I have been reading about the concept of the relativity of simultaneity which states that whether two events, separated by space, occur at the same time is relative to the observer's reference frame. While this is pretty incredible, it is something I can wrap my head around, particularly at the extremes of the reference frames (i.e. an observer traveling near the speed of light).

My question is, is there such thing as being absolutely simultaneous? If we assume there is no observer, can I not sit down and say that something is happening everywhere in the universe right now? If no reference frame is involved (i.e. there is nobody actually observing anything) is there not simply an in-my-mind now?

I'm not sure if ignoring the observer and reference frame even makes any conceptual/real sense, but while reading about the concept there seems to be a lot of emphasis placed on when the observer would see the event occur, but I'm simply asking about the event occurring. It would be impossible to know the events are simultaneous (or not) without the observer, but can I not consider a reference-less point-in-time now existing across the universe?

A somewhat similar way of asking is this: Imagine if it where possible to freeze time instantly, everywhere. If I could do this and had infinite frozen-time to go around and observe everything, could I see what was simultaneously happening everywhere in the universe at the moment time had been frozen?

  • $\begingroup$ Physical events occur in space-time. In SR and GR, that space-time has three spatial dimensions and one time dimension, which means that a physical event can be denoted by four numbers. But the values of these numbers depend on what frame of reference you use. In a given reference frame, then, you can define simultaneous events as all the space-time points that have the same number $t$ for the time dimension. That would be your "now". But the theory of relativity teaches us that a different frame of reference will generally not give you the same set of events if you use $t$ in the new frame. $\endgroup$ Jun 29, 2021 at 13:46
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    $\begingroup$ Note that no one needs to "observe" or "see" or "measure" anything here. It's just that, in order for two events to be considered simultaneous, they must have the same value of the time coordinate, and that value is always on reference to something. $\endgroup$ Jun 29, 2021 at 13:56
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    $\begingroup$ Relativity of simultaneity is true even after you compensate for the finite travel time of light. That's illustrated by this anim from that page. $\endgroup$
    – PM 2Ring
    Jun 29, 2021 at 17:28
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    $\begingroup$ Can there be "straight ahead" without a reference frame? If there is no observer, can I say that something is "straight ahead" of something else? Is Chicago straight ahead of New York? What about "left"? Without a reference frame can I say that something in the universe is located "to the left" of something else? Learning relativity means internalizing the fact that "simultaneous" is exactly as frame dependent as "forward" and "leftward". $\endgroup$
    – WillO
    Jun 29, 2021 at 20:52
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    $\begingroup$ "If we assume there is no observer, can I…" If there's no observer, then in particular, there's no you. $\endgroup$
    – Sandejo
    Jun 30, 2021 at 9:31

3 Answers 3


can I not sit down and say that something is happening everywhere in the universe right now

You can, but it will be from your point of view. You can also sit down and declare that your point of view is the best point of view and this will define absolute simultaneity.

The problem is others can do this too and they will arrive at different notion of simultaneity and they will most likely not take your declaration seriously, as there is nothing physically special about your point of view.

while reading about the concept there seems to be a lot of emphasis placed on when the observer would see the event occur

It is not really about seeing. You always see past, you never see a present moment. Simultaneity is defined by symmetry of spacetime.

If you send light pulse to event A at time $t_0$ of your watch, there it reflects back and returns to you at time $t_1$, then you declare event A to be simultaneous with event my watch is showing time $(t_1-t_2)/2$. But you are not seeing event A at this time. You only know those two events are simultaneous, because light travels at the same speed everywhere and thus the time to go there must be half of the time to go there and back, not because you see them simultaneously.

In principle, you can come up with whatever definition of simultaneity you wish for as long as it defines equivalence relation on set of spacetime events. The advantage of using method I described above is that this equivalence relation respects the symmetry of spacetime and thus it is well adapted for description of physics and thus it is the definition physicists use.

is there not simply an in-my-mind now

Yes there is. Relativity of simultaneity is relevant for events that are separated from each other by bigger distance. For local phenomena (like your mind) the now is well defined.

if it where possible to freeze time instantly, everywhere

Again it is, but from your point of view. Another observer would think you are freezing universe gradually from one place to another.

It would be impossible to know the events are simultaneous (or not) without the observer

Simultaneity is mathematical property of spacetime and certain class of coordinate systems. It requires nothing physical to be there. At least as far as mathematical model of special theory of relativity goes.


Whenever physicists or physics students start an experiment or thought-experiment, the first thing we do is try to isolate the system from all outside influence so that we can precisely vary initial conditions, let the system go, and watch what happens. Metaphorically, experimentation (and solving textbook problems) is a process of building a tiny universe in a box and looking at it through a peep-hole while we start it running, let the entire history of spacetime in the universe elapse, and then destroy and rebuild the universe if we want to test it again.

Even in relativity, it's convenient to take this approach. We might need a bigger box to fit blueshifting galaxies and space ships zipping past each other at 0.99c, but building a metaphorical box around an isolated section of the universe and putting the observer outside of it, peering in through a peep-hole, works great. We position observers in the box, let them take their measurements, and have them send their measurements to us through the peep-hole.

The approach completely pervades our thinking about how to think about the sciences, to the point that we naturally reach for it whenever we try to answer any question. But it is a fallacy. The observer has to be inside the universe with the experiment, affecting and being affected by the experiment. By definition, a universe is everything that can ever affect and be affected by everything else, so if our observer is really outside of the universe, they can make no observations - or even falsifiable guesses - about what's going on inside.

Therefore the success of the universe-in-a-box method comes not from putting the observer outside, but from using a consistent observer: similar mass, velocity, position, charge, etc... and keeping them far enough away from the subject of experimentation so that their contribution to the initial conditions of the experiment is small.

For cosmology questions, the universe-in-a-box method completely breaks down. We want to ask a question about everything, and the first thing we reach for, because it's worked so well for us so far, is to put the universe in a box and put the observer "outside" - or if we're wise to the fallacy of our own thinking, to put a humanlike observer in a specific place at the edge of the box.

Neither approach works. The naieve approach, observer outside the universe, doesn't work, because the box is the whole universe and there's no "outside" to put the observer in. The nuanced approach doesn't work because wherever we put the observer, they'll be in the exact center of their observable universe, and besides, we don't want to ask a question about what the cosmos is like from the perspective of a human on the edge of the universe, we want to know what the cosmos is like, period. So, when we ask cosmology questions, we have to frame the question to include a variable observer.

Our whole system of doing mathematical physics is to chart a sequential procession of present configurations in a way that can be expressed in a system of equations or charted on a graph. But we can't make any definitive statements about the present configuration of the cosmos, because without a single definitive observer, we don't have access to a single definitive present.

This doesn't stop cosmologists from making mathematical statements about the nature of the cosmos... but it does make their job conceptually and mathematically difficult.


No. Simultaneity only makes sense within a reference frame. The reason is that the finite speed of life means that you can’t directly verify what is happening away from you. All schemes to establish simultaneity rely on waiting on some back and forth xchange of signals and establish simultaneity retroactively. Different schemes (reference frames) will disagree on what events are simultaneous.


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