Since the 1960s, the United States government has been shooting lasers at the moon. No, this is not a covert government conspiracy or a relic of the Cold War. It is NASA’s attempt to prove Einstein’s theories right.
Let me explain.
A little over 100 years ago, Albert Einstein published his theory of general relativity, a set of physical laws that upended our understanding of how gravity works. He suggested that time and space are connected and that large masses (such as the Earth) could distort both of these dimensions. While comprehensive, the theory still had to be tested to establish its validity and to understand its nuances in the real world.
About 50 years later, NASA went to great lengths to make this happen. The Apollo program sent astronauts to the Moon between 1969 and 1972, and while they were there, several astronauts left refractors (basically large mirrors) on the surface. These mirrors enabled astrophysicists down on Earth to use them as reference points for studying the distance between these two bodies, a technique called “lunar ranging,” yielding data that could be used to study Einstein’s theory of general relativity.
What does the distance between the Earth and the Moon have to do with relativity? Well, lunar ranging lets astrophysicists understand every minute detail of the Moon’s orbit around the Earth, letting them test Einstein’s theory about how gravity works against how it actually does. If astrophysicists tried to calculate what the Moon’s orbit should look like based on Einstein’s calculations, and their results turn out to not match what actually happens, Einstein’s theory is probably incomplete. But if they run their calculations and it turns out they did correctly predict how the Moon actually orbits the Earth, this suggests that Einstein may have been onto something.
How does lunar ranging work?
The lunar ranging technique that astrophysicists use to measure Earth’s distance to the Moon is very similar to sonar. With sonar, submarines bounce sound waves off neighboring ships and measure how long it takes for the sound to return. The time delay indicates the ships’ distance away. Astrophysicists use a similar technique with lunar ranging, but they don’t use sound waves – they use light.
The researchers start by aiming a large laser pointer at a refractor on the moon and sending out a very short pulse of light. The light only stays on for one tenth of one billionth of a second, long enough to produce a stream of light that’s about one inch long. Then, at 671 million miles per hour, the light travels to the moon, bounces off a refractor and hits a receiver back on Earth. The whole process takes about two and a half seconds. By calculating the exact amount of time it took the light pulse to make the round trip, researchers can determine the distance between the laser pointer and the refractor on the Moon.
Finding a Fixed Distance
Astrophysicists in the United States and the Soviet Union started bouncing light off the Moon in 1962, before the refractors were in place. However, their measurements were not that precise. These astrophysicists were really interested in measuring the distance between the center of the Earth and the center of the Moon, but since light doesn’t travel through either body, all they could do was measure surface to surface. There was no way they could measure how long light takes to get to the middle of the Moon.
They determined that they could overcome this limitation by calculating the exact distance from the surface of the moon to its center. But since they were not aiming at anything in particular at the time, their lasers could have bounced off anything — from a mountain to a canyon to a small bump on the surface of the Moon — meaning there was no way to really know the true distance between the surface and the center. It was not until astronauts put the refractors in place that the moon’s distance could be measured with precision. The refractors never move, which means they will always sit at a fixed distance from the moon’s center. So, there’s no guessing what that extra distance will be.
Interestingly, it’s not as easy to establish a fixed point here on Earth. Ideally, the receiver should be at a fixed distance from the center of the Earth, just like the reflectors are to the Moon’s center. But the Earth’s crust is always moving – from changes in atmospheric pressure to shifts in the tectonic plates to tides from the sun and moon — the surface of the Earth rises and sinks on a regular basis. Physicists have to pay attention to these factors and account for them in their calculations to determine the true distance between the centers of the Earth and Moon.
With the refractors in place and the Earth’s movements known, astrophysicists can estimate the distance between the Earth and the moon within two centimeters!!
(NOTE: I’ve been trying to think of something more Earthly to compare this to, but there is no comparison. The Moon is 238,900 miles away, which means astrophysicists can estimate the Moon’s distance within 5 billionths of a percent. Imagine that!)
Why Go Through All This Trouble?
Isaac Newton established the first theory of gravity – he suggest that all massive objects attract other massive objects, and that the strength of this attraction depends on 1) the distance between the objects and 2) how massive they are, relative to each other. For instance, everything falls downward on Earth because our lives take place on or near the surface of the Earth and really far away from other massive objects such as the Moon, Mars, and Venus. These other bodies are pulling on us (the Moon, for instance, pulls water up to create tides), but overall, the Earth, by far, has the greatest pull.
Newton’s theory of universal gravitation suggest that gravity always pulls things in a straight line. But Einstein’s theory of general relativity suggested otherwise. He suggested that gravity bends space: that gravity curves around massive objects, and that space and time actually grow and shrink in the presence of gravity. We used to think that a meter is always a meter long, and that a second is always a second long, but Einstein showed that these constants that we take for granted are not that constant when gravity is involved.
This phenomenon isn’t really noticeable in our everyday lives, where everything happens on a relatively small scale in both space and time. But it is easier to see these effects on objects that are farther from the Earth and move at very high speeds relative to us, such as the Moon. This is why NASA went to such elaborate lengths to study Einstein’s theory – they couldn’t see its effects by studying anything here on Earth.
The moon isn’t the only nearby object in space that is subject to the laws of general relativity. Artificial satellites are, as well. In fact, engineers need to understand relativity to be able to measure our exact position on Earth with GPS. This means companies like Google depend on NASA’s lunar ranging experiments to help you figure out where you are.
Global positioning systems and relativity
GPS consists of a network of 24 satellites that all talk to each other. They depend on very precise timing and knowledge of their own locations, relative to you, to accurately estimate your location. The satellites are arranged in orbits such that at least four satellites are visible from any point on Earth at any time, and each satellite has a clock on it that measures time to the nanosecond (one billionth of a second).
Now, imagine you’re driving through an unfamiliar neighborhood, trying to find your way to the highway to bring you home, and your GPS is on, faithfully tracking your location. The GPS on your phone is talking to those four satellites in the sky, determining where they are and how long it takes to receive a signal from each one. From this information, your GPS device is able to basically triangulate your location.
Since your car is talking to satellites so far up in the sky, though, the effects of special relativity kick in and affect your GPS’s ability to accurately measure time. Special relativity suggests that, from the perspective of an observer, like your phone, for objects moving at high speeds (such as a GPS satellite), time slows down. As a result, the clocks in GPS satellites, which are orbiting at about 8,700 miles per hour, actually fall behind the clock in your GPS device by 7 microseconds (seven millionths of a second) per day.
On top of this, these satellites’ distance from Earth also affects how they experience time. Massive objects like Earth stretch space-time, such that time takes longer to happen near the Earth. But as you get farther from Earth, the planet’s influence on space-time diminishes, and time speeds up. Consequently, GPS satellites, which are traveling about 12,500 miles above the ground, get ahead of Earth time by 45 microseconds per day.
Adding all of that up, the clocks on GPS satellites move faster than the clock in your phone in your car by about 38 microseconds per day.
If engineers did not take these relativistic effects into account when they designed your GPS, it would give you a false reading after only two minutes, and these errors would accumulate to about six miles a day. That’s not very useful if you’re trying to find your way home!
What does this have to do with shooting lasers at the moon?
As the GPS example demonstrates, physicists need to understand exactly how gravity works to develop new technologies that are affected by general relativity. The moon’s orbit is a great system for testing theories of gravity against. If we understand the moon’s orbit, we can test whether a theory of gravity, such as Einstein’s theory of general relativity, fits what is actually happening in space. By fine-tuning Einstein’s theories, we could develop more and more precise GPS devices – maybe even ones that can find your location down to the inch! Think about that the next time you’re using the “find my iPhone” app and it tells you your phone is…in your house somewhere.