is slowing down erratically, like a drunk determined to make it to the streetlamp.

[SirPeebles]

today's xkcd feels relevant.

[Ozle]

-----------------------------------------------------

FACT OF THE DAY


The world Limbo record is 8.5 inches!

[SirPeebles]

Set the bar pretty low for today's fact.

[Kuildeous]

Set the bar pretty low for today's fact.

Oh, SirPeebles. Jamaican me crazy!

[Ozle]

Ohh yes, new fact:


If you take all of the space out of all of the people in the world. The remaining atoms would all fit into the size of a sugar cube

[KingZog3]

Ohh yes, new fact:


If you take all of the space out of all of the people in the world. The remaining atoms would all fit into the size of a sugar cube

Doesn't that apply for everything on the planet, not just people? Maybe even all the matter in the universe. It's something crazy like that.

[Kirian]

Ohh yes, new fact:


If you take all of the space out of all of the people in the world. The remaining atoms would all fit into the size of a sugar cube

Can I assume you mean "all the remaining particles"?  Because the whole point of the "empty space" thing is that individual subatomic particles are incredibly small compared to atomic volumes.  But calling the remaining atomic volume "empty space" is just horribly incorrect.

But assuming that's what you meant, we're talking a factor (volume) of 10¹⁵.  At that factor, humanity scales down to about 0.3 mL... definitely smaller than a sugar cube.

[Ozle]

Hmm, yes it should have been particles.

But upon further research prompted by your comment I don't like this one.

Lets strike it form the record and i'll try again

[Kirian]

Hmm, yes it should have been particles.

But upon further research prompted by your comment I don't like this one.

Lets strike it form the record and i'll try again

Eh, it's all right.  I mean, it formed the premise of one OK movie (Honey I Shrunk the Kids) and two terrible sequels.

[Ozle]

Hmm, yes it should have been particles.

But upon further research prompted by your comment I don't like this one.

Lets strike it form the record and i'll try again

Eh, it's all right.  I mean, it formed the premise of one OK movie (Honey I Shrunk the Kids) and two terrible sequels.

And a TV show apparently!