What is gravity?

• We all know about gravity• We see and experience its

effects every day• Not just on Earth…

http://grcimagenet.grc.nasa.gov/GRCDigitalImages/

1995/1995_02395L.jpg[NASA]

• But in the solar system…and beyond

[NASA]

• Gravity is a force that attracts everything to everything else

• The strength of the force depends on:– The mass of the objects– The distance between

them• So planets like Earth and

Jupiter have nearby moons orbiting them

• While they in turn orbit the distant, but more massive Sun

[NASA]

• But what actually is gravity?• Does it work the same over

cosmic distances as it does in our solar system?

[NASA]

• Newton made a good job of describing how gravity acts between objects, and his ideas work fine in most situations

• Einstein made a breakthrough by describing gravity as a bending of space-time…

• …so how does that work?

[NASA]

• Imagine a canon ball sitting on a trampoline – that’s like the Sun bending space-time

• Flick a marble along the trampoline past the canon ball and it travels around the canon ball in a circle – that’s the Earth orbiting the Sun

• From the marble’s point of view, it’s going in a straight line along a flat surface

• OK, the marble slows down and falls in towards the canon ball because of friction, but take away the friction and it would carry on orbiting the canon ball forever

• Einstein’s view of gravity is holding up well to experimental evidence

• But it means gravity is a different kind of force to the other three fundamental forces in quantum physics, which each have a mass less “carrier particle”

• The electromagnetic force, for instance, is carried by photons

• Could there be a particle that transmits the force of gravity?

• A graviton?• If so, it will be hard to spot

because gravity is a very weak force at the atomic scale

• In 2011 a fleet of 3 laser-linked probes will search for gravitational waves travelling through space

• They’re ripples in space-time caused by galaxies merging or black holes colliding

• Einstein’s theory predicts they should exist

• But could they also provide evidence for gravitons? [NASA]

• Particle detectors at the LHC might reveal phenomena that support the existence of gravitons

[CERN]