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Is a light year a different distance if measured from a moving object?
The Effects of Moving Matter Across Light-Year distancesSpeed of light and distanceRelative motion at almost the speed of lightIs distance always 0 relative to an object moving at speed of light $c$?If all motion is relative, how does light have a finite speed?Is there a system of units that replaces time with light-distance?Making observations from a particle at 0.99c, will all “slow” objects move at 0.99c?
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The speed of light is absolute, but time is relative. So would a light-year for us on earth be a different distance from a light-year on a different uniformly moving object? Why or why not?
speed-of-light relativity distance
edited 51 mins ago
David Z♦
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$begingroup$
The speed of light is absolute, but time is relative. So would a light-year for us on earth be a different distance from a light-year on a different uniformly moving object? Why or why not?
speed-of-light relativity distance
edited 51 mins ago
David Z♦
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asked yesterday
Elliot ChanceElliot Chance
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$begingroup$
The speed of light is absolute, but time is relative. So would a light-year for us on earth be a different distance from a light-year on a different uniformly moving object? Why or why not?
speed-of-light relativity distance
edited 51 mins ago
David Z♦
64.9k23 gold badges141 silver badges258 bronze badges
asked yesterday
Elliot ChanceElliot Chance
2092 silver badges6 bronze badges
New contributor
Elliot Chance is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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$endgroup$
The speed of light is absolute, but time is relative. So would a light-year for us on earth be a different distance from a light-year on a different uniformly moving object? Why or why not?
speed-of-light relativity distance
speed-of-light relativity distance
edited 51 mins ago
David Z♦
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asked yesterday
Elliot ChanceElliot Chance
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edited 51 mins ago
David Z♦
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asked yesterday
Elliot ChanceElliot Chance
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edited 51 mins ago
David Z♦
64.9k23 gold badges141 silver badges258 bronze badges
edited 51 mins ago
David Z♦
64.9k23 gold badges141 silver badges258 bronze badges
edited 51 mins ago
David Z♦
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64.9k23 gold badges141 silver badges258 bronze badges
asked yesterday
Elliot ChanceElliot Chance
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Elliot Chance is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked yesterday
Elliot ChanceElliot Chance
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asked yesterday
Elliot ChanceElliot Chance
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2092 silver badges6 bronze badges
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Elliot Chance is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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5 Answers
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votes
$begingroup$
The distance light travels in a given period is the same for every observer. That's the whole point of relativity.
You can figure out for yourself almost all the effects predicted by relativity if you start with that assumption and think through its consequences.
Indeed, the reason why times have to be relative is to allow observers who are moving relative to each other to agree on the speed of light.
Take the classic set-up where you are on a railway carriage and I am on the platform of the station you are passing.
As you pass me I flash a laser along the platform.
After what seems to me to be 100 nanoseconds, light has traveled to a certain point 100 feet along my platform (a foot is a light-nanosecond, hence a shorter version of a light-year). To you, however, that point is not 100 feet away, as you have been travelling relative to the platform, so it is some other distance.
If we both agree that the speed of light is the same, ie the ratio of the distance it has covered to the time it has taken, when we each think it has covered a different distance, then we must disagree about the elapsed time, ie time is relative.
answered yesterday
Marco OcramMarco Ocram
1,3092 silver badges14 bronze badges
$endgroup$
1
$begingroup$
Thank you for the explanation, it's easier to visualise (after reading it a few times). However, what if the platform was 100 feet long and on the other end of the platform was a wall. If the person on the train started a clock at the start of the platform and timed 99ns then dropped a ball out of the window. Would that ball immediately hit the wall or would it appear before or after the wall?
$endgroup$
– Elliot Chance
23 hours ago
$begingroup$
Hi Elliot, it is impossible to answer that question unless you know what speed the train is travelling compared with the platform (and its direction!). Let's assume the train is travelling towards the wall at 100 feet per second. After 99ns it will hardly have budged.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
However, I think I know where you are coming from. Imagine the speed of light was quite low, say a foot per second, and the train was travelling an inch per second. And imagine that all along the carriage you have mates sitting ten feet apart. The light leaves you and travels along the carriage, and your mates record the time it passed them. When you compare notes afterwards your pals see that there are ten second gaps between the successive times they have noted (assuming their watches are good)...
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
I on my platform have mates ten feet apart, and we record our times too. Again we find that ten seconds pass on our watches every time the light goes past another of my mates as it travels along the platform.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
Imagine your friends made chalk marks on the platform as light based them, and mine made marks on the side of the train. Having done the experiment we reverse the train to its original position, and we compare the positions of our chalk marks and the times we each noted. We will find that the chalk marks on the train and platform don't line up, and the times we each recorded for light passing from one of our friends to another don't agree. If we all think the speed of light is a foot per second, how do we explain the discrepancies? Try working that out.
$endgroup$
– Marco Ocram
21 hours ago
|
show 5 more comments
$begingroup$
A light-year is exactly 9,460,730,472,580,800 meters, the distance light travels in one Julian Earth year. Time dilation is irrelevant to its definition.
answered yesterday
G. SmithG. Smith
23.4k2 gold badges41 silver badges77 bronze badges
$endgroup$
1
$begingroup$
Yes, a simple Google search returns that. My question is, will that distance differ for an observer is in a different uniformly moving body? So would the distance still be relevant if we gave this distance to someone on mars (even if the different was negligible)?
$endgroup$
– Elliot Chance
yesterday
$begingroup$
Every observer thinks that a meter stick, or a light-year stick, at rest relative to that observer is the length it is supposed to be. But observers measure moving meter sticks or light-year sticks as being length-contracted.
$endgroup$
– G. Smith
yesterday
1
$begingroup$
Alright, well I guess don't understand the question well enough to ask it then.
$endgroup$
– Elliot Chance
yesterday
3
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
1
$begingroup$
Ignore the words "light" and "year" and think of it in the same you think of "meters" or "kilometers" and any confusion should disappear
$endgroup$
– BlueRaja - Danny Pflughoeft
12 hours ago
|
show 1 more comment
$begingroup$
In 1983 the General Conference on Weights and Measures determined the definition of a meter to be the distance light travels in a vacuum in 1/299,792,458 of a second, it's current definition. So since all reference frames see light speed the same, a light year cannot change, it is 9,460,730,472,580,800 meters, and the length of a meter is based on the speed of light, a constant, therefore a light year is a constant.
answered 23 hours ago
Adrian HowardAdrian Howard
1,1182 silver badges10 bronze badges
$endgroup$
2
$begingroup$
That makes sense in terms of a description, but if I wanted to travel one light-year and I was traveling at 0.5c, how long would I travel for? Still a year?
$endgroup$
– Elliot Chance
23 hours ago
3
$begingroup$
An observer at your starting point would observe you to have travelled for 2 years. However, due to time dilation, the trip would take you only 1.73 years, subjectively. Since you both agree on the distance travelled, you will assume you were traveling at ~0.58c. On the other hand, if you measure yourself to be moving at 0.5c, you will not be surprised to find that you have taken exactly 2 years to arrive, but the observer will see you to have taken longer than 2 years, and thus traveling a bit slower than 0.5c.
$endgroup$
– chepner
16 hours ago
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
$begingroup$
@JiK: you misread it, the speed of light is the constant, therefore distances based on it cannot change
$endgroup$
– Adrian Howard
15 hours ago
2
$begingroup$
@AdrianHoward The asker already knows that the speed of light is constant. He is confused about time dilation, and this answer does nothing to explain it.
$endgroup$
– JiK
15 hours ago
|
show 2 more comments
$begingroup$
Anybody in an inertial reference frame (i.e., considers themselves to be at rest), who fires a light beam at a mirror and sees the reflection arrive after exactly 1 Julian Earth year according to their reckoning, will conclude that the light has travelled one light year of distance.
Another person, watching the same events (light beam leaving flashlight at event X, hitting mirror at event Y, reflection arriving back to flashlight at event Z) will also conclude the light traveled one light year of distance and that it took exactly 1 Julian Earth year of their personal time for it to happen. However, they may disagree about the other user having remained at rest, and about what the other user's clock will read.
answered 10 hours ago
Ross PresserRoss Presser
3931 silver badge11 bronze badges
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$begingroup$
The definition of a meter is not constant. Presume that one day we scientists determine a way to more accurately measure the speed of light in a vacuum. The constant (c) would not change in this scenario but rather the definition of a meter.
answered 5 mins ago
Bipedal Beacon of UnorthodoxyBipedal Beacon of Unorthodoxy
11 bronze badge
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5 Answers
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votes
5 Answers
5
active
oldest
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active
oldest
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active
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votes
$begingroup$
The distance light travels in a given period is the same for every observer. That's the whole point of relativity.
You can figure out for yourself almost all the effects predicted by relativity if you start with that assumption and think through its consequences.
Indeed, the reason why times have to be relative is to allow observers who are moving relative to each other to agree on the speed of light.
Take the classic set-up where you are on a railway carriage and I am on the platform of the station you are passing.
As you pass me I flash a laser along the platform.
After what seems to me to be 100 nanoseconds, light has traveled to a certain point 100 feet along my platform (a foot is a light-nanosecond, hence a shorter version of a light-year). To you, however, that point is not 100 feet away, as you have been travelling relative to the platform, so it is some other distance.
If we both agree that the speed of light is the same, ie the ratio of the distance it has covered to the time it has taken, when we each think it has covered a different distance, then we must disagree about the elapsed time, ie time is relative.
answered yesterday
Marco OcramMarco Ocram
1,3092 silver badges14 bronze badges
$endgroup$
1
$begingroup$
Thank you for the explanation, it's easier to visualise (after reading it a few times). However, what if the platform was 100 feet long and on the other end of the platform was a wall. If the person on the train started a clock at the start of the platform and timed 99ns then dropped a ball out of the window. Would that ball immediately hit the wall or would it appear before or after the wall?
$endgroup$
– Elliot Chance
23 hours ago
$begingroup$
Hi Elliot, it is impossible to answer that question unless you know what speed the train is travelling compared with the platform (and its direction!). Let's assume the train is travelling towards the wall at 100 feet per second. After 99ns it will hardly have budged.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
However, I think I know where you are coming from. Imagine the speed of light was quite low, say a foot per second, and the train was travelling an inch per second. And imagine that all along the carriage you have mates sitting ten feet apart. The light leaves you and travels along the carriage, and your mates record the time it passed them. When you compare notes afterwards your pals see that there are ten second gaps between the successive times they have noted (assuming their watches are good)...
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
I on my platform have mates ten feet apart, and we record our times too. Again we find that ten seconds pass on our watches every time the light goes past another of my mates as it travels along the platform.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
Imagine your friends made chalk marks on the platform as light based them, and mine made marks on the side of the train. Having done the experiment we reverse the train to its original position, and we compare the positions of our chalk marks and the times we each noted. We will find that the chalk marks on the train and platform don't line up, and the times we each recorded for light passing from one of our friends to another don't agree. If we all think the speed of light is a foot per second, how do we explain the discrepancies? Try working that out.
$endgroup$
– Marco Ocram
21 hours ago
|
show 5 more comments
$begingroup$
The distance light travels in a given period is the same for every observer. That's the whole point of relativity.
You can figure out for yourself almost all the effects predicted by relativity if you start with that assumption and think through its consequences.
Indeed, the reason why times have to be relative is to allow observers who are moving relative to each other to agree on the speed of light.
Take the classic set-up where you are on a railway carriage and I am on the platform of the station you are passing.
As you pass me I flash a laser along the platform.
After what seems to me to be 100 nanoseconds, light has traveled to a certain point 100 feet along my platform (a foot is a light-nanosecond, hence a shorter version of a light-year). To you, however, that point is not 100 feet away, as you have been travelling relative to the platform, so it is some other distance.
If we both agree that the speed of light is the same, ie the ratio of the distance it has covered to the time it has taken, when we each think it has covered a different distance, then we must disagree about the elapsed time, ie time is relative.
answered yesterday
Marco OcramMarco Ocram
1,3092 silver badges14 bronze badges
$endgroup$
1
$begingroup$
Thank you for the explanation, it's easier to visualise (after reading it a few times). However, what if the platform was 100 feet long and on the other end of the platform was a wall. If the person on the train started a clock at the start of the platform and timed 99ns then dropped a ball out of the window. Would that ball immediately hit the wall or would it appear before or after the wall?
$endgroup$
– Elliot Chance
23 hours ago
$begingroup$
Hi Elliot, it is impossible to answer that question unless you know what speed the train is travelling compared with the platform (and its direction!). Let's assume the train is travelling towards the wall at 100 feet per second. After 99ns it will hardly have budged.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
However, I think I know where you are coming from. Imagine the speed of light was quite low, say a foot per second, and the train was travelling an inch per second. And imagine that all along the carriage you have mates sitting ten feet apart. The light leaves you and travels along the carriage, and your mates record the time it passed them. When you compare notes afterwards your pals see that there are ten second gaps between the successive times they have noted (assuming their watches are good)...
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
I on my platform have mates ten feet apart, and we record our times too. Again we find that ten seconds pass on our watches every time the light goes past another of my mates as it travels along the platform.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
Imagine your friends made chalk marks on the platform as light based them, and mine made marks on the side of the train. Having done the experiment we reverse the train to its original position, and we compare the positions of our chalk marks and the times we each noted. We will find that the chalk marks on the train and platform don't line up, and the times we each recorded for light passing from one of our friends to another don't agree. If we all think the speed of light is a foot per second, how do we explain the discrepancies? Try working that out.
$endgroup$
– Marco Ocram
21 hours ago
|
show 5 more comments
$begingroup$
The distance light travels in a given period is the same for every observer. That's the whole point of relativity.
You can figure out for yourself almost all the effects predicted by relativity if you start with that assumption and think through its consequences.
Indeed, the reason why times have to be relative is to allow observers who are moving relative to each other to agree on the speed of light.
Take the classic set-up where you are on a railway carriage and I am on the platform of the station you are passing.
As you pass me I flash a laser along the platform.
After what seems to me to be 100 nanoseconds, light has traveled to a certain point 100 feet along my platform (a foot is a light-nanosecond, hence a shorter version of a light-year). To you, however, that point is not 100 feet away, as you have been travelling relative to the platform, so it is some other distance.
If we both agree that the speed of light is the same, ie the ratio of the distance it has covered to the time it has taken, when we each think it has covered a different distance, then we must disagree about the elapsed time, ie time is relative.
answered yesterday
Marco OcramMarco Ocram
1,3092 silver badges14 bronze badges
$endgroup$
The distance light travels in a given period is the same for every observer. That's the whole point of relativity.
You can figure out for yourself almost all the effects predicted by relativity if you start with that assumption and think through its consequences.
Indeed, the reason why times have to be relative is to allow observers who are moving relative to each other to agree on the speed of light.
Take the classic set-up where you are on a railway carriage and I am on the platform of the station you are passing.
As you pass me I flash a laser along the platform.
After what seems to me to be 100 nanoseconds, light has traveled to a certain point 100 feet along my platform (a foot is a light-nanosecond, hence a shorter version of a light-year). To you, however, that point is not 100 feet away, as you have been travelling relative to the platform, so it is some other distance.
If we both agree that the speed of light is the same, ie the ratio of the distance it has covered to the time it has taken, when we each think it has covered a different distance, then we must disagree about the elapsed time, ie time is relative.
answered yesterday
Marco OcramMarco Ocram
1,3092 silver badges14 bronze badges
answered yesterday
Marco OcramMarco Ocram
1,3092 silver badges14 bronze badges
answered yesterday
Marco OcramMarco Ocram
1,3092 silver badges14 bronze badges
answered yesterday
Marco OcramMarco Ocram
1,3092 silver badges14 bronze badges
1,3092 silver badges14 bronze badges
1
$begingroup$
Thank you for the explanation, it's easier to visualise (after reading it a few times). However, what if the platform was 100 feet long and on the other end of the platform was a wall. If the person on the train started a clock at the start of the platform and timed 99ns then dropped a ball out of the window. Would that ball immediately hit the wall or would it appear before or after the wall?
$endgroup$
– Elliot Chance
23 hours ago
$begingroup$
Hi Elliot, it is impossible to answer that question unless you know what speed the train is travelling compared with the platform (and its direction!). Let's assume the train is travelling towards the wall at 100 feet per second. After 99ns it will hardly have budged.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
However, I think I know where you are coming from. Imagine the speed of light was quite low, say a foot per second, and the train was travelling an inch per second. And imagine that all along the carriage you have mates sitting ten feet apart. The light leaves you and travels along the carriage, and your mates record the time it passed them. When you compare notes afterwards your pals see that there are ten second gaps between the successive times they have noted (assuming their watches are good)...
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
I on my platform have mates ten feet apart, and we record our times too. Again we find that ten seconds pass on our watches every time the light goes past another of my mates as it travels along the platform.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
Imagine your friends made chalk marks on the platform as light based them, and mine made marks on the side of the train. Having done the experiment we reverse the train to its original position, and we compare the positions of our chalk marks and the times we each noted. We will find that the chalk marks on the train and platform don't line up, and the times we each recorded for light passing from one of our friends to another don't agree. If we all think the speed of light is a foot per second, how do we explain the discrepancies? Try working that out.
$endgroup$
– Marco Ocram
21 hours ago
|
show 5 more comments
1
$begingroup$
Thank you for the explanation, it's easier to visualise (after reading it a few times). However, what if the platform was 100 feet long and on the other end of the platform was a wall. If the person on the train started a clock at the start of the platform and timed 99ns then dropped a ball out of the window. Would that ball immediately hit the wall or would it appear before or after the wall?
$endgroup$
– Elliot Chance
23 hours ago
$begingroup$
Hi Elliot, it is impossible to answer that question unless you know what speed the train is travelling compared with the platform (and its direction!). Let's assume the train is travelling towards the wall at 100 feet per second. After 99ns it will hardly have budged.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
However, I think I know where you are coming from. Imagine the speed of light was quite low, say a foot per second, and the train was travelling an inch per second. And imagine that all along the carriage you have mates sitting ten feet apart. The light leaves you and travels along the carriage, and your mates record the time it passed them. When you compare notes afterwards your pals see that there are ten second gaps between the successive times they have noted (assuming their watches are good)...
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
I on my platform have mates ten feet apart, and we record our times too. Again we find that ten seconds pass on our watches every time the light goes past another of my mates as it travels along the platform.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
Imagine your friends made chalk marks on the platform as light based them, and mine made marks on the side of the train. Having done the experiment we reverse the train to its original position, and we compare the positions of our chalk marks and the times we each noted. We will find that the chalk marks on the train and platform don't line up, and the times we each recorded for light passing from one of our friends to another don't agree. If we all think the speed of light is a foot per second, how do we explain the discrepancies? Try working that out.
$endgroup$
– Marco Ocram
21 hours ago
1
1
$begingroup$
Thank you for the explanation, it's easier to visualise (after reading it a few times). However, what if the platform was 100 feet long and on the other end of the platform was a wall. If the person on the train started a clock at the start of the platform and timed 99ns then dropped a ball out of the window. Would that ball immediately hit the wall or would it appear before or after the wall?
$endgroup$
– Elliot Chance
23 hours ago
$begingroup$
Thank you for the explanation, it's easier to visualise (after reading it a few times). However, what if the platform was 100 feet long and on the other end of the platform was a wall. If the person on the train started a clock at the start of the platform and timed 99ns then dropped a ball out of the window. Would that ball immediately hit the wall or would it appear before or after the wall?
$endgroup$
– Elliot Chance
23 hours ago
$begingroup$
Hi Elliot, it is impossible to answer that question unless you know what speed the train is travelling compared with the platform (and its direction!). Let's assume the train is travelling towards the wall at 100 feet per second. After 99ns it will hardly have budged.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
Hi Elliot, it is impossible to answer that question unless you know what speed the train is travelling compared with the platform (and its direction!). Let's assume the train is travelling towards the wall at 100 feet per second. After 99ns it will hardly have budged.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
However, I think I know where you are coming from. Imagine the speed of light was quite low, say a foot per second, and the train was travelling an inch per second. And imagine that all along the carriage you have mates sitting ten feet apart. The light leaves you and travels along the carriage, and your mates record the time it passed them. When you compare notes afterwards your pals see that there are ten second gaps between the successive times they have noted (assuming their watches are good)...
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
However, I think I know where you are coming from. Imagine the speed of light was quite low, say a foot per second, and the train was travelling an inch per second. And imagine that all along the carriage you have mates sitting ten feet apart. The light leaves you and travels along the carriage, and your mates record the time it passed them. When you compare notes afterwards your pals see that there are ten second gaps between the successive times they have noted (assuming their watches are good)...
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
I on my platform have mates ten feet apart, and we record our times too. Again we find that ten seconds pass on our watches every time the light goes past another of my mates as it travels along the platform.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
I on my platform have mates ten feet apart, and we record our times too. Again we find that ten seconds pass on our watches every time the light goes past another of my mates as it travels along the platform.
$endgroup$
– Marco Ocram
22 hours ago
$begingroup$
Imagine your friends made chalk marks on the platform as light based them, and mine made marks on the side of the train. Having done the experiment we reverse the train to its original position, and we compare the positions of our chalk marks and the times we each noted. We will find that the chalk marks on the train and platform don't line up, and the times we each recorded for light passing from one of our friends to another don't agree. If we all think the speed of light is a foot per second, how do we explain the discrepancies? Try working that out.
$endgroup$
– Marco Ocram
21 hours ago
$begingroup$
Imagine your friends made chalk marks on the platform as light based them, and mine made marks on the side of the train. Having done the experiment we reverse the train to its original position, and we compare the positions of our chalk marks and the times we each noted. We will find that the chalk marks on the train and platform don't line up, and the times we each recorded for light passing from one of our friends to another don't agree. If we all think the speed of light is a foot per second, how do we explain the discrepancies? Try working that out.
$endgroup$
– Marco Ocram
21 hours ago
|
show 5 more comments
$begingroup$
A light-year is exactly 9,460,730,472,580,800 meters, the distance light travels in one Julian Earth year. Time dilation is irrelevant to its definition.
answered yesterday
G. SmithG. Smith
23.4k2 gold badges41 silver badges77 bronze badges
$endgroup$
1
$begingroup$
Yes, a simple Google search returns that. My question is, will that distance differ for an observer is in a different uniformly moving body? So would the distance still be relevant if we gave this distance to someone on mars (even if the different was negligible)?
$endgroup$
– Elliot Chance
yesterday
$begingroup$
Every observer thinks that a meter stick, or a light-year stick, at rest relative to that observer is the length it is supposed to be. But observers measure moving meter sticks or light-year sticks as being length-contracted.
$endgroup$
– G. Smith
yesterday
1
$begingroup$
Alright, well I guess don't understand the question well enough to ask it then.
$endgroup$
– Elliot Chance
yesterday
3
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
1
$begingroup$
Ignore the words "light" and "year" and think of it in the same you think of "meters" or "kilometers" and any confusion should disappear
$endgroup$
– BlueRaja - Danny Pflughoeft
12 hours ago
|
show 1 more comment
$begingroup$
A light-year is exactly 9,460,730,472,580,800 meters, the distance light travels in one Julian Earth year. Time dilation is irrelevant to its definition.
answered yesterday
G. SmithG. Smith
23.4k2 gold badges41 silver badges77 bronze badges
$endgroup$
1
$begingroup$
Yes, a simple Google search returns that. My question is, will that distance differ for an observer is in a different uniformly moving body? So would the distance still be relevant if we gave this distance to someone on mars (even if the different was negligible)?
$endgroup$
– Elliot Chance
yesterday
$begingroup$
Every observer thinks that a meter stick, or a light-year stick, at rest relative to that observer is the length it is supposed to be. But observers measure moving meter sticks or light-year sticks as being length-contracted.
$endgroup$
– G. Smith
yesterday
1
$begingroup$
Alright, well I guess don't understand the question well enough to ask it then.
$endgroup$
– Elliot Chance
yesterday
3
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
1
$begingroup$
Ignore the words "light" and "year" and think of it in the same you think of "meters" or "kilometers" and any confusion should disappear
$endgroup$
– BlueRaja - Danny Pflughoeft
12 hours ago
|
show 1 more comment
$begingroup$
A light-year is exactly 9,460,730,472,580,800 meters, the distance light travels in one Julian Earth year. Time dilation is irrelevant to its definition.
answered yesterday
G. SmithG. Smith
23.4k2 gold badges41 silver badges77 bronze badges
$endgroup$
A light-year is exactly 9,460,730,472,580,800 meters, the distance light travels in one Julian Earth year. Time dilation is irrelevant to its definition.
answered yesterday
G. SmithG. Smith
23.4k2 gold badges41 silver badges77 bronze badges
answered yesterday
G. SmithG. Smith
23.4k2 gold badges41 silver badges77 bronze badges
answered yesterday
G. SmithG. Smith
23.4k2 gold badges41 silver badges77 bronze badges
answered yesterday
G. SmithG. Smith
23.4k2 gold badges41 silver badges77 bronze badges
23.4k2 gold badges41 silver badges77 bronze badges
1
$begingroup$
Yes, a simple Google search returns that. My question is, will that distance differ for an observer is in a different uniformly moving body? So would the distance still be relevant if we gave this distance to someone on mars (even if the different was negligible)?
$endgroup$
– Elliot Chance
yesterday
$begingroup$
Every observer thinks that a meter stick, or a light-year stick, at rest relative to that observer is the length it is supposed to be. But observers measure moving meter sticks or light-year sticks as being length-contracted.
$endgroup$
– G. Smith
yesterday
1
$begingroup$
Alright, well I guess don't understand the question well enough to ask it then.
$endgroup$
– Elliot Chance
yesterday
3
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
1
$begingroup$
Ignore the words "light" and "year" and think of it in the same you think of "meters" or "kilometers" and any confusion should disappear
$endgroup$
– BlueRaja - Danny Pflughoeft
12 hours ago
|
show 1 more comment
1
$begingroup$
Yes, a simple Google search returns that. My question is, will that distance differ for an observer is in a different uniformly moving body? So would the distance still be relevant if we gave this distance to someone on mars (even if the different was negligible)?
$endgroup$
– Elliot Chance
yesterday
$begingroup$
Every observer thinks that a meter stick, or a light-year stick, at rest relative to that observer is the length it is supposed to be. But observers measure moving meter sticks or light-year sticks as being length-contracted.
$endgroup$
– G. Smith
yesterday
1
$begingroup$
Alright, well I guess don't understand the question well enough to ask it then.
$endgroup$
– Elliot Chance
yesterday
3
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
1
$begingroup$
Ignore the words "light" and "year" and think of it in the same you think of "meters" or "kilometers" and any confusion should disappear
$endgroup$
– BlueRaja - Danny Pflughoeft
12 hours ago
1
1
$begingroup$
Yes, a simple Google search returns that. My question is, will that distance differ for an observer is in a different uniformly moving body? So would the distance still be relevant if we gave this distance to someone on mars (even if the different was negligible)?
$endgroup$
– Elliot Chance
yesterday
$begingroup$
Yes, a simple Google search returns that. My question is, will that distance differ for an observer is in a different uniformly moving body? So would the distance still be relevant if we gave this distance to someone on mars (even if the different was negligible)?
$endgroup$
– Elliot Chance
yesterday
$begingroup$
Every observer thinks that a meter stick, or a light-year stick, at rest relative to that observer is the length it is supposed to be. But observers measure moving meter sticks or light-year sticks as being length-contracted.
$endgroup$
– G. Smith
yesterday
$begingroup$
Every observer thinks that a meter stick, or a light-year stick, at rest relative to that observer is the length it is supposed to be. But observers measure moving meter sticks or light-year sticks as being length-contracted.
$endgroup$
– G. Smith
yesterday
1
1
$begingroup$
Alright, well I guess don't understand the question well enough to ask it then.
$endgroup$
– Elliot Chance
yesterday
$begingroup$
Alright, well I guess don't understand the question well enough to ask it then.
$endgroup$
– Elliot Chance
yesterday
3
3
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
1
1
$begingroup$
Ignore the words "light" and "year" and think of it in the same you think of "meters" or "kilometers" and any confusion should disappear
$endgroup$
– BlueRaja - Danny Pflughoeft
12 hours ago
$begingroup$
Ignore the words "light" and "year" and think of it in the same you think of "meters" or "kilometers" and any confusion should disappear
$endgroup$
– BlueRaja - Danny Pflughoeft
12 hours ago
|
show 1 more comment
$begingroup$
In 1983 the General Conference on Weights and Measures determined the definition of a meter to be the distance light travels in a vacuum in 1/299,792,458 of a second, it's current definition. So since all reference frames see light speed the same, a light year cannot change, it is 9,460,730,472,580,800 meters, and the length of a meter is based on the speed of light, a constant, therefore a light year is a constant.
answered 23 hours ago
Adrian HowardAdrian Howard
1,1182 silver badges10 bronze badges
$endgroup$
2
$begingroup$
That makes sense in terms of a description, but if I wanted to travel one light-year and I was traveling at 0.5c, how long would I travel for? Still a year?
$endgroup$
– Elliot Chance
23 hours ago
3
$begingroup$
An observer at your starting point would observe you to have travelled for 2 years. However, due to time dilation, the trip would take you only 1.73 years, subjectively. Since you both agree on the distance travelled, you will assume you were traveling at ~0.58c. On the other hand, if you measure yourself to be moving at 0.5c, you will not be surprised to find that you have taken exactly 2 years to arrive, but the observer will see you to have taken longer than 2 years, and thus traveling a bit slower than 0.5c.
$endgroup$
– chepner
16 hours ago
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
$begingroup$
@JiK: you misread it, the speed of light is the constant, therefore distances based on it cannot change
$endgroup$
– Adrian Howard
15 hours ago
2
$begingroup$
@AdrianHoward The asker already knows that the speed of light is constant. He is confused about time dilation, and this answer does nothing to explain it.
$endgroup$
– JiK
15 hours ago
|
show 2 more comments
$begingroup$
In 1983 the General Conference on Weights and Measures determined the definition of a meter to be the distance light travels in a vacuum in 1/299,792,458 of a second, it's current definition. So since all reference frames see light speed the same, a light year cannot change, it is 9,460,730,472,580,800 meters, and the length of a meter is based on the speed of light, a constant, therefore a light year is a constant.
answered 23 hours ago
Adrian HowardAdrian Howard
1,1182 silver badges10 bronze badges
$endgroup$
2
$begingroup$
That makes sense in terms of a description, but if I wanted to travel one light-year and I was traveling at 0.5c, how long would I travel for? Still a year?
$endgroup$
– Elliot Chance
23 hours ago
3
$begingroup$
An observer at your starting point would observe you to have travelled for 2 years. However, due to time dilation, the trip would take you only 1.73 years, subjectively. Since you both agree on the distance travelled, you will assume you were traveling at ~0.58c. On the other hand, if you measure yourself to be moving at 0.5c, you will not be surprised to find that you have taken exactly 2 years to arrive, but the observer will see you to have taken longer than 2 years, and thus traveling a bit slower than 0.5c.
$endgroup$
– chepner
16 hours ago
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
$begingroup$
@JiK: you misread it, the speed of light is the constant, therefore distances based on it cannot change
$endgroup$
– Adrian Howard
15 hours ago
2
$begingroup$
@AdrianHoward The asker already knows that the speed of light is constant. He is confused about time dilation, and this answer does nothing to explain it.
$endgroup$
– JiK
15 hours ago
|
show 2 more comments
$begingroup$
In 1983 the General Conference on Weights and Measures determined the definition of a meter to be the distance light travels in a vacuum in 1/299,792,458 of a second, it's current definition. So since all reference frames see light speed the same, a light year cannot change, it is 9,460,730,472,580,800 meters, and the length of a meter is based on the speed of light, a constant, therefore a light year is a constant.
answered 23 hours ago
Adrian HowardAdrian Howard
1,1182 silver badges10 bronze badges
$endgroup$
In 1983 the General Conference on Weights and Measures determined the definition of a meter to be the distance light travels in a vacuum in 1/299,792,458 of a second, it's current definition. So since all reference frames see light speed the same, a light year cannot change, it is 9,460,730,472,580,800 meters, and the length of a meter is based on the speed of light, a constant, therefore a light year is a constant.
answered 23 hours ago
Adrian HowardAdrian Howard
1,1182 silver badges10 bronze badges
answered 23 hours ago
Adrian HowardAdrian Howard
1,1182 silver badges10 bronze badges
answered 23 hours ago
Adrian HowardAdrian Howard
1,1182 silver badges10 bronze badges
answered 23 hours ago
Adrian HowardAdrian Howard
1,1182 silver badges10 bronze badges
1,1182 silver badges10 bronze badges
2
$begingroup$
That makes sense in terms of a description, but if I wanted to travel one light-year and I was traveling at 0.5c, how long would I travel for? Still a year?
$endgroup$
– Elliot Chance
23 hours ago
3
$begingroup$
An observer at your starting point would observe you to have travelled for 2 years. However, due to time dilation, the trip would take you only 1.73 years, subjectively. Since you both agree on the distance travelled, you will assume you were traveling at ~0.58c. On the other hand, if you measure yourself to be moving at 0.5c, you will not be surprised to find that you have taken exactly 2 years to arrive, but the observer will see you to have taken longer than 2 years, and thus traveling a bit slower than 0.5c.
$endgroup$
– chepner
16 hours ago
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
$begingroup$
@JiK: you misread it, the speed of light is the constant, therefore distances based on it cannot change
$endgroup$
– Adrian Howard
15 hours ago
2
$begingroup$
@AdrianHoward The asker already knows that the speed of light is constant. He is confused about time dilation, and this answer does nothing to explain it.
$endgroup$
– JiK
15 hours ago
|
show 2 more comments
2
$begingroup$
That makes sense in terms of a description, but if I wanted to travel one light-year and I was traveling at 0.5c, how long would I travel for? Still a year?
$endgroup$
– Elliot Chance
23 hours ago
3
$begingroup$
An observer at your starting point would observe you to have travelled for 2 years. However, due to time dilation, the trip would take you only 1.73 years, subjectively. Since you both agree on the distance travelled, you will assume you were traveling at ~0.58c. On the other hand, if you measure yourself to be moving at 0.5c, you will not be surprised to find that you have taken exactly 2 years to arrive, but the observer will see you to have taken longer than 2 years, and thus traveling a bit slower than 0.5c.
$endgroup$
– chepner
16 hours ago
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
$begingroup$
@JiK: you misread it, the speed of light is the constant, therefore distances based on it cannot change
$endgroup$
– Adrian Howard
15 hours ago
2
$begingroup$
@AdrianHoward The asker already knows that the speed of light is constant. He is confused about time dilation, and this answer does nothing to explain it.
$endgroup$
– JiK
15 hours ago
2
2
$begingroup$
That makes sense in terms of a description, but if I wanted to travel one light-year and I was traveling at 0.5c, how long would I travel for? Still a year?
$endgroup$
– Elliot Chance
23 hours ago
$begingroup$
That makes sense in terms of a description, but if I wanted to travel one light-year and I was traveling at 0.5c, how long would I travel for? Still a year?
$endgroup$
– Elliot Chance
23 hours ago
3
3
$begingroup$
An observer at your starting point would observe you to have travelled for 2 years. However, due to time dilation, the trip would take you only 1.73 years, subjectively. Since you both agree on the distance travelled, you will assume you were traveling at ~0.58c. On the other hand, if you measure yourself to be moving at 0.5c, you will not be surprised to find that you have taken exactly 2 years to arrive, but the observer will see you to have taken longer than 2 years, and thus traveling a bit slower than 0.5c.
$endgroup$
– chepner
16 hours ago
$begingroup$
An observer at your starting point would observe you to have travelled for 2 years. However, due to time dilation, the trip would take you only 1.73 years, subjectively. Since you both agree on the distance travelled, you will assume you were traveling at ~0.58c. On the other hand, if you measure yourself to be moving at 0.5c, you will not be surprised to find that you have taken exactly 2 years to arrive, but the observer will see you to have taken longer than 2 years, and thus traveling a bit slower than 0.5c.
$endgroup$
– chepner
16 hours ago
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
$begingroup$
This seems to me like a circular argument: "A light-year is constant because a light-(1/299729458 seconds) is constant"
$endgroup$
– JiK
16 hours ago
$begingroup$
@JiK: you misread it, the speed of light is the constant, therefore distances based on it cannot change
$endgroup$
– Adrian Howard
15 hours ago
$begingroup$
@JiK: you misread it, the speed of light is the constant, therefore distances based on it cannot change
$endgroup$
– Adrian Howard
15 hours ago
2
2
$begingroup$
@AdrianHoward The asker already knows that the speed of light is constant. He is confused about time dilation, and this answer does nothing to explain it.
$endgroup$
– JiK
15 hours ago
$begingroup$
@AdrianHoward The asker already knows that the speed of light is constant. He is confused about time dilation, and this answer does nothing to explain it.
$endgroup$
– JiK
15 hours ago
|
show 2 more comments
$begingroup$
Anybody in an inertial reference frame (i.e., considers themselves to be at rest), who fires a light beam at a mirror and sees the reflection arrive after exactly 1 Julian Earth year according to their reckoning, will conclude that the light has travelled one light year of distance.
Another person, watching the same events (light beam leaving flashlight at event X, hitting mirror at event Y, reflection arriving back to flashlight at event Z) will also conclude the light traveled one light year of distance and that it took exactly 1 Julian Earth year of their personal time for it to happen. However, they may disagree about the other user having remained at rest, and about what the other user's clock will read.
answered 10 hours ago
Ross PresserRoss Presser
3931 silver badge11 bronze badges
$endgroup$
add a comment
|
$begingroup$
Anybody in an inertial reference frame (i.e., considers themselves to be at rest), who fires a light beam at a mirror and sees the reflection arrive after exactly 1 Julian Earth year according to their reckoning, will conclude that the light has travelled one light year of distance.
Another person, watching the same events (light beam leaving flashlight at event X, hitting mirror at event Y, reflection arriving back to flashlight at event Z) will also conclude the light traveled one light year of distance and that it took exactly 1 Julian Earth year of their personal time for it to happen. However, they may disagree about the other user having remained at rest, and about what the other user's clock will read.
answered 10 hours ago
Ross PresserRoss Presser
3931 silver badge11 bronze badges
$endgroup$
add a comment
|
$begingroup$
Anybody in an inertial reference frame (i.e., considers themselves to be at rest), who fires a light beam at a mirror and sees the reflection arrive after exactly 1 Julian Earth year according to their reckoning, will conclude that the light has travelled one light year of distance.
Another person, watching the same events (light beam leaving flashlight at event X, hitting mirror at event Y, reflection arriving back to flashlight at event Z) will also conclude the light traveled one light year of distance and that it took exactly 1 Julian Earth year of their personal time for it to happen. However, they may disagree about the other user having remained at rest, and about what the other user's clock will read.
answered 10 hours ago
Ross PresserRoss Presser
3931 silver badge11 bronze badges
$endgroup$
Anybody in an inertial reference frame (i.e., considers themselves to be at rest), who fires a light beam at a mirror and sees the reflection arrive after exactly 1 Julian Earth year according to their reckoning, will conclude that the light has travelled one light year of distance.
Another person, watching the same events (light beam leaving flashlight at event X, hitting mirror at event Y, reflection arriving back to flashlight at event Z) will also conclude the light traveled one light year of distance and that it took exactly 1 Julian Earth year of their personal time for it to happen. However, they may disagree about the other user having remained at rest, and about what the other user's clock will read.
answered 10 hours ago
Ross PresserRoss Presser
3931 silver badge11 bronze badges
answered 10 hours ago
Ross PresserRoss Presser
3931 silver badge11 bronze badges
answered 10 hours ago
Ross PresserRoss Presser
3931 silver badge11 bronze badges
answered 10 hours ago
Ross PresserRoss Presser
3931 silver badge11 bronze badges
3931 silver badge11 bronze badges
add a comment
|
add a comment
|
$begingroup$
The definition of a meter is not constant. Presume that one day we scientists determine a way to more accurately measure the speed of light in a vacuum. The constant (c) would not change in this scenario but rather the definition of a meter.
answered 5 mins ago
Bipedal Beacon of UnorthodoxyBipedal Beacon of Unorthodoxy
11 bronze badge
New contributor
Bipedal Beacon of Unorthodoxy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
add a comment
|
$begingroup$
The definition of a meter is not constant. Presume that one day we scientists determine a way to more accurately measure the speed of light in a vacuum. The constant (c) would not change in this scenario but rather the definition of a meter.
answered 5 mins ago
Bipedal Beacon of UnorthodoxyBipedal Beacon of Unorthodoxy
11 bronze badge
New contributor
Bipedal Beacon of Unorthodoxy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
add a comment
|
$begingroup$
The definition of a meter is not constant. Presume that one day we scientists determine a way to more accurately measure the speed of light in a vacuum. The constant (c) would not change in this scenario but rather the definition of a meter.
answered 5 mins ago
Bipedal Beacon of UnorthodoxyBipedal Beacon of Unorthodoxy
11 bronze badge
New contributor
Bipedal Beacon of Unorthodoxy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
The definition of a meter is not constant. Presume that one day we scientists determine a way to more accurately measure the speed of light in a vacuum. The constant (c) would not change in this scenario but rather the definition of a meter.
answered 5 mins ago
Bipedal Beacon of UnorthodoxyBipedal Beacon of Unorthodoxy
11 bronze badge
New contributor
Bipedal Beacon of Unorthodoxy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
answered 5 mins ago
Bipedal Beacon of UnorthodoxyBipedal Beacon of Unorthodoxy
11 bronze badge
New contributor
Bipedal Beacon of Unorthodoxy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
answered 5 mins ago
Bipedal Beacon of UnorthodoxyBipedal Beacon of Unorthodoxy
11 bronze badge
answered 5 mins ago
Bipedal Beacon of UnorthodoxyBipedal Beacon of Unorthodoxy
11 bronze badge
11 bronze badge
New contributor
Bipedal Beacon of Unorthodoxy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Bipedal Beacon of Unorthodoxy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment
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add a comment
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Elliot Chance is a new contributor. Be nice, and check out our Code of Conduct.
Elliot Chance is a new contributor. Be nice, and check out our Code of Conduct.
Elliot Chance is a new contributor. Be nice, and check out our Code of Conduct.
Elliot Chance is a new contributor. Be nice, and check out our Code of Conduct.
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