Wayward Satellites Test Einstein's Theory of General Relativity (scientificamerican.com) 99
An anonymous reader quotes a report from Scientific American: In August 2014 a rocket launched the fifth and sixth satellites of the Galileo global navigation system, the European Union's $11-billion answer to the U.S.'s GPS. But celebration turned to disappointment when it became clear that the satellites had been dropped off at the wrong cosmic "bus stops." Instead of being placed in circular orbits at stable altitudes, they were stranded in elliptical orbits useless for navigation. The mishap, however, offered a rare opportunity for a fundamental physics experiment. Two independent research teams -- one led by Pacome Delva of the Paris Observatory in France, the other by Sven Herrmann of the University of Bremen in Germany -- monitored the wayward satellites to look for holes in Einstein's general theory of relativity.
Einstein's theory predicts time will pass more slowly close to a massive object, which means that a clock on Earth's surface should tick at a more sluggish rate relative to one on a satellite in orbit. This time dilation is known as gravitational redshift. Any subtle deviation from this pattern might give physicists clues for a new theory that unifies gravity and quantum physics. Even after the Galileo satellites were nudged closer to circular orbits, they were still climbing and falling about 8,500 kilometers twice a day. Over the course of three years Delva's and Herrmann's teams watched how the resulting shifts in gravity altered the frequency of the satellites' super-accurate atomic clocks. In a previous gravitational redshift test, conducted in 1976, when the Gravity Probe-A suborbital rocket was launched into space with an atomic clock onboard, researchers observed that general relativity predicted the clock's frequency shift with an uncertainty of 1.4 x 10-4. The new studies, published last December in Physical Review Letters, again verified Einstein's prediction -- and increased that precision by a factor of 5.6. So, for now, the century-old theory still reigns.
Einstein's theory predicts time will pass more slowly close to a massive object, which means that a clock on Earth's surface should tick at a more sluggish rate relative to one on a satellite in orbit. This time dilation is known as gravitational redshift. Any subtle deviation from this pattern might give physicists clues for a new theory that unifies gravity and quantum physics. Even after the Galileo satellites were nudged closer to circular orbits, they were still climbing and falling about 8,500 kilometers twice a day. Over the course of three years Delva's and Herrmann's teams watched how the resulting shifts in gravity altered the frequency of the satellites' super-accurate atomic clocks. In a previous gravitational redshift test, conducted in 1976, when the Gravity Probe-A suborbital rocket was launched into space with an atomic clock onboard, researchers observed that general relativity predicted the clock's frequency shift with an uncertainty of 1.4 x 10-4. The new studies, published last December in Physical Review Letters, again verified Einstein's prediction -- and increased that precision by a factor of 5.6. So, for now, the century-old theory still reigns.
Re:my small brain.. (Score:5, Insightful)
Re:my small brain.. (Score:5, Interesting)
Re: (Score:2)
Re:my small brain.. (Score:4, Informative)
That isn't an opinion, that is a prediction. A prediction that is reliable for particles of any given speed. A reliable prediction.
Scientific Fact (Score:5, Informative)
Your opinion is that muon showers are reaching the Earth due to time dilation therefore proving that time dilation is real.
It's not his opinion, it is established scientific fact. A muon at rest decays with a lifetime of 2.2 microseconds. Travelling at the speed of light this means that, without any time dilation, the muon would travel 3e8*2.2e-6 = 660m. However, muons are typically generated at around 15 km about the surface and we also see a lot coming it at angles meaning that they have travelled even further than this.
Looking at muons produced directly overhead, which have the shortest distance to travel, without time dilation this is well over 22 lifetimes and so the probability of survival of 1.35e-10. This will be even lower for muons produced at non-vertical angles and so have to travel further. We observe a rate of 1 muon per second per cubic centimetre at the Earths surface so to produce this rate without time dilation we would need such a high intensity of cosmic rays (comparable to early accelerator beam intensities) hitting the atmosphere that plane travel and mountain climbing would be death sentences from the massive radiation at altitude.
The lack of acute radiation sickness in pilots and mountain climbers therefore conclusively rules out that the muon lifetime does not change with relative speed. From our point of view the muon's lifetime is dilated by relativity. From the muon's point of view, the thickness of the atmosphere is Lorentz contracted making it appear far thinner to the muon.
Re: (Score:2)
I really have trouble with the phrase "established scientific fact". There is no such thing even possible. What can be shown is that repeated tests have not revealed any exceptions that can't be explained by known or suspected external causes (which need to be identified and listed).
I know that it's often used as short-hand by people who understand what is really meant, but when read by those who don't understand it tends to lead to confusion, and fixated beliefs. So it's dangerous short-hand, yielding a
Then there are no facts (Score:2)
I really have trouble with the phrase "established scientific fact". There is no such thing even possible.
Don't be daft. Relativity, which is what explains the muon's survival, is the most precisely tested physical law that has ever existed. If you cannot call this "a fact" then absolutely nothing is a fact, scientific or otherwise, because there is nothing else which has been tested and confirmed to such an amazing degree of precision. Even if this fact is not absolutely right any change due to new physics will be at a tiny level (less than one part in a quadrillion) because that is about the level of accura
Re: (Score:2)
The fact is the observation of the muon. Relativity is the explanation used to explain it. One is a fact, the other is a theory. Facts can never be refuted. Explanations of facts ("a small epileptic attack caused the UFO siting") can (at least in principle) be shown to be incorrect. They can never be shown to be correct.
Re: (Score:2)
If you have been following the argument, it requires BOTH relativity and quantum theory to explain why those muons were detected where they were.
And I also disagree with your first paragraph. Read my prior post, as you don't seem to have read it before. A fact is not a theory, and conversely. A theory is an explanation for a fact. It may or may not be the correct explanation, and the theory that explains any particular fact will almost(?) always need to be a union of less specialized theories (plus a few
Re: (Score:2)
Wrong again. While it is not an "opinion", his statement is a fact that is wrong, a false-fact.
You have things confused. The statement that the OP was claiming was an opinion was "muon showers are reaching the Earth due to time dilation therefore proving that time dilation is real". This is most definitely true.
Re: (Score:1)
Einstein's theory predicts time will pass more slowly close to a massive object, which means that a clock on Earth's surface should tick at a more sluggish rate relative to one on a satellite in orbit.
This is so obviously and absurdly wrong, that it is really astounding. A clock does not measure time.
To the contrary. Time is defined as that which (ideal) clocks measure.
If the clock on a satellite moves more slowly, it is because something, in this case gravity, has caused the mechanism of the clock to move more slowly.
You can, if you like, think of general relativity this way, as long as you keep firmly in mind that gravity will slow down all clocks, regardless of the physical mechanism, including mechanical clocks, biological clocks, the frequency of light emissions from atoms, radioactive decay, and any other possible clock, even using mechanisms you haven't thought of yet.
(This is emulating Lorentz's thinking, who interpreted the Lorentz time dila
Re: (Score:1)
Time and Space (Score:2)
A clock does not measure time. Time is a concept. Not a thing.
Time is not just a concept it is part of the physical reality of the universe in the same way that space is. Indeed, relativity tells us that space and time and both relative.
If the clock on a satellite moves more slowly, it is because something, in this case gravity, has caused the mechanism of the clock to move more slowly. Time has not slowed down.
Part right but part wrong. If you were stood next to the clock you would not notice any difference in the tick rate so, in one sense, the clock has not slowed down. However, if you are somewhere else deeper or higher in the gravitational well of the Earth, or the clock is moving relative to you, then you will see that the ticking of
Re: (Score:2)
Time and space are relative because velocity is finite. Velocity is finite because time and space are relative. It's beautiful.
Re: (Score:3, Interesting)
Think of a coordinate system with the three usual dimensions (x, y and z) and then one other dimension orthogonal to the other three, this extra dimension is time (t).
An object at rest describes a vector where x= y = z = 0 and t = c (the speed of light).
As an object moves the vector rotates to point in a new direction and therefore the resolution of the (now rotated) vector on the t dimension is smaller than when the object was at rest. Therefore as the object moves time passes more slowly for the objec
Re: (Score:2)
IIRC it involves doing a cross-product and projecting the results from a 4-space down to a 3-space. (Unless that was during the part of the derivation that was being done in a 16 dimensional space. If so Eigenvectors are involved which is sort of similar, but leaves you in a vector space rather than returning a scalar.)
Of course, different modes of derivation use different notations. The one I followed was based on matrices. I hit tensors and bounced two or three times.
Why... (Score:1)
Because (Score:5, Insightful)
Is there something the EU member want to do that the current GPS network cannot or declines to do?
Yes. Not have an important piece of technology controlled by a (potential) rival nation. Maybe not an ideal reason but NIH [wikipedia.org] is sometimes a strong motivation.
Re: (Score:2)
NIH applies to cellular equipment or nuclear weapons. This is "not controlled here". There's no question about whether the US will exercise control over GPS, because we know the answer to be yes.
Re:Because (Score:5, Informative)
It's also much higher precision - by an order of magnitude. The US system cannot be trivially upgraded, you have to replace all of the satellites.
Re: (Score:1)
> Is there something the EU member want to do that the current GPS network cannot or declines to do?
Being independent of a warmonger that has all the incentive to scramble GPS when they see fit.
Re:Why... (Score:5, Informative)
...does there need to be an "answer" to the US GPS? Is there something the EU member want to do that the current GPS network cannot or declines to do?
Yes: when President Clinton opened the high-resolution GPS up to all users (instead of just military) in May 2000, he reserved the right of the U.S. to selectively turn off the GPS system in the event of war or another national emergency (specific words [archives.gov] were: "capability to selectively deny GPS signals on a regional basis when our national security is threatened"). The Europeans at that point committed to making their own system, which they could control, and turn of when they think it's necessary, not us.
Re:Why... (Score:5, Informative)
There are a lot of reasons for having their own system, including control. But regardless of the reasons, right now there are pieces of five different positioning systems in operation right now. There's GPS (US), GLONAS (Russia), Galileo (Europe), Beidou (China), and QSZZ (Japan). New crops of GPS units, including the very impressive ZED-F9P chip from U-Blox, can see all of these satellites, allowing for more accurate and faster fixes. Also more satellites provides more redundant information for weeding out any bad satellite data, overcoming deliberate jamming, or when a country intentionally degrades the signal. Apparently recently near Georgia and South Carolina there was some GPS jamming going on as part of a naval exercise. Having more systems to work with mitigates this somewhat, although they all use similar frequencies to GPS's L1, L2, and L5 bands.
And recently the FCC has finally allowed American users of GPS receivers to be able to use these other satellites. Odds are your phone is now using GPS, Galileo, and Glonass for positioning. It's a really a win win for those that rely on this technology. I can't see a downside, either for end users or countries to have more of these systems up and running, other than cost.
Re:Why... (Score:4, Insightful)
Yes. GPS has low reliability and is controlled by a lunatic. By having an alternative, high-precision, system that actually works and is not controlled by a lunatic, you have what's called a benefit.
Re: (Score:2)
Yeah, but the more systems you have available, the more chance there is that at least one won't be controlled by a lunatic, or at least one will be controlled by a lunatic who's roughly aligned with your interests today.
Other than predicting any orbits accurately (Score:2)
Re: (Score:2)
Re: (Score:2)
Stars are predicted just fine, as are galaxies. Don't know what you're bitching about. There are a few exceptions, where the difference is predictable and follows a fixed rule. There is a little uncertainty as to why that rule is there. This may be Dark Matter, MOND, Emergent Gravity, etc. Precisely the same reason Newton's laws broke down. So unless you reject gravity and believe in mutant space weasels pulling things around, that's simply not even remotely passable as an excuse. So go find one that is acc
Re: (Score:2)
Re: (Score:2)
Do you happen to know a hypothesis that does provide better predictions as a whole?
That kind of is what physics is about. We choose apply and test the models that have proven to be accurate enough for a lot of things. And then you may have some exceptions here and there.
There's probably a good deal of trust in Relativity because the models provided by relativity are se
Re: (Score:2)
Re: (Score:2)
Not being able to predict orbits of objects lightyears away with accuracy is mostly irrelevant for me for example. If you have a better explanation for the rest of the cosmos and can't also account for the relativistic effects of motion and gravity, your model won't be us
Re: (Score:2)
Re: (Score:2)
The mathematics and the current system is enough to give us a certainty of about 5m, which is used in conjunction with various types of accelerometers and fibre optic gyroscopes. Accurate enough. We probably would be able solve devia
Gravitational redshift vs expansion redshift (Score:1)
I haven't heard of the former. I already assume there's no conflict with the claim that the universe is expanding because distance light is more red than expected but I just kinda want to confirm. Is that so? =P
Once upon when learning about gravitational waves / bosons / whatever there was something I wondered but eventually forgot. I hate that because I wanted to have it answered =P
Re: (Score:2)
Gravitational redshift was the first version predicted, and has been more thoroughly tested. Expansion based redshift was predicted later, and is harder to test directly (because we lack a time machine + FTL drive so we can't look at the same source from two significantly different times and places). But it's passed every indirect test so far.
In a way, this is a pity, because we know that either relativity, quantum theory, or both are flawed, because they differ in their predictions over things we have no
Mine bitcoins in asteroids (Score:1)
Re: (Score:2)
Well, far from the sun would help, but if you're also far from Earth you'll need to find some other way to shed heat. Radiation is pretty slow unless you build a huge emitter, and use a heat pump to move the heat from your CPU to the radiator.
Special relativity vs. general relativity (Score:3)
So I was wondering about the time slowing effect of the speed of the satellites vs. our on the surface of the Earth closeness time slowing effect. So the effects partly cancel each other out. General relativity is the norm/aka the general rule (with gravity comes acceleration) and special relativity is the special case one I learned in high school where speed with no acceleration slows down time. I really love this stuff.
This guys explains it well I believe:
https://www.quora.com/If-an-at... [quora.com]
Quote:
"Keith Norfolk
Keith Norfolk, former Educational Specialist at European Space Agency
Answered Aug 22, 2017 Author has 250 answers and 87.4k answer views
There are actually two effect that (partly) cancel each other. Yes, the satellite is moving at a particular speed and than means that from Earth its clock will run slower (according to special relativity). However it is also higher in the Earth’s gravitational field and this is the domain of general relativity is needed (special relativity is only valid in inertial reference frames (i.e. no acceleration and no gravitational differences). According to general relativity, time deep in a gravitational well will run slower and so, reversing the reasoning, time for the satellite (that is higher in the gravitational well) will run faster.
From the point of view of an observer on Earth the two effects partly cancel each other but not fully and so there is a time rate difference for the satellite and the observer on Earth. This is why GPS satellite clocks have to be set to the ‘wrong’ rate in the factory so that they will run at the right rate when operating on orbit.
Curiously, the higher the satellite is, the greater the rate differential is for the gravitational effect. However the higher the satellite is the slower its orbital velocity will be and so the smaller the special relativistic effect will be. So, there should be an orbit at which the two effects exactly cancel each other out. Now that would make an interesting question!
By the way, it is not that there is Special Relativity on the one hand and General Relativity on the other. Special Relativity, as I said earlier, is only valid if there is no acceleration. General Relativity allows for situations where acceleration (e.g. gravitational fields) are present. Special relativity is a special case not the general case. That’s where the two names come from!"
Re: (Score:2)
Earth has no significant acceleration and relativity only applies in an accelerating frame of reference.
Re: (Score:3)
Earth has no significant acceleration and relativity only applies in an accelerating frame of reference.
Huh?
Gravitational time dilation is the g_00 term in the metric, in which the deviation from 1 is proportional to gravitational potential. You get the same time dilation whether you are deep in a gravity well at high surface acceleration or at low. (In fact, you would get gravitational time dilation even if you were at the exact center of a planet, where acceleration is zero.)
Re: Special relativity vs. general relativity (Score:1)
There are actually special- and general-relativistic effects that need to be taken into account, though. The fact that the satellites are in motion relative to the ground means that the clocks run slow by 7 microseconds/day, but the fact that they experience a lower gravitational field means that they would run fast by about 45 microseconds/day. So, combined they're fast by 38 microseconds/day.
How elliptical are the orbits, I wonder. (Score:3)
I'd love to find a source of ephemerides for their satellites. It'd be interesting to plug it into the GPS coverage software I wrote back in the '80s. Just to see for myself how "useless" the Galileo system is because of this $11B screw-up.
Re:How elliptical are the orbits, I wonder. (Score:4, Informative)
Apparently this data can be found in RINEX format here: ftp://gssc.esa.int/gnss/data/h... [esa.int].
Despite these satellites being lost as far as the constellation usability is concerned, the ESA plans to have the system completed by 2020, and that would mean 100% coverage across Europe and most of the world. Right now my phone uses Galileo as well as GPS and Glonas. I just noticed that Glonas reports nearly 100% coverage of the globe right now also.
Some of the GPS units I'm working right including the U-Blox M8T with RTKLIB and the ZED-F9P (integrated RTK) see satellites from GPS, Glonass, Galileo, Beidou, and QZSS. In fact I was able to briefly get an RTK fix on my M8T (Reach RS+) using only Beidou observation data from my base unit, apparently. With cheap receivers like the ZED-F9P, lots of satellite constellations, it's really a golden age for low-cost, high-accuracy GNSS work for agriculture, drones, etc.
Ephemeris (Score:1)
Ephemeral data from NASA Here:
https://cddis.nasa.gov/Data_and_Derived_Products/GNSS/broadcast_ephemeris_data.html
Free on-line trial of STK here (need to create an account)
http://licensing.agi.com/stk/
To check for oddities (Score:3)
At this point with all the overwhelming evidence, most would agree that Einstein is probably correct. The reason they keep doing things like this is to see if they can find something unusual or unexpected. It's often the weird / unexplained phenomena that leads to new theories or even technology that we can use. So yes, it seems redundant but it's how new things are discovered.
Re: (Score:2)
No. Nobody knowledgeable would assert that Relativity is correct. Just that every prediction that it has made that we have checked is correct.
The problem is that Quantum Theory has a track record at least as good as Relativity, and they disagree about predictions for some things we can't check. So one of them has to be wrong.
OTOH, they are both extremely, extremely, extremely good theories. They've both been checked in a huge number of instances, and they've both passed every check. They are both used
I would rephrase (Score:2)
Re: (Score:2)
Everybody knowledgeable agrees that relativity is peculiar.
making the best of a bad situation (Score:2)
i love it!