Spacetime combines the three dimensions of space (length, width, height) and the one dimension of time into a single four-dimensional continuum. This continuum is often visualized as a flexible fabric, like a rubber sheet, that can bend and curve in response to mass and energy.
- Spacetime and light are closely related insofar as the speed of light is constant in all frames of reference. This means that the speed of light is the same for all observers, regardless of the speed and direction in which each observer is moving.
- This constancy of the speed of light as it travels through spacetime means:
- The speed of light in a vacuum is 299,792,458 meters per second (m/s). This is believed to be true for all observers.
- There is no absolute reference frame for space or time, in other words, everything is in motion relative to everything else and so regardless of the place or speed at the moment of measurement, the speed of light always appears the same. As a result, light travels at the same speed, regardless of whether an observer is moving towards or away from the light source.
- If the speed of light is a constant then it must be spacetime that curves.
- The idea that spacetime is curved refers to the idea that if the speed of light is a constant then spacetime must be dynamic.
- In practice, the path of light through space is affected by gravity, and gravity causes spacetime to bend. For example, the curvature of spacetime around a massive object, such as a star, will cause light rays to bend. This is known as gravitational lensing.