Similar to the portable atomic clock used for an experiment in the 1971 to prove Einstein's theory of relativity and...time travel: "Atomic clocks are extremely accurate clocks that can measure tiny amounts of time—billionths of a second. In 1971, scientists used these clocks to test Einstein's ideas. One atomic clock was set up on the ground, while another was sent around the world on a jet traveling at 600 mph. At the start, both clocks showed exactly the same time.
What happened when the clock flown around the world returned to the spot where the other clock was? As Einstein had predicted in a general way, the clocks no longer showed the same time—the clock on the jet was behind by a few billionths of a second. Why such a small difference? Well, 600 mph is fast but still just the tiniest fraction of the speed of light. To see any significant differences in time, you'd have to be traveling many millions of miles an hour faster." (source)
In recent hunts for oddities, I happened to stumble upon this 1960's Patek Philippe & Hewlett Packard Atomic Cesium clock! 
Auction info; HP 5061A Fantastic complex scientific instrument. Actually a mobile mass spectrometer monitoring the hyperfine transition frequency of Cesium isotope 133 as atoms are hit with microwaves, and uses that output to stabilize a quartz crystal oscillator.This was the most accurate clock on earth at the time of manufacture in the late 60's. Still hard to find a more accurate clock. Frequency stability per manual is 8x10 minus 13 power. Cost was around $60,000. HP still makes a very similar model for over $120k. For those of you not around in 1967 $60,000 would buy a luxury home. Has beautiful bevel crystal glass cover for the Patek Phillipe Swiss manufactured analog clock. Makes a solid tick sound with each second. Has 1 and 5 MhZ and 100KHz outputs and 1PPS outputs. Unit is fully functional and frequency locks in 12 minutes and stays locked. Cosmetically excellent. Cannot read hour meter, but appears to have had low usage based on condition inside and out,and function. Weight 67# before packing,18 3/8" X 16 3/4" X 8 1/4". Has space for internal backup battery, not included. Includes Original A/C and D/C plugs. External D/C power can be used for backup power. Repro Operation and Service manual included.
If you do get it, be sure to consider converting it to a very rare Atomic Wristwatch, like this guy-->Link
More info on Einstein's experiment-->Link and-->Link
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Probably one of the more interesting objects you could own from Albert Einstein. Einstein once said, "the separation between past, present, and future is only an illusion, although a convincing one." Too bad we can't ask him what a real timepiece should look like...
