1. Hey guys,

So I'm studying Cosmology in my school in 9th grade, and I just don't get the concept of light year.

I heard that a light year is "the distance that light travels in a year"

But isn't the speed of light constant? so the distance it travels would always be the same.

If something is 4.2 light years away, what does that mean? Does it mean that it would take light 4.2 years to get there?

Thanks! Help is greatly appreciated!

2.

3. You heard correctly. A light year is the distance light will travel in one year and, yes, it will always travel the same distance in one year, since - as you have noted - the speed of light is constant. If something is ten light years away then it would take ten years for light to travel from there to here. Or 4.2 light years. Etc. It is a convenient distance measure since if we quoted those same distances in kilometres we would have a large, unwieldy number.

4. roughly speaking one light year is about:

six million million miles
6,000,000,000,000 miles
or
nine and a half million million kilometers
9,500,000,000,000km

4.2 lightyears is about = 25,200,000,000,000 miles or 39,900,000,000,000km

5. So, that means: the distance light travels in a year = the time it would take for light to travel there?

6. Originally Posted by GalaxyGamingMalaysia
So, that means: the distance light travels in a year = the time it would take for light to travel there?
Well, the time, in this case, is a year (you have said "in a year").

Speed, distance and time are related: speed = distance / time; therefore distance = speed * time.

As light speed is, as you say, constant this means we have a constant relationship between time and distance: 1 light year is roughly 9.5 trillion kilometres; 2 light years is roughly 19 trillion kilometres, etc.

7. Not quite. It is a distance not a time.
It is the distance light would travel in one year. This might seem like a minor point but you should avoid thinking of a lightyear as a time measure.

Edit:
I think Strange just explained it better than I did.

8. This is one of those ideas that you get so used to, that you forget it can trip people up when they first encounter it. But I hope between us we have made it clearer!

How about a simpler example: measuring distance using the unit of how-far-you-can-walk-in-an-hour (HFYCWIAH). Which is about 4 miles. My nearest shop is about 1/4 HFYCWIAH from here (1 mile). The nearest city is about 3 HFYCWIAHs from me (12 miles). Does that help?

9. Yes, that did help. So does that mean that if light travels at about 187,000 miles per second, and a star is, lets say, 5 light years. It means that the star is 187,000 * 5, or 935000 km away?

10. Originally Posted by GalaxyGamingMalaysia
Yes, that did help. So does that mean that if light travels at about 187,000 miles per second, and a star is, lets say, 5 light years. It means that the star is 187,000 * 5, or 935000 km away?
Not quite.
Since it's miles per second then it's ~187,000 x 5 (years) x ~365 (days in a year) x 24 (hours in a day) x 60 (minutes in an hour) x 60 (seconds in a minute) = 2.93925E13 miles/ 4.730264E13 km.

11. No not quite, you seem to have got the concept but in calculating numbers you need to keep your units consistent. In any equation the units must be dimensionally consistent on each side of the "=" sign.

If the star is 5 light years away it is:

5 * (number of seconds in a year) * (c in miles/sec) = X miles (not km)

You see in this example the "number of seconds in a year" cancels the 1/sec in the speed of light leaving you with "miles"

To ensure your equation wil work either use "miles" for everything (i.e. distance in miles and speed in miles/sec) or kilometers (distance in km, speed in km/sec) and if you are using speeds as distance/second you need to use the number of seconds in a year to convert the distance from light years).

Ninja'd again!

12. why light was chosen to be used as "distance measuring thing" between stars? why not sound, energy, work, force, torque etc

13. Originally Posted by sir ir r aj
why light was chosen to be used as "distance measuring thing" between stars? why not sound, energy, work, force, torque etc
I hate to say it but ... because it is a fundamental constant.

Speed of sound is a variable (and doesn't work in a vacuum). The others don't have any obvious relation to speed or distance.

14. Originally Posted by Strange
Originally Posted by sir ir r aj
why light was chosen to be used as "distance measuring thing" between stars? why not sound, energy, work, force, torque etc
I hate to say it but ... because it is a fundamental constant.

Speed of sound is a variable (and doesn't work in a vacuum). The others don't have any obvious relation to speed or distance.
can we use gravity in distance measuring process?

15. Originally Posted by sir ir r aj
Originally Posted by Strange
Originally Posted by sir ir r aj
why light was chosen to be used as "distance measuring thing" between stars? why not sound, energy, work, force, torque etc
I hate to say it but ... because it is a fundamental constant.

Speed of sound is a variable (and doesn't work in a vacuum). The others don't have any obvious relation to speed or distance.
can we use gravity in distance measuring process?
Theoretically, yes: gravitational waves travel at c.
Practically, no: gravitational waves are very difficult to produce, measure and they do not come back to the source (unlike light, they do not reflect). So, radar measures via gravitational waves is not doable.

16. Originally Posted by xyzt
they do not come back to the source (unlike light, they do not reflect)
While it is correct that gravitational waves don't reflect, it is nonetheless possible to make them back-scatter, i.e. deflect them by 180 degrees back to the source. This can be done with long wavelength waves on small, very massive objects such as black holes. And then of course there is the Heisenberg-Coulomb effect ( [0903.0661] Do Mirrors for Gravitational Waves Exist? ), although that is purely hypothetical.

17. Originally Posted by Markus Hanke
Originally Posted by xyzt
they do not come back to the source (unlike light, they do not reflect)
While it is correct that gravitational waves don't reflect, it is nonetheless possible to make them back-scatter, i.e. deflect them by 180 degrees back to the source. This can be done with long wavelength waves on small, very massive objects such as black holes. And then of course there is the Heisenberg-Coulomb effect ( [0903.0661] Do Mirrors for Gravitational Waves Exist? ), although that is purely hypothetical.
Very interesting (though speculative). I wonder why this paper has been languishing unpublished.

18. Originally Posted by Markus Hanke
Originally Posted by xyzt
they do not come back to the source (unlike light, they do not reflect)
While it is correct that gravitational waves don't reflect, it is nonetheless possible to make them back-scatter, i.e. deflect them by 180 degrees back to the source. This can be done with long wavelength waves on small, very massive objects such as black holes. And then of course there is the Heisenberg-Coulomb effect ( [0903.0661] Do Mirrors for Gravitational Waves Exist? ), although that is purely hypothetical.
Has anybody detected gravity waves?
I know people have been trying to detect them but I was not aware that anybody had succeeded.

19. Originally Posted by dan hunter
Originally Posted by Markus Hanke
Originally Posted by xyzt
they do not come back to the source (unlike light, they do not reflect)
While it is correct that gravitational waves don't reflect, it is nonetheless possible to make them back-scatter, i.e. deflect them by 180 degrees back to the source. This can be done with long wavelength waves on small, very massive objects such as black holes. And then of course there is the Heisenberg-Coulomb effect ( [0903.0661] Do Mirrors for Gravitational Waves Exist? ), although that is purely hypothetical.
Has anybody detected gravity waves?
I know people have been trying to detect them but I was not aware that anybody had succeeded.
No, not yet.

20. Originally Posted by xyzt
Originally Posted by dan hunter
Originally Posted by Markus Hanke
Originally Posted by xyzt
they do not come back to the source (unlike light, they do not reflect)
While it is correct that gravitational waves don't reflect, it is nonetheless possible to make them back-scatter, i.e. deflect them by 180 degrees back to the source. This can be done with long wavelength waves on small, very massive objects such as black holes. And then of course there is the Heisenberg-Coulomb effect ( [0903.0661] Do Mirrors for Gravitational Waves Exist? ), although that is purely hypothetical.
Has anybody detected gravity waves?
I know people have been trying to detect them but I was not aware that anybody had succeeded.
No, not yet.
So gravity waves are not really real yet, right?

21. Originally Posted by dan hunter
Originally Posted by xyzt
Originally Posted by dan hunter
Originally Posted by Markus Hanke
Originally Posted by xyzt
they do not come back to the source (unlike light, they do not reflect)
While it is correct that gravitational waves don't reflect, it is nonetheless possible to make them back-scatter, i.e. deflect them by 180 degrees back to the source. This can be done with long wavelength waves on small, very massive objects such as black holes. And then of course there is the Heisenberg-Coulomb effect ( [0903.0661] Do Mirrors for Gravitational Waves Exist? ), although that is purely hypothetical.
Has anybody detected gravity waves?
I know people have been trying to detect them but I was not aware that anybody had succeeded.
No, not yet.
So gravity waves are not really real yet, right?
This is not what I said, what I said is that gravity waves have not been detected YET. The equipment is not sensitive enough (and the waves have very low energy, making them tough to detect).

22. Originally Posted by dan hunter
Originally Posted by Markus Hanke
Originally Posted by xyzt
they do not come back to the source (unlike light, they do not reflect)
While it is correct that gravitational waves don't reflect, it is nonetheless possible to make them back-scatter, i.e. deflect them by 180 degrees back to the source. This can be done with long wavelength waves on small, very massive objects such as black holes. And then of course there is the Heisenberg-Coulomb effect ( [0903.0661] Do Mirrors for Gravitational Waves Exist? ), although that is purely hypothetical.
Has anybody detected gravity waves?
I know people have been trying to detect them but I was not aware that anybody had succeeded.
While they have yet to be measured directly, there is indirect evidence for their existence:

3. The Hulse-Taylor Pulsar - Evidence of Gravitational Waves

23. Originally Posted by dan hunter
So gravity waves are not really real yet, right?
An interesting philosophical position...

24. Originally Posted by dan hunter
So gravity waves are not really real yet, right?
I am fairly confident that we will get a direct detection from either DECIGO ( launch in 2027 ), or eLISA ( launch in 2034 ). Either one of these two should in principle be sensitive enough to find gravitational radiation from strong astrophysical sources.

25. Originally Posted by Markus Hanke
I am fairly confident that we will get a direct detection from either DECIGO ( launch in 2027 ), or eLISA ( launch in 2034 ).
I may be able to give you an answer before then. Just remember, one knock for yes, two knocks for no.

26. Shouldn't this thread be moved back into the physics subforum after cutting out the disruptive posts? Just wondering.

27. Originally Posted by scoobydoo1
Shouldn't this thread be moved back into the physics subforum after cutting out the disruptive posts? Just wondering.
MODERATOR NOTE : Good point. Thread moved.

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