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#1
Oct907, 06:05 AM

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Can someone clear up what is observed in Shapiro time delay experiments.
As I understand it a delay is seen in the round trip signal time of a radar signal sent to a distant planet on the opposite side of the sun due to gravitational effects. The signal path would be longer than the straight line path to the calculated position of the planet in its orbit due to deflection by the gravitational field of the sun. This longer path would introduce a geometric delay. Would it be right to assume the Shapiro time delay is a measured delay over and above the the simple geometric delay? 


#2
Oct907, 12:39 PM

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As I understand it, the Shapiro time delay will include both the longer geometric path, and the effects of gravitational time dilation.



#3
Oct907, 12:52 PM

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Eugene. 


#4
Oct1507, 05:32 AM

P: 1,473

Shapiro time delay?
From the vantage point of the earth observer, the speed of light passing near the sun's surface of radius r is c[(12GM/(c^2)r] but the local speed for an infinitely small physical distance and time is still c.



#5
Oct1507, 01:32 PM

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Eugene. 


#6
Oct1507, 02:30 PM

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If the local speed of light were not constant, General Relativity would be falsified. General Relativity to date has a fairly impressive amount of experimental support, certainly it does a good job of modeling transit times and light paths in the solar system, including the shapiro effect. Note that "the speed of light" near the sun as seen from Earth is not even measurable unless one devises some synchronization scheme. What Yogi computes above is the "coordinate speed" of light in the Schwarzschild coordinate system, i.e. dr/dt, where r is the Schwarzschild r coordinate, and t is the Schwarzschild t coordinate. This can be and usually is interpreted as the coordinate speed of light viewed from an "observer at infinity", (so Yogi is talking about the Earth as if it were infnitley far away from the sun here), but there are conceptual ambiguities about talking about speeds at distant events. Measuring dr/dt is not really a very fundamental way to measure speed, because (among other reasons) r is not really a distance measure, it is just a coordinate. The situation is rather similar to someone who claims that the speed of naval ships increases near the poles of the earth, because d(longigutude)/dt is greater, i.e. the ship can cover one degree of longitude in a shorter time near the poles than it can at the equator. If you look at the actual local speed of the naval ship relative to the ocean, it is constant  the apparent increase in d(longitude)/dt near the poles is an artifact of the curvature of the Earth. Philosphically, lattitude (usually denoted by [itex]\phi[/itex]) and longitude (usually denoted by [itex]\lambda[/itex]) on the Earth are coordinates, not distances. The metric of the curved Earth's surface converts small changes in these coordinates into actuall distances. Because the Earth's surface is curved, the metric coefficients are not constant, assuming the Earth's surface is constant one can write this metric as [tex]ds^2 = r^2 d\lambda^2 + r^2 \cos^2 \, \phi d \phi^2[/tex] (I've taken the liberty of assuming one converts from degrees to radians rather than write out the conversion explicitly) here ds is the distance on the Earth's surface, and [itex]\phi[/itex] and [itex]\lambda[/itex] are the lattitude and longitude as previously mentioned. Compare this to the Schwarzschild metric: it's very similar as far as the angular terms go. (Note that equator of the earth is [itex]\phi=0[/itex] while the equator in Schwarzschild coordinates is [itex]\theta=\pi/2[/itex].) Similarly, the local speed of light is a constant everywhere  what could be called the "change" of dr/dt with r is an artifact of the curvature of spacetime, not an physical change in speed, just as the change in [itex]d\lambda/dt[/itex] for our naval ship as it got closer to the poles (i.e. as [itex]\phi[/itex] increased) was. The confusion comes about in large part by not distinguishing between coordinates: on the Earth, lattitude and longitude, in space: Schwarzschild r, theta, phi, and t coordinates, and distances. Distances are computed from small changes in the coordinates via the metric. Note that in relativity, one actually computes the value of the spacetime interval. 


#7
Oct1507, 04:10 PM

P: 1,746

Hi pervect,
you are correct that my post about the varying speed of light is not consistent with the ideas of general relativity. I simply wanted to note that the Shapiro time delay can be explained without GR's assumption of the geometry change in gravitational fields. An alternative explanation  that the speed of light slows down according to [tex] c' = c(1\frac{2GM}{c^2 r}) [/tex] also works. Eugene. 


#8
Oct1507, 06:18 PM

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http://xxx.lanl.gov/abs/astroph/0006423 takes a somewhat similar point of view, I think. We like to talk about peerreviewed theories here. This is a peer reviewed theory that I think (hope) is simlar to what you are talking about.
It's also an example of a nongeometrical interpretation of something that is locally the same as GR. Unfortunately, this is not quite the same as being equivalent to GR. It is a different thoery because it doesn't necessarily have the same global topology as GR does. The above paper unfortunately doesn't go into or even mention the topology issues. There used to be some criticism of this theory, informally called "funky fields in a Minkowski spacetime" that I would refer people to on the WWW, but it disappeared :(. You can probably find some past PF discussions if you google. I think the idea has promise for getting rid of pesky time machines in GR, but that's a personal opinion, and there are still some unanswered questions as to how one deals with the topological issues, and it is also not quite clear how the theory deals with black holes. (Do said "funky fields" become infinite on the event horizon? Or what?). 


#9
Oct3007, 03:42 AM

P: 3,967

Thanks for all the thoughtful replies and sorry for the delay replying (family issues).
We all seem to be agreed there would be a measurable time delay compared to a constant speed of light over the the coordinate distance. Yogi and Eugene interpret the cause as due to the speed of light slowing down in a gravitational field while Pervect has the more formal point of view that the speed of light is constant in a gravitational field when measured in 4 dimensional curved spacetime rather than the 3 dimensional space coordinates we are used to. This aproximation has the interesting property that the solutions for r < GM/(c^2) do not involve imaginary numbers. It also implies negative gravity and negative time for r < GM/(c^2). I wonder if any real experiments have been carried out that can distinguish f' = f[1GM/(c^2)r] from f' = f sqrt[12GM/(c^2)r] ? I would like to suggest a thought experiment with 2 scenarios to explore the possibilities and further clarify what is thought to be happening in a strong gravitational field. Thought experiment: Suppose we had optical fibre ring that had a radius of [tex] \frac{1}{2\pi} [/tex] light seconds when the ring is far from any gravitational fields. A device is spliced into the ring that can inject a photon into the optical fibre ring and times the interval for the photon to complete one loop and this should presumably be one second. R is calculated by a distant observer from the angle subtended by the ring when viewed through a telescope and the distance of the observer from the ring. At this point both local and distant observers agree on the radius of the ring, the apparent speed of light and that the circumference is given by [tex] {2\pi r} [/tex] . Now suppose we had a convenient (non rotating) gravitational body which just happens to be a snug fit for our optical ring around the equator of the body. Assume the mass of the body is such that the gravitational time dilation factor is 1.25. Also assume there is atmosphere that would give rise to optical effects. Please ignore practical considerations such as this body is probably a neutron star and all local instruments and observers would be crushed ;) What would the local and distant observers measure? Scenario A: (Assuming horizontal gravitational length dilation). Here we assume the circumference of the ring has expanded by a factor of 1.25 The local observer measures the time for a photon to travel within the optical fibre ring as one second due to his slow clocks relative to the distant observer. The distant observer measures the photon as taking 1.25 seconds to travel a distance of 1.25 light seconds. Both agree on the same number for the speed of light in metres per second so neither sees an apparent slowing down of the speed of light in a gravitational field. Scenario B: (No length dilation) The distant observer measures the circumnavigation time of the photon within the optical fibre ring on the equator of the gravitational body as 1.25 seconds. From the distant observers point of view the circumference is 1 light second so the speed of light seems to have slowed down to 0.8c If the distant observer defines distance by light travel times then he could say that that the circumeferance of the body is not given by [tex] {2\pi r} [/tex] The local observers on the surface of the body (with the slower clock) measures the photon travel time as 1 second and so the local speed of light appears to be c to them. Note: Self gravitational lensing I have not taken self gravitational lensing into account in either scenario. It should be apparent that the strong gravitational field of the body will magnify the apparent size of the body to a distant observer. The body and ring will appear to subtend a larger angle in the telescope the distant observer. I can find formulas for the amount light is bent by a gravitational field as it passes a massive body. However I can not find a formula for how much a body would be appear to be magnified by its own gravity. It may be possible that when the distance of the observer from the body, the mass of the body and the gravitational dilation factor on the surface of the body and at the location of the distant observer (assuming he is not at infinity) are all taken into account then the optical magnification would make the speed of light be c to both local and distant observers. For example, in scenario B the distant observer would see the radius and circumference of the body optically magnified by a factor of 1.25 so that the circumnavigation time of 1.25 seconds would be consistent with an unchanged speed of light even to the distant observer. So, is scenario A or B (or neither) more consistent with the accepted wisdom? Does anyone have a formula for self gravitational lensing of a body? (My interpretation is that it would be something like [tex] s = s'\frac{\sqrt(1\frac{2GM}{c^2 d}}{ \sqrt(1\frac{2GM}{c^2 r}} [/tex] where s is the self gravitational optical magnification of the body and d is the distance of the observer from the body) In measuring r and d I have assumed there is no dilation or contraction of length in the vertical direction in a gravitational field. Is that a reasonable assumption? Thanks 


#10
Oct3007, 01:32 PM

P: 1,746

I used f' = f[1GM/(c^2)r] instead of f' = f sqrt[12GM/(c^2)r] simply because (as far as I know) all existing experimental tests have precision of not better than [itex]c^{2}[/itex]. So, keeping only the first order in [itex]c^{2}[/itex] seems to be a prudent approach. Your ideas about experimental tests for gravity effects on length and speed of light look interesting, however (you would probably agree) completely unrealistic. Is it possible to modify these tests, so that they become realizable with modern technology? Eugene. 


#11
Nov207, 06:08 PM

P: 3,967

In 2005 I posted a document on the web that conjectured "What if f' = f[1GM/(c^2)r] if the true relationship rather than the conventional f' = f sqrt[12GM/(c^2)r] ?" As you point out the accuracy of existing tests can not rule out f' = f[1GM/(c^2)r] (as far as I know). In strong gravity fields such as near a black hole the differences become significant and give rise to a different type of black hole. Using f' = f[1GM/(c^2)r] the event horizon is half the conventional Schwarzschild radius. Below the event horizon gravity is negative giving rise to a solid shell with radius GM/(c^2) rather than the singularity of the conventional interpretation. Quantum fluctuations at the surface of the shell would allow radiation from the surface of black hole but that radiation would be at very long wavelengths and very faint due to extreme time dilation at the surface. (The alternative black hole would still be a very dark object) Gravitational lensing due to a black hole would probably be less than the conventional interpretation. One interesting thought is that if all the mass of the known universe was contained within a Planck radius there would extreme negative repulsive gravity giving rise to a simple model of the inflation/expansion of the big bang. The negative time dilation at such extremes in this model is not a problem as it is the cause of negative gravity. Objects fall away from each other instead of towards each other in negative time. Anyway, I have not posted a link to my document as I admit it is all idle speculation and conjecture. (Just an interesting thought) In my last post I asked a lot of questions (perhaps too many). To keep things simple the main question I am asking at his point is: Do objects length contract or expand in a strong gravitational field as measured by a distant observer ...or not? Any expert in GR should be able to answer that simple question :P 


#12
Nov207, 07:20 PM

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P: 2,340

Idealize Earth and Venus to have neglible mass, i.e. imagine that the ambient gravitational field is due to the Sun. This suggests treating the problem in the (exterior of the) Schwarzschild vacuum solution by considering two world lines representing the motion of the Earth and Venus as test particles. Since the round trip takes on the order of ten minutes we can even idealize them as static test particles. The problem is now reduces to studying null geodesics connecting the two world lines. By symmetry, we can consider the two legs of the journey to be almost identical. The line element written in the standard Schwarzschild chart is [tex] ds^2 = (12m/r) \, dt^2 \, + \, \frac{dr^2}{12m/r} \, + \, r^2 \, \left( d\theta^2 + \sin(\theta)^2 \, d\phi^2 \right), [/tex] [tex] \infty < t < \infty, \; 2 \, m < r < \infty, \; 0 < \theta < \pi, \; \pi < \phi < \pi [/tex] The world lines of static observers in this chart have the form [itex]r=r_0, \, \theta=\theta_0, \, \phi=\phi_0[/itex]. The spatial hyperslices orthogonal to such observers have the form [itex]t=t_0[/itex] and they are all identical (by the time translation symmetry of the Schwarzshild vacuum). The null geodesics are planar, so without loss of generality we can suppress the coordinate [itex]\theta[/itex] by setting [itex]\theta=\pi/2[/itex] (the locus of the equatorial plane). If we project the null geodesic arc representing outward leg of the journey of the radar pip into [itex]t=0[/itex], we obtain a curve which I'll call "the path". It is the projection of a null geodesic in spacetime, but it is not a geodesic in the metric on t=0 which is induced by restricting the Schwarzschild metric tensor to this hyperslice (with [itex]\theta[/itex] suppressed): [tex] d\sigma^2 = \frac{dr^2}{12m/r} \, + \, r^2 \, d\phi^2, \; 2 \, m < r < \infty, \; \pi < \phi < \pi [/tex] This induced metric can be considered a perturbation of the usual polar coordinate chart for the euclidean plane, and also a good approximation to it, in the region we are considering (outside the surface of the Sun). The path appears to be "bent" slightly as it passes near the origin (the location of the Sun). As everyone knows, extending the path to infinity in both directions we get a angle between two asymptotes, which Einstein computed ("light bending"). The two asymptotes in fact are rather close to the path itself, so to simplify the computation we can (subject to justification upon demand!) treat the path as two line segments. We can write any straight line segment in a polar chart in the form [itex]R = r\, \cos(\phi)[/itex], where R is the distance of closest approach to the origin. Differentiating gives [itex]r \, d\phi = \cot(\phi) \, dr[/itex] and plugging this relation into the line element of the Schwarzschild vacuum (still working in the equatorial plane) and setting [itex]ds^2 = 0[/itex] (since we are working with a null geodesic arc) gives [tex] dt = \frac{r}{r^2R^2} \left( 1 + 2m/r  2mR/r^3 \right) \, dr \; + \; O(m^2) [/tex] Now suppose that the Earth is at [itex]r=R_1[/itex] (angle irrelevant) and integrate from [itex]R_1[/itex] to [itex]R[/itex]. Proceding similarly for [itex]R[/itex] to [itex]R_2[/itex], where Venus is at [itex]r=R_2[/itex] (angle irrelevant), and adding, we obtain an expression consisting of the flat spacetime "height" (travel time) of our broken null geodesic path, namely [itex]\sqrt{R_1^2R^2}+\sqrt{R_2^2R^2}[/itex], where of course [itex]2 \, m \ll R \ll R_1, \, R_2[/itex], plus some correction terms, of which the largest is a logarithmic term. (Closer examination would show that it overwhelms the small errors introduced by the broken line approximation.) By symmetry, the return leg should give the same result, so twice the logarithmic correction term gives the desired time delay. This is the increased time it takes for the round trip journey, as measured by an ideal clock on Earth, in case the radar pip travels near the limb of the Sun, as compared to when it does not. It's a very concrete effect! Here's a pointer for your original question: suppose we model bending a paper clip using some one parameter family of curves. The tip moves quite a bit as we vary the parameter, but the change of length due to the bending is comparatively modest.
kev, I urge you to simply ignore what meopemuk said; IMO he's adding confusion, not clearing it up. Due to the difficulty of nonlinear mathematics, approximations are often neccessary to make even limited progress. Unfortunately, approximation is a tricky art form and this is where fallacies most often seem to creep into "mathematical physics discourse". Unfortunately for the present context, approximations which are somewhat tricky even in flat spacetime often become much trickier in curved spacetimes. There are in fact a number of "fiber optic" thought experiments offered in arXiv eprints, some of which IMO are misleading and have led to incorrect conclusions. Some authors are much more careful than others; preprints posted to the grqc section seem to exhibit a particularly large range of quality 


#13
Nov207, 07:58 PM

P: 1,746

The claim is that from the point of view of distant observer D objects in the gravity field (F) have the following properties: 1. All clocks go [1+GM/(c^2)r] slower, i.e., the duration of one F second is equal to [1+GM/(c^2)r] D seconds 2. All objects become shorted, i.e., what F observer perceives as 1meter rod, the D observer sees as a [1GM/(c^2)r] meter rod 3. Light is slower in the gravitational field, i.e., observer D measures the speed of light near observer F as c[12GM/(c^2)r], where c is the speed of light in the vicinity of D. If these conditions are satisfied, then measurements of observer F would also yield the speed of light as c. If F takes a rod of 300.000.000m long (which, according to D is shorter than necessary) and sends a light pulse (which, according to D is slower than usual) along it, then signal would arrive to the other end exactly in one Fsecond (which, according to D is longer than normal). So, both D and F would measure the speed of light as 300.000.000 m/s in their local frames. Eugene. 


#14
Nov207, 08:04 PM

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P: 2,340

kev, to state the obvious, it makes good sense to master the mainstream view before exploring/debunking fringe viewpoints (should you desire to do that). 


#15
Nov207, 09:59 PM

P: 1,746

I wouldn't say that "time slows down" in the gravitational potential (because I don't know what's the meaning of the expression "speed of time"). However, it is well established that identical clocks run slower in the gravitational potential than far away from it (e.g., the PoundRebka experiment, GPS,...). This difference in the clock rates is described well by formula [tex] T' = T(1+\frac{GM}{c^2r}) [/tex] where T' is the period of one clock tick in the potential. It is also known that light takes longer time to propagate between two points (e.g., Earth and Venus) if there is a massive body (Sun) on its way. This Shapiro time delay can be explained within GTR, as you demonstrated. But one can offer a simpler explanation as well. One can simply assume that the speed of light depends on the gravitational potential as [tex] c' = c(1\frac{2GM}{c^2r}) [/tex] (independent on the light direction) It can be shown that the numerical value of the EarthVenusEarth time delay comes out exactly as in GTR. Another fact is that the speed of light appears the same to observers in different gravitational potentials. All these facts can be reconciled by the assumption that the length of any rod decreases in the potential as [tex] d' = d(1\frac{GM}{c^2r}) [/tex] The slowingdown of clocks and the reduction of the speed of light can be explained within a simple Newtonlike theory of gravity, where gravitational interactions are distance and velocitydependent. Of course, this theory has nothing to do with GTR, and doesn't belong to the mainstream. However, in contrast to GTR, it is perfectly compatible with quantum mechanics, so, in my opinion, it can't be dismissed lightly. I have no idea how to explain the shortening of lengths in the field. I think it would be great to design an experiment to show whether this length shortening exists or not. This question makes sense no matter which theory of gravity is correct. And it would be a great independent check on the general theory of relativity. Here I agree with kev. Eugene. 


#16
Nov207, 10:54 PM

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http://xxx.lanl.gov/abs/astroph/0006423 might be a place to start. However, it's at a level that I wouldn't want to throw it at the average lay person. It also is arguably flawed, to boot, though I think it's interesting nonetheless for the advanced reader. It would unfortunately probably only confuse someone who didn't already know quite a bit about GR :(. It's problematic with respect to PF guidelines to promote ideas unless they've been published in a peerreviewed journal. Because GR is very technical, I think that respectable popularizations are also acceptable. It's also very handy to be able to point interested readers at respectable published sources. They can usually explain things in a lot more depth than a post can, and they are also something that interested readers can point to when *they* want to talk about the topic with someone else. 


#17
Nov3007, 03:01 PM

P: 39

But the space can be curved (stretched or compressed) even if it is straight for example, rubber band can be stretched without bending You can say that space is flat but ruler shrinks in gravitational field Anyway, if you measure the length of the path with rulers, you definitely need more rulers In my opinion, if you ignore the curvature of space, you will get only half of the Shapiro delay 


#18
Dec107, 06:14 PM

P: 39

In the book, Black Holes & Time Warps (page 400), Kip Thorne says: "What is the real , genuine truth? Is spacetime really flat, or is it really curved? To a physicist like me this is an uninteresting question because it has no physical consequences. Both viewpoints, curved spacetime and flat, give precisely the same predictions for any measurements" 


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