So c, little c, the speed of light, probably one of the most famous symbols in physics.
Certainly it appears in the most famous equation: E=m c squared
The c in there is the speed of light. It really is one of the most fundamental constants.
- c, small c, means the speed of light.
That's what, three times ten to the eighth metres per second.
So three hunderd million meters per second. So it's fast, but not infinitly fast.
- It really is the ultimate constant in physics.
In the sense that it really is the same, whoever measures it.
That's what lies at the heart of special relativity,
this idea that the universe is a relative sort of place.
One of the precepts of special relativity is that the laws of physics
are the same in any reference frame, if you're stationary or moving
you should see the same laws of physics.
- Why is the speed of light c?
- Why is it said to be c or called c? That I don't know.
I really don't know why that constant has been defined as being called c.
I'd be interested to know if Mike knows.
- Maybe because it is thought to be a fundamental constant.
c for constant, but I don't really know.
- People are kind-of used to the idea that speeds add.
So that if you see an aircraft taking off from an aircraft carrier,
if you want to know how fast the aircraft is going,
you have to add how fast the aircraft is moving relative to the aircraft carrier,
to how fast that was moving forward, so the total speed is the sum of the two.
People are very used to that and you use that kind of calculations,
eg. to calculate how fast a car is coming towards you.
You're measuring realtive speeds of things.
But the weird thing about light is, because the speed is always the same, it doesn't add in that way.
So if you're moving in a train and shine a flashlight from the train,
and you measure the speed that the light comes out of the flashlight at,
it comes out at exactly the same speed as if you weren't on the train.
So, speeds don't add. The speed of light, whoever measures it,
however you produce the light, always is the same value, c.
That's not quite true, that's the speed of light in a vacuum.
There are ways to slow light down.
If light travels through air it travels a little more slowly.
It can never go faster than in a vacuum, you can make it slow down.
People will do sophisticated experiments in a laboratory, to slow down light to a snails pace,
by messing around with the refractive index of a material.
Messing around with the properties of the material it is propagating through.
You can make it slow down, you can never exceed the speed that it goes in a vacuum.
So c is in that sense a cosmic speed limit, it it the fastest there is.
- Does it have much to do with in your speciality, is c very important to you?
- For my evening, weekend thinking about physics, yes.
Because I like to read books about cosmology and thinking about things like that.
For most of the experiments we do in this lab, we're looking at electrons in semiconductors
which are moving well below the speed of light, typically about one hunderdth of the speed of light.
- How do we measure the speed of light?
- It's a quite difficult thing to measure, for a long time the experiments were quite crude
and the constraint we had on the speed of light were quite limited.
Some of the best measurements we have today are by using radar echos off things.
You can bounce a radarbeam of things, because it is called the speed of light,
but ofcourse all the electromagnetic waves travel at the same speed.
If it is a radio wave, an x-ray, optical light it all travels at the same speed.
By bouncing radio waves off things, we can measure the speed pretty accurately.
Because light travels extremely fast, 3 times 10 to the eighth metres per second,
that's 300 million metres every second light travels at.
Obviously you can't sit there with a stop-watch and measure it that way, it's too fast.
Unless you can find something that's a long way away.
So bouncing light off the moon or other planets is by using strong radar signals,
is a very good way of measuring how fast that light's going.
- Do you know exactly what it is?
I can't tell you exactly because it is defined with an afwul lot of decimal places.
And one of the nice things about astronomy is we rarely need to know things very accurately.
For astronomers purposes it is 300 million metres per second,
but actually it isn't exactly 300 million metres per second.
Yes, it's something like 2.99 ... eight ... times ten to the eighth metres per second.
but I can't remember that exactly, three point zero zero or 3.00 times 10 to the 8th is a very good approximation.
- Oh yeah 3 times ten to the eighth metres a second.
- I cheat, I've a pocket calculator with all these physical constants in it.
I just press the little button marked 'c' to get it in the display.
transcription: merijn.vogel@gmail.com