Time and tide

I don't know if you've had the experience of running into a relatively straightforward concept that your brain just doesn't seem to be able to wrap itself around.

One such idea for me is the explanation for tides.  I've gone through it over and over, starting in high school physics, and I keep having to go back and revisit it because I think I've got it and then my brain goes, "...wait, what?" and I have to look it up again.

The sticking point has always been why there are two high tides on opposite sides of the Earth.  I get that the water on the side of the Earth facing the Moon experiences the Moon's extra gravitational attraction and is pulled away from the Earth's surface, creating a bulge.  But why is there a bulge on the side facing away from the Moon?

Now that I'm 64 and have gone over it approximately 482 times, I think I've finally got it.  Which is more than I can say for Bill O'Reilly:

So, let's see if I can prove Mr. O'Reilly wrong.

Consider three points on the Earth: A (on the surface, facing the Moon), B (at the center of the Earth), and C (on the surface, opposite the Moon).  Then ask yourself what the difference is in the pull of the Moon on those three points.

Isaac Newton showed that the force of gravity is proportional to two things -- the masses of the objects involved, and the inverse square of the distance between them.  The second part is what's important here.  Because A, B, and C are all different distances from the Moon, they experience a difference in the gravitational attraction they experience.  A is pulled hardest and C the least, with B in the middle.

This means that the Earth is stretched.  Everything experiences these tidal forces, but water, which is freer to move, responds far more than land does.  At point A, the water is pulled toward the Moon, and experiences a high tide.  (That's the obvious part.)  The less obvious part is that because points B and C are subject to a difference in the gravitational attraction, the net effect is to pull them apart -- so from our perspective on the Earth's surface, the water at C pulls away and upward, so there's a high tide there, as well.

There's practically no limit to how big these forces can get.  On the Earth, they're fairly small, although sometimes phenomena like a seiche (a standing wave in a partially-enclosed body of water) can amplify the effect and create situations like what happens in the Bay of Fundy, Nova Scotia, where the difference in the water level between high and low tide can be as much as sixteen meters.

But out in space, you can find systems where the masses and distances combine to create tidal forces that are, to put it in scientific terms, abso-fucking-lutely enormous.  This, in fact, is why the whole subject comes up today; the discovery of a binary system in the Large Magellanic Cloud made up of a supergiant with a mass thirty-five times that of the Sun, and a smaller (but still giant) companion ten times the mass of the Sun.  They're close enough that they orbit their common center of gravity about once a month.  And the combination of the huge masses and close proximity creates tidal bulges about three million kilometers tall.

That's over three times the diameter of the Sun.

You think the people living along the Bay of Fundy have it bad.

Artist's conception of the system in the Large Magellanic Cloud [Illustration by Melissa Weiss of NASA/Chandra X-Ray Observatory/Center for Astrophysics]

And that's not even as extreme as tidal forces can get.  If you were unfortunate enough to fall feet-first into a black hole, you would undergo what physicists call -- I'm not making this up -- spaghettification.  The tidal forces are so huge that they're even significant across a small distance like that between your head and your feet, so you'd be stretched along your vertical axis and compressed along your horizontal one.  Put more bluntly, you'd be squished like a tube of toothpaste, ultimately comprising the same volume as before but a much greater length.

It would not be pleasant.

Be that as it may, I think I've finally got the explanation for tides locked down.  We'll see how long it lasts.

At least I'm pretty sure I'm still ahead of Bill O'Reilly.

****************************************

Time and tide

I don't know if you've had the experience of running into a relatively straightforward concept that your brain just doesn't seem to be able to wrap itself around.

One such idea for me is the explanation for tides.  I've gone through it over and over, starting in high school physics, and I keep having to go back and revisit it because I think I've got it and then my brain goes, "...wait, what?" and I have to look it up again.

The sticking point has always been why there are two high tides on opposite sides of the Earth.  I get that the water on the side of the Earth facing the Moon experiences the Moon's extra gravitational attraction and is pulled away from the Earth's surface, creating a bulge.  But why is there a bulge on the side facing away from the Moon?

Now that I'm 62 and have gone over it approximately 482 times, I think I've finally got it.  Which is more than I can say for Bill O'Reilly:

So, let's see if I can prove Mr. O'Reilly wrong.

Consider three points on the Earth: A (on the surface, facing the Moon), B (at the center of the Earth), and C (on the surface, opposite the Moon).  Then ask yourself what the difference is in the pull of the Moon on those three points.

Isaac Newton showed that the force of gravity is proportional to two things -- the masses of the objects involved, and the inverse square of the distance between them.  The second part is what's important here.  Because A, B, and C are all different distances from the Moon, they experience a difference in the gravitational attraction they experience.  A is pulled hardest and C the least, with B in the middle.

This means that the Earth is stretched.  Everything experiences these tidal forces, but water, which is freer to move, responds far more than land does.  At point A, the water is pulled toward the Moon, and experiences a high tide.  (That's the obvious part.)  The less obvious part is that because points B and C are subject to a difference in the gravitational attraction, the net effect is to pull them apart -- so from our perspective on the Earth's surface, the water at C pulls away and upward, so there's a high tide there, as well.

There's practically no limit to how big these forces can get.  On the Earth, they're fairly small, although sometimes phenomena like a seiche (a standing wave in a partially-enclosed body of water) can amplify the effect and create situations like what happens in the Bay of Fundy, Nova Scotia, where the difference in the water level between high and low tide can be as much as sixteen meters. 

But out in space, you can find systems where the masses and distances combine to create tidal forces that are, to put it in scientific terms, abso-freakin-lutely enormous.  This, in fact, is why the whole subject comes up today; the discovery of a binary system in the Large Magellanic Cloud made up of a supergiant with a mass thirty-five times that of the Sun, and a smaller (but still giant) companion ten times the mass of the Sun.  They're close enough that they orbit their common center of gravity about once a month.  And the combination of the huge masses and close proximity creates tidal bulges about three million kilometers tall.

That's over three times the diameter of the Sun.

You think the people living along the Bay of Fundy have it bad.

Artist's conception of the system in the Large Magellanic Cloud [Illustration by Melissa Weiss of NASA/Chandra X-Ray Observatory/Center for Astrophysics]

And that's not even as extreme as tidal forces can get.  If you were unfortunate enough to fall feet-first into a black hole, you would undergo what physicists call -- I'm not making this up -- spaghettification.  The tidal forces are so huge that they're even significant across a small distance like that between your head and your feet, so you'd be stretched along your vertical axis and compressed along your horizontal one.  Put more bluntly, you'd be squished like a tube of toothpaste, ultimately comprising the same volume as before but a much greater length.

It would not be pleasant.

Be that as it may, I think I've finally got the explanation for tides locked down.  We'll see how long it lasts. 

At least I'm pretty sure I'm still ahead of Bill O'Reilly.

****************************************

The turning of the tides

It's tempting to think that conditions on Earth have always been like they are now.

On one level, we know they weren't.  When people picture the time of the dinosaurs, it usually comes along with images of swamps and ferns and rain forests.  (And volcanoes.  Most of the kids' books about dinosaurs illustrate them as living near erupting volcanoes, which seems like a poor choice of habitat.)

But the basics -- the air, water, soil, and so on -- we picture as static.  It's been the basis of hundreds of science fiction stories; people go back into the distant past, and although there are (depending on when exactly they went to) often giant animals who want to eat you, you have no problem breathing or finding food.

I had a neat hole punched in that perception last week when I read Peter D. Ward's book Out of Thin Air.

 

Ward is a paleontologist at the University of Washington, and his contention -- which is well-argued and supported by a wealth of evidence -- is that the oxygen content of the atmosphere has varied.  A lot.  It's at about 21% at sea level now, but hit a staggering low of 13% immediately after the Permian-Triassic extinction, comparable on today's Earth to being at an altitude of 12,500 feet (think the High Andes).  Humans time-traveling back then would have a seriously difficult time breathing, and life was probably confined to areas that were near sea level -- and those areas would be completely isolated from each other by higher ground in between where there was not enough oxygen to survive.

There were times when it was much higher, too.  Ward says in the late Carboniferous Era, the oxygen content suddenly spiked to around 30%, which explains why coal formation stopped; at 30% oxygen, dead plant matter will combust with little encouragement, resulting in little left behind to form coal seams.

If you'd like to find out more, I highly recommend Ward's book, which is not only an argument for the fluctuating-atmosphere model, but is a good overview of the major events in Earth's history.

Parasaurolophus skeleton [image courtesy of the Wikimedia Commons]

I had another blow delivered to the static-Earth perception from a study that was published last week in Geophysical Research Letters, called "Is There a Tectonically Driven Super‐Tidal Cycle?", by Mattias Green, J. L. Molloy, H. S. Davies, and J. C. Duarte, which considered the possibility that even the tides haven't always been as they are today.

What their study did was to look at a model of the dispersal of tidal energy, and they found that when all the continents were joined into a single land mass (Pangaea), which last happened at the end of the Permian Era a little over 250 million years ago, it represented a tidal energy minimum.  This meant that the tides were smaller than today, and that the majority of the (single) ocean was effectively a stagnant pool of water, with little vertical mixing of nutrients.  Stagnant, low-nutrient, low-oxygen water generally has little biodiversity -- a few species that can tolerate such conditions do exceptionally well, but the rest die out.  So this could be yet another reason that the cataclysmic Permian-Triassic Extinction happened, in which (by some estimates) 90% of the species on Earth became extinct.

What the Green et al. study suggests is that we're near a tidal maximum.  As the press release about the study put it:

In the new study, scientists simulated the movement of Earth’s tectonic plates and changes in the resonance of ocean basins over millions of years. 

The new research suggests the Atlantic Ocean is currently resonant, causing the ocean’s tides to approach maximum energy levels.  Over the next 50 million years, tides in the North Atlantic and Pacific oceans will come closer to resonance and grow stronger.  In that time, Asia will split, creating a new ocean basin... 

In 100 million years, the Indian Ocean, Pacific Ocean and a newly formed Pan-Asian Ocean will see higher resonance and stronger tides as well.  Australia will move north to join the lower half of Asia, as all the continents slowly begin to coalesce into a single landmass in the northern hemisphere... 

After 150 million years, tidal energy begins to decline as Earth’s landmasses form the next supercontinent and resonance declines.  In 250 million years, the new supercontinent will have formed, bringing in an age of low resonance, leading to low tidal energy and a largely quiet sea, according to the new research.

It's a little humbling to think about, isn't it?  The processes that shape the continents, drive the tides, control the chemistry of the atmosphere, will keep chugging along long after we're a paleontological footnote in the textbooks of our far distant descendants.  It's not that what we're doing now isn't critical; in the short term, the out-of-control fossil fuel burning is doing things to our atmosphere that will certainly cause us grievous harm, not to mention the short-sighted pollution of the very resources we depend on.

But if we do succeed in wiping ourselves out, which lately has seemed increasingly likely, the processes that govern the Earth will keep on going without us.  So will natural selection; the survivors of the current mass extinction will evolve into other "forms most beautiful and most wonderful," as Darwin put it in The Origin of Species.

Not that this will be much consolation to us, of course.  But I do find it comforting, in a strange way.  However important we think we are, on the scale of the natural world, we're pretty tiny.  Whatever damage we do, eventually the Earth will recover, with or without us.  And the atmospheric, geological, and tidal ups and downs will continue -- world without end, amen.