What is time?

[bren10]

Is time nothing more than how we perceive change in the universe? Or, is time a fundamental aspect of the cosmos?

This is a question I've struggled with for a while. I have a hard time conceptualizing time as anything more than how we perceive change. Time obviously has a mathematical importance, but is it anything more than that? I'm not sure if this question is scientific or philosophic. If time IS generally thought of as a real thing by the scientific community, as real as up/down/left/right, I'm curious to know the reasoning behind this assumption. Or, is this a subject of debate? If it is, I'm curious to know other people's thoughts on it.

[Jeff Root]

It is certainly the subject of a great deal of debate, but
the debate seems to me to be largely pointless.

My opinion is that time is one of those things which are
so fundamental that the only way you can learn about it
is through experience.

How could time possibly not be real? If it weren't real,
we wouldn't experience anything. Nothing could change.

-- Jeff, in Minneapolis

[Jens]

Is time nothing more than how we perceive change in the universe? Or, is time a fundamental aspect of the cosmos?

I don't think the nature of time is clearly understand any more than the nature of space is. There are some interesting issues, though. Physical laws are generally symmetrical in time, so for example, a mass moving in one direction is the same as the mass moving in the opposite direction, but backwards in time. So there doesn't seem to be any definite reason that time flows in one direction. I remember Stephen Hawking writing that the reason we perceive time in one direction is because of entropy, in other words that the forward direction is the direction from less to more entropy, and that information creates entropy. I guess that's pretty much accepted, but I'm not really sure.

[bren10]

How could time possibly not be real? If it weren't real,
we wouldn't experience anything. Nothing could change.

-- Jeff, in Minneapolis

Couldn't time be just be our perception of change? Things are changing at a certain pace around us, and thus our biological bodies perceive time through those changes. In my mind, I perceive time as having no real fundamental relationship with the universe. Thus, there is only here and now, the past doesn't exist, the future doesn't exist, only the now. As particles in space move, we perceive the forward progression of time. That is how I picture it in my head at least.

[bren10]

I don't think the nature of time is clearly understand any more than the nature of space is. There are some interesting issues, though. Physical laws are generally symmetrical in time, so for example, a mass moving in one direction is the same as the mass moving in the opposite direction, but backwards in time. So there doesn't seem to be any definite reason that time flows in one direction. I remember Stephen Hawking writing that the reason we perceive time in one direction is because of entropy, in other words that the forward direction is the direction from less to more entropy, and that information creates entropy. I guess that's pretty much accepted, but I'm not really sure.

I'm familiar with the theories that treat time as mathematically symmetrical. Does that necessarily imply that time could be run backwards though? In math, you can turn almost any equation negative, but does that indicate that the physical flow of time we perceive in the universe could run just as easily in reverse? It makes sense, at least from an anthropomorphic point of view, that if there was a flow of time, it would be a vector.

[Jeff Root]

I remember Stephen Hawking writing that the reason
we perceive time in one direction is because of entropy,
in other words that the forward direction is the direction
from less to more entropy, and that information creates
entropy. I guess that's pretty much accepted, but I'm
not really sure.

It is true that the forward direction is from less to more
entropy, but entropy is not the reason we perceive time
in that direction. We perceive time in the direction it is
going. You can imagine a situation in which the entropy
of your body and everything around you decreases, either
by magic or because the mechanism of the decrease is
hidden from you, and entropy in some other location that
you can't see is increasing. Time would still go forward
for you, and you would still perceive it as going forward.

You may notice that I haven't said what a correct view
would be. That is so, I can't. All I can say is that time
is not dependant on entropy; it's the other way around.

However, as I like to say, time is a factor in everything
that happens, but it isn't the only factor in anything that
happens.

-- Jeff, in Minneapolis

[Jeff Root]

Couldn't time be just be our perception of change?

How?

Nothing could change without time. Our perception
of change is how we measure the passage of time,
not the other way around. As time passes, things
change, we perceive the change and interpret it as
indicating the passage of time.

Things are changing at a certain pace around us,

Which couldn't happen without time.

and thus our biological bodies perceive time through
those changes.

Yes.

In my mind, I perceive time as having no real
fundamental relationship with the universe.

No you don't. You have an idea that time has no real,
fundamental relationship with the Universe. You don't
perceive it. Ideas are products of the mind. Time is
not a product of the mind.

Thus, there is only here and now, the past doesn't
exist, ...

Not anymore.

... the future doesn't exist, ...

Not yet.

... only the now. As particles in space move, we perceive
the forward progression of time. That is how I picture
it in my head at least.

Of course we actually only perceive events which are in
the past. I had a graphic demonstration of that just a
day ago when I was pulling a shirt over my head, and I
felt something bang against the top of my head. I had
the impression that I hit it on the underside of something
like a ceiling beam (of which there are none here) or a
protruding shelf (which there is, but not near the place
I was standing). I quickly ducked down. A very small
fraction of a second later I felt my hand hit something.
It was what hit the top of my head. The sensation from
my hand reached my brain significantly later than the
sensation from the top of my head, and in the interval
I had time to react by ducking. Of course, it took some
time for the sensation to reach my brain from the top
of my head, too, and more time for me to process it
into the mistaken necessity to duck.

-- Jeff, in Minneapolis

[loglo]

"It is utterly beyond our power to measure the changes of things by time ... time is an abstraction at which we arrive by means of the changes of things; made because we are not restricted to any one definite measure, all being interconnected." Ernst Mach

The only scientific consensus on the nature of time since Mach wrote that in the late 19^th century is that "clocks measure it". The only physicists who study time are mostly quantum gravity researchers looking for a solution to the "problem of time", which appears when trying to integrate GR and QM. QG researchers can't agree amongst themselves whether time is emergent or fundamental. String theory is no help there apparently so the issue remains unresolved.

[noncryptic]

Nothing could change without time

That presumes time -is- a fundamental property, not an emergent one.

If time is emergent, then it is change that causes time, not the other way around.

[Jeff Root]

Okay, I do presume that time is fundamental, not emergent.
Perception of time, however, is emergent.

I will re-assert that change could not happen without time.
So saying that change causes time is absurd. Unless what
you call "time" is what I would call the perception of time.
I don't know why you would do that. My perception of a
thing is not the thing.

-- Jeff, in Minneapolis

[noncryptic]

Perception of time, however, is emergent.

Human perception of time, yes. But that's not necessarily the same as a fundamental scientific concept of time.

I will re-assert that change could not happen without time.

In my perception, time, space and stuff (any form of matter/energy) are intrinsically linked just as much as mainstream theories imply.

Any one of those can not exists without the others. Space without any stuff (known or unknown) is pointless, might just as well not be there. If there is stuff then there is space and interaction between stuff; ie change or if you will, "time".

In other words: (scientifically) time and change are the same thing. I suppose that means it is a fundamental property of the universe.
It seems to me that disagreements about what time is stem from confusing human perception of time with a scientific notion of time.

As someone else has noted: scientifically the only 'mistery' regarding time is in the mathematics involved in trying to unify quantum theory with relativity. All the rest is poetry.

[KiwiBiggles]

As a (non-mathematical) layperson, the interesting thing in popular literature about Time is the notion that for a massless particle like a photon, time itself is said to 'stop' or 'not exist'. Or, sometimes it's related in terms that in a photon's frame of reference (should such a thing exist) a photon can be said in some way to exist everywhere in the universe all at once (along it's direction of travel). You see it again when the uncertainty principle is described as a quantum phenomenon having a certain probability of being anywhere in the universe at any moment.

It's striking (to me anyway) that the question of Time, Fundamental Constants (c for example), Mass... they all seem to be deeply related in some interesting way we haven't understood as yet...

[KiwiBiggles]

Another thought is that SR and GR were said to have dispelled all notions of a separate Time or Space. Didn't Einstein say that from now on, we should only ever consider a unified Spacetime? Is it the case now that quantum theory has since reinstated Old Man Time all on his own?

If not, a gambling man wouldn't bet against Einstein!

[John Mendenhall]

Couldn't time be just be our perception of change? Things are changing at a certain pace around us, and thus our biological bodies perceive time through those changes. In my mind, I perceive time as having no real fundamental relationship with the universe. Thus, there is only here and now, the past doesn't exist, the future doesn't exist, only the now. As particles in space move, we perceive the forward progression of time. That is how I picture it in my head at least.

I think I agree with time being our perception of change, but for the opposite reason. Everything we perceive has happened in the past, even if only the time for light to reach our eyes. Perhaps time is only our perception of the resolution of quantum states.

Whoa, now there's a heavy thought. Time would be emergent from quantum mechanics. There would be no smallest unit of time. I think the physical universe would be emergent from quantum mechanics.

I need some time to think about this.

Regards, John M.

[BadTrip]

.... Things are changing at a certain pace around us, and thus our biological bodies perceive time through those changes. .....

But that "pace" can be altered, even locally. My understanding is that it's a result of the local gravity, but if one were to move across a sharp gravitational gradient would one's perception of the rate of time's progression change? Or would it only appear to change to an observer outside my own frame of reference? I've flown on high-altitude airplanes and never felt that the passaage of time was different, but it's been proven that time dialation does occur. So clearly, TIME is not simply a perception any more than gravity or space is simply a perception.

...... In my mind, I perceive time as having no real fundamental relationship with the universe. Thus, there is only here and now, the past doesn't exist, the future doesn't exist, only the now. As particles in space move, we perceive the forward progression of time. That is how I picture it in my head at least.

If the past doesn't exist then how would you explain the fact that, for example, you're typing on a keyboard that was manufactured 6-months/3 years ago? You're sitting in a char that was manufactured months/years ago. You're living in a house/apartment that was assembled months/years ago. The past indeed happened and to question it's "reality" is not logical. How do you look a at an old picture of an event that happened in the past and recall memories of that event? Yes, I understand that the recall is simply manifestations of synaptic activity in your brain, but you have physical evidence in your hand that confirms what those synapses are telling you. If the past wasn't reality how would you discuss those past events with another person who was there and had memory of that same event?

The fact that the past is not accessible to us via our "standard modes of perception", any more so than the future, is not evidence that the past doesn't exist.

[BadTrip]

I think I agree with time being our perception of change, but for the opposite reason. Everything we perceive has happened in the past, even if only the time for light to reach our eyes. Perhaps time is only our perception of the resolution of quantum states.

Whoa, now there's a heavy thought. Time would be emergent from quantum mechanics. There would be no smallest unit of time. I think the physical universe would be emergent from quantum mechanics.

I need some time to think about this.
Regards, John M.

*blink....blink* ............ indeed.... have you investigated/researched the theory of the time passing in discrete quanta? i.e. what's the smallest "tick of the clock" so to speak. A universe emergent from QM..... that's scary. The cat might actually be dead AND alive......and neither! ... hoi!

[BadTrip]

I think I agree with time being our perception of change, ......Regards, John M.

So then logically...if something is not perceived, no time has passed? i.e. no change has taken place? Does this fit with your reasoning?

[KiwiBiggles]

If one were to move across a sharp gravitational gradient would one's perception of the rate of time's progression change? Or would it only appear to change to an observer outside my own frame of reference? I've flown on high-altitude airplanes and never felt that the passage of time was different, but it's been proven that time dilation does occur.

As I understand it the literature says that time dilation occurs in the observers frame of reference, so I'm not sure that, in itself, dilation argues for the 'real existence' of time.

TIME is not simply a perception any more than gravity or space is simply a perception.

This is the clincher for me - relativity argues that time is no more or less 'real' than space and gravity. How real are space and gravity? Well, as a nice old lady said once, that's really gonna bake your noodle later on...

[gkell1]

O

I will re-assert that change could not happen without time.
So saying that change causes time is absurd. Unless what
you call "time" is what I would call the perception of time.
I don't know why you would do that. My perception of a
thing is not the thing.

-- Jeff, in Minneapolis

I am not hanging around Jeff but just a few comments.The older societies,even up to the recent past,considered concepts which appeared to the mind as infinite whereas today it is popular to consider time and space as finite,these societies did so for the simple reason that Edgar Allen Poe explains -

Let us begin, then, at once, with that merest of words, "Infinity." This, like "God," "spirit," and some other expressions of which the equivalents exist in all languages, is by no means the expression of an idea -- but of an effort at one. It stands for the possible attempt at an impossible conception. Man needed a term by which to point out the direction of this effort -- the cloud behind which lay, forever invisible, the object of this attempt. A word, in fine, was demanded, by means of which one human being might put himself in relation at once with another human being and with a certain tendency of the human intellect. Out of this demand arose the word, "Infinity;" which is thus the representative but of the thought of a thought.
As regards that infinity now considered -- the infinity of space -- we often hear it said that "its idea is admitted by the mind -- is acquiesced in -- is entertained -- on account of the greater difficulty which attends the conception of a limit." But this is merely one of those phrases by which even profound thinkers, time out of mind, have occasionally taken pleasure in deceiving themselves. The quibble lies concealed in the word "difficulty." "The mind," we are told, "entertains the idea of limitless, through the greater difficulty which it finds in entertaining that of limited, space." Now, were the proposition but fairly put, its absurdity would become transparent at once. Clearly, there is no mere difficulty in the case. The assertion intended, if presented according to its intention and without sophistry, would run thus: -- "The mind admits the idea of limitless, through the greater impossibility of entertaining that of limited, space."

http://xroads.virginia.edu/~hyper/poe/eureka.html

In the matter of time itself as we perceive it through clocks and watches,the older societies too considered the matter carefully,not so much time itself but why the pace of things ?,why does life, all natural processes and movement play out the way it does and no individual movement in itself defines time and,in today's world,the encompasses ideas such as 'big bang'.There is nothing dated about the old arguments and somehow their return would be welcome in this era -

Succession or repetition gives us Number- dyad, triad, etc.- and the
extent traversed is a matter of Magnitude; thus we have Quantity of
Movement- in the form of number, dyad, triad, decade, or in the form
of extent apprehended in what we may call the amount of the Movement:
but, the idea of Time we have not. That definite Quantity is merely
something occurring within Time, for, otherwise Time is not everywhere
but is something belonging to Movement which thus would be its
substratum or basic-stuff: once more, then, we would be making Time
identical with Movement; for the extent of Movement is not something
outside it but is simply its continuousness, and we need not halt upon
the difference between the momentary and the continuous, which is
simply one of manner and degree. The extended movement and its extent
are not Time; they are in Time. Those that explain Time as extent of
Movement must mean not the extent of the movement itself but something
which determines its extension, something with which the movement
keeps pace in its course. But what this something is, we are not told;
yet it is, clearly, Time, that in which all Movement proceeds. This is
what our discussion has aimed at from the first: "What, essentially,
is Time?" It comes to this: we ask "What is Time?" and we are
answered, "Time is the extension of Movement in Time!"
Plotinus

[Cougar]

I'm familiar with the theories that treat time as mathematically symmetrical. Does that necessarily imply that time could be run backwards though?

No. It means the laws of physics appear unchanged IF you run the film backwards. This is easy to envision if your film is of a (frictionless) pool table. It is considerably harder to see if your film is of an egg rolling off a table and dropping to the floor and splattering. Oddly (very oddly, if you ask me), physicists claim that the laws of physics do not prohibit the broken egg from being given just the right impetus to bring it all back together into a whole, unbroken egg rolling to a stop on the table top. And it's not just some crank physicist who claims this.

Anyone really interested in this question of "time" would get a lot of their questions answered by checking out Sean Carroll's 2010 book From Eternity to Here: The Quest for the Ultimate Theory of Time.

[bren10]

I feel like a lot of people misinterpreted what I said I envision time as. I'm not considering human perception of time at all in what I'm saying, only the existence of time in the universe, outside of our consciousness. It's a difficult subject to talk about without bringing up our perception of time.

I don't think time is a real thing. When things move around, they are no longer in their previous state. The universe will never be in that state again. Every instant is the present, once a "frame of time" has passed, it is gone forever and will never occur again. The universe exists solely in the present. We can see the "past" because things take "time" to reach us, but the universe is perpetually existing in the present. The stuff we see in space no longer exists, only the photons that are racing towards us exist.

I think time is a human creation. Because we perceive things happening in an order, we create a timeline of events, but that exists only in our heads. Like has been mentioned previously, things DO change at different speeds. But I don't think that is relevant to "time". While things at different speeds and gravity do change at different rates, there still exists a "present" for all things in the universe. Equations that have a value in them for time, are basically using the term time for how fast something changes.

Hopefully that better explains what I think.

I started this thread because I want to hear others opinions though. I'd love to have my opinion changed, I think that's the most exciting thing in science, to have an idea, perception, or whatever, and to have that opinion or theory changed. Thought this topic does seem to not be conclusive.

[Shaula]

If time is a human creation that would rather complicate Noether's theorem. Symmetry of physical laws with respect to time leads to the conservation of energy. Similarly conservation of momentum is linked to symmetry with respect to spatial co-ordinate. So essentially if time is a human construct so must space, rotation, angular momentum, energy and momentum at least. Noether's theorem also applies to the basic forces, so charges of all types and forces must be human constructs - essentially treating time differently makes everything a human construct.

[Jeff Root]

I don't think time is a real thing.

Why?

Without time, change would not be possible.

When things move around, they are no longer in their
previous state. ...

Without time, motion would not be possible.

... The universe will never be in that state again.
Every instant is the present, once a "frame of time"
has passed, it is gone forever and will never occur
again. ...

Yes, that's time for you.

... The universe exists solely in the present. We can
see the "past" because things take "time" to reach us,
but the universe is perpetually existing in the present.
The stuff we see in space no longer exists, only the
photons that are racing towards us exist.

I think time is a human creation.

Why? I've seen the assertion before, but it appears to
be completely without any basis in either observation
or reason. It makes no sense.

Because we perceive things happening in an order, we
create a timeline of events, but that exists only in our
heads.

So you are saying that things do not happen in any
order, but we perceive them as if they did happen in
some order? Why would we do that? How could we
perceive anything without time passing? How could
there be anything to perceive?

Like has been mentioned previously, things DO change
at different speeds.

If things DO change, then time must be passing.

But I don't think that is relevant to "time". While things
at different speeds and gravity do change at different
rates, there still exists a "present" for all things in the
universe. Equations that have a value in them for time,
are basically using the term time for how fast something
changes.

Time is measured by comparison to a standard, the same
way length is measured by comparison to a standard, the
same way mass is measured by comparison to a standard,
and the same way everything else is measured.

Is everything not real? Or is it just time that isn't real?

-- Jeff, in Minneapolis

[noncryptic]

Oddly (very oddly, if you ask me), physicists claim that the laws of physics do not prohibit the broken egg from being given just the right impetus to bring it all back together into a whole, unbroken egg rolling to a stop on the table top. And it's not just some crank physicist who claims this.

I agree that's a odd, given that (as far as i understand) the laws of physics predict the probability of subatomic particles interacting in such a way that an intact egg emerges from a splattered egg is extremely low. Likewise, the laws of physics do not "prohibit" the emergence of Bolzmann Brains.

Can it be argued that the direction of time follows from quantum probability and the principal of least action?

[RandyD123]

There is a 53 minute vid of "The illusion of time" here: http://www.youtube.com/watch?v=ghHts...eature=related

[antoniseb]

There is a 53 minute vid of "The illusion of time" here...

That's a very long video for you to ask people to watch without pointing out what you think is valuable or otherwise noteworthy about it.

[mc^2]

Hi bren10,

Is time nothing more than how we perceive change in the universe? Or, is time a fundamental aspect of the cosmos?

Time as a physical quantity is fundamental to our description of the cosmos. However, the directionality of time is emergent.

Time is a measure of relative changes in energy distribution. When I say it took me 5 seconds to walk across a room, I am really saying that a clock on the wall changed its energy distribution 5 times compared with my single change in energy distribution when I moved position from one side of the room to the other.

Time, therefore, is not a dimension like our three spatial dimensions. We often represent time mathematically or graphically as a dimension resembling space, however you must understand this is purely for convenience. This mathematical utility does not describe what time is.

The direction that time may take can depend on one's choice of coordinate system and the energy distribution under consideration. Time, in and of itself, has no inbuilt directionality. There is no inherent directionality built into the known laws of physics either, so why does time appear to be so linear and always move in the "forwards" direction?

There are thee main reasons:

1. The Big Bang itself was a source of extremely low entropy. Since that event, the universe has been constantly evolving to higher-entropy states.

2. Gravity drives the transition from low-entropy states to higher entropy states.

3. Radiation - atoms prefer to be in their "ground state", or lowest energy level. Radiation, or photons, emitted by atoms now are going to be absorbed by other atoms later on. This is only possible because most other atoms in the universe are in their "ground state". This statistical effect causes the dominance of radiation flowing in the forward direction in time.

As the universe expands and cools, more and more atoms transition to their ground state. It is this asymmetry, between earlier (hotter) epochs of the universe and future (colder) epochs that is responsible for the perceived arrow of time.

Eggs don't unscramble themselves simply because it is a far less likely event than starting with a whole egg and making yourself breakfast

I hope this answers your question.

Regards,

mc^2.

[ZunarJ5]

Is space considered an emergent or fundamental property? I would guess fundamental, but I ask because I am a layman.

If my guess is correct, that space is fundamental, then shouldn't time also be? I am just surprised that time being emergent is even proposed... unless I just don't understand the definitions of "emergent" and "fundamental" when applied in this way.

It seems to me that time is simply the 4th dimension of movement in the universe. Our perception of time is determined by its physical qualities but more rigidly by the nature of our consciousness, which is a mystery in itself. Are up, down, back, forth, and side to side all "real" things? Sure, and so is time. Is it anything more than this? I would be surprised and skeptical if the physicists claimed it to be so.

[Cougar]

I think time is a human creation. Because we perceive things happening in an order, we create a timeline of events, but that exists only in our heads.

Well, I think it exists in our heads because that's what's happening. Yes, only "now" seems to ever exist. But we remember the past, not the future.

[cosmocrazy]

When things move around, they are no longer in their previous state. The universe will never be in that state again. Every instant is the present, once a "frame of time" has passed, it is gone forever and will never occur again. The universe exists solely in the present. We can see the "past" because things take "time" to reach us, but the universe is perpetually existing in the present. The stuff we see in space no longer exists, only the photons that are racing towards us exist.

I think time is a human creation. Because we perceive things happening in an order, we create a timeline of events, but that exists only in our heads. Like has been mentioned previously, things DO change at different speeds. But I don't think that is relevant to "time". While things at different speeds and gravity do change at different rates, there still exists a "present" for all things in the universe. Equations that have a value in them for time, are basically using the term time for how fast something changes.

But who's present? In Relativity time is not constant between different reference frames. If time is not constant which has been proven to highly accurate measurements then how can any one present be said to be the "now" state? Consider time as a dimension much like the 3 spacial ones. This is what Einstein envisioned when the term "spacetime" was coined. Time like space is all relative. Although we can appear to move backward in space, is this nothing more than an illusion? If you get up from your desk take a few steps away then stop and take the same steps back you may consider this to be moving "back" in space. But you have only moved relative to the desk in your own personnal reference frame. In the meantime the desk, yourself and everything else in the universe has since moved in spacetime relative to. So perhaps we only ever move in one direction in "spacetime" and we have evolved (matter) to experience spacetime as 4 dimensional. Can we consider it all to be an illusion? Yet things feel real, matter has substance, space has dimension and time passes by. Time I believe is a fundamental part of the universe, intrinsically entwined with space, energy and matter. But is it ermergent or fundamenal? Why not both? maybe it emerged from the beginings of the universe itself and became a fundamental at that moment.