Physics Help Forum Relativity: What conditions define a spacetime path

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Jan 11th 2019, 12:17 PM   #2

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Your question is "What conditions determine a space-time path." The answer is that all an object needs to do is exist. It will sweep out a path in space-time whatever it's motion is. (Barring falling into a singularity or traveling faster than c.)

The sample answers suggest that you are not telling us the whole question. All of the suggested answers deal with the Twin Paradox, which doesn't need to be addressed as part of your question.

So what is your full question?

-Dan
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Last edited by topsquark; Jan 11th 2019 at 12:20 PM.

 Jan 11th 2019, 01:45 PM #3 Senior Member     Join Date: Jun 2016 Location: England Posts: 964 Since the velocity is relative, it cannot be said that one is stationary and the other is moving. Thus each will see the other in an equal identical time dilation. I think that in all the twin paradox examples I have seen it is actually the acceleration of one twin while the other remains un-accelerated which defines the difference. __________________ ~\o/~ Last edited by Woody; Jan 11th 2019 at 01:51 PM.
 Jan 11th 2019, 02:25 PM #4 Junior Member   Join Date: Jan 2019 Location: ottawa Posts: 9 Since you're from England I tried to look up the equivalent term for "strike one" in cricket but couldn't find one. Allow me to elaborate. Spacetime paths are depicted by spacetime diagrams (SDs or STDs) of which there are 3 types: Loedel, Minkowski and Epstein. The loedel comes the closest to depicting both parties as moving however the relative velocities of each are depicted at "half speed". For example half speed of .6c is 1/3 c. In the Minkowski, there are two analyses which must be performed with each party depicted as stationary. It doesn't mean either is stationary. Acceleration has nothing to do with establishing age difference. Frame jumps can be done without acceleration by passing off clock info at close range and a twin paradox is still in effect. Acceleration may be equivalent to the force of gravity in GR but time dilation due to gravity in GR is not the same as time dilation due to acceleration in SR even though SR is a special case of GR. In fact I keep hearing conflicting reports that acceleration can't cause any type of time difference in SR although I find that hard to believe as acceleration can be linearized into a constant relative velocity. I should add this to my list of questions I need determined. Having spent years on other forums, I've learned the surest way to get banned is trying to help people understand relativity or any other science because their beliefs are sacred to them, math be damned. I'm afraid I'm on this forum for purely selfish reasons so I want to keep my interactions to a minimum unless they're with experts. Last edited by ralfcis; Jan 11th 2019 at 04:09 PM.
Jan 11th 2019, 04:17 PM   #5

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 Originally Posted by ralfcis Since you're from England I tried to look up the equivalent term for "strike one" in cricket but couldn't find one.
This is a place where anyone can ask a question. It is not some kind of contest where we are to answer your question and have it be judged. If something is wrong with a response then feel free to tell us. But right or wrong, be polite.

I must assume, then, that you are indeed looking for information about the Twin Paradox. There are two problems that most people have in talking about this. The first is that, even if you "pass the clock" to a person for the trip back you are still dealing with multiple reference frames. The second problem is that we using an "indefinite metric." Things that seem obvious can easily be wrong.

My scanner is in the pits again so I'll have to risk your displeasure and refer you to a diagram on Wikipedia. Here. Look for the diagram in the "Relativity of Simultaneity" section. If you like, don't bother to read the passage, but my explanation is going to be basically the same.

The problem is that the outgoing reference and incoming reference frames are very different. The time axes meet at an angle and that causes a huge change in the time rates. The frame of the traveling twin changes dramatically (even if you make the change of direction slowly) and is the cause of the difference in aging between the two twins. This can be seen in how the blue lines, representing how the traveling twin sees time, change very quickly at the turnaround point into the red lines. Going from blue to red sweeps out a large time in the stationary twin's frame.

It's actually a bit easier to see using GR. There is a big difference in the two frames. Everyone can agree that the traveling twin undergoes motion in a gravitational field (via the Principle of Equivalence) and this changes his clock; a change that the stationary twin does not undergo.

Call me an expert or a wannabe. The acceleration is in fact the determining factor in the age difference.

-Dan
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Last edited by topsquark; Jan 11th 2019 at 04:20 PM.

 Jan 11th 2019, 04:42 PM #6 Junior Member   Join Date: Jan 2019 Location: ottawa Posts: 9 Let's use the example of Alice leaving on a 3 ly roundtrip at .6c. As I said many have an opinion of how the age difference unfurls. Yours is the swing of Alice's line of simultaneity at her turnaround means Bob ages from 3.2 to 6.8 yrs and Alice would have aged 1.6 yrs if she had remained in constant velocity with Bob hence Bob ends up ageing 3.6-1.6 =2 yrs more than Alice. If you have a different way to arrive at the same result I'd be very interested in seeing it. Now I know for a fact, which doesn't appear in any wiki article or book, that age difference cannot be determined in relativity until the two re-unite. But you are saying all the age difference happens in a huge instantaneous swing at the turnaround point. Of course I'm assuming that is your contention based on many discussions I've had in the past but the one expert I trust because he is an expert said this is not a correct viewpoint. (Neither is the acceleration multi-frame belief.) But he would not answer what is the correct interpretation and that's what I'm trying to find. I have my own explanation which is outside of relativity so I can't discuss it here. Yes I'm trying to control my rudeness so I did kindly answer Woody's question but I do not want side issues to distract from my goals here.
 Jan 11th 2019, 05:39 PM #7 Junior Member   Join Date: Jan 2019 Location: ottawa Posts: 9 You can pre-moderate this if you want. I haven't seen a forum yet that pre-moderates and it worries me a lot that you're both a participant and a moderator which I view as a huge conflict of interest. As a moderator, you are tasked with protecting the participants at large from fake science. But as a participant your knowledge of certain subjects is limited so you would view what you don't know as fake science. With pre-moderation you can block what you see as fake science from the eyes of anyone who has the expertise to make that determination. If this is the modus operandi of this forum, I'll just move on. Unless of course you do have real experts on relativity and we can both get over this initial rough patch. Other forums I've rejected have admitted to having no such experts in hard science (as opposed to biology or earth sciences) so I'm wondering if this is the case here. If it is, do you know of any physics forums. I'm just going down a google list hoping to find one. Yes, I'm terribly arrogant if you're wondering.
 Jan 11th 2019, 05:52 PM #8 Junior Member   Join Date: Jan 2019 Location: ottawa Posts: 9 I love your post of Einstein's original paper. I'm on page one and I don't know if it was a fluke that he used a magnet and a coil to start it off but, if not, it was a stroke of genius that I think, the implications of which, most people have missed. The relative motion to EM fields says all you need to know about relativity which would not have come out if he used some example of Newtonian motion between two teacups for example.
 Jan 11th 2019, 07:54 PM #9 Forum Admin     Join Date: Apr 2008 Location: On the dance floor, baby! Posts: 2,781 I don't have time at the moment to read through all your replies but I will say this: I am not a Moderator. I am an Administrator and that means I have to use my best judgement about what threads are going to be allowed. There are currently no Mods...Basically I'm it. I will say that I work hard to keep my "Ad" job distinct from my "Help" job. If you post something I think look suspicious I will poke around and let the member make a certain number of posts (which vary) to determine what's what. If you think that I've done something Ad-wise you don't agree with please feel free to let me know over a PM. I make mistakes every now-and-again and I'm not above apologizing when I'm wrong. -Dan __________________ Do not meddle in the affairs of dragons for you are crunchy and taste good with ketchup. See the forum rules here.
 Jan 12th 2019, 08:42 AM #10 Junior Member   Join Date: Jan 2019 Location: ottawa Posts: 9 Ok so my last post was lost so I'll re-type. We can exchange "You're wrong, no , you're wrong", all thread long but I'm sure you must have a relativity expert on board who can settle this. Your interpretation of relativity is about the 2nd most prevalent misconception about age difference and how it unfolds. Let's do an example. Alice leaves Bob for a 3 ly roundtrip at .6c. Now you've said there are multiple frames in even a clock handoff (which is false) and you probably believe and therefore should be able to do the calculations of a rounded off turnaround acceleration to show how the acceleration is the culprit in causing age difference. Actually if you assume the acceleration part is a small percentage of the overall time travel then there's no way, no matter how much math you throw at it, that you can support your contention. Now while you're trying to work that out, let's just break down the problem into an idealized state where the turnaround is instantaneous. Surely you would have some math to support the age difference should be infinite since you contend it's all due to acceleration. But if you use the same diagram as you pointed out in the wiki article, you'd see relativity's only valid answer is when they re-unite, Bob is 10 and Alice is 8. So apparently infinite acceleration does not result in infinite age difference, there's a limit. There are many wrong ways to arrive at these numbers. The wrong way you have chosen is that Alice's line of simultaneity swings from t=3.2 to t=6.8 on Bob's timeline and that's where the age difference occurs. Except that equals 3.6 years, not two. I'm able to account for the discrepancy but can you? I sincerely doubt it but even if you can, your interpretation would still be completely wrong. Now you won't believe me or the math so please get your expert to weigh in.

 Tags conditions, define, path, relativity, spacetime

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