This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
On the razor edge between reality and fiction, there is a realm in which worlds we have never seen could, indeed might, exist. I first became interested in these worlds in 1990, when my older son, James, was 5 and inundating my wife and me with "What if?" questions (for more details, see here). As an intellectual exercise, creating those worlds has been especially satisfying for me, second only to making actual scientific discoveries. Both require that I apply and expand the known models of nature, without violating them. The consequences of creating "what if?" worlds, as you can see in my books What if the Earth had Two Moons? and What if the Moon Didn’t Exist? are very different planets from Earth as we know it today. What was equally satisfying for me was that creating alternative worlds also gave fascinating insights into the properties of Earth as it is today.
For example, consider what we can learn about Earth by asking what our planet would be like if the Moon had never formed. To understand the consequences of changing the cosmos, I began by considering Earth as it has actually been transformed by the presence of the Moon over the past 4.5 billion years. As Sir George Darwin deduced back in 1897, the Moon is spiraling away from Earth under the influence of the ocean tides that the Moon creates here. The fact that the Moon is moving away from us has been proven over the decades since 1969 when Apollo 11 astronauts left a reflector array on the Moon. By sending laser beams from Earth to the Moon and timing their round trips, astronomers have determined that the Moon is spiraling away from Earth at a rate of about 3.82 cm (1.5 in) per year. This means, of course, that the Earth and Moon were closer together in the past.
The recession of the Moon from Earth implies that the Moon is gaining energy, just as a ball can rise in the air only if it is given energy by being thrown upward. That energy given to the Moon comes from Earth, namely from our planet’s rotation – we are spinning more slowly today than we were when Earth first formed. Earth loses energy because the Moon creates tides on our rapidly spinning planet. These tides are not in a straight line between the centers of the two worlds. Rather, the high tide closest to the Moon is pulled ahead of the Moon by Earth’s rapid rotation. In turn, the water in that high tide gravitationally pulls the Moon in the direction of the Moon’s orbit, giving the Moon energy (like you would give a ball hanging on a string energy if you held the string and spun around). As a result the Moon (and the ball on the string) spiral outward. Earth, in turn, slows down because of friction between the water in the high tides and the body of Earth.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
How fast Earth was originally spinning depends on how close Earth and Moon were when the Moon formed, a distance that is not yet known. The first "aha" moment for me in going through this argument came when I took reasonable assumptions for the Moon’s original distance and, using the conservation of angular momentum, calculated plausible spin rates for the young Earth. The length of the day was originally between 5 and 10 hours. Work done by Jihad Touma and Jack Wisdom (Figure 1, in a paper published after What if the Moon Didn’t Exist?) suggest a 5-hour day.
As an example of how much care one must use in creating alternative, scientifically plausible, worlds, let’s continue on with the scenario of there never having been a Moon. If it had never existed, then the tidal force from the Moon that slowed our Earth would not have occurred and Earth’s rotation rate would not have slowed due to a lunar tidal effect. Are there any other sources of tides on Earth that would have slowed it down? The answer is yes: the Sun and the planets, especially Jupiter, create tides here. But would they alone slow Earth’s rotation significantly? The tides from all the bodies except the Sun are negligible for our purposes. The Sun creates about 1/3 of the tides on Earth today, so it might seem that Earth would have slowed down 1/3 as much as it did in real life with the Moon in play. This is not correct. The vast majority of the rotational energy lost by Earth came when the Moon was young and very close to Earth. This realization led to the second "aha" moment about Earth as it is that I felt while creating a Moonless Earth. If the Moon was originally 10 times closer than it is today, the equations reveal that tides on Earth back then were 1000 times higher than they are today! (Tides vary inversely as the third power of the distance between the worlds.) Therefore, when they were young, the Moon would have been spiraling away and Earth would have been slowing down much faster than they are today. Taking this into account, tides created by the Sun on Earth without the Moon over the past 4.6 billion years would cause Earth to now be rotating with somewhere between a 7 and 12 hour day.
Two takeaway insights for me about our world as it really is, gotten from asking "What if the Moon Didn’t Exist?", are that Earth was initially rotating somewhere between 2½ to 4 times faster than it is today and that the tides were once between a few hundred and a thousand times higher than they are now. These facts then provide insight into the evolution of life on our Earth, as I discussed in What if the Moon Didn’t Exist.
As noted above, every new world I have created provides deeper understanding of our present world: I gained insights into the relative simplicity of our present tidal cycle from What if the Earth had Two Moons? and into the complexity of biological clocks from What if the Earth were a Moon? (both in the book What if the Earth had Two Moons?). "What if?" gives us important insights into "What is."