sabato 11 dicembre 2010

Newton's Law and General Relativity

Both of them describe behaviour of a mass (the second also of the e.m. radiation) in presence of a gravitational acceleration field. Gravitational constant G allows to calculate , with Newton’s law, the mutual actraction between masses or the gravitational potential generated by masses in the space. And not matter that General Relativity, that is so more precise as to allow to correct temporal gaps of satellitar location systems (GPS), provides for that gravitational acceleration fields are due to a space time curvature in presence of masses, but they aren't generated by masses. However Newton’s law allowed us to land on the Moon and to send space probes in the extreme boundaries of solar system.

But let's return to “Global Position System”. Temporal gaps of satellite signals sent to the Earth, really are two with opposite sign. One is due to Special Relativity that provides that clock on board of satellite is slower than the one on the Earth. The other is due to GR that provides that clock on the Earth is slower than the one in orbit on board of satellite. Adding the two gaps we obtain the right correction of the time, that is fundamental for right working of GPS. The sign of correction will be positive or negative according as predominates SR or GR. In every case they are relative gaps and not absolute one because, as in the paradox of Einstein’s twins, time on the Earth and on satellite goes on normally. Otherwise the same word tells it : relativity, and GR is a natural extention of SR to not inertial systems, as we can observe in the first mathematical steps of theory. In particular let's observe that :"in regions of four dimensional space time indefinitely small, for which is possible an acceleration of coordinates system in the way that doesn’t be produced any gravitational field, remains valid Restricted Relativity", that is the "unchanging" of the term : ds^2 = - (dx1)^2–(dx2)^2 –(dx3)^2 + (dx4)^2 where, obviously, dx4 =cdt.

So in GR all the space time metrics has four coordinates, three spatial and a temporal one. To Lorenz contractions always remains associated temporal dilatation. What’s the difference ? It’s that in Special Relativity the gaps are measured between inertial systems, while in GR between not inertial ones. Really in each of them space doesn’t contract and time doesn’t expand.If things go in this way then we wouldn’t feel weight here on the Earth or on a black hole horizon of events.

Complex interferometers (VIRGO, LIGO) try to detect gravitational waves expected by Einstein’s theory. GW would be space time ruffles come since to us from deep space and emitted by highly perturbed systems as the binary stars ones, from pulsars or catastrofic events like supernovae. As far as now anything has been detected. Why ? Because if they really would be space time ruffles it would be possible to measure a Ds^2 = - (Dx1)^2–(Dx2)^2 –(Dx3)^2 + (Dx4)^2 in the same reference system where the phenomenon happens that, for the same theory, is impossible.

But there is another question. Time is the human measuring unity of matter state variation. For example the "second" is definied as “duration of 9 192 631 770 periods of radiation corrisponding to transition between two iperfine levels of Caesium atom fundamental state”.
So absolute or relative time is not a physical size therefore it can’t contract or expand. And without time contractions or expantions don’t exist the space ones, at least according to Einstein. Only if we could measure a temporal slowdown in the same system falling down in a gravitational field we could adfirm that time expands and consequently that gravitational field is due to a space time curvature. To do it we would have to measure in the local system (for example on board of a spacecraft going on towards a gravitational field), a slowdown of a physical process, as the decay of a radioactive atom, respect to the espected theoretical value.

So what happens ? Nothing special about it. Only that, as we have said in the beginning, GR, as Newton’s law, describes gravity, but doesn’t explain it.

Marius tries to make it.

Stefano Gusman

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