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Questions tagged [arrow-of-time]

A concept related to the asymmetry of time, usually related to the second law of thermodynamics, which says that entropy always either increases or stays the same.

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Why is there an arrow of time? [duplicate]

I believe I read on the internet that quantum mechanics predicts that the fundamental laws are time invertible. The web page probably really meant one theory predicts that. Does that theory combined ...
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Is there a connection between the spreading of the wave packet of the universe and the time arrow?

Entropy of position and wave packet spreading. Why do wave packets spread out over time? Wave packet of the universe and its  spreading. Is there a connection between the spreading of the wave ...
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Is reversal of velocity always equivalent to reversal of time?

Let us imagine there is a container full of small particles which are allowed to collide with each other and the container walls (in 2D). If I initialise the system with given velocities and positions ...
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Second Law of Thermodynamics in General Relativity

I recently came across some posts in StackExchange, which say that simultaneity doesn't have any meaning in General Relativity (see Is simultaneity well defined in general relativity?). I came up ...
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What does the time reversibility of the laws of physics mean for causality?

Does the fact that the fundamental laws are symmetric with respect to direction of time show that causation does not exist? Since causality always requires the cause to precede the effect, but laws of ...
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Increase in entropy and integer factorization (How much work does one have to do to revert “irreversible” processes?)

This is not necessarily a direct physics question, but I am hoping, that by posting it here, someone can give a hint from the physics perspective. It is known, albeit not proven, that integer ...
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Question about a mirror Universe

In the article We've seen signs of a mirror-image universe that is touching our own in the popular magazine “New Scientist” (which describes a real scientific theory though, though in a popular form, ...
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Why do we perceive time?

Since everything (at least what we observe) is travelling through the time in the same direction and speed why do we even perceive the time? Of course, there is the time dilatation, but it is ...
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Antiparticles travelling backward through time [duplicate]

Could someone please explain to me Feynman and Wheeler's theory of waves acting forward and backward in time? I have read about their idea of an antiparticle behaving as a particle travelling ...
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How does the direction of time work with timeless wave functions?

In thermodynamical theory, if we have a set of states for example: A) gas all in top left corner of box. B) gas spread out near the left. C) gas spread out to fill entire box. We can give the ...
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Arrow of time and CPT symmetry

It's been a few years since my physics degree. But I've been wondering: if you look at pictures of collisions at CERN, isn't it obvious which way time flows - simply on a probability basis? The ...
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Is reversing the arrow of time that same as going backward into the past?

I read an article today that somwhat confused me. Is this the same as time travel...which I have always thought was a contradiction. Are they talking about reversing law of thermodynamics? In which ...
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Have researchers managed to “reverse time”? If so, what does that mean for physics?

According to press releases, researchers have reversed time in a quantum computer and violated the second law of thermodynamics. What does that mean for physics? Will it allow time travel? Further ...
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Graviton particles travelling backwards

I know that time travelling backward particles are anti-matter particles but what about graviton time travelling backward?
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Are Mirrored Universes With Opposing Directions of Time Theoretically possible? [closed]

I found an article that talks about this here, entitled "Big Bang May Have Created a Mirror Universe Where Time Runs Backwards". I know any notion of backwards time is probably a dead horse in this ...
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Physics of time running backwards

Although it would seem weird to analyze physical phenomena when time runs backwards, it seems to have a logical sense, at least for me: Entropy would tend to decrease: two balls having energy ...
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What goes wrong, theoretically, when we reverse time?

(Please bear with me if this is a stupid question; I'm not a physicist, just a curious student.) I know that Noether's Theorem links symmetries to conserved quantities: the fact that the laws of ...
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Must the whole universe have the same entropic arrow of time?

Theoretically, could it be possible for one galaxy to have low entropy in the past, but another nearby to have low entropy in the future? I understand that, cosmologically, there almost certainly are ...
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Why does a sign difference between space and time lead to time that only flows forward?

Ever since special relativity we've had this equation that puts time and space on an equal footing: $$ds^2 = -dt^2 + dx^2 + dy^2 + dz^2.$$ But they're obviously not equivalent, because there's a ...
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Brouwer fixed point theorem

How does the Brouwer fixed point theorem (which provides a solution to the photo camera paradox of a causal loop) generalise to show that objects in closed time-like curves do not violate the second ...
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Can time flow backwards? [duplicate]

Is there any equations in physics or laws that say time can flow backwards instead of forward?
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What is the relationship between how time is viewed in thermodynamics and how time is viewed in general relativity?

From my limited understanding of physics, it seems that the second law of thermodynamics, in which entropy never decreases over time in a closed system, relates to how time can only go forward i.e. ...
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Is time travel a violation of second law of thermodynamics?

According to the increase of entropy principle, the entropy of the universe is always increasing. So, does going back in time violates the second law of thermodynamics? Because entropy of the universe ...
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Confusion about entropy when applied to the whole universe. What are the macrostates?

I'm really confused about the concept of entropy when applied to the whole universe. The often hear that the universe started with very low entropy and as the entropy increases the universe will ...
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Can you reverse the arrow of time?

Has there been any attempt to reverse the arrow of time?
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What distinguishes time directions from spacial directions?

Other than the fact that one may move in only one direction in time but can move in two directions in each spacial dimension (e.g. arrow of time), are there any other physical characteristics that ...
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Will a shrinking universe have a reverse arrow of time?

I'm not a physicist so forgive me if this question is silly. I'm reading (actually listening) to Mysteries of Modern Physics: Time by Prof. Sean Carroll. I'm not sure if the concepts in this book ...
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How can we determine time flows in one direction? [duplicate]

Suppose that time could flow in either direction, forward or reverse, similar to a movie. How could our brain possibly record that time had moved backwards? Would there be any way to tell if time had ...
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Is Time the same as The arrow of Time? [duplicate]

I asked this question here. Some doubts emerged in my mind from the comments. Here they are: Are time and the arrow of time, the same thing? All of modern physics, seems to treat time as just another ...
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Understanding time [duplicate]

Am sorry if this is a silly question. I have heard something like this from cosmologists: "There is no intrinsic arrow of time. Today, we believe that the direction of this arrow is due to our ...
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Thermalization in Classical Mechanics: A Paradox

I was wondering to know whether there is any argument shows how a classical system (a system with a Hamiltonian and Poisson bracket) approaches its equilibrium and how the entropy increases. Newton's ...
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Do past states of a system have lower entropy?

It's often said that the second law of thermodynamics is the only time asymmetric law in physics, namely $S(t_2) \geq S(t_1)$ if $t_2 > t_1$. But it seems to me that the concrete application of ...
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Does time matter?

Aside from the second law of thermodynamics it seems physics just doesn't care about the direction of time. So does time really matter? What if instead of 3 spacial and one time dimension. 3+0 or 3+2....
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Validity of Boltzmanns Equation and $H$-function theorem?

A while ago I came across a resource (which I have forgotten) on the validity of Boltzmann's equation. It talked about the fact that the Boltzmann's equation is valid at the extrema of the $H$-...
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Is there a mathematical relationship between time and entropy? [closed]

If there is a relation between time and entropy, what is it? Are there limitations for this equation? Or if there is no relationship between them, what is the current state of research?
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Redo the experiment 'Reversal of thermodynamic arrow of time'

In a recent paper "Reversing the thermodynamic arrow of time using quantum correlations" by Micadei et al, an experiment was carried out to show a reversal of time. Basically they prepare a mixed ...
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1answer
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Can two distinct quantum universes ever have the same configuration, and what does it mean for many-worlds? [closed]

First, I hear that, on a whiteboard, one may casually invert causality and run time in reverse. Next, I hear that there are interpretations of QM, like Chaitin's Great Programmer interpretation or de ...
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Is Quantum Mechanics time-symmetric?

In physics we have the CPT-theorem which guarantees time-reversal symmetry of dynamical evolution (although in some edge cases we will also have to reverse parity and charge). It seems clear enough ...
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Does time reversal symmetry hold in systems that use the geometry (shape) of a molecule as an input parameter to determine a subsequent output? [closed]

Does time reversal symmetry hold in systems that use the geometry (shape) of a molecule as an input parameter to determine a subsequent output? A system such as a synapse of a neuron that responds to ...
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Is there a scheme for secure physical time-stamping?

Is there a physical phenomenon that could be used to record digital information in such a way that it has the following properties: it relies on some immutable physical law and does not rely on any ...
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Is entropy a function of time?

Is entropy a function of time? Since universe is expanding with time (entropy increases) and contracting on reversal of time. Can we say entropy somehow related with time in addition to state?
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Explanation of why doesn't heat flow spontaneously from a colder to a hotter body [duplicate]

The second law of thermodynamics says that heat cannot spontaneously flow from a colder to a hotter reservoir but only with the expenditure of mechanical energy. This is taken as a postulate or law in ...
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What was the reasoning behind Stephen Hawking's thought that time would reverse if the Universe started to contract?

Once upon a time, Stephen Hawking had in mind that time would stop and reverse as soon as the Universe would pass from a state of expansion to a state of contraction. What was the reasoning behind ...
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What does it mean to reverse time? [duplicate]

Experiments are done to see if a process is symmetric (or asymmetric) with respect to the reversal of time. So if a process happens if the particles move in one direction, but not the other way round ...
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CPT-symmetric initial state

Consider a hypothetical state of the universe such that it coincides with its CPT “mirror-image”. I.e., let us reflect positions of all objects by some plane (parity inversion), reverse all momenta (...
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The Feynman diagram for Pair production

The Feynman diagram above is for pair production and at the vertex of photon materialization, the arrow representing positron is against the time axis whereas the second arrow at that vertex which I ...
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How second-order differential equations do not violate causality?

The second order differential equations are time reversible. That means: they don't distinguish the time arrow direction. There is no reason for the time to flow forward. My professor told me that ...
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628 views

Can time run backwards? [closed]

Although equations of physics do not prohibit the backward arrow of time but the movement is arrested by the law of entropy...which makes the backward movement of time less likely but it is not ...
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What exactly is the physical picture of Time Reversal Symmetry

What's the exact meaning for time reversal symmetry in classical mechanics and quantum mechanics, respectively? Why is it right for just single or just a few particles but wrong for many particles or ...
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Entropy: the arrow head of time [closed]

So recently I started reading thermodynamics and on the entropy concept. I understand that the entropy universe always increases. The entropy increases in a particular direction which is the direction ...