# The nature of the observer in special relativity [closed]

In special relativity, is it silently assumed that the observer is only a “measuring machine” without an innate conception of space and time, so what he thinks about motion is exactly what he measures, only this, and nothing more?

So i’m also asking whether the other option is silenty assumed: that the observer is a human, having his innate conception about space and time (that absolute - Euclidean – Newtonian space and time are the foundations of reality) but when he attempts to verify it, he finds unexpected resistance, for, in the physical world, he needs light in order to see things and perform measurements (not so in the geometry of his mind’s eye), but the physical world refuses, for unknown reasons, to allow objects to move faster than light, so the poor observer, dumbfounded by the physical barriers he encounters, arrives, as a physical philosopher, at a strange double identity: a Newton in his mind, an Einstein in his experience.

## closed as unclear what you're asking by WillO, Jon Custer, ZeroTheHero, David Hammen, YashasJun 21 '17 at 13:54

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• I am not sure exactly what you are asking. It seems more like a description (not entirely adequate for the reasons I will mention) of a person who is under the delusion of spacetime being Newtonian. The reason it is not adequate is because it's not the basis of relativistic physics that we need light to see things. The "boundaries" put by relativity are fundamental and are applicable to all the measurements/observers in a very fundamental sense. – Feynmans Out for Grumpy Cat Jun 19 '17 at 22:16
• In SR, to observe, e.g., an object with relative motion, requires no human at all, only rods and synchronized clocks at rest with respect to each other. From the Wikipedia article Observer (special relativity): "The effects of special relativity occur whether or not there is a sentient being within the inertial reference frame to witness them." – Alfred Centauri Jun 19 '17 at 22:19

An observer in Special Relativity is a set-up of the following kind:

There are three rigid rods orthogonal to one another and with equi-spaced marks in a Euclidean sense and extending indefinitely. This is equivalent to a $3 D$ grid. Now, there is a clock at each and every point of this grid synchronized with one another in a symmetric fashion. This means that if a clock $A$ reads $t_1$ when it sends a signal towards clock $B$, clock $B$ reads $t_3$ when it sends a signal towards clock $A$, clock $A$ reads $t_2$ when it receives the signal from $B$ and clock $B$ reads $t_4$ when it receives the signal from $A$ (then and only then) $t_1-t_3=t_2-t_4$.

This is all we mean by an observer. There is no need for a human to "observe" thing in any way. A perfectly automatic set-up would do this. Also, none of the special relativistic effects like time delay or relativity of simultaneity have got to do with the fact with even if something has happened somewhere, a point over here will take some time to know about it because light has to reach here. No. Our observer set-up is, as you can see, an indefinitely extended set-up. It knows something happens the moment it happens according to that frame. So, the special relativistic effects are very real and inherent to the spacetime structure in a very fundamental way.

The effects of a person/object over here knowing something some time after the thing actually happened over there are essentially Doppler effects. They are there even in Newtonian Physics.

• @ Dvij: We know from everyday experience that a triangle which appears right-angled to one observer, can appear obtuse to another, viewing it from a different viewing-angle. Likewise, by relativity theory, a triangle which appears right-angled to one observer can appear obtuse to another. So, what is more profound in Relativity than considering it just like a “change of viewing angle” which alters the appearance of things, not by way of position but by way of velocity? – J.Avaris Jun 20 '17 at 11:50

The word observer is a slightly unfortunate choice because in everyday life to observe something means to see it. That means light from the event has to reach our eyes, be processed by our brain, etc. This is, I suspect, what you are getting at in your second paragraph.

However an observer in relativity (both flavours) is something entirely different. In relativity observation is just the process of assigning events to spacetime coordinates.

if we want to describe when and where something happened we need to specify its position and time. So we might set up a coordinate system with a time $t$ measured by our clock and distances $x$, $y$ and $z$ measured using our rulers. Then for any event in spacetime we can assign it a position $(t, x, y, z)$. When a relativist talks about observing something they just mean assigning it spacetime coordinates.

This might seem a bit trivial, but where the fun starts is that different observers will assign different coordinates to the same event. All the weird effects like time dilation and length contraction happen because observers moving at different speeds assign different coordinates to the same events and they end up disagreeing about the separation between events.

• @ John Rennie: We know from everyday experience that a triangle which appears right-angled to one observer, can appear obtuse to another, viewing it from a different viewing-angle. Likewise, by relativity theory, a triangle which appears right-angled to one observer can appear obtuse to another. So, what is more profound in Relativity than considering it just like a “change of viewing angle” which alters the appearance of things, not by way of position but by way of velocity? – J.Avaris Jun 20 '17 at 11:50
• @J.Avaris: in everyday life a triangle can appear rotated in space when we look at it from a different angle. In relativity a triangle can appear rotated in spacetime. That means some of its corners rotate into the future or past as well as moving in space. It is this that causes all the weird effects associated with SR. For example see my answer to “Reality” of length contraction in SR. – John Rennie Jun 20 '17 at 12:01