# Light & Relativity: 2 Questions Answered

• Dragohunter
In summary, the conversation discusses the concept of frame of reference in relation to the speed of light. It is stated that there cannot be a frame of reference moving along with a beam of light and that the change in distance between two objects cannot exceed the speed of light. It is also mentioned that while objects can be moving at a rate greater than c, it is impossible for us to observe them.
Dragohunter
2 questions

Does light have a frame of reference?

Although there is no speed greater than c, can it be said that the change in distance between two identifiable objects changed at a rate greater than c?

For example one person I know said this:

If you are able to do any simple example in special relativity, you will be able to verify this very easily for yourself. Just look at the distance between two photons moving in opposite directions in an arbitrary inertial reference frame.

Last edited:
Dragohunter said:
Does light have a frame of reference?

Are you asking whether it's possible to have a frame of reference that's moving along with a beam of light? If so, then the answer is no. For example, the $\gamma$ factor linking this frame to the frame of any material object would be infinite.

Although there is no speed greater than c, can it be said that the change in distance between two identifiable objects changed at a rate greater than c?

For example one person I know said this: "If you are able to do any simple example in special relativity, you will be able to verify this very easily for yourself. Just look at the distance between two photons moving in opposite directions in an arbitrary inertial reference frame."

Your friend is correct. What's forbidden by relativity is much more specific. For example, suppose you have an observer who takes measurements in a certain reference frame. Relativity forbids a particle from whizzing right past the observer's location at a speed greater than c. It doesn't forbid distant objects from moving at greater than c relative to the observer. For example, you can say that galaxies beyond the edge of the observable universe are moving away from us at greater than c; however, it's impossible for us to observe those galaxies.

1. Does light have a frame of reference?

According to Einstein's theory of special relativity, light does not have a frame of reference. This means that light does not experience time, distance, or any other physical properties in the same way that objects with mass do. Light always travels at a constant speed of c (the speed of light in a vacuum) regardless of the observer's frame of reference. This is one of the fundamental principles of special relativity and has been extensively tested and confirmed through experiments.

2. Can the change in distance between two identifiable objects change at a rate greater than c?

No, according to special relativity, the speed of light is the maximum speed at which any object can travel. This is known as the cosmic speed limit and is a fundamental principle of the theory. This means that the distance between two objects cannot change at a rate greater than c, regardless of the observer's frame of reference. In the example given, the distance between two photons moving in opposite directions will always remain the same, as they are both traveling at the speed of light. This has also been confirmed through experiments and is a crucial aspect of our understanding of the universe.

## 1. What is the speed of light?

The speed of light is approximately 299,792,458 meters per second in a vacuum, also known as c. This is currently considered the fastest speed at which anything can travel in the universe.

## 2. How does light travel?

Light travels in a straight line in a vacuum. It is a form of electromagnetic radiation, meaning it is made up of electric and magnetic fields that oscillate in a perpendicular direction to each other as it moves through space.

## 3. How does the theory of relativity explain the behavior of light?

The theory of relativity, specifically the special theory of relativity, explains that the speed of light is constant in all inertial reference frames. This means that no matter how fast an observer is moving, they will always see light moving at the same speed. This also leads to the concept of time dilation, where time appears to slow down for objects moving at high speeds.

## 4. What is the difference between special and general relativity?

The special theory of relativity deals with objects moving at constant speeds in a straight line, while the general theory of relativity includes the effects of gravity on objects in motion. Both theories explain the behavior of light and have been extensively tested and supported by experiments and observations.

## 5. How does the speed of light affect the measurement of time and distance?

The speed of light is considered a universal constant, meaning it is the same for all observers regardless of their relative motion. This leads to the concept of time dilation, where time appears to move slower for objects moving at high speeds. It also affects distance measurements, as the distance between two points will appear to be shorter for an observer moving at high speeds compared to a stationary observer.

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