More: Seven things that don’t make sense about gravity
Take a moment to try a jump into the air. Have you ever thought about how remarkable it is that so little effort is required to jump a few inches off the ground. Your puny muscles, weighing just a few kilograms, can overcome the gravitational force of the Earth, all 6 × 1024 kilograms off it. Gravity is a real weakling – 1040 times weaker than the electromagnetic force that holds atoms together.
Although the other forces act over different ranges, and between very different kinds of particles, they seem to have strengths that are roughly comparable with each other. Gravity is the misfit. Why should this be so?
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So far, our best explanation comes from string theory, the leading candidate for a “theory of everything”. String theory requires that the universe has more than the three spatial dimensions that we experience, and possibly as many as 10. According to string theorists’ best ideas, gravity is so weak because, unlike the other forces, it leaks in and out of these extra dimensions. We only get to experience a dribble of the true strength of gravity.
The proof of this might come through experiments that probe the gravitational attraction between objects that are very small distances apart. String theory suggests that the unseen dimensions are hidden from view because they are rolled up small, or lie “end-on” to our dimensions, making it hard to detect their presence. These compactified dimensions could alter the gravitational attraction between two bodies if they are very small distances apart. Experiments have got down to about 0.06 millimetres, but have failed to see anything so far.
One of the big hopes for the Large Hadron Collider at CERN near Geneva, Switzerland, is that it will tell us why gravity is so weak. “The LHC’s purpose, more or less, is to understand this question,” says Lisa Randall of Harvard University.
Though it is not likely to provide a complete answer, the case for gravity residing in extra, hidden dimensions would be strengthened if the LHC finds evidence for particles called Kaluza-Klein states.
These were mooted as far back as the 1930s by theorists attempting to unite electromagnetism and gravity. Kaluza-Klein states arise when familiar particles slip into an extra dimension. As they rattle around there, they create an “echo” that would manifest itself as a heavier particle.