Relativity Question

[Vilas Tamhane]

The number of atoms and the shape matter. Taken together, you can define a density (number per unit volume); a sphere has the smallest critical density. In the moving frame you will not see a supercritical volume. It will be critical as well.

 

It is true. Criticality increases with density, but it is not impossible to think of a length contraction paradox. Consider following set up.

 

In the rest frame, source of light is so arranged that it sends photons in a spreading beam in the x-direction. On the opposite side, at a distance d, there is a photo detector connected in the electric circuit C1. It receives light from the source, intensity of which will increase with reducing distance d. Proportional to light intensity, current I1 is circulated in C1 which can be further amplified. In the circuit C1, magnetic switch sw is connected. Normally off terminals of sw are in another circuit C2, connected to a separate voltage source. A bulb B is connected in the circuit C2. Current I1 is not enough to activate the switch sw and so the circuit C2 remains open. The bulb B therefore remains off.

 

For the observer moving in the direction x, distance d between light source and the detector is reduced. Photo detector will now receive more light and current in the circuit C1 will increase. This current is sufficient to activate the switch sw and so current in the circuit C2 will switch on the bulb B. Hence the paradox. In the rest frame B is off but in the moving frame it is on.

 

 

[tar]

Dr Rocket and Swansont,

 

I truely do yield to the logic and thouroughness that scientists and mathematicians have used to discover stuff about our world and our universe. And I have always found logic and completeness in the explanations and formulae "once I understand what they mean."

 

But not having a complete understanding of the steps, and the explorations built on established findings, built on previous explorations and established finding, I sort of have to take the journey again, myself, insight by insight, to get to the same place as I would be if I just "accepted" the findings.

 

Its not that I don't trust science, or that I do not realize that the sensible thing to do, is to yield to the many more agile minds, that have in concert investigated and documented their findings and reviewed the findings of others to assure the "reality" of what is "known" to be objective reality and its nature.

 

But what it is, is a desire to be part of the investigation. To add something. To "check" if an insight I have come to, has already been had or not. If it has already been noticed, fine. I just need to be told. If I am wrong about something, I might need some reasoning as to why I am wrong.

 

I often seem to "not get" what is considered "proof" of something. Some obvious things to me are "not scientifically" proven, and some "scientifically" proven things seem fanciful and unreal to me.

 

I have a general inability to discern when something is a questioning of somebody else answer, and when something is an answer to somebody elses question.

 

For instance I always thought that E=MCsquared WAS an answer to what the universe was about...till I read about this or that person's "solution" of Einstein's field equations.

 

How do you know when an equation is a statement of fact, and when its a question, that needs to be answered?

 

If E=MCsquared is a statement of fact, can't you just express it in real units, and do some algebra and substitutions and express anything involving energy, in terms of mass time and distance. Anything involving mass in terms of energy time and distance, anything involving time, in terms of energy, mass and distance, and anything involving distance in terms of energy, mass and time?

 

If E=MCsquared is a question, what is it asking that we don't already know the answer to?

 

Regards, TAR

Edited July 21, 2011 by tar

[swansont]

It is true. Criticality increases with density, but it is not impossible to think of a length contraction paradox. Consider following set up.

 

In the rest frame, source of light is so arranged that it sends photons in a spreading beam in the x-direction. On the opposite side, at a distance d, there is a photo detector connected in the electric circuit C1. It receives light from the source, intensity of which will increase with reducing distance d. Proportional to light intensity, current I1 is circulated in C1 which can be further amplified. In the circuit C1, magnetic switch sw is connected. Normally off terminals of sw are in another circuit C2, connected to a separate voltage source. A bulb B is connected in the circuit C2. Current I1 is not enough to activate the switch sw and so the circuit C2 remains open. The bulb B therefore remains off.

 

For the observer moving in the direction x, distance d between light source and the detector is reduced. Photo detector will now receive more light and current in the circuit C1 will increase. This current is sufficient to activate the switch sw and so current in the circuit C2 will switch on the bulb B. Hence the paradox. In the rest frame B is off but in the moving frame it is on.

If you haven't actually done this experiment, then you don't know that this will happen. This is what you expect will happen, but if this is meant to challenge the theory of relativity, there's a problem: is this what relativity predicts will happen? And the answer is no. You have misapplied the theory, so it's no wonder you get the wrong answer.

 

You have failed to take into account other effects. Length and time are directly affected, but then so are any physical properties that depend on them. There are a few invariant properties that will be the same for both observers, but most will not. So the length changes, but so does time, and the energy of the photons, and so will the activation properties of the circuit. You know the values for the rest frame, but you don't know what they are for someone in the moving frame. You can't just assume everything else is the same.

 

DrRocket already covered this: it is not a paradox. It is just a mistake.

[Vilas Tamhane]

If you haven't actually done this experiment, then you don't know that this will happen. This is what you expect will happen, but if this is meant to challenge the theory of relativity, there's a problem: is this what relativity predicts will happen? And the answer is no. You have misapplied the theory, so it's no wonder you get the wrong answer.

 

You have failed to take into account other effects. Length and time are directly affected, but then so are any physical properties that depend on them. There are a few invariant properties that will be the same for both observers, but most will not. So the length changes, but so does time, and the energy of the photons, and so will the activation properties of the circuit. You know the values for the rest frame, but you don't know what they are for someone in the moving frame. You can't just assume everything else is the same.

 

DrRocket already covered this: it is not a paradox. It is just a mistake.

 

 

Unlike your last post, this reply is not helpful. Time doesn’t play any part in this thought experiment. I am also unable to see how energy of photons can change and in what way activation process can change for a moving frame.

 

Was a similar thought experiment posted to which DrRocket replied? If so will you please let me know how to locate it?

 

 

[swansont]

Unlike your last post, this reply is not helpful. Time doesn’t play any part in this thought experiment. I am also unable to see how energy of photons can change and in what way activation process can change for a moving frame.

 

Was a similar thought experiment posted to which DrRocket replied? If so will you please let me know how to locate it?

 

You have not analyzed how time might play a role. How often are the photons emitted to have a given flux? That doesn't depend on time? 1 photon per second in one frame becomes 0.5 photons per second if there is a dilation factor of 2. If length contracts, time must dilate in the other frame. That is, if I shoot 10 photons in my frame, at 1 per second, someone in the other frame will say that it took 20 seconds and my clock was running slow. That cuts the photon flux down by a factor of 2.

 

Frequency shifts because of relative motion. That changes the measured energy of the photons.

 

Material properties must change, because a certain thickness of material will attenuate light, but if the thickness changes from length contraction and yet we still agree on how many photons are absorbed and how many pass through, the absorption coefficient must vary according to which frame you are in — it's not an invariant quantity, but there's no reason to expect it to be. Assuming that it is, is a mistake.

 

A contradiction in a thought experiment means that there is an error somewhere. You have several in the premise, so this cannot be used to show a contradiction in the theory. The theory, at its core, is just math (Lorentz transforms), so you won't find a contradiction in the theory alone. You can only show that theory does not match up with experiment.

 

Saying you are unable to see merely means you have yet to understand relativity at the necessary level. That does not, however, mean that relativity is wrong. It is erroneous to have any individual's understanding be the metric by which the correctness of a theory is gauged. Posting a poorly-constructed thought experiment only shows your mistakes.

[J.C.MacSwell]

Unlike your last post, this reply is not helpful. Time doesn’t play any part in this thought experiment. I am also unable to see how energy of photons can change and in what way activation process can change for a moving frame.

 

Was a similar thought experiment posted to which DrRocket replied? If so will you please let me know how to locate it?

 

Every process works in such a way that the exact same events take place in each frame, even non-inertial frames. Only the "look" and timing is different.

[Vilas Tamhane]

You have not analyzed how time might play a role. How often are the photons emitted to have a given flux? That doesn't depend on time? 1 photon per second in one frame becomes 0.5 photons per second if there is a dilation factor of 2. If length contracts, time must dilate in the other frame. That is, if I shoot 10 photons in my frame, at 1 per second, someone in the other frame will say that it took 20 seconds and my clock was running slow. That cuts the photon flux down by a factor of 2.

 

Frequency shifts because of relative motion. That changes the measured energy of the photons.

 

Material properties must change, because a certain thickness of material will attenuate light, but if the thickness changes from length contraction and yet we still agree on how many photons are absorbed and how many pass through, the absorption coefficient must vary according to which frame you are in — it's not an invariant quantity, but there's no reason to expect it to be. Assuming that it is, is a mistake.

 

A contradiction in a thought experiment means that there is an error somewhere. You have several in the premise, so this cannot be used to show a contradiction in the theory. The theory, at its core, is just math (Lorentz transforms), so you won't find a contradiction in the theory alone. You can only show that theory does not match up with experiment.

 

Saying you are unable to see merely means you have yet to understand relativity at the necessary level. That does not, however, mean that relativity is wrong. It is erroneous to have any individual's understanding be the metric by which the correctness of a theory is gauged. Posting a poorly-constructed thought experiment only shows your mistakes.

 

I am assuming that photon emission is not affected by time dilation. Because this will give rise to another paradox as follows,

 

A collimated beam of light falls on a sufficiently large photo detector in the rest frame (so that length contraction does not play any part). Current that is produced is passed through the resistor. Its temperature has to be same for both the frames.

 

According to you, there will be frequency shift on account of the velocity between source and the moving frame. However this takes place in the rest frame only if there is relative motion between the source and the detector. Photons are not received by the moving frame. Even otherwise, direction of the moving frame can be made such that velocity of the moving frame is from detector to source, so that effect of increased intensity is enhanced.

 

So far as thickness of the cell material is concerned, it can be selected sufficiently large so that no photons are missed.

 

 

[Janus]

Unlike your last post, this reply is not helpful. Time doesn’t play any part in this thought experiment. I am also unable to see how energy of photons can change and in what way activation process can change for a moving frame.

 

 

 

Of course time plays a part. For instance, it takes time for the light to travel from the source to the target. In that time the target moves according to one of the frames. Since light travels at c relative to all frames as measured from that frame, the distance the light travels from the source to the target in the frame where the source and target are moving is going to be greater than just the length-contracted distance between the source and and target in that frame.

 

When you apply all the rules of Relativity to the problem, you will find that no paradox arises and that all frames agree as to the ultimate outcome.

 

Photon energy changes are due to the Doppler effect which changes the frequency/wavelength of light for sources moving with respect to the frame from which it is being measured. This this places even if you don't take Relativity into account (though Relativity predicts a different amount of change).

 

It always amazes me when laymen seem to think that they've found the loophole in Relativity that has somehow escaped the attention of countless physicists for over a hundred years. And invariably, is always, something ridiculously simple, that if actually true, would have resulted in Relativity being pronounced DOA upon inception. Rarely do see someone who says " This seems to be a paradox; what could I be doing wrong or what is that I am not understanding?" But rather you are much more likely to see " This is a paradox, Relativity must be wrong!" A position they hold dogmatically no matter how carefully their mistake is explained to them.

 

The fact of the matter of the matter is that you are never going to find a flaw in Relativity by mere thought experiment. Its logic is airtight and it is completely self-consistent(no matter how much it offends some peoples intuition). It is also consistent with every real world observation made to date. At this point, the only way to uncover a flaw in Relativity is to get results from a real-world experiment that conflict with what Relativity predicts.

Edited July 21, 2011 by Janus

[Vilas Tamhane]

 

You missed the whole point. Let's see if I can put it another way. What we measure as "reality" is just a projection of a "larger picture". Its like the 2 dimensional shadow made by a 3-D space. If the space rotates, the shadow changes.

 

Imagine there are two balls in that space each projecting its own shadow. The only way those balls interact with what we call "reality" is those shadows. Those shadows are the reality for us. If we change our orientation to the space and the "light", the relative positions of the shadows change. In our reality, they can change distance. That is the equivalent of us changing reference frames. Changing reference frames does not physically alter anything in that 3-D space, but it does alter the projection it makes on our reality.

 

Another way to look at it is to imagine a featureless plane. You are standing on the plane. There are two other objects on that plane. One is 1meter directly to the front of you and the other on meter directly to the right. You turn 45° to the right. Now one object is 0.707 meters in front of you and 0.707 meters to your right and the other is 0.707 meters in front of you and 0.707 meters to your left.

 

Replace the front-back direction with time,left-right with distance and the the objects with events, and you have a closer analogy to Relativity. As you turn (change reference frames), the events go from occurring at different times to occurring at the same time, and their distance from each other(left-right being the only measure of spacial separation) changes. The thing to keep in mind is that your whole reality is based on just the measurements made along those two axes. The events really do shift in time and distance. Someone facing in a different direction will measure a different time and distance separation for those events, and his measurements are just as much "reality" as yours.

 

In the first three examples, you are talking about components of a vector. Only one component cannot represent reality of a vector. Vector is real and components are not. These are human constructs. If one dimensional creature says that x-component is the only reality, then his perception is wrong.

 

In the fourth example, length cannot have component in time. When we talk about length alone, time doesn’t come into picture. Please come out of the world of mathematics and explain the question of length contraction. Explain how a rod can contract if I run along it. What makes it contract? Considering this real world situation, please tell me how can rod contract for me? In real world we don’t have any projections to depend on.

 

Mathematics is for calculations and not for description.

 

 

[DrRocket]

The fact of the matter of the matter is that you are never going to find a flaw in Relativity by mere thought experiment. Its logic is airtight and it is completely self-consistent(no matter how much it offends some peoples intuition). It is also consistent with every real world observation made to date. At this point, the only way to uncover a flaw in Relativity is to get results from a real-world experiment that conflict with what Relativity predicts.

 

Right.

 

Relativity (special or general), just like classical mechanics, can be viewed in two ways:

 

1) As a mathematical theory, subject only to the foundational assumptions and rigorous mathematical logic.

 

2) As a predictive model of natural behavior.

 

 

When viewed in the first way, both are airtight, mathematically consistent theories. No disproof by thought experiment is possible.

 

When viewed in the second way, it has already been proved that classical mechanics is "wrong", and the evidence in favor of special relativity is that proof.

 

If special relativity is wrong then the disproof will have to come in the form of experimental data. Since special relativity is just a local approximation to general relativity and neglects gravity, we know that it is not absolutely correct -- but it is still an excellent model when properly applied.

 

We also know that general relativity (GR) and quantum mechanics are incompatible. Thus it is likely that general relativity is also not the last word. But it has the support of a huge body of experimental data and is a very good model when quantum effects are not important.

 

Even in the absence of quantum effects there is a competing theory of gravitation. It is called Einstein-Cartan (EC) theory. EC theory differs from GR in not making the a priori assumption that spacetime is torsion free. This greatly complicates the mathematics of the differential geometry. At the current level of measurement technology the predictions of EC theory are indistinguishable from GR, and because the mathematics is so much more difficult EC theory is not so well known. It has the intriguing feature that at least some of the singularities associayed with exotic GR solutions do not occur in EC theory. The point here being that again we have two mathematically consistent theories, and only an advance in experimental capability will be able to determine which is the better model or "correct". Thought expereiments are not sufficient.

[J.C.MacSwell]

In the first three examples, you are talking about components of a vector. Only one component cannot represent reality of a vector. Vector is real and components are not. These are human constructs. If one dimensional creature says that x-component is the only reality, then his perception is wrong.

 

In the fourth example, length cannot have component in time. When we talk about length alone, time doesn’t come into picture. Please come out of the world of mathematics and explain the question of length contraction. Explain how a rod can contract if I run along it. What makes it contract? Considering this real world situation, please tell me how can rod contract for me? In real world we don’t have any projections to depend on.

 

Mathematics is for calculations and not for description.

 

No math:

 

How can your measurement of the rod, which remains stationary in it's rest frame not contract if you accelerate in that direction?

 

Please answer using accepted laws of physics, or at the very least assumptions that have not been proven to be incorrect.

[Vilas Tamhane]

Of course time plays a part. For instance, it takes time for the light to travel from the source to the target. In that time the target moves according to one of the frames. Since light travels at c relative to all frames as measured from that frame, the distance the light travels from the source to the target in the frame where the source and target are moving is going to be greater than just the length-contracted distance between the source and and target in that frame.

 

When you apply all the rules of Relativity to the problem, you will find that no paradox arises and that all frames agree as to the ultimate outcome.

 

Photon energy changes are due to the Doppler effect which changes the frequency/wavelength of light for sources moving with respect to the frame from which it is being measured. This this places even if you don't take Relativity into account (though Relativity predicts a different amount of change).

 

It always amazes me when laymen seem to think that they've found the loophole in Relativity that has somehow escaped the attention of countless physicists for over a hundred years. And invariably, is always, something ridiculously simple, that if actually true, would have resulted in Relativity being pronounced DOA upon inception. Rarely do see someone who says " This seems to be a paradox; what could I be doing wrong or what is that I am not understanding?" But rather you are much more likely to see " This is a paradox, Relativity must be wrong!" A position they hold dogmatically no matter how carefully their mistake is explained to them.

 

The fact of the matter of the matter is that you are never going to find a flaw in Relativity by mere thought experiment. Its logic is airtight and it is completely self-consistent(no matter how much it offends some peoples intuition). It is also consistent with every real world observation made to date. At this point, the only way to uncover a flaw in Relativity is to get results from a real-world experiment that conflict with what Relativity predicts.

 

If you wish to worship theories, nobody can force you not to. I expect the same liberty in our free world.

 

Obedience you expect from laymen is wrong. Consider them as students and counter their criticism with your expertise.

 

I am also amazed at the anger we generate when we put up our critical viewpoint. Anger is a sign of intolerance that is most common among the faithful. In fact you should encourage criticism. That is the only way, we the heathens can learn.

 

 

[swansont]

Its temperature has to be same for both the frames.

It won't be.

When we talk about length alone, time doesn’t come into picture.

One of the implications of relativity is that you cannot do this. Time and length are related.

 

Any contradiction you come up with will be because you have assumed something that is incorrect. That will be the error that causes the apparent paradox.

 

If you wish to worship theories, nobody can force you not to. I expect the same liberty in our free world.

 

Obedience you expect from laymen is wrong. Consider them as students and counter their criticism with your expertise.

 

I am also amazed at the anger we generate when we put up our critical viewpoint. Anger is a sign of intolerance that is most common among the faithful. In fact you should encourage criticism. That is the only way, we the heathens can learn.

 

Nobody is worshipping theories. They are recognizing that math actually works; a self-consistent theory cannot be shown wrong by a thought experiment. If you could do that it means math itself is broken. Self-consistent theories can only be shown to not work the same way that nature does, and this requires an actual physical experiment. They support relativity being correct.

 

Obedience is not being asked. Understanding is. I'm confident people would consider you as a student if you behaved as one — someone asking questions to improve their knowledge and clear up misconceptions. But that's not how you are acting. You admit no misunderstanding when you declare the theory to be wrong, yet it is clear that you do not understand it.

 

Nobody has shown any anger toward you. On the contrary, the length of the thread demonstrates patience. The accusation is inappropriate.

[Vilas Tamhane]

No math:

 

How can your measurement of the rod, which remains stationary in it's rest frame not contract if you accelerate in that direction?

 

Please answer using accepted laws of physics, or at the very least assumptions that have not been proven to be incorrect.

 

I haven’t understood how length can contract. It is not the question of measurements alone. Problem is not mathematical. I find it impossible that a physical rod can contract. Imagine three trains running, with different speeds, between stations A and B. I am perplexed because you tell me that the stations come closer by an amount different for three different trains. But standing on the platform I can see that there is no contraction and so I conclude that the measurements of the train drivers are apparent.

 

 

[DrRocket]

I haven't understood how length can contract. It is not the question of measurements alone. Problem is not mathematical. I find it impossible that a physical rod can contract. Imagine three trains running, with different speeds, between stations A and B. I am perplexed because you tell me that the stations come closer by an amount different for three different trains. But standing on the platform I can see that there is no contraction and so I conclude that the measurements of the train drivers are apparent.

 

 

 

It would be very educational for you to read a book on special relativity so that you might comment on what the theory actually says rather than what you think it says.

 

An Introduction to Special Relativity by Wolgang Rindler would be a good place to start.

[J.C.MacSwell]

I haven’t understood how length can contract. It is not the question of measurements alone. Problem is not mathematical. I find it impossible that a physical rod can contract. Imagine three trains running, with different speeds, between stations A and B. I am perplexed because you tell me that the stations come closer by an amount different for three different trains. But standing on the platform I can see that there is no contraction and so I conclude that the measurements of the train drivers are apparent.

 

As long as "apparent" is understood in the context that your observation is apparent as well, then I think that is fine. If you think that your view is somehow "real" in a context that the others is not, then you cannot support this scientifically.

 

As for whether the rod contracts (or expands, if the relative speed of the observer decreases), I would say it depends, again, on what is meant by "contract" or even "physically contract". It cannot mean precisely what we mean in our daily usage as the mechanism is different from that of any type of everyday usage. But the length or distance gets measured as shorter when applying a consistent set of physical laws.

[tar]

Swansont,

 

I am sometimes perplexed by the nature of experiments designed to test, say gravitational redshift, in that the setup is complex and many assumptions are made, where it is hard to know what is being already taken into account and what has possibly been overlooked, when there seems to be simple experiments that would do the same job. For instance, wouldn't the Sun provide a sufficient enough gravity well, that we could consider that the adherence of nature to the scheme of general relitivity would cause clocks on Earth to tick faster than at the sun, measure a hydrogen absorption line from some Earthly hydrogen, compare it to the same hydrogen absorption line from some solar hydrogen, and say "There." "See the difference?" "That proves that hydrogen ticks faster out here, than it does deep in the Sun's well". "And look!" "It fits the formulae exactly!"

 

Regards, TAR2

 

But wait. Wouldn't that mean that the hydrogen on Earth is "older" than the hydrogen at the Sun?

 

And if we calculate the age of the universe, should we go by age of the Earth's hydrogen, or by the age of the Sun's hydrogen?

[J.C.MacSwell]

Swansont,

 

I am sometimes perplexed by the nature of experiments designed to test, say gravitational redshift, in that the setup is complex and many assumptions are made, where it is hard to know what is being already taken into account and what has possibly been overlooked, when there seems to be simple experiments that would do the same job. For instance, wouldn't the Sun provide a sufficient enough gravity well, that we could consider that the adherence of nature to the scheme of general relitivity would cause clocks on Earth to tick faster than at the sun, measure a hydrogen absorption line from some Earthly hydrogen, compare it to the same hydrogen absorption line from some solar hydrogen, and say "There." "See the difference?" "That proves that hydrogen ticks faster out here, than it does deep in the Sun's well". "And look!" "It fits the formulae exactly!"

 

Regards, TAR2

 

But wait. Wouldn't that mean that the hydrogen on Earth is "older" than the hydrogen at the Sun?

And if we calculate the age of the universe, should we go by age of the Earth's hydrogen, or by the age of the Sun's hydrogen?

 

Assuming they were the same age prior to becoming part of the Earth and Sun, then yes.

 

The oldest (least accelerated in lowest gravitational field, spending it's "life" in the cmbr isotropy) defines the age of the Universe. (correct me if that is wrong)

 

You are right that it is not easy to separate out SR and GR effects when doing experiments.

Edited July 21, 2011 by J.C.MacSwell

[swansont]

Swansont,

 

I am sometimes perplexed by the nature of experiments designed to test, say gravitational redshift, in that the setup is complex and many assumptions are made, where it is hard to know what is being already taken into account and what has possibly been overlooked, when there seems to be simple experiments that would do the same job. For instance, wouldn't the Sun provide a sufficient enough gravity well, that we could consider that the adherence of nature to the scheme of general relitivity would cause clocks on Earth to tick faster than at the sun, measure a hydrogen absorption line from some Earthly hydrogen, compare it to the same hydrogen absorption line from some solar hydrogen, and say "There." "See the difference?" "That proves that hydrogen ticks faster out here, than it does deep in the Sun's well". "And look!" "It fits the formulae exactly!"

 

Regards, TAR2

 

But wait. Wouldn't that mean that the hydrogen on Earth is "older" than the hydrogen at the Sun?

 

And if we calculate the age of the universe, should we go by age of the Earth's hydrogen, or by the age of the Sun's hydrogen?

"It fits the formula exactly" is valid if the effect from the sun and from earth can be distinguished. Time dilation depends on the gravitational potential and you can measure that at various places and see how it varies. It varies as predicted. If it depended on some other set of variables, it wouldn't behave that way.

[Vilas Tamhane]

As long as "apparent" is understood in the context that your observation is apparent as well, then I think that is fine. If you think that your view is somehow "real" in a context that the others is not, then you cannot support this scientifically.

 

As for whether the rod contracts (or expands, if the relative speed of the observer decreases), I would say it depends, again, on what is meant by "contract" or even "physically contract". It cannot mean precisely what we mean in our daily usage as the mechanism is different from that of any type of everyday usage. But the length or distance gets measured as shorter when applying a consistent set of physical laws.

 

 

Between the three frames of the trains and myself on the platform, I know that my frame is a preferred one. This is because I know that the trains accelerated, not me. I cannot accelerate away from the train. This is not only a physical impossibility but I cannot accelerate differently for three different trains. This is also a fact because I never accelerated w.r.t. the trains in the first place.

 

Reality is that, in my frame, there are innumerable objects that are moving with different velocities. Though velocity is relative, we can tag accelerated frames and that is what we do when we shift from theory to experiments.

 

 

On the reality of the actual contraction, your reply is not satisfactory. You said,

 

“It cannot mean precisely what we mean in our daily usage as the mechanism is different from that of any type of everyday usage.”

 

There cannot be two definitions and two mechanisms for length contraction. Length contraction in SR is just a mathematical derivation and it never takes place because it cannot take place.

[J.C.MacSwell]

I haven’t understood how length can contract. It is not the question of measurements alone. Problem is not mathematical. I find it impossible that a physical rod can contract. Imagine three trains running, with different speeds, between stations A and B. I am perplexed because you tell me that the stations come closer by an amount different for three different trains. But standing on the platform I can see that there is no contraction and so I conclude that the measurements of the train drivers are apparent.

 

 

Between the three frames of the trains and myself on the platform, I know that my frame is a preferred one. This is because I know that the trains accelerated, not me. I cannot accelerate away from the train. This is not only a physical impossibility but I cannot accelerate differently for three different trains. This is also a fact because I never accelerated w.r.t. the trains in the first place.

 

Reality is that, in my frame, there are innumerable objects that are moving with different velocities. Though velocity is relative, we can tag accelerated frames and that is what we do when we shift from theory to experiments.

 

 

On the reality of the actual contraction, your reply is not satisfactory. You said,

 

“It cannot mean precisely what we mean in our daily usage as the mechanism is different from that of any type of everyday usage.”

 

There cannot be two definitions and two mechanisms for length contraction. Length contraction in SR is just a mathematical derivation and it never takes place because it cannot take place.

 

If you cannot keep your own examples straight it is going to be pretty tough to learn anything.

[Vilas Tamhane]

 

Even in the absence of quantum effects there is a competing theory of gravitation. It is called Einstein-Cartan (EC) theory. EC theory differs from GR in not making the a priori assumption that spacetime is torsion free. This greatly complicates the mathematics of the differential geometry. At the current level of measurement technology the predictions of EC theory are indistinguishable from GR, and because the mathematics is so much more difficult EC theory is not so well known. It has the intriguing feature that at least some of the singularities associayed with exotic GR solutions do not occur in EC theory. The point here being that again we have two mathematically consistent theories, and only an advance in experimental capability will be able to determine which is the better model or "correct". Thought expereiments are not sufficient.

 

Surely and steadily, mathematicians are dragging physics to gallows. In India students are rapidly loosing interest in physics. According to reports, situation is similar in developed countries. This is how great civilizations die. Sanskrit is a highly accomplished language (just like Latin). It is now dead.

 

 

[tar]

Swansont,

 

OK then. Back to the twins. There are assumptions made and frames switched all along the way, and certain SR effects and GR effects that are confounded, depending on your "starting" point in each consideration.

In many cases, as with the cesium clocks that have moved at different velocities, all the clocks, whether they are going up or coming down, moving with or against the spin of the Earth, or staying at the airport are constantly under the combined effects of the rest of the universe, with which they "need" to stay in tune.

 

It struck me in your response to my hydrogen absorbtion line question, that "assuming the hydrogen in the Earth and Sun STARTED at the same age" is not necessarily a correct assumption. The "path" that each particle, that created each hydrogen atom, and the subsequent path each hydrogen atom took to get from the Big Bang, to our solar system, define a different history. Almost impossible to consider the "actual" age of each sub component, from any given starting point, considering that the starting assumed age may be incorrect. There does seem to be however, ways that the universe balances things out, as in the SR and GR equations, where something can only be given up here, if it is gained over there. The total age of the universe will be conserved. None of the universe should be able to get too far ahead, or fall too far behind, because each part is affected by and conversly affects, the rest of it, at the speed of light.

 

When we pick a frame and consider all points starting at the same age in the same place, the assumption is not completely correct. The stuff at one end of the frame, is at a different place than the stuff at the other end. The math gets incredibily complex. The universe however, can figure it out, in an instant and know exactly how to behave, considering ALL the "effects" of motion, ALL the time.

 

I believe my problem with the twins is that I can not conceive of the travelling twin actually leaving the universe, and becoming 5 years younger than ALL of it. I would think that inorder to make the trip, she would have to take a substantial part of the universe with her. Considering how much energy it takes to get a tiny particle up to relativistic speeds, she would have to rearrange a lot of the local universe, to get herself and her ship up to that speed. And the universe would find a way to balance things out, so that she would not fall substantially behind.

 

Regards, TAR2

[imatfaal]

With respect Vilas - physics without a rigorous mathematical underpinning probably deserves to die, it is mere word salad.

 

BTW in the UK applications to read physics at university are at a high (especially in Manchester and for female students - whether this is due to Manchesters success at modernising curriculum to gain engagement from young women combined with their recent Nobel success OR because they fancy Brian Cox is debatable!)

[swansont]

For any particle, its own frame is all that matters. There is no "staying in tune" with anything else.

 

The total age of the universe will be conserved.

That's a nonsensical statement. When switching frames one speaks of invariants (conserved is within a single frame only) and time is most certainly not an invariant.

 

The idea that things will "even out" is philosophy, not science.