- 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 is curved. The idea that spacetime is curved refers to the idea that if the speed of light is a constant then spacetime must be a variable. 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.
- The curvature of spacetime also affects the motion of objects. Objects will always follow the shortest path through spacetime, which is called a geodesic. In curved spacetime, geodesics are not necessarily straight lines. As a result, gravity affects the paths of objects, such as planets and asteroids.
- The fact that the speed of light is constant is one of the key pieces of evidence that supports Einstein’s theory of general relativity. If spacetime were not curved, then the speed of light would vary depending on the gravitational field. However, observations have shown that the speed of light is the same in all gravitational fields.

##### Spacetime and quantum field theory

- The concept in quantum field theory is that the fabric of spacetime is not a smooth, continuous medium, but is instead made up of discrete quantum fields, and one such field is the spacetime field.
- The idea that spacetime is made up of quantum fields has a number of implications including that gravity is not a fundamental force but is instead a property of spacetime. This means that gravity is not something that is exerted by one object on another but is instead a property of the fabric of spacetime itself.
- The gravitational field: This field is responsible for the curvature of spacetime.
- The Higgs field: This field gives elementary particles their mass.
- The electroweak field: This field is responsible for the electromagnetic force and the weak nuclear force.
- The strong nuclear field: This field is responsible for the strong nuclear force. The quantum fields that form spacetime are still a matter of research, but some possible candidates include:
- It is also possible that spacetime is made up of some combination of these fields, or even of other fields that have not yet been discovered.

- One way to reconcile the idea that gravity is a property of spacetime with the idea that gravity is a fundamental force is to view gravity as an emergent force. An emergent force is a force that arises from the interactions of more fundamental particles or fields.
- From this viewpoint, gravity could be seen as an emergent force that arises from the interactions of the quantum fields that make up spacetime.
- Physicists are still working to develop a unified theory of physics that can explain the inter-relationship between all of the fundamental forces, including gravity. Until such a theory is developed, it is possible that we will not have a complete understanding of how gravity relates to the other fundamental forces – the electromagnetic, weak and strong forces.

Summary