Central European Science Journals

Central European Journal of Physics

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CEJP 2(4) 2004 687-697

Simulation model for Shapiro time delay and light deflection experiments

Abhijit Biswas, Krishnan RS Mani*

Department of Physics,

Godopy Center for Scientific Research,

Calcutta 700 008, India

Abstract: The time delay experiment proposed by I.I. Shapiro in 1964 and conducted in the seventies was the most precise experiment of general relativity until that time. Further experimentation has improved the accuracy level of both the time delay and the light deflection experiments. A simulation model is proposed that involves only a simple mass and time transformation factor involving velocity of light. The light deflection and the time delay experiments are numerically simulated using this model that does not use the general relativistic equations. The computed values presented in this paper compare well with recent levels of accuracy of their respective experimental results. © Central European Science Journals. All rights reserved.

Keywords: time delay, light deflection, Newtonian deflection, relativistic deflection,

numerical relativity

PACS (2000): 04.25.Dm; 04.90.+e

1 Introduction

Einstein's General Relativity Theory (GRT) leads to almost identical results within the solar system as compared to Newton's theory. Only near very dense objects, e.g., black holes, or on a cosmic scale does general relativity lead to large changes. Thus, Einstein could think in 1916 of only three tiny potential manifestations of general relativity: starlight deflection, perihelion precession, and gravitational redshift.

Thereafter, for over four decades, there were only these three effects that could offer experimental verification of GRT. The sixties ushered in a new era in relativity experiments, utilizing new technologies - radar, lasers, inertial instrumentation, hydrogen maser

* E-mail: godopy@vsnl.com

Received 29 July 2004; accepted 16 September 2004

clocks, and space. As it stands now, these three Einstein tests seem secure. The red-shift has been confirmed. The Starlight deflection is established at an accuracy level of a fractional percent; a closely related new test, the Shapiro time delay experiment was proposed by I. I. Shapiro in 1964 as a theoretical consequence of GRT, based on radar ranging measurements to planets and spacecraft. This experiment, often termed as the fourth test of GRT, and its most recent improved repetition is considered to be one of the most precise experiments of GRT. According to GRT, the travel time of a radar signal should be longer if the radar beam travels near an isolated massive object such as the sun, and this extra amount of time is reported as the time delay.

There are two approximate methods without making standard symmetry assumptions, to solve the Einstein field equations: namely, the post-newtonian approximation (p.n.a) and weak-field approximation methods. While the former is used for non-relativistic slow moving objects, the latter is used for relativistic particles. However, both the methods involve complex mathematics to analyze the problems and it was felt necessary to evaluate a simpler approach to analyze the experiments so far conducted to prove GRT. One phase of such work culminated in this paper, which presents the results of numerical simulation that does not use general relativistic equations, but leads to results which are in agreement with those predicted by GRT and recent accurate experimental results.

It may be mentioned here that no such work using a similar approach for derivation of the time delay and light deflection effects without using GRT could be found in the literature.

2 Experiments for light deflection and their accuracy levels

2.1 Eddington's Eclipse experiment

While many eclipse experiments have been conducted to date with higher and higher accuracy, Eddington's experiment is still the most famous one. The basic tenet of Einstein's GRT is that a light ray grazing on the surface of the sun will be deflected by 1.75 arc-seconds. The experiment involved taking photographs of fixed stars during a near-total solar eclipse and comparing them with those taken when the light ray travels quite far from the sun. Eddington's eclipse expedition of May 29, 1919, though was not blessed with a good weather, analysis of its data and photograph of the stars yielded results corresponding to a deflection for a grazing ray of 1.60 ± 0.31 arcseconds, or 0.91 ± 0.18 times the Einsteinian prediction [1].

2.2 Sobral Expedition

This Eclipse expedition simultaneous to Eddington's expedition, was blessed with a better weather, and could produce a much better agreement of 1.13 ± 0.07 times the Einsteinian prediction [1].

2.3 Radio interferometry

Radio interferometry experiment in 1975 yielded results agreeing to 1.00 ± 0.01 times the Einsteinian prediction [1].

2.4 Very Long Baseline Interferometer

During the 1990s, the Very Long Baseline Interferometer (VLBI) array has been used that yielded results agreeing to 1.000±0.001 times the Einsteinian prediction [2].

To summarize, one can say that the most accurate light deflection experiment conducted to date has shown agreement with the GRT prediction within an accuracy level of 0.1%.

3 Experiments for Time Delay and their accuracy levels

3.1 Shapiro time delay experiment

This experiment, based on radar ranging measurements to planets and spacecraft, has been executed very precisely. The spacecrafts Mariner 6 and 7 were launched on February 24 and March 27, 1969, respectively, with a goal of obtaining new information on the orbital parameters of the Earth and Mars. On December 8th, NASA approved an extended mission to conduct the Shapiro time delay experiment. This relativity experiment, which started in December 1969 and continued till the end of 1970, made thousands of ranging measurements on each spacecraft. The heaviest concentration of these measurements was made around the time of each superior conjunction - April 29, 1970 for Mariner 6 and May 10, 1970 for Mariner 7. For Mariner 6, the distance of closest approach of the radar signal at superior conjunction was about 3.5 solar radii, corresponding to a Shapiro time delay of 200 microseconds out of a total round-trip travel time of 45 minutes. For Mariner 7, the radar signals came no closer than about 5.9 solar radii, giving a slightly smaller time delay of 180 microseconds. Computer calculations showed that the measured delays agreed with the GRT predictions to within 3% [1].

3.2 Planetary radar ranging to Venus and Mercury

Data from the Venus Conjunction in January and February 1970 as well as the numerous Mercury conjunctions between 1967 and the end of 1970 yielded relativistic time delays in agreement with the GRT predictions to within 5% [1].

3.3 Viking Landers 1 and 2

These two landed spacecrafts on Mars generated sufficient data during the November 1976 conjunction through September 1977 using both S-band and X-band radio frequencies to

take care of the solar corona effects. After a year and a half of data analysis, the final results showed a measured time delay in agreement with the GRT prediction with an accuracy level of 0.1% [1].

3.4 Pulsar PSR 1937+21

Outside our solar system, observations of the Pulsar PSR 1937+21 have provided an independent verification for the Shapiro time delay effect to within about 5% of the GRT prediction [3].

To summarize, one can say that that the most accurate Shapiro time delay experiment conducted to date has shown agreement with the GRT prediction within an accuracy level of 0.1%.

4 Discussion on the formulation of the problem and the equations used

A photon is not a material particle and its energy level can be found to have a relativistic transformation factor given by (1-F), as is evident from eqns. (1) and (2) below. This relativistic transformation factor was also adopted for transformation of time. This is the only initial assumption that has gone into the model. However, the epochwise simulation results have amply and consistently proven that this transformation factor is valid even for velocity transformation.

Photon was considered as a point-mass moving along its trajectory in the solar system. Its mass at a distance (r) from the center of the heliocentric co-ordinate system (c.s.) was taken to be equal to its relativistic mass (m) obtained from the energy balance equation, given by

m =-t-^, (1)

v2_ (GM> '

where:

h = Planck's constant,

v = frequency of the photon at a infinite distance from the massive gravitating body, v = magnitude of photon's velocity at a distance r from the center of the heliocentric c.s., G = gravitational constant, and M = mass of the sun.

In the heliocentric c.s., the Red Shift Factor, F, at a radial distance r, due to solar gravitational potential is given by

v2 • r

The relativistic transformation factor for time can be written as:

dt' = ■ dt (3)

1 — F

The gravitational acceleration of photon due to only the central body c, is given by

r = — G ■ M + m) ■ £ . (4)

G = gravitational constant. M = mass of the sun, m = mass of the photon,

r = Position vector of the photon moving in the heliocentric c.s.

The double dots above a symbol signify the double differential throughout this paper, while the single dot above a symbol signifies the single differential with respect to time.

Equation of the gravitational acceleration of photon due to all bodies in the solar system, other than the central body is given by

j = E G

mj (rj - r) mj rj |rj - r|2 |rJ - r| rj rj

mj = Mass of the J-th body in the solar system,

rj = Position vector of the J-th body in the heliocentric c.s.,

r = Position vector of the photon.

The resultant gravitational acceleration vector due to all bodies including the central body is given by

.. .. G ■ (M + m) r

r = r J--~2--r (6)

The resultant gravitational acceleration is numerically integrated to obtain the photon's epochwise velocity and position vectors.

4.1 Light deflection effect

The measure of bending of the photon's trajectory due to the Newtonian gravitational effect, which is termed as the first bending effect, is obtained from the dot-product of the unit vector of the instantaneous velocity with that of the velocity at initial epoch. The formula is given as

= arccos (vo ■ vt) , (7)

vo = the unit vector of the velocity v at time t = 0, and vt = the unit vector of the velocity v at time t = t.

Let us define vr and v$ as the radial and tangential components of velocity v respectively. The relativistic effect that causes bending of the photon's trajectory is due to the relativity of mass of the photon. The orbital angular momentum of the photon (m.r.v$), is conserved as per the conservation law of angular momentum. The mass of the photon m given by eqn. (1) above, has a time-varying nature as caused by the energy balance equation, involving the well known relativistic effect: the gravitational red-shift. The conservation law for angular momentum generates a torque given by (m.r.v$), in addition to the torque due to the inertial forces.

This torque due to conservation of angular momentum causes the relativistic bending effect which really is the angle of precession of the radial velocity vector vr, and can be obtained from the following eqn.

fa = / ^ dt' = / ^ • dt. (8)

J m • vr J m • vr (1 — F )

The total bending effect can be obtained by summing up the two effects:

0 = fa + . (9)

4.2 Time delay effect

The distance (d) traversed by the photon along its trajectory is obtained by numerically integrating the magnitude of the photon's velocity vector (v), using the relativistic transformation factor for time.

d = J vdt' = J V F) dt, (10)

The difference between the epoch times corresponding to the initial and the final points of the trajectory gives the journey time of the photon as per DE202 time scale [7] measured in seconds.

Time delay is calculated as the difference between the time obtained from the epoch time difference (as above) and the "calculated journey time" obtained by dividing distance (d) by the standard velocity of light (i.e., 299792458 m/sec), which would be the velocity had there been no gravitating mass near the trajectory to influence the photon's motion.

4.3 Numerical simulation

The computations were done on the heliocentric ecliptic-of-date co-ordinate system. Heliocentric positional data of the planets were taken from JPL's (Jet Propulsion Laboratory, Caltech) ephemeris DE202. The calculations were not done on a complete General-relativistic framework; instead, these were done using Newtonian equations of motion while incorporating the few basic principles of relativity, as explained earlier.

Ordinary differential equations (ODE) were generated for the equations of motion and the actual distance traveled by the photon particle. Another ODE was generated

for solving the relativistic bending angle. All the ODE's were solved simultaneously using a variable step differential equation solver. For extracting and interpolating the ephemeris data from DE202, we have used the appropriate codes provided by JPL. For differential equation solver, we have used Gear's method [4]. Our sub-program using Gear's method maintains the single step error using Euclidean norm, at a maximum value of 1.0E-10. This method has been tested against various JPL integration algorithms and is comparable to the best available in the field for astro-dynamics in controlling the integration error within the specified value. All the Fortran codes, other than that provided by JPL for using ephemeris data, have been developed by us using our own mathematical model as presented above, while making use of a few of the algorithms for rotation matrices given in JPL's Technical Report [5] and of DE102 by Newhall et al [6]. The results of the computation are presented and discussed in the following section.

5 Discussion of the computation results presented in this paper

Even though Light-deflection and Shapiro Time delay are two different experiments, for a light ray grazing on the surface of the sun both phenomena occur simultaneously. Hence, during each case of computation it is possible to compute both effects. Thus, based on the data available in the public domain, a total of three cases of computation were selected as mentioned below:

• Light Deflection experiment - For simulating this experiment, a light ray grazing the surface of the sun was selected. The closest approach of this light ray to the sun's center was equal in length to the solar radius.

• Shapiro Time delay experiment - For this experiment, the following two cases were selected, which correspond to the two cases of Shapiro's original experiment conducted in 1970, for:

Mariner 6, for which the distance of closest approach of the radar signal at superior conjunction was about 3.5 solar radii to the center of the sun. Mariner 7, for which the distance of closest approach of the radar signal at superior conjunction was about 5.9 solar radii to the center of the sun. The computation results are presented in the tables below.

5.1 Light Deflection

For the Light Deflection experiment, it can be seen from Table 1 that the computed value differs from the GRT prediction by about 0.0086 %, as compared to the recent experimental accuracy of ± 0.1 %. The values compare well within the levels of recent experimental accuracy.

For the two cases of Shapiro Time delay experiment, our computed values of deflection angle are presented in Table 1 below. Computation of deflection angle were done at additional points and the results are presented in graphical format in Figure 1 below.

Computation case corresponds to experiment

Light Deflection Shapiro Time delay Shapiro Time delay

Closest approach of light ray to sun's center expressed in terms of solar radii 1.0 3.5 5.9

Deflection Angle computed by us, Arcsec. 1.75041 0.500068 0.296603

Deflection Angle predicted by GRT, Arcsec. 1.75026 0.500075 0.296655

Recent experimental accuracy compared to GRT prediction ± 0.1%

Deviation of our computed value from the GRT predicted value 0.0086 % - 0.0014 % - 0.0175 %

Remarks Match well within limit of experimental accuracy No experimental data available No experimental data available

Table 1 Computation Results for Light Deflection effect for the three chosen cases of computa-

Light Deflection

1.90-,-

0.15 - i i i i i i i i i | i i i i i i i i i | i i i i i i i i i | i i i i i i i i i | i i i i i i i i i | i i i i i i i i i 0.0 0.9 1.8 2.7 3.6 4.5 5.4

Closest approach of light ray from sun's surface in terms of solar radii

Fig. 1 Computed Light Deflection angles versus the closest approach of light ray to solar surface.

5.2 Time Delay

For a Shapiro Time delay case when the closest approach of light ray to sun's center is equal to 3.5 times the solar radii, Shapiro's Experimental value was 200 microseconds with an experimental accuracy of ± 3 %, and its best experimental accuracy was ± 0.1 % during its recent experimentation; our computed value in Table 2 corresponding to this case was 199.836, which falls well within the accuracy limit.

For a Shapiro Time delay case when the closest approach of light ray to sun's center is equal to 5.9 times the solar radii, Shapiro's Experimental value was 180 microseconds, and its best experimental accuracy was ± 0.1 %; our computed value in Table 2 corresponding to this case was 180.18, which checks well within the accuracy limit.

For the Light Deflection experiment, our computed value for time delay was 249.355 microseconds as presented in Table 2 below. However, no experimental data for this case was available for comparison. Computation of time delay were done at additional points and the results are presented in graphical format in Figure 2 below.

Computation case corresponds to experiment

Light Deflection Shapiro Time delay Shapiro Time delay

Closest approach of light ray to sun's center expressed in terms of solar radii 1.0 3.5 5.9

Time delay computed by us, Microseconds 249.355 199.836 180.18

Time delay measured by Shapiro's experiment, Microseconds 200 180

Shapiro's experimental Accuracy ±3% ±3%

Recent experimental Accuracy ±0.1% ±0.1%

Deviation of our computed value from Shapiro's Experimental value -0.08% +0.10%

Remarks No experimental data available Matches within limit of not only Shapiro's Experimental accuracy level, but also the recent experimental accuracy level. Matches within limit of not only Shapiro's Experimental accuracy level, but also the recent experimental accuracy level.

Table 2 Computation Results for Time delay effect for the three chosen cases of computation.

5.3 Newtonian and Relativistic components of Light Deflection

In Table 3 below, are shown the Newtonian and relativistic components of the Deflection Angle computed by us. It can be seen that these components compare well with each other (up to four to five significant digits) for the different computational cases given in separate tabular columns.

6 Conclusion and discussion

The simulation model proposed here utilizes only a single mass and time transformation factor involving velocity of light. Equations of motion, and equations for time delay and light bending angles for a photon particle passing near the sun, have been generated using this model and have been solved numerically.

Time Delay

256.27 -,-

173.27 -|—i—i—i—|—i—i—i—|—i—i—i—|—i—i—i—|—i—i—i—|—i—i—r

0.0 0.9 1.8 2.7 3.6 4.5 5.4

Closest approach of light ray from sun's surface in terms of solar radii

Fig. 2 Computed Time Delay versus the closest approach of light ray to solar surface.

Computation case corresponds to experiment

Light Deflection Shapiro Time delay Shapiro Time delay

Closest approach of light ray to sun's center expressed in terms of solar radii 1.0 3.5 5.9

Comparison of Deflection Angle computed by us

Newtonian component, Arcsec. 0.875206 0.250036 0.148307

Relativistic component, Arcsec. 0.875203 0.250032 0.148295

Remarks The components are equal up to the first five significant digits. The components are equal up to the first five significant digits. The components are equal up to the first four significant digits.

Table 3 Comparison of the Newtonian and Relativistic components of computed Deflection Angle.

The results of computation presented in the previous section compare well with the GRT predictions for both the Light deflection and Time delay, when a light ray passes near a massive object like sun; the computed values also compare well with the respective experimental results within their recent accuracy levels.

The computed results further show that the deflection of light has both a Newtonian component and a relativistic component and that these components are almost equal (up to four significant digits) as brought out by Einstein in his early papers on GRT.

References

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