How can the past, the present and the future coexist at the same time? [closed]

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We all heard it many times, the theory of Time suggest it is parallel rather than being linear. This leaves the door opened for alternate realities and well, immortality. However, even after reading tones of lines and watching tones of documentaries, I still cannot understand how it makes sense while everybody in the scientific community seems to accept a theory they yet cannot prove. I don't understand how the past, present and the future can exist at the same time. I cannot warp my head around the idea that our younger and grown selves are somehow "living" at the same time creating different versions of ourselves in an infinite loop that is. How can the 5 year old me still be out there while the 28 year old me is here? That means the 56 year old me is already out there? Does it mean that our lives are already predefined? I have so many questions but I will end it here in order to stay relevant to the topic.

$\begingroup$ I have a final exam in 3 hours, that is what I need now: some past and some future :) $\endgroup$

$\begingroup$ "while everybody in the scientific community seems to accept a theory they yet cannot prove." FALSE. It is probably "everybody in the science fiction community" that you mean. All these logical speculations should be addressed to people who are trying to write science fiction that logically hangs together. "Tons of words ( or videos) do not science make". Einstein in the link is not talking of multiple worlds and such stuff. Just of the way he viewed the mathematics of general relativity. $\endgroup$

3 Answers 3

This is a good question and more philosophical in nature: Philosophy SE may be a better home for this question (there are indeed solid and knowledgeable physicist who loiter at that site). Essentially you are talking about the Eternalist or Perdurist conception of time - if you've not heard these terms before they may give you some terms to search by. Famous living subscribers to it include Hilary Putnam. Not so famous ones include the person writing these words.

But the essential idea is this: since simultaneous is a relative notion (different observers in different motion states will disagree on whether two events are simultaneous and even, for some pairs of observers, disagree about the time order of events), it makes little sense to say that the past, as defined by one particular observer, doesn't exist given that this past will be in the future of some relatively moving observer. Likewise for the future of any given observer. There could be, theoretically, an observer in a relative motion state to you whose "present" time slice includes the five year old you. However, that does not mean you could communicate with the five year old you through this observer (see below). It thus becomes difficult to say what is "real" or "extant"- by whose standpoint would you define the existence of things?

Physicists don't generally think to these terms: the notions of "real" and "extant" in this context are more complicated than needed: physics simply either describes the outcomes of experiments or tries to find rules to foretell those outcomes. It is however true that one set of these outcome-foretelling rules, namely special and general relativity, take on their simplest description if you think of spacetime as a unified whole (unified by the geometrical notion of a spacetime manifold).

Some Technical Details

Let's look at some technical details about the above, and we'll see why most of the above discussion lies outside physics and is needfully philosophical and why you can't talk to or see your five year old self.

1. Discussion of Eternalism is usually made in flat, or Minkowski, spacetime. There are three reasons for this: (1) it is a very good approximation (large scale) to our own universe (2) it is simple and (3) it is a valid solution to the vacuum Einstein field equations. So deductions made from it are properties of gravitation theory;
2. We assume the experimentally supported observation that there are no time machines. You cannot communicate with your five year old self: more generally and concisely, any event in spacetime can only influence other events that lie in the the former event's future light cone. Remember I said above that for relatively moving observers may not agree on the order of events. One of the elegant properties of relativity, though, is that if event B is in the future light cone of event A, then this is true for every observer. Event order only changes for events that lie outside each other's future (or past) light cones. They lie in the so-called "elsewhere" or "elsewhen" on each other's spacetime diagrams. This is very important, for it means that we can make our everyday observation that causes do not come before effects consistent with relativity if we postulate that causally connected events must lie in each other's future light cones. $A$ can only be a cause of $B$ if $B$ lies in $A$'s future light cone. Indeed this postulate is the reason why relativity says that locally something cannot move faster than $c$: if it could, it could see the order of causally linked events swap and therefore a notion of causality could not be consistent with relativity.

Now, with all this out of the way, let us look at what I mean when a relatively moving observer can be "contemporary" with the five year old you. Suppose an observer is some distance away from you and begins to move towards you. Because our spacetime is flat, both observers can set up so called affine co-ordinate systems - these are roughly those defined by laying rigid measuring rods end to end to give a grid co-ordinate system. These co-ordinates transform by the Lorentz transformation when you switch to a relatively moving frame, and what the Lorentz transformation shows is that the $t=0$ hyperplane for the observer moving towards you now intersects your world line at some point in the past. Likewise, your $t=0$ plane now intersects their world line in their past. If you are space a great distance from the observer, the effect can be very large: your $t=0$ hyperplanes now intersect each other's world lines in the distant past. Over cosmological distances, this effect can easily be great enough that that past goes back to your five year old self.

None of this, unfortunately, allows you to communicate with the five year old through the moving observer as a go-between, because, although the five year old is on the observer's $t=0$ timeslice, they are not in the observer's future light cone. So that, by the time the observer reached you, you would have aged more than 28-5 years, so the effect cannot be used to visit or communicate with your 5 year old self or indeed any younger version of you. Indeed, if causality is true, no experiment done by the five year old you can be influenced by choices that you make as a 28 year old (the converse, of course does not hold: the five year old can most certainly influence the 28 year old by the choices she makes - in technical jargon - the five year old has a causal, forwards-in-time-pointing link with the 28 year old).

So you can see that the questions addressed by Eternalistic thinking are essentially about the "reality", or otherwise, from the standpoint of any observer, of events that lie outside that observer's future and past light cone. As we have seen, if causality is true, then no experiment can shed light on the answer to this question and so the question becomes one outside physics and in the realm of philosophy. Events that are "contemporary"(on our $t=0$ hyperplane in flat spacetime) with us but at a distance from us cannot be influenced by us, by the universal speed limit $c$. Eternalism essentially asserts that these events do have a reality, even if if can't be tested by physics. Reasons for subscribing to an Eternalistic standpoint include (1) not wanting to have "reality" being an observer-dependent notion (this doesn't bother physics, because the events in question cannot be probed by experiment and no two observers that can eventually communicate can disagree about mutually observed events), (2) some events outside our future light cone can influence events in their future light cones that will, far enough into the future, intersect our future light light cone. So to deny their reality seems a little weird. I liken this very much to the philosophical position that rejects solipsism: we cannot probe certain events experimentally now, but we may eventually see their effects, so it seems simplest to ascribe to those antecedent events a reality in the same way the one cannot prove one is not the only person in the universe and all else their imaginings, but the simplest explanation for the World we see is that there are indeed other people. As Richard Gregory once quipped: solipsists were welcome to study in his department, but they must abide by the special solipsist's law that they must always buy the beer for everyone on Friday evenings.

As for whether all this implies that the Universe is "predefined" or "fore-ordained" that is indeed a new question. A literal reading of the general relativistic description above seems to say so, but another branch of physics, quantum mechanics and quantum field theory, seems to say that the notion of an experiment's outcome's existing before the experiment is done is a meaningless notion. You can begin your research into this new question by looking up the notion of Counterfactual Definiteness.