What are the real-life applications of General Relativity?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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!

Answer 5

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

Answer 6

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.