Of Particular Significance

Did BICEP2 Detect Gravitational Waves Directly or Indirectly?

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/30/2014

A few weeks ago there was (justified) hullabaloo following the release of results from the BICEP2 experiment, which (if correct as an experiment, and if correctly interpreted) may indicate the detection of gravitational waves that were generated at an extremely early stage in the universe (or at least in its current phase)… during a (still hypothetical but increasingly plausible) stage known as cosmic inflation.  (Here’s my description of the history of the early universe as we currently understand it, and my cautionary tale on which parts of the history are well understood (and why) and which parts are not.)

During that wild day or two following the announcement, a number of scientists stated that this was “the first direct observation of gravitational waves”.  Others, including me, emphasized that this was an “indirect observation of gravitational waves.”  I’m sure many readers noticed this discrepancy.  Who was right?

No one was wrong, not on this point anyway.  It was a matter of perspective. Since I think some readers would be interested to understand this point, here’s the story, and you can make your own judgment.

I’ll describe

  1. a past observation of gravitational waves that everyone agrees is indirect;
  2. a future observation of gravitational waves that we expect to happen fairly soon, one that I believe everyone will agree is direct;
  3. BICEP2, and how you can view it either way, depending on your perspective.

A Past Indirect Observation of Gravitational Waves

First, let me describe what everyone agrees was the first observation of gravitational waves, and was definitely indirect.  In 1974, two scientists (Joseph Taylor and his graduate student Russell Hulse) discovered a pulsar.  A pulsar is a city-sized neutron star (made entirely from neutrons and resulting from a Type IIa supernova) that spins rapidly — rotating many times per second — and, due to its powerful magnetic field, sends strong radio beams into space, which sweep past the Earth as the pulsar spins.  We observe this as a pulsing radio signal from the location of the star.

Pulsars are common, but this one was special.  Its frequency of pulsing (i.e. how many times per second does it pulse) varied slightly, growing and shrinking every 7 hours and 45 minutes.  It quickly became clear this was due to the Doppler effect for radio waves; the pulsar was sometimes moving toward us, and sometimes away, because it was in orbit around something else.  Detailed study (using the Newton/Einstein laws of gravity) allowed Hulse and Taylor to infer that what they were seeing was a pulsar orbiting a second neutron star.  They could even figure out the orientation and size of the orbit!

Having figured this out, they could do one more thing.  Einstein’s laws of gravity predict that the gravitational waves — waves in space itself — that are created by these two stars as they orbit one another, and these waves should be carrying energy out into space, reducing the energy available to the two stars.  The effect of this loss of energy would be a very mild reduction in the time (or “period”) that it takes for the two stars to orbit each other — but not by very much!  The period of the orbit, about 28,000 seconds, is predicted by Einstein’s equations to be shrinking by a bit more than one second per year.

Fortunately, pulsars are stable enough, and Hulse and Taylor’s measurements were easily accurate enough, that this change of about a second per year was relatively easy for them to measure during the ensuing decade.  And they could compare their measurements of the change in the period with the predictions of Einstein’s theory of gravity.  Remarkably, the agreement of the theory with the data is excellent!  For this confirmation of Einstein’s theory’s prediction of gravitational waves, Hulse and Taylor received the Nobel Prize in 1993.

Graph showing the cumulative shift of periastron time for PSR 1913+16. This shows the decrease of the orbital period as the two stars spiral together.
Hulse and Taylor’s data (black dots) for the reduction in the period of the neutron stars’ orbit compared to Einstein’s theory’s prediction (solid line) of how the orbit should change due to the emission of gravitational waves. (General Relativity is the name of Einstein’s Theory of gravity.) Notice how remarkably precise is the agreement over decades!

Hulse and Taylor had thus observed the effect of gravitational waves for the first time in human history. But they hadn’t observed the waves themselves; they’d observed the loss of energy, in the neutron star pair, due to the waves, but not the waving of space, compressing and expanding as the waves move by. Clearly, this detection of gravitational waves was indirect.

A Future, Likely Direct Detection of Gravitational Waves

A direct search for gravitational waves is underway now, at experiments known as LIGO and VIRGO.  When a gravitational wave passes by the Earth, space itself grows and shrinks a little bit, and the distances between objects increases and decreases.  It’s an incredibly tiny effect even for powerful gravitational waves; you and I would never notice it.  But this shrinking and growing of space can potentially be observed with extremely stable, carefully designed lasers looking for the distance between two mirrors to shift by less than the radius of a proton, which itself is 100,000 times smaller than the radius of an atom!  [The principles involved are not so different from those used in the famous Michelson-Morley experiment — but the experimental requirements are vastly greater!]

When the repeated changing of the distance between mirrors due to a stretching and compression of space is actually observed, that will clearly be direct observation of waves of space itself — gravitational waves.  This hasn’t happened yet, but the “Advanced” phase of LIGO is coming up very soon, starting this year.  We may well see LIGO make discoveries within the decade.

BICEP2: Direct or Indirect?

I think it’s very clear that BICEP2 — IF the experiment’s results are correct (they have not been confirmed by another experiment yet) and IF they are correctly interpreted as due to gravitational waves (which is still an open question) —represents an advance over the Hulse-Taylor discovery.  But it’s not as direct as LIGO, either.

BICEP2’s measurement [see here for some details]  is actually of the polarization of light  that was released 380,000 years after the Hot Big Bang began, at the time when the universe cooled enough to become transparent. This light has now become the “cosmic microwave background” [CMB] which we observe today coming from all directions in the sky.  So really they’re directly observing light (microwaves rather than visible light), not waves in space itself — gravitational waves.

But the nature and size of the polarization effect they observe (“B-mode” polarization, across large swathes of sky) is believed to have only one possible source: gravitational waves, created in the early universe and ringing for 380,000 years, and then interacting with the light that is now the CMB.  It is the squeezing and stretching of space within which the light is moving that causes the light to end up polarized in a unique way.

In this sense, you could say that the CMB is providing a sort of unusual photograph of gravitational waves, taken at 380,000 years post-Big-Bang.  It gives far, far more detail about their nature than does the Hulse-Taylor measurement; it confirms more and different things that Einstein predicted, such as the fact that these gravitational waves have “spin two”, which is necessary for them to give B-mode polarization.  If you think of it as a photograph, BICEP2’s measurement seems pretty direct.

But on the other hand, it’s nowhere near as direct as LIGO would be, where mirrors that humans have set up will actually move back and forth as a gravitational wave’s crests and troughs pass by.  Far, far more detail will be available when that happens — and there will be little or no ambiguity about the interpretation of the data.  For BICEP2, it’s still conceivable (though no one has thought of anything specific) that the B-mode polarization actually is not due to gravitational waves but is due to something else.  The very fact that this is conceivable — that maybe the polarization comes from something other than waves in space itself — reflects the fact that the BICEP2 data involves looking at something that happened billions of years ago in very distant locations, and drawing inferences.  BICEP2 isn’t itself seeing space shrink and expand; it’s observing polarized light created long ago, and then scientists are inferring that the pattern of its polarization is due to space shrinking and expanding. From that point of view, BICEP2’s detection is still rather indirect.

So call it what you will, it’s clearly (if correct and correctly interpreted) more direct than Hulse and Taylor’s measurement, and less direct than a detection at LIGO would be.  Maybe we should call it “(…nnnn)direct”?  In any case, what we call it isn’t important; what’s important is to figure out whether it’s correct, and what it means.

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57 Responses

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  4. It’s hard to see what the controversy is here. The detection, if that is what it was, was indirect. No one has detected gravitational waves. What has been detected, both in the CBR polarization measurement and in the neutron star orbital decay, is the indirect effect of otherwise unseen gravitational waves.

  5. It seem obvious that the quality of “direct/indirect” is used to label observations and theories that people find likable/strong or unlikable/weak. See the BICEP2 analysis here, or the problem that “direct” detection of gravitational waves are done in instruments that uses light to read of changes in lengths caused by the waves, ultimately converting them to descriptive data after arduous analysis.

    If I ask for a measure or better yet a test of the quality, I don’t seem to get an answer.

    I don’t necessarily agree with the idea that if something isn’t quantifiable, it isn’t science, a physics theory, an idea described here previously. In a weakened sense of science, anything that can be used as constraint, even qualitative properties, should be amenable. That is, if the core of science is hypothesis testing descriptions of observations and theories.

    But if “direct/indirect” isn’t quantifiable, it seems to point to a pre-science history. Scientists seems to find it useful, so maybe it shouldn’t be dumped outright. But [why] isn’t there a measurement theory describing “directness” or strongness?

  6. Last Friday (May 2) Marc Kamionkowski (Johns Hopkins Univ.), who developed the concepts underlying the BICEP experiment, gave a colloquium at the University of Maryland. His talk was extremely clear and cautious. First, about the mechanism. Consider a gwave (from inflation or not) having a long wavelength at the time of emission of the radiation that we now receive as the Cosmic Microwave Background. At one moment, a single linearly polarized gwave compresses physical distances along one axis, while expanding those along the perpendicular axis (where both axes are perpendicular to the wave’s travel). Half a cycle later, the formerly compressed axis becomes the expanded axis, and vice versa. This produces a quadrupole perturbation in the local temperature. The light that scatters from the electrons having that perturbed distribution of velocities is linearly polarized thereby, and we detect that polarization when the light enters our instrument. It follows directly from the mechanism described above that the pattern of the directions of the linear polarization of the scattered light directions will curve around, so that the field of little polarization vectors will have a curl. Approximately quoting Kamionkowski, “Density perturbations have no handedness, so they cannot produce a polarization pattern with curl. The perturbations produced by long wavelength gwaves do have a handedness, so they can and do produce a polarization with curl.” Based on models of inflation, Kamionkowski and colleagues had predicted predicted in the 1990s that the angular scale of the curled patterns would be about half a degree. (Their power spectrum would peak at el = 200.) Several other distinctive features were predicted. They all seem to be borne out by the BICEP2 data. But, as noted in the comment by Terry Ambiel, before concluding that the observed curling patterns of linear polarization are due to gwaves, it is essential to be sure that they are not mainly an artifact of scattering by the uneven distribution of dust in our own Galaxy. (The mere existence of that possibility verifies the indirectness of BICEP2’s indication of gwaves.) The BICEP2 teams has worked very hard on that possibility. Depending on how you count them, four to six tests all suggest that dust does not dominate the production of the observed pattern. None of the tests are individually conclusive, because anything concerning interstellar dust suffers from significant uncertainties. It is reassuring – but not conclusive – that none of the tests suggest dust as the cause. Other experiments now in progress should reduce the ambiguities considerably. One of them is named CLASS.

  7. A related question is, could the CMB provide a direct detection of gravitational waves? I think in principle it could. The scale at that time was about 300,000 light years, and of course gravitational waves move at c, so in principle the pattern would move at ~ 3 x 10^-6 radians / year, EXCEPT that these patterns are (of course) redshifted by a factor of ~ 1500, so the pattern would move (for us) at ~ 2 x 10^-9 radians / year. I think that’s beyond the resolution of our instruments, but if you waited (say) a million years, it would be pretty observable. I think that would qualify as a direct detection.

  8. Sorry, the data from the latest run of the LHC contradict everything in this post.

  9. Im sorry but this does not answer my question please read it through again if you wish to give it another go.

  10. The ensembled phenomena of Higgs mechanism is, unphysical scalar particles called the Goldstone bosons who have a derivative (thermodynamical axioms) mixing with the massive gauge bosons.
    Once the axioms are fixed, you can discuss the existence of undecidable propositions – like spontaneity in spontaneous symmetry breaking.

    The set of equations designed to describe some aspect of nature may have multiple `vacua’ (i.e. multiple solutions that each represent different ways that the universe could be configured — what empty space could be like, and what types of fields, forces and particles could be found in the universe).

    “it seemed obvious that the strong interactions are not mediated by massless particles” – Weinberg.

    Does photon interacts with higgs field to make photo electric effect (particle nature with physical force)?

    Incomplete ZERO mass into complete non-ZERO ?

  11. I have heard about how particles with fixed rest mass is just a consequence of quantum mechanics. But then why are photons completely free from such fixed energy’s and this must have something to do with the higgs field because interaction which the higgs field is what gives rest mass and thus sets the bar for how much energy it takes to make a particle with that rest mass. This is just a clarification of my previous post, i don’t want to cause confusion.

  12. I have an unrelated question concerning the masses of particles. I have heard that the reason that the masses of particles are fixed and not a mushy mix of energy is due to the Schrödinger equation, for example it takes 0.511 MeV to make one electron and only one because 0.511 MeV is the electrons rest mass. But does this apply to photons? or gluons? I mean the electromagnetic spectrum suggests that photons have absolutely no quantization in terms of how much energy a photon can have.(what i mean is there is no specific amount of energy that you require to make a photon eny amount of energy seems to do something to the electromagnetic field unlike the electron field. The main question is that does this have something to do with the higgs field. What i mean is that if the electron interacted with the higgs field more than it does now (therefore having more mass) then it must take more energy to create an electron from the electron field. The idea that I have basically fabricated is that the higgs field is responsible for the energy in particles when there at rest, and that the amount of energy it takes to make a particle from its underlying field is the energy that it takes to have a wave in that field that can exist while it interacts with the non-zero higgs field. And the reason it can take an arbitrary amount of energy to make a photon is that the photon particle does not interact with the higgs field. Please correct me if I am wrong because the whole quantization of electron quarks ect.. and the seemingly unquantization of photons (in terms of energy) has bean very confusing.

    1. A particle is a ‘stable, long-lived’ wave in a field. In order to create a particle you need to fulfill several conditions.(For example you need to ‘wiggle’ the field in a regular way so the resulting wave is nice and neat.) If you do not meet these requirements you will get a ‘virtual particle’ which cal ‘fall apart’ and give you your energy back.

      For massless particles energy is not a condition needed to make a particle. (There are some minor issues with uncertainty, two particles of similar energy will be indistinguishable because the uncertainties in their measurements will overlap.) In theory you could have photons and gluons of infinitely low energy.

      The Higgs field can be considered as taking two ‘photon-like’ fields and connecting them.so that a stable wave in one creates a stable wave in the other. But this requires ‘extra’ energy if the waves are to both be stable; it adds another condition. So while I can excite the electron-left field with any amount of energy the wave I create cannot be stable unless I give it enough energy (0.511 MeV or more) to stably excite the electron-right field too.

      It may help then to view the Higgs field as a ‘destabilizer’ that requires extra energy to overcome.

      1. Many thanks Mr. Kudzu, /there are some minor issues with uncertainty, two particles of similar energy will be “indistinguishable”* because the uncertainties in their measurements will overlap.
        The Higgs field can be considered as taking two ‘photon-like’ fields and connecting them.so that a stable wave in one creates a stable wave in the other./
        So one of the “photon-like field” was polarized (distinguished*) during inflation by gravitational waves ?

        In the spontaneity of spontaneous symmetry breaking (weak mixing angle), W± and Z0 bosons, and the photon, are produced by the spontaneous symmetry breaking of the electroweak symmetry, by the Higgs mechanism.
        “I spent the years 1965-67 happily developing the implications of spontaneous symmetry breaking for the strong interactions”- Weinberg.

        Higgs field stabilize the “rest mass” or shielding the containment – unlike gluons, which have its own antiparticle to have the short distance force. In weak interaction’s short distance force, it needs Higgs mechanism.
        Higgs mechanism was used to unite weak and electromagnetic interactions – making, short distance weak interaction, but not for photons, due to zero mass.
        This means “gauge bosons can get rest mass through Higgs mechanism”.

        But due to polarization, photons can take arbitrary amount of energy or give energy back (radiation) – means, photon field is massless (ZERO) and stable but Higgs field is massive and non-ZERO (distabilizer) ?

        1. A few points:

          When I was talking about ‘photon-like’ fields I meant those of particles affected by the Higgs. Gravitational waves polarize light by a different mechanism. It is rather tricky and I am not sure I understand it entirely but basically since the waves alter space itself in a certain way as they pass by they can ‘tweak’ the polarization of light in that space (which is initially undefined.) It is similar to how a lens can focus light from everywhere onto one point. (And gravitational lenses can also change the polarization of light.)

          Electroweak symmetry breaking is different from the Higgs mechanism. (If you like the Higgs mechanism happens with what is left of the Higgs particles AFTER electroweak symmetry breaking.) It does not unite the weak and electromagnetic forces; instead it MAKES them from two totally different forces (Isocharge and isospin) Both of these forces *were* long range with massless bosons but now only the electromagnetic one is.Symmetry breaking in effect created a ‘crippled’ weak force.

          The Higgs field doesn’t ‘stabilize’ rest mass, it *causes* it. (And not all of it either, the Higgs boson for example has a rest mass, but it doesn’t give itself that rest mass.) It can do this because it is nonzero all across the universe. Imagine if the electron field were like that; the universe would be filled with electricity, protons would not repel each other, all their positive charge would be blocked and so on.

          Polarization is a property of waves, they don’t need to be massless . If you wiggle a piece of string up and down that is polarization. Photons are massless because that is how ALL particles are ‘at the start’ unless something changes them and nothing changed the photon. (Well, it’s a little more tricky than that actually, but close enough.)

  13. I have a maybe pretty hairy question: Is there anything like fine-tuning of the universe for intelligent life? I know that there is the thory that a mutliverse allows for all possible options to be relaized and we just happen to be in this version of a universe where human life is supported. Is that the final answer?

    1. This comes in two flavors of ‘anthropomorphic principle’; the weak which says ‘Isn’t it interesting that the universe is just right for us to exist’ and the strong which says ‘The universe can be no other way than it is now’

      For the universe to support human life specifically a number of variables need to have rather specific values. Gravity is remarkably weak for example. So it can be argued the universe is ‘fine tuned’ but this begs the questions ‘why?’ and ‘by whom?’ As a christian you can guess my take on the matter.

      But there are many objections to this. Firstly if the universe is different it is quite possible life would be different. There might be living stars the size of basketballs sitting and marveling at how gravity was just strong enough to allow their existence. We simply do not have a good enough grasp at present as to what other options are out there.

      Secondly scientists hate a ‘just because’; all values of all variables should arise ‘naturally’ out of a theory and there are a lot at present that ‘just are’ with no explanation as to why they take those values. It is the hope of physics that at some future point a ‘theory of everything’ will be found that shows how our universe isn’t special at all and in fact could not be anything else.

      Multiverse theories are one attempt at doing this (As well as tackling a number of other problems\ideas); in this case the values aren’t fine tuned, our universe would *have* to appear *somewhere* so it’s not special.But these theories are mostly speculation with no definitive evidence at present.

      1. Well, “gap theory” makes your magical agency very powerless, doesn’t it? From “making” universes” to hiding in our not yet elaborated science.

        I would say that the gaps has gone, since inflation both made our observable universe from a spontaneous end of inflation going back to blown up quantum fluctuations and populated it with structure from similar quantum fluctuations. Indeed, the “magical seed” of a spacetime with physics that you propose, has been diluted by at least 10^100 which is far more insanely wrong (or weak, in your interpretation) than standard 10^30 dilution of the “magical seed” in homeopathy!

        Mostly, I wish that a science site wouldn’t have this unnecessary discussion of non-functional magical ideas. Why is it seen as acceptable? We aren’t pre-enlightenment pre-science pre-functional anymore…

        1. I find it a strength in this blog that such ideas are discussed. In many places there are strong limits imposed on what can be discussed. And these ideas are far from some lunatic fringe, they are real questions and issues people have. They should be addressed.

          Dismissing them as ‘pre-science pre-functional’ is about as useful as trying to convert atheists by saying ‘I’ll pray for you.’

    2. If the universe was fine-tuned for life, it obviously should have more that the 0.00000000001 % or so habitable volume than it has. =D

      There is one or two possibly fine-tuned parameters (cosmological constant, lifetime of the vacuum), all else has an open parameter space. E.g. Victor Stenger, and others, find that covarying parameters leave ~ 50 % or so livable universes. Then you can ask why those correlations, but no one has pinned such to anthropic (environmental) theory yet.

      1. I don’t see logically what the percentage habitable volume of the universe has to do with anything. A knife was meant to cut things but only a tiny percentage of it is cutting edge, the rest is uncutting support for that edge or a handle.

        Of course maybe most of the universe *is* habitable… for dark matter photino birds and we’re just some weird accident.

  14. Hello Matt, I red in your post that the fact that the gravitational waves have “spin two”, is necessary for them to give B-mode polarization.
    It is possible for you to give us some explanation or hint about why spin two is necessary for B-mode polarization ?
    Thank you very much for your time

  15. Matt, You say, “But the…polarization effect they observe … is believed to have only one possible source: gravitational waves, created in the early universe and ringing for 380,000 years….”

    When I think of “ringing” waves I think of standing waves, which implies a structure with reflective boundaries–which in this case would be ??? Or when you said “ringing” did you mean a traveling gravitational wave that was still propagating across the universe 380,000 years after the Big Bang?

  16. Matt: Question about interpretation of BICEP2. Some people say these gravitational waves follow from classical GR. So it has nothing to do with quantum gravity. Some others say that actually quantum fluctuations took place in Gravitational field. So it proves quantum gravity. What is your opinion?

    1. Yes, I would love to see a discussion of this. I think I know the answer, but so many people have said strange and contradictory things about it that I feel that there is an urgent need for clarification from someone who knows what he is talking about…

    2. Isn’t that a moot question? Krauss and Wilczek has found that it can only be quantized waves.

      “Thus the gravitational radiation background, measured invariantly, is proportional to ¯h^2. Since this is a positive power of ¯h, we infer the essentially quantum-mechanical nature of that phenomenon. Since no field other than gravity is involved, we infer that quantization of the gravitational field is an essential ingredient.” [ http://arxiv.org/abs/1309.5343 ]

      1. Oh, I forgot: They also ask for consistency checks, the ones BICEP2 has avoided so far.

  17. Folks I show in my paper http://www.worldscientific.com/doi/abs/10.1142/S0219887814500595 that gravitons are propapagated through spacetime in the same manner as heat. This implies that LIGO is actually detecting the thermal activity of gravitons. If my research is correct we will never detect an undulation propagating at light speed which is what we are expecting to see in all gravity wave detectors.

      1. That’s just rude. You may not think much of Lubos Motl, but he’s a well-known figure in HEP theory, having taught at places like Harvard that mostly don’t tend to hire doofuses, having obtained right academic degrees to have an opinion worth considering (versus we’ve no idea about you), he works in this area (versus we’ve no clue whatsoever what if anything you do), and I expect Matt Strassler appreciates his reaction to this article.

  18. It’s my understanding that gravitational lensing is another source of B-Mode polarization, however its contribution to polarized light can be predicted (or measured?) Or maybe it is more correct to say that one can place an upper bound on the contribution of gravitational lensing to B-Mode polarization? In any case, I beieve that the success of the BICEP2 exaperiment hinges on establishing (in the 5-sigma sense) that the amount of B-mode polarization that was experimently detected exceeds whatever contribution gravitational lensing could possibly provide.

  19. Minor correction, the period of the binary pulsar is not changing by 1s per year. That’s the cumulative change in periastron time.

  20. Hi Mat,

    As a layman I’m a great fan of your website.

    I read recently something about newly discovered galactic foreground structures that are not included in BICEP2 foreground models and hence may have an Impact on B-mode polarization measurement results… (galactic foreground emission caused by magnetic dipole radiation from dust grains enriched by metallic iron etc.) See: http://arxiv.org/abs/1404.1899

    Well, you’d pointed out repeatedly in earlier posts to be cautious with BICEP2 results (and Interpretation) unless they are confirmed by other experiment…

    Is this paper just ‘experts’ talks’ about ‘details’ or can this have a major Impact on BICEP2 results? Any idea?

  21. I heard someone from BICEP2 -I think it was Chao-Lin Kuo- describe the detection as semi-direct. I thin that is fair.
    He made the comparison with detecting waves on the surface of the ocean. You could measure them by watching a floating object move up and down (what A-LIGO will do) or you could take a still picture and see the wavy pattern on the surface (sort of the BICEP2, although of course the analogy breaks down in that specific palarization patterns require tensors).

  22. To me, a direct measurement of gravitational waves requires in some fashion measuring or observing the change in spacetime due to the waves. The binary pulsar decay doesn’t do that, but the Pulsar timing array, should it have success, would and thus would be a direct observation of gravitational waves. From that perspective, the BISON2 results come close, but they are still not there, and so are indirect.

  23. The BICEP2 results actually deal with the only gravitational waves thinkable in context of 4D general relativity: i.e. these stationary ones. From this perspective the BICEP2 observation is about true gravitational waves.

  24. I’m pretty sure vector modes sourced by cosmic strings can also produce low-l primordial B-mode polarization. See arxiv:1403.6105. Those aren’t tensor perturbations of the metric, so we don’t usually think of them as gravitational waves. But we might just have a semantic difference somewhere.

  25. At this point we’ve left physics and are only talking about nomenclature. If you want to be pedantic about it, all experimental measurements are indirect. We take physical phenomena and convert them to other types of physical phenomena which we’re better at measuring.

    The simplest example is converting a force on a spring into a displacement. A spring scale is a force-displacement transducer. A more modern version uses piezoelectric elements to convert (force) into (frequency shift) into (voltage) into (bits).

    Now the indirect BICEP2 measurement: (gravitational waves) into (quadrupolar radiation fields) into (polarized radiation) into (current in a phased array antenna) into (raising the temperature of a load on a bolometer) into (resistivity of a TES) into (magnetic flux through a SQUID) into (volts) into (bits).

    Compare that to what LIGO will do: (gravitational waves) into (electric field phase difference in an interferometer) into (power incident on a photodiode) into (volts) into (bits). Still indirect, just fewer steps in the process.

    But again, this is now nomenclature, not physics.

    1. You’re not even wrong here. The crucial difference between an indirect and a direct measurement is how well the physics of the intermediate steps are known. The early universe might do bizarre things we have no idea about; this is even expectable. Whereas, how a gravitational wave might cancel out its own signal in an interferometer would require a very contrived explanation or new physics. Slipping into solipsism is the result if your logic is followed.

      1. You are not completely wrong here. =D The number of steps isn’t crucial, just pointing out the problem of quantifying “direct” in a measurable way.

        Your “how well the physics of the intermediate steps are known” has a similar problem. Ordinarily knowledge is that goes into specifying uncertainty based on random and relative errors. But you allude to “unknown unknowns” and how contrived they can be. By definition those are unmeasurable and contrivance impossible to specify, again raising the head of solipsism. Kudos for “new physics” vs “no new physics” though. But that would go into the area of open field/possible not yet eliminated alternative theories (physics)/possible not yet eliminated ingressing mechanisms, which by definition is the constraint a new measurement always work with.

      2. Also, about “new physics”. That (possible ingressing mechanisms) is peculiar of this new measurement. Others may take place in more familiar territory.

  26. How come gravitational waves cause light passing through them to end up with a particular polarization?

    1. This is due to geometry and statistics. It is similar to how a lens can focus light from many points into one focal point; the original light comes from random directions while the focused light heads towards a specific point. Gravitational waves alter space that light is passing through and (very, very roughly speaking) ‘lens’ the polarization in a specific kind of way.

      This is a poor explanation but I hope it suffices.

    1. The characteristics of “empty space” suggest to me that this space or “vacuum” is indeed quantized. And that it is the dynamic structure of “empty space” that determines the universal basis for time and the Higgs field. The “CMB” is evidence of the dynamic nature and “temperature” of the structure of empty open space; variations in the “CMB” is indicative of the shape of this structure.

  27. If space is the medium for those waves , then space is a thing ….is it possible that spaceness is not fundamental ? Does the status of space as a continuous or discrete medium affect what we see ? Is space is Quantized what does that mean for those waves ?

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