Editor’s Note: Don Lincoln is a senior scientist at Fermi National Accelerator Laboratory in Illinois. He is the author of “The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind” and produces a series of science education videos. Follow him on Facebook. The views expressed in this commentary are his own. View more opinion articles on CNN.
To state the obvious, Albert Einstein was one of the smartest and most impactful scientific minds in history, with a long track record of staggering predictions. Those predictions have been tested in the real world, and they’ve always checked out.
Of course, in science, you’re only as good as your last successful calculation and that means that scientists are constantly working out new ways to test Einstein’s ideas. Astronomers have recently made an exciting and novel measurement of his theory of relativity.
Einstein’s theory of relativity, which he developed over a period of years, makes mind-blowing predictions: that moving clocks tick more slowly than stationary ones and that objects are measured to be shorter the quicker they are moving. When he added gravity to his thinking, he deduced that clocks run slower when they’re in regions of high gravity. He also predicted that light emitted in a place where the gravity is strong will redden as it moves to locations where gravity is weaker.
But that doesn’t mean contemporary scientists complacently accept his theory of relativity as unquestioned fact. They have now sought to answer a new question: Are Einstein’s predictions valid in an environment with a gravitational field far stronger than any found on Earth?
A new experiment needed doing – and black holes, the burned-out hulks of massive stars with gravitational forces so strong that light cannot even escape – provide the perfect environment to put, once again, Einstein’s theory of relativity to the test.
At the center of nearly every galaxy is an enormous black hole. In our own Milky Way galaxy sits one with a mass about 4 million times that of our sun. It’s called Sagittarius A*, after its location in the constellation Sagittarius.
The black hole isn’t alone but rather surrounded by stars that orbit it very closely and sometimes in highly elliptical orbits. And that is key to the new measurement. A star by the name of S2 has passed close by Sagittarius A*, traveling through a region of gravity that is about a million times higher than can be experienced on Earth.
There is a central feature of general relativity called local position invariance, or LPI, which states that any measurement performed on a freely falling object should be identical whether it is in a strong gravitational field or none at all. That sounds a bit complicated, but it’s really no different than saying that a skydiver’s wristwatch should tick the same during a parachute jump as the same watch on an astronaut in deep space, where there is no gravity. That watch should function just the same for a hypothetical skydiver on Jupiter, which has much stronger gravity than Earth.
Essentially, if LPI is true, then measurements should be entirely blind to whether they occur in a gravity field or not. If a measurement invalidates LPI, it also invalidates Einstein’s theory.
Getting a stopwatch near a distant star or black hole isn’t possible, so scientists needed to come up with a different means of comparison. Stars are made predominantly of two atomic elements, hydrogen and helium. Every element emits a unique set of colors, and each color has a different frequency. This has been measured well in the low gravitational environment of Earth, and scientists wanted to see whether the elements emitted the same colors in a strong gravitational field. If they didn’t, that would invalidate LPI.
So, to perform the test, scientists watched the light emitted from S2 as it dove into Sagittarius A*’s strong gravitational field. Now, because the light emitted from the star moved from the strong gravity near a black hole to the weak gravity on Earth where the scientists were making their observations, it is expected that the colors will appear redder on Earth than they were when they were emitted by the star. However, the color of light emitted by hydrogen and the color of light emitted by helium should be reddened by the same amount.
And that’s exactly what scientists observed. Both elements emitted the same wavelengths of light as they fell through the black hole’s strong gravitational field as they emit in the weak gravitational field on Earth. Einstein’s theory of general relativity was again validated, and the findings were published last month in Physical Review Letters.
While this measurement is clearly a scientific triumph, researchers aren’t resting on their laurels. A new facility, called the Extremely Large Telescope, will be able to make even more precise tests of general relativity, so we can be sure more measurements are to come.
You might wonder why scientists constantly question well-established theories such as Einstein’s, but that’s simply the nature of science. No scientific prediction is ever taken to be sacrosanct. Just as Sir Isaac Newton’s ideas of gravity that were developed in the 1670s were replaced by Einstein’s, scientists fully expect Einstein’s theories will eventually be replaced by something better. Until then, Einstein’s theory of gravity will continue to reign supreme.