What is Vection?

When you're traveling in a vehicle such as a car, train, or bus, and you experience an illusory feeling of movement, this phenomenon is known as vection. Although there are many well-explained physics phenomena, vection is a cognitive illusion that challenges our understanding of motion. Simply put, your brain is constantly trying to make sense of your surroundings, and when a significant portion of your visual field (like a train, bus, or truck) is moving, your brain has to decide whether you are moving or the world is moving. Without any other information, your brain defaults to the conclusion that you are moving, thus creating the perception that you are in motion.

Contesting this illusion can be as simple as looking out the window of the same vehicle and seeing that you are stationary. By providing additional visual cues, you can break the illusion and realize that the vehicle you were observing is actually moving, not you.

The Concept of Relative Velocity

The experience of vection is closely related to the concept of relative velocity, an essential aspect of understanding how we perceive motion in our environment. To grasp the intricacies of relative velocity, it's crucial to understand the frame of reference - the perspective from which observations are made. Typically, we take the earth as the frame of reference and assume it to be stationary for most observations. However, when we consider multiple observers or moving objects, our perception of relative velocity can be disrupted.

Frame of Reference

A frame of reference is a specific point or object chosen as a reference for the study of a physical phenomenon. In everyday life, we often assume that the ground or the earth is our frame of reference. However, this assumption can be challenged in certain situations, leading to different perceptions of relative motion.

Let's consider a simple example with three people:

A is sitting on a chair B is running towards A at 5 m/s C is watching both A and B, either stationary on the ground or moving at 5 m/s

Observations from Different Frames of Reference

When C is stationary on the ground, C observes B approaching A at 5 m/s. When C is moving at 5 m/s with B, C might perceive B as stationary, assuming A is moving towards B.

This highlights how the frame of reference can influence our perception of motion. Just as we often believe the sun is moving while it is actually stationary in the universe, our perception of relative velocity can be influenced by our frame of reference.

Understanding the Illusion of the Sun's Motion

One of the most common examples of the illusion of relative motion is the apparent movement of the sun across the sky. The sun appears to rise and set, but in reality, it's the rotation of the earth that causes this perception. When we are observers on the surface of the earth, we assume that the earth is stationary, and the sun is moving. This is a classic example of the cognitive illusion of relative velocity.

Vegetable Characteristics

For a bit of a playful and light-hearted variation, let's consider the analogous situation with our fruit or vegetable friends:

A is standing on the ground, B is a car moving at 5 m/s, and C is a rocket flying at 5 m/s. If C is flying towards B, C might perceive B as stationary, just like how a stationary observer on the ground perceives the sun to be moving when in reality, it is the earth's rotation causing the perception.

Image source: Vectors drawn by Sachin Ganesh

These examples help us understand that our perceptions are always our brain's best guess about the environment around us. Sometimes, this guess can be incorrect, especially when the information available is limited or misleading. Vection is a fascinating example of how our perception of motion can be fundamentally altered based on the information our brain receives.