Lecture excerpt: On the subject of light speed
“A famous experiment was conducted just over a century ago. Two gentlemen, Michelson and Morley, decided to try and measure the speed of the Earth as it moved through space. The plan was to measure the planet’s speed against the fixed background of the ‘ether’, the medium through which light was thought to travel at that time. By shining a light in one direction and measuring its speed, and comparing this with the speed of light measured in the opposite direction, they hoped to get a value for the Earth’s velocity. It was roughly analogous to throwing a tennis ball out of the front of a speeding train, measuring its speed, then comparing this against a ball thrown with equal force towards the back of the train. The difference between these two results would of course allow you to calculate the speed of the train. In the Michelson-Morley case, the train is replaced by the Earth, and the tennis balls by beams of light.
“To their consternation, however, they found that their results were consistently nonsensical. There was apparently no difference in the speed of light whichever direction they looked. They knew the Earth had to be moving, after all it goes round the Sun, and the Sun moves round the Galaxy and the Galaxy moves through the Universe. So the speed of light they measured should surely be different in some directions than others. But it wasn’t. They were confused.
“What does this mean to the likes of you and I then? Well imagine, to continue with our locomotive analogy for the time being, that you are on a train. Shine a torch at a mirror on the train’s ceiling and measure the time the beam of light takes to bounce back to the torch. Remember that to you, the path of light appears to be straight up and straight down however fast the train is going, so let’s say it goes 6 feet up and 6 feet down. Now, imagine a schoolboy sitting on the platform kicking his heels as the train goes past. To him, this all looks very different. The train is travelling forwards - let’s say it is going very fast, close to the speed of light. To the schoolboy, the light obviously looks like it is taking a diagonal path because of the train’s motion - forwards and upwards to the mirror, then forwards and downwards back to the torch. This is a substantially longer distance than the 12 feet you measured, BUT, because this is light, it travels at the same speed from his point of view as from yours. And because it travels at the same speed, but the length it has travelled appears much longer to the schoolboy, when he measures how long it takes for the light to return to the torch he gets a bigger answer than you do. To the schoolboy, the light’s journey appears to take longer than it does to you on the train. How can this be? Two people measure the elapsed time of the exact same event yet both get different answers. And, although not in agreement, both are perfectly correct. Confusing, isn’t it?
“Now, Einstein (yes, him again) realised that if light truly did appear to travel at a constant speed no matter from where it was emitted or which direction you observed it, then this really would represent an absolute universal speed limit. Nothing, not even a particle speeded up for eternity in an infinitely powerful particle accelerator, could go faster. And the repercussions of this... well, we’re talking huge here. Big, big repercussions.
“This is the crux of the Theory of Relativity - there is no fixed time that defines when an event happens. The apparent time that you may measure is entirely dependant on how you are moving in relation to the event. It depends on what is called your ‘frame of reference’.
“It also means that when you are travelling on a train, to any people you may pass on a platform you are to all intents and purposes travelling in a time machine.
“Which may help to explain the timetabling difficulties we always seem to experience on our railways these days.”
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Eradani wondered if perhaps she shouldn’t have lied to Dr. Richter about Cofomaristics and the school of computer sciences. He was bound to find out the truth sooner or later and she could only imagine the trouble she’d be in when that time came.
She dragged herself down the corridor wearing a look of concern on her face. A mysterious but friendly man walked by and cheerfully told her to perk up, but she didn’t take any notice. She continued her march through the labyrinthine corridors of the physics department, not feeling at all happy with the situation.
Arriving at the laboratory door, she took a couple of deep breaths, ran her hands through her hair, then reached forwards.
She felt her hand faintly touch the door handle. Summoning up her will-power, she grasped it firmly and gave it a sharp twist.
It broke off in her hand.
She looked at the door resignedly and gave it a shove.
It opened easily.
She entered.
The door slammed behind her.
Dr. Richter looked up from the table with a start. ‘Ah,’ he said. ‘Glad you could make it. Problem with the door?’ He looked back down at his work without waiting for a reply.
Eradani hoiked herself onto a desk that had been pushed haphazardly up against a wall and and tossed the broken door handle aside. Examining Dr. Richter’s experimental apparatus, she noted at least fifty mirrors carefully lined up in two rows along the length of the table. At the top left corner she could see a standard laser; low power, low intensity, so as not to cause too much damage to the retina if you accidentally looked directly into it. At the bottom right corner of the table there was a photo-sensitive meter, and on the adjoining table there was a laptop computer. She was disappointed to find it displaying a screensaver that sported the sardonic slogan: ‘AARGH! IT HASN’T WORKED AGAIN!’
Dr. Richter still had a few mirrors to set up. Eradani could see by the time it took him to align just one of the mirrors that he must have been in here for hours.
‘You see,’ he said, ‘the light is bounced between each of these pairs of mirrors, slowly working its way down the table.’ He stopped while he examined the position of the mirror he was working on. Eradani had already guessed how the experiment worked, but she let him finish anyway. ‘When it reaches here,’ he flung his arm out vaguely in the direction of the meter, knocking over a couple of his carefully aligned mirrors, ‘the computer over there measures the time between emission and reception.’
‘So you know the exact distance between the mirrors, then?’ asked Eradani, losing interest slightly and picking up the broken door handle to play with.
‘Yes I do,’ responded Dr. Richter, managing to snap his mirror in two. He threw it into a bin just beside him, which Eradani could see was already half full of shattered glass. ‘I’m going to use two methods to calculate the speed of light,’ he continued. ‘First, I’m going to measure the time and divide the distance by it, as you normally do. But then, I’m going to instruct the computer to look for anything travelling at minus four and a half miles per hour.’
Eradani nodded. She knew the experiment was a waste of time. So did Dr. Richter. If there was one thing that modern physics has taught the world, it is that the speed of light is not minus four and a half miles per hour. Cofomaristics, however, begged to differ and was attempting to teach the world that the speed of light was minus four and a half miles per hour. This was the theory’s centrepiece conclusion, and the final three chapters were devoted entirely to its derivation and to its obviously shattering consequences.
And it was this prediction that Dr. Richter had selected to try and confirm experimentally. He moved across to the computer, wiggled the mouse about a bit, selected a couple of things and hit Return. Eradani watched as he proceeded to the measuring device and ensured it was correctly configured. He turned to his student confidently. ‘That’ll do,’ he said.
‘Cracking,’ said Eradani, twisting her door handle absent-mindedly. Dr. Richter gave the set-up one final check and retired to the computer. He made a face at Eradani and her infuriatingly squeaking door handle, then moved his mouse pointer over an important option. Eradani couldn’t see the monitor from where she was, but there you go, you can’t have everything.
The doctor pressed the left mouse button. Eradani heard the laser powering up. She finally lost interest in the door handle and focused her attention on the experiment instead.
Dr. Richter hit the mouse button again. The laser screamed into action. An ultra-thin beam of red light raced from the curiously boxy apparatus and slammed into the first mirror, then rebounded onto the second. It continued, crashing into one mirror after another, reflecting back and forth across the table until, ultimately, it fell upon the detector.
‘We have a result,’ sang Dr. Richter, feverishly clicking away with his mouse, trying to get the computer to display the result in units other than millimetres per second.
‘Here we go. Six hundred and seventy million miles per hour. Rather fast - as it should be.’ He once again grabbed the mouse and bombarded it with a series of heavy clicks and violent sweeping moves.
‘So, it looks like Cofomaristics is wrong, then,’ said Eradani.
‘That it does,’ said Dr. Richter. ‘Although we do still have one more experiment to perform. Maybe that’ll turn up something.’
Eradani could sense a hint of sarcasm in his voice. Dr. Richter was not usually given to being overly sarcastic, so on those rare occasions when he was it was quite noticeable. It was also apparent that he didn’t have the slightest belief that the experiment would work. Obviously, the radical ideas presented in Cofomaristics were a bit hard to take seriously; after all, they were diametrically opposed to every single thing ever developed in physics. Indeed, in every science. If the predictions of Cofomaristics were ever demonstrably verified, then society as we know it would crumble and something entirely new would need to rise in its place.
Eradani mulled that notion around in her head. She decided she quite liked the idea. She willed the experiment to work.
Dr. Richter had finished his preparations. ‘Round two commences,’ he said. He hit the mouse. Miraculously, it survived intact. The laser once again warmed itself up, ready to start shifting electron orbits and generate all those coherent photons.
Dr. Richter clicked the button once again. For the second time the laser beam bounced its way along the mirrored path set out neatly for it and collided headlong with the detector device.
The computer beeped.
A single, solitary, tuneless beep.
Eradani ignored it. Computers often beeped. She always thought they kind of enjoyed beeping. Whenever she used a computer, all it did was beep at her. Really, to get just a single solitary beep would have been an exceptionally good computing session. She looked across at Dr. Richter. It seemed that he didn’t think getting a beep was very good at all. He was lying in a crumpled heap on the floor.
‘Dr. Richter?’ she tried. Nothing. She tried again, a bit louder. Still nothing. She walked over to him and prodded him. Nothing. She kicked him. Nothing. She grabbed his ears and twisted them as hard as she could. He shot upwards, yelping in pain.
‘What are you doing?’ he said, rubbing his ears.
‘You fainted,’ said Eradani bluntly.
‘So you twist my ears?’ he said incredulously.
‘Well, I couldn’t wake you up.’
Dr. Richter just looked at her, totally speechless. ‘Why did you faint, anyway?’ asked Eradani.
Dr. Richter’s speechlessness turned to shock. He remembered the computer. He looked away from Eradani. ‘Look at the monitor,’ he said simply.
Eradani stepped over him and walked to the computer.
‘AARGH! IT HASN’T WORKED AGAIN!’
The screen saver was on.
Annoyed, she shook the mouse around until the proper display came back
It said ‘Specified velocity detected.’
Eradani looked at Dr. Richter.
Neither knew what to do next.
‘By the way - I’m not entirely sure we can open the door,’ Eradani pointed out, figuring now was as good a time as any to mention it.