Light Years

[midgetwars]

Is it true when you look at a star/planet/black hole...anything that is light years away that when we look at it when it was anytime before?

 

e.g. Say there is a planet 2 light yrs away, then we invent a telescope and look at the planet, we would then be looking at the planet 2 yrs ago. So when we look at the technically the light is lagging?

[Gilded]

Yes, that's about right, and is obviously quite evident at even smaller scales. For example if the Sun exploded, we would be completely unaware of it for the ~8 minutes it takes light from the Sun to reach us.

[midgetwars]

yes, Gilded thnx for the answer,but if the sun did explode it will be heading for us but is wouldn't take 8 min for us to know, since it is coming closer to US!!!

 

RIGHT??????

[CaptainPanic]

The sun is 8 light-minutes away.

 

We only see anything that happens at the sun 8 minutes later. That would include the sun exploding - which is not the most likely event.

 

It's commonly accepted that, unless something really weird happens, the sun will first grow to a red giant (and swallow the earth in the process), before it'll really explode in a supernova. that process will take thousands or millions of years. And it will happen billions of years from now.

 

Gilded was talking about the sun exploding because some other event (for example aliens and their cursed SunBusterBombs blowing up our sun).

[freegoldrunner]

Cool!!!!

[midgetwars]

I know what Gilded meant i was pointing out that it wouldn't take around 8min for us to know that the sun was expanding into a red giant, since it is coming closer to earth every second. and it would take less than 8min.

 

To another point if theoretically if we find a planet that's 13.7 billion light years away would we see the creation of the universe????

[swansont]

I know what Gilded meant i was pointing out that it wouldn't take around 8min for us to know that the sun was expanding into a red giant, since it is coming closer to earth every second. and it would take less than 8min.

 

No, because the speed of light is a constant. You don't add the object's speed to it.

 

 

To another point if theoretically if we find a planet that's 13.7 billion light years away would we see the creation of the universe????

 

 

You have to account for the expansion of space; this effect wasn't accounted for in the previous examples since it's negligible on those scales.

[Klaynos]

I know what Gilded meant i was pointing out that it wouldn't take around 8min for us to know that the sun was expanding into a red giant, since it is coming closer to earth every second. and it would take less than 8min.

 

To another point if theoretically if we find a planet that's 13.7 billion light years away would we see the creation of the universe????

 

No, it would take 8mins. Even something heading towards us at 0.5c the light it sent at us would still be at c.

 

There is a dark phase before which we cannot see any further back in time... I can't remember the origins of this atm as I've only heard it in research talks as this is not my area.

[DrP]

This is kinda relavent - You can see/hear the 'time delay' effect with sound rather than light in a thunder storm. If lightening strikes 3 miles a way you SEE it instantly (more or less) but there is a 3 second delay before you hear it crack as thunder. If it strikes 10 miles away then you get a 10 second gap. Similarly, as explained above, light from the sun takes about 8 second to reach us across the 93 million miles it travels.

[gonelli]

This is kinda relavent - You can see/hear the 'time delay' effect with sound rather than light in a thunder storm. If lightening strikes 3 miles a way you SEE it instantly (more or less) but there is a 3 second delay before you hear it crack as thunder. If it strikes 10 miles away then you get a 10 second gap. Similarly, as explained above, light from the sun takes about 8 second to reach us across the 93 million miles it travels.

 

The analogy works perfect, but (for the sake of those who will read this post and retain this information if ever someone else asks them a similar question) it would take longer than 3 seconds to hear thunder from a lightning strike 3 miles away. Probably closer to 12 or 13 seconds.

[DrP]

Yea - your right - sorry. I was just recalling what they tell children to occupy them during thunder storms. Gives them something to do rather than being scared during a storm - - the lightning strikes and then they count the second to the thunder.. But you are of course right, at 340 m/s the sound will take 4 to 5 seconds to travel 1 mile.

[npts2020]

We always guestimated about 1000 ft/sec (its actually about 1200 ft/sec).

[swansont]

We always guestimated about 1000 ft/sec (its actually about 1200 ft/sec).

 

Right. The thumbrule is 5 miles per second of delay.

[Sayonara]

I know what Gilded meant i was pointing out that it wouldn't take around 8min for us to know that the sun was expanding into a red giant, since it is coming closer to earth every second. and it would take less than 8min.

 

Not really. If material/energy ejected at light speed from the sun's explosion is halfway to Earth, then it will arrive in another four minutes. But it's already taken four minutes to get to where it is, so the "heads up" time doesn't change.

[npts2020]

Right. The thumbrule is 5 miles per second of delay.

 

Uh 5 seconds per mile. (gotta do that now I dont often see you make that kind of mistake)

[midgetwars]

You have to account for the expansion of space; this effect wasn't accounted for in the previous examples since it's negligible on those scales.

 

 

What does that mean????

[Edtharan]

Not really. If material/energy ejected at light speed from the sun's explosion is halfway to Earth, then it will arrive in another four minutes. But it's already taken four minutes to get to where it is, so the "heads up" time doesn't change.

If the light was released from it when it was half way to us, then yes.

 

But if the Sun exploded, then the light form the explosion would occur at the surface where it already is, and this is 8 light minutes away. As nothing can travel faster than light, then we would see this light before the light released by the matter at the half way point.

 

So we would know about it 8 minutes after the sun exploded because the explosion occurs where the sun is now/I] and that light would take 8 minutes to reach us.

[swansont]

What does that mean????

 

Space is expanding. The visible universe is larger than 13.7 billion light years across.

[Martin]

...To another point if theoretically if we find a planet that's 13.7 billion light years away would we see the creation of the universe????

 

...You have to account for the expansion of space; this effect wasn't accounted for in the previous examples since it's negligible on those scales.

 

What does that mean????

 

Space is expanding. The visible universe is larger than 13.7 billion light years across.

 

I agree.

Midgetwars the most distant material that we can see at present---that we are right now getting light from---is the stuff that emitted the Cosmic Microwave Background radiation that we are getting.

 

That material is now about 45 billion lightyears from us.

 

The light from it has taken 13.7 billion years to reach us.

 

That stuff has by now almost certainly collected into stars and planets and clusters of stars etc etc, but we see it as it was 13.7 billion years ago---a uniformly spread out hot gas.

 

So we can't look back, as you imagined, and see a planet. When we look way back what we see is stuff the way it was before planets happened.

==============

 

The reason it's 45 instead of 13.7 is exactly what Swansont said:

distances have increased during the history of the universe

and that somewhat magnifies the distance the light appears to have traveled.

 

On its own, in a non-expanding universe, it only have gone 13.7 billion lightyears. But it appears to have clocked more mileage than that, namely 45 billion, because of the general expansion of distances that has taken place.

 

If you are satisfied with our answers, fine. If you still want more explanation, that's fine too, just keep asking questions. Somebody, not always the same people, will probably answer.

[swansont]

Uh 5 seconds per mile. (gotta do that now I dont often see you make that kind of mistake)

 

D'oh! Yeah, I inverted that.

[J.C.MacSwell]

This is kinda relavent - You can see/hear the 'time delay' effect with sound rather than light in a thunder storm. If lightening strikes 3 miles a way you SEE it instantly (more or less) but there is a 3 second delay before you hear it crack as thunder. If it strikes 10 miles away then you get a 10 second gap. Similarly, as explained above, light from the sun takes about 8 second to reach us across the 93 million miles it travels.

 

More like 500 seconds.