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Since everything (at least what we observe) is travelling through the time in the same direction and speed why do we even perceive the time? Of course, there is the time dilatation, but it is negligible between you and me, for example. If it is so, why do we even observe the effects of time when we do not observe changes of something which is moving through the space with the same velocity (or a little bit different) as we do?

What is the difference between existence and non-existence of time? What is the difference between time and space?

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    $\begingroup$ FWIW, the existence of "time" as something that has physical quality is not something that is unarguably accepted by physicists; you can handle all of the physical equations without adding any real sense to the $t$ variable. We need it to describe processes with entropy >0 etc., but from an elementary particle's POV, time doesn't pass at all, only its environment changes. see en.wikipedia.org/wiki/Philosophical_presentism & en.wikipedia.org/wiki/Time_perception $\endgroup$ – vaxquis May 19 at 12:54
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    $\begingroup$ everything ... is travelling through the time in the same direction and speed This idea of things traveling through spacetime at $c$ is mostly something you see in popularizations, e.g., Brian Greene. It's not necessarily wrong, just not very meaningful or useful. If you insist on using it, then (a) it's motion through spacetime, not through time, and (b) it's not all in the same direction -- different things travel through spacetime in different directions, but all such directions are timelike. $\endgroup$ – Ben Crowell May 19 at 18:43
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You are able to detect a change in things around you—that an egg can break, the sun can rise, a colour can fade. So, you can detect different states of things. And those state are obviously not all present simultaneously. What should we call this "phenomenon", this "feature" of the world? We call it time.

The fact that everything doesn't happen at once, is given the name: time.

So, it is not obvious when you claim that "everything [...] is travelling through the time in the same direction and speed". Because time is only noticeable when things change. Hypothetically, if everything was slowed down equally much - all events and behaviour, all chemical reactions - then we wouldn't notice. Because what we measure time from - the changes in our surroundings - would have been slowed down just as much as our perception.

You seem to be aiming towards the conclusion that we shouldn't be able to notice anything changing since we all move equally fast through time. But this idea is an upside-down way of thinking, because if we really couldn't detect changes, then we wouldn't have had a need for a concept like time in the first place.

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    $\begingroup$ This is a bit of a circular argument. Without time there is nothing. The fact that everything doesn't happen at once conjures up an image of a static spatial universe somehow, and this is wrong. SciFi gets this wrong all the time and this just adds to that misconception. A universe without time would be indistinguishable from nothing. Light would not move, there would be no energy, and no part of the universe could communicate with any other part. Nothing would happen. Nothing would exist. It would just be a void whose size and 'contents' would have no meaning. $\endgroup$ – J... May 19 at 14:03
  • $\begingroup$ @J... That sentence you quote does not rule out the fact that without time, there would be nothing. Just like there would be nothing without space. Existence requires both, agreed. But explaining time meaningfully is tough due to its fundamental nature - I like the explanation of time being the fact that everytything does not happen at once, and space being the fact that everything is not at the same place. Because they are simple and I have not heard better explanations for laymen. $\endgroup$ – Steeven May 20 at 5:26
  • $\begingroup$ Many things that are palatable are not necessarily good. If everything does happen at once it still suggests that everything and its happening are two separate things. They are not. If everything is in the same place then there still remains no degree of freedom for anything to evolve, and so again it is misleading. A good explanation should be easy to understand, but if it fails to refine our mental model of the universe then it is not helpful. $\endgroup$ – J... May 20 at 10:29
  • $\begingroup$ @J... You are entitled to your opinion, sir. I disagree largely with this explanation not being helpful for a general idea of the notion of time. $\endgroup$ – Steeven May 20 at 11:32
  • $\begingroup$ Consider that we all naturally have an intuitive sense of time that already tells us as much. This explanation only serves to reinforce that incomplete intuition. Our motion in time defines everything about us. Should we try to slow our passage through time we eventually lose the ability to communicate with the rest of the universe and become a black hole. Space collapses without time, time stops without space. Time is what allows energy to evolve through space. Space is what results when energy evolves. Time is much more than just a ruler we mark the past and future on. $\endgroup$ – J... May 20 at 15:55
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Once you ask about perception, it is no longer physics question.

Anyway we do not percieve time. Look around and all you will see is present moment. The time is abstraction given by the memory of changes that were once happening. And to derive from this memory the flow of time is an abstraction. All the percieved memory is here and now, but there are also links that connects which events predated the next.

Take for example a diary. You know that one event was sooner than later because the later is written on the next page. From that you abstract away the flow of time.

Now to return to physics:

The time is defined by its measurement, and we choose such definition to make physical laws as simple as possible. So let us have some clocks, that have such a period of repeated events, that we are satisfied to call the time intervals between those events as same, f.e. pendulum.

to say something happened at some time, you will say the n-th click of the pendulum and the event happened at the same time. Then you write down to your diary (or better yet - lab report) at this click this happened. At the end of measurement you have huge diary with a lot of associations between clicks numbers and events. If the events are position (x) of some object, then you have graph $x(n)$. By defining time interval between two clicks f.e "1 second" you can get graph of position vs time $x(t)$ and then seek the laws that are consistent with the graph, f.e. $F_g=m a$, where $a$ is acceleration of the object, $F_g$ gravitational force and $m$ mass of the object.

In Newtonian physics saying that something happened at same time as click of some ideal clock is absolute. Einstein however realized, that this notion is absolute only if the two things happened not only at the same time, but also at the same place. Then you can say they happened at the same time, because you have seen them both happened at once. If they are not at same place, you must adjust for light speed and here all of the magic of special relativity comes in.

So time in physics appears only as a set of data together with their relations to clicks numbers. But the data are always percieved whole at any moment, you never see any past or future or flow of time anywhere.

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  • $\begingroup$ There are some questions about perception that are valid physics questions. For example, asking about how light is received by the retina, how the retina converts light intensities into electrical signals, how those signals are carried to the brain, and how the brain processes those signals are all questions that can be answered within the realm of (bio)physics, and indeed are all parts of the answer to the question, "How do we see things?". It's specifically questions about things that inherently aren't experimentally accessible that aren't valid scientific questions. $\endgroup$ – probably_someone May 19 at 16:21
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    $\begingroup$ 'Imagine you are in a room with no windows, no doors, no sounds, no outside interference at all' - In this case, the perception of time fails. Time could simply not exist, and the person in the room would know no different. $\endgroup$ – djsmiley2k May 19 at 17:28
  • $\begingroup$ @djsmiley2k Not true. The person could still feel their own heartbeat. $\endgroup$ – probably_someone May 19 at 19:00
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Some things to consider:

  • Humans exist because we evolved over time. That is, humans at time A makes it more likely to find humans at time B, because we reproduce.

  • If we evolved over some other dimension, might we perceive that dimension as time? I don't know.

  • Our brains have components designed to measure time. We know from daily life that time perception has functional utility.

  • It is not hard to imagine how an object within time can nevertheless perceive it. A pendulum clock "perceives" time through a differential equation that relates position and acceleration. It "remembers" the time because the gears in the clock advance only in one direction with each pendulum swing.

  • If you stretch your imagination, maybe you can say the ocean "perceives" space by the variation in pressure from floor to surface. Like the pendulum clock, it creates a variable that varies over the dimension to be measured. Whether this counts as perception depends on how you characterize it.

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