Time

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Dark Tranquillity
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Time

Postby Dark Tranquillity » Sun Nov 28, 2010 4:37 am UTC

Hey guys, physics newbie here. I've read a lot on it yet never really understood some things. One of these is how time itself can begin and end. I've always read and heard that time began with the big bang, and would cease to exist with the death of the Universe. Can someone explain this to me in layman's terms?

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thoughtfully
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Re: Time

Postby thoughtfully » Sun Nov 28, 2010 7:39 pm UTC

There's two approaches. The classical analogy is viewing the Universe as a 4D sphere, with time as the radial dimension. The center is obviously a fixed endpoint. You can also arrange things so that time is more like North/South on a globe. You can't go south from the South Pole.

The more "quantumy" approach supposes there is a smallest interval of time about which it makes any sense to talk. This would be on the order of the Plank Time. "Time" begins when the Universe reaches this age, because you can't say anything about a Universe that's younger.
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Re: Time

Postby Technical Ben » Mon Nov 29, 2010 12:44 pm UTC

Would a more simple explanation not be: Without anything to measure, how can we measure time? So if there was nothing before the universe, and nothing after, how can we say there is time also?
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Re: Time

Postby Moose Hole » Mon Nov 29, 2010 3:30 pm UTC

I'm not sure what you mean about the end of time, because even after the heat death of the universe there is still space, and therefore there is time. Time before the universe began isn't measurable because we don't know if there was anything before that and have no evidence about it.

Another way to think about this beginning of time is absolute zero. Absolute zero means the lowest temperature possible, because no heat is being produced, because nothing is moving. Things can't be doing less than not moving, so the temperature can't be any less than that. Things can always move more, so there's no maximum temperature, I think.

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Re: Time

Postby Tass » Mon Nov 29, 2010 5:24 pm UTC

Moose Hole wrote:I'm not sure what you mean about the end of time, because even after the heat death of the universe there is still space, and therefore there is time. Time before the universe began isn't measurable because we don't know if there was anything before that and have no evidence about it.

Another way to think about this beginning of time is absolute zero. Absolute zero means the lowest temperature possible, because no heat is being produced, because nothing is moving. Things can't be doing less than not moving, so the temperature can't be any less than that. Things can always move more, so there's no maximum temperature, I think.


But if there was a big crunch (there wont be) then there would be such an end.

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Re: Time

Postby Scyrus » Mon Nov 29, 2010 5:55 pm UTC

Time is a quantity that can be measured, and it is the order in which events happen chronologically. It's passage may vary as other fundamental quantities of the universe vary (space).

I've always thought about it this way: Time, along with Space, form a single entity called spacetime, and they cannot exist independently.
Right before the Big Bang, if there was no space, then there is no time, and it is meaningless to ask how it was before there was space.
(If, however, there was an infinitesimal but larger than zero portion of space, then time would also have a say in the universe).

If the ultimate fate of the universe is heat death, Big Freeze or Big Rip, both space and time will still exist.
If it is a Big Crunch, there are two outcomes: Big Bounce, or if it simply converges and ends, it is also meaningless to ask how or why it ends because at such time nothing will matter.


By the way, "beggining" and "ending" are comcepts themselves completely dependant and derived from the concept of Time. Time is a property, a quantity that can be measured, and like any quantity it has an origin. We don't know whether it is finite or infinite, but there is probably no way to know, since when it ceases to exist, everything will.
So we do not really know if it can begin or end, science is all about rewriting itself.


Tass wrote:But if there was a big crunch (there wont be) then there would be such an end.


There won't be? Why?

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Re: Time

Postby doogly » Mon Nov 29, 2010 7:02 pm UTC

Scyrus wrote:Time is a quantity that can be measured

I wouldn't be so sure! For one thing it is not a quantum mechanical observable. It seems like there isn't any direct way to measure it. See Barbour's essay: http://www.platonia.com/nature_of_time_essay.pdf
I am generally in favor of a fundamental time, but it is not at all obvious.

Scyrus wrote:
Tass wrote:But if there was a big crunch (there wont be) then there would be such an end.

There won't be? Why?

This would require a higher energy density in the universe than we observe.
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Dark Tranquillity
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Re: Time

Postby Dark Tranquillity » Mon Nov 29, 2010 11:56 pm UTC

thoughtfully wrote:The more "quantumy" approach supposes there is a smallest interval of time about which it makes any sense to talk. This would be on the order of the Plank Time. "Time" begins when the Universe reaches this age, because you can't say anything about a Universe that's younger.


That makes some sense. I think part of my problem was thinking there was something before the big bang, though of course we have no idea whether that's true or not.

Moose Hole wrote: Wouldn't a maximum temperature be obtained when the atoms of a substance are vibrating at/just below the speed of light? Not to get off topic.

Scyrus wrote:I've always thought about it this way: Time, along with Space, form a single entity called spacetime, and they cannot exist independently.
Right before the Big Bang, if there was no space, then there is no time, and it is meaningless to ask how it was before there was space.


This is what I'm looking for. I've yet to read up much on spacetime directly, so that's a newer concept for me to grasp. Thank you.

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Re: Time

Postby Moose Hole » Tue Nov 30, 2010 5:10 pm UTC

Dark Tranquillity wrote:Wouldn't a maximum temperature be obtained when the atoms of a substance are vibrating at/just below the speed of light? Not to get off topic.
Movement is one thing that can cause heat (through friction), but matter does not have to be present at all in order for there to be heat, because heat is energy. The sun heats the earth using radiant heat. If there were enough atoms between us and the sun for it to directly conduct heat to us, we'd be a lot warmer.

I believe temperature can be expressed as a function of volume and pressure. I believe the value of absolute zero was obtained by projecting the value of absolute zero pressure. If you have a fixed volume, and you can add pressure, then you can add temperature. I guess the maximum temperature would be all the pressure of the universe packed into a zero point volume, so maybe the beginning of the big bang was the maximum temperature possible, or something.

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Re: Time

Postby Sir_Elderberry » Tue Nov 30, 2010 6:00 pm UTC

There is no maximum temperature because temperature comes from kinetic energy, not from velocity. Velocity is limited at c, but kinetic energy can increase without bound.
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Re: Time

Postby Soralin » Wed Dec 01, 2010 9:18 am UTC

Sir_Elderberry wrote:There is no maximum temperature because temperature comes from kinetic energy, not from velocity. Velocity is limited at c, but kinetic energy can increase without bound.

You know something is really hot when you have to take relativity into account to determine how hot it is. :)

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Re: Time

Postby jeffer » Sun Dec 05, 2010 1:37 pm UTC

Everything is relative. Time is relative with respect to you also. When you born then your time will start and when you die your time will be end. So time is relative with respect to anything. It is the concept of relativity. I hope you will understand the basic thing. For further information you can send your required question.
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Re: Time

Postby thoughtfully » Mon Dec 06, 2010 1:44 am UTC

jeffer wrote:Everything is relative. Time is relative with respect to you also. When you born then your time will start and when you die your time will be end. So time is relative with respect to anything. It is the concept of relativity. I hope you will understand the basic thing. For further information you can send your required question.
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jeffer

That's assuming a coherent definition for what an individual is. The partickles that make up an individual are constantly flowing in and out, and have existed for billions of years, and will go on for billions more.

And that's expressing a lot of understanding of what relativity is and isn't. Einstein never liked the term anyway, since the theory is really about invariants.
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