Thread: Light years.....

I realise that a light year is the measure of distance a particle of light can travel in a year from the point of view of an external observer, but this is highly subjective. If Einstein's theory of relativity is correct, the closer you get to light speed the more time slows down. So why isn't a light year measured taking relativity into account and going by a year from the point of view of the light particle? I'm interested to hear your thoughts on this.

Perhaps it is easier for basic calculations to use a simpler value for a light year rather than taking into account relativity.
i find relativity quite a difficult topic to make other people understand. but perhaps this just shows my inadequacy as a teacher.

Perhaps it is easier. But since when does science follow the path of least resistance?

Light does not have a valid interial reference frame to judge time from. IOW the phrase "time from light's point of view" has no meaning.

In any other frame, light always travels at c relative to to that frame.

Ok maybe time from lights point of view was a bad choice of words. What I meant was if you were yourself travelling at light speed would why would you not measure a light year based on your perception of time rather than from an external observer.

Originally Posted by Dimension

Ok maybe time from lights point of view was a bad choice of words. What I meant was if you were yourself travelling at light speed would why would you not measure a light year based on your perception of time rather than from an external observer.

It is equally meaningless to talk about your perception of time and distance while traveling at the speed of light. It's not what is traveling at c, it is the very speed itself that makes for a non-valid frame of reference.

How so?

I just want to add that we seemingly have only used devices to test the idea that time slows down as you go faster, such as a photon clock, but we have never accelerated anything more than particles to C. If we were to send a visible piece of matter to C, then the effects could change considerably.

Originally Posted by Dimension

How so?

Consider what happens when when you enter c for v in the equation for time dilation and length contraction:

Time dilation:

T = T`/sqrt(1-c²/c²) = T`/sqrt(1-1) = T'/sqrt(0) = T'/0

Division by zero is undefined and has no answer. IOW meaningless.

Length contraction

L = L`* sqrt(1-c²/c²) = L`*sqrt(1-1) = L'*sqrt(0) = T'*0 =0

Here, at least you avoid division by zero, but it's still no good. Length contraction is what you measure as happening to that which has a relative motion to you. Meaning, for an object traveling at c, from its frame of reference, it is the universe that contracts. But if the universe shrinks to zero length there is no distinction between 1cm, 1meter, 1 kilometer 300 billion kilometers, Etc. they are all the same distance. Distance loses all meaning. And how can you even say that you have a velocity of c with respect to the universe, when there is no distance to plot against time by which to measure velocity.

Again, you get a meaningless answer.

Interesting.

Originally Posted by Janus

Originally Posted by Dimension

How so?

Consider what happens when when you enter c for v in the equation for time dilation and length contraction:

Time dilation:

T = T`/sqrt(1-c²/c²) = T`/sqrt(1-1) = T'/sqrt(0) = T'/0

Division by zero is undefined and has no answer. IOW meaningless.

Length contraction

L = L`* sqrt(1-c²/c²) = L`*sqrt(1-1) = L'*sqrt(0) = T'*0 =0

Here, at least you avoid division by zero, but it's still no good. Length contraction is what you measure as happening to that which has a relative motion to you. Meaning, for an object traveling at c, from its frame of reference, it is the universe that contracts. But if the universe shrinks to zero length there is no distinction between 1cm, 1meter, 1 kilometer 300 billion kilometers, Etc. they are all the same distance. Distance loses all meaning. And how can you even say that you have a velocity of c with respect to the universe, when there is no distance to plot against time by which to measure velocity.

Again, you get a meaningless answer.

isn't it c^2/v^2?

Maybe they should try making a new equation?

Originally Posted by Cold Fusion

Maybe they should try making a new equation?

There's no reason to.

But as you showed it gives a meaningless answer....why not try to improve it so that it gives a reasonable answer?

I believe that the 'time' it takes for light to travel in one year can be somewhat misleading. now, let me first say, that i am by no means even close to pretending to be an expert on the matter. However, it seems to me that 'time' is relative only because we created this 'time' as a measurement, so that does not necessarily affect the speed of the light. hmm...kinda confusing, let me see if i can clarify this.

no matter what you call it, in a given time frame, absent of any black holes or anything of that sort, light will always travel from point A to point B, in a given amount of time that will always be exactly the same. However, time gives us the illusion of slowing down the closer we get to the speed to light, simply (and i know this is WAY over simplification), we are going so fast. Yes we can measure how much time slows down at these speeds, but that still doesn't effect how long this light takes to get from point a to point b...

hope this helps, and if i'm totally wrong, please feel free to bash me

Originally Posted by Cold Fusion

But as you showed it gives a meaningless answer....why not try to improve it so that it gives a reasonable answer?

Because there is no reason to believe that there is a non-meaningless answer to be gotten.

I do not understand why folks insist on making heavy weather out of this concept. A light-year is a measure of distance, period. It was defined before relativity was understood. Relativity plays no part in the distance between two points.* Convert the distance to kilometers and this point will become quite clear.

*Some folks will quibble that yardstick traveling at near light speed will change its dimension. So what? The distance between the Sun and Alpha Centauri (4.3 light-years) has not changed. The folks who stayed home will confirm this fact.

Surely it depends upon your reference frame?

Originally Posted by Ophiolite

Surely it depends upon your reference frame?

Only if you seek unnecessary complications. If the distance between A and B is xxx km, will this distance ever change according to your frame of reference?

Are we measuring an observed light-year? Or just a convenient calculation tool for measuring distance?

If we get in a car that can travel at the speed of light, how far will it go if we drive it for a year?

299,792,458 x 31,557,600 = 9,460,730,472,580,800m.