Faster than a speeding bullet or how to measure the speed of a beam of light.

Do you remember how fast we are travelling? I did tell you back at the start of this journey.

“Well that was a while ago Albert, but as we’re travelling on a beam of light we must be going at the speed of light.”

If I told you we’d covered 18,382,965,104,070,000 miles in just over three thousand years could you work it out from that?

“I guess that means we must be going very fast.”

186,000 miles every second. Light moves so fast that it seems to arrive without any delay at all. Light could get from London to New York in just two hundredths of a second. That's faster than a human blink.

“So what’s our speed in miles per hour, I can’t really imagine a speed in miles per second.”

Oh, let’s see. Almost 671 million miles per hour.

“Wow, how could you measure something going that fast?”

It’s difficult but not impossible. How would you do it?

“I don’t suppose we have a speedometer with us do we? Would a stop watch work?”

Not a bad idea. You can measure the speed of anything if you can record how far it goes in a second. That idea works fine for checking the speed of people running or even cars but as light goes 186,000 miles in a second it gets a bit difficult. Can you think why?

“Because I couldn’t see that far? I’d have to be able to see the start and the finish line to see when the light beam started and finished.”

Of course you can see that far. The moon is further away than that. The furthest object in the sky you can see, the Andromeda galaxy, is 2.5 million light years away. The real problem is that the light coming from the starting line would take exactly the amount of time to reach you as the beam of light you were trying to measure. So by the time you saw it leaving it would have arrived at your position and appear to arrive with no delay at all.

“That’s weird.”

But true. Imagine a bullet being shot at you and travelling faster than sound. You would be hit by the bullet and dead even before you the sound of the gun being fired reached you.

“Kind of unfair, but I can picture that.”

So if the bullet was travelling at exactly the same speed of sound you would hear the bullet at the same moment it hit you. It would seem to have arrived without delay. It would be the same if the gun was two feet away. The sound and the bullet would always arrive at the same time.

“I could look for the flash from the gun. That would arrive faster and use that to measure the speed of the bullet.”

True but for measuring light you would need something much faster than light.

“How about radio signals?”

No, radio waves travel at the speed of light because they are part of the same electromagnetic spectrum as light. That is the one of the starting points of my theory of relativity, nothing travels faster than light.

“So just by knowing that nothing travels faster than light, I can understand a little bit of your theory of relativity?”

Not just a little bit, a very important part of relativity. Thinking about the speed of light and when things seem to happen is the heart of special relativity. I told you earlier that one of my scientific heroes, Galileo, was the first person to start thinking about relativity almost four hundred years ago. Galileo was also the first person to challenge the ancient Greek ideas about how light travelled by measuring the speed of light. It was clear to the ancient Greeks that light and sound don't travel at the same speed. The delay between lightning and thunder when a storm isn't overhead showed that light travels much faster than sound but they thought it just arrived without any delay.

“How did Galileo do it. I thought you just explained that you couldn’t measure the speed of light?”

Not at all. I was trying to show that it is difficult but not impossible. Galileo managed to solve the problem we were talking about by being at the starting line and finishing line at the same time.

“How can you be in two places at the same time?”

It happens all the time in a lap race. The start and finish are usually at the same place aren’t they?

“Yes, but light travels in straight lines”

Usually yes, but you can reflect it back or get someone to send another light beam back, like a relay race. That’s what Galileo tried. He sent an assistant to a hill a mile or so away and positioned himself on another hill so there was nothing blocking their view of each other. Galileo had sorted out lamps for himself and his helper and the plan was for Galileo to switch his lamp on and start a clock. As soon as the assistant saw the first lamp light go on, he would turn his lamp on. By measuring how long it took before the light from the second lamp to get back, Galileo hoped to work out how fast light was travelling. Unfortunately even a remarkable man like Galileo couldn't spot the 5 millionths of a second delay while the light travelled from one hill to another a mile away.

"So he just needed a better clock."

Well a better clock would have helped but that kind of accuracy was centuries away, but a much bigger distance for the light to travel would help. With a little thought a Danish astronomer by the name of Ole Rømer had a go at measuring the speed of light in 1676 and came up with a speed of 133,000 miles per second.

"But he got the wrong answer?"

OK, so he got the wrong answer by 50,000 miles a second or so, but back then that was a pretty impressive achievement. But he did show that light took time to get places, it didn’t just arrive.

"Better than a million miles off I suppose. So how did he measure the speed of light?"

He used a clock but didn't need to measure millionths of a second because the light he was measuring wasn’t coming from a nearby hill but from the planet Jupiter which is 400 million miles away at its nearest point to Earth. Rømer carefully measured the time it took for Jupiter's moons to circuit the planet. He noticed that the times when the planet Jupiter's moons disappeared from view behind the planet, like an eclipse, seemed to be different at different times of the year. Since the Earth's moon is regular as clockwork this seemed odd. It only made sense if light from Jupiter's moons took some time to reach the Earth. At those times of year when the Earth was nearer to Jupiter, the light had a shorter distance to travel so the eclipses of Jupiter's moon seemed to happen earlier. Later on when Jupiter was getting further away, the light had further to go so it arrived later. It looked as though the moon had disappeared behind Jupiter later than it should have done.

“So how did scientists come up with the answer of 186,000 miles per second?”

A Frenchman called Leon Foucault used an updated version of Galileo's idea in 1862 but rather than use an Italian servant to open the shutter on a lamp, he used a rotating mirror.

"How does that help?"

If the light beam is reflected off a spinning mirror before being sent off to another mirror some distance away, then by the time the light gets reflected back to the spinning mirror it will have rotated a fraction.

“So?”

So the reflected light beam doesn’t get back to where it started but ends up a small distance away because by the time it gets back the rotated mirror….

“…rotated.”



Exactly so the light beam is reflected off at an angle. It's easy to measure the distance the light beam has moved with a ruler and how fast the mirror is spinning than it is to measure a few millionths of a second. So much easier that Foucault only needed to bounce light from a mirror sixty feet away. He got an answer pretty close to the actual figure but was finally beaten by an American in 1879. Albert Michelson improved on the Frenchman's machine by borrowing $2,000 from his father-in-law to built a bigger better version. This was no mean feat as $2,000 was a huge sum in those days and would be the same as almost a million dollars today. Michelson’s final answer was a speed of light of 186,355 miles per second which is impressively close to the figure accepted today of 186,282 miles per second or 299,792,458 metres per second.

“Impressive but does measuring the speed of light that accurately really help anyone?”

It was an important step for physics. A few years later in 1887 Albert Michelson also helped finally to bury the idea that light needed a strange substance called ether to carry it through space.

"Who's Ether?"

Ether, or aether, is a non-existent substance invented because no one could imagine waves without water.

"I don't follow."

Remember I explained that light can act like a wave.

"Like a sound wave or a wave on the ocean?"

Precisely, but what is left of an ocean wave if all the water is taken away?

"The fish?"

Very funny, the real answer is absolutely nothing. When you take away air and make a vacuum, sound waves disappear like ocean waves without water. A Mexican wave in a football stadium is carried by the spectators, with an empty stadium there is no conceivable way of having a Mexican wave. So if space is empty then how can light waves cross it? Some people were so attached to light being a wave that they said – ‘Simple, space isn't empty, it is full of ether and that's the stuff that carries light waves’.

Proving ether didn't exist rested on the simple idea that swimming in a river with the current is faster than swimming against the current. As the Earth rotates around the sun each year then it must be sweeping through the ether if it exists, effectively producing a flow of ether in the same way as wind whistles past a moving car even on a windless day. Light travelling in the same direction as the movement of the Earth will be travelling into this ‘headwind’ and so get slowed down. Light travelling sideways compared to the Earth won't face any extra resistance. In 1887 Michelson and Morley measured light travelling in these two directions and showed there is no difference in the speed of light. Discounting the remote possibility that all the ether in the universe was rotating around the sun in exact unison with their laboratory on Earth, this showed that light can't be travelling in anything like ether.

“So what difference did that make?”

With ether gone, the time was right for a complete rethink about how the universe works. That rethink was my theory of relativity. Now give your brain a rest for a while. Next time I’m going to throw some really strange ideas at you.

Quantum mechanics for cat lovers – Newton strikes back.

Let me you ask you a question. If you close your eyes does the world still exist?

“Of course it does. What a daft question.”

How do you know?

“Well, I can feel the chair I am sitting on. I can hear noise from the street outside.”

Yes but what if a tree falls in the middle of a forest and no-one sees or hears it falling?

“It still happens because the world exists, we are just part of it.”

You believe that the world is a physical reality?

“Of course, why are you asking these crazy questions?”

To show why I had such a hard time believing in Quantum mechanics. At the start of all this back in the 1920’s all of us theoretical physicists were excited by what we discovered about light and atoms. Then some people like my friend Niels Bohr took quantum mechanics to an extreme and claimed that nothing exists until it is measured. A tree wouldn’t really have fallen until someone went to see.

“So the big bang didn’t happen until someone came along and could measure it?”

Crazy idea, huh?

“Raving. If the universe couldn’t have been born until someone checked it had happened where did that person come from?”

Brilliant, now you’re thinking. Do you know what we call that? A paradox, where something contradicts itself or common sense. Quantum mechanics is full of them and I spent a lot of time tormenting Niels Bohr with paradoxes but he still believed in quantum mechanics. The crazier it got the more he believed in it. Niels Bohr once said if quantum mechanics hasn't profoundly shocked you, you haven't understood it yet.

“Well I’m shocked and I’m still not sure I understand it. How did they even start to believe this?”

In quantum mechanics any situation is a blend of every possible option of what might happen and this blend is called a wave function. This seems to work for light. Sometimes light can act as a particle and sometimes as a wave. Niels Bohr and his friends showed that atoms seem to follow the same rules. As the world is made of atoms, the world must follow the rules of quantum mechanics. Obviously in the real world doesn't spend its life sitting on the fence, things just happen. But in quantum mechanics things happen only when this wave function collapses and only one possibility is left.

"What on earth does that mean?"

Sorry that’s the sort of jargon quantum mechanics use all the time. It means that at some point a situation has to stop having every possible outcome. When an event is observed then all the other possibilities suddenly disappear.

"Hmmm. Still not sure I get this at all."

It's like saying that the universe is based on chance. One enormous casino. What happens next is based on chance not on an absolute certainty. Imagine the universe as a horse race with lots of evenly matched horses. Until the race is over you can't tell which horse is going to win. With quantum mechanics the idea is that the race isn't over until someone decides to check on the result. This is where the science fiction idea of ‘parallel universes’ comes from. If every possible outcome is waiting to happen perhaps it really does happen in another quantum universe. Every horse wins in some reality.

“Gamblers must love quantum mechanics, but it seems too weird to be true.”

That’s what I started to think. But it wasn’t just me. A friend of mine Erwin Schrödinger was the man who first discovered the equations that quantum mechanics relies on. Even he couldn’t believe the idea that nothing happens until someone looks to check it. He invented the most famous cat in science - Schrödinger's cat. If nothing happens until it is observed then imagine the following. A cat is put in a box with a small gadget that will release poison.

"A real cat?"

No this is just an imaginary cat, so whatever happens the cat doesn't really get harmed. Like this journey, it's what is called a ‘thought experiment’ as you have to imagine it happening.

“OK, I’m sure I want to even imagine poisoning a cat but let’s hear where this is going.”

This poison will be released by something that is controlled by the laws of quantum mechanics, for example radioactive decay. Radioactive atoms are ones that are unstable and spontaneously break down into smaller atoms. So there is a lump of radioactive material and a device to detect if an atom has broken down. This atomic break-up has a 50:50 chance of happening in one hour. According to quantum mechanics, until the box is opened an hour later both outcomes should co-exist. The cat should be both dead and alive at the same time until someone observes the result.

"Can't the cat tell if it's dead or not?"

Only if it's alive.

"Hmmm. That’s as daft as the ancient Greeks thinking that seeing involved feeling rays coming out of the eyes."

Well despite what some people think, this story was meant to show how Niels Bohr’s interpretation of quantum mechanics was wrong. It was just an interpretation. I think there is an easier way of thinking about this. Quantum mechanics does seem to explain a lot of things about atoms and light. This craziness of a cat that is both dead and alive only applies if you stick to the idea that everything happens until it is measured by a person. There is no paradox if you just change to the idea that a quantum event happens when the result interacts with anything. When the radioactive atom in the box decays, the cat will only die when the radioactivity detector in the box detects it. When a particle that follows quantum mechanics interacts with anything it has to commit to being one thing or another. So a quantum mechanic event can set up a sequence of events that end up with a cat that is dead or alive without needing it be both at the same time.

“I thought you didn’t believe in quantum mechanics?”

Well I didn’t believe the extreme version, but perhaps in my re-creation inside this computer I’ve mellowed a bit. All this cat really tells us about quantum mechanics is that trying to use quantum mechanics to explain normal day-to-day life doesn't work. Understanding atoms doesn't help you understand a whole cat, but then again understanding cats doesn't help you understand atoms, so it works both ways. At the end of the day quantum mechanics does make sense in its own realm and offers explanations for strange effects that have no other explanation. My problem with quantum mechanics was summed in the my idea that 'God doesn't play dice'. Everyone seems to remember that but do you know not what Niels Bohr said in reply?

“No.”

It is not the job of scientists to prescribe to God how he should run the world. Not a bad reply I think. My real problem with quantum mechanics was that I couldn’t see why the universe would have one set of rules for big objects and another set of rules for the particles inside atoms. I spent most of the second half of my life trying to join this all together into one beautiful theory of everything.

“Did you get there?”

No. Once or twice I thought I was close but it slipped away, like sand through my fingers. Someone out there will solve it I’m sure one day.

“The world needs another Albert Einstein or Isaac Newton to solve that.”

Well the world needs a lot of things more than another Einstein or Newton. Peace, kindness and fewer weapons would be a good start. Mind you, I don't suppose Isaac Newton would have been too happy with the Schrödinger's cat experiment either. One of Newton's less well known claims to fame is as the inventor of the cat flap. In the simple understandable universe that Newton described, the cat would have got bored and left out of the flap at the back, leaving the quantum mechanics scratching their heads and wondering where the cat had gone.


(No cats were harmed in the writing of this blog post. In fact one was fed, let out of the kitchen door, let back in and back out again. I don't have a cat flap.)


CHAT WITH ALBERT 2.0's CAT MIMI