Gravity Waves!

Physicist: A few days ago we managed to detect gravity waves for the first time.  Gravity waves were predicted a century ago by Einstein as a consequence of his general theory of relativity.  This success isn’t too surprising from a theoretical stand point; if
your theories are already batting a thousand, then when they bowl yet another field
goal for a check mate no one is shocked.

What is amazing is not that gravity waves exist, but that we’ve managed to detect
them.  The effect is so unimaginably small that it can be overwhelmed by someone
stubbing their toe a mile away or filing their taxes wrong.  Gravity is literally
the geometry of spacetime: very particular, tiny increases and decreases in
distances and durations.  There’s a fairly standard technique for doing this: light
is bounced back and forth along two separate paths between mirrors (in this case the
length of those two paths are each 4km) dozens of times.  The light from these two
paths is then brought together and allowed to interfere.  If the difference in the
length of the two paths changes by half a wave length, then instead of destructive
interference we see constructive interference.  The actual path difference is
substantially less than half a wave length, but it’s still detectable.

LIGO is b

A gravity wave detector is a device that very, very carefully measures the difference between the lengths of two long paths.  (Left) The tiny difference is detected by looking at the interference between lasers that travel along each path.  (Right) What the detector in Livingston, LA looks like from above.

When a gravity wave ripples through the Earth, the lengths of the two paths change
by about one part in 1021 which is a tiny fraction of the width of a proton over 4 km.  Keep in mind that the light that’s doing the measuring is bouncing off of mirrors that are made of atoms (each of which is much bigger than a proton) and that those atoms are constantly jiggling, because that’s what any level of heat does to matter.  This level of precision is the most impressive part of this whole accomplishment.  Your heart beat is currently throwing around the building you’re in by a lot more than a proton’s width.  And yet, despite the fact that the literally everything in the world is a source of experiment-ruining noise, LIGO is able to filter all of it out and then go on to detect the ridiculously faint signal of a couple of black holes a fair fraction of the universe away and even sort out details of the event.

The signal that we’re hearing about now was actually detected in September.  The cause appears to be the merging of two black holes about 1.3 billion lightyears away (which puts the source well outside of our backyard).  These black holes started with masses of around 36 and 29 times the mass of the Sun and after combining left a black hole with a combined mass of about 62 Sun-masses.  Astute second graders will observe that 36+29>62.  This is because gravity waves carry energy.  In this case the final event turned about 3 Sun’s worth of energy into ripples in spacetime that are “loud” enough to literally (albeit very, very slightly) rattle everything in the universe.  So, if we ever contact aliens from the other side of the universe and they also have nerds, then we’ll have something to talk about.  By the way, this signal (unlike so many in physics) has a frequency well within the range of human hearing.  Properly cleaned up, it sounds like this.

(Top) The signal as detected at the two observatories. The noise is bad enough that without at least two observatories it would be much more difficult to see it. (Middle) The signal as predicted by our understanding of general relativity. (Bottom) The remaining noise after the signal has been subtracted. Notice that it is now fairly constant. (Picture on the bottom) This is a plot of the strength of the signal using color vs. frequency on the vertical axis and time on the horizontal axis.

(Top) The signal as detected at the two observatories. The noise is bad enough that without at least two observatories it would be much more difficult to see it.
(Middle) The signal as predicted by our understanding of general relativity. (Bottom) The remaining noise after the signal has been subtracted. Notice that it is now fairly constant.
(Picture on the bottom) This is a plot of the strength of the signal using color vs. frequency on the vertical axis and time on the horizontal axis.

This is the first direct measurement of gravity waves, but it isn’t the first evidence we’ve seen.  If you have two really heavy masses in orbit around each other, you’ll find that they’ll slowly spiral together.  This is strange because it implies that the masses are losing energy.  But to what?  We first measured this effect with pulsars, which are a kind of neutron star (the next densest things after black holes).  Pulsars are so named because they produce radio pulses that are extremely regular.  You can think of them as giant space clocks.  They’re precise enough that they allow us to figure out exactly how they’re moving using doppler shifts, and they’ve shown that closely orbiting pairs lose energy in exactly the way we’d expect based on our theoretical understanding of gravity waves.

So what can we use this for?  So far we’ve been able to “hear” black holes merging (several more times since September).  We’re not only detecting the spiraling in, but also the process of the black holes coalescing.  Once they come in contact they briefly form an unshelled-peanut-shaped black hole before assuming a spherical shape.  This process is called the “ring down” and it also creates audible gravity waves that give us information about the behavior of black holes.  But beyond heavy things in tight orbits and ringing black holes, what will we hear?  Short answer: who knows.  If you go out in the woods you’ll hear trees falling over when no one is around and lots of bears shitting, but there’s no telling what else you’ll hear.  The only way to find out is to go out and listen.  As our gravity wave detectors get better and more plentiful we’ll be able to hear fainter and fainter signals.  We can expect to hear lots of black holes merging; not because it’s common, but because it’s loud and the universe is big.  Soon we’ll start hearing things we don’t expect and that’s when the science happens.  It’s nice to have our theories regarding gravity waves proven right, but being right isn’t the point of science.  As long as you’re right, you’re not learning.  It’s all the things we don’t expect that will be the most exciting.

Gravity wave astronomy is only the third way we have of observing the distant universe: light, neutrinos, and now gravity waves.  We didn’t know what we’d find with the first two and it’s fair to say we don’t know what we’ll learn now.  Exciting times.

You can read the paper that announced the achievement here.  And check out the author list: there was more collaboration on this than a Wu Tang album.

Update (6/20/2016): And again!

This entry was posted in -- By the Physicist, Experiments, Physics. Bookmark the permalink.

10 Responses to Gravity Waves!

  1. Glen Lancaster says:

    If you were close enough to the two orbiting black holes that were source of these gravity waves, would it be possible to see this effect of spacetime rippling with the naked eye? If so, how close to the black holes would you have to be to see the effect?

  2. Antonio Carlos motta says:

    More a window opened for the researches of the cosmos,neutrinos and now the gravitational waves already thinker by Einstein in theirs equations for gate.demonstrate that these gravitational waves carry energy and with surely particles,as the graviton so,that might to explain the quantum field theory,beyond the standard model.these echoes capped of the colliding black holes,demonstrating the existence of the black holes to the energy level observed by the LIGO,in the part of frequency of the proton.the ripples in the spacetime altere the structure of the curvatures of spacetime.these distortion if propagate in the space with the speed of light,as well as the gravity.we are awaiting news discovered in the in the field of the particle physics as the tentative of meet the graviton stand explain the essence of the gravity,extra dimensions,dark matter,dark energy…

  3. George D Conger says:

    Assuming the common acceptance of “spacetime” is correct.
    I believe there are more modern and better theories that use field, field energy and similar concepts that do not necessarily show that “distance” can be literally altered.
    More likely, it is an energy thing such as K.E. of the wave that gets translated down the perpendicular LIGO apparatus that simulates Length shortening.
    George

  4. Clive Gifford says:

    Glen Lancaster can find some discussion related issue to his question (above) about how close you might need to be to actually see the ripples here: http://www.scottaaronson.com/blog/?p=2651

    I believe the answer can be summarised as “extremely close”, very likely “too close for comfort”!

  5. David says:

    Albert Einstein is once again and finally proven totally correct (except he didn’t think this day would come gravity waves being so small) and any newer theories must include ways to account for all the Einsteinian effects. Congratulations everyone and especially Albert Einstein. We’ll miss you.

  6. Rich says:

    I like the joke a physics major friend of mine made:

    “Gee, I hear that the Michaelson-Morley experiment finally got a positive result.”

    (In that vein, it would be amusing for someone to publish a short paper noting that the LIGO data refutes the existence of the aether to some umpty-silly number of decimal places/)

  7. Brooks says:

    I was wondering… this recent ‘discovery’ or confirmation made by LIGO seems to suggest that we detected the gravitational waves made by two black holes colliding 1.3 Billion years ago. After exhaustive online research I still have questions.

    I will try to be brief and specific.

    1. Is my assertion above that we detected the waves from this particular event? Meaning the event of two black holes colliding 1.3 Billion years ago?

    2. If this is true – how long do those waves stick around? For example – if I throw a rock into the middle of a perfectly round pond measuring exactly 50 meters in diameter, and I’m standing on a conveniently placed rock at exactly 25 meters from the center, then I would expect the waves to meet me and eventually pass me, and then, after a period of time the pond would be (relatively) still again – at least where I’m standing – meaning no waves to detect. Is this the case with us standing on our rock (Earth) in the pond of space? So are they passing by us like me standing on that rock in the pond and eventually there would be no detectable waves from this event?

    3. If they are passing by – will they pass by (meaning will they be detectable by us) for millions of years? hundreds of years? will they be gone tomorrow? will my grandchildren live in a world where we can’t detect these any more or are there just so many out there all the time it’s just up to us to get better at detecting them?

    I thank you in advance for your response.

  8. Casey says:

    Would it be possible for us to see the effects of gravitational waves by looking at the sun, either time-wise or distance-wise? What would happen if two of the largest supermassive black holes collided? What will happen if and when our supermassive black hole collides with the one in Andromeda?

  9. kevin says:

    if we were in the same star sytem as the two black holes would we be affected by them as we would be so much closer ??????

  10. John Roberts says:

    I wonder how the age of the event was derived. Was it from the apparent vs absolute magnitude of the event as described by the supercomputer model for the mass conversioin? or was it from a redshift of the gravity waves as seen in the “chirp”, again based on a change in frequency distribution from the modeled event?

Leave a Reply

Your email address will not be published. Required fields are marked *