# The Naked Scientists Forum

#### thebrain13

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« on: 15/06/2006 21:10:40 »
lets say there are two spaceships containing one pilot each, they are identical twins of course. We'll call them pilot a and pilot b. pilot b fires his engines and travels at 99 percent the speed of light away from pilot a for one year. Then turns around and comes back. Einstain says that from pilot a's perspective pilot b would have aged less than him. However Since velocity is relative to each observer, pilot b sees pilot a speeding away from him and then returning. So pilot b should see a younger pilot a. So my question is when the two meet each other after the trip which one is younger?

#### another_someone

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« Reply #1 on: 15/06/2006 21:57:34 »
Speed is relative, but acceleration is not.

When pilot b switches on hit rocket motors, he will feel a kick up the backside (and every other part of him touching his accelerating space ship), while pilot a will still be floating in zero gravity.

George

#### thebrain13

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« Reply #2 on: 15/06/2006 22:43:26 »
Although i am not sure what you mean by your statement that acceleration is not relative, but doesnt that imply that there is some prefered speed of objects, or absolute motion? the way i understand it, you are saying that an object somehow keeps track of wether it has been accelerated for the entirety of its' lifespan, in order to make sure that pilot a is in fact older than pilot b. Are you saying that the twin paradox occurs only in the period in which an object is accelerated. so if an object accelerates for a period of one second, does that mean that is the maximum amount of time in which his time can be off for the twins? because once they stop accelerating all frames of reference are equal right?

#### another_someone

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« Reply #3 on: 16/06/2006 00:28:32 »
quote:
Originally posted by thebrain13
Although i am not sure what you mean by your statement that acceleration is not relative, but doesnt that imply that there is some prefered speed of objects, or absolute motion?

What I mean by acceleration is absolute is that: F=mA, where F = force, m = mass, and A = acceleration.

You can measure force without reference to any outside object (if you were blindfolded, you would still be able to know whether you were accelerating or remaining at a constant velocity, therefore acceleration must be absolute and not relative to something else).

quote:

the way i understand it, you are saying that an object somehow keeps track of wether it has been accelerated for the entirety of its' lifespan, in order to make sure that pilot a is in fact older than pilot b. Are you saying that the twin paradox occurs only in the period in which an object is accelerated. so if an object accelerates for a period of one second, does that mean that is the maximum amount of time in which his time can be off for the twins? because once they stop accelerating all frames of reference are equal right?

In a sense, this must be so.

I don't know the exact ways it resolves itself, but what one can say is that pilots a and b do not, and cannot, exist in isolation, but exist within the context of the wider universe, which does influence the space within it, and so these pilots, in their space ships must have a motion relative to the motion of the total universe.

George

#### ukmicky

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« Reply #4 on: 16/06/2006 01:21:15 »

http://www.weburbia.com/physics/acceleration.html

The "paradox" is only a conflict between reality and your feeling of what reality "ought to be". - Richard Feynman
Michael
« Last Edit: 16/06/2006 01:37:25 by ukmicky »

#### thebrain13

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« Reply #5 on: 16/06/2006 02:09:34 »
i've read, the stella and terrance phenomena for an explanation of the twin paradox. And to me it seems like a way of masking the problem. Both points of view are still equally applicable, the article states that when stella is leaving terrance, terrance views a redshift when shes moving away and a blue shift when shes coming forth. BUT WHY DOESNT STELLA VIEW A REDSHIFT. The article doesn't say that in order for this to work terrance is the only one to observe the redshift. Are we now stating that if you were ACTUALLY accelerated, then you are free from observing the redshift and you are in fact, the actual, prefered, absolute, frame of reference?

#### another_someone

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« Reply #6 on: 17/06/2006 01:35:09 »
quote:
Originally posted by thebrain13

i've read, the stella and terrance phenomena for an explanation of the twin paradox. And to me it seems like a way of masking the problem. Both points of view are still equally applicable, the article states that when stella is leaving terrance, terrance views a redshift when shes moving away and a blue shift when shes coming forth. BUT WHY DOESNT STELLA VIEW A REDSHIFT. The article doesn't say that in order for this to work terrance is the only one to observe the redshift. Are we now stating that if you were ACTUALLY accelerated, then you are free from observing the redshift and you are in fact, the actual, prefered, absolute, frame of reference?

In essence, that is the interpretation that I would read into the explanation (if not actually a case of not seeing the red shift, but merely seeing less of the red shift, while seeing more of the blue shift).

One consequence that appears to me to come from this is that if two objects are moving relative to each other in space,m one cannot tell what the relative time references (and thus the true red/blue shift) is of the two objects unless one knows the history of the object (i.e. which object accelerated away/towards which?).  This must have an inference for the astronomical observation of distance objects in the universe, objects that are assumed to be receding at very high velocities from us – but did we start moving away from it, or did it start moving away from us?

George

#### Roy P

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« Reply #7 on: 17/06/2006 12:29:14 »
quote:
Originally posted by thebrain13
BUT WHY DOESNT STELLA VIEW A REDSHIFT.

I'm trying to get my head around this. How do we *know* Stella doesn't see a redshift?

__________________________________________________________
Roy P

#### another_someone

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« Reply #8 on: 17/06/2006 12:38:13 »
quote:
Originally posted by Roy P
How do we *know* Stella doesn't see a redshift?

I suppose that until someone actually sends Stella out there at 99% the speed of light, we do not actually know, we merely predict that this is what would happen.  If it did happen, then we would simply have to ask Stella when she got back what it was she saw out of the window.

George

#### ukmicky

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« Reply #9 on: 17/06/2006 13:18:31 »
Ok i'm not too up on this it can be very confusing.

but if stella is traveling at 99% of c wouldnt the space in front of her be Length Contracted shortening the wavelength of any photons in front of her, blue shifting everything.
Michael
« Last Edit: 17/06/2006 13:20:07 by ukmicky »

#### Roy P

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« Reply #10 on: 17/06/2006 15:38:46 »
quote:
Originally posted by ukmicky
but if stella is traveling at 99% of c wouldnt the space in front of her be Length Contracted shortening the wavelength of any photons in front of her, blue shifting everything.

Now that's what I would have thought, but the theory says no. Well, no according to Stella's POV. It's why I have my doubts about 'redshift' overall when it comes to speed and its effects on light.

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Roy P
« Last Edit: 17/06/2006 15:39:57 by Roy P »

#### ukmicky

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« Reply #11 on: 17/06/2006 22:54:36 »
where's soul surfer when you need him
Michael
« Last Edit: 19/06/2006 00:08:33 by ukmicky »

#### Solvay_1927

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« Reply #12 on: 19/06/2006 02:22:28 »
I'm a little confused as to what the problem is here.

The theory (of Special Relativity) and the link (about Stella and Terrance) both say that Stella DOES see the light flashes (from Terrance’s clock) as being redshifted on the outward journey and blueshifted on the return journey.

quote:
The regular pulses are redshifted to lower frequencies during the Outbound Leg, and blueshifted to higher frequencies during the Inbound Leg … And Stella computes the same for Terence … Stella sees what Terence sees: a slow clock on the Outbound Leg, a fast clock on the Inbound Leg.

Or have I misunderstood – is it some other part of the (Stella / Terra) link that’s confusing everybody?

#### thebrain13

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« Reply #13 on: 19/06/2006 02:36:56 »
Okay, so if they both see redshifts on the outbound leg, and they both see blue shifts on the inbound leg, whys one older?

#### another_someone

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« Reply #14 on: 19/06/2006 02:55:59 »
quote:
Originally posted by thebrain13

Okay, so if they both see redshifts on the outbound leg, and they both see blue shifts on the inbound leg, whys one older?

I think the crucial bit the Paul (Solvay_1927) left out was:

quote:

All well and good, but this discussion at first just seems to sharpen the paradox!  Stella sees what Terence sees: a slow clock on the Outbound Leg, a fast clock on the Inbound Leg.  Whence comes the asymmetry between Stella and Terence?
Answer: in the duration of the Inbound and Outbound Legs, as seen.  For Stella, each Leg takes about a year.  Terence maintains that Stella's Turnaround takes place at year 7 at a distance of nearly 7 light-years, so he won't see it until nearly year 14.  Terence sees an Outbound Leg of long duration, and an Inbound Leg of very short duration.
So there's the fundamental asymmetry: the switch from redshift to blueshift occurs at Stella's Turnaround.  Stella sees Terence's telescopic image age slowly on her Outbound Leg, but the image more than makes up for its dawdling on the Inbound Leg.  Terence sees Stella's image off to a slow start too, but here the image's final burst of rapid aging comes too late to win the race.

George

#### Solvay_1927

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« Reply #15 on: 19/06/2006 02:59:20 »
To quote from the link again:
quote:
All well and good, but this discussion at first just seems to sharpen the paradox!  Stella sees what Terence sees: a slow clock on the Outbound Leg, a fast clock on the Inbound Leg.  Whence comes the asymmetry between Stella and Terence?

Answer: in the duration of the Inbound and Outbound Legs, as seen.  For Stella, each Leg takes about a year ... Terence sees an Outbound Leg of long duration, and an Inbound Leg of very short duration.

So there's the fundamental asymmetry: the switch from redshift to blueshift occurs at Stella's Turnaround.  Stella sees Terence's telescopic image age slowly on her Outbound Leg, but the image more than makes up for its dawdling on the Inbound Leg.  Terence sees Stella's image off to a slow start too, but here the image's final burst of rapid aging comes too late to win the race. [In other words, Terrance sees Stella's clock as not going fast enough on the return leg to catch up with the time on his own clock.]

#### Solvay_1927

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« Reply #16 on: 19/06/2006 03:00:25 »
Oh.

Timing was never my strong point.

#### thebrain13

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« Reply #17 on: 19/06/2006 03:55:14 »
history seems to be repeating itself, ive read multiple explanations on how stella is able to seperate her frame of reference from terrances', based on their relative velocities from one another. Too me it seems that all the explanations disguise the problem in a different way. first explanation, stella was accelerated. Second explanation, stella was redshifted. And lastly, stella was taking place in the turnaround event. All subtle ways to provide terrance with a superior frame of reference while keeping einsteins postulate that, there is no prefered intertial frame, intact(meaning all motion is relative) My rebuttle has been consistent.

Granted all frames of reference are equally valid

WHY WOULDN'T STELLA VIEW TERRANCE ACCELERATING?
WHY WOULDN'T STELLA VIEW TERRANCE REDSHIFTED?
WHY WOULDN'T STELLA VIEW TERRANCE'S TURNAROUND EVENT?

Anybody want to become part of the fourth bold statement?[8D]

#### another_someone

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« Reply #18 on: 19/06/2006 14:49:25 »
quote:
Originally posted by thebrain13
All subtle ways to provide terrance with a superior frame of reference while keeping einsteins postulate that, there is no prefered intertial frame, intact(meaning all motion is relative) My rebuttle has been consistent.

Granted all frames of reference are equally valid

WHY WOULDN'T STELLA VIEW TERRANCE ACCELERATING?
WHY WOULDN'T STELLA VIEW TERRANCE REDSHIFTED?
WHY WOULDN'T STELLA VIEW TERRANCE'S TURNAROUND EVENT?

Anybody want to become part of the fourth bold statement?[8D]

First correction, Special Relativity does not say all frames of reference are equal – you got it right the first time: all inertial frames of reference are equal.  An accelerating frame of reference is not an inertial frame of reference.

The point about “WHY WOULDN'T STELLA VIEW TERRANCE ACCELERATING?” rather misses the point.  You are only concentrating on the visual sense, and I agree that the visual sense cannot tell the two apart.  As a comparison, it is often the case that if you sit in a railway carriage, and look at an adjacent train begin moving past you in the opposite direction, it sometimes  takes a moment for you to work out whether it is your train that has started to move forward or the other train that has started moving in the opposite direction.  As you rightly surmise, visually the two experiences are identical.  On the other hand, if you close your eyes, you can sense whether your train is accelerating or not, and if your train is not accelerating, then it must be the other train that is accelerating.

As for  “WHY WOULDN'T STELLA VIEW TERRANCE REDSHIFTED? “ and “WHY WOULDN'T STELLA VIEW TERRANCE'S TURNAROUND EVENT?”, neither of these two questions are correct.  It is not the case of whether Stella will not see Terrance redshifted, because she will see Terrance redshifted – the question is, for how long will Terrance be redshifted.  Nor is it a question of whether Terrance will observe Stella to turnaround, but rather when will he see the turnaround event.

I totally accept your point that having 3 different explanations for the same supposed phenomenon does seem a bit of a fudge.  It might be easier to understand if we could actually perform the experiment; but alas we do not have the wherewithal to undertake such an experiment, so we seem to be left with a number of fudge solutions, with the argument being that in the physical world you cannot actually tell which is the right explanation because to the observers they would all look the same, even though the underlying explanation may be radically different.  It seems more a matter of pragmatic science than creating a unique model of events (I suppose one could argue the same problem between the particle/wave duality in a different scale in science, except that with the particle/wave duality, the two models do create different answers in some situations).

George

#### Soul Surfer

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« Reply #19 on: 19/06/2006 19:15:38 »
I did not come into this argument because I thought that the first reply was adequate.  The initial question is also a bit ambiguous because it is not clear whether the one year journey is from pilot a's or pilot b's point of view although it seems most likely that it was pilot b's in which case the pilot that does not accelerate will experience a period of more than one year before pilot b returns.  On the practicalities of the experiment people are not like clocks and you can only tell age differences superficially if the are quite large their respective digital watches (which I presume they are carrying) will be a much more accurate measure of the difference in time experienced between them.  On a practical point human beings do not likeaccellerating much faster thane 1G and getting up to 99% of the speed of light takes quite a long time and then b has to take just as long to decelerate and acelerate and decelerate on the journey back.

Learn, create, test and tell
evolution rules in all things
God says so!

#### thebrain13

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« Reply #20 on: 29/06/2006 00:17:56 »
okay i've been away for a little while, where to start. first off, your initial rebutal says, acceleration is not relative, or you can tell that you are accelerating. This is simply not the case, you can only tell you are accelerating relative to another object. For example, if you are driving a car and you hit the gas, you consequently feel a push from the seat, this is not because YOU are accelerating but the car is, and is pushing you forward. So lets say you painted the windows black and you hit the gas again, this time some force from within your body is pushing you forward, so you and the car are accelerating equally. In this case, you dont feel any push, so how do you SENSE you are accelerating?

Secondly, lets not deflect attention to acceleration because although einstein does say, only inertial frames of reference are equal, einsteins' time dilation equation is not based on acceleration, it is based on relative velocities. And this phenomena does not soley exist in the moments an object is non inertial.

It seems like I still see the same mistakes I was talking about in my last post, with your rebuttal of my statement, why doesn't stella view a redshift. you say it is a question of how long terrance will be redshifted. So my fourth bold statement is,
WHY WOULDNT STELLA VIEW TERRANCE REDSHIFTED AS LONG?

You also state that it is not a question of wether terrance will view stella to turn around but when.
So my fifth bold statement is
WHY WOULDNT STELLA VIEW TERRANCE TURNING AROUND AT THE SAME TIME?
To get to the heart of my argument, how can you distinguish between two objects using relative motion, when both motions have to be exactly equal at all times?

you also say I am paying to much attention to the visual, I dont think so, all experiments are observed not sensed. Although im not sure where you were going with that.

Also, I just glanced over your second post, you say,I don't know the exact ways it resolves itself, but what one can say is that pilots a and b do not, and cannot, exist in isolation, but exist within the context of the wider universe, which does influence the space within it, and so these pilots, in their space ships must have a motion relative to the motion of the total universe.

This is a blatant example of absolute motion. Given newtons third law, every action has an equal an opposite reaction, the overall motion of the universe has to remain constant, or absolute, you might say. so anything relative to the universe as a whole is as absolute a velocity as they come. Secondly, this is clearly not what einstein was saying, he says any non inertial frame of reference is equally valid, not just the frame of reference which is, the entire universe.
« Last Edit: 09/07/2006 22:48:41 by thebrain13 »

#### thebrain13

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« Reply #21 on: 29/06/2006 00:17:56 »
okay i've been away for a little while, where to start. first off, your initial rebutal says, acceleration is not relative, or you can tell that you are accelerating. This is simply not the case, you can only tell you are accelerating relative to another object. For example, if you are driving a car and you hit the gas, you consequently feel a push from the seat, this is not because YOU are accelerating but the car is, and is pushing you forward. So lets say you painted the windows black and you hit the gas again, this time some force from within your body is pushing you forward, so you and the car are accelerating equally. In this case, you dont feel any push, so how do you SENSE you are accelerating?

Secondly, lets not deflect attention to acceleration because although einstein does say, only inertial frames of reference are equal, einsteins' time dilation equation is not based on acceleration, it is based on relative velocities. And this phenomena does not soley exist in the moments an object is non inertial.

It seems like I still see the same mistakes I was talking about in my last post, with your rebuttal of my statement, why doesn't stella view a redshift. you say it is a question of how long terrance will be redshifted. So my fourth bold statement is,
WHY WOULDNT STELLA VIEW TERRANCE REDSHIFTED AS LONG?

You also state that it is not a question of wether terrance will view stella to turn around but when.
So my fifth bold statement is
WHY WOULDNT STELLA VIEW TERRANCE TURNING AROUND AT THE SAME TIME?
To get to the heart of my argument, how can you distinguish between two objects using relative motion, when both motions have to be exactly equal at all times?

you also say I am paying to much attention to the visual, I dont think so, all experiments are observed not sensed. Although im not sure where you were going with that.

Also, I just glanced over your second post, you say,I don't know the exact ways it resolves itself, but what one can say is that pilots a and b do not, and cannot, exist in isolation, but exist within the context of the wider universe, which does influence the space within it, and so these pilots, in their space ships must have a motion relative to the motion of the total universe.

This is a blatant example of absolute motion. Given newtons third law, every action has an equal an opposite reaction, the overall motion of the universe has to remain constant, or absolute, you might say. so anything relative to the universe as a whole is as absolute a velocity as they come. Secondly, this is clearly not what einstein was saying, he says any non inertial frame of reference is equally valid, not just the frame of reference which is, the entire universe.
« Last Edit: 09/07/2006 22:48:41 by thebrain13 »

#### thebrain13

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« Reply #22 on: 09/07/2006 22:49:23 »
Come on, you gonna let me get away with that?

#### ukmicky

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« Reply #23 on: 10/07/2006 01:06:22 »
quote:
Originally posted by thebrain13

okay I've been away for a little while, where to start. first off, your initial rebuttal says, acceleration is not relative, or you can tell that you are accelerating. This is simply not the case, you can only tell you are accelerating relative to another object. For example, if you are driving a car and you hit the gas, you consequently feel a push from the seat, this is not because YOU are accelerating but the car is, and is pushing you forward. So lets say you painted the windows black and you hit the gas again, this time some force from within your body is pushing you forward, so you and the car are accelerating equally. In this case, you dont feel any push, so how do you SENSE you are accelerating?

But how would that force act equally on each individual molecule or atom of your body negating the sense of acceleration , and if you could provide that force as your in a non inertial environment all that force would do is move you towards the steering wheel.

And how can an internal force push you forward.

Michael

#### Solvay_1927

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« Reply #24 on: 10/07/2006 01:15:04 »
Mr (or is it Ms?) Brain,

Does Newton’s 1st law help?  Every object remains in its inertial state (i.e. stationary or in uniform linear motion) unless (and only unless) acted upon by an external force.

So you know you’re accelerating because a force is acting on you.  And if there were NO force acting on you, you would NOT be accelerating.

For Stella to start moving away from Terrance, a force has to act on her. And for her motion to be changed again at the turnaround point, another force must act upon her.

But throughout this experiment there are NO forces acting on Terrance.

Stella may see Terrance receding away from her at an ever increasing rate (i.e. he “looks” like he’s accelerating), but the fact is that only Stella is experiencing a force being applied to her.

So acceleration is NOT relative.  Both Stella and Terrance may think that the other person “looks” like they’re accelerating away, but only one of them (Stella) really IS accelerating, as only one of them has a force acting on them.  This removes the symmetry in their situations, and explains why they don’t have symmetrical experiences (i.e. why they don’t measure the same passage of time).

As for the whole redshifting/blueshifting description in the above link, although I think it does give an explanation of the twin paradox, I don’t think it’s the best explanation (it’s a bit too easily confused and misunderstood).

Unfortunately, there aren’t really any easily understood explanations for the twin paradox.  Which is probably because, to fully understand the resolution to the paradox, you have to get your head around the fact that the whole concept of “simultaneity” is changed by relativity.

In case it’s of interest, here’s a link to another explanation of how relativity theory causes the breakdown of simultaneity:

http://www.incentre.net/tcantine/TP.html

Hope it helps. (Or maybe it will just make your brain whirl even more, as it does for me!)

#### The Naked Scientists Forum

« Reply #24 on: 10/07/2006 01:15:04 »