Einstein Light
These are the English subtitles for the multimedia presentation, Einstein Light: a brief illumination of relativity (www.phys.unsw.edu.au/einsteinlight). 
We should like to include subtitles in major world languages*, to augment our current set (Chinese, English, French, German, Italian, Japanese, Russian, Spanish). If you are fluent in such a language, and know or can find the technical terms used (eg proper time, thought experiment etc) and if you would like your language represented, please contact us. As the webmaster who adds the subtitles will not speak your language, it is important that the subtitle text be divided in the same way that the original text is divided, and included below the English text, as shown in the first examples below. If a new font is required, we should like to know where we can obtain it.
Thanks very much in advance!
George and Joe.  
georgeh@unsw.edu.au	J.Wolfe@unsw.edu.au 
* We don't wish to seem antagonistic to less widely spoken languages. However, it is likely that any speaker of Dutch (for example) who is interested in relativity will also read English well.

Subtitles.
Relativity began with Galileo, who said that, in the laws of mechanics, only relative motion is important. 
La relativité commence avec Galilée, qui dit que, pour les lois de la mécanique, seul le mouvement relatif est important.

In  other words, mechanical forces are the same for the person on the train and the platform. 
En d'autres termes les forces mécaniques sont les mêmes pour le passager sur le train et celui sur le quai. 

All they need do to convert their measurements is to add or subtract the relative velocity.

If you can juggle on the ground, you can also juggle on a smoothly moving ship, a train or even a plane: 

the laws of physics – the forces involved – are the same, even if a bystander sees juggler and balls all whizzing past at high speed.

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But what about electricity and magnetism? 

Could Galileo's sailor – or our train travellers – use electromagnetism to tell whether they are moving? 

Do the laws of electromagnetism work for both observers? 

Let's do what Einstein called a thought experiment.

Imagine that these two balls each carry positive electric charge.

What is the force between them?

Like charges repel so you can calculate the electric force.

However, moving charges create magnetic fields and, in this case, the magnetic force is attractive.

So you see an electric repulsion, and also a magnetic attraction.

But now look at it from my point of view. 

With respect to me, the charges are not moving, 

so from my point of view there is no magnetic force.

You see electric repulsion, reduced by the magnetic force. 

I see only the electric force. 

So your answer is less than mine. 

This difference is very important. 

If electromagnetism works in different frames, 

then different observers get different answers for the force, solely due to relative motion.

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So, how to reconcile Galilean relativity and electromagnetism? 

We might say that Maxwell's equations of electromagnetism only work in one special frame of reference. 

But what would be the preferred frame?

Einstein's intuition was that Maxwell's equations were so beautiful that they had to be true, and true in all frames, 

even if this meant replacing the relativity of Galileo and Newton.

So Einstein started from the principle that 

the laws of electromagnetism, as well as mechanics, are the same for all observers. 

But, as we've just seen, 

different observers will have to make more complicated conversions when they compare measurements. 

Let's see the consequences.

Light is an electromagnetic wave so, if it obeys the same laws in all frames, then 

all observers will see light travel at the same speed.

Now, fortunately for Einstein, two American physicists 

had recently tried to measure the effect of the Earth's rotation and orbit on the behaviour of light. 

Their results showed... no effect. 

Now this is weird and counter-intuitive. 

Can it really be that light from the headlights has the same speed with respect to the driver and to a pedestrian?

But our common sense is developed for everyday speeds, 

so there's no reason why our intuition should work near the speed of light. 

So either follow these links, or else just accept Einstein's principle, and see where it leads.

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Deep down, most clocks are electromagnetic. 

Here's another thought experiment using a simple, electromagnetic clock:

A light pulse reflects between two mirrors—inside a car. 

Zoë is in the car and sees the mirrors to be stationary. 

For her, each 'tick' of this clock is the time light takes to cross the car: the width divided by the speed.

This we call the proper time: the time observed by someone travelling with the clock.

For Jasper, who sees the car pass by, the light pulse follows the hypotenuse of the triangle. 

For him, the light travels further but, according to Einstein, it travels at the same speed. 

Consequently, an observer with respect to whom the clock is moving observes that the clock ticks more slowly. 

This is called time dilation. It's weird, but it's true, as many experiments have shown. 

So there is no absolute measurement of time. 

It follows that there's also no absolute measurement of distance: 

You and I can measure distance in light years: the distance travelled by light in a year. 

But since we both see the same speed of light, but disagree on the time taken, we'll disagree on the distance as well. 

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When we apply a force to accelerate an object, the work we do becomes its kinetic energy. 

But observers who disagree on length and time must also disagree on acceleration and work, 

so relativity gives different results: 

in Einstein's physics, momentum and kinetic energy increase rapidly as the speed approaches that of light, as this animation shows.

That famous equation is coming up, so please forgive us some maths

When we calculate kinetic energy, mc2 appears as a term associated with a mass that has no relative motion. 

Einstein's insight was that this term is its proper energy: 

an energy associated purely with its mass, independent of its motion, and that thus 

mass and energy might be exchanged or transformed. 

This proper energy, mc2, is the energy equivalent of mass m.

This  explains why nuclear power and nuclear weapons need so little fuel. 

The speed of light c is large, so the conversion of a tiny mass produces great quantities of energy. 

But relativity is everywhere: without it, there would be no strong forces in the nucleus, and therefore no chemistry.

Relativity is needed in engineering, too, 

such as this electron source used to treat cancer patients and in satellite navigation systems.

Relativity has revolutionised our thinking about time and space. 

It is weird and counter-intuitive, but if you want to get the right answer when speeds or energies are high, you simply must use it. 

Scientists and engineers do, every day.

(For the front page)
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