In the same river, you can still enter twice

Do you think that one river can not enter twice? Modern physics considers otherwise

We all heard the saying: “you cannot enter the same river twice” [in the original there’s no time like the present, that is, “there’s no other moment right now” / approx. trans.]. In a broad sense, this means that it is necessary to “seize the moment,” or end up doing procrastination. From a psychological point of view, this makes sense. People feel the “passage of time”, and the current moment has a special quality. Discarding hypnosis and dreams, there is no way to directly experience the past or the future, just as we experience the present. But is the saying true? Does modern physics agree with the fact that you cannot enter the same river twice?

Our best to date theory of space and time is the theory of relativity. Before the Einstein revolution, which happened more than a hundred years ago, physics considered time as an “external parameter” - an independent, fundamental feature of reality, which is not influenced by any other factors of the Universe. Whether the course of time is real or apparent (this philosophical dispute is terribly old, it went before Einstein, and his theories do not allow this discussion), we now know that time intervals are not external, and are not defined everywhere the same way. Time is an internal component of the physical system, a dimension interlaced with three spatial. Together, they form "space-time", and they are influenced by various factors, including speed (relative to other observers or systems) and gravitational forces. Since the theory of relativity postulates the constancy of the speed of light for all observers (even moving relative to each other), space-time itself must be deformed, and the concept of a time interval becomes flexible.

As a result, there is no universal notion of current moment that applies equally to all observers. What seems real to me may be in the future of another person and in the past of the third. Simultaneity is relative. Suppose your spouse went on a business trip and is on the other side of the globe. Just at that moment, when you think about how you missed her, she presses the “send” button in the mail client in a letter with the words about how she missed you. Romantic, isn't it? But although these two events occur simultaneously from your point of view, for a newcomer flying in a spacecraft orbiting the Earth, the temporal order may differ. If the ship flies towards your spouse, the stranger will see sending an email before you get a dreary thought. If the ship flies away from your spouse, her letter will be sent after that. Therefore, due to the theory of relativity, there is nothing special or unique in the current moment. In fact, there are so many things that are similar to what is happening right now - other observers will call your present your future or your past.



And that is not all. Although GR is very well supported by experiments, it is not a complete theory of the Universe. It does not include lessons learned from another pillar of physics of the 20th century - quantum mechanics. And these two theories are indeed incompatible in their current form. Studying, for example, black holes or the origins of the Universe, physicists need to describe very massive objects (which GTR does well) packed in a very small space (belonging to the domain of quantum mechanics). Such scenarios require a rethinking of physics, and one can hope that one day they will lead to the discovery of the quantum theory of gravity. One of the difficulties of such a search is that in these two theories time is treated differently. In canonical quantum mechanics, an earlier approach to time is used as an external parameter that the Universe does not affect. But we know that time cannot be considered in this way, since GR tells us that gravity affects time - and quantum physics does not explain this. Therefore, the new theory of quantum gravity is likely to again turn our view of time, as Einstein did a hundred years ago. The revolution in physics of the XXI century can occur very soon.

And although different versions of quantum gravity have different theories about the nature of time, perhaps one of the most interesting perspectives is the theory of shape dynamics. The dynamics of form generally refuse time and try to explain the effect of gravity through the evolution of spatial forms, each of which tells the cause-effect history of the appearance of the Universe. The theory gives us an uncountable number of presents, "now." Each of these moments is similar to a photograph from an album containing historical records of how we have reached the present state, captured in separate photographs.

Proponents of form dynamics call these timeless instant "nows" "time capsules," and argue that our brain stitches them together, which leads to the appearance of the illusion of the passage of time. Think about how we perceive the movement of a jumping cat: our eyes perceive instantaneous "snapshots" of a cat at different points in the air, and the brain stitches them together to create the illusion of movement. The dynamics of form assume that a similar process is responsible for the perception of time itself.

Regarding our aphorism, the dynamics of form agrees with the theory of relativity: it is impossible to say that it is impossible to enter the same river twice. If the dynamics of the form is true, then the Universe consists of a huge number of “now” moments, and no constant course of time exists at all.

But form dynamics is far from the only approach to quantum gravity. What is the fate of time, for example, in string theory? It turns out that here it is impossible to talk about the uniqueness of the current moment. String theory asserts that the fundamental point particles in physics are best modeled as one-dimensional extended objects capable of taking the form of a loop or segment of "strings". The string theory platform requires the adoption of additional spatial dimensions (at least ten, depending on the type of theory), closed in on themselves so that they are not visible in the three-dimensional world we are used to. An analogy with a garden hose can help here: if you look at it from afar, for example, from the window of the second floor, the hose lying in the courtyard seems to be one-dimensional, having only a length. But if you approach him, you will understand that he has a second property, width.

Just as spatial dimensions arise from string theory, so time arises from a timeless element in a suitable version of this theory. In this sense, time is not a fundamental concept in string theory, and we cannot say that the present time is unique because there is no time at all.

And now, excuse me, I have to redo a whole lot more, and I have to catch the present moment, whether it really exists or not.