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Sci Fi Tides

Does it bug you when science fiction movies get the science part all wrong? I’m not one of those nit pickers who can’t suspend my disbelief long enough to just relax and enjoy the movie. I understand that creating a compelling movie is paramount and certain liberties can be taken with scientific accuracy. Take faster than light propulsion for example. This is one of those future technologies that is so overwhelmingly unlikely that we may as well say it’s impossible. There’s just no getting around the fact that it takes an ever increasing amount of energy to realize the same percentage increase in speed. Ultimately, infinite energy would be required to hit that sweet spot called the speed of light. Still, many science fiction plots require our heroes to travel vast distances within time frames that make sense to the plot. No problem. Just come up with a cool sounding futuristic propulsion and run with it.

Don’t get me wrong, the stupid mistakes do get to me. Take Independence Day for example. Remember that one? Jeff Goldblum’s character writes a computer virus to disable the alien mother ship. Give me a break. We can barely make our different human-derived computer operating systems communicate yet he can quickly cook up a batch of code to screw with the computers created by aliens. And you thought PCs and MACs were different from each other?

One of the things I love about science fiction movies, whether their science is accurate or not, is that they can be a springboard for me to learn more about the science topics they touch upon. I’m sure I had Star Trek’s warp drive in the back of my mind the very first time I investigated the feasibility of faster than light travel.

Independence Day similarly helped me delve more deeply into some aspects of real science because they got it wrong. There was a subtle mistake in this movie that I didn’t pick up on the first time I saw it. Remember that scene early in the movie when the Brobdingnagian mother ship with 25% the moon’s mass enters earth orbit and disgorges its 15 mile wide city-buster ships? Cool scene but talk about a massive waste of resources. To wreak havoc, the mother ship needed to do nothing more than simply stay safely in orbit and let tidal forces do all the work. I knew enough about this force that this made sense to me when I read about it in Lawrence Krauss’ fascinating book “Beyond Star Trek : From Alien Invasions to the End of Time”. It seems though that this mistake made by the movie made me appreciate tidal forces more and perhaps caused me to read about it more than I normally would have. I’ve come across some interesting things about it that hopefully you’ll find as interesting as I did.

Tidal forces arise from the differential gravitational pull a body experiences by being at varying distances from a gravitational source at the same time. A Tidal Force is not a force unto itself but is a secondary effect of the gravitational force. In this it is similar to the strong nuclear force binding protons and neutrons together which is really just the residue of the Color force binding together the even more fundamental quarks.

The classic example of tidal forces is of course the oceanic tides that coast dwellers experience every day. How this effect is produced however is not as straightforward as simply a differential gravitational pull. The ramifications of this phenomenon are fascinating and have played a role in the earth-moon system since its birth and will continue for many billions of years after we’re dust.

Have you ever wondered why there are two high tides a day? I always wondered about this and was never fully satisfied with any of the explanations I read. What was missing from my understanding was the contribution of the centrifugal force to this phenomenon. The earth is not only spinning on its axis; it’s also revolving around the center of mass (barycenter) of the earth-moon system (about 1100 miles below earth’s surface). This revolution produces a centrifugal (pseudo) force that is identical for all points on the earth.

The moon’s gravitational pull on that part of earth closest to it overwhelms the centrifugal force on that side. On the other side of the earth, the moon’s gravity is weaker because it is farther away. On that side the centrifugal force dominates. You might think that these two forces on opposite sides of the earth lift the water and therefore cause the tides. You’d be wrong. Earth’s gravity is far too strong to let these forces directly lift the water against its mighty pull. So what causes the tides then? It’s the horizontal motion of the water caused by the moons attraction on one side of the earth and the centrifugal force on the other. Earth’s gravity has no real say about the water’s horizontal motion, only it up-down motion away and towards the center of the planet. This heaping effect piles up the water on each side of the earth causing our twice daily high tides.

Moon tides do not just affect water. The same process actually causes tides on land as well. The land is actually uplifted by many centimeters twice a day just like the oceans. The same is true for the atmosphere as well even though it is even more subtle.

The most dramatic demonstration of tides can be found near black holes. The gravitational pull on the near-side of someone approaching a black hole is so much stronger than the far side that some astronomers call the effect it would have on a human body as “spaghettification”.

The tides caused by the moon may seem powerful now but they pale in comparison to their former strength. Soon after the moon’s formation billions of years ago when the moon was a fraction of its current distance from earth (perhaps only 14,000 miles away and 5 times larger in the sky) tides were 10,000 foot mountainous swells of water that would scour the landscape hundreds of miles inland bringing all the collected detritus back into the ocean. It is this mixture, often referred to as the primordial soup, that later spawned the first life that earth ever saw.

Tidal forces also work ceaselessly to increase the distance between the earth and moon and to slow the earth until, at some point billions of years in the future, one unlucky side of the earth will never face the moon again. This is caused by those oceanic tidal bulges mentioned above. The bulge closest to the moon is massive enough to gravitationally affect the moon. Due to the spinning earth, the bulge is not directly below the moon but slightly ahead of it. The moon is attracted by this gravitational force which increases its orbital speed, called tidal acceleration. The old orbit cannot support this more energetic moon, only a bigger orbit can do this. The moon therefore is moving about an inch further away every year into a more distant and energetic orbit. Conversely, the earth is slowed down by this effect, called tidal braking, which has caused our day to increase from less than 10 hours to the currently in vogue 24. At some point in the future, the moon will reach its maximal distance from us and our rotation will decrease until we become forever tidally locked to the moon as it has been to us for billions of years.

So thanks to Roland Emmerich and his Independence Day bad science about tides.  I guess that’s why science geeks like me enjoy the science fiction genre so much – whether they get the science right or painfully wrong it gives you something to think about.

9 comments to Sci Fi Tides

  • fredeliot2

    For me, an easier explanation is this. Imagine a tubular satellite in orbit with one end always pointing towards the earth. In the tube there are 2 small masses, one above the center and one below the center. The one below the center is traveling at an angular velocity that is too low to maintain its orbital position and will start to drop towards the earth. The one above the center is at too high an angular velocity and will rise.

  • Jon Blumenfeld

    Bad sci fi science:

    The original Battlestar Galactica has to be the worst offender, in just about every possible category. The fleet runs out of fuel and ‘grinds to a halt’ in space. Huh? Vipers travel interplanetary and even interstellar distance using what appears to be plain old rocket fuel. Time units mean nothing. How long is a ‘centon?’

    Oh, and Star Trek ‘Enterprise’ had at least one major howler I can think of. A shuttlecraft lands on an asteroid not much larger than itself, and falls into a hole. The Enterprise has a hell of a time getting it out because it’s so heavy. WHERE DID ALL THAT GRAVITY COME FROM? What’s that asteroid made of, anyway, pure Neutronium?

    Don’t even get me started on the original ‘Dune’ movie.

  • DLC

    Longstanding dumb science-fiction idea:
    Artificial gravity. We know little about gravity even today, and yet in the future a space ship will have internal gravity at the flip of a switch. Or maybe they’ll all wear Heavy Boots.

  • Speaking of ‘Enterprise’, I was watching an episode just the other night where they were going down to a planet which was “about the size of Earth’s moon”. So obviously they were jumping around like nobody’s business…. Or then again, possibly not. That show seems to have a fraught relationship with gravity.
    I’m not too picky about liberties taken with science in movies, but it really annoys me when they could avoid making stupid mistakes simply by keeping their mouths shut, and it wouldn’t affect the story in the least.

  • Matthias Samland

    @DLC: Artificial gravity as it is portrayed in sci-fi movies is indeed rather unlikely. But it would be possible to create “gravity” by rotation. As Einstein pointed out with his equivalence principle: “the states of accelerated motion and being at rest in a gravitational field (for example when standing on the surface of the Earth) are physically identical”.
    Thus you could use the centripetal force to create what would appear to us to be gravity.

  • Clintsc9

    What bugs me most with Science Fiction stories is not so much the impossible future science, but when the writers ignore current technologies. A case in point is one of the Star Trek movies where Data and Picard have to get through a security door to be able to be transported back to the Enterprise. Data runs many combinations through the lock in super-quick time and they just make it. Watching, I wasn’t the least worried about the super calculations or the speed of his fingers or artifical gravity still working on a dying ship or even the teleportation. I was annoyed that the Borg didn’t use the “three misses and you’re out” simple technology we readily have on nearly all security doors today.

  • Thrawn

    One show which was fairly accurate is the anime Planetes. The only artificial gravity is from huge centrifuges, ships only use thrusters to change their velocity, and there’s even one episode where the protagonists are able to survive because a robotic arm they used was so old the computer hacking into it couldn’t cut off the manual control. Comparisons to 2001 could definitly be made (especially since a major plot arc is the construction of the first manned vessel to reach Jupiter)

  • Tom Woods

    I once read that a good movie maker understands that an audience is willing to believe the impossible, but not the improbable. We can easily suspend disbelief about FTL travel because it is simply impossible. Jeff Goldblum hacking into an alien computer system? Not just impossible but very highly improbable. Will Smith stealing a helicopter, flying to Orange County CA from Nevada, somehow locating his girlfriend amid all of the rubble, and flying back to Nevada, all without refueling, also seems highly improbable. Ultimately, these are much more distracting than for me than the violations of natural laws.

  • […] wind, magnetic storms, and even space dust. The biggest contributor though is something called tidal breaking. As the moon and earth’s tidal bulge pull on each other, our rotational friction is […]

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