Thursday, July 26, 2012

The Journey of a Gravitational Wave II: GWs Get Bent

What happens to a gravitational wave between when it is produced and when LIGO can detect it?  It turns out not much, which makes it a key new medium in which to observe the Universe!

Last week, I began discussing what happens to a gravitational wave as it makes its way from its source to Earth; specifically that gravitational wave can travel through matter and come out the other side unchangedToday's post talks about how the gravitational effects of other masses in the Universe can deflect the gravitational wave from its otherwise straight path.


GRAVITATIONAL LENSING

Let's think again about what happens to light on its way to Earth.  We know from the last post that any matter light comes into contact with will reflect or absorb at least part of the light.  There is also another effect called gravitational lensing that causes light to bend around massive objects due to the massive object's gravitational influence.  This is caused by light following its natural path on curved spacetime (or light being bent by a gravitational field since we've previously established that the curvature of spacetime is a representation of the strength of the gravitational field there).  The first thing that pops into my mind that illustrates something following its natural path on a curved surface is miniature golf:

This is hole 13 at Safari Mini Golf in Vero Beach, FL.  This image is taken from a review of this course and can be read here.
Consider the example hole in the above image.  After you get your ball past the three bumps, there is a wonderful bowl-like curved portion behind the target hole (if you look very closely, you can see the hole directly after the last bump and in the center).  If you hit your ball into this area, the ball's trajectory will change from a straight path to a curved one.  If your ball begins its path on the left side of the bowl, it will curve right; if your ball enters the bowl from the right, it will curve left.  The same thing happens for light and gravitational waves that pass by a massive enough object to cause a significant depression in spacetime (i.e. a strong gravitational field):

The bending of light from a star that is really behind the Sun but appears to be to the side of the Sun.
[Credit: Ethan Siegel of Lewis & Clark College, OR]

The the light from a star, galaxy, or other source travels through the depression in spacetime made by a nearby massive object (in the image above it is the Sun).  The path that light takes is curved just like the golf ball in the miniature golf example above.  But our brains are wired to assume that any light that enters our eyes has come to us in a straight line (which is how light usually travels) so we perceive the location of the source to be directly behind where it appears to be.  Therefore, while the star is really behind the Sun (at point A in the image above), it appears to us to be to the side of the Sun (at point B).  This bending effect is called gravitational lensing and it applies to gravitational waves just like it does to light.


KINDS OF GRAVITATIONAL LENSING

The example of gravitational lensing given above was one of the first observational proofs that Einstein's general relativity was correct.  Before relativity, there was already a prediction of the bending of light due to Newtonian gravity (what we use in our everyday life) but Einstein predicted the bending effect should be twice that predicted without relativity.  In 1919, there was a total eclipse of the Sun which would allow those stars that are near the Sun to become visible.  Images of the eclipse were taken and it was seen that the shift in the position of stars near the Sun was indeed twice that of the shift predicted by Newtonian gravity.

There are also kinds of lensing that produce much more dramatic effects than shifting the position of stars!  Things like large galaxies and clusters of galaxies can cause the light from objects behind them to be split up to form multiple, separate, and complete images. 

[Image from NASA]

The image above shows gravitational lensing of a quasar and a galaxy by a distant galaxy cluster SDSS J1004+4112 (SDSS indicates that it was discovered by the Sloan Digital Sky Survey).  Each image of the quasar is of the same single object; the same is true of the galaxy! 

You may notice that each of the images are a little different from each other.  This is due to the distortion that a gravitational lens can cause.  This effect is illustrated well in the simulation below of a black hole creating a gravitational lens as it passes in front of a galaxy:

[Image from Wikipedia]

Note the circular distortion of the light from the galaxy as the black hole passes by.  When the black hole is directly in front of the galaxy, there is a circular halo of lensed light around it.  This halo called an Einstein Ring can can be caused by any extremely massive object (black hole, galaxy, galaxy cluster, etc.).
[Image credited within the image and retrieved from Wikipedia.]


HOW LENSING AFFECTS THE SEARCH FOR GRAVITATIONAL WAVES

Gravitational lensing affects both light and gravitational waves.  This produces spectacular images of objects using light, but LIGO will not produce images and the sources that produce gravitational waves are more point-like (a black hole, a star exploding, etc.) than large scale objects (like galaxies which are thousands of light-years across).  The effect that will most likely be seen in gravitational waves is their focusing; the bending of gravitational waves can produce more intense gravitational waves from the lensed source (similar to a magnifying glass focusing light to a smaller point).  This paper suggests that the galactic center of the Milky Way could increase the intensity of a source in our galaxy (bur behind the galactic center) up to 4000x.  Also, if a gravitational wave is emitted from a galaxy that has multiple images from lensing, then that gravitational wave will come from each image!

However, most sources will not have appreciable lensing.  While this is something we will always need to consider while conducting gravitational-wave astronomy, it isn't something that is likely to change the information contained on the gravitational wave noticeably (and that's a good thing)!

Thursday, July 12, 2012

The Journey of a Gravitational Wave I: GWs Cast No Shadows!

What happens to a gravitational wave between when it is produced and when LIGO can detect it?  It turns out not much, which makes it a key new medium in which to observe the Universe!

In order to make this information more digestible, I will address one aspect of a gravitational wave's journey through space.  Today's topic discusses how the Universe is essentially transparent to a gravitational wave.  Future editions will discuss how matter can bend gravitational waves (gravitational lensing) and how the expanding Universe can stretch out (redshift) gravitational waves.


GRAVITATIONAL WAVES CAST NO SHADOWS:

First, let's think about what happens to light.  As light travels through the Universe, any time that it encounters other matter, some of the light is absorbed by the matter or reflected away from its original path.  The opposite happens for a gravitational wave; it can pass through matter and come out the other side unchanged (although there are some negligible effects)!  That means that there is no such thing as a gravitational-wave shadow and nothing can obscure our detection of a gravitational wave!


The Spacetime Explanation:

But why is this?  In a previous post, I described a gravitational wave as a change in the gravitational field moving out into the Universe.  This change in gravitational field is often illustrated as a ripple, or wave, on spacetime (where the steepness of the curvature of spacetime represents the strength of the gravitational field, or the gravitational force a mass would feel, there).  Let's look at what the Earth sitting on space-time looks like:


This picture isn't a perfect representation since the size of the Earth will affect the shape of the depression and this has no effect on real spacetime.  Also, this is a simplified 2-dimenional representation of 3-dimensional (or a snapshot of the 4-dimensional spacetime) space.  But if you were to imagine giving a corner of this flexible grid (spacetime) a swift shake, the Earth in the middle would be affected by it but it would not impede the wave.  So, this is an example of how matter doesn't interact with gravitational waves, but I am still somewhat unsatisfied with this since you may think that the Earth will bounce after a wave passes creating more waves of its own (here is another aspect where this representation of spacetime is not perfect).

FYI: A better 3-dimensional representation of spacetime is shown in this clip from the American Museum of Natural History's short documentary called Gravity: Making Waves (which can be seen in its entirety on my Viewing Fun! page).  This animation shows a grid-like scaffolding filling space in which there is a depression caused by mass.  While it still isn't a perfect representation of spacetime, it is much better than the trampoline approximation above.

video



The Lunar Eclipse Explanation:

Recently, I thought of another more intimate example that most of us can identify with: a total lunar eclipse (which I have also blogged about).  This is a situation where the Sun, Earth, and Moon line up in that order so that the Moon is completely in the Earth's shadow:

Image from Wikipedia

When the Moon is completely in the Earth's shadow (or the umbra in the diagram above), a viewer on the Moon would not be able to see any part of the Sun.  If the gravitational field from the Sun were blocked by the Earth, then the Moon's orbit would appear to change.  Since there is no change in the Moon's orbit during a total lunar eclipse, then the Earth does not block the Sun's gravitational field.  By extension, the Earth also would not be able to block any changes in the gravitational field (which are gravitational waves).  If you are familiar with physics, this is an application of the principle of superposition.


Great!  But Why do We care?

Since mass does not absorb or reflect gravitational fields, the Universe is transparent to a gravitational wave.  This is a huge advantage when using gravitational waves to make astronomical observations since nothing can block our view of a gravitational wave!  If you have ever seen the our own Milky Way galaxy in the sky on a clear, dark night, you've seen the billions and billions of stars that live in our "backyard": 

Fish-eye mosaic of the Milky Way galaxy as seen from Chile.  [Image from Wikipedia]

While this is beautiful, it also it almost impossible to see past all the stars and dust there to observe what is behind our "backyard".  We will be able to see right through the Milky Way with gravitational waves! 

P.S.  We can also detect gravitational waves from the other side of the Earth with LIGO since gravitational waves can travel through matter.  Read more about this in this previous post (under the subheading of "Why are there 2 LIGOs?")!

Thursday, July 5, 2012

What Is a Higgs Boson?, What Did CERN See?, and Why It's a Big Deal!

This is my disclaimer - I AM NOT A PARTICLE PHYSICIST!  Therefore, this subject does not fall into my realm of expertise.  However, I do have a very basic training in the physics behind all of this so I would like to share with you a little bit about why all of us physicists have been so excited of late...


WHAT IS A HIGGS BOSON?

The Higgs boson (regular Wikipedia entry, Simple English Wikipedia entry) is the elementary particle that gives matter mass.  Many of us have probably heard about Einstein's famous equation:

This image is taken from Wikipedia.
While this equation is famous, the true meaning behind what it means is often not fully appreciated (I used to see it and simply think "Einstein!").  It means that mass can be converted to energy and energy can be converted to mass, also known as the mass-energy equivalence.  The energy, E, from an amount of mass, m, is equal to the mass multiplied by the speed of light, c, squared (c2 = c*c, c is about 670,616,629 miles/hour).  If you were to convert 1 oz of matter into energy, you would have about 2,500,000,000,000,000 Joules of energy and this would keep a 100 W light bulb lit for over 807,389 years!  So, a little bit of matter can be converted into an immense amount of energy!  It is this conversion of mass to energy that makes nuclear weapons so destructive.

But, if we convert energy into mass, where does this mass come from?  The Standard Model, which is the working description of how the fundamental particles interact (I talked about this in my post discussing gravitons), says it comes from the Higgs field.  This is a field similar to the electric field and the magnetic field (more on fields in general here).  At the instant after the Big Bang, all particles moved at the speed of light (c from above) since none of them had mass because there was no Higgs field.  In the next instant (about a trillionth of a second later), the Higgs field came into existence and produced a resistance to particles based on what they were made of.  This resistance manifests itself as mass: the slower a particle moves through the Higgs field, the more massive it is.  The Standard Model also allows fields to manifest themselves as particles (the photon is the particle associated with the electric and magnetic fields).  Therefore, this Higgs field should also manifest itself as a particle and we call it the Higgs boson.

The only particle from the Standard Model that has not been detected is this Higgs boson.  (Note that gravity is not described by the Standard Model.)  This is because the Higgs boson is very massive for a fundamental particle at about 133 times the mass of a proton.  The amount of energy conversion needed to produce this mass is much larger than we have been able to create in the past.  That is, until CERN built the Large Hadron Collider (LHC)...


WHAT DID CERN SEE?

View of the CMS experiment [note the person near the center] (© CERN)

CERN is the home to several experiments including CMS and ATLAS.  Both of these experiments smash together protons with very large energies.  The protons and their energy can change into a Higgs boson (if there is enough energy).  The Higgs boson decays (changes into something else) almost immediately, so the LHC experiments look for the Higgs boson's signature in the resulting particles.  There are 5 pairs of particles that result from the decay of a Higgs boson, including 2 photons.

Both CMS and ATLAS saw the Higgs signature in 2 of the 5 different resulting decay pairs.  Only CMS had sufficient data to look for all 5 kinds of decay pairs and saw with high certainty signatures in 4 of the pairs.  More research is needed for the signature CMS didn't see.

Diagram of the ATLAS experiment [note the people on the bottom left] (© CERN)


WHY IT'S A BIG DEAL!

The official conclusion is that CERN has indeed observed a Higgs-like particle.  "Higgs-like" does not mean that they are unsure if they found what they were looking for.  Instead, it appears that the Higgs boson they observed has more properties to it than the Standard Model predicted.  The LHC has found what it is looking for as well as hints that there is new physics to be discovered.

So this is the big deal: the population of the Standard Model is complete but the model itself doesn't appear to describe everything.  That is something we knew before, but now we are seeing it with the apparent complication in what was and wasn't observed in the discovery of the Higgs boson.

I am a physicist because every time you discover something new, not only do you understand the Universe better, but you have even more exciting questions to answer!  Soon enough, it is going to be gravitational waves stirring up excitement like this and I am going to be in the midst of it all!


Side Note:
IT'S NOT THE 'GOD PARTICLE'

The Higgs boson is sometimes referred to as the "God particle," especially by the media.  However, it has nothing more to do with God than any other particle in the Universe.  The origin of this misnomer is a book by Nobel Laureate Leon Lederman titled The God Particle: If the Universe Is the Answer, What Is the Question?.  Likening the Higgs boson as a God particle refers to it being the origin of all matter's mass.  Physicists (like me as well as Lederman himself) generally dislike this nickname since it places much more import on this particle than it deserves.  But above that, it is offensive to people of faith and makes us look like we are trying to replace God.  We aren't. 


Want More? ...
Watch the announcement seminar
Read the official CERN press release