A Blog for the Curious and the Scientifically Perplexed

This is the story of a great journey that started with a great thought. One day in 1895 a boy looked into a mirror and wondered what the universe would look like if he could travel on a beam of light. That sixteen year old boy was Albert Einstein and that one thought started him on the road to discover his Theory of Relativity. The great man has been reinvented as Albert 2.0 to come back and blog about a journey through space on a beam of light and explain the science behind everything from atoms, blackholes to global warming. If you've just joined and want to start at the beginning use the index on the left. If you're bored try these links below just for fun.


Tuesday, July 3, 2007

Relatively simple. An easy guide to relativity

“Albert, do you think you can really explain the Theory of Relativity to me?”

Of course but don’t worry if it seems confusing. It is. It’s not until you start thinking about things and getting confused that you can really start stretching your brain. Thinking is like exercise, if it hurts it must be good for you.

“As long as my brain doesn’t implode, I don’t mind a bit of an ache for a good cause.”

Good. Now, the first thing you need to know is that there are two theories of relativity.

“Two? That means it must be twice as hard to understand as I thought.”

Not at all. The two types of relativity are special relativity and general relativity. The special version was the first one I came up and is a little a simpler than general relativity.

“What’s the difference?”

Special relativity deals with things moving at a constant speed, like light or trains or cars. General relativity was much harder to sort out and took me another ten years to get right. This general theory explains how gravity works.

“How about starting by explaining what the word relativity means.”

It’s called the theory of relativity because it is based around the idea that there is no such thing as moving or staying still. All movement is relative to something else so you can never really know if you are moving or not. There is no such thing as absolute movement.

"But we're moving."

Or are the stars just coming towards us? In a station when the train next to yours starts to move, you might be convinced for a while that you are moving when you are not. That is relative movement or relativity.

“But I know a train really moves because it ends up somewhere else.”

True, but what else is happening during your train journey?

“I don’t understand what you are getting at.”

It is not just the train that is moving. The track and station are moving too.

“No they’re not.”

Of course they are. The earth is rotating and does one complete spin everyday.

“But that isn’t real movement because you can’t feel it.”

You can’t feel the movement but can see the effect, that is why the sun rises and sets. The important thing about relativity is that you need measure your movement against some fixed point. What scientists call a frame of reference.

“What’s the frame of reference on a train?”

Usually the track. The problem is that the track is moving as the world spins. If you were on a train near the equator, say in the middle of Africa, the track itself would be moving at almost a thousand miles an hour around the centre of the earth.

“So the centre of the earth must be a fixed point.”

No the whole Earth is going around the sun, that is where the seasons come from but that’s another conversation. Around the sun your train track would be moving over 60,000 miles per hour.

"Wow…so the Sun is a fixed frame of reference?"

Sorry, the Earth's Sun and the whole solar system are rotating around the centre of the galaxy. Your train track is travelling at a speed of almost a million miles per hour around the centre of the galaxy.

"Is the centre...."

Sorry again, all galaxies are moving away from each other; remember the Big Bang and everything moving apart.

"Doesn't the universe have a centre?"

I'm afraid not. My theories also showed that space is curved. If you travel far enough in a straight line you end up back in the same place. So how can something that has no edge have a centre?

"Hmmm, so what can you rely on?"

Much less than you think. If movement is relative, there can be no such thing as a fixed place because you could never know if you were still or moving. The other important thing I realised in developing the theory of relativity is that there is no such thing as absolute time.

“You are saying time is relative? Relative to what?”

Movement and position can affect the time things seem to happen at. Imagine an explosion seen by two people, one is very close and the other a hundred miles away and both have synchronised watches. Who will see it first?

“The light will reach the closest person first so they will see it a fraction before the person who is further away.”

So they won’t agree on the time of the explosion.

“No but if there were in the same place they would.”

So if two people at the same place they will always agree on timing.

“Yes. I think so”

OK, now imagine two explosives joined by long wires to one detonator placed exactly in the middle. What happens if you push the detonator.

“Both explosives will explode at the same time.”

Good. So what if at the exact moment of the explosion there is another person driving in a fast car past the detonator towards one of the explosives.

“He’ll see the two things explode at the same time if he can manage to look in two directions at the same time.”

As this is a thought experiment will give him the power to see in opposite directions at the same time, but he won’t see the explosions happening at the same time.

“Why not?”

He’ll see the explosion he is heading towards first because in the time it takes the light from both explosions to reach the detonator he will have moved a bit himself towards that explosion. Just like you agreed before as he’s nearer to one explosion he’ll see that explosion first and then other explosion later. So the moving person sees the explosions at slightly different times but they will look simultaneous to someone standing by the detonator. If you can’t get people to agree that things happen simultaneously how can you rely on time?

“Hold on Albert, that’s a trick. If the moving man reaches the detonator at the same time as the light from the two explosions they’ll look simultaneous. Ahah, got you!”

I’m afraid not. The light takes a little bit of time to get to the detonator so if the car is at the detonator just as the light reaches it, then at the time of the explosion the car would have to be a little bit away from the detonator. You already agreed that things will only appear to happen at the same time if people are in the same place. So the time things seem to happen at is different for two people if one is moving relative to the other. There is no such thing as absolute time.

“My brain is definitely about to implode.”

Hold on, you might need your brain for a bit longer. We’ve still a long way to go and lots of things to talk about. The good news is that there is one fixed thing in the universe.

“What’s that?”

The speed of light. That is the heart of my theory of relativity. After convincing myself that time and movement have no fixed meaning, I decided to see what would happen if God had decided the speed of light was the one fixed thing in the universe.

“So what does happen?”

Some very strange but real things. But I think it might be safer to let your brain cool down for a bit before I start telling you about that.

Enjoyed it? Then Digg it.


  1. Raúl said...
    Fantastic Blog, I'm enjoying every post.
    Clear and fun, it reminds me of Carl Sagan's "Cosmos".

    About light speed, I have a question:
    It's velocity is an absolute, it starts at it, there is not acceleration (and that is so weird), so when reading I had an idea...

    Light can be affected by gravity, if is strong enough it bends and change paths, and in a Black Hole it can't escape so what happens if -using our imaginations- a light beam is emitted from a point, and after some seconds the point of origin transforms in a gravity source really strong.

    What is going to happen?

    If is strong enough, it should affect the light... will it make stop and reverse direction, now towards the gravity source?
    And if that happens, it will first decelerate to zero and then reverse, or will it just flip directions?

    Maybe a gravity source so strong is not possible, but well...

    Keep writing!
    albert2.0 said...
    Great question. If gravity is strong enough to "stop" light you would have to be within Schwarzchild radius, so it will be trapped and just disappear from the visible universe. If it is just outside that distance it will escape but strange things will happen if the gravity is very strong.

    What will be seen from a distance is that the colour of the light will shift to the red which is the gravitational version of the red shift. Something we'll cover next time is the effect of speed and gravity on time. Time will slow down near the gravity source, so it will take a longer time to escape even though it is still moving at the speed of light if you measure it at that point. Just at the point of disappearing in the black hole or escaping it will seem to take an infinte amount of time to escape. If the gravity source is rotating, like most black holes, the light will also seem to spiral away rather than move in a straight line Strange huh?
    Raúl said...
    So, if I understand correctly, if you "stop" light it disappear...

    Unless you manage to slow down time, and "trap" the photons... from a outsider point of view, because to light, all is normal an traveling at normal speed.

    shesss... so Light is really a constant, for what I understand, it just cannot exist if is not moving at what is supposed to.

    If the gravity source is rotating, like most black holes, the light will also seem to spiral away rather than move in a straight line. Strange huh?

    The black hole is twisting the time and space so much?

    I love this stuff, is so good for the brain.

    And just as a side note, I found your site thanks to your interview in TWIS.
    Anonymous said...
    Come on Albert, don't you know that it is far easier for people to read black on white, and difficult to read white on black? I wanted to join, but I cannot stand the strain on my eyes... I will come back later... and if it changes to black on white for easier reading I will join in the blogs.
    Allan Hansford
    albert2.0 said...
    Sorry you found black a problem. It's a big problem out here in space. Everything is pretty black.
    Todd said...
    I've read numerous accounts of people saying that the speed of light is actually not a constant. They have been measuring it over the years and the speed of light is degrading just like everything in the universe. What kind of impact does that have on your theories?
    albert2.0 said...
    Well, its not constant as it slows in glass water and most other things, but it is constant in space at least in terms of metres per second. Why? Because the metre is now definfed in terms of the speed of light. Any real change now in the speed of light will change the length of a standard metre instead.
    Anonymous said...
    Getting to the original question, where light is emitted from a point, and then seconds later that point turns into a point of extreme gravity, like a black hole: NOTHING WOULD HAPPEN. Gravity waves travel at the speed of light. Since the light emitted from the original point already had a head start, the warping of the newly formed black hole would not catch up to the original beam.

    Please someone correct me if I am wrong, but Einstein, you yourself showed that gravity waves move at light speed: If the sun dissapeared we would see it and feel it at the same time. The earth would not be shaken, then 8 minutes later the sun flickers off.
    albert2.0 said...
    Quite right, the effect of gravity would spread out as a wave so it normally wouldn't catch-up! Could a gravity wave overtake a light wave?
    Yes if the light wasn't travelling directly away from gravity source. It
    could also catch up if the light were travelling through a material (like water) that slowed it down more than gravity waves.
    Anonymous said...
    I know that light travels slower in different mediums, but is there any theory (perhaps yours) that predicts if the speed of a graviton travels slower in a different medium? Or is there just not enough evidence to support a suggestion like that for this elusive particle?
    David said...
    If everything in the universe is traveling, thus making everything relative in time as well as space, shouldn't the speed of light also be relative? Light would seem to be moving faster if slower in the same direction as the universe is expanding; however, if moving toward the center of the universe (away from the expansion), the light would appear to be moving faster.
    HumbleOne said...
    Good question, David. The trick to understanding the expansion of the universe is to realize and accept that there is no dimensional "direction" of the universe's expansion, nor is there a physical "center" of the universe. Imagine for a moment that our round Earth were expanding. Would this mean that there must be a "center" of the Earth's surface? The Universe's expansion is measured in the direction of time we call the past, not in a physical dimension of space. The idea which someone else posed helped me understand this is: imagine that the universe is on the surface of a balloon, which is being filled with air. Take an uninflated balloon, and make dots on it with an ink pen. Now inflate the balloon. You will notice that, as the balloon gets larger, all the dots get further apart. Is there a center or edge of the balloon's surface? No. The only center is in the past, a direction backward in time. Is there an edge of space? No, there is only a future larger size. And as Albert said, if you were to travel in any direction far enough, you would come full circle, as you would if travelling around the world. Imagine what our ancestors thought back when the Earth was believed to be flat, and you will understand the challenge of seeing both perspectives. We now take for granted that the Earth is round; The next step is to accept that space is also curved. We can travel indefinitely in any direction on the surface of the Earth and never percieve that we aren't going in a straight line, but a curved one. The same applies to traveling through space.
    Anonymous said...
    If photons can never be stopped to be examined then where do they come from orginally?
    Anonymous said...
    First of all thank you for your blog. I found it very useful as it was explained in a very easy and understable way.

    The thing I don't understand is that why mass of an object must increase with its speed. I know that if a big object travels faster or in the speed of light its mass becomes infinite. But why?
    Anonymous said...
    you stated that the seasons occur because of our orbit around the sun.
    The seasons actually occur due to the tilt of the earth ( about 23 degrees)the orbit of the earth around our star has very little effect if any as we are in a nearly circular orbital pattern
    albert2.0 said...
    Have a look at this posting:

    Without the tilt there wouldn't be seasons but without the earth orbiting the sun the "seasons" wouldn't change.
    darthcricket said...
    Anonymous said...

    Come on Albert, don't you know that it is far easier for people to read black on white, and difficult to read white on black? I wanted to join, but I cannot stand the strain on my eyes... I will come back later... and if it changes to black on white for easier reading I will join in the blogs.
    Allan Hansford


    Highlight the text and you have dark text on white background.

    Thanks albert 2.0 for the work
    Anonymous said...
    The two types of relativity are special relativity and general relativity. Special relativity deals with things moving at a constant speed, like light or trains or cars and general relativity deals with gravity. Space is curved and if you travel far enough in a straight line you will end up back in the same place.

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