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As we all know, the most famous application of GR and SR is the GPS guidance system, where time dilation can be corrected. But it seemed that everywhere I go, GPS is the only answer to how GR can be used in real life, and to me sounds kind of sad. Are there any other possible real-life applications of GR?

To narrow the discussion, I would like to constrain the answer with the restriction that: any daily life phenomenon which can be explained by GR should be avoided like most of the answers that can be found on this website: https://www.livescience.com/58245-theory-of-relativity-in-real-life.html#:~:text=The%20theory%20explains%20the%20behavior,planet%20Mercury%20in%20its%20orbit.

I want examples that are artificially applied to human technology and would be able to benefit most, if not all, of mankind.

Hopefully, someone here could answer my question.

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    $\begingroup$ Physics is about trying to understand how nature works, not about making new technology. As currently written, this looks like a question about engineering, not about physics. Would Engineering.SE be a better home? Or if this question meant to help you understand something about general relativity, can you clarify what you're trying to understand? $\endgroup$ Sep 12, 2021 at 13:36
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    $\begingroup$ I don't think there are any practical applications of GR other than GPS (at present). GR is more relevant for cosmological observations. $\endgroup$
    – KP99
    Sep 12, 2021 at 14:23
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    $\begingroup$ @KP99 I'm not sure whether you'd call it a practical application but the whole field of gravitational wave astronomy is kind of a practical application. As in it is a new technology to map the sky that we couldn't have had without GR. Of course, this technology is not useful in everyday life. $\endgroup$
    – ACat
    Sep 12, 2021 at 15:36
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    $\begingroup$ To the OP: One can learn physics just fine without knowing anything about its applications. In order to discover new physics, you need experiments and they need technology. But, you don't need GR based technology to discover new GR related things. As long as there is some technology that enables you to do the desired experiments, be it EM based or optics based, it's perfectly fine. $\endgroup$
    – ACat
    Sep 12, 2021 at 15:42
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    $\begingroup$ Re, "No one learns physics without knowing any applications." Serious scientific investigation into electricity and magnetism started circa 1600. The first practical applications (telegraphy, outdoor lighting, electrochemistry, etc.) did not appear until around two hundred years later. $\endgroup$ Sep 12, 2021 at 22:19

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One wouldn't get very far in many engineering projects without taking gravity into account, and general relativity is our best theory of gravity, so in that sense there are many practical applications of GR indeed. But I assume you want to ask about scenarios in which the Newtonian approximation to GR is insufficient. Those are rare, for the good reason that Newtonian gravity is a very good approximation to GR for the relatively weak gravity we encounter in our everyday lives.

Time dilation is the most obvious difference between Newtonian gravity and GR, and that can be directly measured even by amateurs. For example, this person took his family on a trip up a mountain with some atomic clocks bought from e-bay, and was able to measure the extra time they gained (around 22 nanoseconds). That was done a few years ago, and using clocks bought from e-bay. State of the art atomic clocks can detect different time rates arising from height differences of less than a meter (see for example this story).

Edited to add: 22 nanoseconds may not seem like much, but that was over only 3 days -- over a longer period larger errors would accumulate. There are applications (such as high frequency trading) where microsecond accuracy of clocks is required, and the global timekeeping apparatus definitely depends on knowledge of relativistic effects to keep clocks in sync.

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Aside from pure scientific knowledge, there are no applications of GR as far as I know - at least if applications are limited to things that we couldn't have had if we didn't understand GR.

That includes GPS, since clock drift in the satellites can be measured and corrected whether the cause is known or not.

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    $\begingroup$ The clock drift would have made the GPS unusable though until the cause (and magnitude) was figured out. As it was the GPS was able to work from day one because all relativistic effects were properly accounted for. $\endgroup$
    – Eric Smith
    Sep 12, 2021 at 23:51
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    $\begingroup$ @EricSmith There wasn't really a day one. Relativity in the Global Positioning System says that when NTS-2 was launched, "it was recognized that orbiting clocks would require a relativistic correction, but there was uncertainty as to its magnitude as well as its sign", so they made it tunable in orbit, and measured the effect empirically (which was easy, since there was an atomic clock on board). (cont'd) $\endgroup$
    – benrg
    Sep 13, 2021 at 5:49
  • $\begingroup$ @EricSmith The article also says that satellites containing rubidium clocks were still (in 2003) tuned in orbit "as these are subject to unpredictable frequency jumps during launch". They tune the cesium clocks on the ground, but they don't have to. And they could tune them on the ground using empirical data from previous satellites even if they didn't know the reason for the effect. It's admittedly an odd counterfactual because I can't imagine humans with this technology not having figured out GR yet, even if Einstein had never existed. $\endgroup$
    – benrg
    Sep 13, 2021 at 5:49
  • $\begingroup$ Eh... I think "GR and SR are needed to correct the GPS signal, with correction factors that are fit empirically" is a better description of what's going on that "there's a weird effect we worked out has a certain parametric dependence on velocity and height that we can correct empirically." Even in a hypothetical universe where the latter was discovered to work without knowing GR/SR, I think people would be very uncomfortable with this unless it was explained physically and there would be more uncertainty in the prediction. $\endgroup$
    – Andrew
    Sep 13, 2021 at 15:07
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Space craft (S/C)navigation is one, esp. to the gas giants. All interplanetary missions use parameterized post Newtonian formalism (https://en.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism) or something similar.

For Mars mission, the nav teams delivers the S/C to the "B-plane" (The B-plane is a roughly 1 square km imaginary target:https://www.jpl.nasa.gov/images/mars-2020s-b-plane). They promise an S/C will come though it within a one second window, at which point it becomes Entry-Descent-Landing's problem. Straight newtonian physics is not good enough for that.

Interplanetary navigation also uses delta-DOR (https://en.wikipedia.org/wiki/Delta-DOR), which is similar to differential GPS, but it doesn't compare satellite signals, it uses quasars. Nobel laureate Richard Smalley described it as both, "The only particle use of black holes" and "Jonny Cool stuff".

Finally, the entire coordinate system, JPL's barycentric dynamical time (https://en.wikipedia.org/wiki/Barycentric_Dynamical_Time), used for coordinating position and timing for orbit insertions, attitude burns, communication links, instrument observations, and everything else, is rife with general relativity.

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Negatively charged pi mesons (pions) have been used experimentally in radiotherapy for cancer treatment. They are unstable particles with a half-life of only 26 nanoseconds, which means that, if not for general relativity, a pion beam whose particles are traveling near light speed would decay by 50% every 30 cm. Within a few meters from their creation, almost no pions would be left for theraputic purposes. But in fact, general relativity's time dilation allows pions near light speed to travel hundreds of meters before they decay by a significant amount, making radiosurgery with intact pions feasible.

EDIT: As Ben51 points out, this is an example of special relativity, not general relativity. Apparently, it's all "relative" to me!

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    $\begingroup$ That’s SR, not GR $\endgroup$
    – Ben51
    Sep 13, 2021 at 1:25
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There are quite a few applications of GR, outside of pure scientific study. Every measurement done in a gravitational field, of time and/or space is affected, so if you want precise measurements of e.g. your position, you have to take GR into account.

Synchronising clocks is also an important aspect as mentioned previously.

Space travel is one other thing where GR takes a not insignificant seat. The perhelion of Mercury and all planets is precessing. This is an effect with a measurable GR component. If you want to do precision space travel and narrow slingshot manoeuvres you ought to take that into account.

Finally, if we ever do find a black hole at a safe distance, that's spinning sufficiently fast, we will be able to exploit that to our advantage, and extract huge amounts of energy from it. Penrose process

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    $\begingroup$ @AndrewSteane You are correct. Fixed it. $\endgroup$ Sep 16, 2021 at 20:19
  • $\begingroup$ FWIW, Newtonian gravity gives Mercury one precession cycle per ~243,460 years. General relativity speeds up the precession, so we get one extra precession cycle per ~3 million years. See astronomy.stackexchange.com/q/44654/16685 $\endgroup$
    – PM 2Ring
    Sep 16, 2021 at 21:04
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I don't think that GPS's devices need GR to work. They evolve from ship's navigation systems, and rely on differences of $\Delta t$ between device and different satellites. Not in the difference of the time of the device and the time of each satellite (not each $\Delta t$). In this case GR would be essential.

One practical application of GR is gravitational lensing, what allows to observe galaxies too faint to be seen even with the best telescopes. The path of light across gravitational fields is one of the results from GR.

Well, it is useful to astronomers, and they are part of mankind.

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