Placeholder Image

字幕表 動画を再生する

  • 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 in-uh.. during the launch..?

  • Yes - it's not complicated, but, it's a long chain of events..

  • On august, 21., 2014, two satellites were launched by the European Space Agency.

  • They're called Galileo Satellites 5 and 6.

  • They were intended to become part of the Global Navigation Satellite System, or GNSS.

  • This is the European version of the American GPS systems.

  • Now, after successfully blasting off into space,

  • The satellites were launched with the Russian 5-6 rockets;

  • The final stage of the rocket was set to inject the satellites into circular orbit around 23000 kilometers above the Earth.

  • But that - - is when something went wrong

  • Uhh. there was thermal breach between a line of cold helium, and a line of propellant.

  • So, the propellant, it froze;

  • This caused the failure of the altitude control thrusters.

  • The satellites seemed to have been injected into some random direction.

  • It was launched, but in the wrong direction.

  • This sent the satellites into highly elliptical, and seemingly useless orbits.

  • At their low point, the satellites didn't really get a full view of earth..

  • Earth sensors, which enabled them to orient their navigation antennas - stopped working,

  • because the earth just filled their field of view.

  • At the other extreme, the satellites went too high, experiencing significant radiation exposure

  • due to the van Allen belts.

  • There was a threat, that they would just shut them off..

  • Okay, who do we have to talk to, to make sure they keep these satellites alive?

  • They had an idea, for how to use them to make the best tests of general relativity to date;

  • This was a strike of luck for us, we had been proposing such missions.

  • When we saw that this accident happened, we were very happy about it, of course.

  • Now, the satellites did have propellant on board,

  • intended to allow for periodic course corrections over their planned 10 year lifespan.

  • And they could use that fuel to attempt to correct their orbits.

  • So they used the propellant on board -

  • They did some of these maneuvers to bring them into stable orbit -

  • But they didn't have enough to completely turn their elliptical orbits into circular ones.

  • They couldn't circularize completely, thankfully for our project.

  • According to general relativity, relative to a reference clock, clocks tick slower in stronger gravitational fields.

  • That is to say, closer to large masses, like the Earth, deeper in gravitational wells,

  • so, clocks in satellites should tick faster relative to those on Earth,

  • because they're in weaker gravitational fields

  • Here, I'm ignoring the special relativistic effect that works the other way -

  • making clocks in satellites tick slower than those on Earth,

  • because they're moving so much faster.

  • This velocity effect is very well tested, so we focused on the gravitational parts.

  • The gravitational effect is hard to measure precisely, for satellites in circular orbit,

  • but satellites in elliptical orbit, have an advantage - -

  • In every orbit -

  • they go from their lowest position - perigee,

  • to the highest position - apogee, and back.

  • If you want to test the gravitational red-shift of a clock, you need two things - -

  • you need a very accurate clock, and you need a large change in gravitational potential.

  • The satellite goes from 17000 to 26000 kilometers.

  • The difference in altitude is almost 9000 (!!!) kilometers,

  • meaning they are rapidly, and repeatedly going from lower gravitational potential,

  • to higher gravitational potential, and back.

  • So clocks on board, should be ticking slower when they're closer to the Earth,

  • and then faster when they go to their high point,

  • and they'll keep oscillating back and forth; slower and faster relative to clocks on Earth.

  • If you compare a clock on the ground, and a clock on the satellite,

  • then you will have this variation of time..

  • Now, what's great about this, is it allows you to eliminate a lot of sources of error -

  • because, you don't really care about the absolute accuracy of the clocks;

  • All you want to know, is the difference between the rate of ticking

  • at the low point compared to the high point.

  • And since it's the same clock making the measurements at both locations,

  • you can eliminate a lot of errors, like the noise in the clock, or a systematic drift,

  • and that's what allows the scientists to achieve such incredible precision.

  • Why we could do it - is because it's a very predictable effect.

  • Due to the eccentricity of the orbit - the signal we were looking for, is really the modulation.

  • All the other effects - that are at other periods - they will not have an influence on your measurements.

  • Now, I should point out, that if you were traveling with the satellite,

  • for you - time would not speed up and slow down; time would be passing at a constant rate.

  • You wouldn't be able to measure any change in the rate the clock is ticking.

  • The relativity comes when you compare two clocks which are distant enough to feel the curvature of space and time.

  • The clocks on the satellites - there are a couple of different types - are all atomic clocks.

  • The primary clock typically is a passive hydrogen MASER clock.

  • A MASER - is just like a laser, except it uses microwaves.

  • Atoms of hydrogen interact with one specific frequency of microwaves -

  • A photon of precisely this frequency, will flip the spin of an electron.

  • So by tuning the microwaves, so that they best interact with the hydrogen atoms,

  • and then counting up exactly this number of cycles of that radiation,

  • that's one second. You can keep track of time with incredible stability.

  • In fact, over a 30 million years, a clock like this -

  • would not be out by more than a second.

  • Again, if you were traveling with the satellite, you would always observe the frequency of this radiation to be

  • the same. But, if you sent this radiation out - -

  • to a distant observer -

  • who is not in a strong gravitational field,

  • they would observe the frequency of your microwaves to be slightly lower than those from their hydrogen maser.

  • In other words - red-shifted.

  • And the closer the satellite is to Earth, the more red-shifted the microwaves would appear.

  • And hence the slower time would pass, relative to that distant observer.

  • Locally, you can not see any relativistic effect

  • This is called the equivalence principle.

  • It's only when you compare the satellite clock to one on Earth,

  • that you would find the yo-yo-ing rate of the satellite's clock due to it's oscillation back and forth

  • in Earth's gravitational well.

  • This gravitational redshift was previously measured most precisely in 1976.

  • That's right, for over 40 years, we haven't improved our measurement of the gravitational effect on time.

  • In 1976, Gravity Probe A was launched aboard a sub-orbital rocket -

  • it went up in a parabolic trajectory, reaching a maximum altitude of 10000 kilometers,

  • and then it came down

  • That gives you quite a lot of modulation

  • and gravitational potential.

  • The whole time it was in contact with the Earth, through the microwave signal of the on-board hydrogen maser.

  • And then they did a direct frequency comparison, so they really did a two-way microwave link.

  • This allowed for a direct comparison of the rate of which a clock would tick on a rocket,

  • relative to a clock on Earth.

  • The results matched the predictions of general relativity down to a 140 parts per million.

  • The scientists I'm talking to were eventually able to convince those in charge of the satellites,

  • to let them use their misfortune to test the gravitational redshift predictions of general relativity.

  • But actually carrying out the tests, wasn't easy; one of the biggest sources of error was the positions of the satellites.

  • You'd think, in the emptiness of space, the satellites would perfectly

  • maintain their orbits.

  • But that neglects the power of sunlight.

  • The photons of the sun, bouncing on the satellites, is the biggest source of error.

  • That's right, the momentum of photons hitting the satellites was enough to significantly impact

  • the measurements. Careful modeling plus laser ranging to the satellites, brought the orbital uncertainties

  • down to an acceptable level.

  • One way to improve the statistics was by collecting data over 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 that we both agree that we did not prove Relativity

  • There isn't that we confirm the General Relativity

  • Unfortunately

  • They were able to reduce the uncertainty in the measurement

  • by a factor of 5

  • over Gravity Probe A So it's a new high score

  • the first in over 40 years

  • But what is the point? You saying 'unfortunately'

  • Yeah

  • You know what we're looking for, is deviation from

  • General Relativity. Because we know

  • this is not the ultimate error.

  • History has taught us that, new physics always lies at the boundaries.

  • that more and more precise tests sometimes

  • reveal brand new aspects of nature, that we never would've observed

  • if we had not made efforts to look

  • and there are good reasons to believe that General Relativity may not be the whole story

  • Both it and Quantum Mechanics are spectacularly

  • successful theories in their own rights

  • But for nearly a century, all attempts to merge the two

  • have been more or less failures. Plus

  • our current

  • world view includes dark energy and dark matter which make up more than 90% of

  • everything there is in the universe. The fact that this is all down

  • is not understood, tells me that maybe, we don't know

  • everything about gravity already. When more tests are planned

  • to further prove the General Relativity and determine if there's a test it can pass

  • a cold cesium atom clock is set to fly in the international

  • space station and it aims to reduce the deviation

  • by a further factor of 10. As for the satellites,

  • their orbits were made more circular using the propellants on board

  • though they are still elliptical. The navigation signals have

  • been tested and are within acceptable parameters.

  • For the moment, they are restricted to 'test mode'

  • awaiting some new software and modifications on the ground, but

  • the hope is that they will be useful for navigation after all

  • and in the meantime, they hold the record for achieving

  • the best test yet of General Relativity

  • Hey this episode of Veritasium was supported by

  • viewers like you on Patreon and by audible

  • and for viewers of this video, audible is offering a

  • free audio book when you start a

  • 30 day free trial. Just go to audible.com/veritasium

  • or text veritasium to 500500. Now

  • the book, I wanna recommend to you today is called

  • 'Atomic Habits' by James Clear. We are

  • approaching that time of year when everyone makes new year's resolutions

  • and wants to change some big things

  • about their lives. But what this books recommends, is actually making

  • tiny changes consistently is

  • the best way to achieve the long term changes that you

  • wanna see in your life and I have found this book already, just a few chapters in

  • umm, really effective in terms of changing my thinking about how

  • I wanna change my life and the things I wanna get done So

  • I highly recommend it and you can download it for free by going to

  • audible.com/veritasium

  • or text veritasium to 500500

  • but audible doesn't just have audio books

  • they also have audio fitness programs and audible originals

  • and every month, you can get 2 audible originals from a

  • changing set of titles. Those are

  • audio programs made specifically for audible members

  • the way membership works, is that every month you get

  • a credit which is good for one

  • audio book and it also gives you exclusive access to those

  • other fitness programs and to audible

  • originals, so if you wanna check it out

  • go try audible.com/veritasium

  • I wanna thank you to audible for supporting me and I wanna thank you

  • for watching

Okay...

字幕と単語

動画の操作 ここで「動画」の調整と「字幕」の表示を設定することができます

B1 中級

一般相対性理論の最高のテスト (2つの見当違い衛星による) (The Best Test of General Relativity (by 2 Misplaced Satellites))

  • 0 0
    林宜悉 に公開 2021 年 01 月 14 日
動画の中の単語