# B Debate about how a light year can be used

1. Nov 21, 2017

### Harshu Pande

If I say that a star will die after 50 million light years, can I also say that that after 50 million EARTH years, the star will die

BECAUSE DISTANCE AND TIME ARE
INTER-CHANGABLE (if a airplane travels from point A to point B and the distance is 200 miles, I can also say that the distance is 2 plane hours, which is a unit of time)

2. Nov 21, 2017

### rootone

3. Nov 21, 2017

### phinds

Then you would be saying that the star will die after approximately 2.5E19 miles, which of course is just silly.

4. Nov 21, 2017

### Harshu Pande

Phinds, so basically you are saying that light year cannot be used as a unit of time right?

5. Nov 22, 2017

### davenn

right

is not correct

6. Nov 22, 2017

### Staff: Mentor

A light year is an explicit way to make a length corresponding to a time. It doesn’t make sense to use that as a time unit again. Just say year.

7. Nov 22, 2017

### r8chard

HP, I would suggest that you consider the problem (as you stated it) in reverse.

What is the distance a photon travels in an Earth Year?

8. Nov 22, 2017

### stefan r

Depends on what the photon bounces off of. Also could be effected by the curvature. For instance in the LIGO apparatus the beam generates photons at the same time but occasionally they do not arrive in the detector at the same time.

9. Nov 22, 2017

### Harshu Pande

All right guys I got the answer I needed! thank you so much for all of your feedback!

10. Dec 2, 2017

### Bigjoemonger

A light year is a unit of distance not a unit of time.
Distance and time are not interchangeable.
If a plane travels 200 miles in 2 hours that means it was traveling 100 mph. Calling it a distance of 2 plane hours is only relevant if the plane is going 100 mph. If the plane were going 200 mph then it's distance would change to 1 plane hour which makes no sense.

We only use the term light year because the speed of light is the universal speed limit and it never changes in the same medium. And saying a distance of 1 light year is much easier than saying a distance of 5,880,000,000,000 miles.

11. Dec 7, 2017

### unusually_wrong

They are not interchangeable. The 2 hours is measurement of velocity and time.