标签: gravity

Engineers and scientists exist in a world defined by many metrology standards and constants. We start with time, mass, and length, and then expand to electric current, temperature, and many others. There are also fundamental physical constants such as the speed of light or Avogadro's number.   While all these constants are important, some of them are far removed from our daily lives. But one is not: the gravitational constant G. Even since Isaac Newton formulated the law of gravitational attraction F = G (mass1 × mass2)/r2, inspired by that apple falling from a tree , the value of G has been of great interest. Given how pervasive and accessible gravity is, it should be pretty easy to measure G accurately, right?   Well, yes and no. It turns out that gravity is easy to measure, but hard to measure with precision. A fascinating article in the latest issue of Physics Today, " The search for Newton’s constant ," discusses the history of measuring G. It looks at the various experimental setups that have been used over several hundred years (torsion-balance, pendulum, beam-balance, and others) and the data spread in results of each. Some of the sophisticated tests by serious researchers produce results with low uncertainty, yet they differ significantly from other tests, which also claim low uncertainty.   While researchers have certainly improved the accuracy and precision of their results, the article explains why G is still so hard to measure. It's not only an interesting, well written article, it's also a sobering and thought-provoking one as well, because you likely assumed that G's value is pretty much nailed down solid, end of story.   Yet, as most engineers and scientists know, getting consistent, accurate results in any test-and-measurement challenge to better than three or four significant figures is rarely easy. Every added significant figure means ever-more-subtle sources of error must be uncovered, understood, calibrated out, or compensated for in the fixture and equipment.   If you're lucky, the test can be structured so some of these errors actually drop out, or self-cancel, much as the value of mass m cancels out in some basic physics experiments and even carnival rides, such as the "rotor ride" or Gravitron (Figure 1) where participants "stick" to the wall via centripetal force and friction. The mass of the person doesn't matter, only the size of the rotor, the speed of rotation, and the coefficient of friction between their clothes and the wall (Figure 2). (If you can't explain why the person sticks, and why their weight is not a factor, go to a basics physics book.)   Figure 1 The Gravitron's rotor spins and pins people to the wall. A functionally similar but more sophisticated version is used in centrifuges to acclimate astronauts to high-G environments. (Source: NASA)   Or maybe there's another explanation about the elusiveness of a precise, accurate value of G, one that keeps physicists and metrologists worrying: Perhaps the "squared" exponent in the denominator of Newton's Law is not exactly 2.0 out to as many places as you care to pick. Or maybe G itself is not a true constant, but actually changes slightly over time and place. Stranger things have happened; just ask those physicists who believed in the absoluteness of time and distance, but had to change their beliefs to accommodate the curvature of time and space, as well as time dilation itself and even E = mc², as Einstein's 1905 paper on Special Relativity became accepted principle.   Figure 2 The rotor ride spins and people inside the cylinder stick to the wall, irrespective of their mass. Riders are subject to three forces: weight, normal force, and frictional force. (Source: stuegli.com )   Have you ever had a constant or fixed assumption in engineering or science that you had to abandon or at least become flexible about? Have you ever stopped and wondered what "gravity" is, as well? What are your thoughts are gravity waves and gravitational frame-dragging, as Gravity Probe B is exploring?

For engineers and scientists, the world is defined by many metrology standards and constants. We start with time, mass, and length, and then expand to electric current, temperature, and many others. There are also fundamental physical constants such as the speed of light or Avogadro's number.   While all these constants are important, some of them are far removed from our daily lives. But one is not: the gravitational constant G. Even since Isaac Newton formulated the law of gravitational attraction F = G (mass1 × mass2)/r2, inspired by that apple falling from a tree , the value of G has been of great interest. Given how pervasive and accessible gravity is, it should be pretty easy to measure G accurately, right?   Well, yes and no. It turns out that gravity is easy to measure, but hard to measure with precision. A fascinating article in the latest issue of Physics Today, " The search for Newton’s constant ," discusses the history of measuring G. It looks at the various experimental setups that have been used over several hundred years (torsion-balance, pendulum, beam-balance, and others) and the data spread in results of each. Some of the sophisticated tests by serious researchers produce results with low uncertainty, yet they differ significantly from other tests, which also claim low uncertainty.   While researchers have certainly improved the accuracy and precision of their results, the article explains why G is still so hard to measure. It's not only an interesting, well written article, it's also a sobering and thought-provoking one as well, because you likely assumed that G's value is pretty much nailed down solid, end of story.   Yet, as most engineers and scientists know, getting consistent, accurate results in any test-and-measurement challenge to better than three or four significant figures is rarely easy. Every added significant figure means ever-more-subtle sources of error must be uncovered, understood, calibrated out, or compensated for in the fixture and equipment.   If you're lucky, the test can be structured so some of these errors actually drop out, or self-cancel, much as the value of mass m cancels out in some basic physics experiments and even carnival rides, such as the "rotor ride" or Gravitron (Figure 1) where participants "stick" to the wall via centripetal force and friction. The mass of the person doesn't matter, only the size of the rotor, the speed of rotation, and the coefficient of friction between their clothes and the wall (Figure 2). (If you can't explain why the person sticks, and why their weight is not a factor, go to a basics physics book.)   Figure 1 The Gravitron's rotor spins and pins people to the wall. A functionally similar but more sophisticated version is used in centrifuges to acclimate astronauts to high-G environments. (Source: NASA)   Or maybe there's another explanation about the elusiveness of a precise, accurate value of G, one that keeps physicists and metrologists worrying: Perhaps the "squared" exponent in the denominator of Newton's Law is not exactly 2.0 out to as many places as you care to pick. Or maybe G itself is not a true constant, but actually changes slightly over time and place. Stranger things have happened; just ask those physicists who believed in the absoluteness of time and distance, but had to change their beliefs to accommodate the curvature of time and space, as well as time dilation itself and even E = mc², as Einstein's 1905 paper on Special Relativity became accepted principle.   Figure 2 The rotor ride spins and people inside the cylinder stick to the wall, irrespective of their mass. Riders are subject to three forces: weight, normal force, and frictional force. (Source: stuegli.com )   Have you ever had a constant or fixed assumption in engineering or science that you had to abandon or at least become flexible about? Have you ever stopped and wondered what "gravity" is, as well? What are your thoughts are gravity waves and gravitational frame-dragging, as Gravity Probe B is exploring?

A few days ago, I received an email from my chum Arthur Smith, one of the world's foremost experts on cosmic rays, who now devotes himself to beekeeping and making homemade soap. Arthur likes to keep his finger on the pulse of happenings in physics and cosmology and suchlike. He directed me to a UniverseToday.com article saying rumours were flying that gravitational waves had finally been detected.   (Source: NASA) As you are doubtless aware, the Russian-American theoretical physicist Professor Andrei Dmitriyevich Linde was one of the first to postulate the inflationary universe theory, along with the theory of eternal inflation and the inflationary multi-verse. The inflationary universe theory explains what may have happened the first fraction of a second after the universe appeared in the Big Bang. As I was writing this post, CNN announced " gravitational waves detected ." We are talking about the detection of primordial gravitational waves. These ripples in the very fabric of space and time carry echoes of the Big Bang from nearly 14 billion years ago. Furthermore, they may offer evidence supporting Professor Linde's theories. But wait, there's more. I also just received an email from my friend Javi Garcia-Lasheras in Spain, who has been doing some rather interesting work at CERN. He also likes to keep his finger on what is happening in the space-time continuum. He pointed me towards a YouTube video in which Chao-Lin Kuo, an assistant professor of physics at Stanford, surprises Linde with evidence that supports his cosmic inflation theory. I have to say that this brought tears to my eyes. Linde and his wife are totally unsuspecting; they recognise Kuo but have no idea why he's there until he says, "We have five-sigma evidence." That basically means "We have an extremely high degree of confidence." When Linde's wife realises the significance of what's being said, the expression on her face makes me want to cry. Physicists around the world are racing to analyse these results further. If they are confirmed, it means another gigantic step towards a theory of everything that unites quantum mechanics and gravitational effects and furthers our understanding of the universe. Just wait until I call my mom in England and tell her what's going on.  

Several years ago, my mother (now 80 years old) studied for four years or so and got a degree in something or other. She didn't need it—she had retired years before—but as she said, she had never had the time or opportunity to get one earlier. I remember her telling me that one of here lecturers posed a question about atoms and she happily gave him an answer – only to discover that everything she knew was out of date. As she informed the lecturer, she had been taught about atoms three times in her life, and every time she was taught something different. The point of this story is that we tend to think we know it all—especially those of us how are involved in science and high-technology—but in reality we've still only scratched the surface of everything that there is to know. For example, I think most of us remember being taught at school that nothing can travel faster than the speed of light. Of course there is the awkward fact that when two particles are entangled at the quantum level, changes to one are instantaneously reflected in the other, which would appear to violate the speed of light. Some folks get around this by saying "It's only information that is propagated faster than light," to which I roll my eyes. If something is going faster than light, I don't care what it is – it's still going faster than light. (Einstein famously referred to entanglement as "Spukhafte Fernwirkung" or "Spooky action at a distance" .) Leaving quantum entanglement aside, however (which is what physicists like to do because they can't explain it – at least not to my satisfaction – although that might be because I'm an idiot), almost every physicist on the planet would say that no atomic particles – including light itself – can travel faster than the speed of light. At least this is what they would have said up until several days ago when scientist at CERN announced that they have, in fact, clocked neutrinos traveling faster than the speed of light ( Click here to see a full-up article). This reminds me about the current conundrum with regard to gravity. We currently have quantum physics to describe the actions of the very small – and we have Einsteinian gravity to describe the actions of the very large (well, the macro level which is larger than the quantum level) – but we haven't yet found a way to tie the two together. It's generally accepted that we don't yet have a complete picture, and one or both of these fundamental theories is either incorrect or incomplete. Now if you suggest that Einstein's theory of General Relativity is not 100% complete, most folks will look at you warily and tend to back away in case you are about to do something crazy. But think of my mother being taught different versions of "the truth" with regard to the atom. We are learning new things all the time. In 1687, for example, Sir Isaac Newton described his theory of Universal Gravitation. At first this theory seemed to completely describe the motions of the planets and the stars and led to the idea of a "clockwork universe". One very interesting aspect to all of this occurred when astronomers began to realise that there was a problem with regard to the "anomalous precession of the perihelion of the planet Mercury" (which is the clever way of saying that Mercury wasn't orbiting the Sun as expected). The folks of the time absolutely believed in the theory of Newtonian gravity, so they looked for an explanation in this context. The idea they came up with was that there was an – as yet undiscovered – planet (which they called Vulcan ) in orbit between the Sun and Mercury. Based on this proposal, many folks devoted huge amounts of effort and ingenuity trying to find a planet that we now know does not exist. Then Albert Einstein came along with his theory of General Relativity. Amongst other things, this accurately predicted the orbit of Mercury without the need to introduce a "fudge factor" in the form of a non-existent planet. For close to 100 years, General Relativity has been accepted by the majority of folks as fully describing gravity. But once again there's a problem. Astronomers have discovered that the stars at the edges of rotating galaxies are travelling much faster than they should be... so fast that they should fly off into space... but they don't. In order to address this, folks have come up with the concept of Dark Matter . The idea in a nutshell is that Dark Matter is something we can't "see" or "taste" or anything like that... except through its gravitational interactions (the posh way to say this is that "Dark Matter is hypothetical 'stuff' that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter" ). Doesn't this seem a little strange to you. It certainly does to me. The idea is that we are so accepting that General Relativity fully defines gravity that when we make observations that don't fit we simply invent some invisible matter to make everything work. And don't even get me started about other galaxies like NGC 4736 which seem to lack Dark Matter. So, if it turns out that the scientists at CERN are correct and some particles do travel faster than the speed of light, then we will have to come up with an explanation for this. Maybe this explanation will refine our understanding of gravity (maybe we will no longer need to use Dark Matter as a fudge factor—or maybe we will gain a better understanding of what Dark Matter actually is). Maybe it will redefine our understanding of quantum physics. Maybe it will allow us to finally come up with a Unified Field Theory that ties "quantum" and gravity together (or explains one in terms of the other, or vice versa ). And maybe ... just maybe ... we may one day have faster-than-light spaceships that will carry us across our galaxy and, possibly, across our universe... See also my book reviews on the following: A Short History of Nearly Everything by Bill Bryson The Smart Swarm by Peter Miller  

Please try to remember my blog on the three puzzles to ponder . Someone wrote me an email with a question regarding this blog to which I'm sure I know the answer, but...   As you may recall, the third puzzle was concerned with Gravity, which – of course – explains the title of this blog. What do you mean "What does gravity have to do with dogs and the groins of strangers?" What do they teach the young people of today? Who amongst us could forge the immortal words of the great American philosopher Dave Barry, who famously said: "Magnetism is one of the six fundamental forces of the universe, with the other five being gravity, Duct tape, whining, remote control, and the force that pulls dogs toward the groins of strangers."   But we digress... the problem I originally posed was to assume that the circumference of the world is exactly 24,000 miles at the equator. We know that the earth spins round once every 24 hours, so at the equator this is 24,000 miles in 24 hours, which equates to a rotational speed at the surface of 1,000 miles per hour (no wonder my hair always looks so mussed up).   Anyway, the question I posed was as follows: "If you are standing at the equator, the spin of the world is sort of trying to throw you off (if you see what I mean). So, assuming that someone has a mass of exactly 100kg if we were to weigh him or her at the North or South Pole, what would the reading on the weighing scale be if our subject was standing on it at the equator?"     Over time we added all sorts of qualifiers, such as the weighing scale being calibrated in such a way that being moved to the equator did not affect it, and so on and so forth.   The thing is that someone who shall remain nameless to protect the innocent – let's call him Simon (you get 10 extra points if you correctly identify the animated film that prompted me to use the name Simon) – emailed me with a query, that spawned a flurry of messages, which evolved into a telephone converzation, whose conclusion left both of us scratching our heads...   Simon: There is of course the difference between mass (an inherent property of matter, independent of gravitational field, velocity, or acceleration) and weight (which is the force felt by the Earth pushing up on your feet, dependent on your mass and the Earth's gravitational field and any vertical acceleration). The mass doesn't change. The gravitational field of the earth doesn't change. Since there is no vertical acceleration, the measured weight will be the same. That's my theory, anyway.   Max: But the Earth is spinning, so the guy will weigh less at the equator than at one of the poles because the spinning earth causes centrifugal (or centripetal?) force...   Simon: Suppose you were standing at the equator when the speed of the Earth's rotation suddenly doubled – you would fall over backwards because the acceleration would be in the horizontal direction – this is elementary stuff they teach at high school.   Max: I must have been off school that day. So here's another example, suppose I have a rock on the end of a piece of string and I'm spinning it round and round really fast in a vertical plane such that it passes my feet and then goes above my head...     Max: So now let's assume that the string breaks (or is cut by a razor) at the point when the rock is directly above my head. In this case the rock has a mix of inertia in the vertical direction and motion in the horizontal direction, so it will fly up and across, sort of thing...     Simon: Of course it won't. At the point that the rock is directly overhead, the only motion it has is horizontal, so it will take off in a horizontal direction:     Max: You are making my head hurt. Let's return to my original problem, but let's assume that there is no atmosphere. Suppose I'm standing on the equator of an airless object like the moon that is spinning really, really quickly. Are you saying that I won't weigh less at the equator than I do at one of the poles?   Simon: That's right.   Max: Suppose that it's spinning incredibly quickly – like 100,000 times an hour, surely it would fling me off into space.   Simon: No it wouldn't because all you would have would be horizontal motion, but no vertical acceleration.   Max: Arrrgggh! Look, imagine that you're looking down on the equivalent of the North Pole on the moon and you see me standing in a spacesuit on the equator. Let's assume that, from your point of view, I'm standing on top of the moon, which is spinning at some rate in a clockwise direction:     Max: If the moon suddenly disappeared at this point in time, I agree that I would carry on traveling in a horizontal direction off to the right. Now, even if the moon doesn't disappear, inertia will cause my body to want to continue travelling in a horizontal direction. However the gravity of the moon will pull me down, but due to my horizontal motion I will appear to be lighter – that is, although I will have the same mass I will appear to weigh less. If I gradually kept on increasing the rotational speed of the moon, there would come a time when my tendency to keep travelling horizontally (my inertia coupled with my horizontal velocity) would be more than the gravity of the moon and my feet would leave the ground.     Simon: No they wouldn't.   Max: Yes they would.   Simon: No they wouldn't.   Max: Yes they would.   ... This is pretty much where we left things ... with me obviously winning the argument (grin) ...   Actually, now I come to think about it, I could use that Physics modeling program Phun to model the one about the string with the rock (. Well, I could if I had the time, but as usual I am up to my ears in alligators fighting fires without a paddle ... maybe you could use Phun to test this out and report back to the rest of us...