Time reversal question

I’ve been watching a lot of PBS Space Time on Youtube and reading Brian Greene’s “Fabric of the Cosmos.” In both, they discuss time reversal and I’m having a difficult time (no pun intended) trying to wrap my head around their examples. In their examples, they describe items or things moving backwards through time. In Brian Greene’s instance, it’s described like an egg that fell off the counter and broke suddenly coming back together and going back up to the counter. They even show this in the PBS special about his book.

However, I don’t understand how this shows a reversal of time. If it takes +1 second for the egg to fall to the floor, it also takes +1 second for the egg to go back up to the counter. Time (from the example I saw) is still moving forward one second at time, but events are just happening differently. If I count when I see the egg fall, I count one second, and if I see it jump back on the shelf, I count one second. How would a true reversal of time actually seem? It seems to me that regardless of whether events are running forward or backward, time still seems to move in a positive direction.

Can anyone explain it better to me?


Nothing travels backwards in time. Any physics which says it does is hogwash.

What kind of phenomena would you like to see that visibly demonstrate time reversal? When you come right down to it, the arrow of time is determined by several fundamental factors, and I suspect the most important of these are causality – the order of cause and effect, and entropy, a fundamental law of nature that says disorder always increases with time, embodied in the second law of thermodynamics but found everywhere. I haven’t seen the examples you cite but ISTM that a broken egg that fell off the counter reassembling itself and leaping back on the counter is reversing both entropy and the order of cause and effect, so that’s time reversal in every perfect sense of the term. Thus, it’s exactly what you see when you run a film of the event backwards.

The discussion isn’t about things traveling backwards in time, it’s about T-symmetry. I have read Greene’s book and it is a very accessible read for the layman.

Some, but not all, physical laws are invariant under time reversal. This is called T-symmetry. For example, the 1980 Nobel Prize was awarded in recognition of work demonstrating such a symmetry violation in the decay of certain particles.

Your egg example illustrates a different, “thermodynamic arrow of time”. The law is that the entropy of a closed system only increases with time. Therefore, if you see it decrease, you know the film must be running backwards.

ETA if T-asymmetry is related to thermodynamics (cf. this kind of stuff), perhaps someone could explain it succinctly?

To be fair, very little in fundamental physics indicates that time has as preferred direction. The second and third laws of thermodynamics are not time symmetric but that is more of a definitional observation; increase in global entropy is always positive as one progresses along the time axis. Similar arguments can be made for cosmological directionality in time around both universal expansion and singularities. The Schrödinger equation, which is the fundamental statement of quantum mechanics, is time symmetric although there are certain arguments in quantum field theory which tend to suggest a preferred direction for interactions although it rarely affects any observable interactions.

We perceive time to move in a particular direction because we are embedded in a universe in which the physical laws and chemical systems that we interact with follow a set of rules that at least at the macroscopic level appear to be causal and follow an “arrow of time”, hence we do not see smashed eggs leaping back up to the counter to reform themselves whole or water swirling out of a drain. But causality is an assumption that we make to deal with the macroscopic world; at the more fundamental level, either causality or locality (or perhaps both) does not appear to hold, and fundamental interactions are for the most part fully reversible, albeit not in the Dr. Who sense of being able to travel back in history and change events and so forth. We will always perceive events as having a causal direction because that is how our perception works, although you can ‘trick’ the brain into seeing time go backward by running a film backword or creating an animation which appears to reverse a causal event.

There are also theoretical ways within the framework of general relativity to create a path that travels to a point in spacetime that is in “the past” or the superluminal “future” (e.g. outside the causal light cone of the observer) with wormholes or closed timeline curves. There is no particular reason that these can’t be physically realizable rather than just artifacts of the mathematics but we don’t know of the mechanism to create such paths using any existing or foreseeable technology. Then again, two centuries ago nobody knew how to make enough electricity to light a filament or to transmit modulated radio spectrum emissions, and today we take these miraculous discoveries and the technical innovations they spawned so completely for granted that most people have no idea at all how they actually work even as their livelihoods depend upon them.


The causal arrow of time and the thermodynamic one are actually the same thing. Effects are higher-entropy states than causes.

Think of it this way:
|seconds it took for the egg to fall and break| = 1
where |x| is the absolute value of x.

ETA: and |seconds it takes for the egg to unbreak and rise| = 1

Well, see, that’s kinda the point: for time-symmetric physics, it would seem exactly the way it does when time is moving forward. That’s just what symmetry means: that you can’t tell the difference.

To help try and clarify, under Newton’s laws of motion, if the dynamics are invariant kinematics will also allow it to be kinematics; if the dynamics don’t allow for it then kinematics won’t either.

force = dynamics
motion = kinematics

The second law of thermodynamics will allow for it if it is a reversible process

The real world will not symmetry under time reversal.

Newton’s laws of motion and Einstein’s field equations in General Relativity are form-invariant under diffeomorphisms and time-reversal is a diffeomorphism.

The point being made in that video is that those theories don’t presume an arrow of time but other theories do which will break the T-symmetry.

Do any theorists conjecture that CP violation is somehow related to the Arrow of Causality?

Looking at Feynman Diagrams, one can see that a particle traveling backwards in time behaves exactly like an anti-particle moving forwards in time.

Almost exactly. To the extent that there’s CP violation in particle physics (only about 1 part in a thousand, and only in certain specific sorts of interactions), there’s also a time asymmetry.

That was my question, and I don’t know the answer. It is not addressed in the paper linked to above, which purports to show that T-violation implies there should be an arrow of time and that the two directions are distinguishable, but does not deal with statistical mechanics or thermodynamics.

Events reversing in time flow also requires the reversal of all other physical aspects. Gravity must work oppositely to draw the egg upwards instead. Disorder becomes order.


Time does not exist as a thing. Only beings that have the capacity of memory can even notice that things happen before or after other things happen. For all other things time does not exist. Just the interaction of things. They interact according to the laws of the universe.

After you die. Your brain stops. Memory stops. Time ceases to exist to you. As it is to anything without memory.

Maintaining a memory and so forth does have to do with entropy and thermodynamic considerations. This is, indeed, a real arrow of time. The question is whether it is related to the one derived from weak particle interactions.

No. When you toss a ball in the air it travels a parabolic path which (ignoring air resistance) is symmetric in time and space. Similarly the reassembled egg will be decelerating as it travels up toward the dropper’s hand.

I think Isaac Newton may have been first to note that the laws of motion are independent of time’s arrow. Ignoring CP violations in the weak force, the 2nd Law of Thermodynamics may be the only “law” of physics which relates to time’s arrow, but 2nd L.O.T. isn’t a pure law like charge conservation — it’s a statistical expectation related to the law of large numbers.

When you play the video of the cracking egg and look microscopically, you’ll see the egg’s kinetic energy transfer to random-like disturbances among the molecules on the floor, raising the floor temperature slightly. When you play the video backwards you’ll see the random heat energy suddenly becoming perfectly correlated (in violation of the 2nd L.O.T., but in compliance with other laws) and propel the egg upwards, overcoming gravity.

Huw Price’s Time’s Arrow and Archimedes’ Point provides a layman’s look at time’s arrow and associated controversies.

There are three known arrows of time in physics. The first is the thermodynamic arrow of time: The past is when entropy is lower, and the future is when entropy is higher. Essentially all of the other arrows of time you might think of are really just manifestations of this one.

The second arrow of time is the cosmological one: The past is when the Universe is smaller, and the future is when it is larger. Almost nobody actually thinks this one has any real significance; if the Universe were to stop expanding and start contracting instead, it wouldn’t make any immediate difference. I mention it only for the sake of completeness.

The third arrow of time is the particle physics one. It’s a very small asymmetry at best, and usually nonexistant, and it’s so subtle that, so far as I know, it’s never been observed directly, only inferred from CP symmetry violation (which has been observed, but which is still only very slight). If you’re not doing particular sorts of particle physics, you don’t care about this one at all.

So far as anyone can tell, these three are completely independent of each other.

Observed in 2012. Phys. Rev. Lett. 109, 211801 (2012) or arXiv:1207.5832.