There’s no such factor as absolute time. Irrespective of the place you might be, how briskly you’re transferring, or how sturdy the gravitational area is round you, any clock you’ve gotten on you’ll all the time file time as passing on the identical price: one second per second. For any solitary observer, time merely flows.
However when you have two totally different clocks, you’ll be able to evaluate how time flows beneath totally different circumstances. If one clock stays stationary whereas the opposite travels rapidly, the fast-moving clock will expertise a smaller period of time passing than the stationary clock: that’s the rule of time dilation in particular relativity.
What’s much more counterintuitive, nevertheless, is that the relative movement of time additionally is dependent upon the distinction between how severely area is curved between two areas. In Common Relativity, this corresponds to the energy of gravity at your explicit location, which signifies that your ft really age at a special price than your head once you’re standing up. Right here’s the physics of how we all know.
One of many issues we depend on is that the legal guidelines of physics are common. Whereas the properties of the Universe may change with time, with power, or together with your location, the foundations and the basic constants that govern it stay the identical. A hydrogen atom positioned wherever within the Universe will all the time have electron transitions occurring on the identical energies, and the quanta of sunshine they emit will likely be indistinguishable from another hydrogen atom within the Universe.
The identical factor is true for ionic, molecular, and even nuclear transitions: the legal guidelines of physics stay the identical always and all locations, and so these transitions that emit or take in photons all the time happen on the identical power. Nonetheless, if the emitter of a photon and the (potential) absorber of a photon aren’t positioned on the identical time and place as each other, there’s a superb likelihood that they received’t agree on the energies they observe.
When it’s as a result of the objects are in relative movement with respect to 1 one other, we all know this impact as a Doppler shift. Most of us expertise the Doppler shift each time we hear an emergency automobile (or an ice cream truck) both approaching us or transferring away from us: we are able to hear the pitch of the siren change. If the automobile is approaching you, its waves will seem like shifted nearer collectively, and also you’ll hear the next pitch; if it’s transferring away from you, its waves will likely be shifted to reach spaced farther aside, and also you hear a decrease pitch.
For mild, it’s a virtually equivalent situation: if the supply and observer are transferring away from each other, the sunshine will get shifted in the direction of longer (redder) wavelengths, whereas in the event that they’re transferring in the direction of each other, the sunshine will get shifted in the direction of shorter (bluer) wavelengths.
Now, right here’s the place issues get bizarre: this identical sort of shift also needs to happen — even when everyone seems to be stationary — when your gravitational area energy modifications from one location to a different.
Simply as you’ll be able to have Doppler redshifts and blueshifts for mild, you can too have gravitational redshifts and blueshifts. For instance, if you happen to ship a photon from the Solar to the Earth, as a result of the Solar’s gravitational area dominates the Photo voltaic System and is stronger close to the Solar than farther away, that photon will lose power (and turn out to be “redder”) because it travels from the Solar to the Earth. If it had been to go in the other way, from the Earth to the Solar, the photon would achieve power and turn out to be “bluer” in shade.
There have been a variety of doubters within the physics neighborhood who thought that this concept — of a gravitational redshift — was utterly unphysical. It’s intricately associated to the speed at which clocks run: the variety of wave “crests” that cross by your location over any time interval decide the frequency of the sunshine you obtain, and if gravitational redshifts are actual, then sending a photon greater or decrease in a gravitational area ought to result in observable penalties. Which means, as is the case for many physics predictions, there’s a strategy to take a look at it.
Let’s say you induce a quantum transition. Both an electron shifts in power ranges or an excited nucleus reconfigures itself, releasing an lively photon. You probably have the same atom (or atomic nucleus) close by, it ought to be capable of take in that photon, as the identical physics that leads to the emission of a photon may result in the reverse course of: the absorption of that photon.
If, nevertheless, you shift the photon to both longer or shorter wavelengths — no matter the way you do it — you received’t be capable of take in it anymore. The legal guidelines of the quantum Universe are fairly inflexible, and if a photon is available in with barely an excessive amount of or too little power, it received’t set off the right excitation.
This led to a outstanding experiment, the Pound-Rebka experiment, that sought to display and quantify the existence of gravitational redshift, and to show that point actually does run quicker at your head than at your ft.
What the experimenters did was arrange a photon-emitting supply inside a vertical tower, after which put that very same materials on the different finish of the tower. If there have been no gravitational redshift — i.e., if time ran on the identical price for everybody — then the fabric on the different finish of the tower ought to take in the photons emitted from the primary finish.
They didn’t, after all, as a result of they’d the unsuitable power, and therefore, the unsuitable wavelength.
However what Pound and Rebka did was arrange an oscillator (principally the inside of a speaker) that allowed them to “increase” the photon-emitting materials at one finish of the tower. In the event that they boosted it by simply the correct quantity, they reasoned, they may tune this induced Doppler shift to precisely cancel out the anticipated gravitational redshift. So far as time goes, it principally added an additional movement (and an additional little bit of time dilation) to compensate for the consequences that gravity introduces.
Hastily, when the proper frequencies had been reached, the (iron) atoms started absorbing these emitted photons from the opposite finish. The preliminary experiment confirmed Common Relativity’s predictions, and was subsequently improved upon by Pound and Snider all through the Sixties.
The general lesson is that this: for each meter of peak that you simply achieve, you want a Doppler shift of ~33 nanometers-per-second to compensate for it. It’s like being decrease on the floor of the Earth requires you to be in movement at a sure price simply to have time cross on the identical price as it could if you happen to had been greater. In different phrases, with out an additional little velocity increase at your ft — with out an additional period of time dilation added in — time passes extra rapidly at greater elevations in Earth’s gravitational area.
Your head, to be blunt, ages extra rapidly than your ft do.
However you are able to do even higher than these unique experiments: by measuring the passage of time immediately utilizing atomic clock expertise. The way in which we outline time has developed over the centuries; what used to depend upon the movement of the Earth rotating on its axis or revolving across the Solar has now been changed by an atomic definition. A second, as we all know it, is outlined by the cesium-133 atom.
In that atom, there’s a hyperfine transition that’s extremely exact, emitting a photon of a really explicit wavelength. That wave, if you happen to take 9,192,631,770 cycles of it, is our fashionable definition of the second.
And but, if you happen to took an atomic clock — whether or not based mostly on cesium, mercury, aluminum, or another ingredient — and moved it to a special elevation, that clock would run at a special price from its unique elevation: quicker at greater elevations (in a weaker gravitational area), slower at decrease elevations (in stronger gravitational fields).
This has been experimentally verified to astonishing precision, as we’ve detected these predicted shifts for peak variations as small as 0.33 meters (1 foot). In Earth’s comparatively weak gravitational area, this can be a outstanding achievement, demonstrating simply how correct timekeeping with atomic clocks has turn out to be.
But when we took this to a extra excessive setting, the consequences would turn out to be huge. No setting within the Universe is extra gravitationally excessive than a black gap. In case you approached its occasion horizon, time would cross so slowly for you that, in a single second (for you), centuries, millennia, and even eons might cross for somebody far-off.
It’s sufficient to make one fear that even when we might efficiently construct a wormhole, the extreme curvature of area might trigger your complete significant a part of the Universe — the place we’ve stars, galaxies, and attention-grabbing chemistry occurring — to cross by whereas the traveler handed by it.
In our Universe, time will cross the quickest for the observer who minimizes their movement by area and is positioned the place the curvature of area is as little as potential. In case you might journey to the area between galaxies, the place you’re far-off from any sources of mass, you’d age extra rapidly than anybody else. Right here on Earth, the farther you might be from the middle, the quicker time runs for you. The results are extraordinarily slight, however measurable, quantifiable, and strong.
This implies, if you happen to ever needed to time journey to the longer term, your finest guess won’t be to take a protracted, round-trip journey at practically the velocity of sunshine, however moderately to hang around the place there’s a variety of spatial curvature: close to a black gap or neutron star, as an example. The deeper right into a gravitational area you go, the slower time will run for you in comparison with those that are farther out. It would solely grant you a number of additional nanoseconds over your total life, however standing up — and conserving your head farther from Earth’s middle — actually offers you a little bit bit extra time than laying down.