Whether it’s a space walk or conducting experiments with bubbles of water floating within the confines of the station, we assume astronauts are not subject to gravity in the same way as we are, and yet nothing could be further from the truth.
Gravity is all the more remarkable for the fact that it seems pretty straight forward and yet we don’t entirely understand it.
Einstein thought of gravity as a consequence of mass distorting space-time. Modern thinking describes it using quantum force/particles called gravitons which are, at this point, undiscovered. Picture a bunch of springs pulling things together and you get a simple illustration of how gravitons are thought to work.
I’ve had a great yarn with a physicist and amateur astronomer in Garland, Texas on this subject. I’m a little old school, seeing the influence of mass on both space and time as evidence for gravity being a consequence rather than a force, but he’s got some persuasive arguments for the unification of relativity and quantum mechanics that would treat gravity as a quantum force like the nuclear forces. Certainly, the discovery of the Higgs Boson tends toward this conclusion. If you ever happen to be driving through Garland, Texas, stop off at the Firewheel Coffee house and enjoy the debate over a cup of the best coffee this side of Andromeda.
For some, the notion of an equation “for everything,” might seem a little grandiose and perhaps even arrogant and egotistical, after all, what about love, or art and culture? But these are emergent properties that can only exist because of the physical world around us.
Once gravity is incorporated into the Standard Model, or the Standard Model is extended by concepts such as String Theory, we will have an accurate understanding of the universe, something that can be articulated in a concise manner.
Why is this important? The technology we use is possible only because we’re harnessing the natural laws of the universe. The flight of a plane, the workings of an MRI scanner or the computer you’re looking at right now are all extensions of our understanding of these laws. Gaining a theoretical mastery of unified physics will extend our technological reach even further.
Gravity, though, has remained elusive, even though we understand its effects extremely well. Getting back to the opening point, there is no zero gravity. Whenever you see someone floating in space you’re seeing someone moving with a constant velocity that counteracts the attraction of gravity. As this may seem counterintuitive an explanation might help.
Newton was the first scientist to think about the implications of firing cannonballs further and further. He realized Earth was curved and so as his imaginary cannonballs flew further and further he realized the planet would drop away beneath them.
Eventually, if enough force was applied, Newton realized his cannonball would be travelling so fast that, as it fell, Earth would curve away beneath it and it would never hit the ground. This was a startling insight, and is the basis behind our modern satellites.
But notice that the cannonball is not in zero gravity. In fact, the reason it is curving as it orbits Earth is precisely because of gravity. If there were no gravity, the cannonball would sail off in a straight line.
Galileo realized all objects are under the same influence of gravity and so will fall at the same rate, only air resistance slows them down. During the Apollo 15 Moon landing, Commander David Scott dropped a hammer and a feather on the Moon and they both struck the Lunar surface at the same time confirming, Galileo’s assertion. Mythbusters replicated this by dropping a hammer and a feather in a vacuum chamber.
Here’s an experiment you can’t try at home, simply because Earth isn’t big enough, but if you took a laser pointer, pointed it horizontally so the light is going to travel parallel with the ground, and then fired it at the same time you dropped a rock, both the rock and your beam of light would strike the ground at exactly the same time (only millions of miles apart).
Earth would have to be pretty darn big to conduct this experiment, but the point is, gravity affects everything equally, cannonballs, hammers, feathers and even light, only light moves so fast it quickly moves beyond a large gravitational mass like Earth so the effect seems small.
Rather than traveling in straight lines, light zigs and zags through space as it passes through various gravitational fields, bending and twisting in extraordinary ways. If light from a distant galaxy passes close to another galaxy on its way to us, it can bend and twist around the intervening galaxy, allow us to see multiple images of the distant galaxy so far, far away. This effect is known as gravitational lensing.
Light from these distant galaxies distorts like the crazy house of mirrors at the county fair. If the same effect occurred on terrestrial scales, you’d be able to stand in the shadow of one house and still see the house directly behind it, only the second house would be curved and compressed around your home.
In this image, we get several stretched out, distorted images of the same (blue) galaxy hidden behind the (yellow) quasar in the foreground.
Getting back to our intrepid astronauts floating “in zero gravity” it’s important to note that “zero gravity” doesn’t exist anywhere in the universe.
If we could build a tower that reached up to the same height as the space station, some 370km above Earth, would we too float like the astronauts?
The answer is, no.
Standing atop our impossibly tall tower we would feel less gravity than we do at sea level, but we wouldn’t float away. Drop a hammer and it will fall at 8.8 meters per second squared instead of 9.8 meters per second squared down near the ground. And, every hour and a half, we’d see the space station go whizzing by considerably faster than a bullet. Blink and you’ll miss it entirely, as the space station is moving at over 10,000 miles an hour, or roughly three miles a second.
Images from the space station, though, would look serene. Astronauts would float gently beside the station, moving slowly about their tasks. Inside the station, astronauts would drift casually through the cabin, blissfully unaware of their breakneck speed as they whiz by our tower. They’ve been falling for soooo long without hitting anything they’ve become accustom to the weightless experience of constantly falling.
The space station is in free fall around a very massive object, Earth. Ride on a roller-coaster, take a airplane flight through a turbulent storm, or jump on a trampoline and you’ll experience the same sense of weightless free fall, although only for a second or so.
Gravity is pervasive. Right now, your body is experiencing the gravitational pull of Earth, the Sun and the Moon, along with all the planets, stars and galaxies spread throughout the entire universe.
If someone walks into a room or walks up to a large building, though, they’re subtly feeling a greater sense of gravitational attraction from these objects than from any of the planets. Gravity is plastic, it ebbs and swells like waves on the ocean as you approach any mass of any size.
The point of all this is that there is no “zero gravity,” but there’s plenty of fast moving objects in free fall around a large mass, you’re on one right now as Earth falls around the Sun and the Sun falls around the Milky Way, and on it goes.