Slashdot Mirror
Wayward Satellites Test Einstein's Theory of General Relativity (scientificamerican.com)
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.
...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?
Social Media Handywoman at Texas Boys Balloo
This is one of the most stupid arguments I have ever read. You state your opinion, then argue that the concept is wrong since it conflicts with your opinion. Look into muon showers caused by cosmic rays hitting Earth's atmosphere. Taking into account the speed they are traveling and the half-life of a muon, they should never reach the surface. But they do, since they are moving near the speed of light and the time dilation involved means that time is happening much slower from the particle's frame of reference. There is no mechanism involved to slow down.
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 object. When the object moves at speed c, time stops for the object.
I leave it for others to translate this model to a gravitational field (it's been too long and I've forgotten).
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 is sometimes a strong motivation.
yes, relativity reigns supreme. Other than not being able to predict the orbits of any stars or galaxies correctly. Look at all this invisible shit over there -> DARK MATTER MAKES MY CALCULATIONS CORRECT!
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
I said, look into it. In other words, read about it and see if it supports your opinion or contradicts it. Muons are created in particle accelerators. The ones created are moving fairly slowly compared to cosmic rays. We can measure the half life of them before they decay, which is 2.2 microseconds. The ones created by cosmic rays hitting the atmosphere are moving at a speed where the muon should go about 450m before decaying. Yet, they reach the Earth's surface from 60-100 km up where they are created in the atmosphere. The reason they last that long and can move that far is due to time dilation, since they are moving at relativistic speeds.
So, if I mine bitcoins far from the sun and earth... I will get more bang for my cpu cycles ;)?
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...
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!"
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.
CUR ALLOC 20195.....5804M
Let me guess.... you're an electric universe nutcase cultist who, once again, has been told you're a fucking idiot by reality? Sorry, electric universe bullshit is completely useless shit, and Einstein was right again... and again... and again...
Fascism: An authoritarian and nationalistic right-wing system of government and social organization. See also: NAZI's
That isn't an opinion, that is a prediction. A prediction that is reliable for particles of any given speed. A reliable prediction.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
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/
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.
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.
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 the clock is now slower. This will happen irrespective of the clock mechanism.
The reason for this is that time and space are relative. Gravitational fields and relative motion both rotate the space-time axes of the distance clock relative to your own. This means that part of the clock time direction now lies along your space dimension. If you imagine that I am holding a piece of paper in front of you and then rotate it so that the top of the paper moves away from you and the bottom moves towards you then, if you were unable to perceive depth, it would look as if the paper were now shorter. This is what is happening here only the length dimension is rotated into your time direction so you cannot see it in the same way and it apears that the clock is now shorter. For the time component, this "shortening" is seen by you as an apparent slowdown of time for the clock.
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.
I think we've pushed this "anyone can grow up to be president" thing too far.
Neither of them is totally right. They are not necessarily wrong either.
Yes, but all of Einstein's clocks are light beam based. So he says that the speed of light is affected, and time is also. But the time he is measuring is only the time it takes for light beams to travel around in a clock. So it seems to be circular reasoning to me. If you build a clock that rotates, like when the perpendicular line from the base to the peak of Mount Everest rotates completely around the earth one time and points at the same star, it does not change due to gravity. In fact the base and the peak would be at different time speeds, even though it is one object.
In the end, I don't see how you can say light speed changes, so the change it light speed means time also changed. If the speed changed, then time didn't, or vice versa, you can't say one is caused by the other but the other causes the one also.
-- ssoorrrryy,, dduupplleexx sswwiittcchh oonn.. -Quote found on actual fortune cookie.
i love it!
On a long enough timeline, the survival rate for everyone drops to zero.
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 accuracy we have tested relativity to. How often can you be sure of a fact to that degree of certainty?