Please explain what we mean by "Light years away"

[paadness]

When we talk about stars, we say they are this much light years away.

So suppose i say a star is 2 light years away, does that mean...

1. We will reach it travelling at c in about 2 years??

2. If aliens are on that star, will they be seeing our absolute past??

3. If Q-2 is true, some planets may be observing the creation of the Earth.??


Please correct me if im wrong.

 

[MobiusPizza]

They are all right.

Alien on that star would be seing earth in 2 years ago.
There would be a 2 year time lag for the observed light.

There would be planets "observing" the creation of earth. However, no matter how sophiscated the telescope is; it would be impossible to be able to reconstruct an image of such a small and non-lumunous object like Earth that far away (several billions light years away)

 

[imported_Soldier]

paadness you are basicaly correct on all points.

Annihilator it would be better to qualify your comment about imaging planets around other stars with "right now". Current technology is moving in the direction of photometry, or measurements of a distant suns light dimming as a planet passes in front of it. Eventualy this is expected to mutate into imaging of the planet itself as it swings behind its sun. The hope is that the brief window where a suns light reflects off the orbiting planets surface will allow direct imaging of that planet. At the very least it should allow spectral analysis of the light reflecting off the other planet which should tell us much about the atmosphere, if any, the planet has.

 

[silverpig]

Actually, both the dimming and brightening of the star due to obscuring and reflected starlight have been used to detect planets. Google the MOST project (one of my profs is the lead on it). There's quite a bit of extrasolar planet work done at my university

 

[MobiusPizza]

I know that we can detect planet. But aren't the planet's reflected light too dim to be able to register on any telescoping device? The background light noises would have make it impossible. And he mentioned viewing the birth of earth which means viewing from some billion light years away of the light of the sun reflected the surface of a planet

 

[silverpig]

Originally posted by: AnnihilatorX
I know that we can detect planet. But aren't the planet's reflected light too dim to be able to register on any telescoping device? The background light noises would have make it impossible. And he mentioned viewing the birth of earth which means viewing from some billion light years away of the light of the sun reflected the surface of a planet

Nope, MOST can easily see reflected light, as well as Hubble and probably a few other telescopes as well. RESOLVING the planet from the star is still not quite possible (although there is one picture that seems somewhat convincing out now), but that will change once the terrestrial planet finder comes online in a decade or so.

While images would be cool, the stuff I'm really looking forward to is getting much higher resolution spectrographs to detect transmitted emission spectra from the planet when it is eclipsing its parent star. Emission spectra will give you the composition of the planet's atmosphere. I actually looked into doing a research project on this topic, but using reflected starlight instead of transmitted, but couldn't find a spectrograph with the right resolution. In a few years perhaps

 

[unipidity]

I wonder what the effect of having a telescope that is so large that the path difference between various bits of it was on the order of the period of change of the object being observed. Heh. I only wonder since I was formulating an argument based around a collecting mirror the size of a galaxy 5 blya.

 

[DrPizza]

Originally posted by: paadness
When we talk about stars, we say they are this much light years away.

So suppose i say a star is 2 light years away, does that mean...

1. We will reach it travelling at c in about 2 years??

2. If aliens are on that star, will they be seeing our absolute past??

3. If Q-2 is true, some planets may be observing the creation of the Earth.??


Please correct me if im wrong.

Weren't you bashing the Discovery channel in another thread as being for children?

Discovery channel is for kids, i dunno why they still have discovery kids.

You seemed to have a somewhat elitest attitude in that thread. Oddly, you attempted to answer a question about Shroedinger's wave equation - generally not discussed in detail until sophomore level college physics. Then, you ask a question about what a "light year" is?? And a comment about the smallest particles being atoms and molecules in another thread? Perhaps you need to watch the Discovery Channel a little more often!

 

[Oifish]

So suppose i say a star is 2 light years away, does that mean...

Actually, if you were to travel at c, the trip, to you, it would seem instantanious. If it is possible to travel at c you are only traveling through space, not time. In the refrence frame of the earth it would take you 2 years. That is what the whole idea of time dialation is about.

 

[TuxDave]

Originally posted by: Oifish

So suppose i say a star is 2 light years away, does that mean...

Actually, if you were to travel at c, the trip, to you, it would seem instantanious. If it is possible to travel at c you are only traveling through space, not time. In the refrence frame of the earth it would take you 2 years. That is what the whole idea of time dialation is about.

Dammit... I was about to say that....

 

[Calin]

You are correct in both your suppositions. Also, a third would be: We see the past of the planets/stars. We see Sun as was 8 minutes ago, Jupiter as was about half an hour ago, and we see distant galaxies as they were millions and millions of years ago

 

[Oifish]

Whats really crazy to think about is that, technically, everything you see is the past. Looking at the computer screen right now I am seeing the past. There is a very very small amount of time taken for the light to reach me.

 

[magomago]

Originally posted by: Calin
You are correct in both your suppositions. Also, a third would be: We see the past of the planets/stars. We see Sun as was 8 minutes ago, Jupiter as was about half an hour ago, and we see distant galaxies as they were millions and millions of years ago

So then...even if intellgent life existed on planets millions of light years away...we wouldn't know it? Because we are observing their planet from millions of years ago (and they are probably doing the same to us)

so we are searching for intellegent life..but either side won't find it till atleast a few million years from now...interesting...

 

[Calin]

We are looking for extraterrestrial life... however, from a planet like ours, from a civilisation like ours, a directed radio transmission (with our possibilities) would reach only some hundreds light years...
You should think that the first TV transmission (Olympic Games in Berlin, 1936) is just at 70 light years, so it hardly reached a couple of stars (maybe some hundreds stars).
Have you seen the movie "Contact" starring Jodie Foster? I recommend it

 

[Velk]

Originally posted by: Soldier
paadness you are basicaly correct on all points.

Annihilator it would be better to qualify your comment about imaging planets around other stars with "right now". Current technology is moving in the direction of photometry, or measurements of a distant suns light dimming as a planet passes in front of it. Eventualy this is expected to mutate into imaging of the planet itself as it swings behind its sun. The hope is that the brief window where a suns light reflects off the orbiting planets surface will allow direct imaging of that planet. At the very least it should allow spectral analysis of the light reflecting off the other planet which should tell us much about the atmosphere, if any, the planet has.

No, I don't think it has anything to do with imaging technology. Consider that planets are spherical - the reflected light from the planet is going to head off in different directions, so to resolve an image of the planet's surface at a distance of hundreds of light years means you need to have an unimaginably large telescope.

 

[Calin]

In an end, you would need to "separate" the image of a planet (that reflects a part of the light it receive from the star) from the image of the star itself.
Assuming Earth reflects half of the light it receive (less in fact), it receives just a billionth of the light Sun generates.
Now take into consideration the difference between the light we get from the Moon, and the light we get from the Sun. And consider that Moon is at 400 000km from Earth and Sun is at 150 000 000 km - a 1:300 ratio.
The difference in light a planet can make against a sun is so small, that one can find the presence of planets only at some light years (less than 100). As about imaging the planet, all you have is one pixel that is a star, a planet would be so small it won't register if it had a pixel of her own.

Calin

 

[Mday]

Originally posted by: paadness
When we talk about stars, we say they are this much light years away.

So suppose i say a star is 2 light years away, does that mean...

1. We will reach it travelling at c in about 2 years??

2. If aliens are on that star, will they be seeing our absolute past??

3. If Q-2 is true, some planets may be observing the creation of the Earth.??


Please correct me if im wrong.

1. yes
2. i doubt any living thing, alien or not, can survive on a star
3. no, since the earth is older than 2 million years old