Special relativity and the lack of a now moment is causing me a bit of grief.

How meaningful is the notion of now here on Earth between each of us? Does the lack of an objective now moment in any way change how we should think about our friends and family and philosophy between the relationships we form?

My friends' and families' notion of now doesn't ever line up with mine; what does this mean for their conscious perceptions? Am I getting something wrong?

  • Regarding things like time dilation, hypersurfaces of simultaneity etc
    – Danny55
    Jan 31 at 16:07
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    you have to write about your perception of now. If your perception doesn't line up to other's notice, that can be, why not. People can be different in memories, perceptions, waitings. This world is different and it is normal that your observer is able to notice this. This is a point of psychological, when your notice is not coincide to many others( more then 2-3+) you have inner conflict to agree with others or to become a "conflict" with group. Our brains usually have primacy society instinct "to agree with group" and if you don't do, you can get distress. So i think this is it. Jan 31 at 16:35
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    Special relativity does not say there is no "now" moment, just that it's much more difficult to establish than we think intuitively. Even if your twin brother takes a very fast rocket and experiments time dilation, provided you know his precise schedule and he abides by it, and you can do the proper calculations based on his rocket acceleration and trajectory, it is perfectly possible to know what he is doing in his rocket at any arbitrary time.
    – armand
    Feb 3 at 1:57

4 Answers 4


Special relativity has no bearing whatsoever on your day to day activities. If you are moving relative to someone else, then yes in theory you will be time dilated in their frame of reference and they will be time dilated in yours, but the effects at everyday speeds are so utterly minuscule compared with the typical reaction time of your brain that you can ignore them completely.

As for the relativity of simultaneity, that too is dependent on speed but it is also dependent on distance, decreasing with proximity, so the effects are again absolutely negligible in day to day life.

In any event, you should not equate the effects of the relativity of simultaneity with the lack of a common local 'now'. If you are not sure what I mean by that, please post a question of physics SE.

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    So do you think this means we should think about the identity of our loved ones in any other way than normally?
    – Danny55
    Jan 31 at 16:33
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    I can confidently assure you that the implications of special relativity should have no bearing whatsoever on your relationships with your loved ones. Jan 31 at 16:35
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    @άνθρωπος thank you. I am a very trustworthy person, except when I am playing intellectual pranks, but feel free to come back to me at any time if you have the slightest doubt about my assurances. Jan 31 at 17:22
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    You certainly were. The twin paradox provides a good example of the fact that everything happens (that we are aware of) at a common local now. The 'Twin Paradox' provides an example of this. During the turnaround, the travelling twin's plane of simultaneity tilts, so that 'now' for the twin becomes a much later date on distant Earth. But when the traveller returns, the twins still reunite at a common 'now' although they disagree on what year it is. Jan 31 at 20:10
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    No, there are not multiple versions of each observer. If you are in a different reference frame to me, you will use different coordinates to specify where I am in spacetime, that's all. I am where I am. It is analogous to using rotated frames of reference to map a country. Two different maps might be oriented at an angle relative to each other, and will therefore use different coordinates to refer to the same places. Feb 1 at 9:52

The moment of a conscious thought is a personal now, which you could line up with a time, say 3pm. If your friend is orbiting Earth in a satellite their clock might tick slower, but it can be adjusted to synchronise with your clock. Using the synchronised time you can raise a glass at the same moment — sharing the 'now' moment. Nevertheless your friend's time is passing slower than yours; they are aging slower.

In more complex space topologies synchronisation becomes problematic, to the extreme of a gravitational singularity. If your friend is in a gravitational singularity you will not be able to synchronise clocks.


All of our perceptions (whether you consider special relativity or not) are about the past. There is always some time between an event and our becoming aware of it, and this is mostly dominated by the bio-electrical processes in our sensory nerves and brain, not by relativistic effects, you can safely ignore them in this context.

So in any case there is no true "shared now". The moment you realize something about the world around you is unrelated to the moment someone else realizes the same thing (except of course that these realizations can only happen after the thing happened).

In relationships, what counts is the memory and the awareness of shared experiences, knowledge, and interaction. Since you know that your friends and family members are individuals with a similar brain, you can reasonably assume that they experience similar perceptions as you do. Your familiarity with their way of reasoning (even if it differs from your own) enables you to "simulate" and thereby anticipate their reactions to the events that you both experience, and this is what creates the notion of a shared "now". When you're not currently interacting, the question of whether something happened simultaneously loses much of its relevance, and the shared "now" becomes much more blurred to something like "today" or "this year".


Even in a nonrelativistic sense, temporal asynchrony is observable. Compare the Sentinelese tribe (Andaman Islands, Indian Ocean) who're, for all purposes and intents, in the stone age and New York, USA, perhaps Silicon valley is more apropos, at the leading edge of 21st century technological innovation.

Same goes for certain luminaries e.g. Siddhartha of Buddhism abd especially Mahavira of Jainism developed ethics only practicable in the distant future when the technology necessary becomes available.

Time is not exactly moving at the same rate for everybody.

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