# B Why Lightyears? I don’t get it

#### Mickeydee

if light is affected by gravity (ie. bend, stretch, curve) inconsistent.
why do we measure distance in lightyear?

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#### pinball1970

Gold Member
if light is affected by gravity (ie. bend, stretch, curve) inconsistent.
why do we measure distance in lightyear?
Space is flat between galaxies and overall (mainly) in the observable universe so it makes sense to measure this way given the enormous distances.

Staff Emeritus
Furthermore, what is your alternative? Even if stacking metersticks were practical, they would have the same issues.

#### PeroK

Homework Helper
Gold Member
2018 Award
if light is affected by gravity (ie. bend, stretch, curve) inconsistent.
why do we measure distance in lightyear?
In modern physics the second has a definition and light speed is defined to be $299,792 km/s$.

So, the metre is in fact defined in terms of the speed of light.

It doesn't matter, therefore, whether we quote a distance in light years or in metres. It means exactly the same thing.

Your question could be rephrased by asking what we really mean when we say, for example, that a star is 25 light years away (or whatever the equivalent is in metres)? And what sort of measurement process are we assuming?

One answer is that you might as well use the path that light takes. As spacetime is curved, no other path makes any more sense and, indeed, the light path might seem the most natural.

#### Janus

Staff Emeritus
Gold Member
The "light year" is a standard. It is the distance light would travel in one Julian year (365.25 days) in a vacuum through flat space. So when we say something is 1 million light years away, we mean that it is 1 million multiples of that distance away, not that the light leaving it took exactly 1 million Julian years to reach us, taking into account any bending of the light path along the way.

Similarly we have the parsec. Which the distance at which an object would exhibit 1 arc sec of parallax. Again, if we say a galaxy is 1 billion parsecs away, we mean that it is 1 billion multiples of these units of distance away, not that we measured its parallax and came up with this distance from that measurement, as parallaxes at this distance are just too small to measure. The fact that we can't directly measure the parallax of this galaxy doesn't preclude us from giving its distance in parsecs.

#### timmdeeg

Gold Member
if light is affected by gravity (ie. bend, stretch, curve) inconsistent.
why do we measure distance in lightyear?
you seem to think of the Shapiro time delay. But if light crosses the whole milky way thereby passing near stars happens extremely seldom presumably.

#### pervect

Staff Emeritus
if light is affected by gravity (ie. bend, stretch, curve) inconsistent.
why do we measure distance in lightyear?
We can measure distances in any units we like - meters, light-years, inches, furlongs.

The constancy of the speed of light makes it a convenient way to standardized our distance measurements, in fact, the definition of the meter used to be the distance that light travelled in 1/299,792,458 of a second . Nowadays this defintion is worded a bit more abstractly, but it boils down to the same thing.

If you'd like to ask why, how, or when we settled on using light as the basis for our defintion, we can attempt to discuss that, but it's not clear if that's what you're asking.

This standard definition of the meter alone does not account for the stretching, bending, etc. of space-time. The defintion applies for small distances, where these effects are negligible. For small distances, the bending, stretching et al doesn't matter, similarly to the way one can draw local maps of the Earth's surface on a flat sheet of paper without difficutlies.

For large distance it does matters, just as for large distances on the Earth one needs to use a globe to get accurate visualizations of navigation and long distances.

The units still don't really matter - if you have a long distance, you have the same issues with regard to curvature, whether or not you call that distance 9.461e+15 meters or if you call it instead 1 light year.

#### FactChecker

Gold Member
2018 Award
Ignoring the recent measurements of gravity waves, the only thing we get from those distances is light (electromagnetic waves), so it is very natural and convenient to use lightyears as units of distance. It immediately tells us how far away it is was at the time of emission and how long ago that was.

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#### timmdeeg

Gold Member
It immediately tells us how far away it is and how long ago it was.
This could be misunderstood by the OP. I'd prefer to say how far it was away at the time of emission, if we talk about galaxies which recede with the Hubble flow.

#### FactChecker

Gold Member
2018 Award
This could be misunderstood by the OP. I'd prefer to say how far it was away at the time of emission, if we talk about galaxies which recede with the Hubble flow.
Good point. I edited it to be clearer.

"Why Lightyears? I don’t get it"

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