What good is general relativity?

[bji]

Please forgive my choosing such a provocative title for this thread. I specifically shortened the question I am trying to answer to this phrase because I believed that it would be the most thought-provoking.

 

The question I am really trying to ask is (and I realize that all answers to this question will be speculation, I'm not looking for the 'right' answer, just people's opinions on the answer, backed as much as possible by expertise in theoretical and applied physics): if Einstein had never been able to come up with the general theory of relativity, and no subsequent scientist had been able to, and there were still to this day no satisfactory theories to explain some of the aberrations of cosmologic observation that general relativity explains, then how would our world be different?

 

The fundamental aspect of this question that I am most concerned about is, what aspects of modern technology would be impossible without general relativity?

 

And, are any of these technologies in any domain other than cosmology?

 

I ask this because my gut feeling, which you are welcome to tell me is entirely wrong because it is just a guess, and not even an educated one, is that the only things that general relativity is required for is explanations of phenomenon that we observe with telescopes and other instruments which measure phenomenon on a cosmologic scale. For example, I know that general relativity can explain the precession of Mercury (which is the fact that we see light from Mercury appearing in a slightly different place than we expect, right?), and also can explain "gravity lenses" and other anomolies of astrophysical observation.

 

But what else would we be unable to explain? Are there fundamental aspects of, say, silicon transistors, or nuclear reactors, or space travel, or high-energy physics, or lasers, or chemistry, or anything else, that would prevent these things from advancing to the state that they have today?

 

My motivation for the question comes from knowing (or at least, feeling pretty sure that I've read in the past, although I forget the details) that special relativity has important implications for many modern technologies, such that if we didn't have the theory of special relativity, we would have been unable to account for aspects of forces and particles that would have made some technologies impossible (for example, never being able to send a spacecraft to the moon because our trajectory calculations would miss relativistic effects and thus be incorrect).

 

However, I am not sure if the same is true for general relativity - especially considering that its sole advancement in human knowledge (in my very limited understanding) is an explanation of how gravity's effect on matter can be modelled; but since none of our modern technologies (that I know of) depend on relativistic effects of gravity, I wonder if we would or wouldn't be in the same place we are today from a technology standpoint if Einstein had never come up with general relativity at all?

 

Thank you!

[swansont]

Assuming that we didn't notice the systematic differences and/or weren't willing to make ad-hoc corrections to technology to correct things, i.e. the effects of GR were undiscovered:

 

No GPS. Your cell phone and the internet probably wouldn't work.

 

The gravitational time dilation of atomic clocks would be unexplained, and if uncorrected, this would make the synchronization necessary for high-speed communication impossible, or at least much more difficult. (You could do point-to-point handshaking and synchronization, but nothing over a network) The errors introduced would give a positioning accuracy in GPS that would be measured in kilometers after only an hour of operation. We'd probably just assume atomic clocks didn't work, and there was some inherent error in how well timing could be done.

[bji]

Assuming that we didn't notice the systematic differences and/or weren't willing to make ad-hoc corrections to technology to correct things, i.e. the effects of GR were undiscovered:

 

No GPS. Your cell phone and the internet probably wouldn't work.

 

The gravitational time dilation of atomic clocks would be unexplained, and if uncorrected, this would make the synchronization necessary for high-speed communication impossible, or at least much more difficult. (You could do point-to-point handshaking and synchronization, but nothing over a network) The errors introduced would give a positioning accuracy in GPS that would be measured in kilometers after only an hour of operation. We'd probably just assume atomic clocks didn't work, and there was some inherent error in how well timing could be done.

 

Thank you for your answer, it was very informative. It prompted me to do a google search for "gravitational time dilation effect on gps", the results of which confirm what you have said.

 

In particular, the Wikipedia article on the issue was very comprehensive and enlightening.

 

I wonder if scientists wouldn't have been able to find a way to measure and predict the discrepancy of atomic clocks at medium earth orbit, even without GR? Total speculation of course, but I'm guessing that someone clever would have found a way to periodically re-synchronize the clocks, so that the error could be bounded by the frequency of re-synchronization. Combined with known measured quantities for the drift (which the wikipedia page says is 45 microseconds per day), which could have been incorporated into the system even without knowing what caused the drift, maybe GPS could have been implemented (perhaps with less accuracy than we have now) without GR (or even SR?).

 

Once again I appreciate the response. Certainly GPS seems to be the best example of a technology that as it is currently implemented requires the theory of general relativity to function properly.

 

Any others?

 

Your cell phone and the internet probably wouldn't work.

 

Oh - and as a follow-up - this part I don't think I believe. I don't think that cell phones or the internet require GPS. Even without any satellite communication at all, we could still have the internet (undersea cables to most places, radio to places that would be uneconomical to reach by undersea cable) - consider that the internet existed before GPS, so clearly it was possible, at least at the limited scope of the internet in the 1970's. And probably we could still have cell phone service too (at the very least, by using the aforementioned internet to carry calls beyond one's local cell tower). But GPS, for sure we couldn't have that.

[John Cuthber]

I think the simple answer is that if Einstein hadn't come up with the theory then, by now, someone else would have.

[the tree]

I don't think that cell phones or the internet require GPS.

No of course not, but they do require synchronisation over large distances.

[swansont]

No of course not, but they do require synchronisation over large distances.

 

Right. Both depend on precise time measurements.

 

I wonder if scientists wouldn't have been able to find a way to measure and predict the discrepancy of atomic clocks at medium earth orbit, even without GR? Total speculation of course, but I'm guessing that someone clever would have found a way to periodically re-synchronize the clocks, so that the error could be bounded by the frequency of re-synchronization. Combined with known measured quantities for the drift (which the wikipedia page says is 45 microseconds per day), which could have been incorporated into the system even without knowing what caused the drift, maybe GPS could have been implemented (perhaps with less accuracy than we have now) without GR (or even SR?).

 

Well, you have to decide what the conditions of your hypothetical situation are. On the one hand, we don't know about GR, but on the other hand, we learn all about GR because of the experiments we do. Way too much wiggle room; you can draw almost any conclusion you want.

 

But, if you didn't have any basis for making the correction, would you put the satellites up there in the first place? You'd notice timing discrepancies from land-based clocks at different elevations. If this was an open question, there would be no reason to think that clocks in space would give you any ability to do navigation.

[D H]

The internet does not rely on precise timing, anywhere. Packets are tagged with an index number precisely because most internet protocols don't give a hoot about timing. Consecutive packets might arrive in reverse order due to timing hiccoughs. It is incumbent upon the receiving machine to sort things out.

 

Timing is a little dicier with things like VOIP, but only to the extent needed to make a conversation sound more-or-less like it would if the people involved were face-to-face. The same goes for cell phones. Timing is only needed to the extent that packets must be presented to the listener in exactly the same order and more-or-less same rate as sent by the speaker. AFIK, extremely precise timing (atomic time accuracy) is not essential.

 

General relativity most certainly is essential for GPS operations.

[swansont]

Try sending any digital data without a good clock on either end. Packets may not have to be in order, but the data within a packet is still ones and zeroes, and you need to synchronize clocks to do that. Rubidium clocks are big business, selling to telecom companies.

[bji]

No of course not, but they do require synchronisation over large distances.

 

But doesn't GR only enter into the equation if the clocks in question are each at a significantly different part of the gravity well?

 

In that case, wouldn't the effects of GR not matter to synchronized clocks on the Earth's surface, no matter how far apart on the surface the clocks were?

 

I think the simple answer is that if Einstein hadn't come up with the theory then, by now, someone else would have.

 

True, but I specifically discounted that possibility when I framed my question. What I'm interested in is, what aspects of modern technology require explanation of the observed phenomena that GR explains? If there were no explanation, and no way to predict these effects, what technologies would not work?

 

So far the only answer has been GPS, because it requires precise synchronization of clocks that are separated by a significant distance in Earth's gravity well. There hasn't been any other technology suggested yet.

 

But, if you didn't have any basis for making the correction, would you put the satellites up there in the first place? You'd notice timing discrepancies from land-based clocks at different elevations. If this was an open question, there would be no reason to think that clocks in space would give you any ability to do navigation.

 

Wouldn't it be possible to measure the discrepancy between clocks at different altitudes, and come up with a formula that would predict the discrepancy for arbitrary altitudes? If so, then even without a theory explaining why the discrepancy exists, it might have been possible to account for it anyway, at least to a certain degree of precision, which may have been enough for GPS satellites.

 

It's all speculation though. I do find it interesting personally.

 

My main motivation for asking this is that, to some degree, in absence of a formal education in physics, I make some decisions about the 'quality' of theories based on how much practical knowledge they have enabled.

 

For example, the laws of thermodynamics, aside from all of the logical and observational evidence which supports them, have enabled technologies that we could never have had without them, because just knowing the "rules of the game" allows one to with so much more precision formulate ideas about practical applications. Without laws such as these, one would have to use random trial-and-error in places that the laws allow knowing ahead of time which approach is going to work, or at the very least, significantly diminishes the size of the space of possible designs that need to be searched through to find the one 'that works'.

 

Another analogy would be architecture; without Euclid's geometry, it would have been very difficult to have build some of the fantastic structures that mankind has invented. Instead of being able to predict which configuration of support structures would be necessary in building a tall building, we'd have to use trial and error, which would be prohibitively slow and expensive.

 

I personally question some of the fundamental assumptions of GR (which is not to say that I reject the theory - I just find that it's unsatisfactorily explained to me, almost certainly because of my limited education in math and physics - but the parts that I do understand, I have some unanswered questions; see my other thread about "Denying Intrinsic Geometry" for an example); and so I'm curious to know how much "practical application" of GR there has been, to help me feel more confidence that the theory's potential validity has been borne out by enabling practical applications.

 

Whereas well-trained scientists evaluate the quality of a theory by examining whether or not it can predict effects that previously were unexplained or, even better, undetected (thus demonstrating that the theory wasn't made just to fit the bounds of existing knowledge, but has explainative powers that extend beyond existing knowledge, thus enhancing existing knowledge, which is the true purpose of science), I like to also evaluate the quality of a theory by examining the technologies that it has enabled; if I can hold something in my hand that does something that would have been impossible without the theory (for example, a computer; or a cell phone; or the steering wheel of an automobile), then I gain confidence in the theory in the same way that scientists gain confidence in a theory by evaluating its pure predictive powers.

 

Sorry to bring religion into this, but this is one reason that I don't put any faith in the Christian Bible; because it hasn't enabled any technologies, it hasn't proven itself (to me) that any of the supposed 'facts' in it (about the origins of the universe, or anything else it tries to explain) have any merit. As a historical document describing events which occurred in the past, it may have some merit; but as an explanation of how our universe works and how it was created, for the reasons I detailed above, it holds no value at all for me.

[swansont]

But doesn't GR only enter into the equation if the clocks in question are each at a significantly different part of the gravity well?

 

In that case, wouldn't the effects of GR not matter to synchronized clocks on the Earth's surface, no matter how far apart on the surface the clocks were?

 

The frequency changes by a part in 10^16 per meter in elevation. So a km difference accumulates a nanosecond in a few hours. And clocks separated by longitude accumulate Sagnac delays when you send signals or move the clocks, and this is tied in to relativity as well, since it's from not being in an inertial frame. (207 ns for the circumference of the earth)

 

 

Wouldn't it be possible to measure the discrepancy between clocks at different altitudes, and come up with a formula that would predict the discrepancy for arbitrary altitudes? If so, then even without a theory explaining why the discrepancy exists, it might have been possible to account for it anyway, at least to a certain degree of precision, which may have been enough for GPS satellites.

 

It's all speculation though. I do find it interesting personally.

 

My main motivation for asking this is that, to some degree, in absence of a formal education in physics, I make some decisions about the 'quality' of theories based on how much practical knowledge they have enabled.

 

For example, the laws of thermodynamics, aside from all of the logical and observational evidence which supports them, have enabled technologies that we could never have had without them, because just knowing the "rules of the game" allows one to with so much more precision formulate ideas about practical applications. Without laws such as these, one would have to use random trial-and-error in places that the laws allow knowing ahead of time which approach is going to work, or at the very least, significantly diminishes the size of the space of possible designs that need to be searched through to find the one 'that works'.

 

 

And that's why you have to wonder if ad-hoc explanations would suffice for someone to base a technology on. An interesting aside in this case is that even with GR known, there were arguments about what the correction would be or even if one were needed, so the first satellite (or an early one) went up sychronized with the ground clocks, and it was seen that a time difference accumulated in accordance with the theory. Then they switched to the corrected frequency.

 

I'm sure it would have been an interesting research project, but without a solid theory behind it, I don't see a technology arising from it. If you don't know why it works, you don't know why it would break and you can't trust it. Remember that this is a military application first and foremost (which is one reason why it's free).

[D H]

Try sending any digital data without a good clock on either end.

You almost certainly posted the the quoted message using a computer with a fairly lousy clock. The SFN server with almost equal certainty has a fairly lousy clock. Fortunately, every receiver on the computers that relayed your post to the SFN server incorporates phase-locked loop techniques that let the receiver synchronize with the sender without any need for an accurate clock.

[swansont]

You almost certainly posted the the quoted message using a computer with a fairly lousy clock. The SFN server with almost equal certainty has a fairly lousy clock. Fortunately, every receiver on the computers that relayed your post to the SFN server incorporates phase-locked loop techniques that let the receiver synchronize with the sender without any need for an accurate clock.

 

Ah, of course. The packet protocol allows you to send shorter bundles of data, which don't require a high level of accuracy.

 

OK, we still have the internet. But I think we still lose digital voice and any other high-speed communication.

[CPL.Luke]

fiberoptics? I'd imagine that depending on the necessity of accracy in the frequency's gravitational redshifts could gunk up the works over extreme distances.

 

 

gravitomagnetism? there aren't any real applications yet however thats a general relativistic effect that can be created in a lab, its only a matter of time.

 

the best one's would be for geology where gravitometers measure the changes in gravity from point to point and find out whats in the ground (like oil).

[seemysince]

We would still be studing the things he explained

i think technology would still be basically the same just appear more primative

:Lanaguage is relative :;was the true meaning of his theories bEcause the words and sounds we produce to some one who hasnt learned our language) wouldnt explain the things we are thinking about and study and remember and are a part of our every day life,., becuase if we are to not be sure if we are even heard by some one just assume that they know what we are thinking when they do what we think they should do,., gives us a progressive mental pattern of selective thought that sees everything as something to add to what or where we are right now,., and those ideas or items would become a part of our world,., words are not things to just give one to anything you can see, sounds mean more like ambulance sirens,,.

[Sayonara]

:Lanaguage is relative :;was the true meaning of his theories

No it wasn't.

[jedaisoul]

What I'm interested in is, what aspects of modern technology require explanation of the observed phenomena that GR explains? If there were no explanation, and no way to predict these effects, what technologies would not work?

 

So far the only answer has been GPS, because it requires precise synchronization of clocks that are separated by a significant distance in Earth's gravity well. There hasn't been any other technology suggested yet.

 

Wouldn't it be possible to measure the discrepancy between clocks at different altitudes, and come up with a formula that would predict the discrepancy for arbitrary altitudes? If so, then even without a theory explaining why the discrepancy exists, it might have been possible to account for it anyway, at least to a certain degree of precision, which may have been enough for GPS satellites.

I agree with you, a "rough and ready" practical compensation would suffice. Also, even with GR and SR compensation, the satelites' clocks need periodic correction. One factor I've not seen stated is the cumulative magnitudes of the "relativistic drift compensation" and the periodic corrections that are required anyway. I.e. Is the relativistic drift compensation actually significant?

[swansont]

I agree with you, a "rough and ready" practical compensation would suffice. Also, even with GR and SR compensation, the satelites' clocks need periodic correction. One factor I've not seen stated is the cumulative magnitudes of the "relativistic drift compensation" and the periodic corrections that are required anyway. I.e. Is the relativistic drift compensation actually significant?

 

Something like 38 microseconds per day (45 one way due to gravitation, 7 the other due to kinematics). This is set into the system's synthesizer, since it represents a positioning error of about a kilometer every two hours. Smaller adjustments are made ~hourly, IIRC, to compensate for orbit/positioning data as well as clock discrepancies.