## Just a few Relativity Questions

Safer than Professor Rex's office hours
testtubegames
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### Re: Just a few Relativity Questions

exfret wrote:So, then, the average person's supervolume is average_volume_in_m^3*average_lifespan_in_seconds*c m^4 ?
Yup, that's the idea.

Not too much unlike the fact we sometimes discuss distance in 'light-years'.

exfret
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### Re: Just a few Relativity Questions

Except I'm asking the distance of time, while 'light-years' are no where near a measure of time. When we measure with 'light-years', they're purely distances, because we're measuring speed (of light) * time (in years), which cancels it out to give us distance. I don't see why this is such a big deal just because it has a word measuring time in it. It's like saying we measure energy in distance, because a common name for the unit of energy is the Newton-Meter.
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testtubegames
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### Re: Just a few Relativity Questions

Sure. "Light-years" are a measure of distance. The unit is based on time (years) and the speed of light is used as the conversion factor to get you to distance (light-years).

So back to your original question, one could say that the distance of the time unit "year" was a light-year. And the distance of a "second" was a light-second.

So when I said:
testtubegames wrote:So 1 second would be 3x10^8m... or more generally, distance=time*c.
That distance is a light-second.

exfret
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### Re: Just a few Relativity Questions

Oh. So is it just a shorthand, or is a second of time really a light-second of meters, or is there no way to know, or does it not really work that way?
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testtubegames
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### Re: Just a few Relativity Questions

exfret wrote:Oh. So is it just a shorthand, or is a second of time really a light-second of meters, or is there no way to know, or does it not really work that way?
Good question. Yes and no.

A second does not equal a light-second. One is time, the other is distance. *But* if you want to convert between time and distance in relativity, you generally do use distance = speed-of-light * time to figure it out.

Now, if instead of talking about relativity, we were talking about something else, like driving, maybe we'd use another conversion factor (like 55mph... so two towns can be two hours apart, or 110 miles apart, interchangeably). And maybe that's a good analogy for thinking about this. It doesn't mean that the distance '110 miles' is literally the same thing as the time '2 hours', but in the context of the car going 55 mph, you can equate the two. (And the answer to "How much longer do we have to drive" can be answered equally well with either)

exfret
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### Re: Just a few Relativity Questions

Oh. That's a little disappointing. I'm kind of confused about relativity now. I wish it wasn't one of those wait-a-few-years-until-you've-learned-college-mathematics topics. ):
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testtubegames
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### Re: Just a few Relativity Questions

I know, right? Nothing beats learning the math of physics...

My big recommendation would be to grab an awesome book if you're yearning to learn a bit more. One I absolutely love that I first read during high school was Black Holes and Time Warps by Kip Thorne. That was a formative read for me, and used copies are pretty cheap/easy to come by.

19683
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### Re: Just a few Relativity Questions

So is the center of the Earth an inertial frame since it is in 'free-fall' around the sun?
Binomial Theorem: ((a+b)^n)= sum k=0->k=n((n!(a^(n-k))(b^k))/(k!(n-k)!))

testtubegames