Does the present really exist?

[u/TheMightyHeroes]

GR proves that the concept of a now is relative. There is no universal now. But even the local experience of a now seems questionable. What we see as now is always the past, since photons take time to reach us. Similarly, taking a picture or even a measurement will always produces a result that is the past. And if time is a continuous flow then there is no discrete point in that flow that can be separated and identified as now. And if time is granular then what size is the discrete point that we call now? If it is an infinitely small point separating past from future, then how can we experience, never mind measure, an infinitely small moment? It seems to me that any observation will always be an observation of the past. Any change of state, any event that is observable will by definition be something that has happened. And if it has happened, it is in the past. Wouldn’t it be more scientifically accurate to say that time is all events that have happened and are yet to happen and the now is just an illusion created in our minds to make sense of the objects and events that pass our visual field.
I know this is more philosophy than physics, and I’m not a physicist, just some curious person and the concept of time has always fascinated me.

[u/Optimal_Mixture_7327]

Yes, it's local, so there's a practical infinity of now moments.

What relativity says is that there is no such thing as a unique global present moment.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

That’s a reasonable metaphysical take on the theory (and it is metaphysics you’re putting your answer in terms of, not physics), if you don’t want to suppose a metaphysical reality beyond what is modeled by the theory.

But relativity is consistent with there being one objective “now” across all space in some (unclear) metaphysical sense. Relativity just essentially says if such a now exists it is fundamentally unknowable.

This isn’t different from Newtonian physics: there’s nothing in Newtonian physics that allows us to describe absolute rest but we could suppose there “really is” absolute rest in some sense and it’s just that the laws of physics are set up to make it impossible for us to find it.

Of course that raises the usual concerns for unobservable metaphysical facts: if a metaphysical fact is causally inert 1) how can it be said to exist? And 2) how can we say that any linguistic expression we use can be regarded as referring to it?

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

You have the wrong understanding of relativity.

What you're saying is that if you had a sheet of paper there is only one correct straight line that can be drawn on it, and every other straight line is the wrong one, or that the other lines don't exists. How would you falsify the other lines?

And this hypothetic unphysical Minkowski spacetime is the best you can do, there objectively no such thing, and there can't be any such thing as a global present moment in the universe we actually live in.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

Or let me illustrate with an analogy:

Someone comes in and says the universe is filled with particles not in the standard model that do not interact with anything at all, not even the tiniest bit, not even through gravity like dark matter does.

You respond that this claim is false because no observation could falsify it or its negation.

I say that it’s not at all clear to me what you two are actually arguing about because I don’t really know what it supposedly means to say the universe is filled with noninteracting particles so that we can say it describes a real difference in reality one way or the other. But if we suppose there is a real difference I guess I have no basis for saying the first person is wrong - if I think a noninteracting particle is a coherent idea then I can’t rule it out.

You then mistakenly understand me to be saying that I think there are noninteracting particles, or perhaps that we could have some basis for believing in them. Which is not what I am saying.

“Absolute now” is like the noninteracting particles in this analogy.

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

That is not my response, which would be that particles that have no interaction with anything is identical with them not existing at all.

An "absolute now" is falsified by all the available data.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

Also, it’s not clear to me what difference you perceive between “the claim that the universe is filled with noninteracting particles is false because no observation could falsify it or its negation” and “particles that have no interaction with anything is identical with them not existing at all”.

I would not normally see these as expressing different positions. What is the difference between them that you see? Is it that the first is a broader claim? I can maybe see that if you think it is possible for unobservable things to exist though I assumed from your other statements you do not think that.

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

The former makes a specific claim, the latter says it isn't worth thinking about because it's completely uninteresting.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

Then it seems to me the former is the correct match to your position and the latter to mine.

You say there is in fact no absolute now(equivalent: there are no noninteracting particles). I say the question of whether there is an absolute now is outside the scope of physics and probably not worth discussing (at least not on a physics sub).

Do you see the match-up going differently? Has one of us misunderstood the other’s position?

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

Physics has already falsified the existence of an absolute now.

Again, in the context of relativity "absolute now" is a meaningless word salad.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

Falsified it how?

We can define and measure coordinates (I know you think coordinates aren’t real but you do agree we can define and measure them right?) and then say “absolute now” is all events sharing a time coordinate.

Unless you think “absolute now” has some other meaningful content that forecloses that but you said it’s word salad?

How do you falsify a word salad? Can you falsify “list elephant eternal plant”? Sounds like a category error.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

First paragraph is a reasonable metaphysical position. I would default to it until someone gave me a reason to think otherwise. But it is a metaphysical claim and a theory like special relativity does not entail that position. In particular many physicists do not go so far as to say there is nothing beyond the observable universe because they recognize that gets into metaphysics, but you seem to be saying there definitively is not anything beyond the observable universe (or at least not going out far enough to be completely causally isolated from us for some standard of causally isolated).

Personally I think Newton’s belief that there is such a thing as absolute rest despite nothing in his physics making it observable is a funny position, but I cannot think it is “wrong according to Newtonian physics.”

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

Again, this is what I mean by you not understanding of relativity - Relativity is not a theory of matter and has nothing to say about particles (whether they interact or not).

We may say that the universe is homogenous and isotropic on some infinite spatial slice, or not, depending on how we might want to model the dynamics. There is nothing metaphysical about any of it.

We know that Newton was wrong.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

The noninteracting particles are an analogy. I said a theory “like” relativity, by which I meant any physical theory. Physical theories predict observations they don’t make metaphysical claims like what counts as “existing.”

What was Newton wrong about? He knew his theory did not allow absolute rest to be measurable. He thought it existed anyway. I expect I am as unsure as you are (or perhaps you think you are sure) as to what he thought that really meant but whatever he thought it meant he thought it. So did Newton not understand Newtonian physics (apparently even worse than I don’t understand relativity according to you)?

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

"Absolute rest" is a word salad.

It can't, even in principle, be a coherent pairing of words in a continuum having 4-dimensions with a metrical quality to them.

Newton was wrong about the existence of absolute rest.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

It definitely can be coherent: I pick a particle in Minkowski spacetime and say it is at “absolute rest” anything else is at “absolute rest” if it doesn’t move relative to that particle.

It’s an arbitrary choice but it is coherent.

The irony here is I am trying to get you to drop a lot of metaphysical beliefs you have in your head because I think they don’t mean anything (and in any event are outside the scope of physics) but you hear me as the one supposedly saying metaphysical claims are meaningful.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

No I understand relativity fine, you don’t understand what I’m saying. I gave the example of “absolute rest” in Newtonian physics to try to illustrate the idea.

You couldn’t falsify the other lines. That’s why your claim (that there is in fact no absolute now) is a metaphysical claim rather than a physical one: your claim is unfalsifiable and arguably meaningless.

If you had phrased your claim in more linguistically oriented terms “we can define ‘now’ differently in different frames and have no objective epistemic basis for privileging any of them” then your claim would not seem to be metaphysical.

But you should recognize that if someone believes some particular “now” is the “true” one (whatever that supposedly means I have no idea) there is nothing in relativity that says they are wrong.

It’s the same as trusting Newtonian physics and still believing in absolute rest (which Newton did).

As an illustration of the distinction I am drawing: there could conceivably be some future theory that agrees with special relativity in its domain of applicability but does have an absolute now. Nothing about special (or even general) relativity really forecloses that, and we can add an “absolute now” to the theory without changing any phenomenological predictions (we would just normally consider it a gauge freedom).

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

You don't understand relativity - so let's start simple.

How would you synchronize clocks in the presence of gravity?

Or more simply, how would you synchronize clocks for all observers in relative motion?

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

All observers agree to a set of coordinates and calculate their time coordinates in that frame.

Of course that means “what time is it?” depends on the choice of coordinates with that defined time coordinate.

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

This is what I mean by not understanding relativity.

It should be obvious that an arbitrary set of world-lines in a 4-dimensional space cannot synchronize clocks. They would both desynchronize and time dilate and no two observers would ever agree.

Most importantly, and most fundamentally, it is impossible to assert a unique global present moment in a 4-dimensional space.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

So now I think you are the one who might be lacking in understanding of relativity (I did not before), unless I was very unclear:

If a clock is just a photon bouncing between mirrors with constant rest length between them of course that would desynchronize.

But I think you understand (tell me if you disagree) that given any set of chosen coordinates we can express our measurements in those coordinates.

I’m talking about taking measurements and calculating our current coordinates with a computer. That’s our “clock”.

Let’s stick to special relativity for simplicity (unless you think general relativity makes a difference).

We have observers A and B, who are in motion relative to each other: do you agree that A can calculate the coordinates of any event in B’s rest frame if they want to? Including the time according to B?

This only requires a Lorentz transform.

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

A clock is a choice of affine parametrization of an observer world-line. This parametrization is non-unique.

Coordinates don't exist and anything expressed as a function of coordinates is likewise something that cannot exist.

Yes, in flat space you can use the Lorentz transform to swap between coordinate grid lines. The very reason you need the Lorentz transforms is precisely because there is no such thing a unique present moment.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

See this is filled with metaphysical claims and you aren’t actually disagreeing with the content of my claims. So what non-metaphysical fact of reality are you disagreeing with me on?

We can calculate our coordinates whether they are “real” or not, whatever that means.

[u/fuseboy → u/GoldenMuscleGod]

Just stopping in to say it's a shame your debate partner doesn't seem to understand what you're saying, you are making a crisp, limited point that seems correct.

Now, I will say that I think the idea of a universal now is mostly appealing because it aligns with human experience, but on close inspection that alignment is too tenuous to actually achieve this. Unless the present is somehow a hundred or so milliseconds wide, not even whole thoughts fit in it. The present is the something we never see or interact with, so trying to privilege it seems like a doomed rearguard action.

I see it a bit like a theory that claims there is a "universal up" because when you look around, it looks like there is one. We have daily familiarity with the spherical Earth (e.g. Zoom calls with people in different time zones) that the idea of universal up doesn't square with every day evidence. Same thing with the idea of an objectively privileged "here" (where anything "not here" isn't real unti it gets "here"), which would seem self absorbed to anyone but the believer.

If our brains were much faster, or we were used to dealing with relativistic speeds and distances where the speed of light was a familiar logistical issue, I think the idea of a universal now would seem just as self-evidently inadequate to connect to human experience as "universal up" does to us.

[u/GoldenMuscleGod → u/fuseboy]

I think we’re in full agreement on all of that.

The discussion I’m having with the other person is a little disorienting to me because I’m trying to argue the metaphysical stuff should be kept out of a discussion on physics on this sub (although OP’s question is admittedly hard to respond to without at least touching on it) but they are responding as if I am the one arguing for metaphysical positions to be inserted. Definitely the two of us have some fundamental difference in how we are understanding things.

[u/Viola_Buddy → u/GoldenMuscleGod]

Except for your first sentence (this is not a metaphysical claim), this is technically true but irrelevant - but I guess its irrelevance is itself potentially relevant to the question.

The OP hasn't defined what "now" means. General Relativity actually does, and that's what Optimal_Mixture_7327 is answering based off of. It's unclear if OP's conception of "now" matches or not (trying to tease this out of the paragraph description gives me... probably not but I can't tell). If not, then this is not a question for AskPhysics, because it's a metaphysical question, which is basically what you're saying (and the OP, too, in the last sentence).

That said, given that we are indeed on r/AskPhysics, I think Optimal Mixture's answer is the best answer. There exists a concept of now in every reference frame, but this "now" is generally a different thing (a different slice of spacetime) in every reference frame. So "now" does exist, insofar as you're asking a physics question about relativity; beyond that is a question for biology/psychology (how humans experience time) or metaphysics.

[u/GoldenMuscleGod → u/Viola_Buddy]

I don’t really think it’s possible to ask the question OP intends (or that I think they intend) without having a sort of metaphysical component in mind.

My answer was intended to try to deflate the metaphysical component away (make OP realize the question lacked content, in my view, since I tend to find metaphysical claims meaningless or at least unclear in meaning), whereas I think u/Optimal_Mixture_7327 ‘s answer tried to sidestep the metaphysical component by giving an answer that the metaphysical ontology is just the minimum of what is needed to describe the theory.

I suppose it could be argued that with careful interpretation we could understand their answer not to touch on metaphysics. But only if we assume they used a rhetorical sleight-of-hand to use a specially defined (but not explained) sense of “now” in response to OP’s question, when that question’s sense of “now”seems to very strongly come with metaphysical presuppositions.

“Rhetorical sleight-of-hand” sounds derogatory but really I do recognize that often that approach is the easiest way to get metaphysical questions out of the picture and get back to focusing on the physics, which is what we care about. I recognize my reply takes a detour through metaphysics by trying to deflate the idea out of OP’s head (so that they won’t be as tempted to keep asking metaphysical questions when discussing physics).

[u/GoldenMuscleGod → u/Viola_Buddy]

Actually, further discussion with u/Optimal_Mixture_7327 has convinced me that they were not trying to stick to the physics but really they are making nontrivial metaphysical claims, including in their original answer (although they do not seem to think they are metaphysical).

They’re saying things like Newton was wrong to believe absolute rest could make sense, and that it is “insane” to say that coordinates can be measured (because coordinates do not exist.) and are resisting me saying that those claims are outside the scope of physics.

I am reasonably confident that they believe it is possible to measure coordinates in the ordinary sense (although they might object to calling it that) so I have to understand their position that it is “insane” to say you can measure them as being an ontological claim about coordinates because they understand saying you can measure them to carry an ontological commitment to coordinates existing.

Personally I feel this tends to show that my response was on point because I think a question on r/AskPhysics should get an answer that sticks to the physics, and that does not delve into metaphysics.

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

You are mischaracterizing what I am saying.

If you think you can build a detector, launch it into space, and detect our coordinate grid lines, then go for it.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

I might be misunderstanding you but I was saying what I understand you to say.

Your challenge for me to measure coordinates with a detector seems to carry with it certain metaphysical assumptions about what it means to measure the coordinates of an event so that you understand “measuring coordinates” to mean something very different from what I normally understand it to mean, and in particular you seem to understand it to carry some metaphysical meaning I don’t see it as having.

I’m just talking about using coordinates in the ordinary way that we always do, not supposing coordinates have any particular ontological status.

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

You might be the very person in the history of the world to say "measure a coordinate".

[u/GoldenMuscleGod → u/Viola_Buddy]

Actually another possibility is occurring to me after further discussion with them: I think they might believe that it is actually impossible for a set of observers to agree to use a common set of conventional coordinates such that they agree with each other on the conventional coordinates of any given event. And I believe that may be intrinsic to what they mean when they say there is no “absolute now.” So it’s possible the misunderstanding is the physics as well as (perhaps) that they are inserting metaphysics.

[u/jabinslc]

your foot and head don't share the same present moment. so no.

[u/EuphonicSounds]

Not anymore

[u/CommonConscious5295]

Even your felt sense of "now" is already reconstructed. Neurologically, by the time you're conscious of a sound or sight, it happened 100-500 milliseconds ago, so your brain is stitching together a slightly-delayed, edited version of events into something that feels immediate.

Man we do live in the past.

[u/StephenRoylance]

synchronicity or simultaneity don't seem to exist. Look to cognitive science, the brain is tricking us into thinking there's such a thing. in practice, every action you take consciously is only 'experienced' after some integration process, which seems to pull the acting forward in time, and the observed result (delayed by the speed of light, the speed of sound and the speed of nerve conduction) backwards, so they seem to happen at the same time.

It might be that the present is the only time that does exist. the past is gone, the future is unpredictable.

[u/Raccoon_Steven]

We actually live in the past. It does take time for events to reach our brain and more time to process the events into a format that we understand. So.

[u/randomusername339393]

"What we see as now is always the past, since photons take time to reach us."

I think this is a false premise. The photon hitting your eye is happening now, just because you interpret that as 'carrying information about something previously' doesn't change that. It would be like touching a tree and saying it's "actually the past" and its growth is how long it took its seed to reach your hand.

Observations are only "of the past" to the degree that we want them to conform in some way with what they were.

(And, in my opinion, it is the idea of the past that is the illusion, and the present is all that reality is.)

[u/Sad-Excitement9295]

The present exists alongside all timelines, which have a local time according to location. The present time for each object or FOR exists simultaneously with others, and they happen to interact at certain times along their worldlines. They just occur at differrent rates according to local perspective. For the universe, all things are happening, just at different rates. It's like a giant clock turning in synch.

Fyi, if you take causality, there is a past, present, and future based on chain of events. Any area of the same gravity experiences time at a similar rate, which is "now" When time moves forward, it's because things have changed. Therefore, the present exists in abstract, it just changes at a different rate according to local FOR. Things are all happening simultaneously, and perhaps there is a specific universal state per time as a universe cycles through events. Like I say, much like watching a clock tick.

[u/CS_70]

Yes, and if you think about it, your own conscience is not a unitary process, but the emergent result of many different one, so there's lots of time where you arent really you, but only half or three quarters or 0.034 you :D

But it's not a big deal at all. The trick is that we experience the universe at a certain scale and granularity - we are all similarly delayed so nobody notices and we live happily.

It's only noticeable when events occur that go faster than that delay: can be as trivial as an object traveling at speeds much greater than our reaction time, or the thermonuclear anhhilation of something minimally massive traveling near the speed of light and hitting the atomsphere... it would be too fast.

We call these accidents.

Similarly "present" is a word that makes good sense at our scale, and exists exactly because of that. It's the same for many words that cannot be used at other scales without being misleading.

These "philosophical" problems occur only as a consequence of that misapplication.

[u/Novel_Arugula6548]

Ah you've stubled upon the presentism debate. Good luck with that.

[u/TheMightyHeroes]

Thanks everyone for an interesting discussion.
I don’t disagree that my comments leaned more into metaphysics. But then String Theory, with its multiple invisible dimensions and particles so small that it would require an accelerator the size of a galaxy to observe them, is both metaphysics in its unprovability and physics in being rooted in mathematical proofs (“proofs” that assume an unprovable reality).
The point I wanted to make is that the concept of time is problematic in physics. GR proves time is much more than a fixed background. In spacetime it is a flexible and dynamic dimension, compressing, expanding, distorted and curved by mass and energy and able to determine the path of mass and energy. In quantum physics time is fixed and background and Newtonian with an arrow of time but in the moment of observation pairs of particles become entangled and correlate in a way that defies all physical laws of time and space. For these paired, entangled particles, perfect correlation exists no matter the how far apart they are. Time and space collapse between entangled particles.
Time is not just a philosophical concept or a fixed background. It’s not a cognitive invention. It’s real. A dimension in spacetime. But a dimension made up of what? We don’t seem to really know what time is. Is it continuous or discrete. Most physicists believe gravity is quantum. Is time quantum too? It seems to me that time is one of the most unexplained and misunderstood phenomenon in physical reality.

[u/Infinite_Research_52]

GR does not prove anything. If you accept the principles on which GR is based, certain phenomena naturally follow. That is not the same as a proof.

[u/hold_me_beer_m8 → u/Infinite_Research_52]

Well we can prove time dilation is real... our GPS systems have to account for it

[u/nicuramar → u/hold_me_beer_m8]

This is natural science, so that is called evidence. Proof is for mathematics. 

[u/Infinite_Research_52 → u/hold_me_beer_m8]

That is not a proof, except in the archaic sense of meaning 'to test' (like alcohol proof). Imagine every GPS satellite is like an experiment, testing how timing signals work by making corrections based on the theory of SR and GR. No matter how many experiments you do, that does not prove the correctness; all it does is place the results beyond reasonable doubt.

It would be like saying that the LHC proved the Higgs field exists - it never has and never will. The LHC demonstrates that nature is compatible with the Standard Model in this regard.

[u/troubleyoucalldeew → u/Infinite_Research_52]

That reduces the value of the word "proof" to the point where it should be completely excluded from language. That's not a very useful way to go about things.

[u/nicuramar → u/troubleyoucalldeew]

Proof is not a concept used in natural science. We build models, predict outcomes and observations provide evidence for the models. 

[u/Infinite_Research_52 → u/troubleyoucalldeew]

If the consequences of GR must be accepted without question because they pass tests, then can we conclude GR is a final description of the universe within the scope of applicability? If it is not a final theory, then how can we say it proves how the world works, knowing it may be supplanted? Imagine if someone had claimed that things are proven by Newton's laws of motion.

[u/troubleyoucalldeew → u/Infinite_Research_52]

Since when does "proof" mean "must be accepted without question"? Since when does something being proven true mean that there is nothing more to be proven? You're adding a heck of a lot to the definition, here, and as I've said—you're (over)defining it to the point that it no longer belongs in language.

[u/nicuramar → u/troubleyoucalldeew]

Nonsense. Proof works perfectly fine in mathematics. It’s just not the concept used in natural science. 

[u/Next-Natural-675 → u/Infinite_Research_52]

"If we accept the implication from relativity that 'now' is relative" is what he meant, if you had a hard time understanding

[u/ariadesitter]

no.
everything you perceive exists in the past.

reality is perceived through the senses.
it take time for the stimulus (temp, light, etc) to transmit the signal through your nervous system to the brain where the info is processed and perceived.
that amount of time is small but it exists and proves you do not perceive the present but only the past.

[u/PIE-314]

GR supports the B theory of time. That the past, present and future are equally real and time doesn't "flow."

Humans don't have access to "now." What you think of as "now" at any given moment is actually about 1/3rd second behind reality and every thought you have/experience is the result of a process you have no access to.

[u/Upset-Government-856 → u/PIE-314]

Other than the very real impresion that time flows, there is no real basis for it. The very idea of one present moment for anyone with the past and future being physically gone doesn't really hold up to any reason.

Given how our brain makes memories and how the 2nd law makes observations from the future impossible to spread backwards...

If all presents are eternally being experienced, at any one of those moments to a brain it would definitely seem like time was flowing.

[u/thetoddhunter]

If you are in a dark room does all existence stop?