INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS

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1 IC/90/ INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS CENTRIFUGAL FORCE IN ERNST SPACETIME A.R.Prasanna INTERNATIONAL ATOMIC ENERGY AGENCY UNITED NATIONS EDUCATIONAL, SCIENTIFIC AND CULTURAL ORGANIZATION 1990 MIRAMARE-TRIESTE T

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3 IC/90/124 International Atomic Energy Agency and United Nations Educational Scientific and Cultural Organization INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS CENTRIFUGAL FORCE IN ERNST SPACETIME * A.R. Prasanna ** International Centre for Theoretical Physics, Trieste, Italy. ABSTRACT In this we show that in Ernst spacetime which represents the gravitational field of a mass embedded in a magnetic field, the centrifugal force acting on a particle in circular orbit, reverses its sign twice, once very close to r = 3TTI, and again later at a distance far away from the centre, depending upon the combined values of M and t, the mass and the strength of the magnetic field. The second reversal would mean that beyond a certain distance, depending upon Bm, in Ernst geometry, no Keplenan motion is possible for test particle. This result may have some possible astrophyiscal consequence. MIRAMARE- TRIESTE June 1990 * To be submitted for publication. ** Permanent address: Physical Research Laboratory, Navrangpura, Ahmedabad , India.

4 Recently it has been shown by Abramowicz and Prasarma 1} (1990) (hereafter AP), that in static spacetimes, the commonly understood 'Centrifugal Force' reverses its sign across the circular photon orbit (which for Schwarzschild solution is at r «= 3 m) at which the geodesic curvature radius tends to infinity. Following this, Abramowicz and Miller 2) (1990) pointed out that the earlier obtained result of Chandrasekhar and Miller 3) (1974) regarding the decrease of the ellipticity of quasi stationarily contracting relativistic MacLaurin spheroids with fixed mass and total angular momentum for r < 5 m, can also be explained through this effect of centrifugal force reversal close to the black hole radius. Prasanna and Chakrabarti 4} * 5) (1990a,b) using the notion of optical reference geometry invoked by Abramowicz, Carter and Lasota 6) (1988) in the Kerr spacetime showed that the centrifugal and Coriolis forces reverses sign at several different locations. More recently Abramowicz and Bicak 7) (1990) considering the effect in the Reissner-NordstrBm spacetime showed that some of the peculiar properties of the circular motion of charged ultra relativistic particles close to the circular photon orbit can be better understood through the centrifugal force reversal. Abramowicz 8) (1990) has pointed out through intutive arguments and gedanken experiment, how one can appreciate this effect in a physically meaningful approach. It is generally believed in the study of extended structures like galaxies that the effects of general relativity are unimportant and further while considering the dynamical effects the role of magnetic field is completely ignored. However, it may not be out of the way to try and understand qualitatively the effects of general relativity and of the weak magnetic field in the spacetime structure of massive extended objects. The only exact solution of Einstein's equations known to represent the spacetime of a massive body M embedded in an otherwise uniform magnetic field Br is due to Ernst 9) (1976) and is given by the metric ds 2 = -A 2 [( 1-2m/r)dt 2 - (1-2m/r)-'dr 2 - r 2 d9 2 } + (r 2 sin 2 6/A 2 )d<f> 2 (1) with A = (1+ B 2 r 2 sin 2 6). m=mg/c 2, B 2

5 Now following the approach of ACL as described in AP we introduce the optical reference geometry through the (3 + 1) conformal splitting: We get thus (2) <D=A 2 (l-2m/r) (3) The circumferential radius f of the projected circular orbit in this 3-space is i/2 rsing / 2m\" 1/2 r 2 sin 2 0 /. 2mV 1 f 1 - J. The geodesic curvature radius 1Z as given in AP (Eq.2.17) K = T/[g ik Vi(r) V k (r)]v 2 (6) turns out in this case (for 6 = TT/2) to be -3m/r) -3S 2 r 2 (l -5m/3r)]. (7) It is simple to recognize that 72, co exactly at the locations of the extrema of the effective potential for photons in the Emst spacetime A Vefr = -^(l-2m/r) (8) being the specific angular momentum. Unlike in the Schwarzschild spacetime where there is only one circular photon orbit at r = 3 m, in this case we find that there can be in principle two such orbits which change with different values of Bm. then If Cl is the angular velocity of a particle as measured by the stationary observer at infinity, 2 ^(l-2m/r)- 1. (9)

6 Using the definition of the 'orbit speed' v, as in AP, one gets the centrifugal force CF acting on the particle 11 r 3 [r 3-2 A 4 (r-2m)] [\ r / V r /J and the gravitational force G? It is important to notice that as r > 2 m, GFremainsnegative irrespective of the value of B whereas CF changes sign at the roots of the cubic f(r) = ~3B 2 m 2 R 3 + 5B 2 m 2 R 2 + R-3-0 (12) The discriminant A of this cubic is given by A = ~^~^~ [13500 B A m A B 2 m ] (13) and thus depending upon the values of Bm, A is either positive, zero or negative. It is easy to see that for Bm > 0.095, A is either zero or positive and for Bm < 0.095, A is negative. Writing Bm in the c.g.s. units onefindsthat Bm = 2 x 1Q~ 53 BrM (14) which shows that Br M has to be extremely large for Bm > 0.095, (in fact for Bm = 0.1 and M ~ MQ Br has to be k, 2.5 x 10 9 Gauss) which is very unrealistic. Thus for any reasonable combination of Bm it is in fact very small and thus less than 0.095, ensuring that all the three roots of the cubic are real. From Eq.(12) it is clear that the cubic can have only two positive roots the other being negative. Hence neglecting the negative root (r 0) onefindsthat the 'centrifugal force' changes sign at two places - one very close to r = 3m, and the other far away depending upon Bm. It is not difficult to understand this behaviour of centrifugal force reversal at two locations if one looks at the structure of the effective potential for photons as depicted in Figs.l and 2. 4

7 Fig.l shows the curves for small values of Bm wherein one can see the minimum of the potential occurring for values < 1 and Fig.2 for extremely small values of Bm, the minimum occurring very far away. As explained in Abramowicz (1990) the centrifugal force attracts particles towards the stable circular photon orbit which occurs at the minimum of the effective potential. Just to illustrate we also give in Figs.3 and 4 the location of the roots of f{r) wherein again it is clear that for Bm there appears a double root and for values less than that, two distinct positive roots. As mentioned earlier if one considers the spacetime to represent the gravitational field of a large structure having mass ~ 10 9 MQ and a uniform magnetic field of the order ~ 10 ~ 7 Gauss, then Bm «4x 10 ~ 17 for which the centrifugal force reversal occurs at around r = 3 m and then again at r w 2.5 x m, which is around lokpc. This means for objects of this nature, in their spacetime, Keplerian orbits for test particles are possible only upto a distance of around ~ 10 kpc and beyond that as the centrifugal force acts along the same direction as gravitational force no Keplerian orbits would be possible. It may be argued that for large scale astronomical structures as galaxies, the mass distributions would be such that it may not be suitable to consider a Schwarzschild like solution for the field. However, the location where the second reversal of centrifugal force occurs, is really far away and the field gradient at this distance is very very small. As can be seen from the numbers associated with the V t n (Fig.2) after the minimum the raise is extremely small and thus for practical purposes one can assume it to be almost flat. This does not in the least change the physical reality of the change in the direction of the force. Though the magnetic field energy density is much smaller compared to the radiation and dust in the outer regions of a structure like a galaxy, the overall qualitative effect of the magnetic field in reversing the force direction may have significant consequences.

8 Acknowledgments The author would like to thank Dennis Sciama and Xiao-He-Zhang for fruitful discussions and Marck Abramowicz for discussions, careful reading of the manuscript and suggestions. He would also like to thank Professor Abdus Salam, the International Atomic Energy Agency and UNESCO for hospitality at the International Centre for Theoretical Physics, Trieste.

9 REFERENCES 1) M.A. Abramowicz and A.R. Prasanna, Mon. Not. R. Astr. Soc. (1990) (in press). 2) M.A. Abramowicz and J.C. Miller, Mon. Not R. Astr. Soc. (1990) (in press). 3) S. Chandrasekhar and J.C. Miller, Mon. Not. R. Astr. Soc. 167, 63 (1974). 4) A.R. Prasanna and S.K. Chakrabarti, Gen. Relat. Grav. (1990) in press. 5) S.K. Chakrabarti and A.R. Prasanna, J. Astrophys. Astr. 11,29 (1990). 6) M.A. Abramowicz, B. Carter and J.P. Lasota, Gen. Relat. Grav. 20,1173 (1988). 7) M.A. Abramowicz and J. Bicak, preprint Nordita (1990). 8) M.A. Abramowicz, Mon. Not. R. Astr. Soc. (1990) in press. 9) F.J. Ernst, J. Math. Phys. 17,54 (1976).

10 I / * / c if / it 1 / -^ jrn-o03_- Bm=0.01 l i i _i Fig.l

11 Fig.2 T

12 "--4.8 \ w\ N \ \^ , \ \ R Fig. 3 10

13 f, V?. I RM.E-15 \ \\ Fig.A 11 T

14 Stampato in proprio nella tipografia del Centro Internazionale di Fisica Teorica