The Help Center recommends I 'fix' this question:

(original question)

"It seems self-evident that everything exist in the Now. Notwithstanding time-dilation and different rates of the passage of time and entropy, doesn't this all still happen in the same universal moment?"

This question was asked with a rusty but apparently adequate awareness of special relativity. However, it may have been mistaken to assume that 'presentism' only refers to the present. There is a fuller definition according to this article:

Is There an Alternative to the Block Universe View?

If one can talk about a widely (explicitly or implicitly) accepted view on reality it is presentism – the view that it is only the present (the three- dimensional world at the moment `now') that exists. This common-sense view, which reflects the way we perceive the world, has two defining features: (i) the world exists only at the constantly changing present moment (past and future do not exist) and (ii) the world is three-dimensional.

According to special relativity the universe is four-dimensional, so presentism in this form is ruled out.

Nevertheless isn't the present universal? For example, regardless of the the observers' frames in the diagram below, they see event A at the same time, say, at t[0] = "the present".

enter image description here

The exception to this post-relativistic presentism comes only from the realms of time-travel, which would permit the past and the present to coexist. Most speculative, of course.

So is this question on presentism correct? (If not blindingly obviously so.)


The following quote from "The Feynman Lectures on Physics, Vol. I, 17-3 Past, present and future" illustrates why the answer might be less than obvious. However, just because the present may be unobservable does not mean it does not exist. As far as I am aware, it does. It seems some physicists discount its existence because it is unobservable, which may be the reason for the confusion.

enter image description here

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    $\begingroup$ I'd call excluding the things that disprove your point "bad philosophy." $\endgroup$ – Kyle Kanos Apr 1 '16 at 10:48
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    $\begingroup$ There is much entertaining but utterly pointless discussion to be had about presentism, eternalism, the block universe etc etc, but such discussions should be confined to the philiosophy SE, bars (or any collection of drunken physicists) or anywhere apart from here. This isn't physics. $\endgroup$ – John Rennie Apr 1 '16 at 10:50
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    $\begingroup$ I'm voting to close this question as off-topic because it's not a question about physics $\endgroup$ – John Rennie Apr 1 '16 at 10:50
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    $\begingroup$ I can write down a metric that describes a wormhole then do quantitative calculations with it. This is using an established mathematical model (GR) and established mathematics (differential geometry) to do calculations that any physicist can reproduce to test my work. The same is not true of maunderings about the philosophy of time. $\endgroup$ – John Rennie Apr 1 '16 at 12:11
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    $\begingroup$ @ChrisDegnen: most (all?) of us will have discussed ideas like this long into the night usually with the aid of alcoholic beverages, and this certainly includes me. It's not that the subject isn't fun to talk about, it's just that this isn't the venue for it. The seven (at the time of writing) downvotes don't mean we hate you, they just mean your question is out of place here. $\endgroup$ – John Rennie Apr 1 '16 at 15:20

According to the classic physics/Newton there was an absolute time in which things happens. But this intuitive perception of reality is left since Einstein proved that there is not an absolute time but a relative one depending on the observers' frame. Not our intuition is valid but what is measured.

According to Newton, absolute time exists independently of any observer and progresses at a consistent pace throughout the universe. Unlike relative time, Newton believed absolute time was imperceptible and could only be understood mathematically. According to Newton, humans are only capable of perceiving relative time, which is a measurement of perceivable objects in motion.

So already Newton would suggest that if you can't measure it it is imperceptible. So if in special relativity one thing happens at two different times only those are valid because measured and experienced. And to measure the time those measurements were made is not possible because the absolute reference time is imperceptible.

So perhaps the common event happened at one moment but there is no access to. In this line see also Kant. But perhaps Kant would even go further and presume that time exists only in our head.

  • $\begingroup$ So do any physicists maintain that the observers' frames all exist simultaneously, in the same instant? $\endgroup$ – Chris Degnen Apr 1 '16 at 11:13
  • $\begingroup$ I think that the words 'moment', 'exist' and 'instant' all refers to the same kind of absolute definition like classic time. So they could exist simultaneously but it is a meaningless word as there is no basic measurement. And for real physics it is mostly a matter of measuring or predicting but rather than of metaphysical kind. $\endgroup$ – Marijn Apr 1 '16 at 13:11
  • $\begingroup$ If quantum-entanglement turns out to permit instantaneous interaction this should happen in the common moment, across all observers' frames. In other words, the 'instant' wouldn't happen at different times for entangled particles. $\endgroup$ – Chris Degnen Apr 1 '16 at 13:28
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    $\begingroup$ That is not true - the entangled particle wavefunction has to be consistent with relativity, and there is not definition of common 'instant' in different frames. $\endgroup$ – Jon Custer Apr 1 '16 at 13:43
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    $\begingroup$ However time gap is between the past events. $\endgroup$ – Chris Degnen Apr 7 '16 at 9:30

Einstein's 1905 invalid argument:

http://www.fourmilab.ch/etexts/einstein/specrel/www/ ON THE ECTRODYNAMICS OF MOVING BODIES, A. Einstein, 1905: "From this there ensues the following peculiar consequence. If at the points A and B of K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its arrival at B the two clocks no longer synchronize, but the clock moved from A to B lags behind the other which has remained at B by tv^2/2c^2 (up to magnitudes of fourth and higher order), t being the time occupied in the journey from A to B."

Why is the moving clock slow and the stationary one fast? No such asymmetry follows from Einstein's 1905 postulates. What validly follows is that the moving clock is slow as judged from the stationary system, and the stationary clock is slow as judged from the moving system. Einstein's conclusion above (the moving clock "lags behind" the stationary one) is invalid - it does not follow from the postulates. So even if Einstein's 1905 postulates were true (actually the second one is false), Newton's absolute time is not refuted by special relativity.

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    $\begingroup$ What you say is the usual interpretation of the words. The only difference between you and him is that you postulate an absolute system of reference (otherwise you can't have an absolute time) and all evidence points against this... $\endgroup$ – Martin Apr 1 '16 at 16:10

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