What is time – and why does it move forward?

February 23, 2016 by Thomas Kitching, Ucl, The Conversation
Why the night sky can tell us a fair bit about time. Credit: Arches National Park/Flickr, CC BY-ND

Imagine time running backwards. People would grow younger instead of older and, after a long life of gradual rejuvenation – unlearning everything they know – they would end as a twinkle in their parents' eyes. That's time as represented in a novel by science fiction writer Philip K Dick but, surprisingly, time's direction is also an issue that cosmologists are grappling with.

While we take for granted that has a given direction, physicists don't: most natural laws are "time reversible" which means they would work just as well if time was defined as running backwards. So why does time always move forward? And will it always do so?

Does time have a beginning?

Any universal concept of time must ultimately be based on the evolution of the cosmos itself. When you look up at the you're seeing events that happened in the past – it takes light time to reach us. In fact, even the simplest observation can help us understand cosmological time: for example the fact that the night sky is dark. If the universe had an infinite past and was infinite in extent, the would be completely bright – filled with the light from an infinite number of stars in a cosmos that had always existed.

For a long time scientists, including Albert Einstein, thought that the universe was static and infinite. Observations have since shown that it is in fact expanding, and at an accelerating rate. This means that it must have originated from a more compact state that we call the Big Bang, implying that time does have a beginning. In fact, if we look for light that is old enough we can even see the relic radiation from Big Bang – the . Realising this was a first step in determining the age of the universe (see below).

But there is a snag, Einstein's special theory of relativity, shows that time is … relative: the faster you move relative to me, the slower time will pass for you relative to my perception of time. So in our universe of expanding galaxies, spinning stars and swirling planets, experiences of time vary: everything's past, present and future is relative.

So is there a universal time that we could all agree on?

It turns out that because the universe is on average the same everywhere, and on average looks the same in every direction, there does exist a "cosmic time". To measure it, all we have to do is measure the properties of the cosmic microwave background. Cosmologists have used this to determine the age of the universe; its cosmic age. It turns out that the universe is 13.799 billion years old.

Time's arrow

So we know time most likely started during the Big Bang. But there is one nagging question that remains: what exactly is time?

To unpack this question, we have to look at the basic properties of space and time. In the dimension of space, you can move forwards and backwards; commuters experience this everyday. But time is different, it has a direction, you always move forward, never in reverse. So why is the dimension of time irreversible? This is one of the major unsolved problems in physics.

To explain why time itself is irreversible, we need to find processes in nature that are also irreversible. One of the few such concepts in physics (and life!) is that things tend to become less "tidy" as time passes. We describe this using a physical property called entropy that encodes how ordered something is.

Imagine a box of gas in which all the particles were initially placed in one corner (an ordered state). Over time they would naturally seek to fill the entire box (a disordered state) – and to put the particles back into an ordered state would require energy. This is irreversible. It's like cracking an egg to make an omelette – once it spreads out and fills the frying pan, it will never go back to being egg-shaped. It's the same with the universe: as it evolves, the overall entropy increases.

It turns out entropy is a pretty good way to explain time's arrow. And while it may seem like the universe is becoming more ordered rather than less – going from a wild sea of relatively uniformly spread out hot gas in its early stages to stars, planets, humans and articles about time – it's nevertheless possible that it is increasing in disorder. That's because the gravity associated with large masses may be pulling matter into seemingly ordered states – with the increase in disorder that we think must have taken place being somehow hidden away in the gravitational fields. So disorder could be increasing even though we don't see it.

But given nature's tendency to prefer disorder, why did the universe start off in such an ordered state in the first place? This is still considered a mystery. Some researchers argue that the Big Bang may not even have been the beginning, there may in fact be "parallel universes" where time runs in different directions.

Will time end?

Time had a beginning but whether it will have an end depends on the nature of the that is causing it to expand at an accelerating rate. The rate of this expansion may eventually tear the universe apart, forcing it to end in a Big Rip; alternatively dark energy may decay, reversing the Big Bang and ending the Universe in a Big Crunch; or the Universe may simply expand forever.

But would any of these future scenarios end time? Well, according to the strange rules of quantum mechanics, tiny random particles can momentarily pop out of a vacuum – something seen constantly in . Some have argued that dark energy could cause such "quantum fluctuations" giving rise to a new Big Bang, ending our time line and starting a new one. While this is extremely speculative and highly unlikely, what we do know is that only when we understand dark energy will we know the fate of the universe.

So what is the most likely outcome? Only time will tell.

Explore further: Was the Big Bang just a black hole?

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MrVibrating
5 / 5 (1) Feb 23, 2016
Sigh... Entropy is dependent upon time. It can provide a metric, but is clearly an epiphenomenon - the map is not the territory.

Just because entropy increases in one direction (and even negentropic systems like us are fueled by entropic gradients (ie. digestion / metabolism)), this cannot tell us anything conclusive about time itself.

Finally, what kind of "infinity" has a finite beginning? If time began with the universe, which ends in heat death, then time has a finite beginning yet no ending. But an infinite number line includes negative as well as positive values..

Over time our definition of "universe" has had to keep pace with developments - we want a word that means "eveything", but initially it was synonymous with "galaxy" etc., yet now we know there are many galaxies, or "island universes" in the cosmos. And just as ours contains many singularities, which are themselves dependent upon time, so there must be a more fundamental substrate.. per M-theory et al.
gunnee1234
1 / 5 (1) Feb 23, 2016
In my humble opinion there is no such physical reality as "time", as in yesterday or tomorrow. That is in my opinion just a consept constructed in our minds. There is no yesterday, now or tomorrow. The particles and force-carriers just moves right, left, or up/down, according to the physical forces that causes them to move. They moves in space in these directions. Where were my particles (Myself) yesterday you can ask? Then I will ask: Yesterday ? Where is that? The only reality I can tell you about me is that I am here in this room/space and moving according to what forces me to move left, right, up/down. There is no yesterday or tomorrow only this same room where we moves in. Particles just moves. Big bang did not happen in some strange humanmade concept called "time". It just happened over there here in this very same room where you and I lives in right here.
billpress11
1 / 5 (1) Feb 23, 2016
Time is just a measurement of change. These changes probably happen in such small Planck scale units that they would be impossible to reverse in the exact order they occur in. Another reason time as we view it seemingly cannot be reversed, time is a localized event, what is happening around you also has an effect on these Planck scale changes that occur to you, from the distant stars to radioactive decays happening around you. These changes around you will never be identical again thereby making it impossible for anyone the reverse the Planck scale changes in the exact order reversing time as we perceive it.
RobertKarlStonjek
5 / 5 (1) Feb 23, 2016
By a common definition used in physics, the 'present' represents the transition between past and future. If you remove either the past or the future then you also remove the present.

Therefore by the common definition of time, time can not begin or end. (as pointed out by Prof. Paul Davis, Prof. Le Poidevin and many others).

The Big Bang does not predict a beginning of time as numerous books and articles on the subject of time claim. The Big Bang model predicts an evolution of the universe from the Planck scale.

Just why so many texts blatantly misrepresent physics theory and the available facts is somewhat mysterious but I assume that once a rumour starts it is hard to stop it, but it appears to be entirely a 'rumour' with no factual basis whatsoever.
RobertKarlStonjek
5 / 5 (1) Feb 23, 2016
On Entropy, The second law is statistical and while it has a particular direction in time with high probability, entropy can fall, indeed, the 'Poincaré recurrence time' is the interval in which entropy returns to its lowest value!!

When entropy falls, which, statistically, it is bound to do to some degree, time is unaffected. That is, there is no causative link between time and entropy whatsoever. Time and entropy are merely correlated, like the number of ice-creams consumed on a hot day. Entropy has no more effect on time than icecreams have on the daily temperature ~ they are simply correlated.
jimsecor
4 / 5 (4) Feb 23, 2016
There are so many misguided assumptions in this article that it's basically meaningless.
Colbourne
not rated yet Feb 23, 2016
Just imagine a simulation of the universe running on a computer. Time has no real meaning except from when the simulation was started. It is local to the program.
RobertKarlStonjek
not rated yet Feb 24, 2016
I don't thin that the assumptions are misguided so much as echoing common misconceptions that are prevalent even in science writing.

For instance the idea that time travels from the past to the future is one view, quite common, the view that time does not flow at all is another, that time begins in the future and not the past is another view eg Tuesday is next week, then it is today, and later is in last week having swept right past you. Causation, on the other hand, commonly starts at time zero and progresses from there.

Confounding causation, change, timelines, clocks, entropy and/or space for time is extremely common. Any or all of these can be considered to be properties of time, but in theories and models of time just one of these properties is considered sufficient to define time thus immediately causing conflict with other models of time that consider some other property to be foundational...
marce1000
not rated yet Feb 24, 2016
Probably one of the most confused concepts in physics is time, the idea that time moves forward is only a 'statistical interpration' from the human mind , observing events around us. Our time concept therefore got a much more important place in physics then it deservers. This narrow view leads to misconcepts as why 'time' cannot go back. Imagine a one-dimensional circular universe, with one particual running around in it. Is there any notion of time then ? If so why ?
lairdwilcox
not rated yet Feb 24, 2016
While it's true that time moves in the direction of disorder there is nothing that says that it has to continue in that direction. Time may be going on first to disorder, then to order, and then back again. Parts of time may be moving in one direction while others are moving in another. I suspect that parts of the universe are moving in different directions (to and from disorder) while time just keeps plugging on in its initial direction.
Noumenon
3 / 5 (2) Feb 24, 2016
There is indeed much confusion about time. It is founded on two distinct notions....

-We have an a-priori intuition of time, that given the nature of mind, is effectively a condition for experience to be possible; given that the mind is a limited biomachine, it has 'built-in' means of synthesizing experience. The mind adds it's own conceptual structure as a means of producing a synthesis of experience, with the result that we have these 'forms of thought',... time, space, causality,.... artifacts of the process of thought. The mind produces these concepts just as it produces the qualia of "redness", "sound", and "pain",... which don't have ontological existence either.

-In science however, we require quantifiably. Time (and space) must therefore be Defined and transposed to a physical representation, a clock or rod. ....

Urgelt
4 / 5 (4) Feb 24, 2016
This article gave me indigestion.

The author casually suggests that there can be a universal time, but if that's so, then Einstein's interpretation of SR is incorrect and Lorentz was right about a universal frame. That opens a can of worms that wiggle all over the place, but the author seems to be oblivious.

It's not obvious that entropy is cause rather than effect, either. That's difficult to prove and amounts to a conjecture.

Completely missing from the article is any discussion of time viewed from a quantum mechanical perspective. That seems like a rather serious omission. Whatever our conception of time, it has to draw from every investigatory approach. You can't just say 'SR' and 'done.'

The author did admit we don't understand time. That's the truest thing in the article.
Noumenon
3 / 5 (2) Feb 24, 2016
Continued from above......

When one uses "time" in physics they are not using it as if it was an ontological entity, but instead only in comparing the congruence of one physical system to that of another. Indeed, there has never been a time-field or time-particle observed. It is not an independently observable ontological entity active in physical dynamics, [that is, independent of its a-posteriori use in relating events]. All that physics can do is find a physical representation of our intuition,... the CMBR is just an elaborate and convoluted clock.

In GR one sees our instrumentalist definition-of-time "running away" from our intuition of time in time dilation. In 'failure of simultaneity' this incompatibility becomes clear also. In QM, in some experimental arrangements like in entanglement where non-locality occurs, our intuitions are again exposed as an artificial synthesis of experience,... QM indeterminacy breaks the concept of time,... etc

Noumenon
3.7 / 5 (3) Feb 24, 2016
The author did admit we don't understand time. That's the truest thing in the article.


The author also repeated that 'time started at the big-bang',... as if that was an ontological fact.

Before the big-bang there is no known physical system that can be used as a representation of time, there is no handle to grasp onto. This is why it appears that time started at the BB. There is no physical system in which to represent that mind-dependent intuition.

bschott
3 / 5 (4) Feb 24, 2016
There are so many misguided assumptions in this article that it's basically meaningless.


Welcome to Mainstream theoretical astrophysics. 99 out of 100 papers can be classified exactly as above. Don't let the zealots cloud your vision.
Noumenon
3.7 / 5 (3) Feb 24, 2016
There are so many misguided assumptions in this article that it's basically meaningless.


Welcome to Mainstream theoretical astrophysics. 99 out of 100 papers can be classified exactly as above. Don't let the zealots cloud your vision.


Most theories make assumptions, or 'postulates'. This of itself is not a defect of science, as long as theory makes valid predictions, as that is what science IS, predictive knowledge. When one speaks of time as an ontological entity unto itself, they enter metaphysics and confusion. If one expects more from science than suppling predictive knowledge, they desire metaphysics instead.

antigoracle
3.7 / 5 (3) Feb 24, 2016
there has never been a time-field or time-particle observed

So, you are telling me, I can't save it in a bottle.
winthrom
5 / 5 (1) Feb 24, 2016
Time and gravity are unexplained phenomena. When we use entropy to illustrate the arrow of time, we assume that the laws of physics that use time are irreversible. We further assume that entropy must be irreversible (except in the 'Poincaré recurrence time' case, which is eventually reversed within larger mass/energy systems). We neglect to explain why nature insists that collections of masses (e.g. gases, etc.) in high energy states must seek lower energy states to occupy, which then creates the disarray we define as entropy. We also neglect to specify the energy effects of gravity, which is in fact, warped space time related to mass. There is correlation here. Assume that the energy dispersed by mass in entropy is related to the energy absorbed by gravity. In fact, let us assume potential energy gravity is a sink for energy taken from mass. Then Space-time is energy taken from mass in the form of gravity. Mass/Energy is the "magnetic" attraction of gravity yielding entropy.
moebiex
1 / 5 (1) Feb 24, 2016
Perhaps its all a matter of perspective. As in - from our (fermionic matter) perspective the Universe is 13.5-ish B years old. How old is it from the perspective of some of those first photons ejected? That might help explain inflation- there was simply no matter around keeping track so while we see/understand (as if) it as the first trillionth of a second of time, from the perspective of that initial photon the periods elapsed up to that point and from then to our current point are the same (ie. 0). As for time moving in one direction- cannot Hiesenberg's Uncertainty principle be interpreted as defining the "present" in time. Every state is a probability up until the point where it happens (or is measured) at which point the probability of it happening becomes "1" . Does it have to be more complicated than that? I look forward to hearing.
antigoracle
5 / 5 (2) Feb 24, 2016
Time and gravity are unexplained phenomena........

Time and gravity are related phenomena and I believe we'll ultimately comprehend gravity when we can explain time.

the energy absorbed by gravity

While I found what you said quite interesting, I disagree with the implication that gravity is like a physical thing, capable of absorbing energy. Gravity is the physical attraction experienced between 2 masses caught in a spacetime gradient and any transfer of energy is between them i.e. energy does not dissipate into gravity.
pppillai
not rated yet Feb 24, 2016
It is time or too late that i have to read basics and question them!
antigoracle
not rated yet Feb 24, 2016
It is time or too late that i have to read basics and question them!

If you must.
bschott
3 / 5 (4) Feb 25, 2016
Most theories make assumptions, or 'postulates'. This of itself is not a defect of science, as long as theory makes valid predictions, as that is what science IS, predictive knowledge.


When a theory's prediction is so far off observations that keeping the theory and making it viable mathematically involves a radical change to the equation, science requires us to re-visit the theory to see why it failed so miserably. Not adjust it mathematically and claim that what the math says is now real.

If one expects more from science than suppling predictive knowledge, they desire metaphysics instead.


If science expects one to "have faith" that the math is correct when reality says something different, science should change it's name to religion instead.
Noumenon
3.7 / 5 (3) Feb 25, 2016
Not necessarily, as validated theory may allow one to discover new things. For example, in order to make the Schrodinger equation linear so that it could be reconciled with SR, Paul Dirac did something wacky with matrices, a purely mathematical operation. This resulted in twice as many solutions than expected. Instead of tossing the theory away, he allowed it 'to speak to him',... thus preficting the positron (he thought it was the proton at first).
Noumenon
3 / 5 (2) Feb 25, 2016
EDIT: "[predicting] the positron "

Whydening Gyre
5 / 5 (2) Feb 25, 2016
Probably one of the most confused concepts in physics is time, the idea that time moves forward is only a 'statistical interpration' from the human mind , observing events around us. Our time concept therefore got a much more important place in physics then it deservers. This narrow view leads to misconcepts as why 'time' cannot go back. Imagine a one-dimensional circular universe, with one particual running around in it. Is there any notion of time then ? If so why ?

if it's a flat circle, isn't it TWO dimensional?
Whydening Gyre
5 / 5 (2) Feb 25, 2016
there has never been a time-field or time-particle observed

So, you are telling me, I can't save it in a bottle.

Dang... Then you can't do that first thing you'd like to do...
jimboz
not rated yet Feb 28, 2016
From what I understand there's no evidence time existed before the big bang. if so then the beginning of time coincided with the separation of 'material', which led to particles in existence so that differences in location needed 'time' to exist.

Star trek 'beaming up' is time travel.

Entanglement appears to be time travel from the point of view before the big bang, so that entangling two objects merely exposes an existing condition essentially a 'time stamp' of the singularity.

Detecting gravitational waves requires 90 degree separation of physical detectors, producing interference that gives us the 'chirp'. Might 'time objects' as revealed by entanglement be used to detect gravitational waves, which include time distortions? So that disturbances produced by time distortions of gravitational waves would be measured against the time stamp as exposed by entangled objects? 90 degree interference provided by measuing entangled objects in a black hole gravitational well.
obama_socks
2.3 / 5 (3) Feb 29, 2016
It is time or too late that i have to read basics and question them!
- ppp
Always question. That is the reason for this website.
obama_socks
2.3 / 5 (3) Feb 29, 2016
Time has ALWAYS existed...long before the so-called "Big Bang". Even before the emergence of God the Creator of the Universe, Time had existed already going forward. Time exists with or without a Universe to record its passing because it is completely devoid of the influence of anything. Time has been conflated with the 3 Dimensions of Height, Width and Depth as included in its math equation as "spacetime". This is erroneous since Time cannot warp, displace or be displaced, bend, stopped or slowed down, reversed, and has no effect on Matter/Energy. Neither does Matter/Energy or Gravity have any effect on Time.
Time is ONLY a means to measure the course of EVENTS and the length of DISTANCE, with the aid of time pieces and a slide rule or tape measure, or some other means of measuring.
Scientists include Time as though it were a commodity that can be harnessed, controlled and categorized as something that it isn't. This is why Science is mainly based on THEORY and not cold, hard facts
obama_socks
2.3 / 5 (3) Feb 29, 2016
"If science expects one to "have faith" that the math is correct when reality says something different, science should change it's name to religion instead." - bschott

You are correct. Even with all the research and attempts at confirming theory without it resulting in hard, concrete evidence, scientists expect us to have faith in their research and their continuing funded work so that the funding continues to pour in ad infinitum.

"What fools these mortals be". - Puck

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