8 ms·

I am not really skilled in physics and have never understood why time would be a tangible dimension like, say, width or height, instead of a mathematical constr

I am not really skilled in physics and have never understood why time would be a tangible dimension like, say, width or height, instead of a mathematical construction to argue about change. I get it that it is convenient to talk of change in something, like movement from state a to state b, under the guise of wrapping it in "time", but is there some physical argument in support of time in general? Honest question. Living beings getting older is their cells becoming more and more inefficient in division and cell repair. One could likely achieve the same effects with chemicals, but doing so would not mean time has run faster. The concept of time for humans seems to be all about observable change, which needs an observer with a memory to compare the current state with previous state to be able to say: time has passed. A rock changes via erosion and such, but it has no memory, and cannot observe anything. Does a rock feel time? Of course not. Does it exist "in time"? Does it, without an observer that somehow measures the flow of time (via changes in Cesium atoms or something)? Or is the rock just existing and under the whims of all forces of nature that might impact it and change it into smaller pieces and eventually to sand, and so on. I guess my question is: what exactly is time, physically, and why should it have to exist as some sort of a physical process in the first place.

ksdale 8y ago
I know next to nothing about physics, but I think time is more concrete than that. There have been experiments in which they've measured discrepancies in different clocks according to what physics predicts. That is, time passes differently according to your position/speed in space. In that sense, time isn't just arbitrarily declared by the guy holding the clock. I'm not sure if that addresses your concerns at all...
- mrhappyunhappy 8y ago
  So, we know nothing.
  - goatlover 8y ago
    We know more about time than people prior to Einstein.
- deleted 8y ago
  [deleted]
mathgenius 8y ago
These are all good questions, and we can add also "what is energy?" and "what is space?". I find it all deeply mysterious. Perhaps the questions are more interesting than the answers. How about this one: "what is is?"
- kylek 8y ago
  https://en.wikipedia.org/wiki/E-Prime https://en.wikipedia.org/wiki/E-Prime
- empath75 8y ago
  Energy is actually deeply connected with time, as it’s the measurable quantity that’s conserved because of the time-symmetric nature of the laws of physics.
  - antidesitter 8y ago
    For anyone who's wondering, this is a consequence of Noether's theorem: https://en.wikipedia.org/wiki/Noether%27s_theorem https://en.wikipedia.org/wiki/Noether%27s_theorem. Similarly, symmetry under spatial translations implies momentum conservation, and symmetry under spatial rotations implies angular momentum conservation.
notduncansmith 8y ago
It seems plausible that time is an artifact of the same general principle behind the halting problem, Gödel’s incompleteness theorems, and quantum mechanics. The only way to find out the final state of a (program|particle-measurement|partial-differential-equation|universe) is to run it.
- antidesitter 8y ago
  I'm not sure how the halting problem and Gödel’s incompleteness theorems are relevant. Could you elaborate?
  - notduncansmith 8y ago
    Here’s a blog post I found that discusses it, you can search the web for more: https://www.scottaaronson.com/democritus/lec3.html https://www.scottaaronson.com/democritus/lec3.html
    - notduncansmith 8y ago
      Replying to myself because I took too much time and now can't edit. I realize you might mean you're not sure how they're relevant to the topic at hand, rather than how they're relevant to each other. I'll attempt to illustrate this connection by providing the simplest possible internally-consistent model of an abstract system that appears to resemble reality as we experience it, including the passage of time. I will define the base components of the system, and compare the emergent interactions of these components with the phenomena we experience. First, we observe that any given physical space can be said to represent some positive quantity of information; a physical space can thus represent the "state" of a system. Second, we assume the existence of "causality", patterns for recognizing and amending portions of a state. We may then refer to a possible application of these patterns to a portion of a given state, which we'll call an "event". For every pairing of initial state and set of possible events, there exists a space of computable states that we may call a "totality". A totality can be represented as a directed graph. The root of this graph is the initial state, edges are detected events from which new states can be computed, and child nodes represent states transformed by events. Now, when we talk about "the passage of time", we mean the iterative computation of successive levels of totality. An event's distance from the root node is its chronological order, and thus a "point in time" is a depth in the graph of totality. The full state of the system at a given point in time is then naturally the union of the state nodes at that depth. Our experience of the passage of time is represented similarly in the brain, with neurons building complex models by generating successive branching layers. We hinge knowledge on key experiences, and we perceive time fractally, in heirarchical levels of detail. It seems intuitive that our brains, which function by reflecting our outward reality internally, would in fact structurally reflect reality. Furthermore, the conclusion that the causality graph becomes more dense as mass in a space grows more dense seems to reflect general relativity. Here's where Gödel and Turing come in. A totality is defined iteratively: there is no way to know its complete value except to evaluate its events, and the only thing to do with it is evaluate its events according to its patterns of causality. Similarly, given a Turing machine (or a consistent system of axioms), there's no way to know the final state (or final set of conclusions) except to run it with some input, and the only thing it can do is run. So back to the question: > what is time, physically, and why should it have to exist as some sort of a physical process in the first place Time, physically, is the continuous evaluation of possible states of a physical space (like ours). It has to exist because it is, by definition (to the point of tautology), the only thing that can possibly happen. It is still a "mathematical construct", but only to the degree that we also are ¯\_(ツ)_/¯
darkmighty 8y ago
If you want to say something changes at a certain rate, you need to introduce a dependent variable under which it changes. If you look at landscape, and saying elevation changes, you don't need the coordinates as variables to associate height with and compare the different (x,y,height) data points -- only a set of {height1,height2,...} would be sufficient. But to do calculus by taking the small differenes (x-x',y-y',height-height'), i.e. looking at rate of changes of height w.r.t. something , you obviously need something (e.g. coordinates). Ultimately as with most physical phenomena the most profound explanation is that "It's a simple way to describe how things work". That and anthropic necessities. Relationships between events are almost certainly an anthropic necessity. First without change at all, of course there would be nothing of interest. Second an universe that's simply a random-looking collection of events {A,B,C,...} without some close functional relationship (e.g. {A,B=f(A),C=f(B),...}) also cannot allow a conscious subsystem (because conscious subsystems are defined as a particular kind of changing system, i.e. with close functional connections between the events that temporally evolve). There's an infinitude of mathematical constructions that may harbor consciousness, so some things are arbitrary. Our arbitrariness is that there appears to be this continuous dependant real variable (time) which permits localizing events in a complicated way described by general relativity and quantum mechanics, and describing relationships between them (temporal evolution).
Razengan 8y ago
> why time would be a tangible dimension like, say, width or height Because time -is- a location? All things move down this one-way (or so we assume) tunnel, and time is their location in that tunnel. Imagine a paused video player where everything moves back and forth only when you move the playback slider/scrubber, or a lenticular photo that changes when you move your head. You have to consider the human ability, tendecy and limitation of perceiving everything in 3 dimension and in terms of distinct points, particles or “snapshots”, as opposed to perpetual streams of potentially many more dimensions, each with different “directions” through which they may be navigated. We can only see and intuitively reason about a very filtered slice of reality. And this is why we need to find and meet other intelligent beings who perceive reality in fundamentally different ways, to expand our own understanding of it. :)
analog31 8y ago
Instead of thinking about gradual, monotonic change, I wonder if it would help to think in terms of periodic phenomena. Examples include the solar and lunar cycles, seasons, heartbeats (more or less), water dripping out of something, etc. In those cases, it might be possible for someone (e.g., a hypothetical primitive person) to notice that the regularities are related to one another, e.g., there are 28 solar cycles for every lunar cycle, and so many lunar cycles between harvests. It might occur to someone that the solar cycle is a useful way to measure something, even if it's not clear exactly what the "something" is. Knowing when to plant and harvest, or when to expect certain kinds of animals to be plentiful for hunting, might be pretty useful. I tend to think about time in terms of the observable relationships between different forms of periodic behavior. This is at least easier for me to comprehend than problems such as memory and gradual change.
- kurthr 8y ago
  Yes, reciprocal time (Hz) is really useful for repeating phenomena... musical instruments (including the voice) are an excellent example. Once you have a note (e.g. A = 440Hz) you can then divide it down to get seconds, days, years. If you want an example of humans able to determine time incredibly accurately over a lifetime, just look a "perfect pitch" musicians who can tune their instruments to match other musicians to a percent from across the world. Although most people would have difficulty matching that accuracy, they wouldn't have difficulty telling an alto from a soprano at any point in their life after they have first heard the difference. Our civilization grew up with music, dance and all sorts of "absolute" human temporal measures.
gus_massa 8y ago
> I am not really skilled in physics and have never understood why time would be a tangible dimension like, say, width or height, instead of a mathematical construction to argue about change. The problem with your theory is that the main idea of Special Relativity is that time is a good tangible dimension, that has (almost) the same properties than the other three dimensions. And this is fully backed by experiments, and the effects are measurable in satellites, atomic clocks on planes, the color of gold, and many many many additional experiments. One important property of the usual dimensions (x, y, z) is that you can mix them. Let's say that we choose z to be the vertical direction. Now we can choose x to be pointing to the east and y to the north. But we can mix x and y x' = x * 1/ sqrt(2) - y * 1/ sqrt(2) y' = x * 1/ sqrt(2) + y * 1/ sqrt(2) Now x' and y' are obtained mixing x and y. You can think that someone else choose to point x in the north-east direction and y in the north-west. (I hope I got the signs correctly.) And all the experiments should be equivalent because the universe has no preferred direction, x and y are as good as x' and y'. [Since the Earth is spinning, we have a small technical problem here, but just stop the Earth to keep the discussion simple.] Now, it is not a good idea to imagine that the east-west axis is a tangible dimension, but the north-south axis is something else. The main problem is that someone else can choose other directions, like (north-east)-(south-west) and (north-west)-(south-east) and get the same experimental results. Now, which one is the tangible dimension (north-east)-(south-west) or (north-west)-(south-east)? So we assume that all coordinates x, y and z are essentially the same thing and no one of them is special. In most common situations, the vertical axis is different of the two horizontal axis. In normal situations, it is clear where it is up and down, so the "z" axis is special. It is not clear the direction of the "x" axis, and for most experiments you simply choose the direction that makes the calculations easier, but "x", "y" or any horizontal direction is as good as the other. [Just ignore again that the Earth is spinning and that it has a magnetic field. The spinning of the Earth and the magnetic field define two special directions that are both the "north" in some sense.] -- Now, even if there Earth is not spinning, and if the Earth had no magnetic field, the problem with the "z" direction going up is that two persons in different continents will disagree, about where is up and down. If you choose (x,y,z=up) in other continent will choose (x', y', z'=up') where x', y' and z' are obtained mixing x, y and z. And both will be totally convinced that "up" is special and there is a clear meaning of "up", in spite each one has a different "up". So it's better to assume that the universe has no preferred directions (x,y,z) and all the differences are due to the details, like a big chunk of dirt we call Earth. If we assume that some of the directions x,y,z have some essential properties that the other doesn't have, it would not be possible to mix them and construct x', y' and z'. -- Now, about the problem with time ... In Special Relativity you can mix the special coordinates and time. It's easier if you always multiply the time by c (the speed of light), and you use the coordinates (ct, x, y, z). Ad it's more simpler if you think that you measure the distance in x, y, z in light-years and that you measure the time intervals in t in years, so you get the same number in ct. In Special relativity you can mix for example x with ct ct' = x * ? + ct * ? x' = x * ? + ct * ? where ? means some coefficients like the 1/sqrt(2) in the x-y example at the beginning of this comment. It's easy to calculate them, but the exact numbers are not important. The important part is that the formulas are mixing x and ct. In one mix you get ct' that is a new time-like coordinate and in the other mix you get x' that is a space-like coordinate. These are the coordinates that someone else sees when is moving at a different speed than you. You and the other person (in a train, plane, spaceship) will disagree about what how the time flows. You see t and the other person will see t' that is a mix of your t and your x. But both you and the other person can do any experiment and get equivalent results, because the universe don't prefer t to t' or vice versa. There is no experiment to determine if t or t' is better. The main difference between the example with (x,y) and (ct,x) is that x and y are mixed in a slightly different way than x and ct. In particular, you can exchange x and y (or better x and -y for technical reasons). But you can't exchange completely x and ct. All the mixes have a new ct' that is somehow more similar to ct than to x, and a new x' that is somehow more similar to x than to ct. Moreover, all the observers agree that there is one time and three special dimensions, but they will not agree about how the time flows (as the will not agree where the directions x, y and z are pointing to). The explanation of why can't mix "completely" x and ct is part of the mathematical details that I'm not writing here. It's somewhat related the fact that you can't move faster than the speed of light. Just get any introductory book about Special Relativity, but continue reading until you reach the chapter about Minkowski spaces. The initial formulas doesn't make too much sense until you reach Minkowski spaces.
bmer 8y ago
Think of dimensions as the number of pieces of information required to uniquely identify the position of an object. In 1D, you only need one number. In 2D, two. In 3D, three. But in all these scenarios, the point in question is "static". What if the position of the object was also changing in time? Then, it wouldn't be sufficient to provide just the spatial coordinates, you'd also need to specify what time it is. In 1D for example: at 12:00 the point was at 0. At 13:00, the point was at 1.0. At 14:00 the point was at 2.0. Thus, "4D" in special relativity comes from 3 spatial dimensions + time, since we care about how things move through "space and time".
- nopassrecover 8y ago
  One difference of course is that while we can always return to a physical position in space, even if the contents of that space have changed, it seems very conceptually challenging to imagine returning to a position in time even if the contents of that time had changed. What would this even mean? Our definition of time seems therefore to be two things: first, a snapshot of physical state (in which we imagine points in time a bit like Back to the Future), and secondly (as other comments have noted) a measure of periodicity. The latter is that which we seem to be measuring experimentally (e.g. with differences in atomic clocks within gravitational fields). However what we are literally measuring seems to be something like “how many times can a given particle move/irradiate relative to another given particle before we bring them back to each other”. Reducing to a pendulum for example, we could imagine that the space was “more dense” so it took longer to travel the same apparent distance, a bit like how light takes longer to travel through particular materials. Another way to see this would be a kind of universal framerate. So based on this possibly weak conceptual understanding, I can certainly imagine a fourth property of any given spacetime location that reflects how frequently events can occur in that space (a kind of EM-field view of time), but to consider this as a geometric “position” or coordinate is something I conceptually struggle to imagine.
antidesitter 8y ago
Because, according to special relativity, your plane of simultaneity (how the universe is "changing" from your perspective) would differ from that of someone walking right past you. See the Andromeda paradox: https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argume...
- TheOtherHobbes 8y ago
  I hesitate to disagree with Penrose, who has forgotten more about relativity than I'll ever know, but... I'm not seeing the paradox. There is no absolute simultaneity in relativity. So the Andromeda Paradox only becomes a problem if you invent a magical FTL chronovisor that allows you to bypass causality and see what's happening anywhere in the universe at your idea of the present time. There's no reason to believe such a device could ever exist. So I don't see why there's any confusion. Both hypothetical observers discover events that happened in the past when the light cone of the events intersects their position in space time. Events in the past aren't uncertain, they're simply unknown in the reference frames of the observers. And because the observers have no causal influence on distant events, they have to watch them unfold passively. It's no different to an incredibly laggy online game. If you're well behind the action, events happen before you can influence them. The fact that someone may suffer twice the lag you do doesn't make the events any more philosophically ambiguous, mysterious, or acausal.
  - antidesitter 8y ago
    I’m a bit confused by your response, especially the lag analogy, so I’ll try to ask a clarifying question: is there a fact of the matter as to whether the Andromedean space fleet has already set off on its journey?
- amelius 8y ago
  By the way, here's a nice video on the topic of simultaneity. https://www.youtube.com/watch?v=wteiuxyqtoM https://www.youtube.com/watch?v=wteiuxyqtoM
mikekchar 8y ago
I have a kind of followup question (which, if I understand correctly, was touched on a bit in the article). If time is a tangible dimension, is it possible that the "passage of time" is an illusion? I remember the past and because of causality, the events are ordered. Time seems to flow from the past to the future and it never flows from the future to the past. This seems obvious to us, but I've always wondered why time doesn't flow backwards. Just to take a silly example, what if all the "points in time" just exist (and are ordered -- I don't propose to break causality)? They don't flow at all. From my perspective, at every "point" along the time axis, I can recall the past and it will be ordered as if it "happened", but each point could be independent (though constant). If I could remember "forward" through time, then this would be obvious, but since I can only remember "backward" through time, at every point it appears as if I have progressed through time. I suppose the interesting thing is that causality is uni-directional. Things can only happen in a certain order in the past. But this is not true of the future. Even if I have perfect knowledge of the present, there are some things I can not predict about the future (quantum mechanics FTW). I wonder why that is (because we are flowing through time? Ha ha!) Sorry for the diversion, but if someone that is better educated than me could shed some light on the matter, I'd be grateful.
- aaaaaaaaaab 8y ago
  You’ve rediscovered eternalism: https://en.m.wikipedia.org/wiki/Eternalism_(philosophy_of_time) https://en.m.wikipedia.org/wiki/Eternalism_(philosophy_of_ti...
  - akvadrako 8y ago
    If eternalism is real it's hardly fair to say he rediscovered it - all the discoveries have always existed.
    - stan_rogers 8y ago
      Discovery is not invention, it's literally just noticing (or uncovering in a metaphorical sense) something that was always there.
jules 8y ago
You're completely right that there is nothing revolutionary about viewing a point in spacetime as a point (x,y,z,t) in 4 dimensions. If that was what special relativity was about then it would be very boring! The x,y,z directions are connected to each other, because the x direction can be rotated into the y direction by rotating in the xy-plane. What special relativity is about is that the t direction can be rotated into the x direction by "rotating" in the tx-plane. Rotating in the tx-plane is acceleration in the x direction. That's what causes all the weird relativistic effects such as time dilation and length contraction when you accelerate objects. Creatures that don't intuitively understand that the x and y directions form a whole could come up with a similar theory: if you rotate an object in the xy-plane it gets x-dilated and y-contracted. The only mathematical difference between a rotation in the xy-plane and a rotation in the tx-plane is a minus sign in the formula for rotation. So the t direction is not exactly like the x,y,z directions, but the only difference is that terms involving t^2 get a minus sign. For example, the squared distance in 3D is x^2 + y^2 + z^2, but the squared "distance" in the weird 4D spacetime geometry is x^2 + y^2 + z^2 - t^2.
howeyc 8y ago
You're right. Time isn't real in the same way as x,y,z. https://en.m.wikipedia.org/wiki/The_Unreality_of_Time https://en.m.wikipedia.org/wiki/The_Unreality_of_Time http://edwardfeser.blogspot.com/2018/05/godel-and-unreality-of-time.html http://edwardfeser.blogspot.com/2018/05/godel-and-unreality-... It's rarely discussed because the implications are more philosophical. The math, and expiriments are very useful when time is a variable. So is daily life, clocks are very useful.

ksdale 8y ago

I know next to nothing about physics, but I think time is more concrete than that. There have been experiments in which they've measured discrepancies in different clocks according to what physics predicts. That is, time passes differently according to your position/speed in space. In that sense, time isn't just arbitrarily declared by the guy holding the clock. I'm not sure if that addresses your concerns at all...

mrhappyunhappy 8y ago
So, we know nothing.
- goatlover 8y ago
  We know more about time than people prior to Einstein.
deleted 8y ago
[deleted]

mathgenius 8y ago

These are all good questions, and we can add also "what is energy?" and "what is space?". I find it all deeply mysterious. Perhaps the questions are more interesting than the answers. How about this one: "what is is?"

kylek 8y ago
https://en.wikipedia.org/wiki/E-Prime https://en.wikipedia.org/wiki/E-Prime
empath75 8y ago
Energy is actually deeply connected with time, as it’s the measurable quantity that’s conserved because of the time-symmetric nature of the laws of physics.
- antidesitter 8y ago
  For anyone who's wondering, this is a consequence of Noether's theorem: https://en.wikipedia.org/wiki/Noether%27s_theorem https://en.wikipedia.org/wiki/Noether%27s_theorem. Similarly, symmetry under spatial translations implies momentum conservation, and symmetry under spatial rotations implies angular momentum conservation.

notduncansmith 8y ago

It seems plausible that time is an artifact of the same general principle behind the halting problem, Gödel’s incompleteness theorems, and quantum mechanics. The only way to find out the final state of a (program|particle-measurement|partial-differential-equation|universe) is to run it.

antidesitter 8y ago
I'm not sure how the halting problem and Gödel’s incompleteness theorems are relevant. Could you elaborate?
- notduncansmith 8y ago
  Here’s a blog post I found that discusses it, you can search the web for more: https://www.scottaaronson.com/democritus/lec3.html https://www.scottaaronson.com/democritus/lec3.html
  - notduncansmith 8y ago
    Replying to myself because I took too much time and now can't edit. I realize you might mean you're not sure how they're relevant to the topic at hand, rather than how they're relevant to each other. I'll attempt to illustrate this connection by providing the simplest possible internally-consistent model of an abstract system that appears to resemble reality as we experience it, including the passage of time. I will define the base components of the system, and compare the emergent interactions of these components with the phenomena we experience. First, we observe that any given physical space can be said to represent some positive quantity of information; a physical space can thus represent the "state" of a system. Second, we assume the existence of "causality", patterns for recognizing and amending portions of a state. We may then refer to a possible application of these patterns to a portion of a given state, which we'll call an "event". For every pairing of initial state and set of possible events, there exists a space of computable states that we may call a "totality". A totality can be represented as a directed graph. The root of this graph is the initial state, edges are detected events from which new states can be computed, and child nodes represent states transformed by events. Now, when we talk about "the passage of time", we mean the iterative computation of successive levels of totality. An event's distance from the root node is its chronological order, and thus a "point in time" is a depth in the graph of totality. The full state of the system at a given point in time is then naturally the union of the state nodes at that depth. Our experience of the passage of time is represented similarly in the brain, with neurons building complex models by generating successive branching layers. We hinge knowledge on key experiences, and we perceive time fractally, in heirarchical levels of detail. It seems intuitive that our brains, which function by reflecting our outward reality internally, would in fact structurally reflect reality. Furthermore, the conclusion that the causality graph becomes more dense as mass in a space grows more dense seems to reflect general relativity. Here's where Gödel and Turing come in. A totality is defined iteratively: there is no way to know its complete value except to evaluate its events, and the only thing to do with it is evaluate its events according to its patterns of causality. Similarly, given a Turing machine (or a consistent system of axioms), there's no way to know the final state (or final set of conclusions) except to run it with some input, and the only thing it can do is run. So back to the question: > what is time, physically, and why should it have to exist as some sort of a physical process in the first place Time, physically, is the continuous evaluation of possible states of a physical space (like ours). It has to exist because it is, by definition (to the point of tautology), the only thing that can possibly happen. It is still a "mathematical construct", but only to the degree that we also are ¯\_(ツ)_/¯

darkmighty 8y ago

If you want to say something changes at a certain rate, you need to introduce a dependent variable under which it changes. If you look at landscape, and saying elevation changes, you don't need the coordinates as variables to associate height with and compare the different (x,y,height) data points -- only a set of {height1,height2,...} would be sufficient. But to do calculus by taking the small differenes (x-x',y-y',height-height'), i.e. looking at rate of changes of height w.r.t. something , you obviously need something (e.g. coordinates). Ultimately as with most physical phenomena the most profound explanation is that "It's a simple way to describe how things work". That and anthropic necessities. Relationships between events are almost certainly an anthropic necessity. First without change at all, of course there would be nothing of interest. Second an universe that's simply a random-looking collection of events {A,B,C,...} without some close functional relationship (e.g. {A,B=f(A),C=f(B),...}) also cannot allow a conscious subsystem (because conscious subsystems are defined as a particular kind of changing system, i.e. with close functional connections between the events that temporally evolve). There's an infinitude of mathematical constructions that may harbor consciousness, so some things are arbitrary. Our arbitrariness is that there appears to be this continuous dependant real variable (time) which permits localizing events in a complicated way described by general relativity and quantum mechanics, and describing relationships between them (temporal evolution).

Razengan 8y ago

> why time would be a tangible dimension like, say, width or height Because time -is- a location? All things move down this one-way (or so we assume) tunnel, and time is their location in that tunnel. Imagine a paused video player where everything moves back and forth only when you move the playback slider/scrubber, or a lenticular photo that changes when you move your head. You have to consider the human ability, tendecy and limitation of perceiving everything in 3 dimension and in terms of distinct points, particles or “snapshots”, as opposed to perpetual streams of potentially many more dimensions, each with different “directions” through which they may be navigated. We can only see and intuitively reason about a very filtered slice of reality. And this is why we need to find and meet other intelligent beings who perceive reality in fundamentally different ways, to expand our own understanding of it. :)

analog31 8y ago

Instead of thinking about gradual, monotonic change, I wonder if it would help to think in terms of periodic phenomena. Examples include the solar and lunar cycles, seasons, heartbeats (more or less), water dripping out of something, etc. In those cases, it might be possible for someone (e.g., a hypothetical primitive person) to notice that the regularities are related to one another, e.g., there are 28 solar cycles for every lunar cycle, and so many lunar cycles between harvests. It might occur to someone that the solar cycle is a useful way to measure something, even if it's not clear exactly what the "something" is. Knowing when to plant and harvest, or when to expect certain kinds of animals to be plentiful for hunting, might be pretty useful. I tend to think about time in terms of the observable relationships between different forms of periodic behavior. This is at least easier for me to comprehend than problems such as memory and gradual change.

kurthr 8y ago
Yes, reciprocal time (Hz) is really useful for repeating phenomena... musical instruments (including the voice) are an excellent example. Once you have a note (e.g. A = 440Hz) you can then divide it down to get seconds, days, years. If you want an example of humans able to determine time incredibly accurately over a lifetime, just look a "perfect pitch" musicians who can tune their instruments to match other musicians to a percent from across the world. Although most people would have difficulty matching that accuracy, they wouldn't have difficulty telling an alto from a soprano at any point in their life after they have first heard the difference. Our civilization grew up with music, dance and all sorts of "absolute" human temporal measures.

gus_massa 8y ago

> I am not really skilled in physics and have never understood why time would be a tangible dimension like, say, width or height, instead of a mathematical construction to argue about change. The problem with your theory is that the main idea of Special Relativity is that time is a good tangible dimension, that has (almost) the same properties than the other three dimensions. And this is fully backed by experiments, and the effects are measurable in satellites, atomic clocks on planes, the color of gold, and many many many additional experiments. One important property of the usual dimensions (x, y, z) is that you can mix them. Let's say that we choose z to be the vertical direction. Now we can choose x to be pointing to the east and y to the north. But we can mix x and y x' = x * 1/ sqrt(2) - y * 1/ sqrt(2) y' = x * 1/ sqrt(2) + y * 1/ sqrt(2) Now x' and y' are obtained mixing x and y. You can think that someone else choose to point x in the north-east direction and y in the north-west. (I hope I got the signs correctly.) And all the experiments should be equivalent because the universe has no preferred direction, x and y are as good as x' and y'. [Since the Earth is spinning, we have a small technical problem here, but just stop the Earth to keep the discussion simple.] Now, it is not a good idea to imagine that the east-west axis is a tangible dimension, but the north-south axis is something else. The main problem is that someone else can choose other directions, like (north-east)-(south-west) and (north-west)-(south-east) and get the same experimental results. Now, which one is the tangible dimension (north-east)-(south-west) or (north-west)-(south-east)? So we assume that all coordinates x, y and z are essentially the same thing and no one of them is special. In most common situations, the vertical axis is different of the two horizontal axis. In normal situations, it is clear where it is up and down, so the "z" axis is special. It is not clear the direction of the "x" axis, and for most experiments you simply choose the direction that makes the calculations easier, but "x", "y" or any horizontal direction is as good as the other. [Just ignore again that the Earth is spinning and that it has a magnetic field. The spinning of the Earth and the magnetic field define two special directions that are both the "north" in some sense.] -- Now, even if there Earth is not spinning, and if the Earth had no magnetic field, the problem with the "z" direction going up is that two persons in different continents will disagree, about where is up and down. If you choose (x,y,z=up) in other continent will choose (x', y', z'=up') where x', y' and z' are obtained mixing x, y and z. And both will be totally convinced that "up" is special and there is a clear meaning of "up", in spite each one has a different "up". So it's better to assume that the universe has no preferred directions (x,y,z) and all the differences are due to the details, like a big chunk of dirt we call Earth. If we assume that some of the directions x,y,z have some essential properties that the other doesn't have, it would not be possible to mix them and construct x', y' and z'. -- Now, about the problem with time ... In Special Relativity you can mix the special coordinates and time. It's easier if you always multiply the time by c (the speed of light), and you use the coordinates (ct, x, y, z). Ad it's more simpler if you think that you measure the distance in x, y, z in light-years and that you measure the time intervals in t in years, so you get the same number in ct. In Special relativity you can mix for example x with ct ct' = x * ? + ct * ? x' = x * ? + ct * ? where ? means some coefficients like the 1/sqrt(2) in the x-y example at the beginning of this comment. It's easy to calculate them, but the exact numbers are not important. The important part is that the formulas are mixing x and ct. In one mix you get ct' that is a new time-like coordinate and in the other mix you get x' that is a space-like coordinate. These are the coordinates that someone else sees when is moving at a different speed than you. You and the other person (in a train, plane, spaceship) will disagree about what how the time flows. You see t and the other person will see t' that is a mix of your t and your x. But both you and the other person can do any experiment and get equivalent results, because the universe don't prefer t to t' or vice versa. There is no experiment to determine if t or t' is better. The main difference between the example with (x,y) and (ct,x) is that x and y are mixed in a slightly different way than x and ct. In particular, you can exchange x and y (or better x and -y for technical reasons). But you can't exchange completely x and ct. All the mixes have a new ct' that is somehow more similar to ct than to x, and a new x' that is somehow more similar to x than to ct. Moreover, all the observers agree that there is one time and three special dimensions, but they will not agree about how the time flows (as the will not agree where the directions x, y and z are pointing to). The explanation of why can't mix "completely" x and ct is part of the mathematical details that I'm not writing here. It's somewhat related the fact that you can't move faster than the speed of light. Just get any introductory book about Special Relativity, but continue reading until you reach the chapter about Minkowski spaces. The initial formulas doesn't make too much sense until you reach Minkowski spaces.

bmer 8y ago

Think of dimensions as the number of pieces of information required to uniquely identify the position of an object. In 1D, you only need one number. In 2D, two. In 3D, three. But in all these scenarios, the point in question is "static". What if the position of the object was also changing in time? Then, it wouldn't be sufficient to provide just the spatial coordinates, you'd also need to specify what time it is. In 1D for example: at 12:00 the point was at 0. At 13:00, the point was at 1.0. At 14:00 the point was at 2.0. Thus, "4D" in special relativity comes from 3 spatial dimensions + time, since we care about how things move through "space and time".

nopassrecover 8y ago
One difference of course is that while we can always return to a physical position in space, even if the contents of that space have changed, it seems very conceptually challenging to imagine returning to a position in time even if the contents of that time had changed. What would this even mean? Our definition of time seems therefore to be two things: first, a snapshot of physical state (in which we imagine points in time a bit like Back to the Future), and secondly (as other comments have noted) a measure of periodicity. The latter is that which we seem to be measuring experimentally (e.g. with differences in atomic clocks within gravitational fields). However what we are literally measuring seems to be something like “how many times can a given particle move/irradiate relative to another given particle before we bring them back to each other”. Reducing to a pendulum for example, we could imagine that the space was “more dense” so it took longer to travel the same apparent distance, a bit like how light takes longer to travel through particular materials. Another way to see this would be a kind of universal framerate. So based on this possibly weak conceptual understanding, I can certainly imagine a fourth property of any given spacetime location that reflects how frequently events can occur in that space (a kind of EM-field view of time), but to consider this as a geometric “position” or coordinate is something I conceptually struggle to imagine.

antidesitter 8y ago

Because, according to special relativity, your plane of simultaneity (how the universe is "changing" from your perspective) would differ from that of someone walking right past you. See the Andromeda paradox: https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argume...

TheOtherHobbes 8y ago
I hesitate to disagree with Penrose, who has forgotten more about relativity than I'll ever know, but... I'm not seeing the paradox. There is no absolute simultaneity in relativity. So the Andromeda Paradox only becomes a problem if you invent a magical FTL chronovisor that allows you to bypass causality and see what's happening anywhere in the universe at your idea of the present time. There's no reason to believe such a device could ever exist. So I don't see why there's any confusion. Both hypothetical observers discover events that happened in the past when the light cone of the events intersects their position in space time. Events in the past aren't uncertain, they're simply unknown in the reference frames of the observers. And because the observers have no causal influence on distant events, they have to watch them unfold passively. It's no different to an incredibly laggy online game. If you're well behind the action, events happen before you can influence them. The fact that someone may suffer twice the lag you do doesn't make the events any more philosophically ambiguous, mysterious, or acausal.
- antidesitter 8y ago
  I’m a bit confused by your response, especially the lag analogy, so I’ll try to ask a clarifying question: is there a fact of the matter as to whether the Andromedean space fleet has already set off on its journey?
amelius 8y ago
By the way, here's a nice video on the topic of simultaneity. https://www.youtube.com/watch?v=wteiuxyqtoM https://www.youtube.com/watch?v=wteiuxyqtoM

mikekchar 8y ago

I have a kind of followup question (which, if I understand correctly, was touched on a bit in the article). If time is a tangible dimension, is it possible that the "passage of time" is an illusion? I remember the past and because of causality, the events are ordered. Time seems to flow from the past to the future and it never flows from the future to the past. This seems obvious to us, but I've always wondered why time doesn't flow backwards. Just to take a silly example, what if all the "points in time" just exist (and are ordered -- I don't propose to break causality)? They don't flow at all. From my perspective, at every "point" along the time axis, I can recall the past and it will be ordered as if it "happened", but each point could be independent (though constant). If I could remember "forward" through time, then this would be obvious, but since I can only remember "backward" through time, at every point it appears as if I have progressed through time. I suppose the interesting thing is that causality is uni-directional. Things can only happen in a certain order in the past. But this is not true of the future. Even if I have perfect knowledge of the present, there are some things I can not predict about the future (quantum mechanics FTW). I wonder why that is (because we are flowing through time? Ha ha!) Sorry for the diversion, but if someone that is better educated than me could shed some light on the matter, I'd be grateful.

aaaaaaaaaab 8y ago
You’ve rediscovered eternalism: https://en.m.wikipedia.org/wiki/Eternalism_(philosophy_of_time) https://en.m.wikipedia.org/wiki/Eternalism_(philosophy_of_ti...
- akvadrako 8y ago
  If eternalism is real it's hardly fair to say he rediscovered it - all the discoveries have always existed.
  - stan_rogers 8y ago
    Discovery is not invention, it's literally just noticing (or uncovering in a metaphorical sense) something that was always there.

jules 8y ago

You're completely right that there is nothing revolutionary about viewing a point in spacetime as a point (x,y,z,t) in 4 dimensions. If that was what special relativity was about then it would be very boring! The x,y,z directions are connected to each other, because the x direction can be rotated into the y direction by rotating in the xy-plane. What special relativity is about is that the t direction can be rotated into the x direction by "rotating" in the tx-plane. Rotating in the tx-plane is acceleration in the x direction. That's what causes all the weird relativistic effects such as time dilation and length contraction when you accelerate objects. Creatures that don't intuitively understand that the x and y directions form a whole could come up with a similar theory: if you rotate an object in the xy-plane it gets x-dilated and y-contracted. The only mathematical difference between a rotation in the xy-plane and a rotation in the tx-plane is a minus sign in the formula for rotation. So the t direction is not exactly like the x,y,z directions, but the only difference is that terms involving t^2 get a minus sign. For example, the squared distance in 3D is x^2 + y^2 + z^2, but the squared "distance" in the weird 4D spacetime geometry is x^2 + y^2 + z^2 - t^2.

howeyc 8y ago

You're right. Time isn't real in the same way as x,y,z. https://en.m.wikipedia.org/wiki/The_Unreality_of_Time https://en.m.wikipedia.org/wiki/The_Unreality_of_Time http://edwardfeser.blogspot.com/2018/05/godel-and-unreality-of-time.html http://edwardfeser.blogspot.com/2018/05/godel-and-unreality-... It's rarely discussed because the implications are more philosophical. The math, and expiriments are very useful when time is a variable. So is daily life, clocks are very useful.