Some Questions on Time Dilation.

[I_am_learning]

I am not interested in knowing the detailed calculation but would be more than happy if you could just answer the simple questions I put.
Suppose in a Universe There are just two peoples A and B in their space ship. Now suppose they are initially at rest and both have a clock that now shows the same time. Now one of the ship starts up and moves away from the other at some speed.
Q. Will now both A and B see each other's clock tick slowly ?
(other questions as per your reply.)

 

[Tac-Tics]

Q. Will now both A and B see each other's clock tick slowly ?

Yes.

 

[aaryan0077]

they are initially at rest

"According to which frame? One wrt other, or both to some other, of none to single?"

Q. Will now both A and B see each other's clock tick slowly ?

"yes"

DETAILS : read the thread "time dilation but for who?"

 

[I_am_learning]

"According to which frame? One wrt other, or both to some other, of none to single?"

Answer:They are initially at rest with respect to each other, Since there are nothing except A and B in this universe.

OK You answered, Both will see each others time slow down in this scenario, Right ?. If they now see each other coming towards each other (perhaps one or both of the ship turned their propeller's direction) then what will they observe? Will they both see each others time slow down this time also?

(I have already learned the theories about time dilation so you don't need to provide links to "basics of time dilation" or the like. But I am still confused . You will help best by answering the sequential questions (which I have organized in a manner that will give me a clear view,) I will continue to post)

 

[ZikZak]

Any clock in motion relative to the observer will be observed to run slow. It makes no difference at all which direction the clock is going.

This "observation" is different from what they "see" through a telescope. In Relativity, an "observation" is made once one takes what one sees directly through a telescope and corrects it for the changes in light travel-time to the observer. The approaching clock will appear though a telescope to be running fast, but once light-travel times are corrected for, the observation will be that the clock is running too slow.

 

[I_am_learning]

This "observation" is different from what they "see" through a telescope. In Relativity, an "observation" is made once one takes what one sees directly through a telescope and corrects it for the changes in light travel-time to the observer

OK, I am also talking about the corrected time. So in this case also each others time appears to be slow, Right ? Now see, since from the beginning of the motion A and B see each others clock tick slowly now when they meet, Both of them should expect less time to have been passed to the other. What I mean is , A sees that only 5 minutes have passed for B when 10 minutes have passed for himself. So, when they meet A should expect B to be younger by 5 minutes. Similarly B should also Expect A to be younger by 5 minutes. But both can't be younger than the other, can they ? So where is the problem ?
(I have used 5 minutes and 10 minutes just as example)

 

[jtbell]

In order for A and B to reunite, one of them must "turn around" by firing his rocket engines. Let's say that this one is B, for the sake of discussion. Suppose that B's rocket engines are powerful enough that he can reverse direction almost instantaneously. He will observe (after correcting for light-travel time of course) that A's clock has advanced by a much larger amount than his own, during his (B's) turnaround.

This does not mean than anything actually "happens" to A while B turns around. A feels or observes nothing in particular. This effect is analogous to the following:

Suppose that you have an x, y, z coordinate system attached to your nose, such that the x-axis points straight in front of you, the y-axis points to your left, and the z-axis points upward. Initially you are looking directly at an object that is 100 m in front of you. It has coordinates (x, y, z) = (100, 0, 0). You turn your head quickly so that after a fraction of a second the object is to the left of you. Its coordinates are now (0, 100, 0). In this coordinate system, the object has apparently moved a large distance, very quickly, starting from rest and ending at rest. Yet nothing has actually "happened" to the object.

In relativity, we use not an (x, y, z) coordinate system, but rather an (x, y, z, t) coordinate system. Changing your speed or direction of motion in relativity is analogous to rotating one's coordinate system in non-relativitistic physics. It has effects on your (x, y, z, t) coordinates of that object, even though nothing really happens to that object itself.

 

[I_am_learning]

In order for A and B to reunite, one of them must "turn around" by firing his rocket engines. Let's say that this one is B, for the sake of discussion. Suppose that B's rocket engines are powerful enough that he can reverse direction almost instantaneously. He will observe (after correcting for light-travel time of course) that A's clock has advanced by a much larger amount than his own, during his (B's) turnaround.

Well jtbell, thanks for your time. But I am still not fully clear.
You answered, When now A and B approach towards each other, both will observe (ofcourse after light correction) each others clock to tick slowly. But depending on Who as ACTUALLY turned over, The person who as turned over will be very backward in time and when they meet he will be much younger. Right ?

Now I am confused by what it means by who has ACTUALLY turned over. How will one know if its A or B who is turning over by mere observation. For A, B would appear to turn Over For B, A would appear to turn Over.
And please don't tell me that-->"the one Who has really gone on acceleration would FEEL it". I question, When we are accelerated towards the Earth when on free fall, do we feel it? Is the feeling any different than standing still in space ?

 

[sylas]

And please don't tell me that-->"the one Who has really gone on acceleration would FEEL it". I question, When we are accelerated towards the Earth when on free fall, do we feel it? Is the feeling any different than standing still in space ?

There is an equivalence between gravity and the forces experienced by an accelerating observer. If you are standing still in the Earth's gravitational field, your clock runs slow also. So in fact, "feeling" the acceleration IS relevant, and it IS a real substantive difference between the two twins, and it DOES stand as a legitimate answer to the question of who has turned around. The one who experiences an acceleration is the one who turns around... given the constraints of the problem.

There's no amiguity as to who turns around. In Special Relativity, there is a special status for an "inertial observer", and every inertial observer can immediately identify who changes direction. There are also several different ways you can tell that you are NOT an inertial observer... one of these is observing a local gravitational field. Another way is that the observer who turns around can observe at the same time a change in the apparent size of the other twin; whereas the twin who remains inertial does not observe any size change.

Cheers -- sylas

 

[I_am_learning]

So in fact, "feeling" the acceleration IS relevant, and it IS a real substantive difference between the two twins, and it DOES stand as a legitimate answer to the question of who has turned around. The one who experiences an acceleration is the one who turns around... given the constraints of the problem.
Cheers -- sylas

So, you mean acceleration isn't relative, its ABSOLUTE. We can always say whether a body is at rest or its accelerating. Right ?
Then like velocity (which is relative) we don't need to say accelerating with respect to what. for eg.instead of saying the car is accelerating at 5m/s^2 w.r.t. the road, we can simply say accelerating at 5 m/s^2 . Do you mean this also ?
And also,
Can you tell me a very simple experiment I can carry out in a space to test whether I am accelerating or not.

 

[Dale]

And please don't tell me that-->"the one Who has really gone on acceleration would FEEL it".

You already know the answers and have already rejected them. I don't know what you expect from the rest of us, but it sounds like you have an agenda rather than a real inquiry.

 

[I_am_learning]

You already know the answers and have already rejected them. I don't know what you expect from the rest of us, but it sounds like you have an agenda rather than a real inquiry.

I am sorry if I have appeared so rude like you have mentioned. But what I wanted was--- to ask the question -:

When we are accelerated towards the Earth when on free fall, do we feel it? Is the feeling any different than standing still in space ?

beforehand, if you would tell me we will feel the acceleration.

 

[A.T.]

So, you mean acceleration isn't relative, its ABSOLUTE. We can always say whether a body is at rest or its accelerating. Right ?

For proper acceleration, yes.

Then like velocity (which is relative) we don't need to say accelerating with respect to what. for eg.instead of saying the car is accelerating at 5m/s^2 w.r.t. the road, we can simply say accelerating at 5 m/s^2 . Do you mean this also ?

No, this is coordinate acceleration (dv/dt), which is relative,

And also, Can you tell me a very simple experiment I can carry out in a space to test whether I am accelerating or not.

Sure, just drop something, and see if it moves away from you. Or get one of these:
http://en.wikipedia.org/wiki/Accelerometer

When we are accelerated towards the Earth when on free fall, do we feel it? Is the feeling any different than standing still in space ?

No, free fall means zero proper acceleration, or advancing straight in curved space-time. It is equivalent to moving inertially in space (tidal effects aside):
http://www.aei.mpg.de/einsteinOnline/en/spotlights/equivalence_principle/index.html

 

[Dale]

So, you mean acceleration isn't relative, its ABSOLUTE. We can always say whether a body is at rest or its accelerating. Right ?

Yes, you can directly measure proper acceleration using an accelerometer.

Then like velocity (which is relative) we don't need to say accelerating with respect to what. for eg.instead of saying the car is accelerating at 5m/s^2 w.r.t. the road, we can simply say accelerating at 5 m/s^2 .

Yes, we can say that the norm of the proper acceleration is some value without reference to any coordinate system. Note that, unless the road is in free-fall the car's proper acceleration will be different than the coordinate acceleration wrt the road.

Can you tell me a very simple experiment I can carry out in a space to test whether I am accelerating or not.

Yes, read the value of an accelerometer.

When we are accelerated towards the Earth when on free fall, do we feel it? Is the feeling any different than standing still in space ?

The proper acceleration of a freefalling observer is zero, it is only undergoing coordinate acceleration. Do you understand the difference between proper acceleration and coordinate acceleration?

 

[I_am_learning]

DO YOU UNDERSTAND THE DIFFERENCE BETWEEN PROPER ACCELERATION AND CO-ORDINATE ACCELERATION

AH! Seems like here is my problem. Whats proper and co-ordinate acceleration? I would be more than happy if you could tell me in simple terms their difference rather than posting a link to a long and confusing topic. By the way I don't demand too simple language, I am a High-school pass student.

 

[A.T.]

Whats proper and co-ordinate acceleration?

Coordinate acceleration is the acceleration relative to an arbitrary frame of reference. Proper acceleration is the acceleration relative to a local free falling frame of reference.

 

[I_am_learning]

local free falling frame of reference.

Oops sorry, but I am afraid I have to ask, what's LOCAL FREE FALLING frame of reference?

 

[I_am_learning]

ALSO Look at this scenario. Suppose A and B are at present moving towards each other at relativistic speed. Further suppose A and B sees each other (after light travel time correction) of same age at this instant. Now, since both see each others time pass slowly they should find each other younger when they meet. Where is the problem?

 

[A.T.]

Oops sorry, but I am afraid I have to ask, what's LOCAL FREE FALLING frame of reference?

What don't you understand? What a "frame of reference" is? Or what "free fall" is?

In simple words: If you drop something, and measure it's coordinate acceleration (dv/dt) relative to you, then your proper acceleration is the negative of that value.

 

[I_am_learning]

In simple words: If you drop something, and measure it's coordinate acceleration (dv/dt) relative to you, then your proper acceleration is the negative of that value.

Then we can achieve proper acceleration only by the forces that will act on us but don't act on objects we drop; Right ?
So its never ever possible to accelerate (proper) by gravitational forces. But we can accelerate (proper) by cars and ships which accelerates us by providing forces through the seats, so that if we drop a coin, it will fly backwards.
Am i correct till now?

 

[A.T.]

So its never ever possible to accelerate (proper) by gravitational forces.

Yes, that's why there are no gravitational forces in General Relativity, because gravity doesn't produce any absolute acceleration. Proper acceleration is the deviation from free fall (or inertial motion).

 

[I_am_learning]

Is it only gravity that can't produce proper acceleration?
I don't understand this. If you are accelerated by a high-current wind, then if you drop something it will also be driven by the wind so you observe it to be at rest. So, are you not accelerating (proper) this time also? Whats a sure-shot test ?

 

[A.T.]

Is it only gravity that can't produce proper acceleration?
I don't understand this.

Any inertial forces which are proportional to the mass do not produce proper acceleration. They are called pseudo forces sometimes.

Whats a sure-shot test ?

Drop something in an evacuated jar. Make sure it is not affected by electric or magnetic forces.

 

[I_am_learning]

Any inertial forces which are proportional to the MASS do not produce proper acceleration

is it just the MASS or any force that is proportional to charge (magnetic, electric or the like) would also not produce proper acceleration.

If I am a charged particle and am accelerating (I think it should be proper acceleration) in an electric field then I drop a particle in an evacuated chamber which is also charged, so it also experience the force so would appear to be at rest. Then I conclude that I am not proper accelerating. Where am I wrong?

(If you like me to shield the chamber from electric and magnetic field then equivalently, I would like you to shield your chamber from Gravitational fields so that the scenario isn't any different from that of gravitational case )

(If you would say that, "You should Drop uncharged particle", but I further go onto supposition that Everything are charged, (not necessarily electrically charged), and I am being accelerated by the force (not known till today ) which accelerates this particular type of charge I am talking about)

 

[A.T.]

Any inertial forces which are proportional to the mass do not produce proper acceleration. They are called pseudo forces sometimes.

is it just the MASS or any force that is proportional to charge (magnetic, electric or the like) would also not produce proper acceleration.

No, just forces proportional to mass don't produce proper acceleration, because they accelerate everything at the same rate.

If I am a charged particle and am accelerating (I think it should be proper acceleration) in an electric field then I drop a particle in an evacuated chamber which is also charged, so it also experience the force so would appear to be at rest. Then I conclude that I am not proper accelerating. Where am I wrong?

Drop two things with the same charge but different masses. Or just buy an http://en.wikipedia.org/wiki/Accelerometer" if you cannot figure out the technicalities.

(If you would say that, "You should Drop uncharged particle", but I further go onto supposition that Everything are charged, (not necessarily electrically charged), and I am being accelerated by the force (not known till today )...

Physics deals with the observed reality so far, not with some potentially to come stuff.

 

[I_am_learning]

No, just forces proportional to mass don't produce proper acceleration, because they accelerate everything at the same rate.

Thanks for all the help you have been providing. Since you have come on a long way to helping me I hope won't leave at the last hour.

Don't feel bad but I don't like the Accelerometer idea, as I am not concerned about actually measuring my acceleration but am interested in knowing the underlying theory.

O.k. I put the question in this way. You are in a outer space and you want to check whether you are proper accelerating or not. You drop a coin. The coin appeared to be stationary so you supposed that you aren't proper accelerating. But in reality what happened was you were being accelerated by Electric Field (Suppose you were quite positively charged) and in the process of dropping the coin the coin also got little charged. So Both of you were accelerated. Since the mass is different, I would suppose that you and the coin were unequally charged so as to produce the same acceleration. So, you are in this case being accelerated by electric field but you think you aren't being properly accelerated. Where is the argument wrong?

As I have already mentioned I see no logic to Demand that --"The coin should be dropped in electrically and magnetically shielded box" because one can equivalently Demand to drop the coin in Gravitationally shielded box so as to prove that acceleration due to Gravity is also proper acceleration.

 

[A.T.]

O.k. I put the question in this way. You are in a outer space and you want to check whether you are proper accelerating or not. You drop a coin. The coin appeared to be stationary so you supposed that you aren't proper accelerating. But in reality what happened was you were being accelerated by Electric Field (Suppose you were quite positively charged) and in the process of dropping the coin the coin also got little charged. So Both of you were accelerated. Since the mass is different, I would suppose that you and the coin were unequally charged so as to produce the same acceleration.

Highly unlikely that the mass/charge ratios will be equal for both. And if both are charged positively they will also accelerate away from each other. Even if you manage to make the total electric force to be proportional to the mass of every particle it acts upon, in a specific scenario, it means nothing. It is not the general case as with gravity.

As I have already mentioned I see no logic to Demand that --"The coin should be dropped in electrically and magnetically shielded box" because one can equivalently Demand to drop the coin in Gravitationally shielded box

You cannot shield gravitation. As I said: physics deals with the reality observed so far.

so as to prove that acceleration due to Gravity is also proper acceleration.

There is nothing to prove. Forces proportional to mass don't cause proper acceleration per definition.

 

[I_am_learning]

In a specific scenario, """it means nothing"""". It is not the general case as with gravity.

It means a lot to me. It means your definition of "proper acceleration" fails if I managed to make the same charge:mass ratio because at that scenario you would say that the body isn't proper accelerating.

You cannot shield gravitation. As I said: physics deals with the reality observed so far.

THEN

There is nothing to prove. Forces proportional to mass don't cause proper acceleration per definition.[/QUOTE]

 

[I_am_learning]

In a specific scenario, """it means nothing"""". It is not the general case as with gravity.

It means a lot to me. It means your definition of "proper acceleration" fails if I managed to make the same charge:mass ratio because at that scenario you would say that the body isn't proper accelerating.

You cannot shield gravitation. As I said: physics deals with the reality observed so far.

I also agree that physics deals with reality observed so far. But I don't think it is limited only to it. For-example there is a ready equations to correct Max-wells 4 electromagnetic equation should any magnetic monopole be invented in the future. The whole Maxwell's theory won't fail should any magnetic monopole be discovered.
BUT AS PER YOUR DEFINITION OF PROPER ACCELERATION, IF IN THE FUTURE A METHOD TO SHIELD THE GRAVITY IS INVENTED THEN THE WHOLE THEORY OF RELATIVITY FAILS.
OR IS IT THAT IT IS PROVED THEORETICALLY THAT GRAVITY CAN'T BE SHIELDED JUST AS IT HAS BEEN PROVED THAT NOTHING CAN TRAVEL FASTER THAN LIGHT?

 

[matheinste]

It means a lot to me. It means your definition of "proper acceleration" fails if I managed to make the same charge:mass ratio because at that scenario you would say that the body isn't proper accelerating.

I also agree that physics deals with reality observed so far. But I don't think it is limited only to it. For-example there is a ready equations to correct Max-wells 4 electromagnetic equation should any magnetic monopole be invented in the future. The whole Maxwell's theory won't fail should any magnetic monopole be discovered.
BUT AS PER YOUR DEFINITION OF PROPER ACCELERATION, IF IN THE FUTURE A METHOD TO SHIELD THE GRAVITY IS INVENTED THEN THE WHOLE THEORY OF RELATIVITY FAILS.
OR IS IT THAT IT IS PROVED THEORETICALLY THAT GRAVITY CAN'T BE SHIELDED JUST AS IT HAS BEEN PROVED THAT NOTHING CAN TRAVEL FASTER THAN LIGHT?

I have been following this discussion but knowing very little about gravitation can add nothing useful. However it is remarkable how often someone comes along claiming to not understand a very basic consequence of Einsteins postulates, in this case time dilation, but knows so much about more advanced aspects of physics. Why not learn the basics first. If you do not undersatnd the basics how can anyone take you seriously when you make statements about more advanced aspects.

Matheinste.

 

[A.T.]

It means a lot to me. It means your definition of "proper acceleration" fails if I managed to make the same charge:mass ratio because at that scenario you would say that the body isn't proper accelerating.

To avoid this misunderstanding one would phrase the definition more exactly: Forces which are not in general proportional to mass cause proper acceleration. Electric forces are not in general proportional to mass.

IBUT AS PER YOUR DEFINITION OF PROPER ACCELERATION, IF IN THE FUTURE A METHOD TO SHIELD THE GRAVITY IS INVENTED THEN THE WHOLE THEORY OF RELATIVITY FAILS.

I guess so. We will worry then.

OR IS IT THAT IT IS PROVED THEORETICALLY THAT GRAVITY CAN'T BE SHIELDED JUST AS IT HAS BEEN PROVED THAT NOTHING CAN TRAVEL FASTER THAN LIGHT?

Nothing is proved in physics. Physical theories can only be disproved.

You can also use light to measure your proper acceleration localy. If a light beam bends in your frame of reference, the frame is properly accelerated. It is not really practical though, because you have to measure a tiny amount of bending along a short path.

 

[I_am_learning]

To avoid this misunderstanding one would phrase the definition more exactly: Forces which are not in general proportional to mass cause proper acceleration. Electric forces are not in general proportional to mass.

How can one exactly define something when he has included the ambiguous word """in general""" in his definition? I am sorry but I am not still satisfied with your definition, AT. And for your suggestion to use light, I would rather go for this coin drop experiment so that it enables me sought out the answer what exactly is the difference between gravitational force and Electric Force in this scenario.
(As I have always been doing, I discard the probable answers. Please Please Please don't ever think that I am going to propose my own theory or I am deliberately trying to prove the existing theories wrong. These are just to help me understand.)

Probable answer: Gravitational Force is proportional to mass but not the electric Force.
my criticism==> Electric Force is also proportional to charge. Probably Gravitational Force is also proportional to gravitational Charge which in turn is proportional to the mass. So there is not much difference except that gravitational charge are always proportional to the mass. (Anyway, Why can't I suppose that gravitational Charge exist?)

 

[A.T.]

Forces which are not in general proportional to mass cause proper acceleration.

How can one exactly define something when he has included the ambiguous word """in general""" in his definition?

"In general" means not only in a specific scenario. The key is rather that "forces" doesn't mean specific forces in a scenario, but types of forces like the electric force.

And for your suggestion to use light, I would rather go for this coin drop experiment

And what if you don't have a coin? Then you cannot measure your proper acceleration and relativity must be therefore wrong? Sorry, you cannot prove that something cannot be measured by restricting yourself to a certain measuring device.

Probable answer: Gravitational Force is proportional to mass but not the electric Force.

Better: Gravitational force accelerates everything equally. The electric force does not. Therefore you can treat gravitational force as an inertial force in general, and not just in a specific scenario.

 

[sylas]

How can one exactly define something when he has included the ambiguous word """in general""" in his definition? I am sorry but I am not still satisfied with your definition, AT.

Well, it's a perfectly normal and correct way of putting it, and not ambiguous at all. It means that one might come up with a contrived case in which charge and mass is proportional (for example, comparing collections of protons) but the relation does not hold in general.

The point is that the mass has two roles. It represents a kind of "gravitational charge", or the amount of gravitational force produced, and it ALSO represents a "resistance to motion", or inertia.

These two turn out to be equal, for all particles, to the most accurate measurements scientists have been able to make... and scientists DO attempt to falsify this equivalence. This equivalence DOES hold in general, both as far as we have been able to test, and also in the physical laws we currently use to describe the natural world.

Cheers -- sylas

 

[ZikZak]

Probable answer: Gravitational Force is proportional to mass but not the electric Force.
my criticism==> Electric Force is also proportional to charge. Probably Gravitational Force is also proportional to gravitational Charge which in turn is proportional to the mass. So there is not much difference except that gravitational charge are always proportional to the mass. (Anyway, Why can't I suppose that gravitational Charge exist?)

The difference is that electric charge is not the inertia of the particle. Gravitational charge is.