# The Naked Scientists Forum

### Author Topic: Light speed  (Read 1136 times)

#### grumpy10

• First timers
• Posts: 3
##### Light speed
« on: 18/08/2011 00:15:55 »
If photons travel at light speed,time does not pass for them.If that is the case, how do we know how far any stars may be?
It is accepted that for any object approaching the speed of light, that time slows down & mass increases, to the point that it would have infinite mass at light speed, which would indicate that photons do not have mass, but why is light diverted by gravity. If time has stood still(regarding to light), there can be no way of telling how far away any celestial body may be !

#### imatfaal

• Neilep Level Member
• Posts: 2787
• rouge moderator
##### Light speed
« Reply #1 on: 18/08/2011 10:28:07 »
If photons travel at light speed,time does not pass for them.If that is the case, how do we know how far any stars may be?
Because we measure light speed in our reference frame (the ref frame of a photon is weird, unrealistic and does not work) and in our reference frame it has a finite speed.  We only know how far stars are away by looking at red shift, comparitive luminosity, and other secondary factors.  We have no way of measuring the transit time of light from a distant star directly - we calculate how long it has travelled from our estimate of the distance.

Quote
It is accepted that for any object approaching the speed of light, that time slows down & mass increases, to the point that it would have infinite mass at light speed, which would indicate that photons do not have mass, but why is light diverted by gravity. If time has stood still(regarding to light), there can be no way of telling how far away any celestial body may be !
Light is affected because it follows a geodesic (the equivalent of the shortest distance between two points) in curved space.

#### The Naked Scientists Forum

##### Light speed
« Reply #1 on: 18/08/2011 10:28:07 »