> If you watched a video of two billiard balls colliding and bouncing away, you would be unable to tell if it was being run forwards or backwards.
I've seen this image used more than once to make the point, but now I think about it, I don't think it works. It is only acceptable if we think of billiard balls as ideal elastic spheres. They are good illustrations of the concept of "ideal elastic sphere" in our material life -- but they aren't, really, and that destroys the thought experiment.
Let's run the tape at super-duper slo-mo, shall we? The two balls slowly approach, collide, rebound. But now at super-duper slow-mo we can see that, being actual collections of atoms, they behave somewhat like water-balloons. At the point of contact, there is deformation, flattening, and rings of compression waves radiate away, to meet at the other side of sphere, interfere, and come back. (Something of the sort happened to Saturn's moon Mimas.)
So if we play the video in reverse, it is quite easy to tell the "before" from the "after". In reverse, we have balls that, as they roll toward each other, develop increasing concentric ripples of deformation that start from the opposite sides and shrink rapidly to the contact faces, disappearing completely at the moment of contact, and two unperturbed balls separate.
The only way the billiard balls image works is if we instead use ideal perfect spheres of adamantium. Even balls of titanium or diamond would have some kind of internal elastic shape change that would exist only "after" and not "before" the collision.
Which is simply to admit that in the real world, there are no collisions that are actually reversible even in principle.
It's a pop sci analogy, not a rigorous scientific statement. Of course real objects deform, get hot, and lose energy to sound.
But that's the point. Entropy changes. If entropy changes, events are irreversible.
As a crude simplification the less an event leaks entropy, the closer it is to being reversible.
Microscopic events - low-energy atomic collisions and particle collisions - can appear perfectly symmetric in time. So much so that microscopic reversibility is considered a thing in physics and chemistry.
What actually happens is more subtle than that, because quantum indeterminacy makes it impossible to exactly reverse a quantum event. Instead of Newtonian precision you have to settle for a probability density of possible outcomes.
But even so, you can't tell which direction time is running in many particle interactions. The particles aren't altered in any appreciable way, and no information/energy leaks to the surroundings.
So even though the difference between before and after isn't precisely deterministic, you still can't tell whether time is running forwards or backwards, because there's no entropic delta to give you a clue.
One one hand, the analogy is imperfect with billiard balls precisely because they are big collections of particles, not actual particles. It might help to imagine instead a collision of, say helium atoms.
On the other hand, if you see a "video" of two warm, vibrating billiard balls colliding, which then cool off and stop vibrating, it's still only a good guess that the video is in reverse. It's only highly improbable that the motion of the atoms in the billiard ball, air, table, etc cancel out to produce more order, not impossible. That improbability, AFAICT, is the linchpin of the Second Law of Thermodynamics.
It's the old,"A farmer has three chickens that won't lay eggs. So he calls a vetrinarian, and he can't help. So he calls a biologist, and she can't help. So finally he calls a physicist.
The physicist takes copious notes, precise measurements, and spends the rest of the day furiously scribbling in his notebook. Finally the physicist shouts excitedly, and rushes into the farmhouse.
The farmer says, "Did you figure it out?" And the physicist says, "I did! But it only works for spherical chickens in a vacuum."
I never understood the argument either, but I am no physicist. Start with the table racked (e.g. order) and break. One can easily discern the direction of time as the table goes from order to disorder.
I think that's more because you are familiar with the original ordered state of the racked balls. If you take out energy lost to friction and heat and allow infinite time, after breaking the balls would bounce around ricocheting ad infinitum, eventually forming the racked state momentarily before continuing into chaos. Just like the monkeys on typewriters, and more frequently broken clocks.
By chance yes. But the frequency of events of order to disorder is less than order to disorder, because ultimately, the universe tends towards entropy.
The point you're missing though, is that even in your example, the backwards case is still physically valid. It is true that no macroscopic interaction is reversible in practice, but this ceases to be the case for a fully defined system where there is no increase in the entropy, and you could argue it is because these do not exist, but there is no physical reason that they CANNOT exist. For one, if you could model all of the energy losses due to vibrations effectively, there's no reason to think that physically one could not use a very precise arrangement of speakers to cause compression waves to converge to the point of contact at the exact instant of the collision and revert the system to the initial conditions. Just because it's backwards doesn't make it physically invalid, except for the friction (and thus heat/entropy thermodynamic part). The backwards scenario still fully obeys the laws of physics. What you're doing here is essentially begging the question by saying that thermodynamics (and thus experience) tells us that these things (elastic collisions) don't happen because they don't happen. What you haven't established, however, is why. The point of this article is that from a physical perspective, there is no reason to believe that the laws of thermodynamics exist. But there are less complicated examples as well.
For instance, if you have an electron placed some distance away from another negative charge source, you could very easily model its dynamics. But there is no fundamental difference between this electron being repelled from the other negative charge compared to a positron that is being attracted to the charge, but moving backwards relative to our perception of time. So it is a forward moving electron or a time-backwards positron? There's no difference, they're both completely physical systems.
Boltzmann's big accomplishment, one that was at his time rejected until Einstein's description of Brownian motion, was his completion of the H-theorem (which Claude Shannon subsequently named his information-theoretical counterpart for), which describes that physical systems under very simple assumptions (uncorrelated position and momentum vectors) will 'relax' to a distribution of energies given by the maxwell-boltzmann distribution. It turns out, however, that these assumptions are not completely valid for physical systems, so the H-theorem is not really considered a 'true' explanation. I highly suggest reading the wikipedia article on Boltzmann's H-theorem that goes into the mathematical derivation of the positive-definite nature of time derivative of entropy of a closed system.
<wild speculation>
So what are the real answers as to the arrow of time? It's an unsolved problem, but I have guesses. The most obvious answer for things like this is based on the anthropic principle. In an informational world, one could assume that everything that we interact with all moves in the same time direction because DNA is effectively some Turing-equivalent string of 1s and 0s that is modified by the 'computation' of evolution. Memory of the timeline is encoded into DNA, which is what we are from an information perspective.
Chances are, however, that things like this will never be proven, possibly because in this case the proof will be non-computable by Turing machines like us. It isn't even unreasonable to think that the 2nd law of thermodynamics and P!=NP are not equivalent statements and equally unprovable. Am I far out there enough yet? </speculation>
If you want to really blow your mind with regards to time, look at QED. I highly recommend the Feynman lectures.
The net result of quantum interference is that light tends to move at the speed of light, particles tend to move forward through time, but only because everything else mostly cancels itself out.
(The rest of this is slightly rambling, but so was the article so it's hard to comment without matching it, so dusting off my very rusted physics degree...)
With Multiple-Worlds, I think the delayed-choice experiments more show that each universal snapshot is inherently self-consistent, rather than retro-casual. Any non-consistent snapshot is just canceled out. Meaning, there was probably a universe in which the 2-slit delayed-choice experiment didn't show interference, but it didn't survive past the delayed choice boundary. Now, the really big question (to which people keep arguing no, but I have doubts) is can you establish faster-than-light communication using a delayed-choice style effect by essentially canceling out any casualty that doesn't agree with your communication.
My personal favorite for explaining the directionality of time is looking at the boundary conditions. If you assume that at time T0 around the big bang, the universe looked like X, then it requires that particles move away from it in spacetime (otherwise it wouldn't look like X anymore). Thus, there is probably an antimatter version going the other direction in time away from us. Away from the boundary, entropy-like effects would prevent the odd positron coming back in time at just the right moment to produce a non-local causality violation.
That sounds like it might be testable by looking at the CMB for evidence of interference with the antimatter universe. It lso looks a bit like a solution to the whole "why is there more matter than antimatter" question. That's a very interesting hypothesis you've got there. :)
It seems that if we speak of information traveling "backwards in time", we must make an assumption of a global time (against which to travel) -- but isn't that (global synchronized time) known to be a myth?
What if each particle/object has its own relative time frame? Doesn't that solve the problem without needing to invoke "retro-temporal causality"?
For example, in the double slit experiment, perhaps the wave/particle and the slits form a local time frame group, within which no further waveform collapse is necessary. When this group interacts with the "environment", it may or may not have to collapse a large set of possibilities into a smaller set. If that environment includes a system which requires particle-like behavior, the interaction resolves to particle like behavior (as opposed to wave interference). The interaction of the "slit-particle/wave group" with the "environment group" would be a "synchronization event" of sorts which actually determines "causality". No backward time travel necessary if the "arrowed" forward motion of time is the act of waveform collapse. Causality of this sort would affect trajectories we perceive as spanning into the "past" (because we tend to want to view time as globally synchronized), even if in reality that whole past-present trajectory was just a superposition of all possibilities, until resolved via interaction of other "quantum timeframes".
Does that make sense to anyone else? Apologies if this is obviously true or false -- I'm not a physicist, but do love thinking/learning about this stuff :)
Assuming you are talking about special (or general) relativity,
Just because simultaneity is relative doesn't mean that before/after is in every case.
My limited understanding is that which of two events happened first is not relative to the reference frame when something moving at or below c could reach one event from the other.
My understanding of that could be missing some details or be wrong, but I think that is how SR works?
In relativity, 'backwards in time' effects are ones that affect things in their own past-ward lightcone [1]. Only affecting things outside that cone might violate the light speed limit, but it won't violate causality.
Not a physicist either, but I will give it a shot. Relativity tells us that their is no universal time ordering of arbitrary events, you can find reference frames where A occurs before B and where B occurs before A if the events occur at different positions in space. There is nonetheless still a global causal ordering of events - events can only be influenced by events in their past light cone and they can only influence events in their future light cone and light cones do not change when you switch reference frames so everybody agrees on this causal ordering.
I also think your view of quantum effects has some misconceptions. A photon is what it is, it does not switch between particle like and wave like behavior. It goes neither through one slit, nor through the other, nor through both or neither. It is really different from a classical wave or a classical particle. But that is just the way the universe works, the strange thing is not what the photon does, but that macroscopic objects do not behave like quantum objects.
I am not so sure about that one, but I think there are not so many people believing that the wave function collapse is a real fundamental thing. It is just incompatible with the unitary evolution of the wave function and causes you all the trouble figuring out what constitutes a measurement and what not. This is obviously an open problem but the collapse postulate is probably viewed as an approximation at best by most.
And the problem of the arrow of time kind of sits on top of the problem what time is in the first place. One view I picked up recently makes a lot of sense to me but I did not have much time yet to really look at it in some depth. Anyway, here we go. Fundamentally all fundamental particles are massless and travel at the speed of light. This kind of removes the need for time, massless particles travelling at the speed of light experience no time and if everything just moves at the speed of light there is no real need to use time to describe velocities, changes of position over time, because it is implied due to everything moving at the speed of light.
But then fundamental particles interact with each other, they attract and repel one and another. This opens the possibility that the composite system of several fundamental particles moves at a speed slower than the speed of light while all the constituents still just move at the speed of light. It are only the composite systems that are not following lightlike trajectories which experience time. One just compares the rate at which processes occur in composite systems to establish a time base.
Take a photon clock, a photon bouncing back and forth between two mirrors a fixed distance apart. All the electrons, quarks and gluons in the mirrors travel at the speed of light but the composite system is at rest from your point of view. The events of the photon hitting one mirror and then the other establish a time base for you. If you then boost the photon clock away from you with a very high speed, maybe close to the speed of light, the photon takes longer to bounce back and forth between the mirrors because it moves still at the speed of light but the mirror kind of runs away from the photon, that is time dilation. And if you boost another person along with the mirror and look at the cells of that person you see the same thing, molecules take longer to move around and reach their destination because all the parts of the cell move with a high speed and velocities do not simply add linearly, the person ages slower.
As said, I had not yet the opportunity to really look into this way of thinking. Especially I am not sure if really all mass stems from dynamic processes or if there are some fundamentally massive things out there. Maybe someone can provide some further insides whether this is a valid or helpful way of thinking about it.
> In other words, a putative "future memory" is fine-tuned to a particular outcome, and must be readjusted if any slight change occurs between now and the appearance of the "remembered" future state. That means it is too fragile to count as a genuine memory.
Or perhaps it's enough if "future memories" just become more and more fuzzy as they refer to events further and further away into the future.
"Yet none of this one-way flow of time is apparent when you look at the fundamental laws of physics: the laws, say, that describe how atoms bounce off each other. Those laws of motion make no distinction about the direction of time."
I'm not certain precisely what laws we're talking about, but I'm having a reaction that this argument may be specious due to the fact that the known fundamental laws of physics are human constructions, not the entirety of all that is possible. Most of the physical laws I know were specifically designed to factor out time; conservation of mass & energy equate state before and after an event, and prove nothing else. This article seems to be suggesting that the known "laws" of physics do encompass everything possible and thus the lack of a law proving the existence of time somehow proves that time doesn't exist (or is bi-directional).
I wanted to like this article, but it seems very confused. The essential premise, that the arrow of time is a QM process rather than a classical thermodynamic one, isn't presented as much as reams of background on the general topic are.
I posted it more for its interest (to me) as a long scientific article from a popular news outlet on a topic I'd never heard about before. It looks like it may be a re-working of this older article :
Actually isn't time's arrow pointing in the direction of information being increased? Entropy as defined in Claude Shannon's information-theoretic sense is actually correlated with information. More entropy means more information. And indeed, intuitively, we have more information as time goes on (about the past) and not the other way around. If you assume that information does not travel backwards in time - then you once again get that information increases in the direction of time. The article stated it wrong.
Am I unusual in being constantly surprised when people assume time's arrow in everyday conversation? I can never quite accept the sheer oddness and asymmetry of it.
Does that mean you are uncomfortable - or whatever I should call it - with the implied time ordering of events like an egg sitting on the table and then falling down and smashing on the floor?
I've seen this image used more than once to make the point, but now I think about it, I don't think it works. It is only acceptable if we think of billiard balls as ideal elastic spheres. They are good illustrations of the concept of "ideal elastic sphere" in our material life -- but they aren't, really, and that destroys the thought experiment.
Let's run the tape at super-duper slo-mo, shall we? The two balls slowly approach, collide, rebound. But now at super-duper slow-mo we can see that, being actual collections of atoms, they behave somewhat like water-balloons. At the point of contact, there is deformation, flattening, and rings of compression waves radiate away, to meet at the other side of sphere, interfere, and come back. (Something of the sort happened to Saturn's moon Mimas.)
So if we play the video in reverse, it is quite easy to tell the "before" from the "after". In reverse, we have balls that, as they roll toward each other, develop increasing concentric ripples of deformation that start from the opposite sides and shrink rapidly to the contact faces, disappearing completely at the moment of contact, and two unperturbed balls separate.
The only way the billiard balls image works is if we instead use ideal perfect spheres of adamantium. Even balls of titanium or diamond would have some kind of internal elastic shape change that would exist only "after" and not "before" the collision.
Which is simply to admit that in the real world, there are no collisions that are actually reversible even in principle.