When Einstein published his General Theory Of Relativity in 1916, there were a few minor technical issues with checking that his ideas about the effects of gravity upon spacetime were accuratel. Chief among them was that man was still four decades from getting an object into orbit.
Now, though, NASA has announced the results of what it’s calling an epic experiment into the subject, the first funding for which was granted in 1963. And the conclusion? Einstein was right.
The very short version of Einstein’s theory is that a body’s mass and motion distorts the space-time around it in a particular manner. The best explanation of what that means in terms we can easily conceive is as follows:
Imagine a sheet of graph paper: while technically three-dimensional, to the human eye and brain it is effectively two dimensional, with marked lines in two directions.
Now the sheet is held in the air and imagine a heavy marble is rolled across it. As it does so, the marble causes part of the paper to dip down, thus making a third dimension clearly visible in the paper. If you then place a golf ball in the middle of the paper and roll the marble again, its path across the paper is visibly altered.
The analogy works in two ways. Firstly, it’s a way of understanding how a fourth dimension can exist, even though we can’t perceive it: by showing that the paper is subject to three dimensions even though without the marble we can only perceive the paper as two-dimensions, it becomes easier to accept that everything around us can be subject to four dimensions even though we can only perceive them in three-dimensions.
Secondly, if Einstein’s theory was right, the effect of the golf ball on the marble’s movements should represent how gravity can affect a fourth dimension (known as space-time.)
Translating the golf ball/graph paper analogy to reality, Einstein then believed that an object such as the Earth creates a similar “dimple” in space-time, which should affect the movement of the space equivalent of the marble. But, Einstein argued, the fact that the Earth is constantly spinning should mean the dimple is more of a swirl effect.
So how did NASA test the theory? The basis concept was to put a spinning gyroscope into orbit, pointed towards a particular star. Without the effects predicted by Einstein, the gyroscope would spin and orbit in such a way that it would always point towards that star. If Einstein was correct, the swirl effects of Earth on spacetime would mean the gyroscope would be disrupted.
To make things certain, NASA sent four gyroscopes into space, each of them 1.5 inches across and made of quartz and silicon. They are described as the most perfect spheres ever made, with even the biggest bump or dimple no more than 40 atomic layers deep or high. That was vital to make sure the effects of Earth could be isolated, rather than any disruption simply being the fault of the gyroscope itself.
Calculations based on Einstein’s theory suggested the gyroscope would be spun out of position by 0.041 arcseconds a year; there are 3,600 arcseconds in one degree. To give an example of how small and precise such a change is, it’s the equivalent of the Earth spinning an extra four feet around the Equator.
Such precision — which NASA described as “like measuring the thickness of a sheet of paper held edge-on 100 miles away” — meant the project took a year to collect the data and almost five more to analyze it.
But now we have a result. Not only did the effect predicted by Einstein prove to have taken place, but the measured variance for the year — predicted at 0.041 arcseconds, turned out to be recorded at 0.039 arcseconds (albeit with a 0.007 margin of error either side).
That’s roughly equivalent to trying to predict the position of a specific spot on Earth in a year’s time and getting it right within 2.5 inches.
No wonder Clifford Will, who chairs an independent panel charged with monitoring the projects, says that “One day, this will be written up in textbooks as one of the classic experiments in this history of physics.”