Gravity Waves

I feel privileged by living in the times I do.  The first Atomic bomb was detonated a few years before I was born.  But I was old enough and in the right place to witness mankind setting foot on the moon for the very first time.  All I had to do to see it was to find a working TV set and watch it.  The whole thing was beamed live to TV sets around the world.  And one of those TV sets happened to reside in the dining room of the house I grew up in.  But then there's the third big event, the detection of gravity waves.  It didn't happen on live TV but it did happen and I was alive and paying attention when it did.

There's a lot to say about this and I am going to skip all the stuff that requires a technical background.  But that still leaves plenty to talk about.  And let me start by talking about gravity.  According to modern scientific thinking there are four basic forces in the universe.  They are the electromagnetic force, the weak force, the strong force, and gravity.  We have known about the existence of gravity for the longest.  But its the force that we know the least about.

Gravity is the force that holds your feet to the ground.  It's everywhere and it is always affecting much of what we do.  And it is unavoidable.  In my previous post I talked about hunter gatherers.  These people had to take into account the effect of gravity when they aimed their spears and arrows.  So they had a good working knowledge of gravity.  A device called a scale has been used to weigh things for millennia.  It's design depends on gravity to operate.  And commerce depends on being able to weigh things accurately.  So commerce has depended on gravity for as long as there has been commerce.

The earliest design for scales involved two pans.  When each pan had the same weight of material in it the "balance beam" was level.  An ancient but newer design substituted a spring for one of the pans.  With a pan scale you balance the weights between two pans.  It's an "all gravity all the time" design.  But you can compare results between a pan scale and a spring scale.  And the fact that you can make a spring scale that agrees with a pan scale means that somehow gravity does something similar to what happens when you distort a spring.  But for a long time people didn't make the connection.  They just used whatever kind of scale was most convenient without thinking any deeper on it.

And people knew the magnitude of the force of gravity at the surface of the earth.  This is a flowery way of saying people knew the weight of things.  And it was Newton that formalized the idea that gravity was just another force.  So in a spring scale the force gravity exerted on the object in the pan is countered by the force exerted by a spring that had been distorted.  Newton understood that there was a difference between the force of gravity and the weight of something.  But he was the first to figure out that gravity was just a force.  On the surface of the earth a specific weight was strictly proportional to a certain amount of force.

He also figured out that the force of gravity was not constant.  Well, actually others had measured the force of gravity in odd places like the tops of mountains and found that it differed slightly from place to place,  But Newton developed his theory of universal gravity.  This allowed him to predict exactly what the force of gravity would be in various places like on the top of a mountain.  And this was true even if the mountain was on a moon of Saturn.  And that's pretty much all anybody knew about gravity.  No one had a clue as to how or why it worked.  They could just describe the rules it followed.  That is, until Einstein came along.  But here I begin a digression.

The second force that people discovered was electromagnetism.  Well, originally there was the electrical force and the magnetic force.  And for a long time people figured they were two different forces.  But they aren't.  If you vary the intensity of an electrical field you will always create a magnetic field.  If you vary the intensity of a magnetic field you will always create an electric field.  So they are both always there.  It's just that if the intensity of the electric field is constant then the corresponding magnetic field has an intensity of zero.  And if the intensity of a magnetic field is constant the corresponding electric field is zero.  James Clerk Maxwell developed a set of equations that unified electricity and magnetism in the 1800s.  Since then scientists have combined them into a single electromagnetic force.

And it turns out that there is often a frequency associated with electromagnetic fields.  And what also initially seemed like separate phenomena were just electromagnetic radiation operating at different frequencies.  We now talk about the electromagnetic spectrum.  Very low frequencies manifest themselves as radio waves.  As you raise the frequency you encounter infrared light, visible light, ultraviolet light, x-rays, and gamma rays.  The first of these bands to be seriously investigated was visible light.

And Newton did pioneering work that resulted in his book Optics.  And for a long time there was a mystery.  Was visible light composed entirely of waves or was it composed entirely of particles.  There was evidence for and against each of these theories.   Most of the results in Optics are best explained by assuming light is composed of waves.  But Newton thought it was composed of particles.  That's how baffling the situation was for a long time.

Consider waves.  If we throw a rock into a still pond we see waves originating from where the rock hit the surface.  They radiate out from that point in all directions.  We can imagine the rock depressing the surface of the water.  As the rock sinks the water floods back.  This gets carried away and for a moment the new surface of the water is actually above the original level.  We can easily imagine this bouncing down and up and soon back down somehow twisting the water and this twisting propagating outward to make the waves.  This is a very natural and intuitive model for how waves work.

And we can imagine this sort of thing happening with air or Jell-O or pretty much anything else.  And the key idea is that something gets twisted.  Since space is a vacuum, "in space no one can hear you scream", as the tag line for the old movie "Alien" has it.  But this presents a problem.  How do you have waves if you don't have something to twist?  Sound waves do not travel through the vacuum of space but light waves certainly do.  The proponents of the wave theory had to admit this was a problem.  Their solution was to invent something called the "luminiferous aether".  Being good scientists they promptly went looking for it.  And the problem is that a series of very clever experiments convinced them that it did not exist.  Bummer, dude.

Einstein rode in to the rescue in 1905.  He said there was a third way.  Light was neither exclusively particles nor exclusively waves.  It was composed of something called photons that under some circumstances behaved very much like waves and under other circumstances behaved very much like particles and in still other circumstances behaved like neither.  And he set out the rules for when it was appropriate to treat light as particles, when it was appropriate to treat it as waves, and when it was appropriate to do neither.

I am going to skip over the weak force and the strong force.  Scientists have been able to devise experiments to prove their existence and determine many of their properties.  But it requires a lot of technical details to understand any of it.  Instead I am going to return to gravity.

In 1915 Einstein published his General Theory of Relativity.  In many ways it was a theory of gravity.  He said that gravity distorted space and that these distortions propagated through space at the speed of light.  And implicit in his theory was a wave model of how gravity worked.   Well, this threw things back to the particle/wave problem for light that had bedeviled scientists for ages and that Einstein himself had fixed only ten years earlier.  If gravity is a wave then don't you need something, say the gravity equivalent of luminiferous aether, for gravity to twist?  Nobody, and I mean nobody, wanted to go there.  So the obvious solution was to come up with some photon-like gravity particle.  Scientists did.  It's called the graviton.

Scientists have done a ton of experiments on light.  Newton did not have the last word on the subject.  It is still an active area of research.  As a result they know a hell of a lot about photons.  They may be weird but scientists think they understand them.  There is even a theory called Quantum Chromodynamics (QCD) that is a kind of Quantum Mechanics (QM) for photons.  Everyone knows that QM is weird.  QCD is every bit as weird or maybe even weirder.  It would be nice if scientists had also done tons of experiments on gravity and gravitons.  But at this point they know so little that they don't even know if gravitons exist.  The problem is that you can't really do lab experiments on gravity.  Why?

It's everywhere isn't it.  Yes, but it's also terribly weak.  Nobody believes this the first time they hear it but it's true.  Remember what's going on when you step on to a spring scale.  On the one hand you have all of planet Earth pulling you down.  On the other hand you have a few gears and levers and a not very big spring pushing you back up exactly as strongly.

A typical pre-computer bathroom scale weighs about a pound.  Yet it can easily generate the same force as the billions of tons of stuff that make up planet Earth.  If you want to generate a gravitational force equal to your weight in the laboratory you need to have a planet Earth handy.  It's really hard to get one or several entire planet Earths to fit onto a standard laboratory bench.  That makes gravity extremely hard to study.

Let me describe one of the few lab experiments involving gravity that scientists have managed to figure out how to actually do.  Take a piece of strong wire and fasten one end down.  Do this to a piece that is an inch long.  Then twist the free end so that it rotates through a one degree arc and measure how much force this takes.  This won't take much force.  But it's something that can be done in a laboratory.  Now fasten a fifty foot long piece of the same type of wire to the ceiling.  (It's a good thing your laboratory has a high ceiling.)  It turns out that it will take one 600th of the force to twist the free end of this much longer wire through the same one degree arc.  Now we're talking about really small forces, forces similar in strength to gravity.

Fasten a strong bar horizontally to the free end of the wire.  Now put identical heavy balls on each end of the bar.  If you do this properly you can get the bar to hang levelly.  Now bring one of a pair of balls close to each of the balls on the end of the rod without having them touch.  (These free balls need to be identical to each other but they don't need to be the same as the fixed balls.)  And bring each free ball close from opposite sides.  This means that the gravitational pull between each free ball and its corresponding attached ball will cause the apparatus to rotate in the same direction.  You have to be very careful that there is no static charge or anything other than gravity exerting a force between the attached balls and the free balls. But if you carefully eliminate all the extraneous forces what's left is the gravitational attraction between the heavy balls.

Careful measurement and careful calculation will allow you to measure the force of gravity.  A group at the University of Washington has done just this.  As have several other groups.  But that's pretty much the only experiment people have been able to pull off.  You can only do this with balls that are big but not too big and definitely not too small.  And you can only do this with balls made from a few different materials.  And your experimental technique must be top rate or you will end up measuring something but it won't be gravity.  That means you are very restricted in terms of the kinds of different experiments you can do.  So it's hard to find out much about gravity this way.

But that has now all changed.  There is now another kind of gravity experiment you can do.  And it's completely different.  The experiment I described above is literally child's play compared to how hard it was to pull this new experiment off.  And this new experiment is called LIGO.

The above experiment involved scientists using extreme care to make very precise measurements.  And they had to worry about things like static electricity and stray air currents and uneven heating of one side of the balls versus the other and a ton of other things.  What the LIGO people had to deal with is easy to describe.  The fact that they pulled it off is almost incomprehensible.  I have been tracking LIGO since the start.  I came to an early understanding of what they were dealing with.  I didn't believe it was physically possible to do what they needed to do.  So I put their chance of success at pretty much zero.  But they pulled it off.  And I am ecstatic that they made me look like a fool.  So what is LIGO.

The Laser Interferometer Gravitational-wave Observatory (LIGO) is actually two facilities.  One is located in Washington State and the other is located in Louisiana.   And they are quite large.  They consist of two perpendicular pipes each of which is 4 kilometers (2.5 miles) long.  The idea is simple.  You shoot a laser beam into a "beam splitter" mirror.  Half the beam goes down one pipe and the other half goes down the other pipe.  At the end of each pipe is a mirror.  So the light comes bouncing back to where it came from.  There the "beam splitter" mirror is run in reverse and the beams are recombined.  This causes the two beams to "interfere", hence the "Interferometer" in the name of the facility.

If both beams take exactly the same time then the beams will be "in phase" and the combined beam will be twice as bright as either beam would be on its own.  If the travel time is different and the time difference is exactly right then the beams will be "out of phase" and the combined beam will be completely snuffed out.  Different amounts of time difference result in more or less cancellation.  This process is called interference.  Interference can be constructive (in phase) or destructive (out of phase).

Light has a frequency.  It's very high.  If you do the math you can turn the frequency into a wavelength, the distance between one cycle and the next.  The wavelength of light is very small.  And what determines whether the interference will be constructive or destructive is where in its cycle each beam is.  And time is distance and distance is time.  If the distance down one path is exactly the same as the distance down the other path then the beams will end up in phase.  If the distance down one path is a part of a wavelength longer or shorter than the distance down the other path the beams will end up out of phase to a greater or lesser extent.  It is easy to measure the degree and kind of interference.  This combined with the very short wavelength of light makes it possible to measure very small differences.

The whole point of the LIGO design is to be able to measure very small differences in the lengths of the two pipes.  And that gets us back to gravity and specifically gravity waves.  Einstein's 1915 theory predicted that various things would generate gravity waves.  But his calculations showed that they would be very small.  But assume for the moment that they are of reasonable size.  Gravity distorts space.  This is a fancy way of saying gravity affects the distance between things.  So if a reasonable sized gravity wave sweeps through the LIGO observatory it should change the length of one or both of the pipes.

To make it simple assume that the waves are moving exactly along one of the pipes.  This means that the other pipe is perpendicular to the waves and it wouldn't be affected.  But the pipe that is parallel to the direction of propagation would definitely be affected.  This should lengthen or shorten that pipe so the light beam that went down it should get put out of phase and this would show up in the interference measurement.

Now a gravity wave would most likely come from a different direction, one not aligned with one of the pipes.  But a little trigonometry would sort this out.  But there are a few directions for which a particular LIGO observatory is blind to gravity waves.  That's one reason why there are two LIGO observatories.  One or both of he observatories should be able to see the gravity waves no matter what direction they are coming from.  The other reason is that if you can detect the same set of gravity waves at both observatories you can get an idea of what direction they came from.  The time difference between when the gravity waves arrive at each observatory can be used to narrow the direction down.  Cool!

So these LIGO observatories have been detecting gravity waves as soon as they came online, right?  Nope!  It turns out that gravity waves are really, really tiny.  The LIGO observatories first started collecting data in 2002.  But they weren't sensitive enough.  This is in spite of the fact that they were really sensitive.  The "earthquake" caused by an ordinary truck driving down a road many miles away was big enough to throw them out of whack.  The first ten years of operation was used to dial the equipment in and figure out how to make them supersensitive.  During this period the best calculations said they would find nothing.  And that's exactly what they found.  Nothing!  That's depressing.

But the only thing that can be said about this is that a lot of people had a lot of faith.  And what they had a lot of faith in was that given enough time and money the LIGO people would come up with ways to amp the sensitivity up massively.  And fortunately they were able to talk congress into having enough faith that they approved shelling out a billion dollars.  That's what it cost to design, build, run, and repeatedly upgrade LIGO.  It's a good thing I wasn't part the group that made the pitch to Congress.

But they did up the sensitivity massively.  They upgraded to the "Enhanced LIGO" setup in 2009.  They further upgrades to the "Advanced LIGO" setup in 2015.  One simple trick was to bounce the light beam up and down the pipe a bunch of times.  This made the effective length of the pipes not 4 kilometers but hundreds of kilometers.  But they had to come up with schemes to isolate the apparatus from those truck earthquakes and a whole lot more.  And now the systems are so sensitive that they can detect a length change of a thousandth of the diameter of a proton (10 to the minus 18th Meters, if you care).  I literally have no idea how they pulled this off.

Anyhow, the Advanced LIGO did the trick.  LIGO detected gravity waves for the first time on September 14, 2015.  This detection was scary because it happened as soon as they turned on the new setup.  There are a million things that could very plausibly have happened that were not a gravity wave detection.  It could, for instance, have been a glitch in the equipment, or someone throwing some test data into the system because they forgot the observatories had gone live, or, or, or.  So the LIGO scientists spent five months going over everything over and over before they finally went public.  All this certainly went through my mind.

But then had a second detection on December 26, 2015.  This was publically announced on June 15, 2016.  That's the point where I became a believer.  And they have since had three more detections for a total of 5.

I'll be honest.  For a long time I was not sure gravity waves even existed.  The people who knew a lot more about this sort of thing believed in their existence.  But a lot of people have been trying for a long time with a bunch of very creative designs to detect them.  And they all came up empty.  And gravity seems like it is really different than the other three fundamental forces.  So I thought it was possible that gravity worked in some ways so that there was no such thing as gravity waves.  Did I have any idea what that "other way" could be?  Not in a million years.

And things have moved very quickly since that first detection.  One problem with having only 2 LIGOs is that you can't nail down the direction very accurately.  So the best thing scientists could do was point to a general direction in the sky.  But that initial detection accelerated plans.  And in 2016 a European detector called VIRGO came online.  If all three sites detected the same set of gravity waves then a much more accurate direction could be determined.  And that's what happened with detection #5.

Together the LIGO/VIRGO people were able to narrow things down to a relatively small part of the sky.  Then a piece of luck that I am not going to go into happened.  This further narrowed things down.  Other astronomers were on the hunt within a few days of the initial detection.  They were able to locate the exact source of the signal because the event that caused the gravity waves was still visible.  With a specific target additional follow up observations in various parts of the electromagnetic spectrum have allowed scientists to assemble a much more detailed picture of the event than would have been possible with just the gravity wave information.

The first four events were caused by two black holes colliding with each other.  The fifth event was caused by two neutron stars colliding with each other.  Both of these scenarios were just theoretical speculation until now.  And the LIGO observations are offline as I write this.  They are undergoing yet another upgrade to increase their sensitivity.  This will help a lot.  The events LIGO detected involving black holes all took place a billion or more light years away and involved large black holes.  One expects these events to be rare.

But increased sensitivity means that events involving these large black holes can be detected even further away.  Or events involving smaller black holes can be detected at similar distances.  The neutron star event was only hundreds of millions of light years away.  Again an increase in sensitivity means events involving smaller neutron stars can be detected at a similar distance or events involving similar sized neutron stars can be detected further away.  And this means that a lot more events should be detected.  Detecting roughly two events per year is a whole lot better than not being able to detect any events at all.  But the more events that are detected the better idea we will have of what is going on.

But wait, there's more.  LIGO can only detect gravity doing it's wave thing.  Does this mean that gravity is all wave and no particle so no graviton?  We just don't know enough to answer that question.  And waves have frequencies.  And gravity waves do share one attribute with electromagnetic radiation.  You can't detect gravity waves of all frequencies with a single device.  LIGO detectors can only detect gravity waves within a relatively narrow frequency band.  Can gravity waves be created at frequencies that LIGO can't see?  Theory says yes!  To detect gravity waves at these other frequencies takes a completely different design.

Before LIGO had detected gravity waves it was hard to talk people into spending large amounts of money on these other designs. And it wouldn't hurt to have more LIGO-type detectors out there.  The good news is that it's full speed ahead on both fronts.  There are a number of LIGO-like facilities under construction around the world.  And chances are good that completely different designs will also be funded.  One of the most interesting ideas involves flying a number of satellites that would very accurately measure their relative positions.  This would allow very low frequency gravity waves to be detected if it can be made to work.

These are truly exciting times.  The closest parallel is when Galileo pointed a telescope at the sky for the first time.  He was able to see things that previously had been literally invisible.  Gravity waves are not electromagnetic waves.  They are a whole different thing.  We can now see them to a very limited extent.  As we get better and better at seeing them we are exploring a whole different way of observing the world around us.  We now know for sure they are out there.  And there is at least one way to see them that we know works.  The odds are now in favor of the proposition that there are more ways to see them.  All that is needed is for someone to figure out how to pull it off.  Now that we can see the sky in an entirely different way who knows what we will find there.  Every time something previously invisible has become visible tremendous discoveries have been made.  So the sky is the limit and the clouds just parted.