# Gravity – From Newton to Einstein, Dark Matter to Supermassive Black Holes – 20/02/12

20th February 2012

This week we’re talking about Gravity! Travelling from the basics with Newton and his apples onto more complicated stuff with Einstein and black holes, gravitational lensing and even how you can measure the acceleration due to gravity in Student Science!

In the 17th Century Sir Isaac Newton published Philosophiæ Naturalis Principia Mathematica, laying the foundations for most of classical mechanics, the sort of thing taught at GCSE and A-level these days…

He also described gravity as a force that attracts things that have mass, supposedly it all started when he was watching an apple fall from a tree but whether that is true or not he did come up with the three laws of motion and he described universal gravitation.

One of his most famous formulas is:

where F is the force, G is the gravitational constant, m1 and m2 are masses and r is the distance between the two masses. Frazer Pearce tells us a little more about G:

This formula was used for many years and helped explain the orbits of the planets around the Sun and can even calculate your weight on the moon!

Even now people are still using Newtonian physics (albeit modified) to try and describe things such as the difference in observed speeds and theorised speed in stars orbits on the outskirts of a galaxy. We caught up with Tony Padilla to find out more about Modified Newtonian Dynamics or MOND as it is more commonly known:

For our student science experiment this week, we sent George off to the lab to see if feathers and hammers fall at the same speed.. as they should do in Newtonian mechanics. It’s not as simple as you think it is!

[video:http://www.youtube.com/watch?v=8RKAb5accC0]In the 20th Century Albert Einstein published his paper on the Special Theory of Relativity. He was bothered by the fact that he had to use the word “special” and started to work on a more general version.

In 1916 he published his paper on the General Theory of relativity. He suggested that the curvature of spacetime is actually shaped by mass and this curvature causes light to bend.

This actually leads to some interesting effects, such as gravitational lensing. This is where the light is bent around large objects giving a false position. This was first proved by Sir Arthur Eddington in 1919 when he observed that stars were out of position behind the Sun during a total solar eclipse.

We can also use Einstein’s theories to infer the presence of dark matter. This mysterious substance is thought to make up to 84% of the matter present in the Universe… but if we can’t see it, how do we know it’s there? We can use gravitational lensing. Take a look at the picture ‘Lensing’. If the star were heavier, the light would bend more. Therefore we can tell that there is extra mass there, even if we can’t see it.

Finally, as part of the show, we sent George off to investigate whether it is actually possible that time travel could occur. We recommend that you try his experiment.

Step 1: Write an invitation to a party, make sure it’s the best sounding party possible (include the promise of jelly and ice cream – no one can resist those). Write the current date and time on the invitation.

Step 2: Invite people from the future by leaving your invitations by burying your invitations so that only people from the future can find them.

Step 3: Wait for people to show up.

If anyone turns up from the future, congratulations! However, we tried this experiment and got no guests whatsoever. It turns out that if you were to travel close to the speed of light (in order to travel back in time, you need to arrive at your destination before light does) thanks to Einstein’s theories, you would become infinitely heavy.

No time-travel for us then.

Or is it still possible? Listen to the show to find out about time travel near a black hole…

Make sure that you listen to the show, Monday 20th February 6-7pm, or check out the podcast.