# Distance of a lightyear?

1. Jan 30, 2010

### HarryDaniels

A lightyear is a measurement of distance of how far light travels in a year relative to our time frame, correct?
Well, how far does light travel if-hypothetically- it had thought and could determine time. As if I were to travel at light speed for a year it may feel like a second. But, iff I were to travel at light speed for a year in my perspective, how far would I travel?

2. Jan 30, 2010

### Nabeshin

Proper time is what is normally used to distinguish the time a clock moving with an object experiences from what is observed by others. In the situation where we have a rocket moving relative to the earth, the time measured between two events (say, leaving earth and arriving somewhere else) by those on board the rocket is the proper time. For a photon, or indeed any massless particle, the proper time between any two points on its trajectory is zero.

Last edited: Jan 30, 2010
3. Jan 30, 2010

### Mentz114

A light-year is the speed of light multiplied by one year ( velocity x time = distance ).

Not true. The proper time for a massive object is always non-zero, and zero for massless objects.

4. Jan 30, 2010

### Nabeshin

Thanks, meant to type massless :) Corrected.

5. Jan 31, 2010

### yuiop

If you were to travel at light speed for a year (measured in another frame) it would feel like zero seconds in your perspective. However far you travel you would never measure a year in your perspective because your clock has stopped and the time to get anywhere by your clock is always zero. Of course no observer with mass can travel at the speed of light so this is completely hypothetical.

Now if you meant travel at (nearly) light speed for a year it may feel like a second, then if you were to travel at (nearly) light speed for 1 year from your perspective, then you could in principle travel many thousands of light years (from the perspective of another observer that measures your relative velocity to be nearly light speed) .

6. Feb 1, 2010

### HarryDaniels

So due to the fact that a photon has no mass, it does not feel time?

7. Feb 1, 2010

### Mentz114

If something travels at c, then the elapsed coordinate time is always equal to the spatial length of the route. This means the proper-interval, (ct)2-d2 is zero. It is a postulate of SR that we can identify the proper-time as the time shown by clocks on the trajectory of a massive body, but not for objects travelling at c.

So, we can't use SR to answer that question. But if a photon of wavelength L travels a distance d, it will oscillate about 2d/L times, and so maybe it does 'feel' time. Or maybe not.

(Does anyone care ?)?

8. Feb 1, 2010

### Nabeshin

Shut up and calculate.