# B A Hypothesis To Observe The Formation Of Earth

1. Aug 25, 2017

### asesena

"Today's astronomers detect objects so far away that their light has taken perhaps 10 billion years to reach us."

-NatGeo May 1974 page 595

If there was a planet and the light that has been reflected from earth was reaching there 4.5 billion year later (that means it would be 42.75 trilliard kilometers away from earth ,according to my basic maths knowledge) Could observers there observe the formation of earth?

That would be so cool if you could light me up

2. Aug 25, 2017

### Orodruin

Staff Emeritus
In principle, yes. However, the Earth itself is too small to be resolved at that distance with a reasonably sized telescope. What we are generally looking at at that distance are much much larger objects.

3. Aug 25, 2017

### asesena

Thank you and one more thing

The light travels 9.5 trillion km per year so in 4.5 billion years it would basically travel 42.75 trilliard km, but do you think it would get slower by time or travel steady?

4. Aug 25, 2017

### Tracey3

Hypothetically... Hell yeah! Now for the sake of the argument lets say that we are aliens far far way to make our lives a little bit easier. Now lets assume that we luckily managed to point our telescope directly at the earth and light wasn't bent by gravity and there was no obstructions. Now using a telescope with aperture which would be probable around 30 light years across, we might be able to make out few pixels from the photons that we catch.

Now there are a lot smarter people than me on here which probably know of a more clever way of going about this :)

5. Aug 25, 2017

### Orodruin

Staff Emeritus
The speed of light is constant (it is actually a matter of definition) so yes, the light travelled $4.5 \cdot 10^9$ light years. (I strongly suggest not using km for astronomical distances and using scientific notation instead of words such as "trilliard")
However, if we see a 4.5 billion year old object today, it is now further away than $4.5 \cdot 10^9$ light years due to the expansion of the Universe and it was much closer to us than $4.5 \cdot 10^9$ light years when the light was emitted.

6. Aug 25, 2017

### asesena

Thanks for the suggestion and the answer, have a nice day!

7. Aug 25, 2017

### asesena

Hahahaha thanks for the cute answer tracey, have a nice day

8. Aug 25, 2017

### Orodruin

Staff Emeritus
How did you get this number? Assuming the Earth reflects a large portion of the Sunlight, I get that about one photon per second would be captured by an aperture of $5\cdot 10^7$ m. The diffraction limit would be significantly worse and require a diameter of the same order as Pluto's orbit. Either way, it is clearly not feasible.

9. Aug 26, 2017

### Tracey3

I personally didn't derive this number, however, someone asked exactly same question at one of the talks held by Michio Kaku years ago. I remembered the number being 30ly then. I tried to fetch the video since yesterday in order to provide more details and post it here.

10. Aug 26, 2017

### Chronos

While anyone can observe light emitted in the past, no one can view their own past light emissions without violating the most fundamental rule of relativity - you cannot even keep up with, much less outrun your own light cone.

11. Aug 26, 2017

### Orodruin

Staff Emeritus
I cannot see that anyone has suggested that we observe ourselves in this thread. The OP's question was regarding whether or not an alien civilisation today theoretically could see the Earth forming.

12. Aug 26, 2017

### Ophiolite

We are already observing accretion discs around other stars. It is likely only a matter of time - and technology advances - before we can witness planetary formation around some of them. The problem then will be that the formation of planets is spread over many tens of thousands of years, or more. A need for time-lapse photogrpahy par excellence.

13. Aug 29, 2017

### stefan r

Would you see a planet forming? I suspect you might just see the disc. Clumpiness in the disc.

We could flip that around. Meteors can be seen showering earth today. So Earth is still "forming" unless by "forming" you mean someplace that did not have a planet becomes a planet while you watch.

It gets worse if you use the international astronomical union's definition of a "planet". The object would form into a crusty sphere but then only become a planet millions of years later after all the small objects leave the orbital neighborhood.