The Large Hadron Collider was successfully switched on in September 2008 and the physics of particle collisions made headlines. Interest has been particularly focused on collisions between near light speed particles. Aspects of The Theory of Special Relativity help explain how exotic, high speed physics can have very surprising, counter intuitive consequences. These include time dilation and the seemingly unbreakable light speed barrier.
Light Speed Collisions
At the LHC, scientists record the collisions of tiny sub-atomic particles, protons, travelling at speeds just below that of light, about 300 million metres per second. Such protons could circumnavigate the Earth more than seven times in just one second. Imagine two such high speed protons, travelling towards one another on a spectacular collision course.
Common sense dictates that two cars, each travelling at 50 kilometres per hour, will collide head on with a combined speed of double their individual velocity, in other words 100 km/h. Does that mean that the combined speed of the two colliding protons, each travelling at near light speed, will be approximately twice the speed of light? Common sense says yes. Common sense is wrong!
The Special Theory of Relativity
The combined or relative velocity of two, colliding, near light speed objects can never be greater than the speed of light. It is an inviolable rule of nature that nothing can exceed light speed. To understand why common sense is defied, when objects move extremely quickly, requires some knowledge of The Special Theory of Relativity, formulated by Einstein in 1905. Astronomer David Darling gives a short but interesting insight into the mathematics of these relativistic effects in his online encyclopaedia.
Amongst Einstein’s bizarre and brilliant revelations is an explanation of the proton’s strange behaviour. Objects approaching light speed become smaller (foreshortened) in the direction of travel. Their so-called relativistic or running mass increases and, strangest of all, time slows down. To an outside observer, when the object actually achieves light speed, it has infinite mass, zero dimensions (at least in the direction of travel) and time stops!
Imagine an observer, sitting on one of the speeding protons as it is approached by its twin, also travelling at near light speed. To measure the speed of the approaching proton, the observer measures the time it takes for the proton to travel a given distance between the two particles. Speed is calculated as the distance travelled divided by the time taken to cover that distance.
Relative to the observer on one proton, time for the other proton has virtually stopped and therefore one second, for example, gets “stretched” out to an infinite degree. This phenomenon, called “time dilation” is not just a wonderful, peculiar idea. It has been measured by scientists using extremely accurate atomic clocks and is a real, observable fact.
Plug the “stretched out seconds” into the speed calculation, the travel time becomes infinitely long and the observed speed therefore drops to zero. Add together the “combined” speeds of the two protons therefore, and we still get an answer which never exceeds light speed.
The odd behaviour of speeding objects only becomes really significant when, as in the example given, they are travelling extremely quickly, i.e. approaching light speed. One should not be fooled, however, into thinking that slower objects are immune from the physics of Special Relativity. Next time you take a trip on a speeding jet or in a fast car, even on a gently moving bicycle, remember that you are gaining mass, shrinking and staying younger compared to the poor folk that you left behind at home!
Andy Allison - Hi, my name is Andy Allison. I am an analytical chemist by profession, having studied at Sheffield and Manchester Universities to gain my ...
