OP is he for me because it is not a title, but only a noun. But if you are female i can speak of "you" instead of the OP.
wegs said:
Second, why do you believe Dave is wrong?
I do not believe, and it should be obvious for all who know how things work.
If you read carefully, you can understand that it is nonsense to say the thing like he did (but many just repeat thinks like he did without understanding anything, so it is not new to me).
If you dont understand, then it mean the answer he gave you does not allow you to understand.
dicart said:
The only thing we should say, in my opinion, is : If light travel during 1 billlion year (duration) then it has travelled one billion light year (distance).
wegs said:
But once it reaches us, wouldn’t it be
No, because here you assume that the galaxy itself is actualy one billlion light year away.
"You dont need this hypothesis" like gallilee would say.
The only thing you can say is :
One day light went from a distant galaxy.
This light started to travel and in our reference frame "it has traveled one bilion years" (a duration, dont know how you know this, but let suppose it).
Here you can assume, because of the definition of the speed of light, that this same light has traveled one bilion light years (a distance).
You can not say this :
Dave said:
So A galaxy one billion light years from us has light reaching us having travelled six billion trillion miles - 10^9 x 10^12 = 10^21 - or six sextillion miles.
To be much clearer let say the same thing with the same units :
"So A galaxy one billion light years from us has light reaching us having travelled one billion light years."
1. There is no galaxy at one billion light year.
2. If you consider that
it was some galaxy (at least) one billion light years away from us when it emmited the light, the light could not have traveled one billion light years, because of the expansion : It has traveled more.