How accurate is an atomic clock...?  Is it accurate enough...?

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Ok, get this:

How an Atomic Clock Works (Theory)

The atom can be pictured as a mini solar system, with the heavy nucleus at the centre surrounded by electrons in a variety of different orbits.

The orbits correspond to energy levels, and electrons can only move between levels when they absorb or release just the right amount of energy.

This energy is absorbed or released in the form of electromagnetic radiation, the frequency of which depends on the difference in energy between the two levels.

This transition is the source of the term "quantum jump", quantum referring to the tiny but precise amount of energy needed to allow the electron to jump to a different level.

By measuring the frequency of the electromagnetic radiation, like counting the number of pendulum swings on a pendulum, we can measure the passage of time

So what is a second?  Currently this is the exact definition:

Since 1967, the International System of Units (SI) has defined the second as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels of the ground state of the caesium-133 atom.

In other words a caesium atom occilates more than 9 trillion times a second and we can measure that exactly.  That boggles the mind and it is by no means the fastest thing in the universe.  Plus, it never changes.  In theory at least, a million years into the future and in another galaxy, a caesium atom should occilate exactly the same amount of times.  Ok, back to the clocks.   How accurate are these clocks?

By tossing caesium clouds upwards over the course of a day and averaging the resulting frequency, the most accurate caesium-fountain clocks, the NIST-F1 clock in Boulder and similar devices at the Reference Systems for Time and Space (SYRTE) lab of the Paris Observatory in France, can now keep time with an accuracy of 1 second in around 80 million years

You'd think that'd be enough for scientists, but they are developing ever more accurate clocks.

With caesium, that is about the best that can be achieved. But caesium is by no means the fastest atomic oscillator. It was chosen as a matter of convenience: it was easy to excite using microwaves, and its oscillation produced a signal of the right frequency to be fed into the counters used in existing microelectronic circuits. Other atoms have transitions 100,000 times quicker, and so might make more precise clocks

...in March 2008, NIST compared a single-ion mercury transition with a transition in trapped aluminium atoms, obtaining accuracies of just 5 parts in 10¹⁷ - about 1 second in 650 million years (Science, vol 319, p 1808). Caesium's 53-year reign as king of the clocks was over.

As clocks get more accurate strange things start to occur. You see, Einstiens theory of general relativity (the best we have to describe how space, time & gravity interact) tells us that:

a clock will tick faster by 1 second in 10¹⁸ for every centimetre it is raised in Earth's gravitational field.  

So if we get clocks any more accurate that they are now...

the time it told would be different according to how far up the wall it was fixed.

But lets one up that; maybe there is no time at all:

The trouble with time started a century ago, when Einstein's special and general theories of relativity demolished the idea of time as a universal constant. One consequence is that the past, present, and future are not absolutes. Einstein's theories also opened a rift in physics because the rules of general relativity (which describe gravity and the large-scale structure of the cosmos) seem incompatible with those of quantum physics (which govern the realm of the tiny). Some four decades ago, the renowned physicist John Wheeler, then at Princeton, and the late Bryce DeWitt, then at the University of North Carolina, developed an extraordinary equation that provides a possible framework for unifying relativity and quantum mechanics. But the Wheeler-­DeWitt equation has always been controversial, in part because it adds yet another, even more baffling twist to our understanding of time.

"One finds that time just disappears from the Wheeler-DeWitt equation," says Carlo Rovelli, a physicist at the University of the Mediterranean in Marseille, France. "It is an issue that many theorists have puzzled about. It may be that the best way to think about quantum reality is to give up the notion of time--that the fundamental description of the universe must be timeless."

No one has yet succeeded in using the Wheeler-DeWitt equation to integrate quantum theory with general relativity. Nevertheless, a sizable minority of physicists, Rovelli included, believe that any successful merger of the two great masterpieces of 20th-century physics will inevitably describe a universe in which, ultimately, there is no time.

No time like the present, I always say.

Stay Tuned...