1. Originally Posted by malaidas
another way to separate this is to drop back to basics. let's take two cars approaching one is doing 70 the other 60. however each will observe the other doing 130. but of course neither of these are correct each is actually doing their measure speed relatuve to the earth that is rotating and travelling through space etc. your observation is always relative to something else, you are stationary.

however despite our everyday observations, space-time isn't so simple. your experience of such ultinately depends upon an absolute velocity even if you cannot observe it. the faster in this sense you go, the more it 'warps' for you. this effect however becomes apparent to observation relatively. the observer is part of the measurement, he is not part of what I actually going on. The net effect of this is that your twins are always experiencing the same things because they are always travelling in the same frame of reference. but they will both observe something different to this. Each will observe time slowing down for the other, but in reality their clocks are going to be identical and equally different to the neutral observer, in a global sense.
Yes, apparently I have missed this post of yours.
Great.

2. its was part of Einsteins genius to recognize both this and how it didn't seem to apply to light properly and as a result rewrite our entire understanding of the universe as a result of solving the paradox.

3. Originally Posted by Tempeststrawberry
"Both Alice and Beth observe the time in the other spaceship to be flowing slower in the outbound and inbound parts of their trips, but because of the "jump forward" effect when they turn around, they will be of the same age when they meet at C."

Yes, this does the job in a spacetime diagram.
Now I am trying to visualize how this 'jump forward' effect will materialize in physical terms, without violating continuity..
Thx..
Here's yet another page with some spacetime diagrams. If you look at the bottom one, it focuses on the light signals that might travel from one ship to the other. So if Bob sends out signals at a constant rate, you can see that the rate at which Alice receives those signals varies throughout the trip. That might help make sense of what each observer would actually see from the other traveller over the course of the journey.

4. Great posts from both of you guys..
Having found you together, another quick question:

Are blackhole event horizons apparent or absolute?

5. Ken's a better person to ask than me on this, but so far as I am aware the event horizon is just a perimeter in space, created as a ramification of the laws of physics, there's no physicality to it beyond this. That is its the point where gravity has become weak enough, beyond which light isn't inherently trapped. However it does have certain other ramifications I think.
Last edited by malaidas; 2017-Mar-10 at 04:21 PM.

6. Originally Posted by Tempeststrawberry
Great posts from both of you guys..
Having found you together, another quick question:

Are blackhole event horizons apparent or absolute?
It might make sense to ask this in a separate thread. But as far as our understanding goes, absolute. You might not necessarily notice anything unusual on the way in (for large enough black holes, there isn't anything especially notable about crossing the event horizon one way), but once across, no signals can make it back out.

7. Established Member
Join Date
Jan 2007
Posts
1,039
Originally Posted by Tempeststrawberry
I am not talking about their absolute ages. Pls read what I said carefully.
I did. You have sound and fury, signifying nothing.

DO THE MATH. Or let this drop.

Relative to me, you are zooming off and coming back 20 years later, having travelled at relativistic speeds.
Thus in my viewpoint you are younger than me, because of relativistic time dilation.
From your viewpoint same aplies, because I have travelled in the same manner and aged less, relative to you..
The formula says so, not me.
In your scenario, which you have not yet worked through, no twin experiences merely a single inertial reference frames. Rather they must stitch together multiple inertial reference frames in order to produce a series of momentarily co-moving reference frames that "match" their perspective. You can see Bernard Schutz's "A First Course in General Relativity" does this in the first chapter (at least in one edition) for the standard twin "paradox" and you are merely doubling the work.

Working through the scenario, one can see that from either twin, one can work out exactly what age they will be when they meet up.

Our absolute ages are irrelevant.
Unless you are referring to the carefully defined "proper time", there is no such thing as absolute age.
I don't always have to plug in numbers to see where the equation is really going.
OK, but in this case, you haven't a clue what the logic of the scenario is. Many people have pointed out your ignorance here and sticking your head in the sand farther won't aid you.
One of us is wrong.
Yes, and I will stick with what I learned and did from courses and textbooks, until you can show me a genuine argument.

You don't need math to know that an apple is bigger than a cherry.
http://www.guinnessworldrecords.com/...aviest-cherry/

I wonder if there has ever been an apple that, at some point, weighed less than 21g?

8. Established Member
Join Date
Jan 2007
Posts
1,039
Originally Posted by malaidas
another way to separate this is to drop back to basics. let's take two cars approaching one is doing 70 the other 60. however each will observe the other doing 130. but of course neither of these are correct
Actually, both are correct. This is the lesson of contemporary physics: we can do the physics from either starting point and get the same answers for what events happen. This is why people get upset at the twin "paradox", since it looks, superficially, like choosing a different starting point gets different answers for what events happen. In reality, it is only by not paying attention to the physics that one gets different answers.

9. Originally Posted by Tempeststrawberry
Are blackhole event horizons apparent or absolute?
Both. The absolute and apparent horizons are the same thing for a quiescent black hole, but for one that is accreting mass-energy there's a difference between the absolute horizon, which is a spacetime invariant, and the apparent horizon, which is observer-dependent.
The absolute horizon separates regions of spacetime in which all emitted photons end up at the singularity from regions of spacetime where some emitted photons can escape to infinity. The apparent horizon separates regions of spacetime in which all outward directed photons fall inwards from regions where some outward directed photons move outwards. The two are different in a "feeding" black hole because a photon that is initially moving outwards can end up moving inwards again when the black hole increases in mass. Such a photon would be temporarily outside the apparent horizon, but inside the absolute horizon.

Grant Hutchison

10. Ok, I have problems with an absolute eh..
Thanks for the help today.
Cu.

11. Originally Posted by Kwalish Kid
Actually, both are correct. This is the lesson of contemporary physics: we can do the physics from either starting point and get the same answers for what events happen. This is why people get upset at the twin "paradox", since it looks, superficially, like choosing a different starting point gets different answers for what events happen. In reality, it is only by not paying attention to the physics that one gets different answers.
ok, yes, but the point I was trying to make is that the velocity you wish to take depends upon the observer, in fact tough neither are correct in an absolute sense seeing as they don't take into account the movement of the earth etc. Every measurement is relative to something else. Ordinarily this is irrelevent but I find it kind of essential to point out in understanding relativity, because the difference between the observed and the observer is so critical here.

12. Established Member
Join Date
Jan 2007
Posts
1,039
Originally Posted by malaidas
ok, yes, but the point I was trying to make is that the velocity you wish to take depends upon the observer,
Also incorrect. The point of relativity in physics is that anyone can take any system of coordinates (almost) and make the physics work out correctly. When I drive, I usually judge things based on speeds where the road is stationary. I have a speedometer that does this for me!

Relativity theory actually doesn't care for observers at all. Usually, talk of "observers" is crude shorthand for a specific system of coordinates in which a particular object is at rest.

13. if you wish to get technical yes, but that doesn't help in giving a helpful visualisation.

HOWEVER, observed velocity is a product of the observer. The speed you see on your dashboard is relative to the earth, the speed you would measure (unless you took account of your own velocity in relation to the earth) for vehicle coming towards you would be the sum of your velocities relative to the earth. all of which are relative amounts next to the 'absolute' velocity that determines ultimately how time will pass for you etc. Given this concept sits at the root of relativity and Einstein's thought experiments you are on dodgy ground if you are going to argue. ETA: never forget that in science observer is equivalent to measurement, so your speedometer is equally an observer as much as yourself.

Yes in technicality it isn;t about the 'observer' per se, its about the frame of reference but, when talking about people the term observer is important, because the question is ulmitately about what they would see and how this relates back to what is actually going to be the case.
Last edited by malaidas; 2017-Mar-10 at 05:29 PM.

14. Originally Posted by Tempeststrawberry
Ok, I have problems with an absolute eh..
Thanks for the help today.
Cu.

15. Established Member
Join Date
Jan 2007
Posts
1,039
Originally Posted by malaidas
if you wish to get technical yes, but that doesn't help in giving a helpful visualisation.
I'm not prepared to support an incorrect visualization just because it is a visualization.
HOWEVER, observed velocity is a product of the observer. The speed you see on your dashboard is relative to the earth, the speed you would measure (unless you took account of your own velocity in relation to the earth) for vehicle coming towards you would be the sum of your velocities relative to the earth.
If I have a radar gun, then I could use that to measure the speed of any object relative to me. If I had access to sensors embedded in the road, then I could get the speed of any object relative to the road. What matters to a collision is the relative momentum, and this can be calculated in any system of coordinates. Objects in space, without any reference to Earth, can have relative velocities.

all of which are relative amounts next to the 'absolute' velocity that determines ultimately how time will pass for you etc. Given this concept sits at the root of relativity and Einstein's thought experiments you are on dodgy ground if you are going to argue. ETA: never forget that in science observer is equivalent to measurement, so your speedometer is equally an observer as much as yourself.
You have a strange view about what is at the root of relativity. Observers are not at the root of special relativity; systems of coordinates and commitments to the constant speed of light are at the root of special relativity.

Yes in technicality it isn;t about the 'observer' per se, its about the frame of reference but, when talking about people the term observer is important, because the question is ulmitately about what they would see and how this relates back to what is actually going to be the case.
What observers might see is vastly different from what most people describe in simple relativistic scenarios. What most people describe is the coordinate locations of events and they ignore entirely what happens to the light that leaves from those events that might be observed. This is why I recommend not using observer talk at all and instead speak of the location in spacetime of events in systems of coordinates.

16. what your saying is uttey correct when doing the math. but its less helpful in getting a basic handle upon what the math actually means in real terms. coordinate systems in 4D space are very abstract. We can therefore talk about observers within that changed coordinate system and talk in terms of the story of what will happen to them relative to observers in a different coordinate system.. and the fact is that because of such each observer will observe things which won't be true when they all try to make sense when they all join the same coordinate system except through then doing the maths.

what I mean by it lying at the root of relativity is that the key observation that led to it is that whilst usually velocity is observed to be relative to sonetjing else, but this is not true for light. It is always measured to be C regardless of your frame of reference. the whole of special relativity grew out of this observation. the result is the mathematical theorem herein changes in coordibate sysyems occur as you say.
Last edited by malaidas; 2017-Mar-12 at 04:29 PM. Reason: typos

17. and the reason of course for the discrepancy between what each will observe about the other during tramsit vs what each will observe when they arrive back in the original coordinate system, is because their observations are subject to a different coordinate system based upon the relative motion of the 2. what they are observing about the other is not what's actually going on relative to their origin coordinate system.

thus whilst not completely accurate, what i am saying about the difference between the observer and the observed is not too far from the truth. that each observes time slowing down for the other relative to themselves is not a paradox it's a by product of the others apparent motion approaching towards C far more than their actual motion is. leading to the other appearing to be in a different coordinate system than they are. but so long as their transit through space time doesn't differ in the actual coordibate systems at any point the final observation when they compare notes will be identical, regardless of how things appeared during transit. indeed so long as the changes were same even at different moments. so you could have them doing inverse speeding up/slowing down times and it'll obviously still add up.

the maths tells the full story in full accuracy but only if you first understand how to apply it.

ETA: and thus what Einstein proved is that there is no difference for light in terms of physics, in spite of the discrepancy we observe, only an apparent difference in these observations. The only difference in actuality is the every day speeds we observe on earth are too close for the differences between a calculation/measurement based upon space and space -time to be observable and equally that observation is not the same as actual motion of the observed. Your observation is relative to you. the actual motion is not.

if you want to take this to adsurdum we can say that your actual motion is relative to an absolute zero or the actual speed of light both of course lead to something of a mathematical problem. so instead we always deal with relative things. What you observe about yourself is relative to some baseline what you observe about someone else is relative to yourself. Hut the universe doesn't care about what you think or how you measure it lol; the reality is you do have some absolute motion, (assuming the universe isn't rotating), you just can never calculate what it is.

As a result of ALL of this you can still work happily with relatives so long as you understand that your observation is always about the relative measurements and your calculations will be relative to something else that hasn't changed.

So yes understanding the difference between motion and apparent motion is at the core of understandng here. Every measurement you make will be based upon upon totality of relative motion, (at that moment ), based upon the objects you are considering at that time and the paths they have taken through space-time to get there. it'll always add up, should you return to the same coordinate system but there will be discrepancies until then. However if they both based their calculations of another common reference point and knew the actual motion of their twin, (relative to this), then thIy could calculate that in fact no relative difference between them was happening in spite of what their eyes are telling them. the differences would be symmetrical to their neutral reference point
Last edited by malaidas; 2017-Mar-12 at 10:41 PM. Reason: punctuation

18. I appreciate that I don't have to explain relativity to you, but i do want you to see how just understanding the math is not enough to give understanding about nature to most people. they need to understand the context within which that maths operates and the empirical evidence that backs it up.

ETA: and I have found that one of the most effective ways of getting this across is with the simplification, that what the observer sees is not the same and what is happening to the thing they are observing. Its not completely accurate but to someone who doesn;t come from a pure maths background it carries more explanatory power than just talking about the actual mathematics of what is going on to actually account for this. Indeed in order to make proper sense of the paradox, the OP put up, you need a third neutral reference point, that allows you to know something definite to tie your observations back to what is actually going on
Last edited by malaidas; 2017-Mar-13 at 11:44 AM.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•