Time dilation and speed of light

[Séamus Ó Muircheartaigh]

Hi,

 

I hope someone can explain where my reasoning is wrong.

 

The faster something travels relative to an observer, the slower time passes for the moving entity.

 

Taking this to what appears to me as a logical conclusion...

 

If I, the observer, see light from Sirius; since it is travelling at the speed of light, the light should arrive instantly since no time would have passed, disagreeing with the light from Sirius taking ~ 8.6 years to reach my eyes on Earth.

[J.C.MacSwell]

 

 

Hi,

 

I hope someone can explain where my reasoning is wrong.

 

The faster something travels relative to an observer, the slower time passes for the moving entity.

 

Taking this to what appears to me as a logical conclusion...

 

If I, the observer, see light from Sirius; since it is travelling at the speed of light, the light should arrive instantly since no time would have passed, disagreeing with the light from Sirius taking ~ 8.6 years to reach my eyes on Earth.

1. Time dilation, and length contraction as well, is frame dependant, so you have to be careful what frame you are using.

 

2. You can't use the reference frame of the light. It does not have one.

 

If something is 8.6 LY away in your reference frame, you would measure and calculate that the light will take 8.6 years to arrive.

Edited December 21, 2012 by J.C.MacSwell

[elfmotat]

Light doesn't have a rest frame, so talking about how much time passes for photons in meaningless.

[swansont]

Hi,

 

I hope someone can explain where my reasoning is wrong.

 

The faster something travels relative to an observer, the slower time passes for the moving entity.

 

Taking this to what appears to me as a logical conclusion...

 

If I, the observer, see light from Sirius; since it is travelling at the speed of light, the light should arrive instantly since no time would have passed, disagreeing with the light from Sirius taking ~ 8.6 years to reach my eyes on Earth.

 

You've answered it already — the dilation is for the object in the moving frame. You do not experience it yourself, since you are at rest.

 

(Also, the others are quite correct that light is not in an inertial frame, so one can't discuss time and dilation for them. It's meaningless.)

[Séamus Ó Muircheartaigh]

Thanks for all the replies. I think I get the point but may be back later.