Gravitational Lensing to Observe Ancient Earth • Hacker News Board

Re: Gravitational Lensing to Observe Ancient Earth

#41

Post by privong » Tue, Mar 05, 2013, 5:58 AM UTC

pkrein wrote: post #17

DeepDuh wrote: post #16

I've never seen that image of our galaxy's central region, it's fascinating! Is "Sgr A" the black hole, Sagittarius A*? If yes, why would it be bright? I thought it's inactive.

Sgr A is the black hole Sagittarius A* as far as I know. The black hole itself isn't bright. As far as I understand, the swirling gas around it (outside the event horizon) is what's sending out all the light. What do you mean by inactive?

Technically Sgr A* is a component of the Sgr A region. There is also Sgr A East and Sgr A West, in addition to Sgr A*.

Re: Gravitational Lensing to Observe Ancient Earth

#42

Post by hakaaaaak » Tue, Mar 05, 2013, 6:31 AM UTC

Posting obligitory link to:

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

Of course someone should mention Father François Brune and his Chronovisor: http://en.wikipedia.org/wiki/Chronovisor

But none of that is real.

Re: Gravitational Lensing to Observe Ancient Earth

#43

Post by ars » Tue, Mar 05, 2013, 6:48 AM UTC

Why not use two black holes, each bending the light approximately 90 degrees? You don't even need a black hole - a large sun would work too.

Re: Gravitational Lensing to Observe Ancient Earth

#44

Post by guelo » Tue, Mar 05, 2013, 6:54 AM UTC

ISL wrote: post #18

You don't want the light reflected by 180 degrees - it needs to be reflected where Earth is going to be ; a much harder problem. For the easy case quoted in the article of 6 light years, any collimated light would need to be properly directed at the sub-nanoradian (milliarcsecond) level (which is possible -- see sig and [0]). Interesting to think about, even if it is an incredible observational challenge. Fun applica…

Actually it makes the problem easier because the angle is less than 180 degrees which means the lensing happens further away from the blackhole with less noise. By my quick googling and back o' the napkin calculations using MACHO-96-BLG-5, we, the Sun and the Milky Way, have traveled about 24 light years in 6,000 years giving an angle of 179.5 degrees! OK, that didn't change much.

Re: Gravitational Lensing to Observe Ancient Earth

#45

Post by ck2 » Tue, Mar 05, 2013, 8:57 AM UTC

If we ever invented FTL travel, even for just probes, this could be done too.

Except you'd need insane levels of resolution or a whole new way to think beyond optics to be able to see details.

I mean what if you could see that far back but the earth's resolution was only the size of a pin head?

Re: Gravitational Lensing to Observe Ancient Earth

#46

Post by lutusp » Tue, Mar 05, 2013, 9:22 AM UTC

ck2 wrote: post #45

If we ever invented FTL travel, even for just probes, this could be done too. Except you'd need insane levels of resolution or a whole new way to think beyond optics to be able to see details. I mean what if you could see that far back but the earth's resolution was only the size of a pin head?

> If we ever invented FTL travel, even for just probes, this could be done too.

If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality.

Remember that the speed of light isn't just a speed limit for light waves -- it's a barrier that sorts out temporal causes and effects.

Imagine a flat plane in the time dimension called the present -- on each side of the plane is a cone. The cone in the past encloses possible causes for present events. The cone in the future encloses possible effects from events in the present. Outside the past cone are events that may not produce results in the present. Outside the future cone are effects that cannot have been caused by events in the present. The slope of the cone surfaces is the speed of light.

Light cone picture:

http://4.bp.blogspot.com/--VBwEngltGc/Tzniy1imbII/AAAAAAAABL...

Re: Gravitational Lensing to Observe Ancient Earth

#47

Post by simonh » Tue, Mar 05, 2013, 9:27 AM UTC

ck2 wrote: post #45

If we ever invented FTL travel, even for just probes, this could be done too. Except you'd need insane levels of resolution or a whole new way to think beyond optics to be able to see details. I mean what if you could see that far back but the earth's resolution was only the size of a pin head?

Except of course if you have invented FTL travel, you can use it to actually go to the earth in the past. FTL drives are also time machines. There's a saying in Physics. Choose two: FTL, causality, or relativity. You cannot have all three.

Re: Gravitational Lensing to Observe Ancient Earth

#48

Post by 98709870987 » Tue, Mar 05, 2013, 11:11 AM UTC

lutusp wrote: post #46

ck2 wrote: post #45

If we ever invented FTL travel, even for just probes, this could be done too. Except you'd need insane levels of resolution or a whole new way to think beyond optics to be able to see details. I mean what if you could see that far back but the earth's resolution was only the size of a pin head?

> If we ever invented FTL travel, even for just probes, this could be done too. If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality. Remember that the speed of light isn't just a speed limit for light waves -- it's a barrier that sorts out temporal…

>>If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality.

You are confused. You are not actually traveling to the past therefore causality is not a problem here.

Re: Gravitational Lensing to Observe Ancient Earth

#49

Post by 98709870987 » Tue, Mar 05, 2013, 11:14 AM UTC

simonh wrote: post #47

ck2 wrote: post #45

If we ever invented FTL travel, even for just probes, this could be done too. Except you'd need insane levels of resolution or a whole new way to think beyond optics to be able to see details. I mean what if you could see that far back but the earth's resolution was only the size of a pin head?

Except of course if you have invented FTL travel, you can use it to actually go to the earth in the past. FTL drives are also time machines. There's a saying in Physics. Choose two: FTL, causality, or relativity. You cannot have all three.

Except we still don't know enough about physics to know if this is true. So far this is only a hypothesis that has never been proven.

Re: Gravitational Lensing to Observe Ancient Earth

#50

Post by andyjohnson0 » Tue, Mar 05, 2013, 11:38 AM UTC

ISL wrote: post #18

You don't want the light reflected by 180 degrees - it needs to be reflected where Earth is going to be ; a much harder problem. For the easy case quoted in the article of 6 light years, any collimated light would need to be properly directed at the sub-nanoradian (milliarcsecond) level (which is possible -- see sig and [0]). Interesting to think about, even if it is an incredible observational challenge. Fun applica…

"it needs to be reflected where Earth is going to be; a much harder problem"

The relative velocity of the ancient Earth, black hole, and present earth would also need to coincide in such a way that you'd get a continuous (or, less desirably, periodic) stream of photons making the complete round-trip. Such a coincidence seems pretty unlikely.

Its a nice idea though.