Time Assymetry of the Weak Nuclear Force--Any Biological Significance?

Factual Questions
1

Do I recall correctly that of the four fundamental forces, only the weak nuclear force is time-assymetric?

Assuming I’m right about that:

If all forces were time-symetric, then time could go backwards (whatever that means) and the resulting procession of events would be indistinguishable from a time-forward procession of events. This means among many other things that a properly poised object that looks a lot like an organism could be such that, in time-reversal, it proceeds in an apparently normal biological fashion–but backwards.

But the weak nuclear force is not time-symetric. If time ran backwards, you could tell by looking at what’s going on with the weak nuclear force.

And my question is, would what’s going on with the weak nuclear force have any effect on organism-like objects like the one I just described, i.e., ones poised such that they would proceed in apparently normal (but backwards) biological fashion when time reversed had all four forces been symetric? Would something about what’s going on with the nuclear force prevent them from proceeding in this apparently-normal-but-time-reversed way?

Well, make sense of that.

2

Ah… Wikipedia article on “Arrow of Time” says something relevant.

A. Is that accurate?
B. The claim is uncited in the article. If it is accurate, I would love to have a citation of the claim onhand. I think a lay-accessible cite would be better for my purposes than a physics textbook or article would be.

3

The weak nuclear force, as I understand it mainly affects nuclear decay. I doubt it has much effect on organisms. Or rather, any effect it has is going to be though the fundamentally random effects of radioactive decay.

4

While QED (the quantum theory of electrodynamics) itself is nominally reversible (time symmetric) there is a good argument [warning: PDF] to be made that it, too, is subject to the second law of thermodynamics. As for weak interaction (the so-called “weak force”) it governs quarks and leptons of left-handed chirality, and is mediated by the massive W and Z gauge bosons. As this all occurs within the nucleus of an atom (quarks don’t exist in an unconfined state separate from the composite nuclear particles they form) it has zero impact on the electrochemistry that comprises the processes of life as we know it, except when decay releases harmful ionizing radiation that does damage to biological structures.

So the answer is no, or at least, not anywhere in our understanding of biology and particle physics.

Stranger

5

I doubt gravity is completely time-symmetric.

The probability is infinitessimally remote that a ball a few meters away from me will start rolling, bounce a few times, and jump into my hands.

6

That process of beginning to roll, bouncing, and jumping into your hands would be perfectly consistent with the laws of physics. This is not as strange as it seems if you think carefully about the fact that the ball is not an undifferentiated mass, but rather, is a conglomeration of particles. As the ball rolls across the floor, slows and comes to a stop, that motion consists in the motions of all the ball’s particles. If you reverse time, all those particles’ motions reverse. And those particles’ motions are going to be exactly as though governed by the normal laws of physics.

Aggregate particle motions that add up to slowing to a stop in the time-forward case end up in the time-reverse case adding up to acceleration. Since the reversed motions of the particles are completely consistent with the laws of physics, it follows that the acceleration of the ball would be consistent with the laws of physics. It would be incredibly improbable, but it would be physically possible.

Similar reasoning applies to the ball’s beginning to bounce and jumping up into your hands. All the particle motions are perfectly consistent with the laws of physics. Its just that their occurence would normally be incredibly improbable. The reversing of time allows for the occurance of improbable events like this. But importantly, in doing so, it preserves the laws of physics.

Made sense?

7

The equations of general relativity are reversible except about singularities where the math breaks down. A quantum theory of gravity very likely will result in asymmetries, but so far no complete and falsifiable theory has emerged.

The probability of a ball doing what you describe is not an example of time asymmetry, but equilibrium. It takes from potential the same amount of energy for the ball to fall out of your hands as it gives away when you pick it up. In other words, if you suddenly applied the same impulse to the ball on the ground equivalent to what it saw during falling (but opposite in magnitude) it could readily follow the same path back into your hands.

Stranger

8

I think the important thing to highlight here is that “the same impulse” doesn’t just involve a single force in a single direction applied to the ball taken as a whole, but rather consists in what you get when you apply all the individual impulses to the particles constituting the ball opposite whatever impulses they had as they slowed to a stop.

9

Well, if you’re getting down to the particle level you are dealing with the ugly, indeterminate muck of quantum mechanics, and that is not reversible in a global sense to a large system owing to the probabilistic nature of QM. However, if we apply an equal and opposite impulse (which, as you point out, is the whole time-force history) to the decoherent system (i.e. one that can be treated as an aggregate) and neglect losses due to deformation of the ball, acoustic and frictional losses, et cetera, such that the only forces acting are gravity and the perfectly elastic rigid bodies of the ball and ground, we will get a fully reversible system.

Stranger

10

I like what I said better. :p;)

11

And even there, the equations allow for the existence of past singularities in perfect symmetry to the future singularities. So far as relativity can tell us, it’s nothing but a quirk of the Universe that it happens to not contain any past singularities.

Back to the OP, there are three known independent arrows of time. The first is the thermodynamic arrow of time: The past is the direction in which entropy is smaller, while the future is the direction in which entropy is larger. The second is the cosmological arrow of time: The past is the direction in which the Universe is smaller, and the future is the direction in which the Universe is larger. The third is the weak arrow of time, which the OP mentions, but it’s so subtle that it’s never been directly observed, and makes almost no difference to anything.

Any other arrow of time you can come up with (for instance, that the past is the time you can remember, while the future is the time you cannot remember) can ultimately be reduced down to one of those three, and it’s almost always the thermodynamic one.

12

Maybe someone can explain what time symmetry is supposed to imply, especially with references to forces. It seems like they are the most assymetric things, with respect to time, in that they are universally attractive in one direction and repulsive in the other (excepting maybe electric charge?).

13

A force is time-symmetric if its operations do not give you a basis for determining which way the arrow of time is pointing.

In other words, it’s time-symmetric if, were you to watch things under the influence of that force unfold backwards in time, they would still appear to be under the influence of that force in the same way they appeared to be under its influence when things were going forward in time.

Gravity, for example, will still appear to be an attractive force, no matter whether things are going forward or backward in time.

Stranger and I have explained how this can be possible in the case of gravity, despite how counter-intuitive it seems. Where in our explanations did you stop following the logic?

Here’s another explanation. Imagine a sun. Lets imagine ourselves at rest relative to it. Now imagine a smaller body in deep space, hurtling nearly in the direction of the sun, but not directly at the sun. The body’s path will curve as it comes near the sun. But imagine the body is going fast enough that it’s not pulled into the sun or into orbit–it escapes the sun’s gravitational field and goes on its merry way.

So the body traces a curved path in space because of the presence of the sun. The path curves in the direction of the sun, because gravity is attractive.

Now run the tape backwards, and what do you see?

Basically the same thing–a body travelling through space, approaching a sun, its path curving as a result, and then going on its merry way. (Do you see it?) And which way is the path curved? Toward the sun.

This illustrates how gravity works the same in both time-forward and time-reversed views. In both cases, it works as an attractive force.

Here’s another way in. Think of a basketball bouncing quite high. It bounces four or five times. This involves movement both upward and downward. (And note that the upward movement is not incompatible with gravity’s being an attractive force.)

But run the tape backwards, and what do you see? Bouncing again, involving both upward and downward movement. Now there may be some mysteries about what you see (for example, why are subsequent bounces higher than previous bounces?) but here’s a question to ponder. If reversing time is supposed to make gravity repulsive, then what explains the basketball’s downward movements in the second half of each bounce in the time-reversed scenario?

14

I guess the examples I was thinking of where it would seem repulsive are actually examples of dissipated heat uniting to impart kinetic energy on objects. I guess it’s just the entropy thing…

15

Yes. :wink: