Concept of Light speed

What exactly is the Speed of light?
The speed of light in a vacuum is 299,792.458 km per second – just shy of a nice round 300,000km/s figure.

The Sun is 150 million (i.e., 150 followed by 6 zeroes - 150,000,000) km away from Earth and light takes just 8 minutes and 20 seconds to travel that far.

Can any of human creations compete in a race with light?
One of the fastest human-made objects ever built, the New Horizons space probe, passed by Pluto and Charon in July 2015. It has reached a speed relative to the Earth of just over 16km/s, well below 300,000km/s.

But humans did make several tiny particles to travel much faster than that but none exceeding the speed of light. In the early 1960s, William Bertozzi at the Massachusetts Institute of Technology (MIT) experimented with accelerating electrons at greater and greater velocities.

How can electrons travel faster than other human-made objects?
Because electrons have a charge that is negative, it is possible to propel – or rather, repel – them by applying the same negative charge to a material. The more energy applied, the faster the electrons will be accelerated.

Can we just apply more energy to make objects go fast?
Well to a certain extent, yes.

You might imagine that you just need to increase the energy applied in order to reach the required speed of 300,000km/s, but it turns out that it just is not possible for electrons to move that fast. Bertozzi's experiments found that using more energy did not simply cause a directly proportional increase in electron speed.

As objects travel faster and faster, they get heavier and heavier – the heavier they get, the harder it is to achieve acceleration, so you never get to the speed of light. So with electrons, an ever-larger amounts of additional energy is required to make ever-smaller differences to the speed the electrons moved.

The team got just a little closer and closer to the speed of light but never quite reached it.

But then how can light travel so fast as it is also made of particles?
Light is made up of particles called photons. The reason photon particles can travel at the speed of light when particles like electrons cannot is because a photon actually has no mass.

No mass and therefore no question of them getting heavier. So, photons can just zip about in vacuums like space without having to speed up.

Is Light an energy or flow of particles?

Does light in an fiber optic cable travel at a speed of light?
No because the photons in there are encountering a kind of interference caused by other photons being released from the glass atoms as the main light wave travels past.

So, nothing can go faster than speed of light and it is constant?
Yes, according to Albert Einstein's theories.

No matter where you are or how fast you are travelling, light always travels at the same speed. If you are in vacuum, it will travel at 299,792.458 km/s.

Given speed is distance travelled per time (e.g. 100km per hour etc), how can speed of light be constant in a given medium (be it glass or vacuum) in the below case?
Imagine shining light from a torch up to a mirror on the ceiling of a stationary spacecraft. The light will shine upwards, reflect off the mirror, and come down to hit the floor of the spacecraft. Let's say the distance travelled is 10m.

Now let's imagine that the spacecraft begins travelling at a hair-raising speed, many thousands of km/s. When you shine the torch again, the light will still seem to behave as before: it will shine upwards, hit the mirror, and bounce back to hit the floor. But in order to do so the light will have to travel diagonally rather than just vertically. After all, the mirror is now moving quickly along with the spacecraft. The distance the light travels therefore increases. Let's imagine it has increased overall by 5m.

That is 15m in total, instead of 10m. And yet, even though the distance has increased, Einstein's theories insist that the light is still travelling at the same speed. Since speed is distance divided by time, for the speed to be the same but the distance to have increased, time must also have increased.

Yes, time itself must have got stretched or dilated (i.e., took more time for light to travel).

What is Time dilation?
It means time travels slower for people travelling in fast-moving vehicles, relative to those who are stationary. For example, time runs 0.007 seconds slower for astronauts on the International Space Station, which is moving at 7.66 km/s relative to Earth, compared to people on the planet.