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A pair of aluminum atomic clocks verify Einstein's twin paradox and time dilation

Scientists from NIST - National Institute of Standards and Technology in the USA are measuring two phenomena from Einstein's theory of relativity.

NIST scientists compared a pair of the finest atomic clocks available to prove that indeed, as relativity predicts, going up a flight of stairs accelerates your aging. Figure courtesy of NIST
NIST scientists compared a pair of the finest atomic clocks available to prove that indeed, as relativity predicts, going up a flight of stairs accelerates your aging. Figure courtesy of NIST

Almost half a century ago, experiments were already carried out with the help of cesium clocks to demonstrate Einstein's twin paradox. Hall of NIST scientists pulled out More accurate clocks. With their help, they demonstrated that we age almost as much if we stand at the top of the high peak.

The study was published today in an issue of Science:
CW Chou, DB Hume, T. Rosenband and DJ Wineland. Optical Clocks and Relativity. Science. September 24, 2010.

The NIST researchers recreated the time dilation experiment by comparing the activity of a pair of atomic clocks. These are the best experimental atomic clocks in the world. The clocks are almost identical to each other and are based on the "tick" of one single aluminum ion (an electrically charged atom) as it vibrates between two energy levels a million billion times per second. The aluminum ion is trapped between electric fields and vibrates at light and ultraviolet frequencies, which are 100,000 times higher than the microwave frequencies used by other atomic clocks, cesium clocks. The two aluminum clocks are so accurate: the first clock lags by one second only after 3.7 billion years and the second is close to it in performance. The two clocks are located in two separate laboratories at NIST and are connected by a 75 meter long optical fiber.

The clocks are called "logical quantum clocks" because they are based on a decision-making technique from the field of experimental quantum computing. They are accurate and stable enough to reveal minute differences that could not be seen until now. The clocks work by illuminating the ions with laser light at optical frequencies, which are higher than the microwave frequencies used in standard atomic clocks based on the cesium atom. The day will not be far off and optical clocks will be able to lead to timekeeping standards that are a hundred times more accurate than today's standard clocks.
The aluminum watches can detect small effects that are predicted by the theory of relativity because of their alarming accuracy and because of their high "Q factor" (no loss of energy): a quantity that indicates the reliability of the ion that absorbs and preserves the optical energy during the change from one energy level to another.

The NIST experiments focused on two cases predicted by the theory of relativity:

1) Gravity: When two clocks are subject to two unequal gravitational forces due to different heights above the Earth's surface, the higher clock above the Earth's surface (which is the one that feels a weaker gravitational force) moves faster. while the lower clock moves slower.

2) Slow motion and accelerated motion: the famous twin paradox. The Twin Paradox: Two twins, who are initially at rest on Earth synchronize their clocks. The first twin remains at rest on Earth throughout the experiment, while the second twin enters a spaceship and travels with it on a long trip through space: initially it accelerates to a speed very close to the speed of light in a negligible amount of time (it turns on its spaceship's engines). Then it continues to travel for a long time at a constant speed. Upon reaching its destination, let's say a star far enough from the Earth, and located at some point, it very quickly slows down for a negligible amount of time and rotates (it turns on its engines again), until its speed is once again uniform in the opposite direction to the Earth. It then travels to Earth for a long time at a constant speed. At the meeting point with the first twin it immediately slows down to rest (it starts its engines again). Let's say that the first twin's clock measures 20 years when the second twin returned to Earth and for the first twin the journey in each direction therefore takes 10 years. On the round trip the second twin's clock will show only 16 years. The paradox started with special relativity and moved to general relativity. The decisive factor here is the acceleration - the passenger twin accelerates and decelerates while making the round trip.

The NIST scientists observed these effects by making certain changes to one of their two logical quantum clocks, the aluminum clocks: they caused a change in one of the ions and then they measured the difference in the relative frequency between the two ions, that is, the difference in the ticking rate between the two ions.

In one set of experiments, the scientists raised one of the clocks by raising the laser table to a height of a third of a meter (about one foot) above the height where the other clock was placed. The higher clock will "tick" at a slightly faster rate than the lower clock, just as expected. Therefore if you are higher you age faster. The researchers said the difference is too small for humans to feel: it is a total of 90 billionths of a second for every 79 years of life. Really not significant and you can stay to live up in the mountains or alternatively at the top of the tower.

In the second set of experiments, the researchers examined the effects of changing the physical movement of the ion in one hour. In the normal state, during the standard operation of the watch, the ion in the watch is almost completely motionless. The NIST scientists excited the ion in one of the clocks and made it gyroscopic so that it moved back and forth at speeds equivalent to several meters per second. The clock will tick at a rate that is slightly slower than the other clock, and this is as the theory of relativity predicts.- The moving ion acts exactly like the moving twin in the twin paradox.

And why are such experiments important? They are important to geodesy, the science of measuring the Earth and its gravitational field, with applications to geophysics and hydrology, and perhaps even applications to tests of fundamental physics theories.

It should be noted that this is not the discovery of a new fact, but rather a further verification of the predictions of the theory of relativity.
These predictions are already being used successfully by us, receiving daily confirmation, in various applications, perhaps the most prominent of which is the GPS.

More on:
Many articles in the world press

27 תגובות

  1. It is worth clarifying that the twin "paradox" can be explained without any connection to accelerations. To understand this intuitively, let's take an example of uniform acceleration to a constant speed, cruise at this speed to a point in space, and back with the same profile. Now pay attention to 3 points:
    1) If we double the acceleration rate then the acceleration time will be reduced by half, therefore, as a first approximation, it is likely that the effect on the clock will not change.
    2) If we double the distance to a point in space, the time change will double.
    3) All this is also true at relatively low speeds, if the distance to the point is large enough. In other words, the duration and speed have an effect, not the magnitude of the accelerations.

    In other words - the paradox exists, and is resolved in the theory of special relativity. In general, it should be emphasized that special relativity also deals with accelerated motion.

  2. deer,

    "2. There is a difference between different systems that are in different gravitational fields - in a system that is in a stronger gravitational field, time will pass more slowly."

    "In this article we talk about case 2 - a system located on the summit of Mount Everest (or at a height of 30 cm) experiences a weaker gravitational field than a system at sea level (or on the floor), therefore time will pass more slowly."

    In the second paragraph it should be "and therefore the time will pass faster in it"

  3. There is a fundamental problem here that has not been addressed.
    The experiment only proves an effect on the way we measure time.
    This does not mean that there is an effect on time itself. Time is not related to the aluminum ion and the electric fields and is not created as a result of it. Such a clock is only an accurate way of measuring time as we perceive it. It is quite possible to develop another clock that measures in a different way and has no effect on it as a result of gravity.
    In my understanding the paradox refers to something completely different.

  4. Is it correct to say that as we move faster, the mass increases, the gravitation increases and therefore time slows down?
    If it was theoretically possible to move fast without increasing the mass, wouldn't time slow down?

  5. Many thanks to Zvi, Ehud and Michael (and others) for the clear and fascinating comments. Often your comments help me, as a reader, to clarify articles and even expand beyond the subject of the news.

  6. Moti,

    The twin paradox has two meanings:
    1. Time will pass more slowly for the twin who traveled and will return to Israel (this is due to being in a system that will accelerate just as Ehud and Michael mentioned).
    2. There is a difference between different systems located in different gravitational fields - in a system located in a stronger gravitational field, time will pass more slowly.

    In fact, the two results are nothing but the same thing, since according to the principle of equivalence, inventions in a gravitational field are equivalent to inventions in an accelerated system.

    In this article we talk about case 2 - a system located on the summit of Mount Everest (or at a height of 30 cm) experiences a weaker gravitational field than a system at sea level (or on the floor), therefore time will pass more slowly.

  7. Another clarification regarding accelerations:
    Beyond the fact that the one who moves away and returns feels the acceleration itself, he also experiences the transmissions between them in a different way.
    The one who travels and returns begins to notice the change in the transmission frequency he receives from the one who is at rest - immediately the moment he starts to return while the one who is at rest will sense the fact that the other has started to return only after receiving the signals sent by the accelerator at the moment he started to return (that is - after this signal has traveled the entire separating distance Including.

  8. Moti:
    I see that Ehud has already answered you.
    As a principle, it is not possible for them to return and meet without at least one of them having been accelerated.

  9. Moti,

    In my opinion, the twin paradox was not explained well in the article and it is possible that this is the reason for your confusion. In the twin paradox, the twin who goes on a journey ages more slowly than the twin who stays on Earth, but on the other hand their movement is relative. The twin in the spaceship sees his twin on Earth moving relative to him (speed is relative), therefore according to his assessment the twin (the one on Earth) should age more slowly and not faster. Each apparent twin believes that the other twin should age more slowly, this is the paradox. But it is solved by the fact that there is no symmetry between the twins. One twin (the one in the spaceship) goes through a series of accelerations (and transitions) and therefore its condition is not equal to that of the twin that remains on Earth.

  10. Michael:
    With the help of two accurate clocks as described in the article, you can tell which body is moving and which body is stationary.
    One clock will be attached to a moving body and the other to a standing body, based on the differences between the clocks it is possible to distinguish who is really the moving body and who is standing. Each clock will transmit its time to the other and taking into account the speed of the communication transfer time.

  11. Joseph

    Your question is correct, but to get an estimate for the relativistic correction from special relativity you have to calculate the relative speed at which the point 30 cm higher moves with respect to the ground. A quick estimate (if I'm not mistaken) shows that this is a relative speed of 2 cm per second. To calculate the relative correction, divide this number by the speed of light and square it. The correction is an order of magnitude of 10 to the power of minus 20, much smaller than the accuracy capability in the said experiment

  12. A note about the accuracy of atomic clocks. One of the experts in this field in Israel is Prof. Yehuda Band from the Chemistry Department at Ben-Gurion University, who wrote an article about the different types of noise in atomic clocks and how much they limit the accuracy that can be expected.

    A note about GPS accuracy. The accuracy of GPS is not dictated by the accuracy of the clocks alone and indeed for these relativistic corrections are made (both special relativity and general relativity) but also (especially) by fluctuations in the atomic number density. Positioning on the ground is done by triangulation of signals from several satellites, when the satellite transmits its position and the time when pulses are sent. The time it will take for the pulse to reach the ground depends on the density of the range through which it passes, therefore the accuracy is limited by fluctuations in the density of the atmosphere. So that more accurate clocks will not necessarily contribute to the accuracy of GPS systems.

  13. I will try to answer several questions that arose in connection with the article, even though it is not really my area of ​​expertise. I am sure that Zvi can answer in a much more convincing and plausible way.
    First to Yehuda's question about factors that can create noise in an experiment and how to avoid them. The experimenters, who performed the experiment, are among the world's leading experimenters in the field and much of their expertise is related to the reduction and assessment of noise in the experiment so that Yehuda's questions will not surprise them.

    First, regarding the ability of temperature differences to affect the experiment. Time measurement in atomic clocks was done in an ultra-cold system cooled by lasers (the cooling methods are reminiscent of those used to produce Bose-Einstein condensation) There is no coupling at all between the system of atoms and the environment since the atoms are in an almost complete vacuum and at temperatures very close to absolute zero.

    Regarding gravitational effects of the sun, the moon or the scientists at the clocks. The experiment on the clocks was carried out on the surface of the earth with one clock raised by about 30 cm from the other clock. On the Earth's surface the main gravitational field is that of the Earth and the main gradient is in this field. The gravitational field of the moon, the sun or the scientists is so weak as to be negligible on the system and in particular is not sensitive to a 30 cm shift, not even at the level of accuracy measured in the experiment. Therefore there is no need to neutralize such effects in the experiment. To get an estimate of how negligible these gradients are in the range of 30 cm regarding the sun and the moon, suffice it to say that they are as small as one of the parts of the distance to the center of the object: sun, moon and proportional to the mass (which disqualifies the researchers from being a factor even if they are particularly fat).

  14. Kafir:
    Invest in what you want and let others invest in what they want.
    The investment here was not in verification but in the development of accurate watches. This investment has many uses.

  15. Moti:
    You shouldn't have been confused.
    One of the conclusions of Einstein's theory is that time is also relative. that's it.
    There is no possibility to distinguish between a body moving at a constant speed and a stationary body.
    If you indicate the way in which you think it is possible to conclude from the above which body is in steady motion and which is not - it will be possible to explain to you what your mistake is in more detail.

  16. So now I am confused, according to what I have learned, I understood that it is not possible to measure absolute speed and that speed is relative to something, or in other words there is no difference between a body moving at a constant speed and a stationary body.
    And according to this experiment, it is possible to measure absolute speed and there is a difference between a body moving at a constant speed and a stationary body.

  17. A fascinating but not noisy experiment.
    Maybe instead of investing money in repeated verifications - invest money in innovative experiments in far-reaching fields?

  18. Blackmail for the speed of publication.
    Since the great accuracy of the GPS systems really demonstrates the Torah every day, it seems to me that this experiment shows more the technology (the ability of accuracy that can be reached) than the science.

    Besides, it's a bit funny to require the twins to synchronize watches and look at the clock. The whole idea of ​​them being twins is that their clocks are synchronized from birth and it is enough for one of them to come back young when the other is a grandfather for them to realize that something happened.

  19. A change in time differences can affect the amount of visible light
    So a galaxy with a larger mass, time will pass more slowly in it,
    Therefore, the amount of light visible from it will decrease significantly.
    Which can explain the meaning of dark matter, as time differences.

  20. The article claims that "Gravity: When two clocks are subject to two unequal gravitational forces due to different heights above the Earth's surface, the higher clock above the Earth's surface (which is the one that feels a weaker gravitational force) moves faster. while the lower clock moves more slowly." As far as I remember from the theory of relativity, there is also an effect of the speed of the movement (referring to the movement relative to the position and not to the rate of change of time) on the time differences between the two clocks. Therefore, the clock located at a higher position on the surface of the earth has a greater speed (V) (due to the larger radius from the center of the Earth) and therefore it should move (rate of change of hands/time) more slowly. It is interesting how much this effect lowers the rate increase resulting from gravity.

  21. This experiment must be very precise. For example:- the gravity of the researchers can have an effect, a deviation in one of the clocks, temperature differences, the gravity of the entire structure around the experiment, and I believe that neither the position of the sun and the moon. It is difficult for me to understand how all the above effects were neutralized.
    But otherwise
    Have a good week and Happy Holidays everyone!
    Sabdarmish Yehuda

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