You were good at Physics at school. You were not bad at Maths. But you
never understood those things about Einstein's relativity, with those
explanations about trains and lightning flashes traveling in all
directions. One problem is lots of authors, journalists and
encyclopedia
article writers don't understand anything either on the subject. They
just
repeat what they heard, reproduce the drawings they saw and suggest
that if you don't understand, that's because you're idiotic. Let's give
it another chance...
The speed of light is 299,792.458 km/s but let's round it to 300,000
km/s inside this text.
What did you see in those articles about Special Relativity? There was
that drawing with that rocket that
looks
shorter because it travels very fast. Then that assertion according to
which a clock inside the fast rocket runs slower.
To straighten things out, let's say you live inside a space train. It
is made of 10 wagons and each wagon is 300,000 km long. Between each
wagon hangs a clock. Those 11 clocks are synchronized: they show the
same hour. This means that if you are in the middle of a wagon and you
look with a telescope at the clock in the front of the wagon and at the
one in the back of the wagon, they show the exact same hour. (But if
you are at the end of the wagon, just below the clock, then the clock
in the front of the wagon will look to be 1 second back. That's because
the image of that clock took 1 second to travel through the wagon and
reach you.) Every passenger inside your train has a synchronized clock.
Whatever speed your train travels,
it obviously has the same length for you. If you go walking from the
front to the rear with very precise measuring instruments, the length
of
the train always is exactly 3,000,000 km. You can even spacewalk out of
the train, billions of kilometers aside of it, and take a photograph of
the whole train. It
perfectly shows to have a length of 3,000,000 km. And of course, all
the clocks aboard obviously run at a normal rate. The mechanical
clocks, the water clocks, the electronic clocks, your hearth beat...
all run at one second per second exactly. You feel it... You can
compare them...
What says Special Relativity? Suppose your train is loosely hanging
high above in Space. If another train, identical to yours, shears along
yours at 150,000 km/s, it will be
shorter. Instead of being 3,000,000 km long, it will be only
2,597,476 km long. No kidding: if you ask the passengers of your
train where the front and the rear of the other train were at exactly
11:34
am,
they will confirm the photograph you took while spacewalking: the other
train is shorter. Its front was behind your front and its rear was
further than your rear. Let's say it once more: at 11:34 am and 00
seconds exactly, a passenger in the
middle of your front wagon was
seeing the front of the other train right through his window. And a
passenger in the middle of your last wagon was seeing the rear of the
other train right through his window. On your photograph, each wagon of
the other train seems as shrunk lengthwise a little bit. Weird... (But
the other train's wagons still have the same height as yours.)
And the clocks inside that train indeed seem to run slower. Let's
survey the clock in the front of the other train. When it reached the
back of your train, it showed 08:00:00 pm exactly (a person in the back
of your train confirms this very tightly). The other train travels at
150,000 km/s, so its front reaches the front of your train after 20
seconds. So then that clock should show 08:00:20 pm. Well, no... a
person in the front of your train says that the clock in the front of
the other train then showed 08:00:17 pm. It obviously runs slower.
Whatever photograph or video you take, will confirm this.
Two things are strange:
The persons living in the other train say that they measured
their train and that it has a length of exactly 3,000,000 km.
The persons living in the other train say their clocks run at
perfectly one second per second.
Actually this is easily acceptable. Their train was shortened by the
high speed but so where the measuring
instruments they use to measure its length. If your shirt shrunk
in the washing machine and a witch shrunk you too during the night, you
will not get the feeling that your shirt has shrunk when you put it on
in the morning.
The same way, if every clock inside the other train is slowed down
exactly the same way, then when comparing them, they will see no
difference. They will get no hint towards a slowing down of their
clocks. They are slowed down themselves.
Now here come the true and frightening paradoxes:
The persons living in the other train say they observed your
train and clearly it was only 2,597,476 km long.
The persons living in the other train say they observed your
clocks and clearly they run slower.
How in Space can it ever be possible that your train looks shorter to
them and altogether their train looks shorter to you? Let's admit their
train is
shorter... but then yours must look to be longer to them!
How on Earth can it ever be possible that your clocks look slower to
them and altogether their clocks look slower to you? Let's admit their
clocks run
slower... but then your clocks must look to run faster to them!
That's the paradox that Einstein solved. He realized that the clocks
inside the other train will not look synchronized to you. Actually, the
passengers inside the other train synchronized their clocks perfectly,
using the exact same methods as you used to synchronize your clocks in
your train. For them, their clocks look perfectly synchronized. But
this makes their clocks be unsynchronized
from your point of view... To express this with precise numbers: when
you see that the clock in front of their train marks 08:00:40, then you
see that the clock at the rear of their train marks 08:00:45. And all their clocks in-between
show a time in-between. There is nothing magical about this. Simply
stand by and observe a passenger in the middle of a wagon inside the
other train. He looks at the clock in the front of his wagon. The image
of that clock speeds towards him at 300,000 km/s. And he speeds towards
the image at 150,000 km/s. So, he will "hit" that image really soon. On the other hand, the
image that originates from the back of the wagon sure travels towards
him at 300,000 km/s, but he flees that image frontwards at a speed of
150,000 km/s. So it will take a lot
more time till that image reaches him. So, if he has to see the
same hour on both clocks, the clock at the back of the wagon must be a
short time later. That way he
will see the same hour on both clocks. He is absolutely sure both
clocks are synchronized. He sees that with accurate precision. But you
see that they are not. But you
understand why he sincerely believes his clocks are synchronized.
And in a flash you understand that he
must see your clocks as being
not synchronized! Though you
see them as perfectly synchronized.
The clocks of the other train are unsynchronized
from your point of view... So, now you understand why they honestly
believed that your train is shorter. They made the measure using their
clocks and they got a result that looks false to you... but you
understand why they made that
"error". From their point of view, it is better to consider things as
they obviously seem to be: your train is shorter. And from your point
of view, their train is shorter, in all honesty too. And if a passenger
of the other train went out spacewalking aside of his train, and took a
picture, on that picture, his train appears to be 3,000,000 km long and
your train appears to be shorter. (And if he sees you and wants to
shake hands with you and he fires his retrorockets to slow down 150,000
km/s to get immobile compared to you, and he turns his head to look at
his train, he will now see, just like you, that it is shorter than
yours. And that your clocks are perfectly synchronized.)
Einstein's main discovery actually is that light always travels at
300,000 km/s. If you see a passenger in the other train fire a bullet
frontwards at 1 km/s, you will see it travel at 150,001 km/s; the speed of the bullet plus the speed of the train. But
what if he lights a torch and directs it towards the front of his
train? You would think that you see that light travel at 450,000 km/s;
which is 300,000 km/s plus the
150,000 km/s of the train. Well, no... you see it travel at 300,000
km/s. But the inhabitants of the train see that light travel at 300,000
km/s too. Are they mad? No... their measuring tools are shorter, their
clocks run slower and they are unsynchronized! So, when you observe
them measuring the speed of that light, you understand why in all
honesty they conclude it travels at 300,000 km/s. And whatever light you produce, in whatever direction,
you will see it travel at 300,000 km/s and the passengers in the other
train too will always see it
travel at exactly 300,000 km/s. And they will understand, while
observing you, why you believe your light and their light always travel
at exactly 300,000 km/s. (This is for the speed of light in vacuum.
Einstein perfectly knew that light travels slower in air, glass,
water...)
The magic Special Relativity tells about our universe is that:
Objects that travel at a some speed compared to you are shorter
(whoever you are, whatever you are, wherever you are and at whatever
speed you travel).
Clocks that travel at a some speed compared to you run slower
(whoever
you are, whatever you are, wherever you are and at whatever speed you
travel).
Whoever measures the speed of light, whoever produced that light,
whatever the difference of speed between the two, the light will always
be measured to travel at exactly 300,000 km/s.
Weird... Seems paradoxical... But things are so that in fact there is
no paradox at all. Everything fits neatly and mathematically together.
This is how our universe is. This has been verified by countless
experiments.
Some further consequences are that:
When an object gets some speed compared to you, it gets heavier.
You were told at school that an object with some speed has "kinetic energy". Imagine an object
that travels really close to the speed of light. It fires a rocket to
accelerate even more. It gets the whole supplement of kinetic energy
the rocket dispenses. It should now go faster than the speed of
light... but it does not. Why? Because the supplement of energy turned
into more mass rather than into more speed.
Nothing can travel faster than light. Even if you try to push an
object constantly, so it goes faster and faster, its mass will increase
so much that it will tend towards infinity when approaching the speed
of
light. Whatever hard you push, you cannot get an infinitely heavy
object to accelerate... (More exactly: nothing can travel faster than
the speed of light in vacuum. If a particle travels at 299,999
km/s and
it enters a block of matter where the speed of light is only 280,000
km/s, it will go on traveling at 299,999 km/s (till maybe it is braked
by the matter).)
Matter contains a tremendous amount of energy. Even matter that
stands still in your hand, is made of a gigantic amount of energy.
That's what makes it has a given mass.