The Best Test of General Relativity (by 2 Misplaced Satellites)

Okay... Hello. Hey. So this is good, this is good. You... you're working, can you see me? I can see you. Do you know what went wrong at... during the launch...? Yes, it's not complicated, but it's a long chain of events. On August 21, 2014, the European Space Agency launched two

satellites

. They are called Galileo Satellites 5 and 6. They were intended to be part of the Global Navigation Satellite System or GNSS. This is the European version of the American GPS systems. Now, after successfully taking off into space, the

satellites

were launched with 5-6 Russian rockets; The final stage of the rocket was to place the satellites in a circular orbit about 23,000 kilometers above Earth.

But that... was when something went wrong. Uhh. There was a thermal gap between a cold helium line and a propellant line. Then the propeller froze; This caused the altitude control thrusters to fail. The satellites seemed to have been injected in some random direction. He launched himself, but in the wrong direction. This sent the satellites into highly elliptical and seemingly useless orbits. At their lowest point, the satellites did not get a complete view of Earth. The ground sensors, which allowed them to orient their navigation antennas, stopped working because the Earth simply filled their field of view. At the other extreme, the satellites went too high and suffered significant radiation exposure due to the Van Allen belts.

There was a threat that they would simply shut them down. Well, who do we need to talk to to make sure they keep these satellites alive? They had an idea about how to use them to perform the

best

test

s of

general

relativity

to date; It was a stroke of luck for us, we had been proposing missions of this type. When we saw this accident happen, we were very happy, of course. The satellites now had propellant on board, intended to enable periodic course corrections over their expected 10-year lifespan. And they could use that fuel to try to correct their orbits.

So they used the onboard thruster. They did some of these maneuvers to get them into a stable orbit. But they didn't have enough to completely convert their elliptical orbits to circular ones. They couldn't circulate completely, fortunately for our project. According to

general

relativity

, compared to a reference clock, clocks run slower in stronger gravitational fields. That is, closer to large masses, like the Earth, deeper into gravitational wells, so the satellites' clocks should run faster relative to those on Earth, because they are in weaker gravitational fields. Here, I'm ignoring the special relativist. effect that works in reverse: making satellite clocks work slower than those on Earth, because they move much faster.

This velocity effect is very well proven, so we focus on the gravitational parts. The gravitational effect is difficult to measure precisely, for satellites in circular orbit, but satellites in elliptical orbit have an advantage: - In each orbit, they go from their lowest position (perigee) to the highest position (apogee) and vice versa. If you want to

test

the gravitational redshift of a clock, you need two things: you need a very precise clock, and you need a large change in gravitational potential. The satellite ranges from 17,000 to 26,000 kilometers. The altitude difference is almost 9000 (!!!) kilometers, which means they are rapidly and repeatedly going from a lower gravitational potential to a higher gravitational potential and back again.

So the clocks on board should run slower when they are closer to Earth, and then faster when they reach their highest point, and will continue to oscillate back and forth; slower and faster relative to Earth's clocks. If you compare a clock on the ground and a clock on the satellite, then you will have this time variation. Now, the nice thing about this is that it allows you to eliminate many sources of error, because I don't really care about the absolute accuracy of clocks; All you want to know is the difference between the tick rate at the low point compared to the high point.

And because it is the same clock that makes the measurements in both places, many errors, such as clock noise or systematic drift, can be eliminated, and that is what allows scientists to achieve such incredible precision. The reason we were able to do it is because it's a very predictable effect. Due to the eccentricity of the orbit, the signal we were looking for is actually modulation. All other effects - which are in other periods - will have no influence on your measurements. Now, I must point out that if you were traveling with the satellite, time for you would not speed up or slow down;

Time would be passing at a constant pace. You wouldn't be able to measure any change in the rate at which the clock ticks. Relativity arises when you compare two clocks that are far enough apart to feel the curvature of space and time. The satellite clocks (there are a couple different types) are all atomic clocks. The main clock is usually a passive hydrogen MASER clock. A MASER: It is like a laser, except it uses microwaves. Hydrogen atoms interact with a specific microwave frequency: a photon of precisely this frequency will reverse the spin of an electron. So by tuning the microwaves to interact better with the hydrogen atoms and then counting exactly this number of cycles of that radiation, that's one second.

You can track time with incredible stability. In fact, more than 30 million years ago, a clock like this would not be delayed by more than one second. Again, if you were to travel with the satellite, you would always observe that the frequency of this radiation is the same. But, if you were to send this radiation to a distant observer who is not in a strong gravitational field, they would observe that the frequency of your microwaves is slightly lower than that of their hydrogen maser. In other words: redshift. And the closer the satellite is to Earth, the more redshifted the microwaves will appear.

And therefore time would pass more slowly, in relation to that distant observer. Locally no relativistic effect is seen. This is called the equivalence principle. Only when you compare the satellite clock with one on Earth do you find the yo-yo speed of the satellite clock due to its oscillation back and forth in the Earth's gravitational well. This gravitational redshift was previously measured with greater precision in 1976. That's right, for more than 40 years, we have not improved our measurement of the gravitational effect over time. In 1976, Gravity Probe A was launched aboard a suborbital rocket: it ascended in a parabolic trajectory, reaching a maximum altitude of 10,000 kilometers, and then descended.

That gives it quite a bit of modulation and gravitational potential. All the time it was in contact with the Earth, through the microwave signal from the onboard hydrogen maser. And then they did a direct comparison of frequencies, so they actually made a two-way microwave link. This allowed a direct comparison of the rate at which a clock on a rocket would tick, relative to a clock on Earth. The results matched the predictions of general relativity up to 140 parts per million. The scientists I'm talking to finally managed to convince those responsible for the satellites to let them use their misfortune to test the gravitational redshift predictions of general relativity.

But actually carrying out the tests was not easy; One of the biggest sources of error was the positions of the satellites. You would think that, in the vacuum of space, satellites would maintain their orbits perfectly. But that ignores the power of sunlight. Photons from the sun, which bounce off satellites, are the biggest source of error. That's right, the momentum from the photons hitting the satellites was enough to significantly impact the measurements. Careful modeling and laser ranging of the satellites reduced the orbital uncertainties to an acceptable level. One way to improve statistics was to collect data for more than a thousand days.

That's almost 3 years. Unlike Gravity Probe A, which spent only 2 hours in space. So... what did they find? I think we both agree that we don't prove Relativity. It is not that we confirm General Relativity. Unfortunately, they were able to reduce the uncertainty in the measurement by a factor of 5 with respect to gravity probe A. So it's a new high score for the first one. in more than 40 years But what's the point? You say "unfortunately." Yes. You know that what we are looking for is a deviation from General Relativity. Because we know that this is not the definitive error.

History has taught us that new physics is always at the limits. that increasingly precise tests sometimes reveal completely new aspects of nature, which we would never have observed if we had not made the effort to look, and that there are good reasons to believe that General Relativity may not be the whole story, both it and Mechanics. Quantum. They are spectacularly successful theories in their own right. But for almost a century, all attempts to merge the two have been more or less failures. Furthermore, our current view of the world includes dark energy and matter, which make up more than 90% of everything in the universe.

The fact that all of this is below is beyond understanding, it tells me that maybe we don't know everything about gravity yet. As more tests are planned to further test General Relativity and determine if there is a test that can pass, a cold cesium atomic clock is ready to fly on the international space station and aims to reduce the drift by an additional factor of 10 In satellites, their orbits became more circular thanks to the thrusters on board, although they remain elliptical. Navigation signals have been tested and are within acceptable parameters. At the moment they are restricted to 'test mode' pending some new software and ground modifications, but the hope is that they will be useful for navigation after all and in the meantime hold the record for achieving the

best

test so far. from General Relativity Hey, this episode of Veritasium was supported by viewers like you on Patreon and Audible.

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