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Old 18th July 2022, 02:05 PM   #1
Skeptic Ginger
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What is a light-year

Why a lightyear is a measure of distance and time.

We all agree it is a measure of distance.

But when talking about things like how much time it takes someone/something to go a certain distance and we are using a lightyear scale, then lightyears can be a measure of time.

Eg: if we are traveling 10% the speed of light then it would take us 40 years to travel 4 lightyears. Provided I did the math right, doesn't that make lightyears a measure of time?

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Old 18th July 2022, 02:12 PM   #2
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Originally Posted by Skeptic Ginger View Post
Why a lightyear is a measure of distance and time.

We all agree it is a measure of distance.

But when talking about things like how much time it takes someone/something to go a certain distance and we are using a lightyear scale, then lightyears can be a measure of time.

Eg: if we are traveling 10% the speed of light then it would take us 40 years to travel 4 lightyears. Provided I did the math right, doesn't that make lightyears a measure of time?
A light year may have been intended to be used as a measure of distance, but it can also be used as a measure of time or speed.

A light-year is the distance light travels in one year.

In other words, distance measured by speed and time.

But can also be used as a measurement of time by distance and speed.

If I know I've travelled 100km at 50kph I can calculate I've travelled for 2 hours (no clock required).
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Last edited by ynot; 18th July 2022 at 02:15 PM.
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Old 18th July 2022, 02:22 PM   #3
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Originally Posted by ynot View Post
A light year may have been intended to be used as a measure of distance, but it can also be used as a measure of time or speed.

A light-year is the distance light travels in one year.

In other words, distance measured by speed and time.

But can also be used as a measurement of time by distance and speed.

If I know I've travelled 100km at 50kph I can calculate I've travelled for 2 hours (no clock required).

velocity = distance / time

The unit of velocity is m/s, and distance is in meters, and time is in seconds.

This relationship doesn't mean that units of distance and time can be interchanged. Which is what you seem to be implying.
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Old 18th July 2022, 02:42 PM   #4
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Originally Posted by Mike Helland View Post
velocity = distance / time

The unit of velocity is m/s, and distance is in meters, and time is in seconds.

This relationship doesn't mean that units of distance and time can be interchanged. Which is what you seem to be implying.

They can be when a specific constant velocity is established or assumed. Such as, that of light in vacuum.

At least, that's what my physics professors and textbooks thought when teaching relativity. I still refer to 12-inch units of displacement as nanoseconds, as often as not.

But it works in more casual usages as well. For instance most people in the US have a general idea of how rapidly one travels on interstate highways, so it's perfectly reasonable and common to say things like, "Minneapolis and Chicago might look close together on a map, but they're seven hours apart."
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Old 18th July 2022, 04:06 PM   #5
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Originally Posted by Mike Helland View Post
velocity = distance / time

The unit of velocity is m/s, and distance is in meters, and time is in seconds.

This relationship doesn't mean that units of distance and time can be interchanged. Which is what you seem to be implying.
I'm not talking about velocity.

If I've travelled 100km (distance) at 50kph (speed) I've travelled for 2 hours (time).

If I've travelled at 50kph (speed) for 2 hours (time) I've travelled 100km (distance).

If I’ve travelled for 2 hours (time) for 100km (distance) I’ve travelled at 50kph (speed).

All these assume speed is constant.
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Old 18th July 2022, 04:46 PM   #6
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Originally Posted by ynot View Post
I'm not talking about velocity.

If I've travelled 100km (distance) at 50kph (speed) I've travelled for 2 hours (time).

If I've travelled at 50kph (speed) for 2 hours (time) I've travelled 100km (distance).

If I’ve travelled for 2 hours (time) for 100km (distance) I’ve travelled at 50kph (speed).

All these assume speed is constant.
Sure. What is there to physics really besides relationships?

speed = distance / time
F=ma
E=mc2

And on and on. Knowing some things helps you find out other things.

But a light year is a unit of distance, not time. End of story.
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Old 18th July 2022, 04:49 PM   #7
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Originally Posted by Skeptic Ginger View Post
Why a lightyear is a measure of distance and time.

We all agree it is a measure of distance.

But when talking about things like how much time it takes someone/something to go a certain distance and we are using a lightyear scale, then lightyears can be a measure of time.

Eg: if we are traveling 10% the speed of light then it would take us 40 years to travel 4 lightyears. Provided I did the math right, doesn't that make lightyears a measure of time?
Can you suggest a practical scenario where I might use a light year as a unit of time?

Under what circumstances would a light year mean a measure of time other than just a year?
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Old 18th July 2022, 04:58 PM   #8
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Originally Posted by Skeptical Greg View Post
Can you suggest a practical scenario where I might use a light year as a unit of time?

Under what circumstances would a light year mean a measure of time other than just a year?
A light year is a unit of length:

https://en.wikipedia.org/wiki/Light-year

It is not a measurement of time, not a year, not anything of the sort. It's a distance.

What is going on today?
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Old 18th July 2022, 05:17 PM   #9
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i was just responding to SkepticGinger's Post

Originally Posted by Skeptic Ginger View Post
Why a lightyear is a measure of distance and time.

We all agree it is a measure of distance.

But when talking about things like how much time it takes someone/something to go a certain distance and we are using a lightyear scale, then lightyears can be a measure of time.

Eg: if we are traveling 10% the speed of light then it would take us 40 years to travel 4 lightyears. Provided I did the math right, doesn't that make lightyears a measure of time?
A couple of other posters seemed to have agreed that it is a valid measure of time..

I'm with you.
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Old 18th July 2022, 06:50 PM   #10
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Originally Posted by Skeptical Greg View Post
Can you suggest a practical scenario where I might use a light year as a unit of time?

Under what circumstances would a light year mean a measure of time other than just a year?
"Other than a year" doesn't make sense.

I'm not sure why there's all this pedantry over lightyears being a measure of time (aka year) and distance (aka the distance light travels in a year).

I gave an example, if X is traveling at 10% the speed of light it will take X 40 years to go 4 light years.

If Y traveled for 40 years at 10% the speed of light, Y would have traveled 4 lightyears.


Last time I checked we weren't students in a physics class with an anal retentive professor. What's the big deal?
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Old 18th July 2022, 06:59 PM   #11
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Originally Posted by Skeptic Ginger View Post
Last time I checked we weren't students in a physics class with an anal retentive professor. What's the big deal?
We're on a skeptic's forum, which is almost as good.
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Old 18th July 2022, 07:00 PM   #12
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Originally Posted by Skeptic Ginger View Post
It's both.
Originally Posted by Mike Helland View Post
A light year is a unit of length:

https://en.wikipedia.org/wiki/Light-year

It is not a measurement of time, not a year, not anything of the sort. It's a distance.

What is going on today?
Sorry Ginger. Mike is right. It's not a measurement of time.

That you might use the variable of speed of light to measure the time it takes to travel a specific distance does not change the fact that a lightyear is a distance measurement. If you can travel at specific percentage of the speed of light doesn't ever change that.
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Old 18th July 2022, 07:04 PM   #13
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Originally Posted by Skeptic Ginger View Post
"Other than a year" doesn't make sense.

I'm not sure why there's all this pedantry over lightyears being a measure of time (aka year) and distance (aka the distance light travels in a year).

I gave an example, if X is traveling at 10% the speed of light it will take X 40 years to go 4 light years.
distance measurement

If Y traveled for 40 years at 10% the speed of light, Y would have traveled 4 lightyears.
distance measurement

Last time I checked we weren't students in a physics class with an anal retentive professor. What's the big deal?
It's not a big deal. Still, we should get it right.
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Old 18th July 2022, 07:08 PM   #14
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Originally Posted by Skeptic Ginger View Post
"Other than a year" doesn't make sense.

I'm not sure why there's all this pedantry over lightyears being a measure of time (aka year) and distance (aka the distance light travels in a year).

I gave an example, if X is traveling at 10% the speed of light it will take X 40 years to go 4 light years.

If Y traveled for 40 years at 10% the speed of light, Y would have traveled 4 lightyears.


Last time I checked we weren't students in a physics class with an anal retentive professor. What's the big deal?
If I'm standing still waiting for my bartender to serve me my drink, how many fractions of a light year have I been waiting? How many fractions of a mile per second have I been waiting?

When you're recording how long since a patient's last dose, do you record in minutes, or in inches per second ? What on earth kind of nurse would record time intervals in units of distance per time at a certain speed?
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Old 18th July 2022, 07:57 PM   #15
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Originally Posted by Skeptic Ginger View Post
Why a lightyear is a measure of distance and time.

We all agree it is a measure of distance.

But when talking about things like how much time it takes someone/something to go a certain distance and we are using a lightyear scale, then lightyears can be a measure of time.

Eg: if we are traveling 10% the speed of light then it would take us 40 years to travel 4 lightyears. Provided I did the math right, doesn't that make lightyears a measure of time?
If you're traveling 10% of 60 mph it would take you 10 hours to travel 60 miles. Ergo miles is a unit of time?
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Old 18th July 2022, 08:10 PM   #16
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Other than academically correct usage (which doesn't have much importance for most people), is there really any difference in saying “It’s a light year’s distance away” and “It’s a light year’s time away”. Aren’t they essentially the same measurement seen from different perspectives? And aren't they both valid/true?
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Old 18th July 2022, 08:51 PM   #17
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Originally Posted by arthwollipot View Post
We're on a skeptic's forum, which is almost as good.
That's no reason to be pedantic.

We all know what a lightyear is. ...


Oh for pity's sake, everybody pile on because it's so important.

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Old 18th July 2022, 08:54 PM   #18
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Originally Posted by acbytesla View Post
It's not a big deal. Still, we should get it right.
Well you go right ahead and get it right. I'm just fine with how I use the term.
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Old 18th July 2022, 08:55 PM   #19
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Originally Posted by ynot View Post
Other than academically correct usage (which doesn't have much importance for most people), is there really any difference in saying “It’s a light year’s distance away” and “It’s a light year’s time away”. Aren’t they essentially the same measurement seen from different perspectives? And aren't they both true?
Thank you.

Like I said, this isn't a physics class. Surely some of you guys have better things to waste your time on.
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Old 18th July 2022, 08:55 PM   #20
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Originally Posted by ynot View Post
Other than academically correct usage (which doesn't have much importance for most people), is there really any difference in saying “It’s a light year’s distance away” and “It’s a light year’s time away”. Aren’t they essentially the same measurement seen from different perspectives? And aren't they both true?
Let's say you travel at 10% the speed of light.

And you travel for 10 years.

You will have traveled 10 years (duration) and 1 light year (distance).

A light year doesn't mean we're talking about light traveling at c. That's how its defined, but that's not an exclusive domain for its usage.
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Old 18th July 2022, 09:00 PM   #21
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Originally Posted by whoanellie View Post
If you're traveling 10% of 60 mph it would take you 10 hours to travel 60 miles. Ergo miles is a unit of time?
Given you are talking about relatively short distances on this planet, your analogy fails. Lightyears are huge distances across space and time.
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Old 18th July 2022, 09:10 PM   #22
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Originally Posted by Skeptic Ginger View Post
Thank you.

Like I said, this isn't a physics class. Surely some of you guys have better things to waste your time on.
Let's hope more amazing images become available soon so this thread can get back on topic.
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Old 18th July 2022, 09:17 PM   #23
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Originally Posted by ynot View Post
Other than academically correct usage (which doesn't have much importance for most people), is there really any difference in saying “It’s a light year’s distance away” and “It’s a light year’s time away”. Aren’t they essentially the same measurement seen from different perspectives? And aren't they both valid/true?
If the question is "how long is the trip", an answer in duration or distance would both be valid.

When a galaxy emits light very very far away, that light will encounter expanding space.

The distance the galaxy was when it emitted the light, let's say N million light years, the light will take N' million years to get there. Ignoring units, N will be less than N'. Considering the units, it's a fools errand to compare distance and duration.

The distance the galaxy is today will be N'' million light years, which will be greater than N million light years, the distance of the galaxy when the light we're receiving now was emitted.
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Old 18th July 2022, 09:25 PM   #24
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Originally Posted by Skeptic Ginger View Post
That's no reason to be pedantic.
It's never stopped us before.
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Old 18th July 2022, 09:29 PM   #25
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Originally Posted by arthwollipot View Post
It's never stopped us before.
Pedantic is my middle name.
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Old 18th July 2022, 10:10 PM   #26
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Originally Posted by arthwollipot View Post
It's never stopped us before.
Originally Posted by acbytesla View Post
Pedantic is my middle name.
That's reasonable. I got no issues on that front.
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Old 18th July 2022, 11:58 PM   #27
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Now that we've established all that, how many parsecs does it normally take to make the Kessel run?

Dave
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Old 19th July 2022, 12:21 AM   #28
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The problem is that once you are using light years then you are considering relativistic velocities. So if you travel ten light years at 80% speed of light it will not take you 12.5 years due to time dilation. It will be 12.5 years to an external observer. So no a light year cannot be used as a unit of time.
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Old 19th July 2022, 12:22 AM   #29
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Originally Posted by Planigale View Post
The problem is that once you are using light years then you are considering relativistic velocities. So if you travel ten light years at 80% speed of light it will not take you 12.5 years due to time dilation. It will be 12.5 years to an external observer. So no a light year cannot be used as a unit of time.
Ding, ding, ding, ding. Correct answer.
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Old 19th July 2022, 01:15 AM   #30
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Originally Posted by ynot View Post
Other than academically correct usage (which doesn't have much importance for most people), is there really any difference in saying “It’s a light year’s distance away” and “It’s a light year’s time away”.

Yes. One of them is wrong.

Quote:
Aren’t they essentially the same measurement seen from different perspectives?

No, a light year is the distance light travels in a year.

Quote:
And aren't they both valid/true?

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Old 19th July 2022, 03:18 AM   #31
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Huh... never heard of it being used as a unit of time before.
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Old 19th July 2022, 04:33 AM   #32
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It's either "light-year" or "lightyear" (or "Lightyear" if you're talking about Buzz, which I'm surprised no one has mentioned yet.) Not "light year".
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Old 19th July 2022, 04:41 AM   #33
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Originally Posted by HansMustermann View Post
Huh... never heard of it being used as a unit of time before.
My colleague used it as that the other day.. something like "that cad model will take a light year to complete". I had no idea if he meant the distance or the time which I guess is 1 year? I didn't bother to correct him.
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Old 19th July 2022, 04:53 AM   #34
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Originally Posted by alfaniner View Post
It's either "light-year" or "lightyear" (or "Lightyear" if you're talking about Buzz, which I'm surprised no one has mentioned yet.) Not "light year".
Damn. Now that you guys had settled the light year thingy, I was hoping you'd move on to find the definition of a heavy year. But never mind.
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Old 19th July 2022, 06:18 AM   #35
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Originally Posted by Skeptical Greg View Post
Can you suggest a practical scenario where I might use a light year as a unit of time?

Under what circumstances would a light year mean a measure of time other than just a year?
Originally Posted by Skeptic Ginger View Post
"Other than a year" doesn't make sense.

I'll try again.

You said a light year would be a measure of time.. How much time would it be?


Originally Posted by Skeptic Ginger View Post
That's no reason to be pedantic.

We all know what a lightyear is. ...
Maybe we all do now.

Quote:
Oh for pity's sake, everybody pile on because it's so important.
If it isn't so important, why were you so argumentative early on when you were informed that a light year is a measure of distance and not time?

We actually had to have a new thread to explain it.
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Last edited by Skeptical Greg; 19th July 2022 at 06:24 AM.
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Old 19th July 2022, 06:47 AM   #36
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Originally Posted by Skeptical Greg View Post
Can you suggest a practical scenario where I might use a light year as a unit of time?

Under what circumstances would a light year mean a measure of time other than just a year?
A commercial year is a 360-day period composed of 12 months of 30 days that is used by some businesses to internally track changes in accounts. Differences in the number of days in each calendar month are adjusted so that comparisons for sales, expenses, etc. It's like a year, but a little light. It can also be used to calculate the orbit of Nereid around Neptune.
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Old 19th July 2022, 07:41 AM   #37
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Old 19th July 2022, 12:41 PM   #38
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The meaning and usage of words aren’t the preserve of science. Colloquially/conversationally words have flexible common usage meanings. Scientifically words have more defined and specific agreed meaning because science needs to be more defined and specific. Yes, we all know what the title of this section of the forum is.

When talking to “the public” many science academics commonly say things like “This image comes from x light years away, so we’re looking back x years in time”. I've heard some say "With this image we're travelling x light years back in time". Anyone here ever told them they’re using light year incorrectly?
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Last edited by ynot; 19th July 2022 at 12:49 PM.
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Old 19th July 2022, 12:50 PM   #39
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What is a light-year

Originally Posted by ynot View Post
The meaning and usage of words aren’t the preserve of science. Colloquially/conversationally words have flexible common usage meanings. Scientifically words have more defined and specific agreed meaning because science needs to be more defined and specific. Yes I know what the title of this section of the forum is.

When talking to “the public” many science academics commonly say things like “This image comes from x light years away, so we’re looking back x years in time”. I've even heard some say "We're travelling x light years back in time. Anyone here ever told them they’re using light year incorrectly?

In the first case they aren’t using it wrong. They’re dividing the speed in light-years by the speed of light, and giving the time in years.

No different than saying “the train travelled at exactly 60mph got 60 miles, so it left one hour ago.” That is explicitly not saying that mph is a measure of time.

Now the “x light years back in time” is incorrect, and I’d like to know where this was used, cause I haven’t heard it from anyone knowledgeable. I suspect the context would make it clear they’re using it as shorthand for the above, though, or poetically/metaphorically. Something like when someone says “how far away is Townsville” and you answer “about an hour’s drive”, it doesn’t mean hours are a unit of distance. It doesn’t make light-years a unit of time.

Light-years are distance, not time. Neither scientifically Nor colloquially.


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Last edited by Hellbound; 19th July 2022 at 12:52 PM.
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Old 19th July 2022, 12:51 PM   #40
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Originally Posted by ynot View Post
The meaning and usage of words aren’t the preserve of science. Colloquially/conversationally words have flexible common usage meanings. Scientifically words have more defined and specific agreed meaning because science needs to be more defined and specific. Yes I know what the title of this section of the forum says.

When talking to “the public” many science academics commonly say things like “This image comes from x light years away, so we’re looking back x years in time”. I've even heard some say "We're travelling x light years back in time. Anyone here ever told them they’re using light year incorrectly?
The ones who are deriving elapsed time from a known distance traveled at a known speed are not using it incorrectly.

The ones who are substituting a known distance traveled at a known speed for an elapsed time are using it... idiosyncratically at best.

Say a nurse in Humboldt county, California, is to administer a dose of medicine to a patient once every 24 hours. Imagine the following bizarre conversation:

"Here's today's dose. I'll be back again tomorrow at 19,200 miles."

"What do you mean?"

"Well, we live at 40 degrees north latitude. Given the rate of the Earth's rotation, and the circumference of the parallel, I can derive the timing of your next dosage in terms of the distance we will have traveled due to the Earth's rotation over the next 24 hours."

"Wouldn't it make more sense to just say you'll be back tomorrow at the same time?"

"My point is that we can use earth-rotation-miles as a measure of time."

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