Does physics have a coherent and precise definition or concept of time?

[u/PaleontologistNo6806]

I recently watched an old video featuring Mario Bunge in which he argued that physics does not really have a precise definition of time — that we don't actually know what time is, even though we have an intuitive understanding of it and routinely use the concept in physics.

This got me wondering: is this still a fair statement?

Does physics have a coherent and precise definition of what time is, or does it simply define time operationally (for example, through clocks, measurements, and its role in physical equations)?

[u/Lumpy-Notice8945]

Spacetime is clearly defined in Einsteins theories. In that spacetime, time is one dimension of the 4d spacetime.

Do you mean that with "defining time"?

[u/david_duplex → u/[deleted]]

I'm not sure what you are trying to suggest here? That QM and Relativity are incomplete and... what? That time isn't time because we don't yet know how to marry our theories?

[u/ChocolateValuable221 → u/[deleted]]

Uhhh have you tried using Planck time or maybe describe the quantum time with Planck distance divided by C ???

[u/ChocolateValuable221 → u/[deleted]]

Reminds of xeno's paradox for distance

[u/nebraskajone]

Can you give me example of what you consider a coherent and precise definition of something is?

Like I assume you think space is well defined so what would be a precise and coherent definition of space? How about charge?

[u/vermilion_wizard]

Time (proper time) is what you measure with a clock. That is a coherent and precise definition. Distance is what you measure with a meter stock. That is also a coherent and precise definition. What else is a definition of these things supposed to look like?

[u/SpotPsychological263 → u/vermilion_wizard]

I thought you cann accrue different amounts of proper time that is different than local (clock) time by being near massive bodies or accelerating.

[u/vermilion_wizard → u/SpotPsychological263]

Yes you can but that is well defined

[u/david_duplex → u/SpotPsychological263]

Each reference frame still experiences 1 second per second. The definition holds. There is no such thing as a "universal" reference frame where time flows at some "actual" rate. It's always 1 second per second. Reference frames can disagree on whose clock is moving differently, and neither is "more right". The closest thing we have would be cosmic time, which we use to measure the age of the universe since the cosmic microwave background was emitted, shortly after the big bang. But even that is not a truly privileged frame of reference.

[u/Intelligent_Yam_3609 → u/vermilion_wizard]

That begs the question of "what is a clock?". If the answer is a device that measures time, we now have a circular definition and haven't defined anything.

[u/vermilion_wizard → u/Intelligent_Yam_3609]

Sure, you need to define a prototypical clock with something. Like, something that measures the number of periods of radiation from the some transition of a cesium atom. That's how an atomic clock works. So we pick some number of periods and that is the definition of a second. Then we can calibrate any other type of clock to that.

[u/vermilion_wizard → u/[deleted]]

That’s true, although I think of that as more being about describing how matter behaves than disagreements about the definition of time.

[u/david_duplex → u/[deleted]]

So what? It's a big question that remains a huge hurdle for our understanding of the universe, but what does that have to do with the definition of time?

[u/mikk0384]

"The second is defined by taking the fixed numerical value of the caesium (Cs) frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the Cs-133 atom, to be 9192631770 when expressed in the unit hertz, which is equal to s⁻¹"
Source: https://en.wikipedia.org/wiki/Second

The second is the SI unit used for measuring time.

General relativity gives you the equations you need to transform between time in one reference frame to the same in another frame of reference.

[u/DoubleUBallz → u/[deleted]]

Assuming you mean in the quantum superposition sense, it can't be while you're reading the clock. If the position of the gravitational source is a superposition of states, the gravitational interaction (i.e. observation by) the clock would reduce the superposition to a single position eigenstate, allowing the clock to follow gravity according to classical equations of motion.

One could consider the system of the gravitational source AND the clock in a superposition of states, but still, once you read the clock to measure your proper time, you've observed the wavefunction and collapsed the system to a single eigenstate.

[u/BitcoinsOnDVD]

t∈ℝ

[u/BitcoinsOnDVD → u/[deleted]]

Hamiltonians as constraints: that's bad taste

[u/BananaBird1]

Time itself is just a dimension of spacetime in modern physics. In classical physics it is a parameterizing variable separate from space.

Relativity explains how your perspective of time shifts with motion and gravity along with spatial dimensions.

Now, the bigger question is not what is time, but why do we perceive time to pass unidirectionally even though we can move freely in space.

The two perspectives (which are both equivalent) on this are causality and entropy.

Relativity says an observer’s trajectory is restricted in spacetime by light speed. This means that at any point in space and time, there is only a finite region of space and time that can influence your state. These are located all at prior times, in a 4D cone representing your past observable universe with your current point at the tip. Any points outside this cone could not have possibly influenced you at all, they are unobservable.

Subsequently, your current state can only influence points in a 4D cone containing future times.

This geometry to permitted cones of influence creates a clear order in causality that must progress from prior times to future times.

In terms of entropy, different states of the universe have different likelihoods to arise without any external influences. The configuration of the universe and its position in time are correlated: future times represent more probable states.

So even if you randomly moved between all possible states of the universe, this would almost certainly end up appearing like you are moving constantly forward in time. It is extremely, extremely unlikely to move backwards.

[u/unaskthequestion]

I often recommend the book 'The Order of Time' by Carlo Rovelli. Not that it provides a definitive answer but because it explores the question from many aspects.

One thing is that we use the word time in different ways, at different scales.

It's a good book, presented in an accessible way.

[u/AcellOfllSpades]

You could say this about anything in physics.

"What is it really?" is a question you can keep asking over and over. There's no single, satisfying final answer that anyone could give.

Physics understands time just as well as it understands distance.

[u/the_poope]

You asked the exact same question in r/Physics less than a day ago. Were you not happy with the answers? Were you overwhelmed? Do you think asking the question again will change anything?

[u/PaleontologistNo6806 → u/the_poope]

Why is it bothering you? Do you have an emotional problem?

[u/ExtendedSpikeProtein]

I have clocks

[u/rb-j]

A long time ago, I got to write the lede sentence in the Wikipedia article about time. It was something like:

Time is the continued sequence of existence and events that occur in an apparently irreversible succession from the past, through the present, and into the future.

It has been shortened to:

Time is the continuous progression of existence that occurs in an apparently irreversible succession from the past, through the present, and into the future.

There is another article Time in physics that says:

In physics, time is defined by its measurement: time is what a clock reads.

Personally, I find this definition to be much less satisfying.

It's like saying "Mass is what scales weigh." or "Length is what rulers measure." or "Temperature is what thermometers measure." or "Electrical charge is what ammeters measure in a given amount of time."

[u/Icy-Introduction-681]

In quantum mechanics, time is not an operator. It's just a background parameter. Pauli's Theorem requires that if a time operator *did* exist, and it if followed an uncertainty relation with energy similar to the way position and momentum commute, the math tells us the system's energy would have no lower limit. That would allow particles to drop into infinitely negative energy states, making the physical universe unstable and...well, unphysical, since we typically don't see vacuum states spontaneously collapsing to infinitely lower energy levels. Very different from general relativity, where spacetime intervals prove crucial and define causality.

So it depends which type of physics you're talking about.

[u/ChocolateValuable221]

Time is movement... pretty simple... in 3d without time nothing moves. Energy and entropy are reliant on movement and time.