Do we overlook “reverse” or “hidden” gravitational lenses?
tl;dr/Abstract: Gravitational lenses were predicted by Einstein, first observed in 1919 and are widely used today as handy tools for measurements of distant galaxies, the distribution of dark matter in the universe and countless other things. Interestingly, it seems that only one of three possible configurations of a gravitational lensing system is discussed and theorized about so far. In this constellation, a massive object which changes the direction of light is located between the emitting object and the observer. In our opinion there have to be two more possible constellations: the massive object could also be located behind the emitter or behind the observer. Depending on the relative distances of these objects, it could have non-negligible effects on many calculations and assumptions that are made about the structure of the universe.
NASA, ESA, J. Richard (CRAL) and J.-P. Kneib (LAM). Acknowledgement: Marc Postman (STScI))
In 1704 Isaac Newton asked: “Do not Bodies act upon Light at a distance, and by their action bend its Rays; and is not this action (caeteris paribus) strongest at the least distance?”[1] So he could be called the father of the theory of gravitational lenses. Two centuries later Einstein published the theory of general relativity with the core concept of spacetime. One effect of spacetime is the fact, that light could be “bent” by space curved around massive objects.
Years later this bending of light was observed during the solar eclipse of 1919[2]. It seems that Einstein himself introduced the term gravitational lensing effect[3], even though for quite some time he assumed that the effects are too small to observe. Thankfully today and especially since the Hubble Deep Field anyone can see the astonishing images of bent and distorted “projections” of galaxies far far away.
In optics, a lens is a medium that refracts light. For an optical lens to work, it has to be located between the observer and whatever she wishes to observe. For a gravitational lens to act similar to an optical lens (with optical effects like an Einstein ring), it also has to be between the emitter and the observer. But this comparison to an object we all encounter in our daily lives seems to really create a problem: In our view, any kind of lens can only work if its position is right between the emitter and the observer. So the wording alone and the pictures that inevitably spring to mind make it difficult to sit back and think of other possible constellations of the objects.
Let’s try anyway.
We postulate the three parts of a gravitational lensing system as observer “O”, the massive object “M” and the emitter “E”. So now we can construct three different and unique constellations: OME, OEM and MOE. In this nomenclature the classical gravitational lensing effect is given in OME.
The next and probably most obvious case is MOE. On Earth, an observer O only needs to take a look away from the center of our galaxy. Currently scientist calculate the mass of the black hole in the center of the Milky Way at about 4.3 million solar masses. Add another 10 million mostly old red giant stars with up to 12 solar masses and some other black holes in the central cubic parsec and you can imagine the torsion of the gravity funnel formed by the center of our galaxy. Our solar system is sitting somewhere on the “rim” of this funnel. Any light originating from the opposite direction and travelling straight to us (on the shortest path, as it does) should “fall” in this gravity funnel — or be bent around the curved spacetime. Stars inside our galaxy, but located behind us, further outside on the “rim”, should therefore shine brighter as they would without the mass in the center, as their light is “concentrated” towards us. The increase of incoming light might be even greater for more distant objects.
As an Observer on earth, the hardest case to detect (and to imagine) is the OEM. Let’s say the observed object is Messier 15, a globular cluster which is calculated to be about 33 600 light years away. Let’s further assume that there is a massive (dark) object M behind this cluster. We have no ability to directly detect this M or its properties (unless stars within or around it suddenly begin moving in suspicious ways). Depending on the mass of M and its distance to the cluster, the diversion of the emitted light of the globular cluster could very well decrease the incoming light and therefore make Messier 15 seem either dimmer or further away to us than it really is.
Now what?
Both constellations (MOE and OEM) can change the amount of light from a distant object we are trying to measure. But in nearly every process of classification of an object, the amount of light we detect is used to determine its properties. Size, distance and age are calculated in relation to the brightness, combined with spectroscopic measurements. Furthermore it is unclear if and how the diversion of light by these kind of constellations should be considered to have an effect on the shift of the light. Should there be any relevant effect, then not only the sizes, distances or ages of the objects could be different from our current assumptions but also their speed/acceleration and direction.
Disclaimer: If you didn’t already notice: We are not scientists! We nevertheless try to understand how the world works. In this case, it is very well possible that we got everything horribly wrong, missed essential papers on the topic[4] (recommendations are always welcome) and are too stupid to google the simplest things. (BTW: Never, ever try to google for “gravitational mirror”)
[1] Selected Queries from Isaac Newton’s Opticks
[2] Solar eclipse of May 29, 1919
[3] A brief history of gravitational lensing
[4] Scholarpedia: Weak gravitational lensing
