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jimmysun
2007-Feb-17, 10:54 PM
If a star is one light year away, does that mean any light we see from that star is delayed by one year, since it takes a year for light to travel from that star to earth?

SolusLupus
2007-Feb-17, 10:57 PM
If a star is one light year away, does that mean any light we see from that star is delayed by one year, since it takes a year for light to travel from that star to earth?

Yeap! That's precisely right, by definition. "Light year" is defined as "the time it takes for light to travel a year". So, if a star were 800 light years away, then it would take 800 years for that light to visit us. Considering that many of the stars in the sky are many many many light years out, that light that we're seeing now is a snapshot into the past!

Now that I've said all that, welcome to the forum, Jimmysun!

EvilEye
2007-Feb-18, 05:48 AM
May I thow in a caveat?

The light that hits your eye may take 10,000 years to hit your retina, HOWEVER... the photons are exactly the same age they were when they left the star.

The light you see is new.

SolusLupus
2007-Feb-18, 05:51 AM
May I thow in a caveat?

The light that hits your eye may take 10,000 years to hit your retina, HOWEVER... the photons are exactly the same age they were when they left the star.

The light you see is new.

The same "age"? What do you mean?

If you mean that they didn't decay or anything, that's one thing. But how is it possible to be "the same age"? How is "age" defined in this context?

Dr.CRClrfld
2007-Feb-18, 06:58 AM
The same "age"? What do you mean?

If you mean that they didn't decay or anything, that's one thing. But how is it possible to be "the same age"? How is "age" defined in this context?

Oh wow, that caveat just hit me too. :D
I guess what he meant is that the photon is going the speed of light so to it, it just left the star pretty recently. To us, it's taking 800 years to get to us.
If we could ride alongside the photon, never mind our atoms would probably fly apart at that speed, we'd get from the star to our earth pretty quickly.

EvilEye
2007-Feb-18, 02:46 PM
Oh wow, that caveat just hit me too. :D
I guess what he meant is that the photon is going the speed of light so to it, it just left the star pretty recently. To us, it's taking 800 years to get to us.
If we could ride alongside the photon, never mind our atoms would probably fly apart at that speed, we'd get from the star to our earth pretty quickly.

That is pretty much what I was saying.

Physics says that motion slows time. And the nearer we get to the speed of light, (the cosmic speed limit) the more it slows. If a photon is traveling AT the speed of light...from its perspective, time has stopped. It hasn't stopped moving, but it is not aging. So when it gets to us, it hasn't experienced any time. It never got enjoy the journey.

The one thing that always made me wonder about photons is where do they go when we observe them? Do they become something else when we collect them in our eyes, and process them into an image?

EvilEye
2007-Feb-18, 02:57 PM
Oh, and back to Jimmysun's question. There is another Caveat.

Lightspeed is an imperfect science. Since there is no such thing as perfect vaccum, we just get as close as we can to defining a light-year. You have things such as gravity, dust, particles, etc... that the photons bounce around off of before it gets to us from a very far distance.

It can take miilions of years for a photon to get from the center of the sun to the surface, before it races toward us.

But since we are trapped in our extremely short lifespan cosmically, our measurements are close enough.

...UNLESS you factor-in how far you are looking.

Delay can build up pretty big back toward the big bang, and that is why we can't see the "beginning". - The light hasn't had enough time to reach us, even though we were a part of it.

My head hurts.

Dr.CRClrfld
2007-Feb-18, 11:05 PM
That is pretty much what I was saying.

Physics says that motion slows time. And the nearer we get to the speed of light, (the cosmic speed limit) the more it slows. If a photon is traveling AT the speed of light...from its perspective, time has stopped. It hasn't stopped moving, but it is not aging. So when it gets to us, it hasn't experienced any time. It never got to enjoy the journey.

The one thing that always made me wonder about photons is where do they go when we observe them? Do they become something else when we collect them in our eyes, and process them into an image?

It is very strange to think about but I guess it does make sense when you think about it. If I played a song, let's say Joe Satriani's Surfing with the Alien, twice as fast as he, I would reach the end of the song while he's still in the middle of it and in a sense, I would see his playing slow down. Relative to mine, that is.
So we see a photon go very fast and it sees us move very slow. Actually it goes too fast for us to see since it's virtually instantaneous and we move too slowly for it to notice we're even moving. :)

Once the photon goes past the eye, I choose to leave the rest of its journey to the neuroscientists. I'm with you, relativity is enough of a headache. Pass the aspirin!