Can we see Earth in the past?

  • Thread starter kevpatts
  • Start date
  • Tags
    Earth
In summary, the speaker discusses the possibility of using computational power to calculate the trajectory of light that may have been sling-shotted around a supermassive black hole located approximately 32.6 light years from Earth. However, the speaker acknowledges that it would be difficult to distinguish between photons from Earth and those from other sources, making it unlikely that we would be able to view light from 75.2 years ago in the future.
  • #1
kevpatts
2
0
Okay, say we are approx 32.6 light years from the supermassive black hole at the center of our universe. Surely then approx 75.2 years ago light left Earth traveling towards said black hole. A certain minute proportion of that light would have been sling-shotted around the black hole in just about every direction.

Using the computational power available these days it should be possibly to calculate exactly where we should have to look to see the light returning from this round trip.

Is it feasible that at some stage in the future we would be able to view this almost infinitesimally small amount of light and effectively watch Earth 75.2 years ago?

Is this far too simplistic? Am I being an idiot??
 
Astronomy news on Phys.org
  • #2
kevpatts said:
Okay, say we are approx 32.6 light years from the supermassive black hole at the center of our universe. Surely then approx 75.2 years ago light left Earth traveling towards said black hole. A certain minute proportion of that light would have been sling-shotted around the black hole in just about every direction.

Using the computational power available these days it should be possibly to calculate exactly where we should have to look to see the light returning from this round trip.

Is it feasible that at some stage in the future we would be able to view this almost infinitesimally small amount of light and effectively watch Earth 75.2 years ago?

Is this far too simplistic? Am I being an idiot??

You seem to have a serious misunderstanding of cosmology.

First, there is no center to the universe, so there is no meaning to the phrase "supermassive black hole at the center of our universe". If you mean our OBSERVABLE universe, then there IS a center, and you are at it. Are you a black hole?

If you mean the black hole at the center of our galaxy, then we are approximately 36 THOUSAND light years from it.

How you figure light waves from Earth would be sling-shot around it and come back the way the came is beyond me, but if they did, they would return to where the Earth was 72,000 years previously so, no we wouldn't see them.
 
  • #3
Good point. Kind of like the notion of a time traveller traveling through time from Earth in one location and in reality ending up out in cold space the next in a different time. But that doesn't work for hollywood movies unfortunaetly or HG Wells.
 
  • #4
welcome to pf!

hi kevpatts! welcome to pf! :smile:
kevpatts said:
… A certain minute proportion of that light would have been sling-shotted around the black hole in just about every direction.

Is it feasible that at some stage in the future we would be able to view this almost infinitesimally small amount of light and effectively watch Earth 75.2 years ago?

light approaching the black hole very close to (but outside) the photon sphere (radius 3M = 3Gm/c2 = 1.5 times the schwarzschild radius) can go round it many times, before coming back out in any direction

in principle, it should be possible to detect the occasional photon from Earth returning to Earth in this way

unfortunately …

i] the photons would be far too infrequent to build up a meaningful signal in a human lifetime!
ii] how on Earth (literally) would you tell which photons had come this way from earth, and which from somewhere else? :redface:
 
  • #5
Thanks Tiny Tim. Yes, you seemed to have understood my question. What I was thinking was that with the computational power we have these days we can predict exactly where planet Earth was relative to the centre of our galaxy at the time approx 75,200 years ago when it would have left earth, we know where the Earth is now, so could the perfect trajectory to achieve this be calculated? If so then we could focus our telescopes near the black hole where our calculations predict the light to be exiting its partial orbit.

I know there are 75,000 years in which any number of variables could be introduced to invalidate the calculations, but the fact that we can clearly see nebulae a billion light years away suggest that this is still unlikely.

I thought that the amount of photons would be an issue all right, we're not a star after all.

This was more of a theoretical question, not a practice one.

Thanks,
Kevin
 
  • #6
Hi Kevin! :smile:
kevpatts said:
If so then we could focus our telescopes near the black hole where our calculations predict the light to be exiting its partial orbit.

yes, but see my ii] above …

our calculations would also predict light with exactly the same exit direction but (infinitely many) different entry directions (none from old-earth) :wink:

(since we've no way of telling when the light approached the black hole)
 
  • #7
tiny-tim said:
Hi Kevin! :smile:yes, but see my ii] above …

our calculations would also predict light with exactly the same exit direction but (infinitely many) different entry directions (none from old-earth) :wink:

(since we've no way of telling when the light approached the black hole)
Exactly? I think this would kill information - outside a black hole.Earth is receiving ~1.4kW/m^2 * pi * (radius of earth)^2 power from the sun, and radiates approximately the same amount, partially in the visible spectrum (~1eV) and partially as infrared (~1/40eV). As an upper estimate, I will assume that all photons are infrared (this overestimates their number). This corresponds to 5*10^37 photons per second emitted from earth.

Now, we have to hit a very narrow region near the black hole to get a significant bending. Let's say it would have to come closer than 3 times the Schwarzschild radius.

The central black hole in the Milky Way has a mass of ~4 million solar masses, which corresponds to a Schwarzschild radius of ~12 million km. It is 26000 light years away.
The squared ratio shows that 2*10^(-20) of our photons come close to it, neglecting any influence of the interstellar medium.
For those photons, assume that they are scattered in all directions. The distance is the same, but if we want them to hit Earth the target is even smaller, giving a ratio of 7*10^(-28). Again, I neglect all other losses.

Multiplying all numbers, Earth receives ~7*10^(-10) photons per second "from earth", or 1 photon every ~50 years. This photon completely vanishes in all the other photons we receive. And even if the remaining universe would be completely black, the origin of the photon on Earth could not be tracked.Smaller, stellar black holes could give a slightly better number as their distance might be reduced by a factor of 1000. Their mass is smaller by a factor of ~10000 or more, but as the distance is taken to the 4th power and the mass is taken squared, the frequency can go up a bit.
 

1. How far back in time can we see Earth?

With current technology, we can see Earth as far back as 13.4 billion years. This is the estimated age of the oldest light in the universe, known as the cosmic microwave background radiation.

2. How do we see Earth in the past?

We can see Earth in the past by observing light that has traveled from Earth to other objects in the universe. This light takes time to reach its destination, so when we look at objects far away, we are seeing them as they appeared in the past.

3. Can we see specific events on Earth in the past?

Yes, we can see specific events on Earth in the past if they were bright enough to be visible from a great distance. For example, the light from a supernova explosion on Earth would be visible to us even if it occurred thousands of light-years away.

4. How accurate is our view of Earth in the past?

Our view of Earth in the past is as accurate as the technology and methods used to observe it. With advanced telescopes and techniques, we can see objects and events on Earth in incredible detail, even from millions of light-years away.

5. How does the distance of an object affect our view of Earth in the past?

The further an object is from Earth, the further back in time we are seeing it. This is because light takes time to travel, so the farther away the object is, the longer it takes for its light to reach us. Therefore, the more distant an object is, the further back in time we can see it.

Similar threads

Replies
5
Views
1K
  • Astronomy and Astrophysics
Replies
12
Views
2K
Replies
5
Views
868
  • Special and General Relativity
2
Replies
37
Views
2K
  • Astronomy and Astrophysics
Replies
18
Views
3K
  • Sci-Fi Writing and World Building
Replies
2
Views
1K
  • Special and General Relativity
4
Replies
115
Views
5K
  • Astronomy and Astrophysics
Replies
2
Views
1K
Replies
2
Views
2K
Replies
9
Views
2K
Back
Top