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Simultaneity = 3D Space.
Observer and observation independent.
Reality vs Solipsism.
A 3D space is made of simultaneous events: all the events of 3D space exist at the same time. That's the definition of a 3D space world.
Does a 3D space world exist, out there, independent of any observer or observation?
We all know that our own individual 'now' moment is real. Your present awareness. Mind. Strictly speaking we do not know whether there exists a world outside our mental awareness. Maybe our body and brain are only mental illusions.
Realists do believe in the existence of a reality independent of our mind. Einstein believed in an observer independent reality. An existence independent of observations. Einstein:
<< Belief in an external world independent of the perceiving subject is the basis of all natural science. >> Einstein, "Maxwell's Influence on the Evolution of the Idea of Physical Reality," 1931, in Einstein, Albert, Ideas and Opinions, New York: Random House, 266.
<< Physics is an attempt conceptually to grasp reality as something that is considered to be independent of its being observed. In this sense one speaks of 'physical reality. >> Autobiographical Notes, 1949, in Schilpp 1949 p.81
<< I am not a positivist. Positivism states that what cannot be observed does not exist. This conception is scientifically indefensible, for it is impossible to make valid affirmations of what people 'can' or 'cannot' observe. One would have to say 'only what we observe exists', which is obviously false. >> Autobiographical Notes, 1949, in Schilpp 1949 p.81
Einstein is not talking about a philosophical viewpoint. He is talking about the need of realism to make sense of science (my underlining):
<< As always the conception of the existence of the real world is fundamental in physics. Without is there would be no borderline between psychology and physics. ... Modern developments have changed nothing in this respect. >> To David Holland, June 25, 1948 Einstein Archives9-305
Einstein was a realist. He believed in the existence of a reality independent of whether we look at it or not. Physics is about observing and measuring the world we live in. We do not know what the world around us is made of, but that does not mean there is no world before an observation is made. In order to be able to observe the world around us, there has to be (exist) a world before we make an observation of that world. Otherwise observation would not be possible.
I do not want to engage into a philosophical or ontological debate here, but I find it necessary to frame a philosophical/ontological paradigm when dealing with SR. Physics rests upon certain pre-supositions about the meaning of 'reality', 'existing', 'observation', etc. Without semantics physics can never give any meaning to words or mathematical data.
To observe an apple involves first the existence of an apple before I see the apple, then lightbeams traveling from that apple to my eyes, hitting my retinae and finally creating a mental image of that apple in my brain. If you see an apple on the table, there was an apple on the table before you saw it: it took time for lightbeams to travel from the apple to your retinae. You see the apple because there was an apple on the table.
The apple as such is 'observer and observation independent':
1/ Observer independent.
For everybody -unless under the influence of drugs etc.- the object on the table is an apple. Nobody will say they see a coconut on the table when I see an apple. Everybody agree there is an apple on the table. No coconut.
Simultaneity is is not observer independent. Different observers, moving relative to each other, consider different events happening simultaneously.
A clock time indication (hands of the clock pointing to a number), part of a specific event (for example: the moving clock is at the tree), is an observer independent fact, in the sense that the time indication of a clock is part of the event (f. ex. clock time when that moving clock arrives at the tree) and is identical for all observers, moving or not. Relativity of simultaneity explains how different events are considered simultaneous. In a later chapter I will show what this means for time dilation.
2/ Observation independent.
The apple is also on the table if you do not look at it. The act of observation is not necessary to create the apple on the table out there in reality. If there would be no apple on the table before I see it, where does the apple come from when I see an apple? The apple on the table is an 'observer and observation independent' physical realty.
The content of an event is absolute, identical for everybody. Therefore: in SR all events are observer and observation independent facts, they are 'absolute'. This is very important.
Is simultaneity observation independent? Is simultaneity created bij observation? This is a difficult one. In theory one can consider different coordinate systems determining different sets of events simultaneous. But together with your wristwatch time (proper time) only one set of simultaneous events will measure speed of light correctly. (This is true if you don't question Einstein's definition of simulaneity). Hence the simultaneous events for a specific observer with his own proper wristwatch time are already simultaneous before he sees them.
I agree that without observation we can never know there is an apple on the table. But that is no reason for not believing in an observer and observation independent world. Because without the existence of an observer and observation world (3D or 4D...) there would be no observation possible, and no need for observers!
Out there in 3D space there are 'now' specific events (such as the apple on the table) that are simultaneous with my 'now', with whatever reference system I measure. It's not because simultaneity is relative that there are no space-like events out there, spatially removed from us!
In SR nothing is observation dependent! 'Observation dependent' for example means: the observed image is not equal to what is out there to be observed because there is -for example- optical illusion involved. Because SR is not about optical illusion I will do not elaborate on this. Observation is only necessary to find out what was already out there before you literally see it.
Einstein uses observations in his train thought experiment (I elaborate on this experiment in another section on this site). In order to understand that experiment we have to agree on what 'observers' and 'observations' mean, otherwise it is pointless to start dealing with observers and observations, or SR at all! Just to make sure we understand each other as far as 'observation' is concerned I describe what it means:
Looking at a passing train you 'observe' (see) that an observer in the train sees a lightning hitting the front of the train because first there was lightning, then light travels to the observer in the train, and finally the light hits the observer's retinae. This is the meaning of 'to observe'. You see lightning hit the front of the train because that event (lightning hitting front of the train) is an observer independent event that is later observed by you (and other observers). Therefore: if you refute an observer independent reality, then there is nothing to observe!
I am flabbergasted when SR experts tell me they are not interested in 'what lays at the origin of the observations'! I do not understand them because "what lays at the origin of the observations" is essential to have observers making observations. If you do not agree with the existence of (observer dependent) simultaneous space-like (observation independent) events before you see them, then you pretend there was no apple on the table until you actually see/observe it. Hence the apple and the table disappear from existence when you close your eyes, and by magic reappear again when you open your eyes. Here the event 'apple on the table' is created through observation: an observation dependent event. This smells solipsism.
A solipsist only believes in the existence of what he is aware of at his 'now' event (his 'hic et nunc', the apex of his light cone on a spacetime diagram) (Berkeley's principle, esse est percipi). A solipsists has a very egoistic point of view. He believes only his mental images are 'real' (and therefore there can in fact only be one solipsist, i.o.w. he himself!). Solipsists believe there is only mind. No external reality. Hence the events of their mental image are nothing more than 'psychological created phenomenae'. If somebody escapes discussions about 'what lays at the origin of observations', he probably is a solipsist.
Solipsism leads to some bizarre consequences. A solipsist should never be afraid of shooting a bullet through his head. If his body and brain are only mental illusions, then his brain is not the creator of the mental images. But see what happens: if he shoots a bullet through his, all is gone. Never try this at home :-)
=> Go to ....
- Relativity for Dummies
- 4D Spacetime Block Universe
- Reciprocal time dilation
- Reciprocal length contraction
- Lengths do not 'appear' contracted!
- Measuring the contracted train
- Constant light speed
- Where is the bullet?
- Train experiment
- Slower and shorter - case study
- Minkowski vs Loedel
- Spacetime video
- Further reading