special-relativity

Live WebGL demo

Simple single HTML version. Source located under static/simple.html. You may find this version easier if you just want to see how the transformation works and apply it to your own projects. It is completely self contained and has no external dependencies.

This project shows how the geometry of the world looks under special relativistic transformations. You can see a live demo here. You can see effects such as Terrell rotation, Doppler shifting and the headlight effect. Note that the page uses WebGL. If you cannot use WebGL, there is a video here.

Accelerating along a street at at relativistic speeds. You can see the geometric effects of abberation and Terrell rotation.

See Explanation for details on the math as well as for information on how to add this to your own code.

Usage

Go to the live demo above. You can rotate with the mouse and you can move the camera with the following keys:

Key	Description
W	Move forward.
A	Move left.
S	Move backward.
D	Move right.
E	Move up.
Q	Move down.

On cell phone you can use your thumbs to touch the screen and two virtual joysticks will appear. You can then use these to move around.

You can also plug in a game controller and use the analog sticks to move around.

Options

When you open the page you will see a bunch of options. They are

Max camera speed: Maximum speed the camera can move at. Should be a number above 0 and below 1 where 1 corresponds to the speed of light.

Relativistic beaming: Shows the "headlight effect" where various effects cause the luminosity of different objects to change with velocity.

Doppler effect: Shows the relativistic Doppler effect.

Assume no light travel time: Shows the geometry of the objects assuming no time delay. This requires us to assume a reference frame because the "now" slice will be different in different reference frames. There are two obvious choices:

• Camera: Assume the world is moving and the camera is at rest. This means the world will look compressed in the direction perpendicular to the direction of motion.
• World: Assume the world is at rest and the camera is moving. In this case the camera's sensor is length contracted, but the light entering the camera is not. This means objects will appear stretched because the light from an object will be spread over a larger portion of the sensor.

Show synchronization: Causes the blue color channel to pulse at the same time in the world frame of reference. Useful for inspecting time delay and time dilation.

Use Galilean relativity: Assumes no special relativity. Note that the rasterizer won't render things correctly for speeds greater than the speed of light.

Assume fixed camera speed: Assumes the camera is moving forward a fixed velocity relative to the environment but without actually moving the camera.

Scene: Either "Dice" which is a set of dice all in a row or Sponza, which is a beautiful atrium commonly used for rendering.

Development

First run pnpm install to install all the dependencies and then run pnpm run start to start a live demo on your computer.

The directory utils contains packages related to development or that only need to be built/executed infrequently. For example wavelength-color-map is used to generate the texture for mapping Doppler shifted colors.

Overview of code

The code is located under src and static. The project uses babylonjs to provide a basic engine and to load the GLTF model. We use shaders to apply the actual special relativistic transformations.

If you are interested in the actual code that transforms geometry according to special relativity, go to static/main.vert and to see the code that transforms the colors, go to static/main.frag. This will require knowledge of Shaders.

static/main.vert is the GLSL shader that applies the Lorentz transform to the vertices.

static/frag.glsl is a basic diffuse shader. Nothing special in here.

src/camera.ts contains code to control the camera and how quickly it speeds up and slows down so that the acceleration is smooth. I decided to work in proper velocity instead of regular velocity since it made for smoother motion.

Sources

I used the fantastic Relativity visualized site to get a better understanding and for the idea of rendering a bunch of dice.

See Chapter 4 of Daniel Weiskopf's dissertation for a great and detailed treatment.

Similar to A Slower Speed of Light.

Sponza model take from glTF-Sample-Models.

Background from NASA/Goddard Space Flight Center Scientific Visualization Studio.

Known issues

• The camera's acceleration is ad hoc. I want a simple way to make the camera speed up to a maximum velocity in the direction of movement when a key is pressed and then slow down to a stop when the key is released.

• Shading model is very basic.

• The Doppler and headlight effects are messily implemented in the Fragment shader. It could probably be done in a separate pass.

• No support objects moving in anything other than a straight line. I tried to implement a rotating wheel but it's difficult to compute the intersection with between the light ray's path and the wheel's position.

• No support for users opening their own models without modifying the code. Two things here: first is adding a UI for opening a GLTF file and then passing it to the babylon loader. The second is to subdivide large triangles automatically so that straight surfaces transform correctly.

• The Galilean relativity isn't quite correct. In particular, the background isn't transforming the same as the foreground.

Contributions

Feel free to open a PR or issue (either for bugs, features or questions) and I will review when I can.