What is gravity?
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Scientists assembling one of the elements of an LHC detector
For pupils up to 16, gravity is presented pretty much as Newton described it - a universal force of attraction between objects. The strength of this gravitational pull is proportional to the mass of the objects, and inversely proportional to the square of the distance between them. Gravity is the force that gives objects weight. And that's about it.
Newton's law of universal gravitation gives a good approximation of how gravity affects objects in everyday life, but it isn't the complete picture. There is a lot more to our modern understanding of gravity, but the bottom line is nobody has really quite figured out what gravity is. Einstein showed how gravity can be explained as a deformation in spacetime. The maths is tough, but the concept is fairly easy. Mass "tells" spacetime how to bend, and spacetime "tells" mass how to move. This description of how gravity acts, provided by the general theory of relativity, has passed every experimental test thrown at it, but it doesn't attempt to account for what gravity actually is.
We tend to think of gravity as a strong force. It's pretty noticeable if you drop a brick on your foot, and it takes a huge amount of energy for a rocket to achieve escape velocity. To particle physicists though, it's a very weak force that only becomes worth bothering about when you are dealing with great lumps of matter (like the Earth). The force that holds positively charged protons together in an atomic nucleus is many orders of magnitude stronger, but only acts over sub-atomic distances.
If you could ask a proton which way was "up", it really wouldn't have a clue what you were on about. But if you asked the three quarks that make up a proton about the force holding them together, they could tell you all about it. This strong nuclear force is mediated by things called gluons, which are force-carrying particles that glue the quarks together. It may be that there is an analogous thing that conveys gravity - there's even a nice name for it: the graviton. If these particles exist, they haven't been detected yet. It isn't thought likely that the LHC experiments will confirm the existence of gravitons, but the LHC may shed fresh light on how gravity fits into the "standard model" in physics that describes fundamental particles and the forces that act between them.
This model has been refined and tested to the point where it gives a pretty water-tight description of three of the four fundamental interactions between the elementary particles that make up all matter. The odd one out is gravity, which stubbornly refuses to fit neatly into the equations. This is a big obstacle to the holy grail of theoretical physics: a grand unified theory that elegantly describes all the particles and all of their interactions.
For pupils, realising that we don't have a complete understanding of something as basic as gravity is a great insight into how incomplete our model of the universe still is.
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