A new world story of the young girl-electron which was going to a party time and distance

Travelling at the speed of light

The speed of light
Nega looked at young Elna with a smile as she asked this question, then slowly answered: - Light has a very high speed. Light travels in vacuum at about 300,000 km/s ! To make a comparison, think that you and your friends ran in the metal at average speeds of about 1,000 km/s. Nevertheless these speeds are only 0.003 times the speed of light. Then think of the super-fast human racing cars that run at 300 km/h, that is 0.08 km/s. They are practically stationary, only 0.0000003 times the speed of light. Yes, you heard me correctly: a zero followed by six zeros after the dot!

    - But Humans build also artificial satellites that reach very high speeds, - said Elna defiantly.

    - The satellite speed around the earth may be around 20 km/s.


(Credit: Partial figure adapted from: Il grande libro della Scienza, Mondadori 1976, 220)		  
This may seem a very high speed to Humans, but it is only 0.000066 times the speed of light! Humans also build electron microscopes, where electrons are accelerated at a much higher speed. Electrons can reach speeds very close to that of light.

    - After all, - continued Nega trying to clarify the concept,  - also the speeds of the planets rotating around the Sun inside the Solar System seem to be incredibly high. The speed of the Earth, for example, is about 30 km/s, but it is just 0.0001 times that of light.

Nega paused for a while, then began again in an almost solemn tone: - The light speed is constant and equal for all observers; for the Human that you see down there, sitting in his laboratory, for the astronaut travelling between the stars and for the possible inhabitants of a distant planet wandering in the Cosmos. And on top of this, the speed of light can't be exceeded.

Elna listened in silence.

Nega, after a further studied pause, concluded: - These are the properties that make the speed of light a real term of comparison. Note that, the closer you get to this speed, the more you can appreciate effects that you had ever noticed before, like that of mass growth (that we have already seen) and many others I haven't yet told you about.

The speed of light can't be exceeded
    - Light is really fast! - exclaimed Elna. Then she added thoughtfully: - Why can't the  speed of light be exceeded?

    - Let me try to explain, - said Nega smiling. - I've already told you that the mass increases as the speed increases; but, the higher the mass, the more difficult it is to accelerate it. Pushing a boy on a bike is different from pushing a truck. The more a body accelerates the more difficult it becomes to continue accelerating it. Think of the muon in  the example I gave you before. At 99.999 per cent of the speed of light, the mass is increased 244 times its original one; at 99.99999999 per cent  it is 70,000 times. The mass increases with no limitation approaching the speed of light. So you would need an infinite energy to reach 100 per cent of this speed. Obviously this is not possible, and so we have to reach the conclusion that nothing can go faster than light!

Nega saw that young Elna had become silent and pensive. To lighten the tone of the talk and to bring a smile back to her face, Nega added jokingly: - I don't know how much of this is true. Anyway, young Peter asserts that there is "something" faster then light. He says that, when he goes to the bathroom, his need is always faster than switching the light on!

Young Elna broke into a laugh and then fell silent.

The speed of light is constant and equal for all the observers
While Elna was laughing another young girl-electron, who had been listening in silence all this time, turned shyly to Nega: - My name is Katiele, though everyone calls me Kate. I was listening to what you said. May I ask you a question?

Nega looked at the girl-electron, a pretty creature with black hair and dark eyes. She was pleased to have aroused the girl's curiosity. After all, the whole time Nega had been speaking to Elna, she had felt the same pleasant sensation she felt while telling stories to her daughters. So she had started to enjoy herself.

    - Ask me whatever you like. I'll be happy to answer, if I can, - replied Nega with her usual kindness.

    - You said that the speed of light is constant and equal for everyone. What does that mean? In practice, does this change anything for us ? - said Kate, gaining confidence.

    - This changes a lot of things. It's a real revolution of the "way of thinking" for the inhabitants of Worlds that usually experience low speeds, like the Human World, - replied Nega calmly.

    Nega saw the astonishment on the face of her young listeners. She continued: - I'll give you an example. Imagine for a second that you are a Human. One day, a crazy man throws a hand-bomb at you while you are walking down the street. What would you do? You would run away as fast as you could. Suppose that the bomb is travelling at a speed of  8 m/s and you run at 5 m/s, you know that the bomb's speed towards you is (8-5)=3 m/s. So it will reach you without any doubt and you have no way of escaping.

    - But, - continued Nega, - let's see what would happen if the crazy man shot at you with a laser gun, so he shot some "light" at you. You can think of the light as formed of many little balls, called "photons". Suppose that these photons travel toward you at the speed of 300,000 km/s. In this case too you would try to run away. Imagine, with a bit more fantasy (!), that you jump onto a space-ship that is able to reach 40,000 km/s. Well, according to common ideas, the speed of the photons coming towards you should be (300,000-40,000 =) 260,000 km/s.


(Credit: Partial figure adapted from: Quark 22, dic. 2002, 78)

    - But this is not what you would experience! Although you are running in the opposite direction, the photon speed in the vacuum is always 300,000 km/s! At first sight, this may seem impossible after what we said in the example with the bomb (and what we could experience in endless other cases in our daily life). But it is not impossible, on the contrary it is possible, it is what happens! You can escape from photons or run toward them: their speed will be always the same.

    - That's incredible! - said Kate. Then, after a while, she added: - Can you give us another example?

     - Yes.let me think. Right, I have it. Do you know baseball, the game humans play? Now, consider these three different situations. Think of catching the ball:

(1st case) ball thrown by a player standing motionless in front of you;

(2nd case) ball thrown by a player coming towards you, for example in a car at 50 km/h;

(3rd case) ball thrown by a player going away from you at 50 km/h.

Well, what do you think the speed of the ball would be? Obviously you expect a different speed in the three cases. This is really what happens. However, if the pitcher threw you photons by means of a laser gun, then you would experiment the photons to have exactly the same speed in all the cases!