Can you please explain to me in simple words why would the fact that time can go through different speeds imply that we don't share the same present? Because from where I stand it seems perfectly possible that we would share the same present with time having different speeds... Where am I wrong?

  • A little context would help.
    – Boba Fit
    Nov 23, 2022 at 15:44
  • @BobaFit Seems pretty clear. Nov 23, 2022 at 16:14
  • personally, I interpreted it as a question about traveling close to the speed of light and experiencing time dilation. A bit of context couldn't hurt.
    – emmagras
    Nov 23, 2022 at 17:07
  • @emmagras It is about that. Different flows of time; but is the present still not singular always? It is difficult either to prove or disprove. Nov 23, 2022 at 17:17
  • Singular in the sense that 'now' on Earth is the same as 'now' everywhere. Nov 23, 2022 at 17:34

5 Answers 5


Let us limber up with some very familiar 2-dimensional geometry. This will help us develop some thought-muscles that will, hopefully, get us over the hill for simultaneity.

Consider a collection of 5-foot-long ladders. (Feet because a 5 foot long ladder is something you can easily imagine, and 5 because of a maths trick that is coming.) How tall are these ladders?

You need to know their arrangement because "tall" refers to distance from the floor to the top. A ladder laying on the floor is not very tall. A ladder standing vertically is, of course, 5 feet tall.

Stand one of these ladders against a wall such that its top is 4 feet up. A little maths and it must stick 3 feet out at the bottom. Now this ladder is 5 feet long, but 4 feet tall, and 3 feet wide. (Pythagoras tells us this.) Give the ladder a kick so its bottom is now 4 feet out, and the top must be 3 feet up. Still 5 feet long.

So the ladder has a property that stays the same, its length. And it has some properties that can change, height and width. But the height and width have a relationship.

H^2 + W^2 = L^2

Height squared plus width squared = length squared. The familiar Pythagorean theorem.

So if you stood a bunch of ladders against the wall, all different angles so all different heights, you would not be perplexed. You know they are all the same length, just arranged at different angle against the wall. So even though it is the same length ladder every time, the height and width are different. It is not startling.

This introduces the concept of an invariant quantity. In this case, the length of the ladder does not change when you lean it at different angles.

Now relativity.

Let's have the definition of an event. An event is a point in space and time. It includes the position and time coordinate values. Say, right THERE =><= on your computer screen, at exactly 1PM today.

| E1
|_________> x

That's my little t-x diagram showing where event E1 is.

Relativity tells us that two observers moving relative to each other will each see the other's clock moving at a different rate to their own. There is a lovely complicated formula. But the ratio is dependent on the relative speed.

So two observers that are separated in space, and moving relative to each other, will disagree about the interval between events. That is, observer A might claim the events were separated by 10 seconds, but observer B might claim only 9 seconds.

That is, they don't agree on how long a time was between the two events. They don't agree on the time of the second event, so they don't agree on what events it is in the same "present time" with.

But they will also disagree about the distance between. That is, they don't agree on which events the second event is in the same location as.

And here comes the critical point.

There is an invariant. Suppose Observer A says 10 seconds between, and 20 light-seconds distance between (a light-second is the distance light travels in 1 second, about 3x10^8 meters). And suppose Observer B says 9 seconds between, how far will he say the two are physically apart?

That's where Pythagoras comes in, though in a different guise. Observer A says the events are (10,20) apart (in time and distance). Observer B says (9,X) where we want to know X.

It goes like so.

square root(20^2 - 10^2) = square root(X^2 - 9^2)

That is, where Pythagoras adds the square all the coordinates, Einstein adds the square all the space coords, but subtracts the square of time. That tells us (by solving for X)

X = 19.51922 (about)

So the moving observers will disagree about space distances as well as time. But there is a quantity that functions very much like the ladder length. And that is the distance squared minus the time squared. In this case, it is square root(300) for both observers. (Work it out for yourself. Take X^2 and subtract 9^2.)

So, just like the ladder on the wall, the invariant does not change.

So what is happening is, a physical thing, the separation between two events, is being observed by two different observers with different coordinates. It is as though one is leaning against the garden wall and so seeing the ladder at a different angle.

So how about the difference in "present time." Each observer will have a "zero" for time. Maybe they were at the same location some time in the past and could synchronize. But since then, they have been moving apart and their clocks going at different rates. So they will agree on the invariant quantities. Like the length of the ladder. But they will not agree on the time, nor space, coordinate values for events. Like the top or distance out of the ladder.

  • Sorry, words salad....
    – Gab Daud
    Nov 23, 2022 at 19:35
  • Re. "since then, they have been moving apart and their clocks going at different rates." In the example of GPS satellites with different clock rates, they adjust their clocks so that they can be synchronised and operate according to the present time. (Earth present, or universal present? is the question.) Nov 23, 2022 at 20:01
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    @GabDaud It's not though. There's an absolute ton of maths I have left out, since this is about 1.5 months of a university level course. But everything I have said follows very directly from relativity.
    – Boba Fit
    Nov 23, 2022 at 20:51

It may well be that the present, as experienced, is universal. A grander title being the nunc stans — the everlasting now.

"As experienced", because cognition is originarily intrinsic. Fine-tuned clocks and Skynet can take over later. Notwithsanding mental impairment affecting temporal perception, the present is internal and external: in terms of Dasein, both authentic (subjectively proper) and inauthentic (such as scientific consensus).

There are counterarguments, for instance, Prof. Dingle's 1937 discourse Science and the Unobservable

the abandonment by Einstein of the idea of the absolute simultaneity of events at different places.

and a critique of the 2008 publication Einstein, Relativity and Absolute Simultaneity

the first collection of essays devoted … to arguing that simultaneity is absolute.

Actually proving that the present is universal is somewhat tricky. The thought example with synchronised GPS satellites is encouraging, but how does it scale in highly warped spacetime? Is a synchronisation even theoretically calculable, or are there too many unknowns?

Nevertheless, at least the universal present is not disproved. To specifically answer the OP's question: "there is not only one present" inasmuch as it apparently cannot be proved.

  • I like this answer, thank you sir.
    – Gab Daud
    Nov 23, 2022 at 19:09
  • @GabDaud Do you know you upvote and/or 'accept' ✓ answers on this forum? Nov 25, 2022 at 16:22
  • I need at least 15 points of reputation to upvote/accept. Hence my comment.
    – Gab Daud
    Nov 25, 2022 at 16:52
  • Nevermind, I thought I had to upvote before I can accept. But I can't upvote. I won't accept it as the answer I really needed as I prefer mine in all honesty. So I should probably just accept mine.
    – Gab Daud
    Nov 25, 2022 at 17:06
  • From your first link:"The first example of such application to arouse general discussion was the abandonment by Einstein of the idea of the absolute simultaneity of events at different places, because of the discovery that it was impossible to determine absolutely whether such events were simultaneous or not. " This is where Einstein failed in my opinion. The fact that we can't determine with an absolute certainty the simultaneity of events doesn't imply that simultaneity doesn't exist. This was a stupid shortcut that should be re evaluated.
    – Gab Daud
    Nov 25, 2022 at 17:51

This issue is called the Relativity of Simultaneity.

It might help you to think about the most extreme case. From an outside observers perspective, for an object at a blackhole Event Horizon, time will stop. For the object time will continue, and the Event Horizon is not something distinct the object will experience passing through.

  • I find that the examples in the wiki page serve well my point. They basically say that events don't occur at the same order depending on the position of the observer. But this is so wrong! The observer has to take into account the distance of the light and to adjust the result knowing c. The ignorance of the observer if he fails to do so is not a proof that simultaneity doesn't exist!
    – Gab Daud
    Nov 24, 2022 at 17:43
  • Also I highly doubt that in the blackhole event horizon an object could exist. In such a place there may be no more time nor space where an object could fit. If the time stop, then it's in a quantum state.
    – Gab Daud
    Nov 24, 2022 at 17:49
  • I can't remember when it was the first time I encountered this argument about the position of the observer, but I do remember how utterly shocked I was by the stupidity of this "argument".
    – Gab Daud
    Nov 24, 2022 at 18:10
  • @GabDaud: "We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct." -Niels Bohr
    – CriglCragl
    Mar 21 at 23:05

The fundamental problem in all of that is that we can't measure time as an absolute and isolated property.

If all you had was camera footage of a blank void without a time stamp or something moving, changing or even being visible you'd have no chance to tell whether you're looking at a screenshot or an eternity of live footage. There would be no sense of time in that.

Like sure you have this idea that you'd be counting in your head "twenty-one twenty-two...", but that just enumerates how long you as an external observer would be looking at the screen it wouldn't tell you how much time has passed in that universe. Like neither whether it's a still or a live stream nor whether it's sped up or slowed down. Without things experiencing change (which includes you as the external observer), there would be no "time".

Like there could be such an observer or an imaginary clock ticking in the background in steady increments but neither are we such an observer to our universe, nor could we tell for sure that such an observer, that counts in their head, could even exist.

So what is it that we call "time"? Well essentially it's how the rate of change of continuous objects relates to other objects. Btw "space" has the same problem you can't really tell how vast a blank void is without some continuous objects that have changing relations towards each other. Like if you'd be looking at a still image you couldn't tell if things are small or just far away, large or just close to the observer.

It's when they move and you don't that you can iron out some ideas of "space" and "time". And obviously those are completely arbitrary and specific to your "inertial frame of reference" that is "you assume that you 'stand still' (are inert) and the rest of the universe moves in relation to you".

However there is nothing preferential about your inertial frame of reference to the inertial frame of reference of any other person. Yeah that's right you could place an observer at any imaginable point in "space" and they'd have their own inertial frame of reference.

And even if you pick the same starting point and the same physical clock mechanism you'd get the twin paradox: https://en.wikipedia.org/wiki/Twin_paradox

Maybe the talk about clocks obfuscates the point a little, but these clocks in the twin paradox aren't some wrist watches that are misaligned it's the fact that they will have physically AGED differently by the time they meet again and not because of increased radiation or stress of space travel but due to the fact that they moved faster than then the other.

Sure you can come up with these simultaneous planes and compute when a message sent by either of the observer will reach the other. So under ideal conditions it's possible to compute the time in the other inertial system using the Lorentz Transformation. But the fact alone that faster travel can make one of the twins be older/younger pretty much messes with the conventional intuition of "universal presence" and things "simultaneous". So why would the two agree on a time that is either biased towards one or unsuitable for both? And while you might find an observer for which the two age simultaneous the more object or people you'd consider the more impossible it would be to find an observer that has equal distance to all inertial frames of reference at any time.

So theoretically it might be possible practically it's largely not feasible or useful.


What I'm saying is the issue scientists have seems to be related to the clocks we use to measure, so let's say in some thought exercise that we invent a clock that can and does regulate itself according to its speed in space or the gravity around it or whatever parameter that distorts time that we're not aware of. Now if I can synchronize 2 of such clocks they will remain synchronized no matter what. The fact that we failed in doing this is not a proof that the universe experiences multiple present times. The fact that we could imagine some thought exercise like this tends more likely to be a proof on its own that there is only one present.

  • Actually, it's something that is already done with GPS satellites: in their orbit at high velocity and higher in Earth's gravity well than us, they experience a very small yet noticeable time dilation. But this dilation is corrected and they are kept synchronized, otherwise the system wouldn't be able to give us our position.
    – armand
    Nov 23, 2022 at 23:23
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    "a clock that can and do regulate itself according to its velocity" velocity with respect to what? Note that the only reason that the GPS stuff that armand mentioned is possible is because, in that case, we have a preferred coordinate system: one attached to the Earth.
    – Sandejo
    Nov 24, 2022 at 5:06
  • I thought velocity was a synonym for speed, I edited it. Also the only thing we correct with the GPS is how it's converting into present after all calculations... So it's not relevant whatsoever for this question, it's more serving my point.
    – Gab Daud
    Nov 24, 2022 at 5:48
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    Yeah, my point was to serve your point. As Sandejo mentionned, we of course need a common frame to define speeds. But once this is done, it's possible to use relativistic equations to synchronize two clocks although they don't experience the same time dilation, which defines the notion of present.
    – armand
    Nov 24, 2022 at 6:25
  • It is where you are wrong, the time dilatation doesn't define the present, it defines the speed of time related to other existing referential. We still can share the same present with different speeds... you would see me going slow for example as I'd see you going fast. Not an issue at all...
    – Gab Daud
    Nov 24, 2022 at 14:47

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