Bruce Allen: What I said in my lecture was that GEO-600 is not the most sensitive of the gravitational wave (GW) detectors, because it is smaller than the others. However it has served as an important laboratory for developing and testing technology that was then later adopted by the other GW detectors. Currently it is the only detector in the world testing "squeezed light". Probably those methods will later be adopted by the larger (LIGO and VIRGO) instruments.
I don't see any paradox. The different detectors world-wide share a common goal: to detect GWs. They have developed somewhat different technology, because the different groups have different resources, different expertise, and most important, different opinions about what methods might work the best.
In gravitational wave physics it is a little more complicated because each group needs the other but they have different histories and different interests. GEO600 is unlikely to contribute much directly to the detections that ought to arriving from the late 2010s because it is so small but the group can contribute a lot to the development of the science that will go into the larger detectors. It may well be the smallness of the group and their detector that allows for the flexibility needed to pioneer new approaches. Quite properly, GEO600 will take its place among the discoverers when the first terrestrial detection comes along. Virgo is much larger, with 3 kilometer arms, and ought to be as sensitive, or nearly as sensitive, as the 4 kilometer detectors built by the Americans due to ingenious features of the design. So far, however, it has tended to lag behind the Americans in the speed of development and in fulfilling the promise of the technology -- perhaps because they have fewer personnel. For this reason, or because Italy has been the source of what are now counted as incorrect claims, Virgo tends to be very cautious in interpreting signals. The Americans, who own the two largest and most advanced detectors -- the devices without which there could be detection -- have shown themselves to be very generous in spirit -- always ready to share the honours with the smaller detectors. I would say the collaboration is very healthy though there is the potential for disagreement when the first detection comes along if it is marginal.
Bruce Allen: Our field will become new science with the LIGO and VIRGO instruments later this decade, not with LISA, which is about 20 years in the future. There is another effort underway to detect very low frequency gravitational waves, using "pulsar timing arrays" or PTAs. This effort might succeed at the same time, or later, or even earlier than the LIGO and VIRGO efforts. It will be scientifically very interesting, but complementary (different sources) to the LIGO and VIRGO efforts. White was talking about PTAs.
What is the chance of Planck to detect primordial gravitational waves through the so-called B-polarization mode?
Bruce Allen: I would really be happy if the Planck satellite could detect the B-mode signature of primordial GWs. However my understanding is that they are probably not strong enough for Planck to detect them: a follow-on mission would be needed.
Would cosmic strings lead to a observable gravitational wave background?
Bruce Allen: Cosmic strings produce GWs. So if they are formed with high enough density, and at the right time in the history of the universe, then yes, they could produce an observable background. This is a topic that I worked on a lot many years ago, and published several papers about. However there is until now no evidence that our universe ever contained cosmic strings. So it's a long shot!
The so-called Anthropic Principle (AP), in its many forms, attempts to explain why our observations of the physical universe are compatible with the life observed in it. From the Weak AP (WAP), which in one form states that "conditions that are observed in the universe must allow the observer to exist", to the Strong AP (SAP) which in one version states that: “The Universe (and hence the fundamental parameters on which it depends) must be such as to admit the creation of observers within it at some stage,” they all try to answer the question of why there is life in the universe, or why the fundamental constants are the way they are. But, do any of these principles add anything to our understanding of the ultimate question of life and the universe?
Perhaps the best answer is embedded in Martin Gardner’s sarcastic proposal of the Completely Ridiculous Anthropic Principle (CRAP): “At the instant the Omega Point is reached, life will have gained control of all matter and forces not only in a single universe, but in all universes whose existence is logically possible; life will have spread into all spatial regions in all universes which could logically exist, and will have stored an infinite amount of information, including all bits of knowledge which it is logically possible to know. And this is the end.” Is this logical conclusion our only chance?
Bruce Allen:: the Anthropic Principle, that the universe is the way that it is, because otherwise we would not be here to observe it, has always seems rather sterile to me. Of course one can't argue against it, but on the other hand there is absolutely no way to falsify it. So I have consider it to be a philosophical rather than a scientific statement. The fact that it can't be falsified, and thus can't be tested, puts it for me in the category of questions like "how many angels fit on the head of a pin" or ""what happens when an unstoppable force meets an immovable object?"
Thank you very much Prof. Bruce Allen and Prof. Harry Collins, specially for sending me this quote for Urania!
"My feeling that I have seen the dark side of the Moon is not so different from my feeling that I have seen a cup and saucer ...
That we do not feel terribly disadvantaged by the indirect quality of the seeing in the case of the dark side of the Moon is quite striking, but the really odd thing about the second spatial dimension of seeing reveals itself, as might be expected, when the seeing gets stormy. In heavily disputed areas, people who are far removed in social space from the instruments of seeing are often more certain about what has been seen than those who actually peered through the instruments. A big part of the sociology of seeing concerns the way that those distant from the instruments of seeing come to learn about what has been seen, and the way that they form their often very strong opinions.
One element in the explanation of this strange phenomenon is that, as a general rule, as you move from the scientific core group the message gets simpler and more straightforward. Those in the core group are aware of every argument and every doubt as it unfolds, whereas those a little more distant have their views formed from more-digested sources. In the nature of things, digested sources must simplify. The medium of transmission has, as it were, too narrow a “bandwidth” to encapsulate the sum total of the core’s activities. Other things being equal, then, “knowledge waves,” at least weak knowledge waves, behave in the opposite way from gravitational waves. Gravitational waves weaken as they spread; knowledge waves get stronger.
Because weak knowledge waves get stronger as they spread, it is hard for most of us to remember how faint they were at their source. In a modern science there are often only a very small number of scientists “looking through” the instrument and handling the strings of numbers that emerge. The study of seeing is about how this indirectness, this faintness, and all this scope for disagreement get turned into the kind of widespread certainty that allows us to say the equivalent of “We have seen the dark side of the Moon” and one day will allow us to say, without fear of contradiction, “Gravitational waves have been directly detected.”
Gravity's Ghost: Scientific Discovery in the Twenty-first Century. University of Chicago Press, 2010. pp. 11-12.
The Wave Hunters
The Wave Hunters - 01 - Expanded and Squeezed
The Wave Hunters - 02 - Stretched and compressed
The Wave Hunters - 03 - Extremely sensitive
The Wave Hunters - 04 - Squeezed light
The Wave Hunters - 05 - New York, New York
The Wave Hunters - 06 - Clean Optics
The Wave Hunters - 07 - Tailor-Made and Remote-Controlled
The Wave Hunters - 08 - High Tech in the Cornfield
The Wave Hunters - 09 - A World Record
The Wave Hunters - 10 - Needles in Space