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I was basically repeating/interpreting what the fine article said:

FRBs show a frequency-dependent dispersion, a delay in the radio signal caused by how much material it has gone through.

I believe interstellar dispersion of electromagnetic waves is caused by interactions with ionized interstellar medium. A quick Google(interstellar dispersion) brought up this page which gives a formula that relates the integral of electron density along the path of the signal to the dispersion of the signal. On that page they assume they know the density of interstellar medium inside our galaxy and use the dispersion of signals from pulsars to estimate their distance. In the FRB experiment they did it the other way around and used the known distance (using the red-shift of the source galaxy) and the dispersion to estimate the integral of the density. Integral measurements such as this usually give much more accurate results than point measurements.

If the interstellar medium were entirely made up of ionized hydrogen then knowing the electron density would give you the total density since there would be one proton for every electron. You need to add corrections because only about 70% of the interstellar medium is hydrogen so you need to estimate the number of neutrons. You also need to make a small correction since the interstellar medium is not 100% ionized.

The reason why the dispersion is related to the electron density is given, for example, in J. D. Jackson's Classical Electrodynamics where the dielectric "constant" (and hence the speed of light) is shown to be a function of the frequency. The electromagnetic wave causes the electrons to wiggle (just a little), the higher the frequency, the less wiggling so the higher frequencies are slowed down less than the lower frequencies. You can think of it like having an eccentricity in a front tire of your car that makes the steering wheel vibrate. When you go fast enough so the frequency of the oscillations is much greater then the resonant frequencies of the components then the amount of vibration you experience goes down because the direction reverses before things can move very far. In the interstellar medium each electron slows down the wave just a little and the total amount of slowing down is proportional to the number of electrons encountered.

Usually, the "closeness" of the electrons is not considered, instead the integrated electron density is used. But it is possible to calculate how close the electrons have to be to a line between the source and the receiver using what is called the 3-point function. The cross section of the volume that contributes to a fixed percentage of the signal will be roughly elliptical with the source and the receiver at each focus of the ellipse. Given the vast intergalactic distances involved, it will probably be extremely wide near the middle by any earthly standards but this width is independent of the calculation of the dispersion. I'd imagine the width would scale as sqrt(d * c * t) where d is the distance between the source and the receiver, c is the speed of light, and t is roughly the duration of the signal (more accurately, the inverse of the bandwidth). The reason the closeness (the width of the 3-point function) doesn't matter is because the more spread out the 3-point function is, the weaker it is. All that matters is what you get when you sum it all up.

And when you dismiss all data that doesn't agree with you-- which is what you're doing-- then it is completely impossible to ever overturn your conspiracy theory that all the science ever done on climate happens to be wrong.

Let me also note that apparently, it is possible to observe solar activity prior to direct observation by measuring carbon 14 in tree rings as a proxy. As a result, it is claimed that there were other periods of lowered solar activity from about 1000 AD through to the Maunder minimum.

So, what you're now saying is that the little ice age is not due to the Maunder minimum, but you're hypothesizing that it might have been due to some other sunspot minimum for which we have only proxy data.

Unfortunately,
(1) proxy data on solar activity is somewhat harder to interpret (see, for example, review article here: http://solarphysics.livingrevi... )
  (2) nobody looking at the record of proxy reconstruction has been able to find a firm correlation to global climate (although there are some regional climate correlations),
(3) there still isn't any accepted mechanism connecting sunspot number to climate,

For example, there were periods of alleged reduced solar activity between 1280 and 1350 and between 1460 and 1550.

This analysis looks like you have a result you want, and you're going back through the data trying to select data to try to fit the result. If this were actual science, you would need a correlation coefficient. How well does the variation in (proxies for) sunspot number fit the variation in (proxies for) climate?

All of the scientists doing the actual studies of this sort say the effects seen are far too small to explain the current warming trend: see, for example Solanki et al. 2004 study of sunspot numbers over the past 11,000 years and climate: http://www.nature.com/nature/j... "we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades", or the review of dozens of studies here: http://www.skepticalscience.co...

But, returning to the topic, we seem to agree: the little ice age cannot be attributed to the Maunder minimum.

Franson's theory cannot be right, as it disagrees with the solar system tests of General Relativity

His Equation 18 predicts a change in the gravitational red shift by a factor of 9 alpha / 64 for photons, where alpha is the fine structure constant (~ 1/137), so the correction is ~ 1.08 x 10^-2. The gravitational red shift has been tested, by GPS and also by Gravity Probe A, with an accuracy of a few parts in 10^-4 (see Figure 3 in that reference). This excludes the Franson correction, and so his theory cannot be correct. Since the Shapiro delay also depends on the gravitational redshift, Franson's theory thus predicts a 1% change in that too, which is also much too large to be consistent with experiment (see Figure 5), again excluding the Franson theory.

So the theory is wrong, and the other problems I have with the paper are irrelevant.

Franson's theory cannot be right, as it disagrees with the solar system tests of General Relativity

His Equation 18 predicts a change in the gravitational red shift by a factor of 9 alpha / 64 for photons, where alpha is the fine structure constant (~ 1/137), so the correction is ~ 1.08 x 10^-2. The gravitational red shift has been tested, by GPS and also by Gravity Probe A, with an accuracy of a few parts in 10^-4 (see Figure 3 in that reference). This excludes the Franson correction, and so his theory cannot be correct. Since the Shapiro delay also depends on the gravitational redshift, Franson's theory thus predicts a 1% change in that too, which is also much too large to be consistent with experiment (see Figure 5), again excluding the Franson theory.

So the theory is wrong, and the other problems I have with the paper are irrelevant.

It might be a bit of an oversimplification to call this a test of relativity.

Relativity consists of special relativity (SR) and general relativity (GR). GR includes gravity.

SR has been tested in many different ways to extremely high precision. Here is a summary of experimental tests of SR. Note that even if the faster-than-light neutrino result from CERN/Gran Sasso is correct, it doesn't necessarily conflict with SR. SR doesn't forbid FTL. It only forbids an object from being accelerated from a speed less than c to a speed greater than c.

Here is an article on tests of general relativity. A nice popularization of this kind of thing is the book Was Einstein Right? by Clifford Will. Although GR has not been as thoroughly tested as SR, it has been tested in many different ways. There is not really a heck of a lot of doubt that it's right in many ways. Alternative theories exist, but they are extremely tightly constrained by observation.

We expect that Sagittarius A* is a black hole, and the definition of black hole basically means that it has an event horizon. If, contrary to everyone's expectations, it turns out not to have an event horizon, the most likely interpretation may not actually be that GR is wrong. It may actually mean that there is something wrong with relativistic particle physics. It's possible that the process of formation that we think leads to a black hole actually stops short of forming a black hole, and instead forms some other exotic object. There are various speculations about these things: gravastars, fuzzballs, quark stars, boson stars, q-balls... If we found out that Sgr A* was one of these hypothetical critters, it would be very exciting for the particle physicists, but it would not disprove GR.

Per the referenced article at http://relativity.livingreviews.org/Articles/lrr-2003-1/ the GPS handles the relativistic effects but the European Space agency alternative to GPS "GALILEO project [25] states that relativistic corrections will be the responsibility of the users (that is, the receivers)" so some GPS analogs don't take into account the relativistic effects. Assuming they used GPS and not GALILEO, the relativistic effect seems not to be the problem. The documentation for my car/marine GPS unit says that it does not work well around tall trees or tall buildings. I would assume mountains might cause problems too. I also understand that humidity or air pressure changes can have an effect.

It's bogus. (Yes, I am a physicist.) OPERA used portable atomic clocks, which were moved to the the two labs and then synchronized via GPS (see this article). GPS thoroughly incorporates general relativity (which includes special relativity). It has incorporated GR ever since it was first built, because if it didn't, it wouldn't work. At all. No, not even well enough for hiking and driving. Here is a review article on relativity in GPS. GPS uses coordinates called Earth-Centered Inertial (ECI). These are coordinates (t,r,theta,phi), where the spatial coordinates are spherical coordinates that rotate along with the earth, and t is the time coordinate of a hypothetical observer in a nonrotating frame at rest relative to the center of the earth. General relativity is completely agnostic about what coordinate system you use, so this choice of a coordinate system is not a choice that has any physical significance; it's just a bookkeeping thing. Van Elburg assumes that GPS was constructed by people who didn't understand relativity, and therefore GPS times need to be corrected for relativistic effects. That's just completely wrong.

Yes and no. GPS would work pretty well without GR and just having tiny adjustments applied periodically (ie: GR shift is simply another source of error to correct for even if the underlying cause isn't known precisely).

This page gives a big stack of math to back that up. GR shift accounts for something like 4 parts in 10 billion. It accumulates over time to be sure, but its not severe enough to prevent GPS working without it.

Of course, I don't know that the full QED is really needed for all of our modern day electronics either, so it my example could be just as overzealous as requiring GR for GPS ;). I suspect though that our ability to control EM and light waves will give QED a much stronger (practical applications) role in the future even if its not entirely there yet.

Then again, there may come a day when our space exploration blossoms past a few unmanned probes here and there. Then GR might become more relevant in day-to-day life (even then though, I'm not sure what scale we have to reach before we worry about it to any great extent. Definitely inter-galactic. Probably inter-stellar. Inter-planetary? Not so sure).

If you look here:

http://solarphysics.livingreviews.org/Articles/lrsp-2008-3/

You will see (in section 4.4) a table (from a 2007 study) listing the lengths of the 19 Grand Maxima in the Holocene. The one that has just ended is the fourth longest. If you look in section 4.1 you will see a graph that quite clearly indicates that the one just ended was the strongest peak since 9000 BCE when there was a double peak (basically one protracted event) following closely on the heels of a similar set of events around 9500 BCE that quite possibly were the proximate cause of the end of the ice age. The events around 6700 BCE were of slightly longer duration but much lower magnitude.

Not to get into duelling experts, but these are rather current results and the review paper covers the science used to obtain them in detail. Usoskin is a Solanski collaborator, so I would have to say that the 2007 results supercede the 2005 results. I'd also say that either way there is no doubt that the twentieth century Grand Maximum was unusual in both its duration and its strength. This is a confounding factor that is universally ignored -- seriously -- in the arguments for GHG-mediated AGW. One of the most basic premises of AGW is that the heat we are experiencing is "unprecedented" (a word frequently used) on a millennial timescale, because if in the past it has been just as warm without CO_2, it confounds the entire argument. This is why so much of their conclusion has depended on "erasing" the Medieval Warm Period -- if it was even close to being as warm in the MWP as it is now with nothing but solar variability (through mechanisms known and unknown) as a cause, CO_2 stops being primary and risks being dropped to secondary -- as you say, responsible for only a relatively small part of the warming of the last century, barely enough to compensate and leave us reasonably warm as the sun returns to its normal state (which is much less active).

To put it in the simplest terms, the argument as usually presented is: "The temperatures are abnormally warm. The Sun's state is normal. CO_2 is abnormally high -- the highest in recorded history. Temperature linearly responds to (correlates with) solar state and CO_2 concentration. Therefore CO_2 is the proximate cause of the abnormal warming."

This argument is obviously, if not deliberately, misleading. The correct statement is: "The temperature is abnormally warm. The Sun's state is abnormally active -- the highest in recorded history. CO_2 is abnormally high -- the highest in recorded history. Temperature responds nonlinearly to changes in solar state and probably to CO_2 concentrations as well. It is therefore probable that both are the proximate cause of the abnormal warming."

The thing about the second argument is that it reveals two serious weaknesses in the hypothesis "CO_2 is responsible". First of all, it makes it clear that this is almost certainly not true. Both are almost certainly factors. Second, it introduces a rather huge degree of uncertainty into the discussion. You clearly have a lot more faith in GCMs than I do, given that they have almost no predictive power and damn little explanatory power. Ultimately, they are nonlinear parametric curve fitting routines that fail to extrapolate. One of the key parameters is the climate sensitivity, which is basically unknown. They are not capable of predicting global temperatures across thousands of years from just solar data because there are so many confounding factors -- it is quite probable that global oscillations such as the PDO are major factors in mean heating and cooling with multidecadal timescales and that those oscillations undergo more or less random/chaotic shifts to completely different modes every thousand years or so, with significant perturbations of the existing modes along the way. We are (in my opinion) far from understanding

Or, the warmers might point out that we finished the quietest sun-spot cycle in well over a century...

Really? Which one was that? Surely none of these: http://solarphysics.livingreviews.org/Articles/lrsp-2008-3/ -- which seem to show the earth as just having finished a patch of solar activity unequalled in eleven thousand years, right back to the very beginning of the Holocene circa 9000 BCE. Or we could zoom in on just the last four hundred years -- http://solarphysics.livingreviews.org/open?pubNo=lrsp-2010-1&page=articlesu28.html -- hmmm, kinda hard to see us "finishing" the quietest sunspot cycle in well over a century -- the last cycle of the twentieth century is what, the fifth or sixth most active over the last four hundred years (and by extension from the previous figure, part of a patch of the most active cycles over thousands of years).

Oh, do you mean the current solar cycle? The one that isn't yet at its peak? Yes, cycle 24 is well on its way towards being the quietest one in a hundred, or even two hundred, years. The end of cycle 23 was also significantly delayed. And if the article (referenced above) is correct, and we're about to start a Maunder minimum -- well, all one can say is that there is a name for the last Maunder minimum, clearly visible on the figures above. It was called "The Little Ice Age". Or at least, that's what it was called before Mann et. al. managed to "erase" it with some clever fiddling and cherrypicking of their proxy data.

Alas, I'm not even a Republican, and I don't bet, but it would be nice if AGW "enthusiasts" could at least keep their solar cycles straight, since they are so very certain that the fact that the twentieth century hosted one of the most intense grand solar maxima of the Holocene is completely irrelevant to those "high global temperatures" observed at the very end of it, after two of the most intense solar cycles in literally thousands of years.

rgb

Or, the warmers might point out that we finished the quietest sun-spot cycle in well over a century...

Really? Which one was that? Surely none of these: http://solarphysics.livingreviews.org/Articles/lrsp-2008-3/ -- which seem to show the earth as just having finished a patch of solar activity unequalled in eleven thousand years, right back to the very beginning of the Holocene circa 9000 BCE. Or we could zoom in on just the last four hundred years -- http://solarphysics.livingreviews.org/open?pubNo=lrsp-2010-1&page=articlesu28.html -- hmmm, kinda hard to see us "finishing" the quietest sunspot cycle in well over a century -- the last cycle of the twentieth century is what, the fifth or sixth most active over the last four hundred years (and by extension from the previous figure, part of a patch of the most active cycles over thousands of years).

Oh, do you mean the current solar cycle? The one that isn't yet at its peak? Yes, cycle 24 is well on its way towards being the quietest one in a hundred, or even two hundred, years. The end of cycle 23 was also significantly delayed. And if the article (referenced above) is correct, and we're about to start a Maunder minimum -- well, all one can say is that there is a name for the last Maunder minimum, clearly visible on the figures above. It was called "The Little Ice Age". Or at least, that's what it was called before Mann et. al. managed to "erase" it with some clever fiddling and cherrypicking of their proxy data.

Alas, I'm not even a Republican, and I don't bet, but it would be nice if AGW "enthusiasts" could at least keep their solar cycles straight, since they are so very certain that the fact that the twentieth century hosted one of the most intense grand solar maxima of the Holocene is completely irrelevant to those "high global temperatures" observed at the very end of it, after two of the most intense solar cycles in literally thousands of years.

rgb

Seriously, just look at this:

http://solarphysics.livingreviews.org/Articles/lrsp-2008-3

The solar activity levels are at their highest in ten thousand years, and have only been this high a handful of times in the entire Holocene. You're trying to understand a curve by looking at noise at the very end, where it isn't even properly detrended. Oh, and you might try overlaying any temperature reconstruction of the last thousand years that you happen to be fond of onto the end of this, so you can actually see the correlation between the grand maximum we have been in for the last 150 years and the increasing temperatures over the last 150 years. If it's one that actually still shows the medieval optimum and little ice age the effect is more dramatic, of course, but suit yourself.

I'm not precisely sure by what measure the last three solar cycles represent a meaningful decrease in solar activity, given that the peaks are all very nearly the same height and are in any event so very, very high compared to the mean over the Holocene. Now this cycle -- it is going to be a low one. Very low, in fact. But don't worry. If, by any chance, the AGW hypothesis turns out to be incorrect and the warming we've seen is almost entirely due to Mr. Sun, then the fact that a Little Ice Age now would only kill what, a billion people, well, what the heck. That's why they call it "science". We'd better just hope that the completely uncontrollable sun behaves itself and doesn't regress towards the mean or anything"unlikely" like that.

rgb

Excuse me, what does the UAH lower troposphere global mean have to do with continental temperatures in Antarctica, specifically?

I wasn't claiming that the Earth's temperatures were or were not warmer -- I was referring to the fact that Steig 2009 finds substantial warming in Antarctica as an artifact of poor methodology. This is, in fact, clear. It is also clear that the mean temperature in continental Antarctica dropped from 1980 to 2010, whether or not it rose elsewhere. The penguins are safe from being cooked where they stand.

There has indeed been warming of the globe since the Dalton minimum in the 1800s. It has been directly associated with an increase in solar activity across most of that time, especially in the late 19th and 20th century (which has had some of the highest solar activity in the last three solar cycles observed in the last twelve thousand years, see: http://solarphysics.livingreviews.org/Articles/lrsp-2008-3/). There is a very strong correlation between solar activity and global temperature over that entire range, one that is completely evident in the "hockey stick" increase in solar activity at the right hand end of the graph (where the blue areas following 1000 CE are the Wolf, Oort and Maunder grand minima) leading up to the peak of a grand maximum, one that looks like it could be the highest in ten thousand years (just like the alleged global temperature). What a coincidence!

Could it be, do you think, that solar forcing could be tied to some aspect of solar state, something that climatologists seem to want to vehemently deny (because if true, everything they've done and claimed for twenty years will turn out to not only be wrong, but dangerously, expensively wrong)? Bear in mind that anthropogenic CO_2 over this interval looks nothing like this and is absurdly decorrelated from global temperature across the entire range except the last 150 years, right there in that confounding pesky solar activity grand maximum.

There are two distinct problems in climate research today. One is that to even a casual observer who actually looks at the data instead of uncritically believing what is being trumpeted in the newspapers every few weeks, global warming is not necessarily "just" due to anthropogenic CO_2. It is a fact that the thermal record of the entire Holocene makes it extremely implausible that CO_2 is solely, or even primarily, responsible for the modest twentieth century temperature increases. This, however, is a point that you will never find mentioned in any of the canonical papers warning of the dire consequences of CO_2 warming egregiously forced with an assumption of extreme climate sensitivity to multiply the otherwise uninteresting temperature increase to alarming. Sensitivity that remains completely unproven, especially given that current models do not include solar state in any significant way. Indeed, the keepers of the global climate models would have you believe that solar state is irrelevant to global temperature in spite of the fact that the overall Holocene data says otherwise quite convincingly.

The second is that climatologists need to learn how to do statistics without bias! I'm not accusing Steig of "lying", but I am accusing the author(s), the journal, and the referees of not just this paper but many of the AGW papers of sloppy statistics, laziness and too-easy acceptance of flawed methodology. Egregious claims are made and backed up with methodology that, when carefully examined, often turns out to be almost painfully flawed starting with MBH. The really sad thing is that climatologists seem unwilling to police themselves and outsiders such as Steve McIntyre have to hammer through the critical examination of methodology when the published results (such as the infamous "hockey stick") are just plain unbelievable, obviously wrong from the beginning if you just

The onboard clocks run slower (and thus need to be corrected) because, for the satellites to be in a geostationary orbit at that altitude (IOW, to keep the same angular velocity than Earth), they need a linear velocity that's much faster than Earth's.

This is incorrect. The satellites are not geostationary. Also, the gravitational time dilation effect is stronger than the special-relativistic time dilation due to motion, so the net effect is that the clocks run faster. Here is a discussion of the physics. The clocks are tuned to 10.22999999543 MHz, which is perceived on earth's surface as 10.23 MHz.

Disclaimer: I work on loop quantum gravity.

Actually there IS something special about loop quantum cosmology - it's theory actively predicts a big bounce instead of a big bang. This comes directly out of the loop quantization of a homogenous and isotropic cosmology. So far all other theories have had to put in a bounce "by hand" - adding extra physics at the singularity in order to get something out of the other side. LQC doesn't do that - it replaces the usual metric and curvature operators with holonomies and flux operators as done in loop quantum gravity (OK, the derivation isn't exact yet, and we've a lot more work to do here).

Once you do this, however (and by using other tricks like using a massless scalar field as your time variable), you see that a contracting branch naturally re-expands once you reach a critical matter density (something like 82% of the Planck density - Ashtekar has a good numerical reason for this IIRC). In these steps you end up replacing the Wheeler-deWitt equation (a continous differential equation) with a difference equation which needs to pick a certain super-selection sector of the theory - in simpler terms the timestep effectively becomes discrete.

The beauty of LQC is that it doesn't need us to speculate about what happens at singularities - it gives us an active way to look at them without needing to invent new physics that only apply there. Sure, it makes a few assumptions - that our basic observables are holonomies and fluxes - but there's no new input directly at the singularity, unlike in other theories (such as ekpyrotic scenarios where two branches are joined artificially across the singularity.

For an introduction, see Martin Bojowald's (one of the founders of LQC) living reviews site:
http://relativity.livingreviews.org/Articles/lrr-2005-11/

If you have questions, please reply and I'll see what I can do to answer them to the best of my ability. If there's enough interest, I might be able to get an "Ask Slashdot" type of thing put to Ashtekar/Bojowald although it'll probably be their post-docs and grad students who end up answering all the questions ;)

Solar pyhsics is not my field, hence I have found it rather staggering to read some of the recent advances. Earlier this year, I bought "The Sun" by Jay M. Pasachoff ("The Complete Idiot's Guide To" Series). The TRACE telescope in space observes the sun using different filters. Some of these are in the extreme ultraviolet, including a 195 Angstrom filter that observes Iron (Fe) that has lost 11 of its 26 electrons, an event that occurs at 1,500,00 degrees C. Rather than the sun just being one giant bar magnet, it also has smaller regions of polarity. From these polarized regions, ionized gas loops out hundreds of thousands of km into space. Here is an article I found that simulates the current prevailing theory "that active regions on the solar surface originate from strong toroidal magnetic fields generated by the solar dynamo mechanism at the thin tachocline layer at the base of the solar convection zone." http://solarphysics.livingreviews.org/Articles/lrs p-2004-1/ whose author looks kinda cute: http://www.hao.ucar.edu/Public/about/Staff/yfan/

Here you go.

http://www-astronomy.mps.ohio-state.edu/~pogge/Ast 162/Unit5/gps.html

(This link is more math intensive)
http://relativity.livingreviews.org/Articles/lrr-2 003-1/node5.html

Decades of work has indicated more than three ways to make QM compatible with GR, including: string theory (AKA M-theory), twistors, and loop quantum gravity (LQG). However, it is suspected by some that, just like St. Patrick would tell us, these three are actually different facets of the same underlying reality. (Just like different interpretations of QM don't actually produce different predictions.)

I will be the first to admit that eventually there will be some limit to how small we can make a transistor (or transistor replacement) it seems that we still have a ways to go.

I knew all that research I did for my novel might come in handy one day. :-)

The theoretical limits of information and computational density (based on quantum density limitations and reletavistic constraints on signalling, i.e. speed-of-light limits) are Bremermann's Limit and the Bekenstein Bounds, and we're one hell of a long way away from that. Practical limitations may be an order of magnitude or two less ... which we're also nowhere near.

The problem with GPB is that it measures a pretty uninteresting effect and takes a lot of money to do so.

Why is the effect uninteresting? According to the parameterized post-Newtonian (PPN) formalism, which describes most reasonable extensions of Newtonian gravity, frame dragging is a combination of only two effects: the amount of curvature of space caused by matter and lack of spatial isotropy, each given by a parameter. In GR, those parameters are 1 and 0, respectively.

Now, we know the amount of spatial curvature caused by a mass. With that, frame dragging of the amount predicted by GR is pretty much a given unless there is significant lack of isotropy.

So, GPB becomes a very expensive test to see whether space is isotropic. But even at that, GPB isn't a very good test: if its results disagree with General Relativity, we learn something, but that result is very unlikely and there would be better ways of looking for anisotropy. If GPB's results agree with GR, however, we have learned nothing, because there are many ways in which this particular experiment could fail to observe anisotropy even if it exists.

GPB's results should agree with GR. If they don't, then the most likely explanation is an engineering mistake. If they do, GPB will be hailed as a great "confirmation" of GR, although in reality, we will have learned nothing.

There are critics of Einstein that are academically serious and not off their rocker like some zero point/tesla fanatics.

There have been critics of Einstein ever since he released his theories.

You don't hear much about them as they are all heaped into one group and astrocized.

There are critics of Einstein that are academically serious and not off their rocker like some zero point/tesla fanatics.

There have been critics of Einstein ever since he released his theories.

You don't hear much about them as they are all heaped into one group and astrocized.

However, there are an infinite number of gravitational theories you can write down, described by the space of post-Newtonian parameters (PPN formalism). GPB puts constraints on the possible values of some of the parameters, parameters which heretofore have been largely unconstrained.

Thanks for the nice response. I did some digging. It turns out that the only two PPN parameters that are related to frame dragging are the amount of space curvature produced per unit mass (gamma) and a parameter related to preferred frame effects (alpha1).

Since the amount of curvature produced by a mass is known fairly well from other observations, that means that GPB mainly becomes an attempt to identify a preferred frame effect (alpha1 significantly different from zero). Note, however, that if the experiment yields a result that agrees with GR, we can't even place an upper bound on alpha1, since there are plausible (indeed, probable) scenarios in which there are preferred frame effects but we happen to get a result that agrees with GR in this particular experiment.

So, GPB cannot exclude any alternative theory to GR: after GPB returns a result that agrees with GR, the same set of alternative theories will be valid as before.

Overall, it seems that we can say that GPB is only a test that, if we are lucky and GR is wrong, may find (but not exclude) of a preferred frame.

(A good site for this stuff seems to be Living Reviews in Relativity.)

Plus other articles on the web.

--jeff++

If you say that this experiment "confirms" GR, then it also "confirms" many theories that otherwise wildly disagree with GR.

I would concur.

The assumption is often that, since a particular mechanism was proposed along with a particular equation, that if the equation is right, the mechanism has to be.

That simply doesn't have to be the case, any more than deriving an equation describing the falling activity of Slashdot threads would lend 100% credence to my hypothesis that the light from the Slashdot home page casts light evenly on the topics, and thus causes activity, until they pass beyond the Oldnewsii Shell at some distance and get progressively more shielded from the light by other articles. :)

There are plenty of means to get to the same sorts of relationships as GR presents in terms of light bending. Etheric, tired light, and other particle, flux, or field-based theories can arrive at the same observations.

Sometimes it's a primacy (who got there first) or popularity contest. That's perhaps sad in a way, but the devil is really in the anomalies - sometimes brushed away as 'error', other times requiring some other body acting on it (that can lead to discovering Pluto, or a need for dark matter). The anomalies are where alternate explanations might really prove their mettle.

Here's an example:

There are anomalies in the in-track acceleration of the LAGEOS satellites.

Do you try for the General Relativity solution (section 3.7.3)? Or something "wilder" (near the bottom)?

I guess it's easier to say "X confirms GR" than "X confirms list of equations here", but it does imply that much more in physics has been 'set in stone' than actually has been.

You may also be interested in reading about loop quantum gravity, an alternative theory of everything. I' not expert, better refer to this reference that I looked up.

I guess we have have some sort of picture of what things were like at one plank time after "time zero". This is something like 10e-43 seconds. Which sounds extremely close to zero. Almost unimaginable close. But then I think, as long as you don't actually go back to the singularity, time is continuous, so there's just as much to be discovered between 10e-86 and 10e-43 seconds as there is between 10e-43 and 1 second (and between that and 10e+43, for that matter). But then I think that it's very naive to assume that time is continuous in the same sense that the real numbers are continuous. In fact, it seems like some theories actually imply that space and time are discrete. Check out Smolin's book, Three Roads to Quantum Gravity for a nice introduction to the topic.

Well, as far as I know, the same thing applies to electrostatic fields (charged bodies) only.

Magnetic fields are excluded from this weirdness, as anybody who looks at the light coming from the sun knows that the sun's apparent position is where it was about 9 minutes ago.

-- but this linear extrapolation is encoded fully in only A's retarded position and velocity.

This is precisely the problem, moron.

Juvenile as ever, Louis.

A's retarded position and velocity was imparted to the propagating field disturbance at the time when the change happened. How did A know of its true position/velocity relative to B at that time, pray tell?

If B responded to A's true instantaneous position or velocity, you wouldn't get something that agrees with either experiment or GR theory.

Bullshit! Newtonian gravity is extremely accurate over galactic scales.

You can also see this in electromagnetism. Coulomb's law is directly analogous to Newton's inverse square law of gravity. But as soon as you try to make Coulomb's law consistent with SR, you move from electrostatics to full Maxwellian electromagnetodynamics -- in fact, this is one way to derive Maxwell's equations -- and in the process, a finite wave propagation speed is enforced. Ad-hoc attempts to keep an infinite propagation speed fail. I seem to recall there is a good treatment of this in a text that discusses early alternatives to Newtonian gravity in the context of Mercury's perihelion precession, but I forget the reference.

The only discrepancy that general relativists have been able to come up with (and this is still being debated by many) is the perihelion advance of Mercury.

This discrepancy can easily be ascribed to the lack of terms for motion-induced and gravity-induced clock-slowing (aka time dilation) in Newtonian gravity.

In any event (and this is not my specialty, so take this with the standard grain o' salt), the reason that a collapsar is expected to stop collapsing at neutron-star stage is that the neutron degeneracy pressure (basically, the Pauli Exclusion Principle in action) is able to resist the mutual gravitational forces up to some limit -- by calculation, 1.4 to 1.8 solar masses, although it appears that a value of 2.3 solar masses has been observed. Clearly, though, greater densities can exist, because if the limiting mass is exceeded the collapse continues -- to form a black hole, if you accept the present standard formulations of the problem. It's just that we don't know of a stronger force than neutron degeneracy, which will be able to resist the gravitational collapse. During the formation event (typically a supernova), if the collaspe forces (gravitation, implosion) exceed the neutron degeneracy forces, there's nothing to stop the continued collapse (through higher densities) to a black hole. Our lack of knowledge doesn't mean there's not a further stable state, though -- only that we don't know about it. Some scientists have speculated that a further point might exist in a quark star, which would consist (at least in its core) of free strange quarks. But some models of quark stars end up with lower densities than neutron stars... the problem is that we just don't know enough, yet.

(BTW, there's good info here on neutron stars, from a specialist.)

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Spacetime explained through knot equations? You may be thinking of loop quantum gravity (slightly easier article also available).