# The Naked Scientists Forum

### Author Topic: Using Gravitational Lensing to see further into the past....  (Read 2833 times)

#### latebind

• Sr. Member
• Posts: 248
• Hello World
##### Using Gravitational Lensing to see further into the past....
« on: 20/03/2010 13:00:49 »
This is my theory of how scientists will be able to see even further into the past by using gravitational lensing in a new way.

OK, so we know that when you see a galaxy that is 2 billion LY away, you can deduce that you are seeing the galaxy as it was 2 billion years ago.

Now suppose you look the opposite way, IE 180 degrees from this galaxy and you find a massive black hole or something with ferocious gravity that is able to create a 360 degree gravitional lense.

Do you see where this is going? Now suppose that black hole is 1 billion LY away, and it is able to comfortably suck in the light rays from our observed galaxy and spit them out again straight back to us. But now these images are not how the galaxy was 2 billion years ago, these images are in fact how the galaxy was 3 billion years ago, hence we can see further into the past by using 360 degree gravitational lensing.

Here is a little bit of simple math to explain the concept....

-Light takes 2 Billion LY to reach earth from faraway galaxy
-travels a further 1 billion LY to reach black hole and get bounced back
-travels a further 1 billion LY to reach earth from the black hole
- hence I think this makes it 3 billion LY since the last billion years of travel (towards earth) was the static picture of the galaxy 3 billion years ago...

Please correct me on the math if it is actually 4 billion, im not sure...

Now I'm not saying the way to do this is to find a galaxy and then look for a massive black hole directly opposite, in fact the best way would be a reversal of that process. Also the light would need to strike the black hole on its perimeter so that it doesnt get sucked in but rather spun around and shot out the other side, straight towards earth.
« Last Edit: 20/03/2010 15:02:08 by latebind »

#### LeeE

• Neilep Level Member
• Posts: 3382
##### Using Gravitational Lensing to see further into the past....
« Reply #1 on: 20/03/2010 15:37:01 »
Unfortunately, a black Hole would be unable to "suck in the light rays from our observed galaxy and spit them out again straight back to us" for a couple of reasons.

First of all, whilst photon spheres seem to exist...

http://en.wikipedia.org/wiki/Photon_sphere

Quote
As photons travel near the event horizon of a black hole they can escape being pulled in by the gravity of a black hole by traveling at a nearly vertical direction known as an exit cone. A photon on the boundary of this cone will not completely escape the gravity of the black hole. Instead it orbits the black hole. These orbits are not stable.

...a passing photon will not be traveling "at a nearly vertical direction known as an exit cone", and only photons created close to the Event Horizon of a BH would seem to apply here.

Secondly, the path of the photons through space will be curved but for the photons to travel out to the BH and then back to us the path would need to be straight.

It's a nice idea though.

#### latebind

• Sr. Member
• Posts: 248
• Hello World
##### Using Gravitational Lensing to see further into the past....
« Reply #2 on: 20/03/2010 16:15:35 »
Unfortunately, a black Hole would be unable to "suck in the light rays from our observed galaxy and spit them out again straight back to us" for a couple of reasons.

First of all, whilst photon spheres seem to exist...

http://en.wikipedia.org/wiki/Photon_sphere

Quote
As photons travel near the event horizon of a black hole they can escape being pulled in by the gravity of a black hole by traveling at a nearly vertical direction known as an exit cone. A photon on the boundary of this cone will not completely escape the gravity of the black hole. Instead it orbits the black hole. These orbits are not stable.

...a passing photon will not be traveling "at a nearly vertical direction known as an exit cone", and only photons created close to the Event Horizon of a BH would seem to apply here.

Secondly, the path of the photons through space will be curved but for the photons to travel out to the BH and then back to us the path would need to be straight.

It's a nice idea though.

Yes, perhaps a black hole is not the ideal candidate, I did mention though that any object with 'ferocious' gravity might be able to make this possible, perhaps dark matter or a really massive star or something like that...

#### LeeE

• Neilep Level Member
• Posts: 3382
##### Using Gravitational Lensing to see further into the past....
« Reply #3 on: 21/03/2010 11:10:41 »
A Black Hole is actually the best candidate, as it produces the steepest gravitational gradient and hence the greatest distortion of space-time, which is what you want for gravitational lensing.  I seem to recall discussing light being returned to its source via BHs in another recent thread and I think we decided that it might be possible if three+ (but probably not just two) BHs were involved.

#### latebind

• Sr. Member
• Posts: 248
• Hello World
##### Using Gravitational Lensing to see further into the past....
« Reply #4 on: 22/03/2010 17:14:32 »
A Black Hole is actually the best candidate, as it produces the steepest gravitational gradient and hence the greatest distortion of space-time, which is what you want for gravitational lensing.  I seem to recall discussing light being returned to its source via BHs in another recent thread and I think we decided that it might be possible if three+ (but probably not just two) BHs were involved.

Well, lets hope a genius out there can prove the theory works!! Maybe we'll be able to use 2 of these phenomena to exponentially increase the number of LY we can see into the past....

#### The Naked Scientists Forum

##### Using Gravitational Lensing to see further into the past....
« Reply #4 on: 22/03/2010 17:14:32 »