Register to reply 
Looking to understand time dilation 
Share this thread: 
#37
Nov410, 12:35 PM

P: 250

Of course, other inertial observers (who are moving at some constant velocity with respect to the above inertial observer) will prefer a different inertial frame (the one in which THEY are stationary). What special relativity says is that there is no single inertial frame that ALL inertial observers prefer. There is no single inertial reference frame that is UNIVERSALLY special. Mike Fontenot 


#38
Nov410, 02:10 PM

Mentor
P: 17,293




#39
Nov410, 03:33 PM

PF Gold
P: 4,745

The term "preferrred frame" has a specific meaning. You are making the same mistake that I am dealing with someone else on in this thread: http://www.physicsforums.com/showthr...=442132&page=2 It doesn't mean a personal preference like "I prefer blondes". Even if everyone in the world preferred blondes, it wouldn't mean they are "UNIVERSALLY special". I hope you don't think the word "special" in Special Relativity has anything to do with the concept of "preferred". This has nothing to do with anyone's preferences. It has to do with whether we base our physics on the concept of an æther. 


#40
Nov410, 03:41 PM

P: 250

The wellknown results of timedilation and lengthcontraction are directly available for your use (when you are an inertial observer), provided that you choose to use the usual Lorentz coordinates, in an inertial frame in which you are stationary. And the time coordinate, with that choice, corresponds to the time shown on your OWN watch. The spatial coordinate, with that choice, correponds to the lengths as reported by your OWN rulers and measuring tapes. There is a REASON why Einstein chose those coordinates, when developing his special theory. In the spirit of general relativity, you are of course free to choose some other set of coordinates, by transforming those Lorentz coordinates in an almost unlimited number of ways, provided that the eigenvalues of the resulting metric are either {1, 1, 1, 1} or {1, 1, 1, 1}, assuming that spacetime is everywhere flat. In those alternative coordinate systems, your "clocks" would behave in very odd ways, compared to ordinary clocks. And likewise for your "rulers". Real workaday experimental physicists would seldom, if ever, choose those types of "measuring devices" to do their experiments in their laboratories. There is a REASON that the phrase "in the laboratory frame" is used so often. Mike Fontenot 


#41
Nov410, 03:56 PM

P: 250

My other use of the term "special", as in "There is no single inertial reference frame that is UNIVERSALLY special", referred to the fact that in flat spacetime, there is no single inertial reference frame that is preferred by ALL inertial observers. Mike Fontenot 


#42
Nov410, 05:17 PM

PF Gold
P: 4,745

You got the word "special" right, I just don't know why you can't get the word "preferred" right.



#43
Nov410, 05:44 PM

PF Gold
P: 4,745




#44
Nov410, 06:11 PM

Sci Advisor
P: 8,470




#45
Nov410, 10:10 PM

Mentor
P: 17,293

So Mike, are you going to continue to dodge the challenge and hide from the issue? After a dozen or so requests you have had plenty of opportunity but you still can't even define your terms let alone demonstrate your claim. I suspect that you know that your claim is wrong. 


#46
Nov510, 12:48 AM

PF Gold
P: 4,745

I think Mike has defined his terms "elementary measurements" and "elementary calculations". Look at this post:
Then, somehow, he slips from a perpetually inertial frame to a noninertial frame when the traveler starts his voyage: 


#47
Nov510, 09:11 AM

PF Gold
P: 252

LT is used to transform measurements: what a clock currently reads, not a calculation of the difference between two readings on a clock. LT is a function of the CURRENT velocity. It has absolutely nothing to do with the history of how their relative speed might have varied over the course of their travel relative to one another. 


#48
Nov510, 09:15 AM

Mentor
P: 17,293




#49
Nov510, 09:55 AM

P: 1,060

"It is only if the traveller keeps on travelling past the home twin on his return that the age difference is apparent and it will be apparent to each of them." These two statements are incorrect. Perhaps I have missed something from your previous posts that puts these statements in context, if so forgive me and point them out. Don't forget you cannot use one LT for the whole of the journey for the travellers non inertial frames. Matheinste 


#50
Nov510, 10:05 AM

PF Gold
P: 4,745

In other words, if there were only one velocity involved, you could use LT directly to calculate the difference in clock readings OR you could use the tick rate multiplied by the time interval and you will get the same answer but when you introduce a new velocity, you cannot use the former method to calculate the difference in clock readings, you have to use the later twice, once for each time interval that the clock was moving at each speed and then add them together. 


#51
Nov510, 10:27 AM

Mentor
P: 17,293




#52
Nov510, 12:01 PM

P: 36

Let me explain what I see here. First let me say two principles I am taking out of the Special Relativity. 1) Any object in the same frame of reference  even if they are very far apart from each other  must agree on the fact that only one of them is younger, even if we have no clue who of them. 2) Any object next two each other must basically agree who is younger even if they are in different frames of reference. Now let A and B move away in linear motion, A and B are far away and you claim that both can claim to be younger. Now let's have C  which is using the same of frame of reference as A  next to B, and B and C will agree that they are the same age. Yet since A and C must agree that only one of them is younger  even if they are far away it follows that B and A must also agree that only one of them is younger, even if they are far away. How is this fitting together with SR? 


#53
Nov510, 01:51 PM

Mentor
P: 17,293

The easiest way to visualize this is to draw a spacetime diagram and use the Lorentz transform to label the A and Bframe coordinates. 


#54
Nov510, 06:11 PM

P: 250

Some members of this forum have expressed the opinion that the coordinates used, in an inertial reference frame, can be chosen in many different ways, and that all such choices are equally good.
The first part of that opinion is true. The second part isn't. To anyone who's opinion includes that second part, I've got these questions for you. We have all seen how the Lorentz equations (which relate the coordinates in one inertial frame to the coordinates in another inertial frame) are derived ... any textbook on special relativity will give a derivation. 1) Have you ever seen a derivation, of the equations relating two inertial frames, that use any coordinates OTHER THAN the "standard Lorentz coordinates"? (By "standard Lorentz coordinates", I mean the case where one of the four coordinates is a TIME coordinate consisting of the readings on ordinary clocks that are stationary in the given inertial frame, and the other three coordinates are SPATIAL coordinates consisting of the readings on ordinary measuring tapes that are stationary in the given inertial frame). 2) Have you ever seen equations, relating two inertial frames, WRITTEN out, and/or USED in actual calculations, where the coordinates in the equation are OTHER THAN the standard Lorentz coordinates? 3) Have you ever seen the wellknown timedilation result (that everyone has heard about, and probably often used), expressed in terms of coordinates OTHER THAN the standard Lorentz coordinates? 4) Have you ever seen the similarly wellknown lengthcontraction result expressed in terms of coordinates OTHER THAN the standard Lorentz coordinates? If you took a poll of all workaday physicists, asking them the above questions, I think you'd get very few, if any, "Yes" answers. Wonder why not? To any forum members who answer "yes" to any of those questions: Pick one of those equallygood sets of coordinates for the two inertial frames (different in nontrivial ways from the standard Lorentz coordinates), and, using those coordinates, write out the equations relating the two inertial frames, and then state the timedilation and lengthcontraction results, in terms of those coordinates. State how the postulates of special relativity would be specified in those coordinates. And then, since those coordinates are equally good, maybe you should consider writing an entire textbook that exclusively uses those coordinates. Mike Fontenot 


Register to reply 
Related Discussions  
I understand time dilation with light clocks, but...  Special & General Relativity  15  
Gravitational time dilation vs velocity time dilation  Special & General Relativity  22  
A good video to help one understand time dilation  Physics Learning Materials  1  
Calculating Gravitational Time Dilation in black hole/Future Time Travel  Special & General Relativity  5  
Wanting to make sure I understand length contraction and time dilation  Advanced Physics Homework  8 