# Optimal shape for manipulation via telekinetic light

I have a species that can emit beams of light from their bodies (think flashlight). If the light lands on an object, they can telekinetically manipulate that object (pull, push, lift, etc.). Now bear with me: the telekinetic force applied to the object occurs at the point of light absorption. Considering an opaque black sphere, for example, the force would be applied along the outer surface on the side catching this light beam. A translucent sphere would absorb light throughout, which would mean force could be applied to the entire volume of the object. A transparent sphere would work similarly, except some light would leak out the far side and therefore be wasted. A reflective sphere would also waste light.

Over generations and generations, this species has developed a specific shape that works optimally with their powers. This ideal shape absorbs as much light as possible, since any light not absorbed is wasted. The shape also absorbs light in such a way that the net force is applied as close to the center of mass as possible. Assume that the object can be kept at a specific orientation in relation to the light beam. Also assume that the object is made from material that can be any color and transparency level throughout its volume. What would this optimal shape be?

For my own part, I have two tentative designs that I think illustrate this problem:

1. A cone that starts with a transparent point and transitions smoothly to a completely black, opaque base. The cone is oriented such that the light beam enters at the clear point and is fully absorbed by the time it reaches the base. With this design, I worry about light leaking out the sides of the cone, which brings me to my second design.
2. A sphere with a primarily black, opaque surface. There is a single transparent entry point for the light beam, somewhat like an eye. The internal makeup transitions smoothly from that transparent entry point to black and opaque. I believe this design absorbs more light than the cone, but would likely be heavier, which may not be a good tradeoff.

Both of my solutions ignore that "transparent" typically means "reflective," which would waste light. I can imagine there are more exotic solutions like shapes utilizing hollow cavities, "egg carton" surfaces, or structural coloration: I am absolutely interested in these types of designs if anyone is feeling their oats, but I recognize that this strays pretty heavily into light physics territory. Mostly I'm wondering if there are key improvements that could be made to either of my designs, or if there are other somewhat obvious designs that would work better. This shape has a high level of cultural and technological importance to this species, so I really want to give it my due diligence. Thanks!

• The cone sounds like a "laser beam dump", which has the goal of absorbing as much light as possible. Braniac75 recently made a video constructing one, if you want to understand what is inside: youtube.com/watch?v=rRIlhskMYAM Commented Jun 9, 2021 at 19:41
• When you say the force is applied wherever the light is absorbed, does this also mean that the direction of the force is the direction the light last came from? Commented Jun 9, 2021 at 19:50
• @IronEagle The light direction is not related to the force direction. At a low level, if a molecule absorbs this light, the creature can then apply force to that molecule in any direction. Commented Jun 9, 2021 at 19:58

There are three options for light hitting an object: it can be transmitted, absorbed, or reflected. You presumably want an object that will absorb as much light as possible, without reflecting or transmitting.

In laser use, there is a device used to absorb as much light as possible, called a beam dump. This is usually composed of a black cone inside a black conical cavity. The light is aimed at the central cone, and most of it is absorbed. Whatever is not absorbed is reflected into the conical cavity, which then absorbs even more of the light. The angle of the conical cavity is generally made so that further reflections either hit the cavity again, or the cone, leading to a further round of absorption. A black surface with an absorption of 90% will, in this shape, absorb around 99.9% of the light.

An additional advantage is that absorbing all this light will tend to warm up an object, with the absorption occurring only at the surface, the rest of the object can be made up of metal and can even be water-cooled.

Due to the absorptive nature, good copyright-free images of this design seem to be unavailable, but Brainiac75 on YouTube has a video constructing such a beam dump that should show the internal structure.

• Thanks, this is a good resource and an intriguing design. I like the "cone in a cone" idea, I think it has a lot of potential due to its simplicity and the fact that it's purely surface-based, which means it can be lightweight. I would have to consider what the center of mass of such an object would be and how it would relate to where light is being absorbed. One drawback of a purely surface-based solution is that it works similarly to glue: the force can only be as strong as the underlying surface can tolerate. However, optimizations could be made by using a slightly translucent material. Commented Jun 9, 2021 at 20:35

Sphere in a sphere.

Outer sphere is clear. Inner sphere is matte black.

The shape is symmetric. It can rotate and bounce all you want. You have a clear shot at the light absorbing center regardless of how the sphere happens to be oriented.

The force applied to the black center must be transmitted to the rest of the sphere. Regardless of the orientation of the sphere this force will be spread over half of the black sphere where it interfaces with the clear one. You reduce the risk of ripping the black sphere out of the transparent one. Which is a risk because the transparent one is made of very soft goo.

This is better than single transparent entry point because it is hard to aim to hit that point and if you hit the shape next to the point you will produce an unwanted vector. Also the shape might rotate so you dont have a clear shot at the entry point.

Almost forgot! The clear sphere has some small flat planes symmetrically at the outer edge. The edges of these planes point at the center and the outer wall. These are useful if you want to rotate the entire thing, which is hard to do using only the big center sphere.

My bigger question: ideal shape for what? What are these telekeinetics doing with their shapes? A baseball bat is a pretty ideal shape for me to swing around but I am limited in the number of tasks I can accomplish with it. I keep trying new ones though.

• Oh you've asked the fun question. :) "Ideal shape" in this case is for an object at rest or moving slowly and meant to maximize the effect of the light beam from a specific direction. A use-case might be a "handle" attached to some other item. I like the idea of the sphere: "crystal ball" type lenses have very interesting optical properties that would work well with a matte black or "smoky" central core. These could be used in less predictable environments when beam direction is not guaranteed, or when multiple creatures are lifting the same object together. Interesting. Commented Jun 10, 2021 at 0:18

You are looking for a black body https://en.m.wikipedia.org/wiki/Black_body

A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light.

An approximate realization of a black surface is a hole in the wall of a large insulated enclosure (an oven, for example). Any light entering the hole is reflected or absorbed at the internal surfaces of the body and is unlikely to re-emerge, making the hole a nearly perfect absorber.

A realization of a black body refers to a real world, physical embodiment. Here are a few.

Cavity with a hole

In 1898, Otto Lummer and Ferdinand Kurlbaum published an account of their cavity radiation source. Their design has been used largely unchanged for radiation measurements to the present day. It was a hole in the wall of a platinum box, divided by diaphragms, with its interior blackened with iron oxide. It was an important ingredient for the progressively improved measurements that led to the discovery of Planck's law.

• I'm upvoting this because it is perhaps a more general solution than the laser beam dump (which is itself a black body approximation). I have already discussed my hangups with this type of design, but I think it's a great foundation for something tailored for my specific use-case. The more responses I get, the more I'm realizing that there is unlikely to be a one-size-fits-all solution. Not a bad thing from a world building perspective, in my opinion. :) Commented Jun 10, 2021 at 18:07