Relativity: Coming to Terms 1: Starting Assumptions

[ralfcis]

Coming to terms with the theory of relativity is a long and difficult process that requires shedding all the popular misconceptions and hype surrounding the subject matter. Science doesn't sell but throw in carefully worded claims of time travel, matter materialization, shape shifting and alternate realities and relativity becomes pretty sexy. I think the truth is sexy if only because so many veiled layers need to be peeled off to get at it. My agenda is to cut to the truth and bare relativity, maybe helping others avoid the same pitfalls I've fallen into. Part of the problem is that stepping into relativity is like stepping into a cathedral where only hushed, reverent tones are tolerated.

I think the biggest pitfall is coming to terms with the fact that the assumptions or conventions which form the basis of relativity are not directly provable, much like beliefs. They're only indirectly provable in that the results of those assumptions follow what the theory predicts of how motion and gravity (being indistinguishable from changes in motion) affect reality. Some results that you'd conclude are real, but do not fall out from the assumptions, are excluded from the theory. A specific example is the theory cannot predict a twin's aging as he's leaving his earthbound twin at high speed until he turns around and returns to earth. Everything before the turnaround is indeterminate (even if there's communication between the two parties) but suddenly becomes real after the turnaround. A second example is the theory can't determine the outbound one way speed of light, despite the fact Maxwell's equations can, because light itself is used to synchronize clocks used to measure the speed of light, an apparent conflict of interest. Interpreting relativity is like negotiating a mine field and requires unwavering focus and careful steps and language.

The 2nd pitfall in coming to terms with relativity is the terminology itself. Relativity piggybacks onto terms used in everyday language but changes their meaning making discussion very difficult. Terms such as relative velocity, reality, mass, length, wave medium, theory, post-processing, the present, past and future mean something very different in the jargon of relativity. This leads to many arguments and misinterpretations based on the wrong assumption that the scientific terms mean the same thing as their language counterparts. The creation of any jargon innocently starts as a shortcut to communication within a group but ultimately ends up as a means of excluding and befuddling those outside the group.

The 3rd pitfall is that while no one will commit to a single story of how Einstein came up with the theory of relativity, all printed versions of the story follow the exact same sequence of events. I had begun to try to understand relativity by recreating the thought process sequence Einstein purportedly used to come to his conclusions. I couldn't find a way to connect the dots that made any sense. That path just led to a myriad of questions that everyone found too tedious to answer. So I'm dropping this tack (for now) and will come up with my own story based on three unprovable assumptions.

The 1st assumption is there's no way you can tell if you're moving unless you change your motion and experience an inertial force (which is equivalent to a gravitational force). Relativity forbids you from placing a marker in either time or space that would give you a way to determine an absolute value of your motion because the absolute motion of the marker itself would be unknown. This assumption is supported by another assumption that both parties in relative motion see the others clock dilating because it is unknowable who is actually moving.

The 2nd assumption is that there's a universal rate of time that passes the same for everyone. The guy on a black hole or traveling near the speed of light will experience themselves aging at the same basic rate as someone free floating in space would experience. But depending on their relative motions, they would see everything outside their own timeframe as aging faster or slower relative to themselves. Time passes slower for all other moving frames outside the "stationary" reference frame unless that reference frame has experienced a change in motion thereby establishing it wasn't stationary. Time would therefore pass quicker for all other frames from its perspective as is seen in the twin paradox example. Relativity dictates that both twins would have to receive messages before the turnaround that each was aging slower than the other. Somehow it would have to go back in time and change the messages for one of the twins after the turnaround that his other twin had been aging faster all along.

A 3rd assumption is that while there's no speed limit on how fast you can travel from point A to point B using your own base time rate, there is a limit to how fast information can travel. This is the main reason the rules of relativity supercede those of a Newtonian universe, to preserve causality while still allowing infinite speed.

Before I give more examples of how the above assumptions contradict each other, I need to know if the assumptions are valid and not just the basis of a strawman argument. Undoubtedly there will be some disagreement of the terminology used which I'll define in subsequent posts.

 

[tionis]

The 3rd pitfall is that while no one will commit to a single story of how Einstein came up with the theory of relativity, all printed versions of the story follow the exact same sequence of events. I had begun to try to understand relativity by recreating the thought process sequence Einstein purportedly used to come to his conclusions. I couldn't find a way to connect the dots that made any sense. That path just led to a myriad of questions that everyone found too tedious to answer. So I'm dropping this tack (for now) and will come up with my own story based on three unprovable assumptions.

Welcome to PF, ralfcis. There is a book tittled: The Genesis of General Relativity. This book contains everything you would ever want to know about how Einstein came out with GR. Contained in the book, is Einstein’s 1912 Zurich notebook. ''The four-volume work represents the most comprehensive documentation and study of the creation of general relativity; one of the fundamental physical theories of the 20th century. It comprises key sources from Einstein and others who from the late 19th to the early 20th century contributed to this monumental development. Some of these sources are presented there in translation for the first time. Einstein’s famous Zurich notebook, which documents the pivotal steps toward general relativity, is reproduced here for the first time and transcribed in its entirety. The volumes offer detailed commentaries and analyses of these sources that are based on a close reading of these documents supplemented by interpretations by the leading historians of relativity. All in all, the facets of this work, based on more than a decade of research, combine to constitute one of the most in-depth studies of a scientific revolution ever written.''

 

[PeroK]

The road to understanding SR is not as long and difficult as you imagine. Get a good introductory book or sign-up for a professionally-taught course.

Most of what you have written, in my opinion, is not correct, and you are already creating a barrier to learning SR properly by repeating a mixture of half-digested ideas. One thing that is true is that you should approach SR thus:

SR is definitely right and I am determined to understand it properly.

 

[stevendaryl]

The 2nd pitfall in coming to terms with relativity is the terminology itself. Relativity piggybacks onto terms used in everyday language but changes their meaning making discussion very difficult. Terms such as relative velocity, reality, mass, length, wave medium, theory, post-processing, the present, past and future mean something very different in the jargon of relativity.

I really don't think that that's true. Relativity assumes that some of the things that people used to believe about those topics are not true, but I don't think that that means that the words have changed their meanings.

I would say, rather, that relativity is hard because it claims that many common-sense, unquestioned beliefs about time, length, etc. are actually false.

 

[ralfcis]

The road to understanding SR is not as long and difficult as you imagine. Get a good introductory book or sign-up for a professionally-taught course.

Most of what you have written, in my opinion, is not correct, and you are already creating a barrier to learning SR properly by repeating a mixture of half-digested ideas. One thing that is true is that you should approach SR thus:

SR is definitely right and I am determined to understand it properly.

I've done the course route worldscienceu.com. I'm trying to understand relativity as stated but it's so easy to misinterpret the statements. So what parts of relativity have I misstated?

 

[stevendaryl]

I've done the course route worldscienceu.com. I'm trying to understand relativity as stated but it's so easy to misinterpret the statements. So what parts of relativity have I misstated?

Your second pitfall is incorrect. Your third pitfall is almost irrelevant to understanding relativity. To understand a theory, there is no need to figure out, historically, how the theory was invented. That's the beauty of science--it's not about personalities, it's about the universe. Once a scientific theory is created, its inventor's opinion about it is no more privileged than anyone else's. Typically, the invention of a new theory involves lots of false starts and misconceptions that must be corrected later. Following the path of its invention is interesting from the point of view of understanding how scientists work, but I don't think it's all that helpful in understanding the theory itself.

As for your assumptions, your second assumption is either wrong, or very misleading. What does it mean to say that there is a universal rate of time? A "rate" is usually the change in something as a function of time. If that something is time itself, then what does a rate mean? There is also an ambiguity as to what you mean by "time". Time has two related meanings that are different in relativity: (1) A coordinate, which is a number given to events to pinpoint their place in history--for example, "January 1, 2016, at 12:00 a.m." (2) The elapsed time on a clock. Those are different things in relativity.

Your third assumption is very convoluted. There might be a way to make sense of it within relativity, but it is a conclusion of relativity, not an assumption.

In my opinion, your pitfalls and assumptions are a very bad starting place for studying relativity.

 

[PeroK]

"I think the biggest pitfall is coming to terms with the fact that the assumptions or conventions which form the basis of relativity are not directly provable, much like beliefs."

Nothing in science is provable. All postulates and assumptions are based on and corroborrated by experimental evidence.

"A specific example is the theory cannot predict a twin's aging as he's leaving his earthbound twin at high speed until he turns around and returns to earth. Everything before the turnaround is indeterminate (even if there's communication between the two parties) but suddenly becomes real after the turnaround.

This is nonsense.

"The 2nd pitfall in coming to terms with relativity is the terminology itself. Relativity piggybacks onto terms used in everyday language but changes their meaning making discussion very difficult. Terms such as relative velocity, reality, mass, length, wave medium, theory, post-processing, the present, past and future mean something very different in the jargon of relativity. This leads to many arguments and misinterpretations based on the wrong assumption that the scientific terms mean the same thing as their language counterparts. The creation of any jargon innocently starts as a shortcut to communication within a group but ultimately ends up as a means of excluding and befuddling those outside the group."

Everything in science and mathematics depends on well-defined terminology. SR, in fact, uses very little specific terminology, although see Stevendaryl's post above.

"The 3rd pitfall is that while no one will commit to a single story of how Einstein came up with the theory of relativity ..."

This is interesting, of course, but largely irrelevant to learning the theory.

"This assumption is supported by another assumption that both parties in relative motion see the others clock dilating because it is unknowable who is actually moving."

This is not an assumption, but simply and logically derivable from the two postulates.

"The 2nd assumption is that there's a universal rate of time that passes the same for everyone."

This is not true at all. This is what was thought to be true in classical physics and which SR superseded.

"The guy on a black hole or traveling near the speed of light will experience themselves aging at the same basic rate as someone free floating in space would experience."

This is nonsense. Moreover, any talk of black holes is way beyond SR. This is the sort of fantasy that will stop you learning anything properly. There is no guy on a black hole.

"But depending on their relative motions, they would see everything outside their own timeframe as aging faster or slower relative to themselves. Time passes slower for all other moving frames outside the "stationary" reference frame unless that reference frame has experienced a change in motion thereby establishing it wasn't stationary. Time would therefore pass quicker for all other frames from its perspective as is seen in the twin paradox example. Relativity dictates that both twins would have to receive messages before the turnaround that each was aging slower than the other. Somehow it would have to go back in time and change the messages for one of the twins after the turnaround that his other twin had been aging faster all along."

This is a mixture of half-truths, at best.

"A 3rd assumption is that while there's no speed limit on how fast you can travel from point A to point B using your own base time rate, there is a limit to how fast information can travel. This is the main reason the rules of relativity supercede those of a Newtonian universe, to preserve causality while still allowing infinite speed."

This is totally wrong.

"Before I give more examples of how the above assumptions contradict each other, I need to know if the assumptions are valid and not just the basis of a strawman argument. Undoubtedly there will be some disagreement of the terminology used which I'll define in subsequent posts."

This is correct: you've put up your own strawman of relativistic half-truths, errors and misconceptions.

I'd say: get a good book and start again. There's a free one here:

http://www.lightandmatter.com/sr/

 

[jedishrfu]

Your understanding of the twin paradox is one example where you state the paradox and then your resolution doesn't really answer the paradox.

I for one had issues with this same paradox until I really started to look into the math and realized what was going on.

We have an Insights article by Orodruin on it that you can read:

https://www.physicsforums.com/insights/geometrical-view-time-dilation-twin-paradox/

and here's another one some great spacetime diagrams of what is happening:

http://vnatsci.ltu.edu/s_schneider/courses/contemp/minkowski.shtml

 

[ralfcis]

I really don't think that that's true. Relativity assumes that some of the things that people used to believe about those topics are not true, but I don't think that that means that the words have changed their meanings.

I would say, rather, that relativity is hard because it claims that many common-sense, unquestioned beliefs about time, length, etc. are actually false.

Here are examples of where I ran into trouble:

relative velocity: If 2 spaceships are coming together at .1c to a point exactly between them their relative velocity is less than .2c but their relative time dilation is 0. If one is stationary and the other is coming at him at .2c, now there is time dilation between them even though it's the same relative velocity. So the term has an added meaning in relativity.

reality; The reality in a ship that's moving experiences no time dilation yet a stationary observer sees his reality in slow motion. There is a contradiction even though both are real. This is not typical in everyday language.

mass: People still talk of mass increasing as one approaches light speed but this mass is not real.

length; Length is measured using time and it contracts (real or not) as time dilates. This idea of length was introduced by relativity and hence the term changed.

wave medium: Light is supposed to have no medium but it is an electromagnetic wave in the electromagnetic medium of space. Its formula for propagation is the same as that for a mechanical wave through a mechanical medium. Maybe the term should be specified that light has no mechanical medium.

theory; relativity is based on several assumptions yet wiki defines a theory as fact. Unproven assumptions are not facts so relativity has redefined the word "theory".

post-processing: In brian greene's course at worldscienceu.com he defines post-processing of data to understand reality and dispel the illusion of perception as only applicable to non-relativistic events because, in relativity, the perception is not an illusion. This is a very little known definition of that term.

the present, past and future; Greene believes all are concurrently real. Just by adjusting the direction of motion to a stationary frame one can have that frame's yet un-experienced future in the moving frame's present. That's quite a different definition.

 

[stevendaryl]

Here are examples of where I ran into trouble:

relative velocity: If 2 spaceships are coming together at .1c to a point exactly between them their relative velocity is less than .2c but their relative time dilation is 0. If one is stationary and the other is coming at him at .2c, now there is time dilation between them even though it's the same relative velocity. So the term has an added meaning in relativity.

That's completely garbled. You seem to be very confused about what Special Relativity says.

For one thing, in SR, there is no such thing as something being "stationary". There is no such thing as something traveling at speed $0.1 c$. Velocity is relative to a reference frame, or to a coordinate system. "Relative time dilation" means something different from the way you're using it, as well. "Relative velocity" is ambiguous. You need to clarify what you mean.

Suppose there are three frames: $F_{alice}$, the rest frame of one object (Alice's spaceship), $F_{bob}$, the rest frame of another object (Bob's spaceship), and $F_{you}$, some third frame, the rest frame of an observer (you) watching those two objects. Then there are a number of different velocities involved:

1. The velocity of Alice as measured by Bob: $V_{bob, alice}$
   
2. The velocity of Bob as measured by Alice: $V_{alice, bob}$ (This is actually equal to $- V_{bob, alice}$
   
3. The velocity of Alice as measured by you: $V_{alice, you}$
4. The velocity of Bob as measured by you: $V_{bob, you}$

So there are several different quantities that you might mean when you talk about "the velocity of Alice relative to Bob":

• You might mean $V_{bob, alice}$ or $V_{alice, bob}$
• You might mean $V_{closing, alice, bob, you}$ = "the closing velocity between Alice and Bob, as viewed by you" = $V_{alice, you} - V_{bob, you}$

Relativity says that $V_{bob, alice}$ and $V_{alice,bob}$ always have to be less than $c$ in magnitude. It doesn't say that closing velocity must be less than $c$.

Similarly, there are a number of different "clock rates" involved:

1. $R_{alice, bob}$: the rate of Alice's clock, as measured by Bob.
2. $R_{bob, alice}$: the rate of Bob's clock, as measured by Alice.
3. $R_{alice, you}$: the rate of Alice's clock, as measured by you.
4. $R_{bob, you}$: the rate of Bob's clock, as measured by you.

So when you talk about Alice's clock rate relative to Bob, you might mean:
$R_{alice, bob}$

or you might mean $\dfrac{R_{alice, you}}{R_{bob,you}}$.

These two notions of "relative clock rates" will be different.

In your two scenarios:

Scenario 1:

• $V_{alice, you} = - V_{bob, you} = 0.1 c$
• $V_{closing, alice, bob, you} = 0.2c$
  
• $V_{alice, bob} = 0.198 c$
• $R_{alice, bob} = R_{bob, alice} = 0.98$
• $R_{alice, you} = 0.995$
• $R_{bob, you} = 0.995$
• $\dfrac{R_{bob, you}}{R_{alice, you}} = 1$

So Bob measures Alice's clock to be running slow by a factor of $0.98$, and Alice measures Bob's clock to be running slow by the same amount. You measure both clocks to be running slow by a factor of $0.95$

Scenario 2:

• $V_{alice, you} = 0$
• $V_{bob, you} = 0.2 c$
• $V_{closing, alice, bob, you} = 0.2c$
  
• $V_{bob, alice} = 0.2 c$
• $V_{alice, bob} = -0.2c$
  
• $R_{alice, bob} = R_{bob, alice} = 0.995$
• $R_{alice, you} = 1$
• $R_{bob, you} = 0.995$
• $\dfrac{R_{bob, you}}{R_{alice, you}} = 0.995$

In this scenario, Alice views Bob's clock as running slow, and vice-versa.

 

[ralfcis]

Welcome to PF, ralfcis. There is a book tittled: The Genesis of General Relativity. This book contains everything you would ever want to know about how Einstein came out with GR. Contained in the book, is Einstein’s 1912 Zurich notebook. ''The four-volume work represents the most comprehensive documentation and study of the creation of general relativity; one of the fundamental physical theories of the 20th century. It comprises key sources from Einstein and others who from the late 19th to the early 20th century contributed to this monumental development. Some of these sources are presented there in translation for the first time. Einstein’s famous Zurich notebook, which documents the pivotal steps toward general relativity, is reproduced here for the first time and transcribed in its entirety. The volumes offer detailed commentaries and analyses of these sources that are based on a close reading of these documents supplemented by interpretations by the leading historians of relativity. All in all, the facets of this work, based on more than a decade of research, combine to constitute one of the most in-depth studies of a scientific revolution ever written.''

Thanks but I'm still stuck on SR and probably will never have the interest in GR.

 

[stevendaryl]

length; Length is measured using time and it contracts (real or not) as time dilates. This idea of length was introduced by relativity and hence the term changed.

That's not true. Einstein did not introduce a new definition of "length". Length is always: "The distance between two ends of an object, measured at the same time". If an object is moving, you can't define length to be "the distance between the two ends of the object" without stipulating "at the same time".

 

[stevendaryl]

Here are examples of where I ran into trouble:

Almost every single thing that you've said about relativity is wrong. What you seem to have done is this: you took somebody's explanation of what relativity says, and then you paraphrased it--put it into your own words. Then you complain that those words are contradictory or confusing. Yes, they are, but that's because your paraphrase was incorrect. So you aren't analyzing the actual theory, you're analyzing a garbled version of your own creation.

In my opinion, your understanding of relativity is much too garbled for you to even post a coherent question about it. Instead of listing a large number of pitfalls and assumptions, try to post a specific question about relativity that you don't understand.

 

[Dale]

relative velocity: If 2 spaceships are coming together at .1c to a point exactly between them their relative velocity is less than .2c but their relative time dilation is 0. If one is stationary and the other is coming at him at .2c, now there is time dilation between them even though it's the same relative velocity. So the term has an added meaning in relativity.

"Relative velocity" has the same meaning, all that changes is the computation. In both relativistic and pre relativistic physics the relative velocity of A and B is the velocity of B in the reference frame of A.

reality; The reality in a ship that's moving experiences no time dilation yet a stationary observer sees his reality in slow motion. There is a contradiction even though both are real. This is not typical in everyday language.

"Reality" is not relativity jargon at all. It is a metaphysics term.

mass: People still talk of mass increasing as one approaches light speed but this mass is not real.

That definition of "mass" has been deprecated for decades, but I do agree that it is a constant source of confusion.

length; Length is measured using time and it contracts (real or not) as time dilates. This idea of length was introduced by relativity and hence the term changed.

"Length" has the same meaning in relativistic and non relativistic physics. It transforms differently, but the meaning is unchanged.

wave medium: Light is supposed to have no medium but it is an electromagnetic wave in the electromagnetic medium of space. Its formula for propagation is the same as that for a mechanical wave through a mechanical medium. Maybe the term should be specified that light has no mechanical medium.

I don't think that "wave medium" is defined any differently, but I disagree with the suggestion to use the term "medium of space". If you so that then you would need to provide an experimental measurement of the speed of that medium, which is not possible.

theory; relativity is based on several assumptions yet wiki defines a theory as fact. Unproven assumptions are not facts so relativity has redefined the word "theory".

All theories are based on assumptions. Either the Wiki is wrong or you misunderstood.

post-processing: In brian greene's course at worldscienceu.com he defines post-processing of data to understand reality and dispel the illusion of perception as only applicable to non-relativistic events because, in relativity, the perception is not an illusion. This is a very little known definition of that term.

I won't dispute this one, Brian Greene is infamous for confusing statements. If you are trying to actually learn relativity, I would look elsewhere.

the present, past and future; Greene believes all are concurrently real. Just by adjusting the direction of motion to a stationary frame one can have that frame's yet un-experienced future in the moving frame's present. That's quite a different definition.

I agree here. More importantly than the redefinition is the fact that they don't determine causality in relativity. The causal structure of relativity is completely different and different terms were developed.

 

[Mister T]

Coming to terms with the theory of relativity is a long and difficult process that requires shedding all the popular misconceptions and hype surrounding the subject matter.

Right. In a later message you stated that you took a course in relativity. That is the way to do it because the hype is just that, hype. You didn't mention, though, how you fared in that course. Do you feel you mastered the subject or just barely got by with a passing grade? There is a big difference there. I would recommend an actual face-to-face course with homework and tests. They help motivate us.

Another route is to get a book such as Spacetime Physics, by Taylor and Wheeler, 2nd edition. Work through it, including all the worked examples and chapter-end problems.

I think the biggest pitfall is coming to terms with the fact that the assumptions or conventions which form the basis of relativity are not directly provable, much like beliefs. They're only indirectly provable in that the results of those assumptions follow what the theory predicts [...]

This is true of every scientific theory and is more a commentary on the nature of scientific knowledge in general rather than relativity in particular. Theories make predictions, experiments yield results. When they match people use the theory to build stuff that works. This is how our society ended up with machines that do work for us rather than having to rely on beasts of burden and the like.

The 2nd pitfall in coming to terms with relativity is the terminology itself. Relativity piggybacks onto terms used in everyday language but changes their meaning making discussion very difficult.

This is also true of not only scientific knowledge in general, but all knowledge. Rarely are new words invented. Instead existing words have their meaning refined.

The 1st assumption is there's no way you can tell if you're moving unless you change your motion and experience an inertial force (which is equivalent to a gravitational force). Relativity forbids you from placing a marker in either time or space that would give you a way to determine an absolute value of your motion because the absolute motion of the marker itself would be unknown.

Right. That's the Principle of Relativity. It's been around since the time of Galileo. No experiment has ever been performed that results in an exception. Therefore it makes sense to build upon it as a fundamental facet of Nature. On the other hand, you can construct a perfectly valid theory of relativity that assumes the existence of an absolute rest frame. It will not make any predictions that differ, and it will not be inconsistent with any experimental results. It's just that there is no evidence to support it so it's not part of science. The same is true of angels dancing on pin heads.

This assumption is supported by another assumption that both parties in relative motion see the others clock dilating because it is unknowable who is actually moving.

That concerns the parties being in motion relative to each other. And it works in the sense that scientists, engineers, and technicians are forced to use it every minute of every day at various places all around the globe. If they ignore it their stuff won't work. Stuff like proton therapy and GPS location fixing.

The 2nd assumption is that there's a universal rate of time that passes the same for everyone.

There is no such assumption. Look at a clock and assume it measures time. That's the only assumption needed.

 

[ralfcis]

Yikes I'm overwhelmed with the number of responses. It'll take me weeks to reply and those replies will generate more responses. I have prepared a bunch of subsequent threads on the topic and some have more detailed explanations but if I release them now I'll be swamped so I won't. I'll have to figure out some way to handle this.

 

[Dale]

I have prepared a bunch of subsequent threads on the topic

That seems a little premature at this point. I think you should carefully consider the points raised (even if you don't respond) and probably read some more material on the topic especially material not by Brian Green or other pop sci authors.

Are you familiar with spacetime diagrams, four-vectors, and the spacetime interval?

 

[jedishrfu]

Based on the OP's last post, I think it's time to close this thread.

I want to say thank you all for your contributions here and to the OP I want to say read over and carefully consider what's been said and then if you have further questions post each one in a separate thread.

The goal here is to strip away the various wrong preconceptions and misinterpretations that people latch onto in trying to understand Relativity theory and begin to see the real physics behind it all. This can only be done with an open mind and a desire to learn what others have discovered in their pursuit of this rich theory of physics.