# On the distinction of past and future: could one theoretically reverse direction of particles and cause time to appear to go backwards?

Based on my understanding of physics after seeing The Distinction of Past and Future on Project Tuva, there is no distinction between past and future on a fundamental level- all particle interactions can occur in reverse. So my question is whether or not one could theoretically reverse the direction of all particles in the observable universe relative to each other and have time essentially go backwards indefinitely.

If you think about it, things could "fall" upwards because the air resistance would be much lower due to the way the air was moving when it fell, and the velocity from the gravity downwards would be reversed as well as air under the ball pushing up (again due to the way the air was moving previous to the switch). I don't see why this same logic couldn't be applied to a more complex system.

Does this logic make sense? If not, where is the flaw? What other constraints would need to be added to make time essentially go backwards other than reversing direction, if it is possible at all, in theory?

• You wouldn't really be making time go backgrounds though. Time would still be going forwards, just in the opposite direction. – David H Feb 21 '14 at 17:54
• @DavidH, understood. That is why I said, "essentially." – David Ball Feb 21 '14 at 18:10
• One word: entropy. If we "reversed time" indefinitely, entropy would be decreasing overall. The laws of physics say this can't happen, therefore, this slight distinction between forward and reverse time makes it essentially impossible to simply turn back the clocks – Jim Mar 23 '14 at 19:24
• @Jim, is that true? I though entropy was just a general trend of events and not an absolute law in all scenarios. Am I wrong? – David Ball Mar 23 '14 at 23:22
• the law that dictates that entropy must always increase overall is one of the most fundamental laws of physics. Many other laws have some exceptions at very small or very large scales. Or you can get around some laws by introducing particles or different models. But this law is immutable. This law is the one law that can make or break a theory. If a theory says energy is not conserved, it's still ok. If a theory says entropy decreases, it's dead. – Jim Mar 24 '14 at 12:23

Yes, as you say, there is a built-in time symmetry in the mechanical laws that underlie our universe. At the moment the most accurate statement seems to be CPT symmetry. Under a CPT reversal (particles -> antiparticles, flip space, flip time), mechanics works identically. On a practical level though, even time symmetry alone holds to a good degree.

It is of course very theoretical. Even in a simple classical picture, you would have a lot of trouble getting in there and reversing all the motions of every molecule.

Quantum mechanics adds even more complications to the requirement of reversal -- it's not enough to just reverse the motions of particles, but you actually have to preserve all the complex wave correlations (entanglements) between the particles. But there's the trick: you can't observe those entanglements, so you need some sort of sneaky time-flip operation that does not involve observation. Moreover, once you observed the time-reversed system you would induce decoherence and destroy its reversed-ness.

• But it is theoretically possible? – David Ball Feb 21 '14 at 19:31
• I think you have the wrong expectation for your answer "it is possible". By the wording "it is", you seem to be thinking of a physical process that would reverse time. But that's not at all what the equations predict. What is true is that if you flip the sign of time (it's actually somewhat more complicated than that) and the sign of charge and then the sign of the spatial coordinates, the equations of physics will look exactly the same. That's all time-reversal means. – mcFreid Feb 21 '14 at 20:22
• Time reversal is possible for some small or very specialized systems. For example the spin echo phenomenon is a sort-of time reversal. The examples you have in mind (such as reversing of friction of a ball falling through air) are theoretically possible, however by "theoretically" I mean only that they are mathematically possible; it is very doubtful that they will ever be practically possible at any point no matter how our technology develops. – Nanite Feb 21 '14 at 22:11
• Be aware that both CP violation (many cases starting with neutral kaon decay) and explicit T violation have been observed in flavor violating reactions. In other words, not quite all of physics obeys time reversal invariance, even neglecting macroscopic thermodynamics concerns. – dmckee --- ex-moderator kitten Feb 22 '14 at 2:18

Full reversibility at the elementary level does not imply what you suggest: new qualitative features appear as the scale of the system and its ability to interact with the rest of the world increase, so that dissipation (irreversibility, loss of "useful" energy) at the level of our everyday experience does not contraddict microscopic reversibility. If a drop of ink falls into water and diffuses, you still could not (in a sense made accurate in Statistical Mechanics) reverse all the particles' motions and recover the original drop, the rest of the world being left unaffected.

• I'm sorry, I don't think you were very clear. Why couldn't the drop example be reversed (assuming the entire observable universe were also to reverse)? – David Ball Feb 21 '14 at 18:20
• The concept of Entropy plays quite a role in that. – Smerdjakov Feb 21 '14 at 18:44
• The concept of Entropy plays quite a role in that. As I said, the reversibility of the elementary is not in contraddiction with the existence of dissipation as experienced everyday. What we call dissipation is, in essence, just the transferal of energy (which is conserved, as reversibility implies) to a portion of the system from which we can hardly recover it. Let us consider a dampened pendulum: a macroscopic observer would err if they thought energy is disappearing. It is not, it simply takes the appearance of heat. – Smerdjakov Feb 21 '14 at 18:49
• The Second Law dictates that this form of energy is different in some sense from mechanical energy, in that it cannot be reversibly transformed back. – Smerdjakov Feb 21 '14 at 18:51
• Quote from en.wikipedia.org/wiki/Entropy_(arrow_of_time): "Unlike most other laws of physics, the Second Law of Thermodynamics is statistical in nature, and therefore its reliability arises from the huge number of particles present in macroscopic systems. It is not impossible, in principle, for all 6 × 10^23 atoms in a mole of a gas to spontaneously migrate to one half of a container; it is only fantastically unlikely." This combined with the idea that all microscopic phenomena have inherent time symmetry makes me think that my scenario and outcome are entirely plausible. Rebuttal? – David Ball Mar 24 '14 at 17:04