Can we see it?

[michel123456]

A star exploded 2 millions years ago. This star is at a distance of 1 million Light Years from us. Can we see the explosion?

[Cap'n Refsmmat]

How far was the star from us when it exploded?

[Sisyphus]

Yes. In the big mirror 500000ly in the opposite direction.

[insane_alien]

we could see the remenants of the explosion.

[michel123456]

How far was the star from us when it exploded?

 

Right. Wrong to me. Correcting:

A star exploded 2 millions years ago.

The star WAS at a distance of 1 million Light Years from us.

Can we see the explosion (today)?

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Merged post follows:

Consecutive posts merged

Yes or No will do.

[Alpha Centauri]

there is a possibility that we can detect the light that originated from the star when it exploded 2 million years ago,so we may or we may not

[Darkchilde]

We have the explosion of the star 2 million years ago. The star's distance at that point in time was 1 million light years from the Earth.

 

What is the distance from the star now? What is the relative movement of the Earth-star system? Are we approaching the star, so we are less than 1 million light years away from it, in which case, we have already "seen" the explosion, or are we receding from the star in which case, we are farther than 1 million light years away from the star, and the radiation from the explosion has not reached us yet.

[michel123456]

We have the explosion of the star 2 million years ago. The star's distance at that point in time was 1 million light years from the Earth.

 

What is the distance from the star now?

The star exploded, lets suppose it dissapeared. Puff.

What is the relative movement of the Earth-star system?

It doesn't matter.

Are we approaching the star, so we are less than 1 million light years away from it, in which case, we have already "seen" the explosion,

which means....what? Do we see it (today) or don't we?

 

or are we receding from the star in which case, we are farther than 1 million light years away from the star, and the radiation from the explosion has not reached us yet

We can not recede faster than light, so the "has not reached us yet" is not an option. The light from the star has reached us 1 million years ago. Can we see it (today)?

[Sisyphus]

Not enough info to answer the question. I guess you thought I was kidding about the big mirror, but the point I was trying to make was that you can't assume that all information travels at the speed of light by the shortest possible route. If it did, you would have no way of learning about anything that has happened on Earth more than a fraction of a second ago.

[Hendrix]

No, we would have missed the explosion by 1 million years. If the star exploded 2 million years ago and was 2 million light years away than yes we would. Ignoring lensing, blockages what have you.

[Darkchilde]

The star exploded, lets suppose it dissapeared. Puff.

 

This is not what happens, though. If the star exploded then it would a white dwarf star, a pulsar, a black hole, something.

 

 

It doesn't matter.

 

Yes it does.

 

which means....what? Do we see it (today) or don't we?

 

We can not recede faster than light, so the "has not reached us yet" is not an option. The light from the star has reached us 1 million years ago. Can we see it (today)?

 

I can't say if we see it today or not; it depends on our distance now from the star. If we are receding from the star then there will be a redshift from its light. If we are approaching it, we will see a blueshift.

 

Even though we cannot reach the speed of light: if 2 million years ago, the star was at a distance of 1 million light years, because of the respective speeds and travel that both our systems have done in the galaxy we may be closer or farther away from the star than we were 2 million light years ago. So the light that started 2 million light years ago may or may not have reached us yet.

 

In 2 million years, the Solar system has moved about 1% of its orbit around the galaxy's centre! The sun has moved about 1500 light years around the galaxy in 2 million light years (if my rushed calculations are correct).

 

So, if we take the other star to be stationary, then the light from that supernova may need more time to reach us (about 1500 years) or we may already have seen that (1500 years ago). Or if the star is at the exact same distance as it was nack then, then the light would be reaching us now.

 

There are also other considerations, such as gravitational lensing, and so on, and so forth.

[Airbrush]

A star exploded 2 millions years ago. This star is at a distance of 1 million Light Years from us. Can we see the explosion?

 

Yes, we can still see the remnants of the explosion. It would be located in a nearby galaxy in our local group, much too far away to be inside the Milky Way. It will also be gravitationally bound to us, so it would NOT be receeding from us. The remnants of that supernova would be orbiting the center of mass of our local group of galaxies, just like we are.

[michel123456]

Hm.

Lots of considerations here.

Personnaly I liked 1 answer:

Hendrix wrote:

No, we would have missed the explosion by 1 million years.

 

That was my point, so simple. Not regarding any other consideration about relative speeds or remnants.

 

If I had asked wether we can see today a living dinosaur chewing grass in a valley of Germany, I guess everybody could agree about the answer: No. (not talking about the remnants, but about the living creature).

Why?

Because the dinosaur is close to us (upon Earth) but in the past.

If the dinosaur was chewing grass upon a planet 2 million LY away from us, 2 million years ago, we could see the living animal today.

[Sisyphus]

We can still gather information about that dinosaur grazing in Germany.

[Airbrush]

"A star exploded 2 million years ago. This star is at a distance of 1 million Light Years from us. Can we see the explosion?"

 

"No, we would have missed the explosion by 1 million years."

 

"...the star IS...1 million LY from us"

 

If it IS 1 million LY away from us now, then on the average it was also about that far from us 2 million years ago (it could have been much closer or much further away when it exploded, but on the average it was not much further than 1 million LY) because it has been locked in this local group since this local group formed. So please explain how light cannot travel 1 million LY in 2 million years?

 

It could have exploded way on the other end of the local group 2 million years ago at a distance of over 2 million LY from us, and migrated closer to us now, so it is possible the light has not reached us yet, but I think unlikely.

Edited March 18, 2010 by Airbrush

[michel123456]

We can still gather information about that dinosaur grazing in Germany.

 

Yes. But we can't see it, nor gather information transmitted by EM radiation.

 

I cannot see today my father who passed away, because he is close to me (upon Earth) and in the past. A fellow E.T. placed upon a planet 20 LY from here can see my father alive.

 

By extension, we cannot see today the explosion of a star 2 millions years ago at a distance of 1 million Light Years from us, because it is too close to us. We would have missed the explosion by 1 million years, as mentionned by Hendrix.

[Airbrush]

"...we cannot see today the explosion of a star 2 millions years ago at a distance of 1 million Light Years from us, because it is too close to us."

 

Come to think of it, 2 million years is a very short interval on the scale of galaxies orbiting the center of the local group. So you are correct. I take back my initial assessment. It is impossible the explosion could have happened much further away on the other side of the local group. If it is 1 million ly away now, then 2 million years ago it was not much further away. We would not see the moment of supernova, but we could see the remnants of the shock wave.

[Sisyphus]

Yes. But we can't see it, nor gather information transmitted by EM radiation.

 

I cannot see today my father who passed away, because he is close to me (upon Earth) and in the past. A fellow E.T. placed upon a planet 20 LY from here can see my father alive.

 

By extension, we cannot see today the explosion of a star 2 millions years ago at a distance of 1 million Light Years from us, because it is too close to us. We would have missed the explosion by 1 million years, as mentionned by Hendrix.

 

I would imagine a lot of the information you gather would be EM radiation. You look at the tracks. You look at the fossils. Etc. Can you intercept a photon that traveled directly from the living dinosaur? Probably not, barring unusual light paths, like looking at a reflection off of something 40ly away.

 

But what's special about that path of information? You gather photons in your eye, and interpret them to deduce something about a past event. That description applies to looking through a telescope at a distance supernova, or examining dinosaur bones.

[Hendrix]

Hm.

Lots of considerations here.

Personnaly I liked 1 answer:

Hendrix wrote:

 

 

That was my point, so simple. Not regarding any other consideration about relative speeds or remnants.

 

If I had asked wether we can see today a living dinosaur chewing grass in a valley of Germany, I guess everybody could agree about the answer: No. (not talking about the remnants, but about the living creature).

Why?

Because the dinosaur is close to us (upon Earth) but in the past.

If the dinosaur was chewing grass upon a planet 2 million LY away from us, 2 million years ago, we could see the living animal today.

 

Oh yeah! 1 for 1. The question was realtively simple, a lot of the brianiacs went complex on it, of course there are many different factors that could effect the answer.

 

I'm no scientist/physicist but I do have a love for science, i've been reading Dawkins - The Oxford Book of Modern Science Writing (excellent book btw) and trying to wrap my brain around Einstein's thoughts on general relativity was a pleasure. I figured I better seek out like minded individuals.. so hi guys!

[Spyman]

We can not recede faster than light, so the "has not reached us yet" is not an option.

That is wrong, according to our best theories, models and observations, space is not only able to expand faster than the speed of light, it actually is accelerating it's rate of expansion to boot.

So it is a valid option to consider distant object to be 'receding' from us faster than light.

 

 

Let's look at two real and observed examples, one with lower expansion rate than lightspeed and one faster:

 

First, here is a link to a cosmological calculator: http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

 

As I understand it, our best estimates of Omega is 0.27, Lambda is 0.73 and Hubble is 71.

(You need to enter those numbers in their corresponding boxes.)

 

Example 1) GRB 080319B was a powerful Gamma Ray Burst detected by the Swift satellite on March 19, 2008.

http://www.internationalreporter.com/News-3384/biggest-ever-cosmic-explosion-observed-7-5-billion-light-years-away.html

How distant from Earth was the star when it went supernova and emitted the GRB we observed in 2008 ?

(Clue: Redshift or z = 0.94 for GRB 080319B.)

 

Example 2) The Cosmic Microwave Background Radiation was emitted during the Recombination ~400 000 years after the Big Bang event.

http://en.wikipedia.org/wiki/Recombination_(cosmology)

How far away from the matter that later became Earth was the emitters of the CMBR we are able to observe now ?

(Clue: Redshift or z = 1100 for the CMBR.)

[Sisyphus]

Oh yeah! 1 for 1. The question was realtively simple, a lot of the brianiacs went complex on it, of course there are many different factors that could effect the answer.

 

Oh, I knew what his point was. I just happen to think it's wrong.

 

I'm no scientist/physicist but I do have a love for science, i've been reading Dawkins - The Oxford Book of Modern Science Writing (excellent book btw) and trying to wrap my brain around Einstein's thoughts on general relativity was a pleasure. I figured I better seek out like minded individuals.. so hi guys!

 

Hi.

[michel123456]

Sisyphus is right on everything he writes, and he knows my point from older posts. He thinks I am wrong. So I have to take the question otherwise. All that stuff mentionned by Spyman is too complicated in comparaison with my purpose.

Forget the star.

Take a pencil, and let it fall down the floor.

Then pick it up, and put it upon the table.

Now, can you see your pencil falling down?

The answer is no. You cannot see it NOW, you saw it, in the past. Now the pencil is upon the table, and you can take the remnants of the pencil in your hand. But you cannot see the past event of the pencil falling down.

Why can't you see it?

Because it is in the past and because it is too close to you.

Can you find someone else that can see it?

The answer is yes. Your friend placed at a distance of a few light-seconds can see your pencil falling down.

As a matter of fact, distance tells exactly what your friend is able to see.

 

Is there anything wrong with this?

[Sisyphus]

Well, again, in the trivial sense that you can't look at that spot and see with your eyes the same thing in the same way, then no, you can't see it. But that doesn't mean that information is lost. In fact, you could get all the same information, if someone was filming it with a good enough camera.

 

In both cases, you're taking in sensory information, and interpreting it to piece together an event in the past. The only difference is that in the first case you are on the "leading edge" of the information cone (or actually not, because light doesn't travel at C through air, and the biological processing takes time, but close to it), and in the second case you're farther behind the leading edge.

 

It doesn't feel like the same thing, because the first one feels like "now," and the second one feels like the past. But really they're both the past, and neither is more "real" than the other.

[michel123456]

I agree, Sisyphus. Information is not lost. I will keep your interesting light cone observation for later discussion, if you don't mind.

 

Now, other question:

Can we see (today) a planet placed in the past 1,5 million years ago, & 1,5 million LY from us? (please try a simple answer, like yes or no)

[Law]

I agree, Sisyphus. Information is not lost. I will keep your interesting light cone observation for later discussion, if you don't mind.

 

Now, other question:

Can we see (today) a planet placed in the past 1,5 million years ago, & 1,5 million LY from us? (please try a simple answer, like yes or no)

 

i think so...we would be witnessing its creation right now...but whats the point?? that lightspeed is impossible?? and is the farthest that planet can see is 1.5 LY away...or is my thinking too simple??

 

btw that father thing was trippy

 

sorry for not giving that simple answer...just imagining what you said got me pondering..lightspeed to me seems to be time (or distance) itself...its the limit BUT what if we found a mirror ball in space and zoomed in on it 1.5 mil LY away??? would we see the future??? past or present of ourselves??

Edited March 20, 2010 by Law
added in last assumption