A simple question about absolute time and Special Relativity

[quantum quack]

I do think I already have the correct understanding but I thought I would seek confirmation from others more versed in the nature of SRT.

The question is:
Is time absolute from a single reference frames perspective?
In that from a single frames perspective all time is absolute and only not so when considering the perspective of another observer who is travelling at relative velocity.

It is true that SRT states non-simultaneity exists when comparing two frames at relative v.

IS it also true that from the RF that is at rest all objects in it's frame of reference share an absolute time?

[Jay-qu]

SRT proved time was not absolute full stop... since theres no such thing as an absolute position i dont think you can have an absolute time

[Little Bang]

Time is one of the least absolute values in the universe.

[geistkiesel]

Originally Posted by Jay-qu

SRT proved time was not absolute full stop... since theres no such thing as an absolute position i dont think you can have an absolute time

Here you are in error Jay-qu.

Using a linear Sagnac effect device an absolute invariant point in space can determined with trivial ease.

Using three postulates of light only the eternally invariant point P is defined.

1. The speed of light is constant.
2. The motion of light is isotropic (light moves in a straight line until acted upon by an external force).
3. The motion of light is independent of the motion of the source of the light.

The figure below is not difficult to nterpret. The Sagnac effect device moving to the right with an as yet unknown velocity v (wrt point P), emits two photons, L and R, from the midpoint of the L and R clock/reflectors at each end of the moving inertial frame. The phyiscal midpoint mP moves to the right at velocity v.

The point P can be verified by at least three separate events in the complete round trip of the photons.

1. The photons are emitted at t = 0.
2. After moving a distance ct The L photons returns to P after reflection form clock/reflector L and after moving another distance ct.
3. The L and R photons return simultaneously to the phyhsical midpoint mP after moving for a complete round trip time of 2ct + t'.

t' is the time the photons move during the transit across the distance 2vt plus the distance the frame travels while the photon is underway. This distance is seen by both photons separated by their respective completed distance of ct. After the photons are emitted and the L photon has moved a distance ct (and arrived at the L clock reflector) the R photon is located adistance 2vt + vt' from the R reflector. After another distance travled of ct byoth photons the L photon is looking at the 2vt + vt' distance away from the physical midpoint of the L and R clock/reflectors, Here the R photon moving oppositely to L is located a distance 2vt + 2vt' from the oncoming mP. The L and R photons converge simultaneously at mP after a travel time of 2t + t'.

If one analyzes the figure with the detail required, they will see that during the time t' immediately after the L photon has reflected from L clock/reflector both the L and R photons are moving in the same direction to the right. This continues until the R photon reflects back to the direction of the invariant point P. This t' is the measured difference in time for the ropund trip of the photons wrt the condition where the frame is at rest wrt the stationary frame. This short duration deflection t' from perfect symmetry is what accounts for the different measured round trip travel times and is the measured proof of motion of the inertial frame versus a condition of rest for the inertial frame.

From the distance the light travels 2vt while the frame is underway, ct' we determine the time t' from ct' = 2vt + vt', where t' = t(2v)/(c - v), or in terms of velocity, v = ct'/(2t + t').

These data are not availabe for analyses prior to the completion of the round trip of the photons and therefore an observer on the movng frame has no data from which he may conclude, one way or the other, whether the frame is in motion or at rest wrt the stationary frame of reference.



Here, we assume the arrival times of the photons at L and R are embedded in the reflected signals which are not available for inspection until the reflected signals arrive at the midpoint mP after the completion of their round trip. On can see the difficulty in separating the signals such that an illusory loss of simultaneity can be claimed by eiother observer. The simultaneous arrival time as observed by the stationary observer cannot be rationalized away using a claim of nonexistent absolute tmes as perceived by the moving observer which can have only one solution. If the stationary observer measures the simultaneous arrival of the L and R photons where the mP is located (which can be determined from experiment) then there is no physical way in whch the moving observer can claim the event was not simultaneous. Similarly, if the stationary observer places clocks on the stationary frame (determined from experiment) at the point where the photons arrive at the L and R clocks on the moving frame, these clocks will show a definite sequential arrival of the photons at L and R that cannot be erased theoretically and have a substituted simultaneous arrival of photons measured.

Even were we to assume the moving clocks to be unsynchronized wrt the stationary frame the fact of time dilation and frame contraction will not hide the sequential arrival opf the photons wrt the moving frame clock measurements. This, of course, would negate any claim of time dilation and frame contraction.

The midpoint of the moving photons holds up to the point of reflection by the L photon. However, the point is still defined bhy the motion of the photons as the L photon returns to the point after reflecting back a distance ct. For the rather short spurts of time and distance considered here the isotropic motion, constancy of speed are maintained to any concevable resolutuion of measurement.

Notice that differences in observation by an observer moving with the inertial frame and an observer on the stationary frame are resolved by the measuremnt of t', which negates the concept that the moving and stationry observer will measure times of the events other than simultaneously.

The convergeance of the photons at the moving midpoint mP is simultaneous as it is when the frame is in the stationary condition. This makes it very difficult for the moving observer to claim any "loss of simultaneity, doesn't it?

If we assume the distance from mP to L and R each at 3000 meters we see the round trip time of flight for the photons in the stationary frame as 2x10^-5 sec. If the frame is moving in the L-to-R direction at 30 km/sec this will add a t' = 4.00040004 x10^-9 sec. In this time the frame will have moved in the order of 6 cm .

Geistkiesel

[Jay-qu]

wow, sorry bout that... but i believe with the expansion of space itself you cant have an absolute point

[Erasmus00]

Geistkiesel, if the distance between the mirrors in some frame is L, and the distance from one mirror to the point where the light beams reconverge is l, then what is frame invariant is l/L and not the point in space, per se. Also, I'm unfamiliar with phrase linear Sagnac frame, as the sagnac effect occurs in a rotating reference frame.
-Will

[infamous]

Originally Posted by Jay-qu

wow, sorry bout that... but i believe with the expansion of space itself you cant have an absolute point

Assuming that space is truly expanding I would have to agree with you Jay-qu. Isn't it frustrating how some will grasp at every opportunity to expound upon their pet theories.

[quantum quack]

Thanks guys, my question was really about how an observer will record the time or duration of any event in his universal reference frame in absolute terms.
Pretty naked stuff really.....and that it is only when we presume the perspective of a relative v observer that time becomes relative....

[geistkiesel]

Originally Posted by Erasmus00

Geistkiesel, if the distance between the mirrors in some frame is L, and the distance from one mirror to the point where the light beams reconverge is l, then what is frame invariant is l/L and not the point in space, per se. Also, I'm unfamiliar with phrase linear Sagnac frame, as the sagnac effect occurs in a rotating reference frame.
-Will

No Will, the physical frame moves wrt P that is and is not invariant. Look at it from the stationary frame3 and you will see what I mean. Google on Sagnac effect for a plethora of Sagnac descriptions. AE even suggested that "rotating frame" be unwrapped into a linear mode. Think about it Will, at what angle does rotating, or circular stop and linear begin? It certainy isn't a "delta function' interface.

I don't understand what youmean by "what is frame invariant is l/L?"
Thanks for the comments.
Geistkiesel

[Erasmus00]

Originally Posted by geistkiesel

No Will, the physical frame moves wrt P that is and is not invariant. Look at it from the stationary frame3 and you will see what I mean. Google on Sagnac effect for a plethora of Sagnac descriptions. AE even suggested that "rotating frame" be unwrapped into a linear mode. Think about it Will, at what angle does rotating, or circular stop and linear begin? It certainy isn't a "delta function' interface.

The Sagnac effect is ONLY observed in non-inertial reference frames. As such, Linear Sagnac makes no sense. If AE is Einstein, then you suggest Einstein thought the Sagnac effect would occur in a linear frame, and I'll need a source for that. If you want a mathematical treatment of the Sagnac effect in SR, let me know, and I'll provide. Now the sentence that ends in "that is and is not invariant" makes no sense. EDIT: I suppose in a non-inertial frame, an accelerating frame, you could have a linear sagnac effect, but that should hardly be surprising. You could even probably build a Sagnac accelerometer.

I don't understand what youmean by "what is frame invariant is l/L?"
Thanks for the comments.
Geistkiesel

You don't have an invariant point in space. Different observers will disagree about where the light reconverged. What you have is an invariant ratio. Lets say in some frame an obersver A measures the seperation beween mirrors L, in another frame an observer B measures L'. Now, A measures the light as reconverging a distance l from the end of the back mirror. B measures the light reconverging a distance l' from the end of the mirror. Then l/L=l'/L', for all reference frames. The point varies from observer to observer, but the ratio stays the same
-Will