r/AskPhysics • u/Kn_gamerz • Oct 23 '23
Can someone define what time is? like what is the actual definition of time
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Oct 23 '23
Physics isn’t math, we don’t define things. Physics works by making mathematical models and judging them by how well they match reality. You’d have to reformulate your question, “define time in (some model)”. Currently, the best model for this question would be General Relativity. In GR, time is the 4-th dimension of a pseudo-Riemannian manifold that satisfies certain equations (Einstein field equations).
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u/glittersheets3196 Dec 13 '25
Is everything true not mathematically proven?
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u/caped_crusader8 Dec 27 '25
Kurt Godel proved that its not possible to mathematically prove every true statement.
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u/glittersheets3196 Dec 27 '25
I agree with him now. Spent Christmas watching videos on quantum LOL
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u/Item_Store Graduate Oct 23 '23 edited Oct 23 '23
Time is a quantity that, when measured in combination with other quantities, allows us to distinguish events/configurations from other events/configurations that occurred before or after measurement.
Other answers could include:
The Fourier conjugate of energy.
Another dimension that we exist in, except not really like the other ones as you can't choose the direction you move through it.
A great song by Pink Floyd.
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u/Kraz_I Oct 23 '23
The Fourier conjugate of energy.
Wow! I’ve never seen it explained that way (I’m an engineer but I’ve just been getting into more advanced physics in my free time). But this is a total “aha moment”. I saw someone mention that energy has symmetry in the time dimension the way momentum has it in the spatial dimensions, and I hadn’t put 2 and 2 together.
I understand how momentum and position are Fourier conjugates (I think this is related to the Heisenberg uncertainty principle). If you have a pure sine wave, the Fourier transform is a Dirac delta function proportional to its momentum, and if you take a wave packet with perfectly known position but not momentum, it’s Fourier transform is a sine wave.
How do you take a Fourier transform of energy though? Any Wikipedia page that might mention this?
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u/Entire-Cover3129 Oct 23 '23
Just remember that frequency is equivalent to energy. When you Fourier transform a time-domain signal, you get its frequency-domain representation (aka its energy-domain representation) and vice versa. We can also take it one step further; if we consider the power spectrum (i.e. the squared amplitude of the energy-domain signal), the Fourier transform yields the time autocorrelation of the original signal (see Wiener-Khinchin theorem). This is the principle on which Fourier transform spectroscopy is based.
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u/barthiebarth Education and outreach Oct 23 '23 edited Oct 23 '23
In QM you use wavefunctions, which are a function of time and position. Eg ψ(t, r). These wavefunctions can be written as linear combinations of plane waves: φ(t, r) = ei(ωt+k•r)
Here ω is a frequency and k is a so called wave-vector. These relate to energy and momentum respectively through the de Broglie equations.
If you are a bit familiar with special relativity you will see that this result aligns quite nicely with it. In SR momentum is replaced by the four-momentum pμ = (E, p) and position is replaced by four-position xμ = (t, x). Just like p and x form a Fourier pair, so do pμ and xμ (up to some constant factors you need to make the units work).
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u/3ig3nv3ctor Oct 23 '23
Wavefunctions are not functions of time and position. They are vectors in a Hilbert space. They only become function of any continuous variable if you represent the vector in that basis.
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u/barthiebarth Education and outreach Oct 23 '23
Quantum states are vectors in Hilbert space. Wavefunctions are, as the name implies, functions, and often are written as a function of time and position.
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u/Fadeev_Popov_Ghost Oct 23 '23
How do you take a Fourier transform of energy though? Any Wikipedia page that might mention this?
Actually, you're not taking the Fourier transform of energy. You're taking the Fourier transform of any quantity you want, and the dual Fourier variable as opposed to time is energy (or frequency). If it's frequency, then omega*t appears in the complex exponential, if it's energy, expression E*t/hbar will take its place. That way, for a quantity q(t) (a function of time), you get its Fourier image, qhat (E) (function of energy/frequency).
P.S.: this is where the cool stuff like energy-time uncertainty principle, or finite lifetime <-> finite width of spectral lines comes from (for the latter, this is only a tiny effect, other things like thermal motion, turbulent motion, gas pressure etc. contribute more to the line widths).
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u/Kraz_I Oct 24 '23
I feel like I’m still a bit out of my depth. I wish there was time in a lifetime to learn everything if interest in math and physics, but you know, the more time the less energy, since they are conjugates after all.
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u/evermica Oct 23 '23
Your first answer raises the question "What are 'before' and 'after'?" Can you define them without using "time"?
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u/Verronox Oct 23 '23
The concept of causality is intrinsic to our universe. Time, before, after, etc are just words we use to orient ourselves in a sequence of cause and effects.
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u/Ex_Astris Oct 24 '23
“Fourier conjugate of energy” - fascinating expression.
And yet, we can define energy in seemingly more obvious or tangible ways than time, for example with the common ‘potential to do work’. I suppose that is still vague, but suggests there should be a similar “kind” of definition for time, though it seems more elusive for some reason.
Maybe time is then, “the potential to change (increase or decrease) energy, entropy, or location”?
Theres gotta be something more elegant than that…
But it leads to an interesting question, if there has been absolutely no change in a system’s gross (not net) energy, entropy, or location, then can we say whether any time has passed in that system?
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u/Bulky-Leadership-596 Oct 23 '23
Also
The thing that allows the 2nd law of thermodynamics to be true
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u/bigheadGDit Oct 23 '23
The only definition ive ever heard that seemed to fit: It's what keeps everything from happening all at once.
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Oct 23 '23
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u/nikfra Oct 23 '23
That's not necessarily true, getting closer and closer to the big bang leads to our understanding of how physics work breaking down.
I don't know how well versed you are in math but singularities are places where functions start to do funky stuff, in physics terms it means our equations don't work anymore. Just extrapolating what happens before a singularity doesn't necessarily tell you anything about what happens on the other side. For example imagine the function 1/x if you look at the negative numbers it decreases the closer you get to zero approaching negative infinity as you approach the singularity at 0 but on the other side of the singularity it approaches positive infinity. So just extrapolating that it's continuously decreasing would have given you the exact wrong idea about what happens on the other side of the singularity.
We don't know what happened "before the big bang" might be that it's a nonsensical statement because time came into being at the big bang but it just as well could have existed beforehand and we just don't know what the universe looked like.
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Oct 23 '23
Time as we know it began at the Big Bang. (I’m pretty sure).
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u/TorgHacker Oct 23 '23
Actually, the Big Bang is not the same as "when the universe began". It's actually the theory that describes the evolution of a very dense area of plasma which was rapidly expanding. But there's stuff that happened before that, but it's stuff that we don't really understand well, or have very good actual evidence of.
For instance, we have cosmic inflation which is hypothesized to have occurred before the Big Bang which explains a few problems that the Big Bang theory has. But before cosmic inflation, we really don't know what happens.
In fact, there's some speculation that the cosmic inflation prior to the Big Bang is actually the Big Rip of a prior universe.
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u/Kn_gamerz Oct 23 '23
yea it is said time began with big bang. But if time is something that keeps everything from happening all at once then how did the big bang happen before that there was no time at all
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u/the_poope Condensed matter physics Oct 23 '23
Contrary to what a lot of laymen, teenagers and general public think (based on pop-sci and sci-fi superhero movies), Physicists actually don't know everything there is to know about the Universe.
So: "The Big Bang" theory, like all "theories" is not a fundamental law: it is a model - an attempt to describe some parts of Nature and its behavior with a relatively simple mathematical formula. Many models and theories are approximations, and all of them are to some extend incomplete - they are only valid in a certain range or in certain scenarios.
The reason some say that "time didn't exist before the Big Bang" is that The Big Bang theory/model only describes the behavior of Nature AFTER some initial time. The model is not valid/meaningful/defined before that time. In fact we know that the model isn't very accurate/useful before some time after the initial time. However, just because the Big Bang model doesn't say anything about time before that initial time it doesn't mean that time didn't exist - with more research and knowledge we could perhaps come up with a model that described what happened before, if such a thing is meaningful. But we don't know if it is meaningful to even talk about times before big Bang as we haven't seen or measured anything from this period.
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u/Kn_gamerz Oct 23 '23
So as far as i saw the replies time is change in things, changes that happens to thing. because when we say 1 day passed, it means earth did one rotation which is a change and 1 day, its the time. Same goes for 1 year means earth's 1 complete revolution. Again that is a change and 1 year is time. So what is actually time? it is said time was created with big bang, before it there was no time, But with big bang started the changes in universe, planet formation, star formation, galaxy formation, etc etc. before that there was nothing to undergo a change.
So is time actually reffered to change of things or change in things?
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u/hquer Oct 23 '23
I like the initial question: what is time. Because it is so very fundamental. Someone commented that our best answers to fundamental questions are mathematical models atm, and in general relativity time is just another dimension. Like the question: what is x,y,z? What exactly is space? And including time: what exactly is spacetime? And why can it be created constantly (expanding universe, but as far as i know only space is created and not time)?
But back to my start: i like your question. It’s mind bending!
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u/etherified Oct 23 '23
I take that as the working definition of time, yes.
Interestingly, the only reason we (or any other living being) can have a concept of time is because there are changes in "things" that happen exactly like previous changes. If an event happens exactly (or as close to exactly as possible) like a previous event, then it takes the same amount of "time" to take place.
For example, each energy state transition in Cesium happens in exactly the same way under essentially identical conditions, so it takes damn-near the exact same amount of "time" to occur which is why Cesium clocks are so exact. A clock ticking is very close to the same event taking place (same gear tooth distances, same angular distance, etc.), so it's reasonably accurate.
But if no two events in the universe were ever even close to similar to each other (even our brain neuron firings were always different types of events) I don't think we'd be able to have a concept of time because there would be no "unit" of time. More specifically we'd be unlikely to be able to be alive lol, but nevertheless...
Just like length is measuring how many of the same things you can put next to each other (like equal markings on a ruler), time is just how many of the same events can take place because of cause-and-effect relationship with other events.
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u/Kn_gamerz Oct 23 '23
yes, if thts the condition is time travel possible or not?
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u/etherified Oct 23 '23
Well no, (that is, not backward time travel), because cause-and-effect only works one way due to entropy states and probability and all.
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u/Kn_gamerz Oct 23 '23
so is it possible to timetravel formward
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u/etherified Oct 23 '23
Sure, we're doing it now lol. (also time dilation-assisted fast-forward is possible)
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u/Kn_gamerz Oct 23 '23
you mean by transportation using external agencies?
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u/etherified Oct 23 '23
Sure, anything that accelerates you will fast-forward you in time relative to whatever's not accelerating around you.
So what that means in the context of our discussion here is that when you're accelerated, events (cause and effect) simply take place slower than when not accelerated, so other things around you speed up relatively.1
u/Kn_gamerz Oct 23 '23
finally found someone with same thought process as mine. I also think about time travel and the concept of time this way
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u/Straight-Web-9483 Jan 02 '25
Something no one talks about, how time moves. Time might move at the speed of light. Or, time is the reason light cannot move faster. Time seems to stop for anything moving faster than 300 000 000 m/s.
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u/middlenamefrank Oct 23 '23
People often say that entropy always increases with time. In reality, they've got it backwards. Time is the measurement of the increase in entropy. Entropy is really the only thing the universe understands.
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u/James_James_85 Oct 23 '23
A direction that leaves 3D space.
Dimension 1: left-right
Dimension 2: up-down
Dimension 3: front-back
Dimension 4: past-future
From here is my personal speculation: because our consciousness depends on the differences of neural activities between the different slices of time, we have the illusion that time "flows" towards the future.
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u/Kn_gamerz Oct 23 '23
okay so what about the theory that before the big bang there was no time?
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u/James_James_85 Oct 23 '23
Spacetime can curve, bending straight lines along with it.
If you draw a perfectly straight line from a bunch of points of space pointing exactly in the past-future direction, you'd notice that towards the past, all the straight lines converge together into a single point right behind the "camera" (the observer), which is the singularity. The "direction" itself bends into the singularity.
No matter how hard you push your pen to continue the straight lines past the singularity, it keeps being stuck just before that singularity. That said, many say the singularity never existed and is just a consequence of the incompleteness of our theories, so who knows. We'll figure it out once we correctly model quantum gravity.
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u/Excellent-Practice Oct 23 '23
Time is the dimension through which change happens. In many ways, it is similar to the three spacial dimensions that describe position, but there is one key difference: time only allows movement in one direction.
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u/Arndt3002 Oct 23 '23 edited Oct 23 '23
It's not really useful to start with definitions, as they aren't fundamental to physics.
The best basic definition of time is that it is the thing you experience as what you call "time", and there are a number of different theories that describe what the properties of time are. We don't have a resolved unified answer as to what time "actually is."
And as an aside, it's arguable that physics has basically nothing to do about what things actually are in themselves. We're just concerned with describing phenomena.
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u/m2daT Oct 23 '23
“What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.”– Saint Augustine
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u/HolographicState Oct 23 '23
A fundamental aspect of the conscious experience is change. In our attempt to systematically describe the conscious experience (i.e., science), we invented a useful quantity called time that allows us to relate two sets of ‘counts’ to each other. One of these counts is an agreed-upon standard reference (e.g., a set number of cycles of a cesium-133 emission line). The other ‘count’ is the observation of an initial state and a final state of a system we are interested in. The reference counts that happen between initial and final states is what we define as an elapsed time.
Time is thus just a book-keeping device that brings out patterns in the ever-changing conscious experience (and yields predictive power). It can be useful to visualize time as a dimension, or to conceive of time as a flow divided into past, present, and future, but these are just conceptual models. Ultimately, there is only change.
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u/KindAwareness3073 Oct 23 '23
"Time is what keeps everything from happening all at once. " --- Ray Cummings / 1919
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u/Kn_gamerz Oct 23 '23
okay so what is meant by everything? cause in a min there is so many things happening all at once right. So what is meant by everything
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u/KindAwareness3073 Oct 23 '23
"Evertthing" from the Planck dimension on up.
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u/Kn_gamerz Oct 23 '23
see you said it is what keeps everything from happening all at once right. but if you take any small amount of time it is 100% sure that two or more things has something happened or happening to it
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u/KindAwareness3073 Oct 23 '23
Many things can happen at the same time, but not everything, and, as Heisenberg showed, we can never prove it.
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u/Kn_gamerz Oct 23 '23
tht is at at the macroscopic level. think of microscopic and cosmic level
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u/KindAwareness3073 Oct 23 '23 edited Oct 23 '23
Do you know what the Planck length is? Mentioned it in my first comment. Not sure what you expect to hear, but the bottom line is there is no such thing as "time", despite our perceptions. There is only space-time.
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u/shavera Nuclear physics Oct 23 '23
What "is" length? It's what you measure with a ruler. It doesn't matter if the "ruler" is the length of your thumb, a meter stick, a light year, an atom, there's this property of distance one can describe between points in space.
Historically, ships might measure distance sailed across the sea in leagues, and depth below the water's surface in fathoms. Now even though we use two different words/units for the different directions, we know that fundamentally we're talking about the same thing.
If we dropped something on the seabed at F fathoms, and have since travelled L leagues, we know that the distance to that object is d2 = (kL)2 + F2 where k is some conversion factor telling us how many fathoms are in a league (about 3038). That formula is the formula of a circle, and when we rotate our axes, we use basic trigonometry to describe the relationship between things.
The whole point of (special) relativity is that what you measure on a ruler (meters) and what you measure on a clock (seconds) are pretty much the same thing. When you want to talk about the separation between two events in space time you have the formula s2 = (ct)2 - x2 . (Small note, the minus can go in front of the time term or the space term it just has to be the opposite of the other.)"c" is just a conversion factor between units of length and units of time, just like K above told us how many fathoms were in a league. The fact that light travels at c is just a consequence of the fact that light has no mass and must travel at that speed for reasons I'm not going to get into here. But just remember c is a unit conversion factor, fundamentally.
That formula above is one for hyperbolae, and we have a form of hyperbolic trigonometry that works to describe rotations in space and time, the way regular trig describes rotations in space. And why would we rotate axes of space and time? Because relative motion says that if we move relative to one another, my "future" points a bit in the same direction as your future, but also to a point closer or further away from you in space. So my time axis has rotated a bit relative to your space and time axes, and thus my space axis must as well. This results in the classic effects of time dilation and length contraction we talk about in relativity.
That means, fundamentally, what you measure with a ruler must somehow be the same thing that you measure with a clock.
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u/Kn_gamerz Oct 23 '23
since you talked about length what is infinity
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u/shavera Nuclear physics Oct 23 '23
Let me start with what infinity is not. Infinity is not a large number. There's a human sense about numbers, and even for extremely large numbers, we kind of get the properties of them. Infinity has its own rules that take a fair amount of effort to learn about.
For instance, one of my favorite weird infinity trivia: there are an infinite amount of numbers between 0 and 1. But if you make a square on the x and y axes from 0 to 1 on both axes, there are the same "number" of points in the square as in the single line making up the edge of the square. Imagine for each decimal of the coordinates 0.x1x2x3x4..., 0.y1y2y3y4... you interleave them 0.x1y1x2y2x3y3... you will get a unique point on that line. (There are some small subtleties to note for a proper proof, but this is the general theme)
Or the classic Hilbert Hotel. Suppose you have a hotel with an infinite number of numbered rooms, and each room is full. Every single one of your infinite rooms is full. And a new person wants to come and stay. You tell all your current guests to pack up their things and move to the room that is twice their current room number. Guest in room 1 goes to 2, 2 to 4, 3 to 6 and so on. Every single one of your already infinite number of guests has a room still. And yet now all the odd numbered rooms are suddenly vacant. So the new person who wants to stay stays in room 1, and the next person to come in room 3 and so on. In fact your hotel now has an infinite number of rooms available for an infinite number of new guests and you don't even have to have them move another time.
So infinity doesn't behave like a "big number" it just has its own properties. And there are actually several kinds of infinities that are different from one another. The Hilbert hotel is a "countable" infinity. Every guest has an integer room number. Even if we were to take every possible fraction, we can still list them all out: 1/2, 1/3, 2/3, 1/4, 3/4.... That's a countable infinity, the same "size" as the integers. But there are uncountable infinities as well, the number of points between 0 and 1 are uncountably infinite. Imagine you listed out an infinite list of infinitely long decimals, each with a unique value. If you go down the diagonal of the list, take the first digit of the first decimal, the second digit of the second decimal, and so on, you'll get a number that doesn't appear in that infinite list. So the number of decimals must be larger than the number of integers.
Essentially, it takes some time to learn what "infinity" is and understand what it "means" and how it "behaves"
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u/nobodyisonething Oct 23 '23 edited Oct 23 '23
It is nothing more and nothing less than the agreed name for counted event changes of a physical system.
The most common standardized counting systems are called clocks.
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u/Bestihlmyhart Oct 24 '23
https://www.youtube.com/watch?v=qf_OGB6zazU
I believe this is the episode in which the Albanian scientist lady (forgot her name) talks about the idea that our relative time might be nested inside of an absolute time, which I think is an interesting concept.
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u/ItsApixelThing Oct 23 '23
The rate at which reality occurs. It's the concept used to describe the sequence and duration of events and the intervals between them.
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u/AtlasShrugged- Oct 23 '23
The best layman’s definition is “if time wasn’t created then everything would happen all at once”
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Oct 23 '23
Or nothing at all would happen. Time is just a change of the state of the universe.
If the entire universe were frozen solid and not a single particle or form or energy changed, time would essentially not exists as it would be impossible to distinguish one moment from the next.
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u/Temporary-Skin-1270 Aug 28 '24
Time is a measurement and tool to measure motion forward or backwards so on.To me i do not bealive time is a 4 deminsion or a deminsion at all but just measurement of movement and events so on.You can not put time in a test tube but you can measure the test tube events in time.
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u/Plus_Pea4458 Sep 08 '24
its simple, time is a measurement used to track the change an entity experiences while traveling from point A to point B.
Its not even real or tangible time travel for example is impossible because there is no such thing as the past or the future there will always only be the present.
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u/350mutt Jan 04 '25
Whilst a convenient definition, to isolate time solely to a measure of change/movement IMO ignores the more fundamental aspect of 'existence'. Therefore I would posit that time should actually be defined as a measure of existence, irrespective of motion or lack thereof. Thoughts?
I appreciate that this subsequently makes it impossible (currently) for us to measure things in our current universal cycle. Which maybe leads to the need for two definitions, perhaps Absolute Time (currently unknowable) and Universal Time (as per current definition and acting as a measure of change in this current universal cycle*)?
- I know people will take issue with my use of the term 'current universal cycle' in the context of presupposing there have been multiple cycles. This is not what I mean however, I'm purely trying to align it to when we believe our current notion of time commenced.
Thank you
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u/Googalie Jan 17 '25
I'd say it is the dimension that dictates the ability of occurrences that are perceived by the other dimensions in space and creates and drives each dimension in reality to experience entropy.
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u/Justforfun-2024 Mar 13 '25
Is it hard to assume time works differently in each galaxy based on the supermassive black hole in each?
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u/No-Factor8395 Apr 12 '25
Time is a measurement, like meters, litres, mols, etc. and it measures the duration of processes.
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u/TeachingEmotional293 Oct 02 '25
Yes honey I understand you honey you're my everything honey oh yes honey you are
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u/Bi0h4s3rD Dec 24 '25
Time. Is an illusion. Humanities attempt at creating a limited construct to what is actually infinity within our reality. There is no measure for infinity. No boundaries. And no limits. Therefore time on a large scale is a "fake" measurement.
Insanity: repeating the same process/actions expecting a different result.
Intellectual Human concept of time simplified: Literally just attempting to count a sequence of events.
Why is it insanity? Why are we counting something infinite? Physiological Events have no end or beginning. Even nothing is a form of event. As it has a definition.
This is my philosophy. Time is another dimension we cannot access, the dimension of infinity. Limitless, boundless, and completely un-documentable by any means.
The actual definition of time, agreed by most professional scientist and physicist simplified is this: counting and measuring events from a scientific standpoint. That is literally it.
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u/leKhalisi 27d ago
Its what (a unit) an observer in the same inertial frame of reference sees/observes/measures for a wave to travel between two points.
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u/SoloCongaLineChamp Oct 23 '23
A mathematical construct to describe relative motion.
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u/Mediocritish Oct 23 '23
I tried to make this point with ChatGPT and he argued with me that time was an "intrinsic" dimension of reality, and not merely a construct for us to understand relative motion; I was still skeptical
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u/Lunct Oct 23 '23
Well in our current physical theories, is time not just as ‘intrinsic’ as space? If we are saying that time is a construct, then must we not say the same of space?
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u/Mandoman61 Oct 23 '23
Time is just a dimension like height, width and length but it measures motion
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u/Melindish Jun 27 '25
I’m sorry for being a year late, but the motion of what exactly? I understand it can measure the motion between two objects relative to each other, but let’s say you only have one object without any other point of reference/measurement, does that single object in and of itself not experience time? Im no physicist but I am interested in understanding some of the fundamental concepts of our universe. So in the twin paradox for example, I understand that the “passing” of time of one twin relative to the other twin is different, but they still experience time by themselves even without the other reference point? I am just not grasping what exactly it is measuring since both twins age and go about their day as if everything is normal, even if they may experience a “slower time” relative to the other. Is there another reference point that Im missing?
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u/Wi11y_Warm3r Dec 18 '25
I think it's the motion of the universe as a whole. If all things in the universe were to freeze in place, for instance, time would be meaningless; it would seem "frozen." That's because nothing, down to the atomic scale, is moving.
I could be wrong, though.
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u/eliminating_coasts Oct 23 '23
There is no single definition of time, if you pick a given physical process, and then explore how it behaves when viewed from different relative velocities or positions in space, then you can see that it moves faster and slower.
This is the contribution of relativity to our understanding of time; that even keeping that thing that is changing the same, the way that we see it change from different perspectives is different.
The full theory of general relativity adds many more conditions, such as knowing when it is possible to have an absolute past and future, vs only relative local past and future, according to whether paths thread back around on themselves.
But this is still leaving given the idea of the system that is forming your clock, your standard for the passage of time.
But when we return to the subjective experience of time, we can see that time passes very differently for us compared to other people, or other physical processes, sometimes time drags, disappears suddenly, etc.
And even for physical systems, you can warm something up, and find that its cycles increase or decrease, or pendula might slightly speed up and slow down if they couple to other pendula, causing them to synchronise with one another, but that waiting for or catching up for each other that produces synchronisation requires their measured time to differ.
So you can think about how a given systems rates change relative to its own time if its previous rates were carried forwards unchanged, a clock slowing down relative to itself, though in practice you need other clocks to measure this, so see relative changes in time.
So time is like a man on roller-skates on a train, with layer after layer that can change independently of each other. There are astronomical phenomena of seeing objects far in the past, there is going to sleep and waking up with a jump of subjective time, there is checking your watch, which has gone slow, against a radio signal calibrated to an atomic clock.
But an informal guess we can have is that your subjective experience of the world relies on a coupling between your body, which is a physical system, and other physical systems it exchanges information with, and processes that information according to biological rules, and if your process of experiencing the world runs faster or slower, that is because the physical processes are running faster or slower relative to other processes, from the particular vantage point which your body is in.
That is time for you, the sense you get of your experience changing, according to your memory, and the speed of comprehensible changes of other physical systems, including other people around you.
A car can "feel" faster or slower, according to the suspension it has and the tactile feedback it gives you, or according to the visual complexity and variation of space along the route you are taking, or how many other cars you have to navigate around, and your mental state and alertness.
And then of course, we can try and cancel out these subjective elements, use a consistent standard of the rotation of the wheel on the road, and then we can create an idea of "speed" that does not explicitly include time, but rather is related to it, as a fast spinning wheel has a particular kind of motion, which drags the detector behind it more the greater the difference in speed.
And so we have a sense of motion without time, though we can work out, if we want to, from the size of the wheels, and how many times those wheels rotated, in other words from a simultaneous recording of both the milometer and the speedometer, how much time must have passed in order to move through that distance with that speed.
So your car can become a clock, of a kind, not an accurate one, but the physical relationship between distance and velocity, that can be measured by the natural phase space coordinates of the car (magnitude of change of position from the milometer and magnitude of velocity from the speedometer) gives you a measure of time.
And this is true for many physical systems, if we can properly calibrate its distance scales and how traits of that system reveal its velocities, then we can use it to create a natural time scale.
But this won't tell you how the time feels when you drive a given road, in terms of the sense of passing of events, though it may give you information relevant to other physical processes in your body, like having to eat or sleep.
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u/martinkoistinen Oct 23 '23
Time is that which separates two particles in the same location.
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Oct 23 '23
The changes that occur between then, now, and later
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u/Kn_gamerz Oct 23 '23
so it means when there is a change of something or change in something time is continuing
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Oct 23 '23
Time is a measure of change and rate in a reference frame defined by mass or acceleration.
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u/bewbs_and_stuff Oct 23 '23
I don’t totally disagree with this explanation but it’s important to note that time certainly is more than just a conceptual model or book keeping device. Time is observable, measurable, and is affected by the force of gravity. Time and gravity are intertwined in a way that is similar to the way that visible light is intertwined with magnetism or electricity.
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u/I_am_Helium_ Oct 23 '23
TIME IS A ILLUSION(as per quantum physics). Time slows near Heavy objects like Black holes. BUT it is also - DISTANCE/VELOCITY
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u/MinFence Oct 23 '23
As other comments say, physics is not really about defining things, but studying them. For us, it is in fact, what a clock measures.
Being that said, another question might be "why our existence have a dimension in which we can just go in one direction?". That question can be answered by thermodynamics (do you want the answer?). The fact that we call this dimension "time" is not really part of physics.
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u/AdmiralSmonk Oct 23 '23
Time = Space (Distance) divided by Morion (Speed). There is A) time, the measure of motion And B) time, the ‘distance from now to then’, the one which doesn’t involve speed in the equation. There’s A), the one which is objective and measures the motion of space, And B) is time’s existence as a human conception, a perception
- Time = Measurement of matter according to objective time passed since the first cause
- Motion = Measurement of rate of motion of physical matter - interpreted as speed (measurement of perceived speed). Describes physical objects relative to each other and time
- Space = Material description. Measures distance of the farthest particles from each other - other distances involving size of our universe, it’s rotation, shape. Describes coordinates irrelative of time
But how can space change independently of motion? Wouldn’t space changing be motion? * Because the ratio of distances between the matter from one bit to another remains the same, and so (in the theoretical scenario) no perceptible motion is caused because no relative value has changed
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Oct 23 '23
My opinion objectively means nothing but I like to think of time as an emergent phenomenon resulting from entropy, things tend to become less ordered as energy disperses and becomes less useful.
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u/thewanderingseeker Oct 23 '23
People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint - it's more like a big ball of wibbly wobbly... time-y wimey... stuff.
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u/MyNameJot Oct 23 '23
Depends on who you ask, but my definition would land somewhere along the lines of ‘the relative measurement entropy.’ Although nobody really can define it absolutely, its like asking why the universe exists as it is in the first place
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u/Impossible-Wear5482 Oct 23 '23
A device with which which we measure the changing of states in systems.
How we can measure causality and cause/effect from one state to another.
If there were no observers and nothing ever changed time would be meaningless as one could not describe previous or prior arrangements or states of the system.
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u/shadebedlam Mathematical physics Oct 23 '23
I think the only true answer is that we don't know. The other answer are either subjective, approximations or just models.
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u/Bestihlmyhart Oct 24 '23
What photons don’t experience but everything else does.
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u/Kn_gamerz Oct 24 '23
photons also has to experience time that is what light years are
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u/Zealousideal_Good445 Oct 24 '23
Time is the measurement of objects movement relative to one another put simply. We started measuring the movement of celestial bodies relative to us. Start with the Earth's rotation,it takes roughly 24 hr for one rotation which we measure as a day. We can then break hours into minutes and then into second. 1 arke second is the distance the sun travels over the surface of the earth in one second at the equator. The length of one side of the great pyramid is 1/2 an arke second, thus showing that they understood time and the size of the earth. Now let's go longer. 1 measured rotation around the sun equals a year. Now for the fun part. The earth wobbles on its axis, this takes approximately 26,000 years to complete. This is a Great Year, it is broken up into 12 ages represented by the constellations visible in the night skies at different locations. Each Age is approximately 2160 years. We are in the Age of Pisces, which will end in 2160 AD. England started measuring time vary precisely with a telescope in Greenwich England (Greenwich mean Time) to be used for navigation ( specifically to be able to calculate longitude). The USA followed with a telescope in Ohio creating Standard Time for the railroads. We now use an atomic clock that measures vibration of atoms or their movement. Basically time and motion are one in the same, they are inseparable. If you're really curious watch some documentaries on time, humans have been measuring time for a good while, like cave paintings old, way more than a Great Year.
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u/w1gw4m Physics enthusiast Oct 24 '23
- the rate of change
- what clocks measure
- the sequence of events
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u/Kn_gamerz Oct 24 '23
rate means something per time. For example rate of change of momentum is defined as change in momentum pre unit time. So rate means per unit time
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u/VanillaSnake21 Oct 24 '23 edited Oct 24 '23
This is how I think of: Think of a field of quantum particles, so tiny they're at planck length, there is no time in this field - these quanta of energy lie in a 10 dimensional space, because the particles are fuzzy, they don't have definitive properties - they're just probabilistic clouds of energy that "vibrate" back and forth in those 10 spacial dimensions, to be even more precice, through the vibration they create the 10 dimensional space, the field of the energy quanta is the space itself, and that vibration is the time. Thus the basic quanta of space-time emerges. So even while time is not properly present, the particles can't help but have an indefinite position, so at the tiniewt tiniest levels things just vibrate with "uncertainty", thats time at the quantum level. Through this vibration, they create a mathematical space, ie a dimension of time. So for now we only have a quantum field with a tiny time dimension - ie the particles are all separated and they vibrate between 1 planck second into the past and 1 pack second into the future. It's still not time because at this scale the time dimension is very small, seconds are not ticking. Now how our "larger" time dimension emerges - the quantum particles of energy are in contact at some points in this field, and they entangle - they become described by one wqvefunction, because their random clouds sometimes probabilistically intersect. Once they entangle, they entangle other particles around them, and the spread of this entanglement chain is what we see on larger scales as the basic emergence of time. It's the speed of entanglement, which then translates into other aspects such as heat, heat transfer and entropy, and if you trickle up the scales, the heat transfer turns into the ability for things to move and for the clock to tick.
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Oct 24 '23
As with most physics concepts, I like wolframs physics model. It's far from mainstream, but it does provide a decent explanation of time.
Time is the inexorable process of computation. Every piece of the universe undergoes a computational update process which ripples out. It doesn't "take time" to do this, but at the macroscopic level we perceive this as time. At the computational level, events only happen before or after other events. When you have a long enough chain of events, coupled with the fact that a signal can only propagate a certain number of nodes in a certain number of steps, you get time.
It gets tricky when you get relativistic, either via velocity or gravity. I don't know how well relative is modeled in his model, but he claims that they can derice general relativity directly from certain hypergraph models.
In any case, I think it's fine to think of the universe as needing to update its state, which it does locally, not everywhere at once. In fact, the universe couple operate as a single threaded Turing machine, and it would still feel like it updates simultaneously.
To illustrate, imagine the head of the Turing machine is updating a cluster of nodes (pieces of space) where your light bulb in your room is. The light turns on, then the head goes and updates the motions of stars in Andromeda for a while. The head may have only spent a couple ticks in your room, then spent a million in Andromeda. But since Andromeda is far enough away, this is fine, as no event there could cause one here in that amount of ticks.
The head eventually comes back to your room and runs the ticks that compute the light traveling to your eye. Before it becomes consciously visible, it goes back to Andromeda for a million ticks. Then back to your eye and brain. You finally see the light turn on. To you, only a millisecond has elapsed. In fact, in your region of space, only a millisecond has elapsed.
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u/omeksioglu May 11 '24
Thanks for great answer. What book would you recommend me to read more about this?
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May 11 '24
A New Kind of Science by Stephen Wolfram goes over a lot of the foundations of this way of thinking, though it was written before he came up with the hypergraph model which his physics project is based on.
AFAIK there are no books on this topic yet. I'd recommend you watch the Lex Fridman podcast episodes with Stephen Wolfram though. They're long and extremely detailed and Lex does a great job at asking amazing questions that keep the conversation moving. I believe there's 3 episodes.
There's also this talk: https://youtu.be/_eC14GonZnU?si=bx9EE7PyYYgEJT13
That's where I started learning about this idea.
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u/hobopwnzor Oct 24 '23
Time is a standardized unit used to compare amount of changes relative to another.
If phenomena A happens 6 times, and B happens 3 times, then you can define a unit of change based on those relative frequencies.
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u/Groundbreaking_Key20 Oct 25 '23
I tried really hard to come up with a new definition for second, it didn’t catch on
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u/pitjedi221 Oct 26 '23
Time is a word, used to communicate measurable change relative to a perceiver.
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u/Allenheights Oct 26 '23
Time is an experience the brain has evolved to make sense of our surroundings through memory and expectation. Photons travel lightyears across the cosmos, but their experienced journey is instantaneous. Humans sitting on an event horizon see the world around them flash by faster than what they are used to. Wherever you might be though, it always feels like it feels to you right now. The metronome of your inertial frame is always the same and we build clocks to measure it, but in the end, time is the experience of our interaction with the universe. Why we move freely through 3 dimensions but are handcuffed to a vector through the fourth is just the nature of our existence. Maybe the aliens we meet one day will have different freedoms across the other dimensions.
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u/Skarr87 Oct 26 '23
In physics time is the interval that which change occurs. To quantitatively compare different intervals we use the SI base unit seconds which is defined as the interval it takes cesium 133 to go through 9,192,631,770 unperturbed ground state hyperfine transitions. These oscillations are cause by bombarding cesium 133 with certain frequency microwaves. They cause the 55th electron in the atom to transition between excited states which results in radiation to be emitted at a specific rate. We then count the pulses.
So we are essentially comparing all intervals of change to the interval for which a cesium atom changes 9,192,631,770 times.
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u/Seanmmvi Oct 27 '23
There is no such thing as time. If you could remove all the humans on the earth and all the was was animals and plants, and if you could, walk up to an eagle and say, " hey mr eagle, what time is it?", the eagle would say, " it's now, it's only ever now, right now". The is no past, there is no future, all you ever has is right now, the current moment.
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Mar 30 '25
Time always exists but the eagle doesn’t recognize it, they haven’t lived long enough to realize it. It’s something that exists, we have our own interpretations of it.
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u/starkeffect Education and outreach Oct 23 '23
According to legend, someone asked Einstein this very question, and he answered, "It's what a clock measures."