# Questioning Light Speed: An Advanced Physics Mystery

• wilders
In summary, the velocity of an object relative to another object cannot exceed the speed of light, but the difference in velocities between two objects as seen by a third object can be greater than c. This is due to the way velocities are calculated in relativity, and the fact that the velocity of an object relative to a photon is nonsensical.
wilders
Hi everyone,

First I want to say that I hardly know anything about advanced physics, and I'm just looking to ask something that I've been thinking about for a while. So I'd also appreciate if you kept the explanation somewhat understandable :P

Anyway, here's my question:

If light from one source goes in the opposite direction of light of another source, wouldn't that mean that they would go 2 times the speed of light (2c), RELATIVE to each other?
I'm pretty sure this is not the case, but what I do not understand is why it isn't the case.

Hopefully you understand what I mean x_x

When we say "velocity of object A relative to object B," we mean "the velocity of A as 'seen' by B" (or more technically "the velocity of A in a reference frame in which B is at rest"). This can never be greater than c.

You're asking about "the difference in the velocities of A and B as 'seen' by a third object C which is not moving along with either A or B." This is a different thing, and it's OK for this to be greater than c. In fact it can be as great as 2c, as in your example.

The reason they're different is that velocities don't "add" in relativity the same way as they do in classical mechanics:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel2.html

There's another issue in that the "velocity of an object relative to a photon", using the definition in the first paragraph above, doesn't make any sense, because there is no reference frame in which a photon is at rest.

## 1. What is the speed of light?

The speed of light is a universal constant, defined as approximately 299,792,458 meters per second in a vacuum. This means that in a vacuum, light will always travel at this speed, regardless of the observer's frame of reference.

## 2. Why is the speed of light considered the fastest speed possible?

According to Einstein's theory of special relativity, the speed of light is the maximum speed at which all matter and information can travel. This is because at the speed of light, time and space become relative, making it impossible for anything to travel faster.

## 3. How is the speed of light measured?

The speed of light can be measured using various methods, including the use of lasers, mirrors, and interference patterns. One common method is to measure the time it takes for light to travel a known distance, such as using a rotating mirror to measure the time it takes for a beam of light to travel to and from a distant mirror.

## 4. Can the speed of light be exceeded?

According to our current understanding of physics, the speed of light cannot be exceeded. However, there are theories that propose the existence of particles, such as tachyons, that could travel faster than the speed of light. These theories have not been proven, and the concept remains highly debated among scientists.

## 5. How does the speed of light impact our daily lives?

The speed of light plays a crucial role in many aspects of our daily lives, including communication, technology, and transportation. For example, the speed of light is used in fiber optic cables to transmit data, in GPS technology to determine location, and in medical imaging technology to create detailed images of the human body.

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