A science blog with Eric Berger

11 for ’13: Henry Pownall believes in gravity














Eric Berger

14 Responses

  1. Old Gringo Stan says:

    So true….just as the mystery of the universe…..only the religious have answers :)

  2. craig says:

    Do we have any idea of even where to start to look for the answers? Is anyone actively seeking them or are we so far away from the answers that scientists are merely hoping to stumble on them in the course of doing what now would seem like totally unrelated studies?

    The ability to control the force of gravity would certainly make space travel more manageable. If our UFO sightings are real than their inhabitants have probably got this figured out.

    • JohnD says:

      Is anyone actively seeking them

      Oh, my, yes. We’ve been looking for the answers for several centuries. Right now, there are several experiments going on that may lead to an answer. The most obvious is the Large Hadron Collider, which may have detected the Higgs Boson which may mediate mass and thus interact with gravity.

      Slightly less well-known outside of the physics community is the continuing negative results at the Laser Interferometer Gravity-wave Observatory. Though it may not seem like it, these negative results are upending what we thought we knew about how gravity propagates and so may lead to an even better theory of gravity which may give us ways of manipulating the force.

      And then there are the dark matter experiments and the DUSEL experiment and…

    • DanM says:

      JohnD: the (so far) negative results at LIGO have not yet up-ended anything. The sensitivity of the LIGO detection system at this point is not enough to rule out anything due to non-observation. So nobody is anticipating any revolutions yet, on the basis of the results so far. Now, after they implement advanced LIGO, if the still don’t see anything, then we may have to start thinking about revolutions. But that result is still at least several years off.

    • JohnD says:

      DanM, when LIGO was first proposed, they said that there would be a discovery each year at a minimum. Though they have described the negative results as positively as possible, it is nevertheless true that what we have observed and what we expected to observe are totally different.

    • DanM says:

      “what we have observed and what we expected to observe are totally different.”

      I think that’s correct, but it is not because of any surprises or modifications to general relativity or anything else that we know about gravity. It is instead because of the difficulty in forecasting the sensitivity of the measurement technique, and to a lesser extent in the frequency of astronomical events that would produce a relatively large signal. It turns out to have been harder than anticipated to make LIGO sensitive enough within the relevant bandwidth to detect events that are, in the end, fairly rare.

      Advanced LIGO should provide the necessary sensitivity to make definitive statements about GR. But current measurements cannot do so. The surprises here (so far) are in the detector performance, not in the physics of gravity.

    • JohnD says:


      I think that we almost agree here. It isn’t just the sensitivity of the detectors that is important; the size of the signal also matters. And the early researchers thought that the signal from the events would be large enough to be detected by LIGO (otherwise, we wouldn’t have built the darn things). Instead, it turned out that any gravity waves generated by astronomical phenomena are much, much smaller than predicted. And that did require a rethinking of how gravity works or at least of how gravity waves are generated.


      And again, that assumes that our new understanding of how gravity waves work and of how LIGO will interact with them is correct.

      It will be very interesting to see how our understanding changes if LIGO fails again.

    • DanM says:

      We are almost agreeing. But there are some important clarifications to make. The idea that gravity waves are interacting less sensitively with the detector than anticipated is not quite right. The design of LIGO has always (from before it was funded) been that it would be capable of detecting certain types of astronomical events (e.g., neutron star coalescences) if they happened above a certain mass threshold and within a certain distance of earth (distance matters because the signals drop like 1/r^2 from the source, just like for any spherical wave). The fact that we’ve seen no events so far is not an indication that the gravity waves are interacting more weakly with the detector – instead, it is an indication that none of the special events that emit large-amplitude gravity waves have actually occurred within the required distance and during the LIGO data runs.

      So, I maintain my original statement: the null result so far has not changed our understanding of the way gravity waves are generated or the ways that those waves interact with LIGO (or anything else). Nothing has changed about the ways that LIGO researchers are computing the expected signals for a given amplitude of gravity wave incident on us from some event in space – the physics of generation, propagation, and interaction with matter are all unchanged by the results so far. The calculations are the same now as they were when the original proposal was submitted.

      On the other hand, it may be that these negative results are changing our understanding of the frequency of the kinds of events that would have generated LIGO signals, if they had occurred. In other words, we are not learning anything new about gravity yet, but we may be learning something new about the likelihood that two neutron stars will run into each other.

    • JohnD says:

      DanM, it appears that we are on different sides of that debate. You are with the group that holds that the gravity-wave generating events are rarer or further away than expected, whereas I am with the group that holds that the number and distance of events is correct but either the size of gravity wave generated by them or the interaction of the wave with the detector is not in accord with previous expectations. (Nature being what it is, the true answer is probably “all of the above”.)

      What is true is that something didn’t go according to plan. And, speaking as a tax-payer, I would really like some assurance that the next generation LIGO will actually detect gravity waves before we spend money on it.

  3. Danzmark says:

    There is no gravity – the earth SUCKS !

  4. DanM says:

    “The absence of gravity in a vehicle orbiting the earth shows us just how dependent we are on gravity.”

    This is what happens when you let an expert in one field comment on a different field, in which he (obviously) is not an expert. As any freshman in physics 101 can tell you, the vehicles in orbit around the earth are NOT experiencing zero gravity. They are experiencing free-fall. It is not at all the same thing. Confusing people by getting this wrong in a discussion about the nature of gravity is, well, shameful.

  5. Jim says:

    If gravity has black holes,
    why can’t magnatism have ‘um?