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

100 thoughts on “The Best Test of General Relativity (by 2 Misplaced Satellites)

  1. I’d be interested to know your thoughts on this format. Would you like to see more ‘Talking to scientists about their recent publications on Skype’? Maybe not on this channel but on Sciencium perhaps?

  2. Why should we, ordinary citizens, care? It is your (scientist) job to check theories and hypothesis of science. We, as average citizens, simply do not care about best experiment for proving general relativity. That is your job. Stop wasting our time.

  3. This is a really nice video, but it bugs me that in the animations of orbits, Earth's size was not to scale, particularly as the orbits should not have been so big that a rescaling of Earth was appropriate for the sake of clarity.
    And realistic representations of astronomical size and distance relations are much too rare in the public.
    Could this not be incorporated in future videos, at least where it is not too bad, say on a scale of Earth and the Moon (of course not on a scale of the solar system)?

  4. So….. does actual time change, or does gravity only affect the way we measure it? Per the experiment it appears the result proves gravity affects the wavelength of the matter used to measure time. Since we simply use means to measure how long an interval between events is, should not the interval between two specific events be exactly the same no matter how you measure it? The only thing that changes is your perception based on the instrument you are using.

  5. Shouldn´t we experience the same kind of time dilation phenomenon during earth´s path arround the sun since earth´s orbit is also eliptical?

  6. How are they getting around the speed effects of relativity? Shouldn't the increased speed closer to the planet exactly negate the gravity well effect?

  7. Derek, you're not measuring time with a clock. you're just measuring motion – the tick. Just because it moves hands and shows you numbers doesn't mean you're measuring time. Time, in the case of a clock, is a man-made perception and invention. The entire reason clocks where invented is because without them, everyone perceives "time" at different rates (the more distracted you are, the slower your personal time passes). you could do the same experiment with a metronome and check to see how much faster or slower the metronome is moving compared to a metronome on earth. Time isn't something you can move through, it's a measure of perceived change.

  8. respected sir I really like your videos and I shall be highly obliged if you read the comment
    sir I want to what major u took as physics and I always wanted a mentor like is there any way I could in touch pls sir

  9. i get the satellite clock and earth clock but sending a microwave to distant observer at 6:03, red shift in such waves is due to expansion of universe, right and that is over large huge distances… otherwise the wave travels at the same speed always in vacuum and hence same number of oscillations and same time

  10. Why doesn't this happen with the clocks on earth if we have an elliptical orbit around the sun? isn´t it posible for us to measure that?

  11. So, for simple humans like me, how much the difference is, in seconds (or obviously much less), between the two clocks of the satellites? I mean, for a second on earth, how much passes on that satellite?

  12. I would like to know how they can compare two clocks to such precission on two different places? Because it is impossible to do two things at the same time without uncertantiy. Even speed of light has its limit for transforming information for syncing two clocks. Even though this would be possible have we checked precission of hydrogen clock on different altitudes from earth?

  13. There is of course the fact that at perigee the satelite goes faster, so that, again, has an opposite effect.

  14. Hi there, thank you for your videos! Could you explain how the clocks just start ticking slower or faster, what changes its internal mechanism so that its hands move slower or faster?
    Thank you again, congrats from Portugal!

  15. if the effect is truly universal then the distant observer recognizing the shifts existence would not experience it from their prospective. assuming the distance is divisible by two and relativity is based on a static observers individual perspective.

  16. What happens to time in darkness?
    I have been searching it for hrs but nothing found
    I thought that there must be some slight difference

  17. 8:50 Relativity not the ultimate theory?! WHAT?! ofc it is! theres nothing better out there, theres just more missing information and forces we don't know about yet. But they are ALL relative to the per-portion of other forces around them, every single one. Even quantum entanglement.

  18. Gravitational difference may have made the clock to tick at different rate. But does it necessarily mean time has changed speed as well? Put a magnet near my watch and it could affect it's speed. What's the difference?

  19. Interesting, how did you cancel out the moons variable gravity shift, due to its rotation around the earth, and the sun's gravity shifts due to the earth's rotation around it. Also, the earth and sun's gravity is not constant, gravity is stronger and weaker on the surface on both objects. Or is your experiment an estimate and not meant to be taken as an exact measurement?

  20. "we had been planning something like this for a while"


  21. this video is the best that happened this week xD my dad once nearly fought me by trying to prove this wrong… im LOVING this. thank you.

  22. What guarantees that red shift is not due to a frequency drift together with the frequency to which the hydrogen atom resonates to due to external gravitational forces?

  23. So does this mean you'd age faster? Or does this mean since there is less gravity, there less mechanical friction in the clocks, to make it move faster? I'm confused haha

  24. Tell this to Dr Jason Lisle PhD (astrophysics) who, after 5 normal papers on solar dynamics, decided to return to his roots in creation "science" and inerrancy in bible BS. He reckons he has solved the "distant starlight problem" (naturally only a problem to YECs) by stating that light goes at different velocities away and towards us.

  25. Scientific experiment successful:

    Religious people who were wrong: this is unfortunate
    Scientists who were right: this is unfortunate

  26. I love how so many things in science follow this format:

    "Well, we set out to do this one thing… and it broke. Badly. So badly, in fact, that we can no longer do the thing we spent so much effort trying to do.

    BUT there's this OTHER really cool thing that we can TOTALLY do with the broken first experiment now, and it's completely by accident!!"

    So much stuff in science was discovered accidentally because of things like that, and it's fantastic.

  27. then why things float inbetween earth and moon.i mean moon rotate around earth because earth bends the space time around it..if we put an object in between earth and moon it is not going around either moon or same time earths gravity pulls moon .why is that??(sorry about my english if any mistakes)

  28. General relativity is true. Great respect to You and Einstein. You have made great experiments by yourself, and I trust You. Don't ask me to trust ESA, NASA, etc, if you want to keep that respect. They are proven liars.

  29. If i watch a 5-min video (say, on a mobile phone) at earth and in space, will I observe any difference in the length of the footage or rise/drop in the frame-rate?

  30. if the gravitation make the time past harder , then …. negative gravitation will make the time past backwards ?

  31. Understand the Quantum GR dimensions, hmmm? Scientist will never, until they look bigger to what’s right inside.

    Hurray, determined time, they did. Big discovery, might show them the way, hmmm maaaahahahha!

    Think bigger, think as big as one can see! Show you the way, it will!

  32. Since the satellite undergoes different rates of acceleration could this indicate that radiation wavelength is dependent on acceleration and time has nothing to do with it ?

  33. Time is not a clock. So a failure in a clock measured in 2 points in the space doesnt prove anything. Even if this clock is an atom clock.

  34. So if I put two fly's on two different satellites both orbiting Earth, one much further out than the other, knowing that both fly's would normally live approximately one week (on the Earth's surface) would I expect the fly on the furthest orbit to die before the other one? If so, does that prove that time is not a human construct but is in fact a property of reality?

  35. According to general theory of relativity, a body with mass stretches the space and thus making a wave to travel more on earth than a wave traveling in a space far away (time shift). If that is true, could we go far enough from celestial body that a wave travels no distance and time stops ultimately.

  36. k and what if u are on a centrifuge? i mean it feels like increased gravity, so shouldnt that affect you aswell? if no then that would mean acceleration-force wouldn't affect time dilation which it allegedly does. so how does it work in a centrifuge? someone explain pls ty

  37. @Veritasium; Could you actually fall into a black hole, or would the black hole 'die' before that happens?

    So according to general relativity, time 'slows down' in areas with a big gravitational pull (such as black holes).
    According to Hawking, black holes lose energy (thus mass) in the form of Hawking radiation but that proces is very slow, 10^70 years lets say (from our perspective).
    But since time slows down the closer you are to the center of a black hole, time needed for a blavk hole to 'die' would greatly decrease.
    My question is, by the time you get to the point of 'no return' or 'spaghettification' of the original black hole you were falling into, wouldn't that border move further towards the center of the black hole (from Hawking radiation), thus not killing you, and wouldn't this proces continue until the black hole evaporates, making you never able to fall passed a certain point?

    Does anyone know anything about this? All the articles about 'falling into a black hole' just say that after you've passed event horizon nothing can escape… I would love to get an answer to this question.

  38. when you say ''clocks tick slower in stronger gravity'' do you mean time itself or just clocks. i.e. would a digital clock run slower in stronger gravity

  39. Hi, thank you for a great video. Just wondering, was it really the photons, or could it be the solar wind with some electrons and protons that affect the trajectory? thanks

  40. To illustrate how strong gravity is no matter the distance. I have a short mind game for ya'll. Lets say there is nothing else in the universe than you and a pack of cigarettes. If we were to place the pack of cigarette as far out away as the known universe horizon. The gravity will pull you closer to that pack no matter the distance if there is just you and a pack of cigarettes existing in the universe. So what is reaching out that far in an instant like that?

  41. From penicillin to Viagra to satellites our mistakes have granted us some amazing discoveries. Too bad my mistakes aren't so beneficial…

  42. Hi Derek 🙂
    Really like your channel. Great stuff !
    One question. How come photons have momentum ?? @07:58


  44. @ 5:40 "Over a 30 million years, a clock like this, would not be out more than a second" … Well, that depends what you compare it to. A duplicate clock in the same place under the same conditions, I presume?
    And, apparently both gravity and speed influence the "passing" of time, but the difference is opposite and is not the same size? And what can you call the "normal" speed of time? The one without influence of any gravity?

  45. We actually know that GenRel and QM aren't complete, exactly because of their "quarrel" with each other. The line of investigation featured here, may not turn up any answers; then again, it just might.
    We have to try every avenue we can, though, to find one that will ultimately bear fruit.


  46. I always thought that time slowed down on the satellite and they had to make adjustments, proving that the faster one moves, slows down time. Like Einstein said about the speed of light. ??

  47. my orbital mechanics is a little rusty, but a satellite going from perigee to apogee slows a bit, and from apogee to perigee excellerates. would not this cause a kinematic time dilation slowing from perigee to apogee, faster from apogee to perigee, the same progression as gravity faster dilating time from perigee/apogee slower time dilation from apogee/perigee, adding to the observed?.

  48. That is such a typical skype thing when starting a call. "Can you see me?" "Can you hear me?" I laughed so hard at it

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