How does an atom of a radioactive substance know when it is to decay?

We know that when we have a certain amount of radioactive material, that after a certain period, half of its atoms will have decayed. But we do not know which atom will decay at what point.
No one knows. Especially not the atom.

It’s a mostly random process, the instability of the nucleus causing it to decay in such a way that half will have done so in a given period of time. There is no know way of telling when a given nucleus will decay.

My guess would be that it is a quantum phenomenon, meaning that the best we can do is to give a probabilistic explanation. Like other probabilistic events, like flipping coins, we can make accurate statements about aggregates of events.

That’s where the term “half life” comes from. In that amount of time, one-half of the radioactive material will have decayed.

I’m still thinking of this atom. Nothing changes for it and yet after an arbitrary period of time it changes and decays. What was different about that instant of time from all the other instants.

Things behave strangely at the quantum level. The position of a particle is given by a probabilistic function. It is possible for a particle to move from A to B without occupying any position between A and B. Atomic decay follows the same pattern. If you find this explanation unsatisfactory you are in good company. Einstein was never able to accept the probabilistic interpretation of quantum physics.

At a fundamental level, reality is probabalistic. Things don’t happen or exist at certain places and times—they have a probability of happening or existing at certain places or times.

We can measure what these probabilities are, just like we can measure the probability of getting a “6” on a six-sided die. Over thousands or millions or throws, we can be certain that the die will land on 6 about 1/6 of the time. But for any individual throw, we don’t know. The die doesn’t “choose” to land on 6, anymore than it “chooses” to land on 6 1/6 of the time over millions of throws.

Yes, it’s weird that reality works this way at the quantum level. But I’m sure to someone living at the quantum level, our layer of reality would look just as weird to them.

A number of physicists have attempted to explain this apparently probabilistic behavior of matter at the atomic and sub-atomic scale by introducing hidden variables as a sort of deterministic internal clock that an unstable nucleus would consult to tell it when to decay. These were largely given up due to Bell’s Theorem.

I can’t quite see where Bell’s non locality theorem comes into this. The idea of a sub atomic particle possessing a clock doesn’t sound right either and why should each particle’s clock be set at an individual time to go off? Sub atomic particles should be identical to one another should they not. If they are indeed different then what are these differences?

Just using the concept of a clock as a metaphor.

I think we just have to agree that this is deeply mysterious and nobody knows. At the quantum level space and time don’t seem to exist in the way we are familiar with.

‘I think I can safely say that nobody understands Quantum Mechanics’ – Richard Feynman

That said we can predict various quantitites with impressive degrees of precission using the mathematical formalisms of the field theories that grew from adding special relativity to quantum mechanics.

Hear and see Feynman lecture for “laypersons” on QED here.

“The universe is not only stranger than we imagine; but stranger than we CAN imagine.”
– J. B. S. Haldane

Thanks for the link to the Feynman lectures and the Haldane quote. Science began by saying ‘I don’t know’ and it looks like it always will.

That’s why people with scientifically trained minds run away from anyone who claims to know all the answers.

I’ve been meaning for some years now to really sit down and work through this paper and this book (don’t let the look of the latter fool you. Real math inside!).

When a nuclear fission bomb is ignited, something causes the rate at which the unstable radioactive material decays (through fission or splitting apart of U235 atoms) to increase exponentially. A chain reaction occurs. Fission of one atom produces particles that fly apart and sometimes collide with other large, unstable radioactive atoms—or rather nuclei. In other words, a given unstable radioactive atom may split apart when it is whacked by something, in turn producing other particles, some of which may whack neighboring nuclei.

But when radioactive atoms are few and far between, or when a rapid chain reaction is not ignited… how does an unexcited Uranium atom split all by itself? Or maybe it lies in its unstable condition until it is whacked. Can Brownian motion do the trick at high enough temperatures? Or particles from the sun?

Do we know that a lone radioactive atom will eventually split by itself at zero degrees Kelvin if shielded from all external radiation?

@ichthus The fission reaction is different from that of normal radioactive decay. Fission occurs when a thermal neutron enters the nucleus of a fissile atom, say U-235 or Pu-239, this causes the nucleus to split, releasing energy and more neutrons. The geometry of this reaction, control of the neutron flux and energy spectrum via reflectors, absorbers and moderators determine whether this is subcritical, prompt critical (like a nuclear reactor) or prompt supercritical (nuclear explosion). It’s all a matter of how neutrons of a certain energy level are created and directed.

There are some radioisotopes that spontaneously fission, but are generally not useful in nuclear reactions.

The higher the rate of spontaneous fission the faster the supercritical mass needs to be assembled or one runs the risk of a premature low-yield detonation. The propensity of Pu-240 to undergo spontaneous fission ruled out using plutonium in a gun-type bomb; the velocity necessary to fire the projectile portion of the fissile mass would have required too long of a barrel. The alternative would have been to isotopically separate out the Pu-240 from the reactor product, but then the whole reason plutonium was followed in parallel with uranium as bomb material by the Manhattan Project was to avoid the cost and effort of having to do this in the first place! So developing the implosion mechanism became key to utilizing plutonium.

An atomic nucleus is much like a liquid droplet with the protons and neutrons free to move about but at the same time strongly bonded together. Likewise, the protons and neutrons themselves consist of three quarks confined, but otherwise free to move about within a tiny volume. This composite swarm of quarks is constantly exchanging virtual particles* with one another, primarily gluons (within the protons and neutrons) and neutral pions (between the protons and neutrons). “Once in a blue moon” virtual particles associated with the weak nuclear force get exchanged and this can cause a spontaneous decay event.

*: in quantum field theories, all forces are manifested by the exchange of virtual particles.

EDIT: I’ve stepped out onto a limb with this one, and I doubt this answer is fully correct after consulting Wikipedia on this topic.

Thanks for the link to the article and for your response. The atom is a good deal more complicated than I had been led to believe. My simple picture I didn’t like as I couldn’t see how it could explain radioactive decay but an explanation is at least possible in a less simple model. The article isn’t clear on how radioactive decay occurs. On the one hand it says that arbitrarily small disturbances from quantum vacuum fluctuations may trigger the decay and the next sentence says a radioactive nucleus is unstable and can spontaneously decay. An arbitrarily small amount is not zero. But either way why the process should lead to a very precise half life is still very mysterious to me.

A physicist, though not a nuclear physicist, recently suggested to me that weak and strong forces within the nucleus create or are related to what I would describe as “wobble.” The model he seemed to present suggested that there is a statistical probability for any given unstable and radioactive nucleus that within any given period of time the wobble would add enough wave crests (so to speak) as to throw some nucleus contents out into an orbit that would permanently leave the nucleus.

I read something similar in a book about Paul Dirac recently. It would mean that atoms are not identical but like snowflakes no two are quite the same . That in itself would be quite surprising.