All posts tagged gravity


Moon over Seattle

Moon over Seattle

Humans have long known of the link between the Moon and the tides. We’re learning that it’s not only the ocean that’s affected.

To picture the effect of the Moon’s gravity on the planet Earth, draw two lines starting at the very center of the Moon which gradually spread away from each other until they just graze the Earth at its opposite edges. That makes a long, narrow “vee” with the Moon at its point and the Earth held between its open ends.

Earth is pulled into, or falls into the vee due to gravitational attraction, so its sides get squeezed in the narrowing slot. This results in the Earth bulging slightly toward the Moon, and on the opposite side, away from it. Imagine gently squeezing a balloon between your hands held in a vee-shape. The sides will flatten and the front and back will bulge.

The same thing happens to the Earth’s ocean, with the bulges being experienced as tides on opposite sides of the planet. There is a similar effect in the atmosphere, though it’s not as noticeable as the ocean tides. As Earth rotates, the tides remain fixed, pointing at and away from the Moon.

Low Tide

Low Tide

Even less noticeable is the same pair of bulges in the Earth’s crust. It’s easy to imagine bulges in the atmosphere or ocean, but the ground beneath our feet seems too substantial to have tides. But if you put the right instruments in place and measure the effects at one spot on Earth, you will detect a twice daily rising and falling as you spin through the opposing bulges.

The bulges aren’t exactly the same. Because it’s a vee-shaped squeeze, the bulge facing the Moon is slightly pointier and the one facing away is slightly broader. This has been observed in ocean tides, which are slightly higher on the Moon side.

It makes for quite a picture to imagine our planet being kneaded by gravity, but that’s not all of it. It seems that the continuous westward movement of the bulges is resulting in a tiny movement of the Earth’s crust itself. Floating on the sticky, molten magma below it, the crust is free to be pulled ever so slightly back against the rotation of the planet, resulting in a very slow general drift westward. The rate might be in the centimeters per century range.

In addition to larger effects like spreading sea floors and upwelling magma plumes, it looks as if gravitational drag is contributing a small but steady force in the constant evolution of the surface of our planet.


Dark Flow

Dark Flow

The known universe is about 27.4 billion light years across. It’s thought that the universe is bigger than that, possibly a lot bigger, but we can’t see farther than 13.7 billion light years in any direction. Our knowable universe is limited by the speed of light and how far it could travel since the Big Bang, 13.7 billion years ago. Since light travels one light year per year, that limits us to a radius of 13.7 billion light years.

That’s pretty big. If it’s all we can see with no way to ever see beyond, who’s to say there is anything else? If we’re to be limited to this, admittedly enormous, bubble of space and time, is there any point in wondering if there’s anything else out there? Of course there is. Even if it’s impossibly out of reach we will reach for it. We humans, as soon as we’re shown our boundaries, will try to see beyond them.

Astronomers think they might have done just that. How do they infer that there is more to the universe than we can see? They do it by detecting its effect on what we can see. Two ways of doing that seem to show positive results. One is the motion of large swaths of galaxies and the other is a peculiar imbalance in the symmetry of space.

The flow of galaxies is called Dark Flow by some, in keeping with other great unknowns such as Dark Matter and Dark Energy. It was found in a survey of galaxy clusters, huge gravitationally bound congregations of hundreds or thousands of galaxies, in an area about two billion light years across. They all appear to be moving in the same direction at about a thousand kilometers per second. The implication of that much matter moving at high speed toward the same point is that there isn’t enough matter in the observable universe to account for the gravitational attraction required. It suggests huge concentrations of matter beyond the known universe drawing our galaxies away.

The peculiar asymmetry, the second effect, is found in the cosmic microwave background(CMB) radiation that fills space. The CMB is the cold, fading glow left over from the extreme heat of the Big Bang. It’s observed more or less evenly spread everywhere, with small fluctuations. In theory even the fluctuations should be evenly distributed, but they’re not. They’re about ten percent more numerous on one side of the sky than the other. This suggests that the observable universe’s structure is affected, distorted or sloped in some way, by other structures much larger than everything we can see.

The “whole” universe, of which our observable bubble is just a small part, would have to be very big. So much bigger that there wouldn’t be room in this article to write out how much bigger. It’s no wonder it affects the part we can observe.



Albert_Einstein_portraitAlbert Einstein didn’t like it. When he worked out his General Theory of Relativity in 1916 he realized that it made for an unstable universe. The General Theory is all about gravity and it has been one of the most successful theories of all time. It has stood up to every observation and experiment that scientists have thrown at it. But Einstein didn’t like the implication that the universe should collapse under its own gravity. He grew up in a world that took it for granted that the universe was infinite and eternal. That it was complete, done, cooked to perfection. When his theory threatened to upset that, he decided that there must be another force working to balance gravity. He stuck it in and called it the Cosmological Constant. Later he was to call it his “biggest blunder.” When astronomer Edwin Hubble announced in 1929 that galaxies in all directions appear to be moving apart, Einstein realized that the universe is expanding. That was why it wasn’t collapsed long ago by gravity. He abandoned the Cosmological Constant, disgusted with himself.

Photo credit - Bell Labs

Photo credit – Bell Labs

In his lifetime he saw the birth and growth of what would become the Big Bang theory. If everything was moving apart then it was logical that it was closer together in the past. Go back far enough and it was all in one place. A little less than 14 billion years ago the universe as we know it burst into existence, according to the Big Bang Theory, and it has been expanding ever since. In the 1960s, a very sensitive microwave receiver used in Bell’s communication network was plagued by persistent background noise. After weeks of trying to track it down, including the eviction of a couple of resident pigeons, the technicians, Penzias and Wilson, realized it was real microwaves coming from space. It was the cool afterglow of the Big Bang explosion.

In the decades since, observations have been refining the data and developing the theory. The Hubble Space Telescope, other observatories and legions of cosmologists have looked deep into the expanding universe and have given us the clearest picture ever. One surprising datum they’ve come up with is that, rather than the expansion slowing down under universal gravitation, it is actually speeding up. Some force seems to exist in the fabric of space which is accelerating its expansion. Some people are calling it the Cosmological Constant.

It wasn’t quite as big a blunder as Einstein thought.