Stargazing: Do the stars really move?


As I write these words, I await the launch by SpaceX of Falcon Heavy, the most powerful rocket on the planet. Its 27 rocket motors will be able to launch 141,000 pounds into orbit.

Falcon Heavy restores our ability to return at last to the moon. It is the first grand step to an eventual human landing on the planet Mars.

It will become the most effective anti-gravity device on planet Earth, and it is, after all, the tyranny of gravity that prevents us from moving beyond our planet to distant worlds.

In that regard, gravity is both a curse and a blessing. It holds us fast, but it also traps our life-giving atmosphere as a thin membrane around the planet.

Our planet is moving at enormous speed around the sun. Its motion ought to take it away from the sun and outward into space, but gravity holds us in a stable orbit around our life-giving star.

In fact, it holds our star together in perfect balance with the explosive forces that define a star and make that star want to fly apart. And that grand balance between gravity and explosion has happened at least 300 billion times in our Milky Way galaxy alone and as many times in each of the perhaps four trillion galaxies that populate our universe.

Gravity holds the universe together, but it has also been one of science’s greatest mysteries – and remains so.

We know what it is, and we can predict its effects. However, we still don’t know exactly how it spreads throughout the universe.

We experience, albeit dimly, the gravitational force generated by the collision of black holes at extreme distances. How did the gravity get from there to here?

The study of gravity begins with the writings of Aristotle, the great Greek philosopher of the fourth century BCE. He believed that gravity had something to do with how much stuff was present in an object, i.e., its mass. We commonly measure mass on Earth by the object’s weight. Aristotle believed that falling objects accelerated toward the earth faster if they were heavier. If you drop a one- and two-pound ball, the two-pound ball should reach the ground earlier.

That notion persisted for centuries because Aristotle failed to consider the effects of the medium – the resistance from the air – through which the objects were falling.

The fundamental change in that idea originated with Galileo, who actually bothered to check the principle in a controlled way. He carefully measured the fall of balls of various weights rolling down inclines. (No, he probably didn’t drop balls from the Leaning Tower of Pisa as the apocryphal story goes.)

The results were astonishing. Balls of various weights rolling down the same incline reached the ground at the same time. Heavier objects do not have a higher gravitational acceleration.

However, mass clearly has something to do with generating gravity in the first place, and it required an enormous leap of intellect to discover the relationship.

Isaac Newton wanted to know what force kept the planets in their orbit despite their enormous speed as they careened through space. The answer was gravity. Newton calculated the exact relationship between mass and velocity, and he showed the way, almost exactly, that the force of gravity diminishes with the decrease in the mass of the two objects attracting each other and their distance from each other.

Astronomers still regularly use Newton’s equations to calculate the orbits of the planets because they produce almost perfect results.

But the results aren’t absolutely perfect. The measured positions of the planets are a tiny bit off from the Newtonian predictions.

Perhaps gravity is not a force at all. Perhaps it is a quality of the universe generated by the mass, the “weight,” of matter.

That quality is present in the fabric of the universe itself, which astronomers have come to call spacetime.

Spacetime is everything and everywhere. It is the “space” between stars, planets and galaxies. It is the stars, planets, and galaxies themselves, which are just portions of spacetime denser with matter than the emptier parts of space.

That second great revolution came with Albert Einstein and his General Theory of Relativity. He suggested that gravity is curvature in the fabric of spacetime. The more massive an object is, the more curvature it generates and the more we experience that portion of spacetime as having more gravity.

Ultimately, an object can become so massive that space curves in on itself and nothing can escape its gravity. Such is the quality of a massive dead star we call a black hole.

Einstein’s theory has been verified by direct observations many times over the years. It produces very accurate predictions of the positions of planets, for example.

One implication of curved spacetime is that an object does not have to be burdened with mass to be affected by gravity. Light is captured by the curved space around very massive objects.

That capture leads to the oddest of astronomical images: an image of a galaxy that is directly behind a gravitationally powerful source like a black hole or another galaxy. We see the more distant object reproduced as a ring, arc of light, or even as multiple images of the object.

Einstein doesn’t explain the exact mechanism that mass uses to create curved spacetime, and physicists still struggle for an answer. That conundrum involves subatomic particles called gravitons and esoteric ideas called string theory, neither of which has yet to provide a definitive answer

As I write the last of these lines, I am looking at a video of the perfect liftoff of Falcon Heavy and a simultaneous controlled landing of its two side boosters in grand defiance of the tyranny of gravity. Take that, curved spacetime!

By Tom Burns


Tom Burns is director of the Perkins Observatory in Delaware.

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