How does time work?

How does science work? And what's all this about quantum mechanics?

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uwot
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How does time work?

Post by uwot » Sun Mar 05, 2017 12:12 pm

Everything you ever wanted to know about time: http://willijbouwman.blogspot.co.uk

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Noax
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Re: How does time work?

Post by Noax » Sun Mar 05, 2017 1:36 pm

Time as a coordinate system. That's good. An event (date w/ stickwoman) expressed in 4D: N, E, up, 7 o'clock. And the horse is totally cute.

Unlike the gravitation field in which a thing finds itself, speed is not a property of a thing, so speed cannot make something happen slower. If you want to teach really basic relativity, a longer comic might be required. Actually nothing ever happens slower. Two events (one of a pair of twins saying goodbye to the other as he boards the rocket, and the event of their subsequent reunion) are separated by different amounts of time depending on the path taken. The path with excessive spatial displacement requires less time displacement to connect the two events. This is unintuitive since it seems to violate euclidean geometry, but so does the gravity thing, so Euclid was already out the window.

The post describes how time is measured/sliced up, and doesn't really offer an opinion about how it 'works'. So perhaps the title is misleading, and I think it would be a mistake to lay a claim as to how it actually works. If you do, a nice roundup of popular interpretations would be nice. There's no one answer to it.

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Noax
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Re: How does time work?

Post by Noax » Sun Mar 05, 2017 1:52 pm

You also nicely describe the pre-standardization era when there were no standard time zones and noon was defined locally by when the sun was highest. My question was if the clocks of those days attempted to account for the longer days (as measured from noon to noon) in January than in July? Were the clocks seasonally adjusted for that, or did they not bother and just let it average out over a year like they do now?

uwot
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Re: How does time work?

Post by uwot » Sun Mar 05, 2017 9:16 pm

Noax wrote:The post describes how time is measured/sliced up, and doesn't really offer an opinion about how it 'works'. So perhaps the title is misleading, and I think it would be a mistake to lay a claim as to how it actually works.
Thanks again for the feedback. It wasn't my intention to be misleading, but it's a fair point; I should perhaps have labelled it pt1. Why time slows down with increased velocity and gravity is on the way and you're right; I'm not so daft to claim that I know how it works; rather the idea is to create a model that can be understood intuitively by people who lack either the time or inclination to study academic physics.
As for seasonal time, you're right, it was common practise, before the invention of the mechanical clock, to divide the period between sunrise and sunset into twelve equal hours. Some early clocks did account for that, the Orloj in Prague dates from the 15th century I think, and is a beautiful example.
Railway time was introduced by the Great Western Railway, which as the name suggests, operated from Paddington and headed west, Bristol being the major destination. In the US, it was a crash in the 1850's between two trains that were on the same track simultaneously because of the different times the two companies operated on, that prompted the move to standardisation.
Time is what we make it, and as far as most physicists are concerned, time is what clocks measure and bugger the philosophy.

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Noax
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Re: How does time work?

Post by Noax » Sun Mar 05, 2017 9:41 pm

uwot wrote:As for seasonal time, you're right, it was common practise, before the invention of the mechanical clock, to divide the period between sunrise and sunset into twelve equal hours. Some early clocks did account for that, the Orloj in Prague dates from the 15th century I think, and is a beautiful example.
Wasn't really speaking of the seasonal change of daylight (which is shorter in the northern hemisphere in January and is due to Earth's tilt). The measurement from noon to noon is also seasonal, and actually longer in January (both hemispheres, and due to the elliptical orbit, not the tilt). I was wondering if any clocks tried to sync to that if the pre-railroad standard was supposed to be from noon to noon.

Clocks don't count rotations of Earth (and your post correctly doesn't say that), but only that has no seasonal change to it. Earth rotates ~366.2 times a year, and the moon orbits ~13.4 times a year, despite there being about 12.4 lunar cycles a year. But all clocks/calendars mark subjective time, which is the light and dark cycles of the daylight and the moon respectively.
Time is what we make it, and as far as most physicists are concerned, time is what clocks measure and bugger the philosophy.
As best I can describe, clocks (like a tape measure) measure temporal separation between two events, but the measurement (again, just like the tape measure) is only accurate if the device is drawn straight between the two measurement points. Yes, clocks measure time as defined by physics, and that is by no means a philosophical statement concerning what time actually is.

It might be interesting to note on the web page that the time axis is no more defined than the x, y, or z spatial axes. They may be arbitrarily assigned so long as they're mutually orthogonal. Doing so helps explain the twins paradox I think. Sure helps me visualize it better.

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Re: How does time work?

Post by uwot » Sun Mar 05, 2017 10:04 pm

Noax wrote:It might be interesting to note on the web page that the time axis is no more defined than the x, y, or z spatial axes. They may be arbitrarily assigned so long as they're mutually orthogonal. Doing so helps explain the twins paradox I think. Sure helps me visualize it better.
The twins paradox isn't really a paradox (technically, philosophical definitions blah, blah) and as Hafele-Keating showed, it is a demonstrable physical reality. Explaining time dilation through geometry and Lorentz transformations is proper physics, but it doesn't follow that is what actually happens; it's an epistemological tool, which may or may not reflect the real world, but I think there is a simpler way to explain it, which again may or may not reflect reality. There's a very rough draft in an old blog I was fooling around with http://willibouwman.blogspot , it's on a page called Why time slows down the faster you go (I think) and there's a follow up called the wacky world of hafele-keating, if you're interested.

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Harbal
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Re: How does time work?

Post by Harbal » Sun Mar 05, 2017 10:59 pm

How does time work? It works like this: You finish work on Friday afternoon, then, the next thing you know, you're getting up for work again on Monday morning. That's how it effing works!

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Greta
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Re: How does time work?

Post by Greta » Sun Mar 05, 2017 11:29 pm

Enjoyably clear and systematic work again, uwot.

Nice analogy - "four football fields per second" (the Earth's rotation speed). Then again, we're moving through the universe at around 3 million kms per hour, or about 870 kms per second - entities zooming around in near-nothingness as they whirl themselves into existence over time.

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Re: How does time work?

Post by Hobbes' Choice » Sun Mar 05, 2017 11:51 pm


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Noax
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Re: How does time work?

Post by Noax » Mon Mar 06, 2017 1:47 am

Greta wrote:Then again, we're moving through the universe at around 3 million kms per hour, or about 870 kms per second - entities zooming around in near-nothingness as they whirl themselves into existence over time.
Out of curiosity, 870 kms relative to what? I've heard a figure something like that as our velocity towards the so called great attractor, but since our velocity relative to the frame in which the CMB is isotropic (the best guess frame for 'stationary' at a given point in the universe), it is more like the great attractor is coming towards us.

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Noax
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Re: How does time work?

Post by Noax » Mon Mar 06, 2017 2:06 am

uwot wrote:The twins paradox isn't really a paradox (technically, philosophical definitions blah, blah) and as Hafele-Keating showed, it is a demonstrable physical reality.
Agree. The word 'paradox' was attached to the description since the effect is unintuitive.
There's a very rough draft in an old blog I was fooling around with http://willibouwman.blogspot , it's on a page called Why time slows down the faster you go (I think) and there's a follow up called the wacky world of hafele-keating, if you're interested.
Unable to follow the link at this time. I don't think time slows down the faster you go since you are always stopped. Speed is not a property of an object, and I can arbitrarily assign any speed to myself without worrying about my watch not keeping proper time. Time slows down for others that have high speed relative to you. So for the twins example, in the frame of the rocket man, time goes slower for his twin that stays on earth, for both legs of the journey since Earth-man is the one with the velocity in both cases. So when does the twin on Earth get so old? It happens when the rocket man turns around! It is simply a redefinition, from rocket-man's new frame, of the time on Earth that is simultaneous with rocket man.

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Greta
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Re: How does time work?

Post by Greta » Mon Mar 06, 2017 5:13 am

Noax wrote:
Greta wrote:Then again, we're moving through the universe at around 3 million kms per hour, or about 870 kms per second - entities zooming around in near-nothingness as they whirl themselves into existence over time.
Out of curiosity, 870 kms relative to what? I've heard a figure something like that as our velocity towards the so called great attractor, but since our velocity relative to the frame in which the CMB is isotropic (the best guess frame for 'stationary' at a given point in the universe), it is more like the great attractor is coming towards us.
Yes, the Milky Way is said to be travelling at over 2 million km/h, most of the roughly 3 mill https://astrosociety.org/edu/publicatio ... wfast.html. I don't understand about how a relatively uniform CMB means that the Great Attractor is coming towards us. My understanding is that the Great Attractor is the gravitational centre of Laniakea. Exciting stuff - reality at these scales is still a frontier for us.

While our local cluster and The Great Attractor are moving closer together (one way or another), Laniakea too must be moving relative to other superclusters. Still, if the universe is a megacluster of swirling superclusters moving relative to each other, with no discernable relativity outside its expanding bounds, then the space and distances would seem less suggestive of the universe's story than the transformations of its material over time (than the constant movement facilitates).

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Noax
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Re: How does time work?

Post by Noax » Mon Mar 06, 2017 1:51 pm

Greta wrote:Yes, the Milky Way is said to be travelling at over 2 million km/h, most of the roughly 3 mill https://astrosociety.org/edu/publicatio ... wfast.html.
I could not find the 3 million kms reference in there at all. The article is American and expresses everything in miles per hour, a few translations to km/h, and nothing per second. I may be American, but I've never done physics or chemistry in those units.
The 2.1m km/h figure is galaxy vs. CBR frame and is about 580 kms. Since we are on the side of the galaxy spinning away from the direction of travel of the thing, we subtract off most of the 220 kms from the rotation putting the velocity of our solar system around 390 relative to CBR frame.
I don't understand about how a relatively uniform CMB means that the Great Attractor is coming towards us.
Well the galaxy is moving at ~580 kms relative to CBR frame, presumably in the general direction of the GA. So if it's moving around 1000 kms towards the GA, the GA must be moving towards us (relative to CBR frame) by nearly as much as we're moving towards it for it all to add up to nearly 1000 kms. I don't admit to knowing a lot about the Great Attractor. I was just trying to find the 3million km/hr (870/sec) reference you quoted.
While our local cluster and The Great Attractor are moving closer together (one way or another), Laniakea too must be moving relative to other superclusters. Still, if the universe is a megacluster of swirling superclusters moving relative to each other, with no discernable relativity outside its expanding bounds, then the space and distances would seem less suggestive of the universe's story than the transformations of its material over time (than the constant movement facilitates).
I think superclusters are far apart enough that the expansion of the universe keeps them from swirling each other. For the most part, superclusters move apart from each other. At some scale, everything has to.

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Re: How does time work?

Post by Melchior » Tue Mar 07, 2017 4:26 am

First comes now, then comes now, then comes now, etc.

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Re: How does time work?

Post by attofishpi » Tue Mar 07, 2017 4:37 am

Greta wrote:My understanding is that the Great Attractor is the gravitational centre of Laniakea. Exciting stuff - reality at these scales is still a frontier for us.

While our local cluster and The Great Attractor are moving closer together (one way or another), Laniakea too must be moving relative to other superclusters. Still, if the universe is a megacluster of swirling superclusters moving relative to each other, with no discernable relativity outside its expanding bounds, then the space and distances would seem less suggestive of the universe's story than the transformations of its material over time (than the constant movement facilitates).
Thanks Greta - love it when i learn something new - i had never heard of Laniakea or the Great Attractor.

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