# What is Arrow of time: Definition and 54 Discussions

The arrow of time, also called time's arrow, is the concept positing the "one-way direction" or "asymmetry" of time. It was developed in 1927 by the British astrophysicist Arthur Eddington, and is an unsolved general physics question. This direction, according to Eddington, could be determined by studying the organization of atoms, molecules, and bodies, and might be drawn upon a four-dimensional relativistic map of the world ("a solid block of paper").Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true. Yet at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) of time.

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1. ### I Entropy Real? End of Time? - Veritasium Video

Is entropy real? It seems like it's not real because it depends on how you group microstates together into a macrostate, and the way you group them can be arbitrary. For example (at 13:04 of the video below), there are 91,520 microstates in the macrostate “9 in left; 1 in right” but 627,264...
2. ### B Exploring the Link Between Entropy and the Arrow of Time in the Early Universe

My understanding is that a sub atomic particle has no arrow of time. Clearly the Arrow of time as understood in our macroscopic world has one direction. Entropy being an arrow in the direction toward uniform distribution of the individual and or sums of all the forms of energy in a system and ...
3. ### B Is time a consequence of 2nd law of thermodynamics?

I have heard from a knowledgeable physics proffessor, time exists independently and it is not a consequence of arrow of time. Could some body explain this?
4. ### I How Does Relativity of Simultaneity Clash w/Thermodynamics?

In special relativity, observers can disagree on the order of events - if Alice thinks events A, B and C are simultaneous, Bob can think A happened before B which happened before C, and Carlos thinks C happened before B which happened before A - provided A, B and C are not causally connected, of...
5. ### I Reason for matter rather than antimatter?

At the end of the inflation period (if it occurred) the potential energy of the inflaton field decays into particle/antiparticle pairs. When a particle/antiparticle pair is created each component of the total 4-momentum of the pair is zero. This must include the time component as well as the...
6. ### Relation between the arrow of time and entropy

What is the relation of the arrow of time and entropy according to thermodynamics?
7. ### I Arrow of time and its reversal on a IBM quantum computer

I would like to know what the implications of this paper are https://arxiv.org/abs/1712.10057. They say " Here we show that, while in nature the complex conjugation needed for time reversal is exponentially improbable, one can design a quantum algorithm that includes complex conjugation and...
8. ### Are all arrows of time special cases of the thermodynamic one?

Are all arrows of time special cases of the thermodynamic one? The arrows of time I am referring to are the psychological arrow of time and the cosmological arrow of time. Thanks.
9. ### B Entropy, irreversability, and the arrow of time

It is said that entropy causes an arrow of time. However, how about the irreversability of a measurement like electron spin. When measured a certain spin, the previous value gets lost. So does that also require an arrow of time?
10. ### B Why does time pass in an isolated vacuum?

I read a little bit on the arrow of time and how some physicists think that it points in the direction of increasing entropy. This made sense until I thought about a vacuum. From what I read, entropy does not increase nor decrease in a vacuum so if we used this definition on the "arrow of time"...
11. ### B Understanding Absence of a Microscopic Arrow of Time

At the particle interaction level, we cannot distinguish a preferred direction of interaction, an arrow of time as they say. I do not understand this if i) in annihilations, there is a manifest disparity between particles before (massive fermions) and after (photons) an interaction and...
12. ### I Behavior that seems to violate the arrow of time, but....

Very simply put, I have an intense desire to understand an experimental result which, on the surface, violates entropy and the arrow of time-- although, since the experimenters predicted exactly that outcome, a deeper analysis must show that it does not actually violate entropy and the arrow of...
13. W

### B Gravity and the Direction of Time

I recently read an interesting article published in Physical Review Letters in October 2014 - "Identification of a Gravitational Arrow of Time." There were also other articles in several general interest science magazines...
14. ### B The Arrow Of Time As Defined By The Forces

I was wondering why the arrow of time can't be defined by the direction in which the forces hold their observed values. It seems to me that if the arrow were to be reversed this would necessitate a reversal of the forces' ability to attract or repel. For example: wouldn't gravity have to start...
15. ### I How is the arrow of time defined?

Physical processes do not require an arrow of time to be defined. Then how does one know for certain that time is unidirectional, that there is a past, present and future?
16. ### A Has the stability problem of changing entropy and time been resolved?

My question is, if somehow it were possible to change the change of entropy such that it were negative, i.e. ##\Delta S <0## wouldn't we go backwards in time, from the future to the past? I have this paper: https://arxiv.org/abs/gr-qc/0503077 I wonder if the stability problem was ever...
17. ### I Arrow of time and travel at the speed of light

If a guy gets on a rocket that travels at the speed of light for a finite period of time (in relation to other observers not moving at C) the guy in the rocket experiences no passage of time. In this scenario his leaving and arrival of the rocket happens at the same time in his frame, correct...
18. ### About Entropy and Poincare Recurrence

I was reading about entropy, Poincare recurrence theorem and the arrow of time yesterday and I got some ideas/questions I'd like to share here... Let's think a about a system that is a classical ideal gas made of point particles, confined in a cubic box. Suppose that at time ##t=0## all the...
19. ### Entropy or Quantum Mechanics ?

is entropy problem easiest than quantum mechanics problem which one is more complicated to understand and to solve ?
20. ### Do you believe that entanglement drives the arrow of time?

https://www.quantamagazine.org/20140416-times-arrow-traced-to-quantum-source/ QUOTE "The idea that entanglement might explain the arrow of time first occurred to Seth Lloyd about 30 years ago, when he was a 23-year-old philosophy graduate student at Cambridge University with a Harvard physics...
21. ### Comments - How Fundamental is the Arrow of Time?

kreil submitted a new PF Insights post Retrocausality: How Fundamental is the Arrow of Time? Continue reading the Original PF Insights Post.
22. ### Memory, Entropy and the Arrow of Time

Sean Carroll has stated several times that the reason we can remember the past and not the future is because entropy is increasing, i.e. because there is an arrow of time. Is this statement justifiable? Remember that life and its processes, including memory, require negentropy. In other words...
23. ### A Thought on the Apparent Asymmetry of Time (the "Arrow of Time")

I had an interesting thought regarding the "arrow of time" and, with the hope of getting the opinion of someone possessing more relevant knowledge than do I, I'm posting it here. Essentially, I thought that it may be productive to contemplate the notion of a unidirectional flow of time. I do...
24. ### Arrow of time and virtual reality

As we know that in classical world we have a clear arrow of time which points towards future..We can see glass falling from table and break but not the opposite in our world and whole universe... If we watch a film of glass being broken and that film is run in both direction we can easily...
25. ### Does the act of observing/measuring define an arrow of time?

i was just reading the following article and had an interesting thought; http://www.ipod.org.uk/reality/reality_arrow_of_time.asp Referring to the following image (http://www.ipod.org.uk/reality/reality_arrow_of_time.asp ) Both Feynman diagrams are said to be mathematically equivalent...
26. ### Exploring the Arrow of Time: The Role of Probability in Moving Forward

Is the arrow of time explained by probability - i mean do we go forward in tiome because there are more possibilities in the forward direction than there are to go directly back to the exact position we can from.
27. ### Can Information Be Preserved on Closed Timelike Curves?

The arrow of time is globally derived from the global increase of entropy. In an information theory sense as a system evolves in time it becomes more random, the system can be in more possible configurations otherwise known as states. Likewise from the second law of thermodynamics a closed...
28. ### Arrow of time in copenhagen interpretation.

Most physicists don't draw a distinction between past, present and future.. this is called the arrow of time. Physicists such as Sean caroll, Paul Davies e.t.c But if all events exist in some sense then the future is <not> open. If the future is not open then determinism is true.. thus...
29. ### Where is the arrow of time if every decision is essentially instant?

In John Gribbin's book 'Schrödinger's Kittens and the Search for Reality', Gribbins describes a quantum interpretation that claims to solve the so called,'spooky action at a distance'. From what I've gathered, perhaps naively, is that quantum entities spontaneously communicate through the act of...
30. ### Question about the arrow of time?

I'm not sure if this is the correct section but I have a question I would like to be answered, I was thinking since a system will have many more possible disordered states than ordered states, then would this mean that if we were normally moving forward in time, and for example, some glass broke...
31. ### Is it possible for the arrow of time to be observed in reverse?

I know that in general the arrow of time does not itself reverse but is it possible for there to exist 'observers' in our universe that see its progression from, what we think as, finish to start? I just read http://arxiv.org/pdf/hep-th/0611088v3" by Brett McInnes so I understand why the...
32. ### Relativity and the arrow of time

Relativity and the arrow of time... It seems to me that the arrow of time is a direct consequence of the Theory of Relativity. Relativity says that everything in the universe is traveling though space-time at the speed of light. Motion through space diminishes motion through time. However...
33. ### Quantum physics, randomness and its influence on the arrow of time

Hi, I have a question which I've been pondering over and would like to have an anwser to. I'm not schooled in physics so I may be using bad terminology which you're welcome to point out of course. Anyway my question mainly has to do with the randomness in quantum physics. I know that the...
34. ### Thermodynamics and the Arrow of Time

Here's a discussion I'm hoping to start. As you may be aware, time goes forward. A shocking thing to say I know- but the universe clearly prefers time going from past to future, some relativity implications non-withstanding. The interesting thing about it is that the laws of physics seem to...
35. ### Many Worlds and the arrow of time

I had a couple of interesting thoughts today on the matter of Many Worlds. First is that, in a way, I think the so-called "branching" posited for MWI could be thought of as a derivative of time. For example, time is the derivative of space (i.e. using change in space with respect to time)...
36. ### Why is there an arrow of time?

I have a question, hope maybe someone has seen some papers on the subject and can point me in a direction. I searched arxiv/astro-ph with the search paprameter "arrow of time" but could not find anything in the following specific area: Hypotheses: a) Time is symmetric. b) When the big...
37. ### Antimatter Problem + Arrow of Time

Hey all, First post so I should introduce myself. I'm a freshman physics+math major at Virginia Tech, probably headed towards particle physics. I do some undergrad research w/ one of my professors, pretty lame though as I only build photomultipliers, but nontheless my foot's in the door...
38. ### Quantum Arrow of Time Solution

There have been a proposed solution to the Quantum vs Thermodynamic arrow of time problem published in the Phys. Rev. Letters. If my understanding is correct, the idea is very simple - time flows symmetrically in both directions, but observers only can remember the forward flow...
39. ### The Arrow of Time: The Laws of Physics and the Concept of Time Reversal

I definitely remember reading something official that said the laws of physics don't distinguish between the past and the future. I thinkit might have been A Brief HistoryOf Time. You could run it backwards and it would still work just as well. But now I've thought about it, there's something I...
40. ### Is the Cosmological Arrow of Time Linked to the Thermodynamic Arrow of Time?

Is the cosmological arrow of time just a special case of the thermodynamic arrow of time, where the universe is the isolated system?
41. ### Arrow of time derived from Markov like model?

I want to find a solid argument why entropy always increases and even more important why it is S=\sum_i p_i \ln p_i I've seen some more or less sophisticated arguments. What I'd find most convincing is an argument based on general converging Markov processes, that show that the above...
42. ### Arrow of Time? I don't understand.

"Arrow of Time?" I don't understand. I don't understand, and I'm not kidding about that, TOO many people that are a hell of a lot smarter than me ponder the question of why there appears to be an "arrow" of time, i.e. "why doesn't time run in reverse every now and then". So I have to figure...
43. ### Question regarding the arrow of time

How about this idea... With regards to this thing called, the arrow of time! Could the universe have different arrows of time for different regions of the universe? This regions of the universe where the Earth is, the arrow of time is going say forward. Could there be other far off...
44. ### Time Arrow: Entropy & Energy - QP Explored

I have only recently started reading about QP and there seems to be a big question mark on whether time has an arrow (direction). There seems to be a tendency leaning towards entropy having an influence on this topic. This is that, time moves in accordance to order vs disorder - the arrow facing...
45. ### Laws of Nature & the Arrow of Time

The abundance of matter and antimatter implies the laws of nature are different for particles and antiparticles. This is shown in the way more b mesons than anti-b mesons decayed into kaons and pions - the weak force does not conserve charge/parity. But if CP symmetry is not conserved, does...
46. ### Understanding the Direction of Time: A Philosophical and Scientific Exploration

This is a topic relating to physics but philosophical in nature. Physicists are talking about explaing why the "arrow of time" flows forward the way it does, instead of flowing in any other direction. My questions are these: 1) How do we know that time flows at all? is it not possible that we...
47. ### Arrow of time/ entropy probability

In Popper's autobiography, published in his contribution of Library of Philosophers, I read the following, after Popper's presentation of Boltzmann's ideas about entropy: A bit further this is formulated as such: Is this all accepted? Any additional comments? (The arrow of time always...
48. ### Medical Is the arrow of time a mental construct?

I think it is, but am sleep deprived (been flying for 24 hours). Its nagging me. Would anyone be happy to think, worry and explain about this, so I can go to bed?:smile:
49. ### What causes the arrow of time ?

Precisely this is the reason that ALSO the problem of arrow of time is still unsolved It is simply false that a non-unitary evolution can be derived from an unitary evolution as a kind of "good approximation". It is mathematically imposible and physically wrong. This is the reason people...
50. ### Thermodynamic vs Psych' Arrow of Time

I've read that there is a thermodynamic arrow of time, based on the fact that entropy can only increase or stay at the same level. But why is it thought to be directed in the same direction as the psychological arrow of time. Could there be an arrow of time directed in the opposite direction to...