Originally posted by Janus
For this time dilation, their relative velocity would be .866c.

If each astronaut slept for 10 hrs by their own clock, they will each figure that they are 8.66 light-hours apart when they wake, and that according to the signal received from the other ship, only 5 hrs has passed for the other astronaut (after correcting for transmission delay and Doppler effect)

Both astronauts will measure that while 10 hrs have passed for them, only 5 have passed for the other.
Ok. You assumed that they both started off from common point with same acceleration, although that wasn't a known. Now suppose one of rockets didn't have same acceleration, but lets say, stayed around earth orbit, thus being very close to frame of earth. Now we have to touch twin issue. Departing ship reaches 0.866c relative to earth and other ship. But, neither ship knows the fact. Can they detect which case is true?

Now, on one hand, the mutually observed time dilation should be measured on both ships as you said, but on other hand, when departing ship returns, it must show real retardation of time. This means, that on one ship, time retardation must have been real, not just measurement due to dopler effect and light travel time for signal.

Astronauts have slept for the whole duration of acceleration, and waken up only when relative inertial motion is achieved.

In this case, the one in the moving ship would measure 10 hrs as having passed for him, and 20 hrs for the other astronaut (near earth).

For the astronaut on near-earth ship, if he measured 10 hrs for him, then he would measure only 5 hrs as passing for the astronaut in the departing ship?

You also assumed that each ship can measure how much time passed on other ship. This implies coded time signals. But lets omit that for a start and only assume that single bzzzz signal is available, thus only clock rate comparison is possible. What kind of bzz signal frequency shifts would either ship measure?

In the second case, I assume that you are talking about putting one ship inot some "time retardation field".

In this case, the one in the field would measure 10 hrs as having passed for him, and 20 hrs for the other astronaut.

For the astronaut outside the field, if he measured 10 hrs for him, then he would measure only 5 hrs as passing for the astronaut in the field.

You wouldn't have the same situation/measurements as you did with the actual relative motion.
This is same case as twin journey. Only that ship in such 'field' would make virtual journey. If not supposedly short signal propagation time between ships, compared to real motion, this would be pretty much like real twin journey.

Now, I'm wondering, trying to keep in mind all of relativity, IS there any special difference between real motion as we know it, and virtual motion that results from time retardation? If time retardation is real, which it appears to be, then, although we currently know of no other way to cause it, would effects of time retardation be equivalent to relativistic motion?

If you are talkng about some Super-duper technology that mimics relativistic effects perfectly while the astronauts sit next to each other, then you are talking about somethign along the lines of "If Pigs had wings".

Such technology would in all likelyhood violate Relativity and causality, and if such technology could exist, then the rules of Relativity don't apply. Therefore invoking such technology does nothing towards increasing understanding of Relativity,
Please, Janus, patience. This is "thought experiment (tm)", where pigs have not only wings, but they actually can fly. Relax, I know its considered impossible technologically. I only want to focus on what either ship CAN measure, and equivalence between the cases that they would detect. Idea I want to test here, is that do they really have any chance to detect whether they are moving relativistically, or sitting next to each other one being in field "where pigs have wings". Because if they can't, then there is really no reason to say there is any real difference, other than that it would be counterintuitive.

Both time and space are relative. Whenever you deal with relativity, you must consider both time dilation and length contraction together.
Sure. Thats why its interesting to consider what would ship inside 'time retardation field' measure, and what would lab equipment outside it measure. Somehow, when we talk about such forced time dilation, we tend to assume that there exists space in which we first have to move, while there really is no separate existence of space without time and interaction.

Relative velocity between two frames as meaured from those frames will always be the same.
Hmm. Are you completely confidently positive here? Timeflow for one of twins is retarded. Signal it generates in such frame, should have higher frequency when received by earth twin. Dopler redshift shifts it back down, but imho there should be difference in measured received signal by standards of each ship. You can transform the effects to common frame only IF you know relativistic velocity first. But without prior knowing it, the only way is to rely on assumption that atomic clocks on both ships are of same type, and find v by comparing clock rates.

Relative velocity between two frames as measured from a third frame moving wrt to the both of the first two, will not be that same as the relative velocity of the two as measure from either of the two.
This depends, as I understand, on geometry of trajectories taken.

Janus
Staff Emeritus
Gold Member
Originally posted by wimms
Ok. You assumed that they both started off from common point with same acceleration, although that wasn't a known.
No, I only assumed that they had a constant relative velocity during the period. (say that neither started his clock until any accleration periods were over, and only compared clock rates for that period. )

Now suppose one of rockets didn't have same acceleration, but lets say, stayed around earth orbit, thus being very close to frame of earth. Now we have to touch twin issue. Departing ship reaches 0.866c relative to earth and other ship. But, neither ship knows the fact. Can they detect which case is true?
You can't have a rocket orbiting the Earth at .866c unless you force it to. Thus you have to apply a acceleration to it which will be felt by the occupant as a force. This force acts just like the turn around force in the Twin paradox does, it will cause the occupant to measure Earth time as passing by quicker, this is not the same as when the rocket has a constant velocity with Earth. ( Remember constant velocity means constant speed and direction. placing the rocket into a forced orbit means you are constantly changing its direction, thus it does not have a constant velocity.)

It is easy to determine between an astronaut traveling at a constant velocity and one traveling in a forced orbit, one won't feel any forces of acceleration and the other will. And they will measure Earth time differently.

Now, on one hand, the mutually observed time dilation should be measured on both ships as you said, but on other hand, when departing ship returns, it must show real retardation of time. This means, that on one ship, time retardation must have been real, not just measurement due to dopler effect and light travel time for signal.

Time dilation is what is left after you compensate for Doppler effect and light signal delay.
In order for the ship to return it must undergo an acceleration, and during this period of acceleration during turnaround, they will see Earth time as passing faster than their own. So from their perspective, Earth time ran slow during the coasting parts of the trip and ran very fast during the turnaround phase. This is just as "real" as the Earth's viewpoint that the ship's time underwent retardation during the trip. You can only say that the time difference between the two twins once they meet is real, there is no absolute on how that difference came about.

Astronauts have slept for the whole duration of acceleration, and waken up only when relative inertial motion is achieved.

In this case, the one in the moving ship would measure 10 hrs as having passed for him, and 20 hrs for the other astronaut (near earth).

For the astronaut on near-earth ship, if he measured 10 hrs for him, then he would measure only 5 hrs as passing for the astronaut in the departing ship?

I already covered this above. If the Astronauts only consider the time periods when neither was accelerating, then both would measure less time as passing for the other during these periods.
[quote

You also assumed that each ship can measure how much time passed on other ship. This implies coded time signals. But lets omit that for a start and only assume that single bzzzz signal is available, thus only clock rate comparison is possible. What kind of bzz signal frequency shifts would either ship measure?
After compensating for Doppler shift, both would measure a lower frequency from the other ship, as long as neither experiences an acceleration.

This is same case as twin journey. Only that ship in such 'field' would make virtual journey. If not supposedly short signal propagation time between ships, compared to real motion, this would be pretty much like real twin journey.
No, as Iv'e already pointed out, putting one twin in a time retardation field would not mimic the measurement made during the twin paradox trip.

Now, I'm wondering, trying to keep in mind all of relativity, IS there any special difference between real motion as we know it, and virtual motion that results from time retardation? If time retardation is real, which it appears to be, then, although we currently know of no other way to cause it, would effects of time retardation be equivalent to relativistic motion?

There is no such thing as "real" motion, in the way that you can say that one object is moving and another isn't. You can only say that the objects have a relative motion wrt each other. Again, you can't say that the time retardation measured by one astronaut is any more "real" than the the pattern of time retardation and time acceleration measured by the other.

Please, Janus, patience. This is "thought experiment (tm)", where pigs have not only wings, but they actually can fly. Relax, I know its considered impossible technologically. I only want to focus on what either ship CAN measure, and equivalence between the cases that they would detect. Idea I want to test here, is that do they really have any chance to detect whether they are moving relativistically, or sitting next to each other one being in field "where pigs have wings". Because if they can't, then there is really no reason to say there is any real difference, other than that it would be counterintuitive.

Sure. Thats why its interesting to consider what would ship inside 'time retardation field' measure, and what would lab equipment outside it measure. Somehow, when we talk about such forced time dilation, we tend to assume that there exists space in which we first have to move, while there really is no separate existence of space without time and interaction.

We are not talking about what is technologically feasable.

I am saying that a time retardation field would not mimic the measurements of Relativistic time dilation.

Also, I don't see how a universe that run by the Rules of Relativity would allow the creation a "field" that would mimic its effects for both someone in the field and out.

Hmm. Are you completely confidently positive here? Timeflow for one of twins is retarded. Signal it generates in such frame, should have higher frequency when received by earth twin. Dopler redshift shifts it back down, but imho there should be difference in measured received signal by standards of each ship. You can transform the effects to common frame only IF you know relativistic velocity first. But without prior knowing it, the only way is to rely on assumption that atomic clocks on both ships are of same type, and find v by comparing clock rates.
Yes, the Relative velocity as measured by either will be the same. And each will measure time retardation in the other while they maintain constant relative Velocity. There is no "absolute" time retardation of either. and both will receive a lower frequency time signal from the other, once you compensate for Doppler effect.

Originally posted by Janus
You can't have a rocket orbiting the Earth at .866c unless you force it to.
Thanks for this, but where did you read I implied that ship near earth orbits at 0.866c? I meant near-earth ship orbiting at normal orbit speed in freefall. Its the departing ship that travels at 0.866c away from earth system. Both ships are in inertial motion. Question is can ships determine who is departing and who stayed near earth? According to you they can't.

Time dilation is what is left after you compensate for Doppler effect and light signal delay.
After you compensate for relativistic Dopler effect, what other time dilation is left?

In order for the ship to return it must undergo an acceleration, and during this period of acceleration during turnaround, they will see Earth time as passing faster than their own. So from their perspective, Earth time ran slow during the coasting parts of the trip and ran very fast during the turnaround phase. This is just as "real" as the Earth's viewpoint that the ship's time underwent retardation during the trip. You can only say that the time difference between the two twins once they meet is real, there is no absolute on how that difference came about.
Problem is that one is forced to mix "what happens really" with "from their perspective", or more precisely, that there is NO "what happens really" at all and questions as such are banned.
While in essence it is as simple as that:
- one twin travels,
- when he returns, he aged less. Period.
Fact: time retarded for travelling twin. This is "real". On this earth at least.

All those details about at which point who 'thinks' what other's time does is irrelevant in comparison to this simple fact. From instant when twin leaves earth and upto instant when he returns, there has to be definite period where his time retarded relative to earth. And it works one way only. Twin can't come back older. And this retardation is definitely as real as it can get.

Acceleration. Saying that time retardation of earth is not any less 'real' than that of twin traveling is pure bs. No actions of twin ship can influence timeflow on earth. Thus any effects of SR must be coupled to traveling twin. How they both detect or how they even are capable of detecting what happens on other ship is completely other issue. Fact of basement is: twin travels - twin ages less. Earth IS prefered reference frame, at least for accounting time.

We receive atom spectral lines from remote stars. But what do we know about natural timeflow rate there? We assume that clock rate of hydrogen is same as here, and based on freq shifts we derive relative velocity. We basically assume that clock rate is same everywhere, and only relative velocity causes changes to it.

We assume all inertial frames equal only when we reduce their clock rate differences to equal ground. When clock rates differ, inertial frames are not equal. They have either relative motion or have different gravity potentials.
And its the clock rate that defines "their perspective".

I already covered this above. If the Astronauts only consider the time periods when neither was accelerating, then both would measure less time as passing for the other during these periods.
Or basically, either ship is measuring its own delirium. Neither what they each 'see' is real, only computationally consistent.

No, as Iv'e already pointed out, putting one twin in a time retardation field would not mimic the measurement made during the twin paradox trip.
But ship of the twin IS time retardation field. How it behaves exactly is matter of details. Its not symmetric, it has direction, sideways time dilation doesn't occur, in forward direction and reverse direction dilation is big.

There is no such thing as "real" motion, in the way that you can say that one object is moving and another isn't. You can only say that the objects have a relative motion wrt each other. Again, you can't say that the time retardation measured by one astronaut is any more "real" than the the pattern of time retardation and time acceleration measured by the other.
Take it step further. When you go to next room, this is real motion. Or perhaps it isn't?
Of course I can say that time dilation of traveling twin is more real than his perspective on earth time dilation. This is proved when they meet again.

I am saying that a time retardation field would not mimic the measurements of Relativistic time dilation.
How do you imagine 'time retardation field' actually? How many assumptions do you pack there before saying that its impossible? I'd say I'm not sure at all. Do you assume that there would be no size dilation, energy change?

Imagine ship, creating 2 fields, in front of it with increased clock rate, behind it, with decreased clock rate. I'd say this would be ship with "gravity-drive", and it would actually move through space.
Also, I don't see how a universe that run by the Rules of Relativity would allow the creation a "field" that would mimic its effects for both someone in the field and out.
Change perspective. You create that field by what we call accelerating your ship. Question is about equivalence - is specific manipulation of spacetime field equivalent to relativistic motion?

Yes, the Relative velocity as measured by either will be the same. And each will measure time retardation in the other while they maintain constant relative Velocity. There is no "absolute" time retardation of either. and both will receive a lower frequency time signal from the other, once you compensate for Doppler effect.
Why are you are stuck with that 'absolute'? Noone ever said its absolute. Let put it this way: clock rate in our frame depends on our position in field potential. The value of potential is irrelevant, as we can only measure relative values here. Now, when twin travels at rel velocity, he climbs potential, he gains kinetic energy, and his real timeflow and clock rate slows, relative to where he comes from, relative to where he goes to! It doesn't matter what velocity earth has relative to any other object in space. We can always compare only relative velocity between objects, and relative timeflow. The limit of relative speed is set by moving object's clock rate approaching zero relative to us.

russ_watters
Mentor
Actually, there are many experts on relativity who dispute several aspects of the theory. But of course those don't count, right?

I know it's not politically correct to affirm that Einstein's theory is not absolutely, perfectly, thoroughly self-consistent. People thought the same of Newton for 300 years, why should it be different this time around? Yet the flaws in Newton's thinking were as obvious to his contemporaries as they are now. Science moves so slowly because it has a lot of inertial mass, it takes too long to convince people of some obvious truths.
Thats a contradiction and thus a flawed analogy.

You're right that it was apparent from the beginning that Newton's theory had flaws and people immediately began to look for an explanation of them. So that means science did NOT have any inertia with respect to Newton's laws, it simply took a long time to figure out WHY the laws were flawed.

Similarly, Relativity has flaws (as others have noted, its relationship with QM) and people are working hard to figure them out. But the things being kicked around here have nothing at all to do with the known flaws in Relativity.

The things being discussed here for the most part are NOT flaws as understood by mainstream physicists but misunderstandings of the theory itself.
Question is can ships determine who is departing and who stayed near earth? According to you they can't.
The one with the engines on is the one departing. The ships can most certainly feel their own acceleration.

Originally posted by russ_watters
The one with the engines on is the one departing. The ships can most certainly feel their own acceleration.
Please, Russ, can you at least pay as little respect as to read before jumping in. inertial departure, engines already off. Sleepy astronauts wandering around trying to figure out who moves who stayed.

Janus
Staff Emeritus
Gold Member
Originally posted by wimms

After you compensate for relativistic Dopler effect, what other time dilation is left?

You didn't say Relativistic Doppler effect, you said "Doppler effect". Relativistic Doppler effect is Doppler effect corrected with Relativistic effects. In most discussions of Relativity, you usually assume that the Doppler effect has been compensated for and you only need to deal with the Relativistic transformations.

Problem is that one is forced to mix "what happens really" with "from their perspective", or more precisely, that there is NO "what happens really" at all and questions as such are banned.
While in essence it is as simple as that:
- one twin travels,
- when he returns, he aged less. Period.
Fact: time retarded for travelling twin. This is "real". On this earth at least.

Reality is what we measure. Relativity effects how we measure. Each astronaut measures different time rates for the other. Neither measured reality has preference over the other, and neither is more real than the other. The end result is the same, but the reasons are different. So no, you can't say that one set of reasons is more real than the other.

All those details about at which point who 'thinks' what other's time does is irrelevant in comparison to this simple fact. From instant when twin leaves earth and upto instant when he returns, there has to be definite period where his time retarded relative to earth. And it works one way only. Twin can't come back older. And this retardation is definitely as real as it can get.

Again, the age difference at the end is real, but does not mean that time redardation for one twin is the only "real" answer for this outcome.

Acceleration. Saying that time retardation of earth is not any less 'real' than that of twin traveling is pure bs.
No,it is Relativity.

No actions of twin ship can influence timeflow on earth. Thus any effects of SR must be coupled to traveling twin. How they both detect or how they even are capable of detecting what happens on other ship is completely other issue. Fact of basement is: twin travels - twin ages less. Earth IS prefered reference frame, at least for accounting time.

The acceleration felt by the twin in the ship effects how he measures the time rate on Earth. On the other side, the relative velocity between Earth and Ship effects how the Earth measures time rate on the Ship.

Both realities are detemined by what is measured by each. And since Reality is determined by what we measure, both are "real".

We receive atom spectral lines from remote stars. But what do we know about natural timeflow rate there? We assume that clock rate of hydrogen is same as here, and based on freq shifts we derive relative velocity. We basically assume that clock rate is same everywhere, and only relative velocity causes changes to it.

No, we assume that the measurements of time rate would the same as long as both are within the same inertial frame, and realtive motion effects those measurements, in both directions .

We assume all inertial frames equal only when we reduce their clock rate differences to equal ground. When clock rates differ, inertial frames are not equal. They have either relative motion or have different gravity potentials.
And its the clock rate that defines "their perspective".
Fine, but you keep wanting to make one perspective more prefered than the other.

[/b]

Or basically, either ship is measuring its own delirium. Neither what they each 'see' is real, only computationally consistent.

[/b]
All we can know about reality is what we can measure, so reality is what we measure. what each measures is real.

Take it step further. When you go to next room, this is real motion. Or perhaps it isn't?
Of course I can say that time dilation of traveling twin is more real than his perspective on earth time dilation. This is proved when they meet again.

No, all this says is that the age difference is real. It says nothing about how it got that way.

How do you imagine 'time retardation field' actually? How many assumptions do you pack there before saying that its impossible? I'd say I'm not sure at all. Do you assume that there would be no size dilation, energy change?

With a simple time redardation field, the viewpoints are not symetric. A person in the field would see things outside the field as running fast the whole time, and at the same rate difference as the outside observer see things moving in the field as moving slow. I the real twin scenerio, one twin sees, the other's time rate as moving slow the whole time, while the second twin sees the other follow a pattern of slow, very fast, slow.

It is a greater assumption that other effects such a length contraction and energy change would occur, than not.

Imagine ship, creating 2 fields, in front of it with increased clock rate, behind it, with decreased clock rate. I'd say this would be ship with "gravity-drive", and it would actually move through space.

Back by what? Just your general feeling that this should be true?

[/b]

Why are you are stuck with that 'absolute'? Noone ever said its absolute. Let put it this way: clock rate in our frame depends on our position in field potential. The value of potential is irrelevant, as we can only measure relative values here. Now, when twin travels at rel velocity, he climbs potential, he gains kinetic energy, and his real timeflow and clock rate slows, relative to where he comes from, relative to where he goes to! It doesn't matter what velocity earth has relative to any other object in space. We can always compare only relative velocity between objects, and relative timeflow. The limit of relative speed is set by moving object's clock rate approaching zero relative to us. [/B]
Whenever you assert a prefered reference frame, you are invoking absolute motion, wheter you realise it or not.

The problem with your statements is that you fail to consider the other side of the coin concerning relative velocity.

Yes, the ship has a relative velocity wrt the Earth, but the Earth has a relative velocity wrt to the ship also, with a corresponding increase of kinetic energy, etc.

You can't say that is only the relative velocity of the ship to the Earth that counts, to do so perverts the very meaning of "relative motion".