# Are there other phenomena (besides light) whose speed is invariant?

• quantumphilosopher
In summary, there are a few phenomena, such as gravitational radiation and possibly gluons, that are expected to have a constant speed independent of the motion of the source. However, any massive particle with a high enough velocity will also appear to have a constant speed, as shown by the recent discovery of the neutrino's mass. Despite the misconception that the speed of a massive particle will always increase with the motion of the source, special relativity explains that this is not the case.

#### quantumphilosopher

Are there any other phenomena (optical or not) whose speed is constant and does not depend on whether the source is moving or not?

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quantumphilosopher said:
Are there any other phenomena (optical or not) whose speed is constant and does not depend on whether the source is moving or not?

Since the neutrino has been found to have mass, it is expected to travel (very slightly) slower than the speed of light.

This leaves gravitational radiation as a form of radiation expected to travel at 'c'. The strong force carriers (gluons) may also be a candidate for moving at 'c' - the major problem is they can't really be isolated as I understand it (not very well, unfortunately), which makes it difficult to measure their speed.

Note that ANY massive particle, with a sufficiently high velocity, will appear to travel at a nearly constant speed independent of the motion of the source. This is how the neutrino, for instance, was thought to be massless for a very long time.

While the speed of an ultra-relativistic massive particle won't be exactly constant, it can be easily shown that the motion of the source does not directly add to the speed. For instance, suppose an object moving with a velocity v1 emits a massive particle with a velocity v2, where v2 is very close to 'c'. It's quite easy to make v1+v2>c, and it is equally easy to perform an experiment to show that the massive particle moves with a velocity v<c by emitting a light beam and showing that it beats the massive particle. This is explainable by how velocities add in SR (they don't add).

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Yes, there are other phenomena besides light whose speed is invariant. One example is the speed of sound in a medium. The speed of sound is constant and does not depend on the motion of the source. This is why we can hear the sound of a moving object, such as a car, at the same speed regardless of whether the car is moving towards us or away from us.

Another example is the speed of gravitational waves. These are ripples in the fabric of spacetime that travel at the speed of light and are not affected by the motion of their source.

In addition, the speed of all fundamental particles, such as electrons and protons, is invariant and does not depend on their motion.

In terms of optical phenomena, the speed of radio waves, microwaves, and X-rays is also constant and does not depend on the motion of their source.

Overall, the principle of invariance of speed is a fundamental concept in physics and applies to many different phenomena, both optical and non-optical.

## 1. What is the meaning of "invariant speed" in physics?

Invariant speed refers to the fact that the speed of a phenomenon remains constant regardless of the observer's reference frame. This concept is a fundamental principle in physics, as it implies that the laws of physics are the same for all observers regardless of their relative motion.

## 2. Is light the only phenomenon with an invariant speed?

No, there are other phenomena besides light whose speed is also invariant. For example, the speed of sound in a vacuum or the speed of gravitational waves are also invariant. However, the speed of light is the most well-known and studied example of an invariant speed.

## 3. What role does the theory of relativity play in understanding invariant speed?

The theory of relativity, specifically special relativity, is the foundation for understanding invariant speed. It states that the laws of physics, including the speed of light, are the same for all observers in inertial reference frames. This theory has been extensively tested and verified, providing a framework for understanding the concept of invariant speed.

## 4. How is the speed of a phenomenon determined to be invariant?

The speed of a phenomenon is determined to be invariant through experimentation and observation. Scientists use a variety of methods, such as measuring the time it takes for light to travel a certain distance or observing the behavior of particles in different reference frames, to determine that the speed remains constant regardless of the observer's perspective.

## 5. Are there any exceptions to the concept of invariant speed?

While the concept of invariant speed is a fundamental principle in physics, there are a few exceptions. For example, in certain extreme conditions, such as near black holes or in the early universe, the laws of physics may behave differently and the speed of light may not be constant. However, these exceptions are rare and do not negate the overall concept of invariant speed.