PDA

View Full Version : Can gravitational lensing be utilized for mapping gravitational sources?

mkline55
2015-Nov-24, 02:05 PM
Imagine you have a steady flow of falling steel ball-bearings, all falling straight down from numerous randomly placed sources somewhere in the air at varying heights above a field. By placing detectors on the ground, you could measure the impact of each ball-bearing, and calculate the 3-dimensional location of each source - horizontal location matching which detector is hit, and height determined by energy of impact.

Now, suppose you took a large magnet and moved it slowly in a line through the air somewhere above the ground, but also somewhere within the field of falling ball-bearings. By analyzing the changes in the patterns on the ground, could you determine the approximate size, height, velocity, and direction of motion of the magnet? I believe you could.

For example, you could determine approximately where, horizontally, the falling pattern is being disrupted, and could track that over time to determine also the direction and velocity of motion of the magnet. You could also calculate an approximate distance above the ground for the magnet by tracking the distances of ball-bearing sources which are and which are not affected by the magnetic field.

The conclusion, then, is that in this scenario, it may be possible to map the entire field of ball-bearing sources as well as the existence of, location, motion and size of the magnet just by recording the impacts of the ball-bearings on the detectors, and applying some math.

Could a similar scenario be utilized to map distant (not necessarily visible) gravitational sources and their motions by examining changes in light signatures from 'visible' sources? (I put 'visible' in quotes, because I don't want to restrict this to just the humanly-visible light spectrum.)

antoniseb
2015-Nov-24, 02:18 PM
The answer is, look up "weak lensing". Almost anytime you see an image with an apparent map of dark matter on it this is how it was mapped. At the moment, this is not a very precise mapping tool, but it is way better than no mapping at all.

mkline55
2015-Nov-24, 02:21 PM
Thanks. I'll add this to my growing collection of unoriginal ideas.

IsaacKuo
2015-Nov-24, 09:05 PM
Bear in mind that the distances and timescales involved make it impossible to look at changes in the distortion over time. It would take millions of years for a useful change due to motion to occur. So instead of looking for changes over time, weak lensing looks at what's essentially a snapshot. Weak lensing relies upon detecting subtle distortions and usually statistical analysis to deduce (very crudely) mass distribution.

In contrast, microlensing involves much shorter distances and very much shorter timescales. With microlensing, one is looking at the effect of something passing between the telescope and an individual star. It's impossible to predict these events in advance, but if you ogle enough stars for long enough, you'll catch some events by luck. In these cases relative movement is fast enough to produce changes in the relevant graviational distortion on short timescales.

chornedsnorkack
2015-Nov-24, 09:33 PM
In contrast, microlensing involves much shorter distances and very much shorter timescales. With microlensing, one is looking at the effect of something passing between the telescope and an individual star. It's impossible to predict these events in advance, but if you ogle enough stars for long enough, you'll catch some events by luck. In these cases relative movement is fast enough to produce changes in the relevant graviational distortion on short timescales.

Not impossible. You might spot the lens before microlensing event, whether by observing a previous microlensing event by the same lens, or just seeing the lensing body and predicting its motion relative to background objects to be lensed.

antoniseb
2015-Nov-24, 10:47 PM
Not impossible. ...
Right, I guess. predicting such events has never been done, and is unlikely to ever be done in the next century, and might take millennia of technological development before they can be predicted frequently, but not impossible.
Post Edit: and, micro-lensing doesn't reveal dark matter.

chornedsnorkack
2015-Nov-25, 05:06 AM
Right, I guess. predicting such events has never been done, and is unlikely to ever be done in the next century, and might take millennia of technological development before they can be predicted frequently, but not impossible.

It has been done with Sun a century ago.
It has also been done with Proxima Centauri:
http://arxiv.org/abs/1401.0239
The first of these events was in October 2014. 13 months ago.
What was observed?

antoniseb
2015-Nov-25, 12:15 PM
It has been done with Sun a century ago.
It has also been done with Proxima Centauri:
http://arxiv.org/abs/1401.0239
The first of these events was in October 2014. 13 months ago.
What was observed?
I stand corrected, though the lack of a follow-up paper suggests that they didn't get time with Hubble to do the test, to see if that 19th magnitude background star was displaced by a few hundred microarcseconds (would not have been significantly brightened), and more importantly to this thread, this doesn't reveal dark matter.

ngc3314
2015-Nov-25, 03:54 PM
The Hubble archive shows a series of observations for 2 proposal to look for this Proxima Centauri lensing (deflection) event 12985 (http://archive.stsci.edu/proposal_search.php?mission=hst&id=12985) and 13466 (http://archive.stsci.edu/proposal_search.php?mission=hst&id=13466), both led by Sahu), with WFC3 imaging from 2012 to late 2014. So they may still be trying to improve ticklish astrometric effects at the milliarcsecond level.