
What Is Time? - calhoun137
https://medium.com/@calhoun137/what-is-time-dee7f911eafa
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FesterCluck
This article does an amazing job of describing the various versions of the
definition of "time" used in physics. The author limits his use of jargon, but
defines them well when necessary. Highly recommended.

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iaw
I wonder if the statement about Newton's second law (there exists an r(-t))
holds true when you include relativistic effects.

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calhoun137
Time is reversible in general relativity, in the sense that if you replace t
by -t everywhere the solution is still valid. I don't remember the full proof
off the top of my head, and the argument I do remember is complicated.

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pentabular
no such thing as backwards time travel, sorry!

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drdeca
Can you clarify? I don't know what you mean.

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briancgreen
"There was a smell of Time in the air tonight. He smiled and turned the fancy
in his mind. There was a thought. What did Time smell like? Like dust and
clocks and people. And if you wondered what Time sounded like it sounded like
water running in a dark cave and voices crying and dirt dropping down on
hollow box lids, and rain. And, going further, what did Time look like? Time
looked like snow dropping silently into a black room or it looked like a
silent film in an ancient theater, one hundred billion faces falling like
those New Years balloons, down and down into nothing. That was how Time
smelled and looked and sounded. And tonight--Tomas shoved a hand into the wind
outside the truck--tonight you could almost touch Time." (Illustrated Man)

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ScottBurson
I read somewhere once that it's not actually true that special relativity
permits time travel into the future. Such time travel would require leaving
Earth, accelerating to a relativistic velocity, travelling some distance, then
accelerating back toward Earth, again reaching a relativistic velocity, then
finally accelerating to match Earth's velocity and position, and landing.

The argument was that although it's true that your time will pass more slowly
than time on Earth while you're coasting, you also clearly have to do a lot of
accelerating, and acceleration itself also has an effect on the apparent
passage of time in your reference frame -- and in this scenario, where you
start and end in Earth's reference frame, that effect cancels out the effect
of velocity (perhaps even more than cancels it out; I don't recall).

Does anyone here know if this is true?

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leephillips
It's not true. In fact, particles "travelling into the future" is routinely
measured in high energy physics experiments: a particle's decay time will be
lengthened exactly as special relativity predicts. Also, the effect has been
measured directly using atomic clocks. The circumferences of your hard drive
platters are younger than the part near the spindle.

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ScottBurson
I'm not questioning the existence of time dilation; I'm asking about the
effect of acceleration on that dilation. Perhaps this is a question for
General Relativity.

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thisjepisje
"Time Is What Prevents Everything From Happening At Once.." \- John Wheeler

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jhrobert
Perfect.

Why?

If effects from causes occurred immediately, versus with a distinct delay,
everything would occur at once (or loop forever, that's a bug).

Hence, because we can observe that things happen "in sequence", there is a
"propagation delay". That's what time is: "Time: the process whereby effects
occur after causes, in a step by step manner".

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pentabular
Nothing but the ongoing interactions of things in the universe. What is a
dimension?

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ca98am79
Time is an illusion

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georgemcbay
Lunchtime doubly so.

(Sorry it took so long to post the punchline to your obvious Douglas Adams
setup; I was at lunch).

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theophrastus
time flies like an arrow (whereas) fruit flies like a banana

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chasing
Fruit flies like an arrow, time flies like a banana.

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dllthomas
In my experience, fruit flies haven't cared much about arrows. What are "time
flies"?

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geographomics
[https://en.wikipedia.org/wiki/Time_flies_like_an_arrow;_frui...](https://en.wikipedia.org/wiki/Time_flies_like_an_arrow;_fruit_flies_like_a_banana)

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dllthomas
I'm quite familiar.

