Why are the effects of Time Dilation permanent but Length Contraction is Not?

[dfaullin]

This question is in regard to special relativity.

From my rudimentary understanding, concerning the twin paradox, if one twin leaves traveling near the speed of light and returns, he will find himself younger than his twin who stayed behind. Hence, the effect of time dilation is permanent.

However, I have never read anywhere that the traveling twin's length will also be permanently adjusted due to length contraction.

How is it that one Lorentz transformed aspect remains while the other one vanishes upon the traveling twin's return?

I apologize if this question has been asked before. If so, and you know where to find the responses, please point me in the right direction.

Thank you!

~Dylan

[russ_watters]

Time and distance aren't the same type of dimensions and they work differently. You can walk down the street and come back to where you started, but you can't move forward in time, then go back to the time you started at. So time dilation's effects are cumulative.

[PAllen]

This question is in regard to special relativity.

From my rudimentary understanding, concerning the twin paradox, if one twin leaves traveling near the speed of light and returns, he will find himself younger than his twin who stayed behind. Hence, the effect of time dilation is permanent.

However, I have never read anywhere that the traveling twin's length will also be permanently adjusted due to length contraction.

How is it that one Lorentz transformed aspect remains while the other one vanishes upon the traveling twin's return?

I apologize if this question has been asked before. If so, and you know where to find the responses, please point me in the right direction.

Thank you!

~Dylan

Welcome to Physicsforums.

This question has been asked, but it is a good question. I don't know of an FAQ to point you to, but I am happy to answer this.

Note that accumulated time for one twin is less, but that once the twins are together, the rate of their clocks is immediately the same. Similarly, the twins height is immediately the same when they re-unite. Age is the sum of moment to moment 'passage of time'. What would be needed for distance would be a measure of moment to moment 'passage of space'. There is nothing as convenient as a clock to measure this. However, if the non-inertial twin measured the integrated distance traveled by the inertial twin, and vice versa, the non-inertial twin would conclude the inertial twin had traveled less distance (compared to what the inertial twin measures for the non-inertial twin). In this sense, there is an analogous 'permanent' length contraction effect for the twin scenario.

[dfaullin]

Ah, very interesting. The traveling twin will have measured a smaller distance relative to the Earth-bound twin's measurement. That helps me feel better about it.

Because of this, will they both measure the same velocity for the traveling twin (since v=d/t)? In other words, does the smaller distance measured by the traveling twin occur in the same proportion as the smaller time measured by the traveling twin?

Also, thinking of time as accumulating (whereas length does not) helps me visualize the situation better too, so thank you for the prompt replies on the matter. I've been racking my brain on day on this stuff and it feels good to finally feel like I've accomplished something (a better understanding).

One last question: While the amount of time that passes for the traveling twin will be smaller than what is measured by the Earth-bound twin, each twin in his/her own frame does not feel time pass any differently. I guess I'm having trouble understanding how the clocks will show different times whereas the twins feel time pass at the same, normal rate in their own frames.

[PAllen]

Because of this, will they both measure the same velocity for the traveling twin (since v=d/t)? In other words, does the smaller distance measured by the traveling twin occur in the same proportion as the smaller time measured by the traveling twin?

Yes.

One last question: While the amount of time that passes for the traveling twin will be smaller than what is measured by the Earth-bound twin, each twin in his/her own frame does not feel time pass any differently. I guess I'm having trouble understanding how the clocks will show different times whereas the twins feel time pass at the same, normal rate in their own frames.

If all processes (clocks, biological aging, chemical processes etc.) are all 'slower' by the same amount, everything seems normal. How can you detect anything out of the ordinary?

[ThomasT]

This question is in regard to special relativity.

From my rudimentary understanding, concerning the twin paradox, if one twin leaves traveling near the speed of light and returns, he will find himself younger than his twin who stayed behind. Hence, the effect of time dilation is permanent.

However, I have never read anywhere that the traveling twin's length will also be permanently adjusted due to length contraction.

How is it that one Lorentz transformed aspect remains while the other one vanishes upon the traveling twin's return?Because it applies to different velocities (speeds). An oscillator's frequency is, apparently, dependent on its speed. So, when the travelling twin is reunited with the earthbound twin, then their clocks (and their biological oscillators) are 'keeping time' at the same rate. But while the travelling twin was travelling at a rate of speed exceeding the earthbound twin, then the periods of his clock and biological oscillators were increased. And that increase was cumulatively, and irreversibly, recorded as a slowing of time and aging.

[dfaullin]

It just seems like 1 second for Twin A is the same as 1 second for Twin B. What is different is the time that Twin A sees passing for Twin B.

I think I just need to think some more about it. I need to reconsider how I view the situation in light of thinking about the distance of the traveling twin being literally less than what Earth-bound twin measures.

Thanks again for your responses.

[DaleSpam]

From my rudimentary understanding, concerning the twin paradox, if one twin leaves traveling near the speed of light and returns, he will find himself younger than his twin who stayed behind. Hence, the effect of time dilation is permanent.

However, I have never read anywhere that the traveling twin's length will also be permanently adjusted due to length contraction.

How is it that one Lorentz transformed aspect remains while the other one vanishes upon the traveling twin's return?Actually, time dilation and length contraction are the same in this regard. A pair of ticks on a clock measures a uniform duration between the ticks. A pair of ticks on a rod measures a uniform distance between the ticks. While the twins are travelling both kinds of ticks are distorted. When they return to rest both kinds of ticks are undistorted.

The difference is simply that we typically keep a running total of the ticks for a clock but not for a ruler. The device which we use to keep a running total of ticks for a ruler is called an odometer. The device which we use to measure ticks of a clock without keeping a running total is called a metronome. So, the proper comparison is between clocks and odometers or between rulers and metronomes.

[harrylin]

Actually, time dilation and length contraction are the same in this regard. A pair of ticks on a clock measures a uniform duration between the ticks. A pair of ticks on a rod measures a uniform distance between the ticks. While the twins are travelling both kinds of ticks are distorted. When they return to rest both kinds of ticks are undistorted.

The difference is simply that we typically keep a running total of the ticks for a clock but not for a ruler. The device which we use to keep a running total of ticks for a ruler is called an odometer. The device which we use to measure ticks of a clock without keeping a running total is called a metronome. So, the proper comparison is between clocks and odometers or between rulers and metronomes.
Actually a metronome produces a beat; we measure it with a frequency meter. But indeed, the comparison is between clocks and odometers as well as between rulers and frequency meters.

[dacruick]

The difference is simply that we typically keep a running total of the ticks for a clock but not for a ruler. The device which we use to keep a running total of ticks for a ruler is called an odometer. The device which we use to measure ticks of a clock without keeping a running total is called a metronome. So, the proper comparison is between clocks and odometers or between rulers and metronomes.

I like this explanation. I think it is useful to make a direct comparison from length contraction to time dilation. If we look at age as the sum of all of the ticks on a clock, then for our length contraction to show its effects in the same way, we would have to define height as the sum of the individual's length over time. It sounds absurd right? I agree...

It is useful to treat time in this way (cumulatively), and it is not useful to treat height like this. Yet I do not think that they intrinsically differ. Hmm...well, those are just my thoughts.

[Elroch]

There's another way to look at it, as well. With time dilation what happens is that two people keep a record of time and their record differs in the end. The same would happen with distance if it was performed in a certain way. Suppose A goes to Alpha Centauri and back, and B stays on Earth. Both measure the apparent distance they travel relative to each other.

When they compare distances at the end, B says the total distance was just over 8 light years, but A says it was actually a lot less. Of course we know the reason that for A the distance to his destination was contracted when he was going fast.

[dacruick]

When they compare distances at the end, B says the total distance was just over 8 light years, but A says it was actually a lot less. Of course we know the reason that for A the distance to his destination was contracted when he was going fast.

I have a question about this. Is it the distance between the planets that is contracted or the length of the spaceship that's contracted? I always assumed that both were possible, it just depended on the frame of reference. In the case that the distance between the planets is contracted, that then implies that the spaceship was stationary, and that both Earth and Alpha Centauri were moving with respect to the spaceship. This then also implies that the observer on Earth was experiencing time dilation to a greater degree than the observer on the spaceship.

Is my recollection of the events accurate?

[ghwellsjr]

I like this explanation. I think it is useful to make a direct comparison from length contraction to time dilation. If we look at age as the sum of all of the ticks on a clock, then for our length contraction to show its effects in the same way, we would have to define height as the sum of the individual's length over time. It sounds absurd right? I agree...

It is useful to treat time in this way (cumulatively), and it is not useful to treat height like this. Yet I do not think that they intrinsically differ. Hmm...well, those are just my thoughts.
Remember, length contraction occurs only along the direction of motion, so unless the traveler is laying down, it's not his height that is contracted but his thickness front to back.

[ghwellsjr]

I have a question about this. Is it the distance between the planets that is contracted or the length of the spaceship that's contracted? I always assumed that both were possible, it just depended on the frame of reference. In the case that the distance between the planets is contracted, that then implies that the spaceship was stationary, and that both Earth and Alpha Centauri were moving with respect to the spaceship. This then also implies that the observer on Earth was experiencing time dilation to a greater degree than the observer on the spaceship.

Is my recollection of the events accurate?
Yes.

Just remember that in any Frame of Reference, nothing is unusual for stationary observers/objects/clocks--it's only those things that are moving in that FoR.

So in the spaceship's FoR, the spaceship is normal but the distance between planets is contracted. In the planet's RoR, the spaceship is contracted.

Also, remember that two observers in constant relative motion directly towards or away from each other will always measure that relative speed to be the same, independent of any FoR and independent of any means by which they make the measurement.

[dfaullin]

Ah, I just made a connection (in my mind)!

Light takes the shortest path through space-time. Thus, the closer you are traveling at the speed of light, the shorter your distance will be.

But now I don't understand this: You say that the distance measured by Earth-bound twin will be greater than the distance measured by traveling twin. How does Earth-bound twin measure that distance? Using a photon? Then shouldn't that measurement give the shortest possible distance, since light is taking the shortest path to the star and back?

Thanks for the great responses.

It also helps to think that the stationary observers will notice nothing unusual ever, only those things which are moving that will exhibit strange behavior. I wish I would have used this forum sooner. I bet I would have done better in Physics when I was in college. :)

[dacruick]

Also, remember that two observers in constant relative motion directly towards or away from each other will always measure that relative speed to be the same, independent of any FoR and independent of any means by which they make the measurement.

So if the person travelling to the other planet measures the distance to be quite small due to length contraction, and the person on earth measures that distance to be large, how can they both measure the same speed relative to each other? Is this reconciled using time dilation?

I can understand how the spaceship travelling at lets say 0.9c has it's length contraction balanced out by the time dilation, but in what way does the spaceship observe a person standing on earth.

[ghwellsjr]

Ah, I just made a connection (in my mind)!

Light takes the shortest path through space-time. Thus, the closer you are traveling at the speed of light, the shorter your distance will be.

But now I don't understand this: You say that the distance measured by Earth-bound twin will be greater than the distance measured by traveling twin. How does Earth-bound twin measure that distance? Using a photon? Then shouldn't that measurement give the shortest possible distance, since light is taking the shortest path to the star and back?

Thanks for the great responses.

It also helps to think that the stationary observers will notice nothing unusual ever, only those things which are moving that will exhibit strange behavior. I wish I would have used this forum sooner. I bet I would have done better in Physics when I was in college. :)
The Earth-bound twin can use light to measure the distance to the planet. He starts a timer when he sends a flash of light (it might actually be a radio signal as used in radar) which reflects off the planet and stops the timer when he receives the return signal. The distance is one half of the time interval times the speed of light. Even though the light take the shortest path, it still takes time for it to make the trip.

[ghwellsjr]

So if the person travelling to the other planet measures the distance to be quite small due to length contraction, and the person on earth measures that distance to be large, how can they both measure the same speed relative to each other? Is this reconciled using time dilation?

I can understand how the spaceship travelling at lets say 0.9c has it's length contraction balanced out by the time dilation, but in what way does the spaceship observe a person standing on earth.
The spaceship can measure the speed that the person standing on earth is traveling away from him by observing the Relativistic Doppler of a signal coming from earth and calculating the speed. It will be the same Doppler and therefor the same speed that the earth observer will measure of the spaceship.

[dfaullin]

Okay, imagine this scenario.

An observer on Earth is going to measure the distance to Alpha Centauri in two ways. First, he will send a photon and calculate the distance as 1/2 ct.

Then, he will send an odometer to Alpha Centauri traveling near the speed of light (say .9c) and have it return and will take 1/2 of the odometer reading.

Which distance will be shorter?

~Dylan

[PAllen]

Okay, imagine this scenario.

An observer on Earth is going to measure the distance to Alpha Centauri in two ways. First, he will send a photon and calculate the distance as 1/2 ct.

Then, he will send an odometer to Alpha Centauri traveling near the speed of light (say .9c) and have it return and will take 1/2 of the odometer reading.

Which distance will be shorter?

~Dylan

Well, there really is no such thing as an odometer to measure travel through empty space. This is why, in my initial answer, what I posed for each twin to measure is the travel distance of the other, as they measure it. So let's say the twin A remains on earth and measures B traveling 4 lightyears away (to some star) and back - total distance 8 ly. Assume B has traveled there at .9c. B measures A traveling away and back to B. The distance B measures for A's trip will be approx 3.5 light years (assuming 'instant turnaround'). However, as soon as B stops at earth again and measures the distance to the star, they get 4 light years. So they say 'whoa, this relativity can be really strange'. Yes, it can.

As to how B can measure A (and the sun) distance as A travel's away and back (per B), any valid method will do (in our situation of constant speed). For example, they could use parallax with the aid of companion traveling along with them at some distance away. This modest distance (to the companion) could also be measured any convenient way (rulers, light travel time, it doesn't matter).

[dfaullin]

Mind blown!

[Elroch]

I have a question about this. Is it the distance between the planets that is contracted or the length of the spaceship that's contracted? I always assumed that both were possible, it just depended on the frame of reference. In the case that the distance between the planets is contracted, that then implies that the spaceship was stationary, and that both Earth and Alpha Centauri were moving with respect to the spaceship. This then also implies that the observer on Earth was experiencing time dilation to a greater degree than the observer on the spaceship.

Is my recollection of the events accurate?

To an observer on the spaceship, the rest of the Universe appears to be squashed in the direction of flight. For example, when a spaceship slows down, in the (non-inertial) decelerating frame, things in front of it can appear to be receding (and things behind it can appear to be receding as well). Of course, it is also true that, to someone observing a spaceship, it appears to be squashed along the direction of (relative) motion.

[Elroch]

Well, there really is no such thing as an odometer to measure travel through empty space.
True. There can be no such thing since there is no aether. But measuring position and speed relative to the midpoint of source and destination seems a natural choice for this problem.

[PAllen]

True. There can be no such thing since there is no aether. But measuring position and speed relative to the midpoint of source and destination seems a natural choice for this problem.

In a generic twin problem, there is not necessarily a well defined destination, certainly not necessarily an obvious mid point (if you consider arbitrary trajectories). The thing that seems natural and completely general for me - best stand in for an odometer - is for each twin to measure the apparent travel distance of the other twin; as I have suggested in post #3 and again in #20.

[Qzit]

WOW, this is a fun thread. What would be the difference between your brain synapsis firing at 16 frames/second, 16 frames / day or 16 frames/year? If everything is relative in your FoR absolutely nothing. Your synapsis is your internal clock for judging time. So the traveling twin’s synapsis is slowed compared to the twin on Earth. Well I suspect so are your relative metabolism and the relative cycles of all the electrons in your body. In fact they can be so slowed that if you had a magic telescope to watch your twin on Earth he would seem to move so fast you would think of him as the comic book hero the Flash. Is the distance contracted? No. Your sense of time is contracted. If you were traveling at 0.9C and a photon closing the distance towards you would be 1.9C. A photon traveling parallel with your ship would have a relative departing speed of 0.1C. Now what is physically happening and what you observe to be happening are two different things because of the speed of your synapsis and the cycling electrons in your FoR.

[ghwellsjr]

Okay, imagine this scenario.

An observer on Earth is going to measure the distance to Alpha Centauri in two ways. First, he will send a photon and calculate the distance as 1/2 ct.

Then, he will send an odometer to Alpha Centauri traveling near the speed of light (say .9c) and have it return and will take 1/2 of the odometer reading.

Which distance will be shorter?

~Dylan
Assuming that Alpha Centauri is 4 light years away from Earth, the observer on Earth will measure 8 years for a signal to get there and back and so will calculate its distance to be 4 light years.

He can send an odometer there and back at .9c and measure the distance that way. He can make an odometer by observing the spectrum of light coming from Alpha Centauri and from the Sun prior to sending away the odometer. Then, as the odometer is traveling, it continuously measures the Relativistic Doppler coming from both stars. One will be the reciprocal of the other. (Only one is required for the measurement but I'm showing that either one or both can be used.) Assuming that one of these ratios is R, the speed of the odometer is:

β = |(1-R2)/(1+R2)

Integrating the speed over time yields distance traveled as a function of time. To make the calculation easier, we will assume that the speed is constant throughout the entire roundtrip which means that we only have to multiply the speed by the total time to get the total distance.

Now let's work out the details for our example:

At .9c, the values of R will be √[(1-β)/(1+β)] and its reciprocal. So R for Earth will be 0.2294 and for Alpha Centari will be 4.359 during the outbound portion of the trip. For the inbound portion of the trip, these numbers are exchanged.

The odometer will use the equation above to determine that β is indeed 0.9c.

Now at 0.9c, the clock on the odometer will be running slow by a factor of 1/γ. We calculate γ as 1/√(1-β2), so 1/γ = √(1-β2) which equals 0.4359. Now we need to calculate how long the trip will take. We do this first in the Earth frame as distance divided by speed which is 8 light years (round trip) divided by 0.9c which equals 8.8889 years. Now we multiply this by 1/γ to figure out what the time will be in the odometer's frame. This will be 8.8889 years times 0.4359 or 3.8747 years. This means that the total distance traveled is 3.8747 times 0.9 c or 3.4872 light years for the round trip or 1.7436 light years for the distance between Earth and Alpha Centauri. As a sanity check, this should be the distance in the Earth frame divided by γ or multiplied by 1/γ which we calculated as 0.4359. Indeed, 4 times 0.4359 is 1.7436 light years.

So to answer your question, the distance measured by the odometer is shorter.

[ghwellsjr]

Well, there really is no such thing as an odometer to measure travel through empty space.True. There can be no such thing since there is no aether. But measuring position and speed relative to the midpoint of source and destination seems a natural choice for this problem.
Well, I just showed how to make an odometer to measure travel through empty space.

And I don't understand why you base your conclusion on the idea that "there is no aether". I don't see the connection.

I also don't understand your comments about "the midpoint of the source and destination".

[ghwellsjr]

WOW, this is a fun thread. What would be the difference between your brain synapsis firing at 16 frames/second, 16 frames / day or 16 frames/year? If everything is relative in your FoR absolutely nothing. Your synapsis is your internal clock for judging time. So the traveling twin’s synapsis is slowed compared to the twin on Earth. Well I suspect so are your relative metabolism and the relative cycles of all the electrons in your body. In fact they can be so slowed that if you had a magic telescope to watch your twin on Earth he would seem to move so fast you would think of him as the comic book hero the Flash. Is the distance contracted? No. Your sense of time is contracted. If you were traveling at 0.9C and a photon closing the distance towards you would be 1.9C. A photon traveling parallel with your ship would have a relative departing speed of 0.1C. Now what is physically happening and what you observe to be happening are two different things because of the speed of your synapsis and the cycling electrons in your FoR.
WOW, I think you're having too much fun. A lot of erroneous ideas here.

What do you mean that there is no difference between 16 frames per second, day or year? It is true that all processes, including biological will be slowed down for the traveling twin, as determined by the rest frame of the Earth, but the traveling twin will also determine that the Earth twin is the one that is experiencing time dilation according to his own rest frame. And you're also mixed up on the speed of a photon being 1.9c or 0.1c according to the traveling twin. I suggest that you do a lot of reading or asking questions instead of trying to answer them until you get yourself up on the learning curve.

[harrylin]

Well, I just showed how to make an odometer to measure travel through empty space.

And I don't understand why you base your conclusion on the idea that "there is no aether". I don't see the connection.

I also don't understand your comments about "the midpoint of the source and destination".
I also wondered about that... My 2 cts: There are no mechanical odometers; I suppose that what was meant, is that there is no material ether on which we could let an odometer roll. Note also that if we could, then any odometer would measure "absolute" lengths, for the wheel is at rest wrt the road at the point of contact!

And I think that the midpoint of source and destination depends on the reference system for two twins in arbitrary motion.

[Qzit]

ghwellsjr
I think you were reading with your mind and not your eyes. You would not perceive a difference in time out in space if your relative synapsis fired 16 times a second, a day or a year. My other question to you is what is the closing speed between two photons traveling on a collision course? This was not in reference to the traveling twin. Of course the traveling twin would see it differently with his clock slowed.

[nitsuj]

WOW, I think you're having too much fun. A lot of erroneous ideas here.What do you mean that there is no difference between 16 frames per second, day or year? It is true that all processes, including biological will be slowed down for the traveling twin, as determined by the rest frame of the Earth, but the traveling twin will also determine that the Earth twin is the one that is experiencing time dilation according to his own rest frame. And you're also mixed up on the speed of a photon being 1.9c or 0.1c according to the traveling twin. I suggest that you do a lot of reading or asking questions instead of trying to answer them until you get yourself up on the learning curve.

I can't see one in his/her post. Could you be more specific since your are being so bold.

The closing distance comment is fairly accurate (#'s aren't). The concept is definitly right (excluding the "distance" contraction / "time contraction" comment).

Of course the 16 seconds, days, year comment is refering to time dilation. Im pretty sure you understand that. It means there is no difference in the traveling observers perception of time passing (tick tock / proper time ect) to that of an observer who is at "rest".

I suggest that you do a lot of reading or asking questions instead of trying to answer them until you get yourself up on the learning curve.

Nice ain't it?

You've helped me improve my understanding of SR on a number of occasions ghwellsjr, it is a shame to see you post a reply like that. When I know you are capable of getting Qzit's interpretations on a more defined track / train of thought.

[DaleSpam]

I can't see one in his/her post. I don't know which ones ghwellsjr noticed, but I saw a few that were either wrong or only conditionally right:

WOW, this is a fun thread. What would be the difference between your brain synapsis firing at 16 frames/second, 16 frames / day or 16 frames/year? If everything is relative in your FoR absolutely nothing. Your synapsis is your internal clock for judging time. So the traveling twin’s synapsis is slowed compared to the twin on Earth. Well I suspect so are your relative metabolism and the relative cycles of all the electrons in your body.So far everything is fine.

In fact they can be so slowed that if you had a magic telescope to watch your twin on Earth he would seem to move so fast you would think of him as the comic book hero the Flash.This would be true on the return journey, but false on the outward journey.

Is the distance contracted? No. False. Distance is contracted too.

Your sense of time is contracted. If you were traveling at 0.9C and a photon closing the distance towards you would be 1.9C. A photon traveling parallel with your ship would have a relative departing speed of 0.1C. True only in the earth frame.

Now what is physically happening and what you observe to be happening are two different things because of the speed of your synapsis and the cycling electrons in your FoR.This is a LET interpretation, not a SR interpretation. However, interpretations cannot be experimentally proven one way or the other, so I have no objection other than a kind of general distaste for LET as a pet theory of crackpots.

[nitsuj]

Yes Dalespam I understand your point and even ghwellsjr's point. This forum like's to be techincaly right, which is awsome and seperates a poor quality physics forum from a great one. Leaving out discussions about biology, conciousness and the sorts is important to point out because of the add difficulty of interpretation among other things.

But do you understand my point?

Qzit's interpretations are not flat out wrong, they are poorly defined. This is a big difference if someone is going to call those interpretations as erroneous. Said differently alot of Qzits interpretations are correct, ghwellsjr implied most were wrong. In a sense, possibly pushing Qzit further down the learning curve.

I wouldn't have an issue if ghwellsjr replied pointing out the technicalities, like you did.

And yes, as I noticed too (a layman of SR) the "distance" contraction isn't right. Big deal, point it out and say good for you in noticing the impact of closing distances. So one wrong comment.

Your last comment, if I understand right, yes if Qzit wants to stay "true" to SR (or physics in general), the "reality" is the observation (measurement +calculation). But it is merely a prefference, and not crackpottery.

[DaleSpam]

Do you understand mine?

Qzit's interpretations are not flat out wrong, they are poorly defined. Yes, I understand your point. Only the one comment was flat out wrong, the rest could be either wrong or right depending on the "poorly defined" details.

[DaleSpam]

Your last comment, if I understand right, yes if Qzit wants to stay "true" to SR (or physics in general), the "reality" is the observation (measurement +calculation). But it is merely a prefference, and not crackpottery.Let me clarify my intention.

Now what is physically happening and what you observe to be happening are two different things because of the speed of your synapsis and the cycling electrons in your FoR.This is a LET interpretation, not a SR interpretation. However, interpretations cannot be experimentally proven one way or the other, so I have no objection other than a kind of general distaste for LET as a pet theory of crackpots.In LET it is true that what is physically happening and what you observe to be happening are two different things. In LET as you move relative to the aether your lengths contract and times dilate such that the measurements you make are incorrect. Specifically, your coordinate time is an incorrect measure of the aether coordinate time, and so forth. Only the physics in the aether frame are "physically happening".

On the other hand, in SR every reference frame is equally valid. So both of our observations are equally valid observations of what is physically happening. The disagreements between our observations are not because one of us "distorted" and the other is not but rather simply because we are each observing the same physics from different but valid perspectives. In this view, your coordinate time is a correct measure of the proper time, and so forth.

Both of the above interpertations are valid, and in principle I have no problem with the LET interpretation. However, most of the proponents of LET that I have encountered are crackpots and my interactions with them are usually fairly unpleasant. So I have a certain "guilt by association" distaste for LET. I was not calling Qzit a crackpot nor even implying it, merely expressing the source of my distaste for LET.

[nitsuj]

Understood Dalespam.

I appreciate what you are saying.

You've been patient with me in that past with such things & George too to a lesser extent. It's really really helped me in trying to grasp the concepts of SR intuitively.

So it kinda burns me to see a post like I've been commenting on. Where a poster is on the cusp of being correct, but is instead "written off" as being not worthy (the interpretations) of a helpfull reply.

[Qzit]

I did not know my thoughts were LET or crackpot ideas Sorry. Please help me understand a little better.
False. Distance is contracted too.
I thought it was only the view that was contracted and not the distance. If your synapsis is slowed than distance would appear contracted but the physical distance remains the same for a photon to travel. How does a space ship actually contract space in front of it? Please help I am confused. I did not understand that a space ship could do anything to the space in front of it. If I were mid-point to a destination would I be in a physical contraction?
Your sense of time is contracted. If you were traveling at 0.9C and a photon closing the distance towards you would be 1.9C. A photon traveling parallel with your ship would have a relative departing speed of 0.1C.

True only in the earth frame.

"C" I thought was a constant. What does not matter where you observe? If there is no observer "C" is always "C". What you observe is an accurate mathematical difference based on relativity. Why do you see it more than an optical affect?
This would be true on the return journey, but false on the outward journey.


If you had a magical telescope (which is impossible to begin with) say your Synapsis on the ship is firing at 16 frames an hour and your twin is firing at16 frames per second on Earth. The Earth twin would always be moving much faster to my way of thinking. This would be real and not just observed I would suspect. If I am wrong please explain.

[ghwellsjr]

I still maintain that Qzit has a lot of erroneous ideas. Most basically is his concept expressed in his last sentence that it's only because of your sluggish neurons in your For that explains why observations differ from reality:
Now what is physically happening and what you observe to be happening are two different things because of the speed of your synapsis and the cycling electrons in your FoR.
In your FoR, your synapses and cycling electrons are perfectly normal. It's the other person's synapses and cycling electrons that are experiencing time dilation. He's got this backwards.

How about this comment:
In fact they can be so slowed that if you had a magic telescope to watch your twin on Earth he would seem to move so fast you would think of him as the comic book hero the Flash. Is the distance contracted? No. Your sense of time is contracted.
He's thinking that because your processes are running slowly, you would see your Earth twin's processes as running fast which is not correct. He's not talking about Relativistic Doppler which is observable with a real telescope, he's talking about a magic telescope that eliminates the Doppler to let you see what's really going on. And he has it wrong, the traveling twin would see the Earth twin's processes as going slow, not fast.

And what do you make of his last comment: "Your sense of time is contracted."? Time is not contracted, it's dilated, but either way, your sense of time is always normal, never contracted or dilated.

Now as to his comments regarding the closing speed of photons which he said is not related to the Twin Paradox:
If you were traveling at 0.9C and a photon closing the distance towards you would be 1.9C. A photon traveling parallel with your ship would have a relative departing speed of 0.1C.
I don't think he understands that the speed of a photon, like the propagation speed of all light, AKA the one-way speed of light, is never observable, it is assigned a speed by a theory, in this case the Theory of Special Relativity, and that speed is always c, not 1.9c or 0.1c. In his later post, he asked:
My other question to you is what is the closing speed between two photons traveling on a collision course?
The answer to that is 2c but it should not be regarded as any measurable or physically real speed, it's just the difference between two speeds in a given FoR. There is nothing traveling at 2c with respect to any FoR or to any other object.

I really want to help Qzit learn SR and if I have misdiagnosed his comments, then I apologize, and he can help me by providing relevant feedback.

[nitsuj]

I did not know my thoughts were LET or crackpot ideas Sorry. Please help me understand a little better.
I thought it was only the view that was contracted and not the distance. If your synapsis is slowed than distance would appear contracted but the physical distance remains the same for a photon to travel. How does a space ship actually contract space in front of it? Please help I am confused. I did not understand that a space ship could do anything to the space in front of it. If I were mid-point to a destination would I be in a physical contraction?

"C" I thought was a constant. What does not matter where you observe? If there is no observer "C" is always "C". What you observe is an accurate mathematical difference based on relativity. Why do you see it more than an optical affect?

If you had a magical telescope (which is impossible to begin with) say your Synapsis on the ship is firing at 16 frames an hour and your twin is firing at16 frames per second on Earth. The Earth twin would always be moving much faster to my way of thinking. This would be real and not just observed I would suspect. If I am wrong please explain.

I am not skilled enough in SR to debate certain SR concepts.

But I can tell you forget about including biology if you plan on learning SR concepts here. Not supprisingly it shouldn't come up. :smile:

Understand that time and length are measurements. It is these measurements that are comparatively different with relative motion. You can as easily works backwards by considering c is constant. c is two measurements, time and length. Simmer that....

For me an easy way to understand this "which perspective (FoR) is "real"" is to consider the simple idea that motion is relative. If you can subscribe to that, the rest of observations made of course have that same principal.

that's about as well as I can explain the concept.

[nitsuj]

ghwellsjr, I think I may have taken your post a little differently then your intent.

You made me realize something; I take back my comment the poster was on the cusp of being correct. That, as I found out myself with SR concepts, is far different from understanding.

[Qzit]

WOW I see my English is very poor. You are correct to gig me for that. Let’s look at contraction a different way. If you had a space ship coming towards at ½ the speed of light and there was a mirror on the front and on the back of the spaceship to bounce returning photons we can see contraction as an optical effect of the finite speed of light. Now we shoot a signal to the ship. Half of the signal hits the first mirror and heads back. The second ½ of the signal moves towards the back of the ship. While the second half of signal is moving towards the back the ship has closed the distance towards the signal by 1/3 of the ships length when the second half of the signal bounces off. On return of the signal to the stationary observer the ship appears to be 2/3 its actual size. Moving away it would appear to be 1 and 1/3 its actual size. This is because of the finite speed of lights optical effect not a physical effect to my way of thinking. Am I wrong?

Put a strobe on in the dark to mimic a slowed synapsis firing. You sit down and watch others move. They appear to be moving faster.

[Janus]

"C" I thought was a constant. What does not matter where you observe? If there is no observer "C" is always "C". What you observe is an accurate mathematical difference based on relativity. Why do you see it more than an optical affect?



It would be more correct to say that c is invariant. c has the same value relative to any inertial frame as measured from that frame.

So in your ship you will measure the light as traveling at c relative to the ship. In other words, the light coming from in front of you will take the same time to travel from the front of the ship to the back of the ship as the light traveling from the back of you will take to travel from back of the ship to the front.

However, the same light as measured from the Earth travels at 0.1c relative to you going in one direction and 1.9c going in the other. This means that according to the Earth, the light passing you from behind takes longer to pass from back to front of your ship than that light you are meeting takes to pass from front to back of your ship.

[ghwellsjr]

WOW I see my English is very poor. You are correct to gig me for that. Let’s look at contraction a different way. If you had a space ship coming towards at ½ the speed of light and there was a mirror on the front and on the back of the spaceship to bounce returning photons we can see contraction as an optical effect of the finite speed of light. Now we shoot a signal to the ship. Half of the signal hits the first mirror and heads back. The second ½ of the signal moves towards the back of the ship. While the second half of signal is moving towards the back the ship has closed the distance towards the signal by 1/3 of the ships length when the second half of the signal bounces off. On return of the signal to the stationary observer the ship appears to be 2/3 its actual size. Moving away it would appear to be 1 and 1/3 its actual size. This is because of the finite speed of lights optical effect not a physical effect to my way of thinking. Am I wrong?
Let's put it this way, you are half wrong. What you are describing is Doppler but it's not like normal Doppler for sound in air where there is in fact no length contraction. With Relativistic Doppler involving light, there is the effect caused by the finite speed of light which is what you are talking about, but when you subtract that out, there remains an effect caused by the contraction of the length of the spaceship.

[Qzit]

Hi ghwellsjr
After you subtract the Doppler how much more contraction is there?

Hi Janus,
How do you measure the one way speed of light in a single direction or is it assumed that inside the ship the speed of light is measured to be the same in both directions?

[DaleSpam]

I did not know my thoughts were LET or crackpot ideas I tried to make it clear. I never said that you were a crackpot, nor did I intend to imply it. I was only mentioning the reason for my aversion to LET.

I thought it was only the view that was contracted and not the distance. If your synapsis is slowed than distance would appear contracted but the physical distance remains the same for a photon to travel. How does a space ship actually contract space in front of it? Please help I am confused. I did not understand that a space ship could do anything to the space in front of it. If I were mid-point to a destination would I be in a physical contraction?Spaceships don't do anything to space. Length contraction is an aspect of the mathematical relationship between different inertial reference frames, just like time dilation. This relationship is called the Lorentz transform. In different reference frames the physical distance is different, even after you correct for all of the view-related and finite-speed-of-light-related effects. You cannot have time dilation without length contraction, they are two sides of the same "coin" (the Lorentz transform).

"C" I thought was a constant. What does not matter where you observe? If there is no observer "C" is always "C". What you observe is an accurate mathematical difference based on relativity. Why do you see it more than an optical affect?If I am travelling at .9 c in Earth's frame heading towards a pulse of light then in the Earth's frame the distance between the pulse of light and me is closing at 1.9 c. In my frame I am travelling at 0 c by definition, so in my frame the distance between the pulse of light and me is closing at c. So your statement above is only true in the Earth's frame, not in my frame.

If you had a magical telescope (which is impossible to begin with) say your Synapsis on the ship is firing at 16 frames an hour and your twin is firing at16 frames per second on Earth. The Earth twin would always be moving much faster to my way of thinking. This would be real and not just observed I would suspect. If I am wrong please explain.It is impossible to start from a false premise like a magical telescope and obtain any valid conclusions. Can you think about your real question and ask it without any impossible premises?

[Qzit]

Hi DaleSpam,

I am beginning to think I think more in line with LET but I do not know the meaning. Between my use of the English language and your replies about LET I am sorry if I was unpleasant. I thought the Lorentz transformations were the same as the optical effect. I did not realize there is a different Lorentz contraction then the one I described with the mirrors. What is the mathematical Lorentz contraction for measuring the length of a ship going ½ “C” to a stationary observer?

[DaleSpam]

The Lorentz transform is given here:
http://en.wikipedia.org/wiki/Lorentz_transformation#Lorentz_transformation_for_ frames_in_standard_configuration

All of the usual relativistic features like time dilation, length contraction, and the relativity of simultaneity are contained in it. The Lorentz transformations are not optical effects. They are what remain after all of the optical effects are accounted for. They are not due to the fact that the speed of light is finite, but rather they are due to the fact that the speed of light is invariant.

[Qzit]

Hi DaleSpam,

All of the usual relativistic features like time dilation, length contraction, and the relativity of simultaneity are contained in it. The Lorentz transformations are not optical effects. They are what remain after all of the optical effects are accounted for. They are not due to the fact that the speed of light is finite, but rather they are due to the fact that the speed of light is invariant.

Relativity of simultaneity is an explanation of visual contraction not physical contraction. The Lorentz contraction says close to the speed of light the contraction is close to 100%. If a ship could go “C” it would have no length. I can explain relativity of simultaneity that shows it to be a visual effect of the finite speed of light. You are in a ship going near the speed of light. You are in the front of the ship and hit the switch that simultaneously turns on the front and back of a pulse emitter. The front goes off first and the signal travels down the ship at the speed of light. When the signal reaches the back of the ship the back reaches the position the first pulse fired. There is your contraction due to the finite speed of a photon. If you set off the signal in the center of the ship there would be no issue with the simultaneity of relativity and no contraction. Instantaneous and simultaneity of relativity are two different things.


Time dilation on the other hand should be a physical issue. Time slowing down suggests that mass dilates, gets larger and less dense. This is definitely an issue of invariance of “C”. If mass dilates and the path of the electron increases the cycles tick slower to match the longer distance a photon has to travel to maintain its invariance. Every other frame uses their own measuring stick and tick of their clock for measurement. If there measuring stick is shorter like in outer space away from mass there clock ticks faster. In this way the speed of light is measured in every frame to be invariant.

[DaleSpam]

Relativity of simultaneity is an explanation of visual contraction not physical contraction.This is incorrect. The relativity of simultaneity is not an explanation of any kind of contraction.

You are in the front of the ship and hit the switch that simultaneously turns on the front and back of a pulse emitter. The front goes off first You are contradicting yourself.

When the signal reaches the back of the ship the back reaches the position the first pulse fired.This is incorrect in every reference frame. There is some time, dt, between when the first pulse is fired and when the signal reaches the back of the ship. In that time the light has moved a distance c dt.

If you set off the signal in the center of the ship there would be no issue with the simultaneity of relativity and no contraction.This is also incorrect. Length contraction depends on the relative velocity between two frames, not on the location of some light source.

Time slowing down suggests that mass dilates, gets larger and less dense.Also incorrect. As v->c time slows down, and lengths contract making objects smaller and more dense.

Every other frame uses their own measuring stick and tick of their clock for measurement. ... In this way the speed of light is measured in every frame to be invariant.At least you got something right.

Qzit. You should stop making assertions and start asking questions. You have some very strangely confused ideas about relativity. I am not sure what the source is. I would like to help, but I can't tell where to start yet.

[Qzit]

Hi DaleSpam,

Relativity of simultaneity is an explanation of visual contraction not physical contraction.

This is incorrect. The relativity of simultaneity is not an explanation of any kind of contraction.


You did not understand my thought experiment.

You are in the front of the ship and hit the switch that simultaneously turns on the front and back of a pulse emitter. The front goes off first

You are contradicting yourself.

I am not contradicting myself if you understand the concept of Relativity of Simultaneity. That is why I mentioned instantaneous as different from simultaneity. You have to understand the inherent relativity of the finite speed of light.

When the signal reaches the back of the ship the back reaches the position the first pulse fired.

This is incorrect in every reference frame. There is some time, dt, between when the first pulse is fired and when the signal reaches the back of the ship. In that time the light has moved a distance c dt.


Of course from a observer at rest the ship and the distance light traveled is the same so the observer at rest sees the two pulses at the same place. The result is the observer at rest sees no length to the ship.

If you set off the signal in the center of the ship there would be no issue with the simultaneity of relativity and no contraction.

This is also incorrect. Length contraction depends on the relative velocity between two frames, not on the location of some light source.


This merely changes the pulse to instantaneous rather than simultaneous of relativity.

Time slowing down suggests that mass dilates, gets larger and less dense.

Also incorrect. As v->c time slows down, and lengths contract making objects smaller and more dense

If your assertion were true than we would not have an invariant speed of light in every frame. A clock that slows down has a longer length measuring stick. The slowed clock has to have enough time for the photon to cover the increased distance of the dilation. If it were smaller the slowed clock would allow the photon to wiz past the measuring stick early destroying the invariant speed of light.

Do you want me to learn from others that are also mistaken?

[DaleSpam]

You did not understand my thought experiment.Probably not. Can you re-explain your thought experiment, this time carefully specifying which frame each quantity is measured in or in which frame any given temporal ordering is?

I am not contradicting myself if you understand the concept of Relativity of Simultaneity.I understand it quite well. However, the relativity of simultaneity doesn't apply from a single reference frame. If you want to invoke the relativity of simultaneity you need at least two different frames. It may be that you intended that, but it wasn't clear which measurements were intended for which frames from your description.

You have to understand the inherent relativity of the finite speed of light.None of the relativistic effects are due to the finite speed of light. They are due to the invariance of the speed of light.

For example, suppose that pre-Lorentz aether theories were correct and Maxwell's equations only held in the aether frame and that other inertial frames were related to the aether frame via the Galilean transform. In that case, the speed of light in other frames would be c+v where v is the speed of the frame wrt the aether, i.e. the speed of light would still be finite but not invariant. Since the Galilean transform holds, there would be no time dilation, nor length contraction, and no relativity of simultaneity.

Therefore the relativistic effects are due to the invariance of c, not the finite-ness of c. All of the features of relativity are effects that remain even after correctly accounting for the finite speed of light.

Of course from a observer at rest the ship and the distance light traveled is the same so the observer at rest sees the two pulses at the same place. The result is the observer at rest sees no length to the ship.This is not correct. I would encourage you to actually work out the math of the Lorentz transform and see for yourself.

If your assertion were true than we would not have an invariant speed of light in every frame. I would encourage you to work out the math for yourself for this also. Light and the Lorentz transform simply do not work the way you think they do.

If you have trouble working out the math for either of these then I can show you. But it will be more valuable for you if you can do it yourself. If you get stuck please let me know and I will try to help.

[Janus]

Hi DaleSpam,


If you set off the signal in the center of the ship there would be no issue with the simultaneity of relativity and no contraction.

This is incorrect. If you set off a signal in the center of the ship, according to someone in the ship, the signal radiates out from the center of the ship and will reach the ends of the ship at the same time. The point of emission and the center of the ship coincide at all times

However for someone on Earth, the signal radiates out from the emission point at c while the ship moves at 0.9c relative to the emission point. The emission point and the center of the ship only coincide at the moment of emission. Thus the rear of the ship is rushing toward the signal with a closing speed of 1.9c and the front of the ship is running away from the signal at with a closing speed of 0.1c. The signal reaches the rear of the ship before it reaches the front of the ship.

This is not an "optical" effect but a real disagreement as to whether or not event at the ends of the ship are simultaneous or not.

There also has to be a contraction. Let's assume that, upon reaching the ends of the ship, the signal is reflected back to the center. The time that passes between emission and return is noted on a clock at the center. We'll say that 1 microseconds elapsed on the clock between emission and return.( .5 us each way for both signals)

According to someone at rest with respect to the Earth, The signal will take 5 us going one way and 0.263 us going the other way, for a round trip time of 5.263 us assuming no length contraction. By time dilation, this means that 0.436 x 5.263 = 2.294 us passed on the ship's center clock.

However, from above, we know that 1 us passes at the ship's center. The ship and Earth must agree as to the reading on the ship clock at emission and return. Since the speed of the signal relative to the observers is invariant, the only way around this is for the length of the ship to be contracted by a factor of 0.436, which reduces the round trip time according to the Earth to 2.294 us and the time passed on the ship clock according to the Earth to 2.294 x .436 = 1us.

[Qzit]

Hi DalSpam

This was a sub note to the reference you gave me.
Note that in this equation it is assumed that the object is parallel with its line of movement. Also note that for the observer in relative movement, the length of the object is measured by subtracting the simultaneously measured distances of both ends of the object. For more general conversions, see the Lorentz transformations. An observer at rest viewing an object travelling very close to the speed of light would observe the length of the object in the direction of motion as very near zero.
Hi Janus
Are you saying that within the frame of the space ship the speed of light is measured to be different heading to the back of the ship than heading to the front? If that is the case then we just have to place the switch in a position where the emitters go off instantaneously and not relatively by simultaneity. Than we will not see a contraction in the rest frame wrt the spaceship frame.
I always thought there was a difference in the forward and backward directions in a moving spaceship. I was always told there was not.
I think DelSpam is suggesting that the visual effect of the Lorentz contraction is a physical contraction of the space ship. What is your opinion? Is the Lorentz contraction just a visual or also a physical contraction?

[Passionflower]

From my rudimentary understanding, concerning the twin paradox, if one twin leaves traveling near the speed of light and returns, he will find himself younger than his twin who stayed behind. Hence, the effect of time dilation is permanent.

When their spacetime paths are different both time dilation and length contraction are in place. However when they essentially travel on the same spacetime path it is not.

I am not sure why you think time dilation is permanent.

[DaleSpam]

This was a sub note to the reference you gave me.Yes. So now you agree with me on the above points?

[Qzit]

Hi DelSpam,

All of the usual relativistic features like time dilation, length contraction, and the relativity of simultaneity are contained in it. The Lorentz transformations are not optical effects. They are what remain after all of the optical effects are accounted for. They are not due to the fact that the speed of light is finite, but rather they are due to the fact that the speed of light is invariant.

Relativity of simultaneity is an explanation of visual contraction not physical contraction. The Lorentz contraction says close to the speed of light the contraction is close to 100%. If a ship could go “C” it would have no length. I can explain relativity of simultaneity that shows it to be a visual effect of the finite speed of light. You are in a ship going near the speed of light. You are in the front of the ship and hit the switch that simultaneously turns on the front and back of a pulse emitter. The front goes off first and the signal travels down the ship at the speed of light. When the signal reaches the back of the ship the back reaches the position the first pulse fired. There is your contraction due to the finite speed of a photon. If you set off the signal in the center of the ship there would be no issue with the simultaneity of relativity and no contraction. Instantaneous and simultaneity of relativity are two different things.


Note that in this equation it is assumed that the object is parallel with its line of movement. Also note that for the observer in relative movement, the length of the object is measured by subtracting the simultaneously measured distances of both ends of the object. For more general conversions, see the Lorentz transformations. An observer at rest viewing an object travelling very close to the speed of light would observe the length of the object in the direction of motion as very near zero.

This was a sub note to the reference you gave me.

Yes. So now you agree with me on the above points?

So we agree that the transformation would be very near zero but the only disagreement is

You think the contraction is a physical effect on the space ship and the space ship is being reduced to very near zero length.

I think it is a visual effect that we see zero length with no physical effect on the space ship. Meaning it remains the same physical length or slightly larger in the very near "C" ship as it is in the resting frame ship.

Do I have that correct?

Hi Janus,

Earth frame observer is watching a space ship made of glass traveling 0.9C. There is a stationary (same as the Earth frame) photon emitter that will emit a photon parallel to the glass ship forward and backward as the ship passes going 0.9C. There is also one inside the glass space ship that will emit a photon forward and backward. When the space ship lines up with the observer and the outside photon emitter they go off in unison. The Earth observer sees the outside photon emitter’s photons departing from each other at 2C or each at C in opposite directions. Now inside the ship the Earths observer, observes the forward emitters photon going the speed of light (relative ship frame= 0.1C=C). He sees the rear emitters photon going 0.8C forward (relative ships frame 0.1C=C backwards).
So to the Earths observer the photons are both going forward at "C" and 0.8C respectively.

Do we give up light is measured to be invariant in every frame or give up the idea that the speed of light is the same in every direction within a frame?
They are mutually incompatible statements for a photon.


Happy Holidays

[Janus]

Hi DelSpam,







So we agree that the transformation would be very near zero but the only disagreement is

You think the contraction is a physical effect on the space ship and the space ship is being reduced to very near zero length.

I think it is a visual effect that we see zero length with no physical effect on the space ship. Meaning it remains the same physical length or slightly larger in the very near "C" ship as it is in the resting frame ship.

Do I have that correct?

Hi Janus,

Earth frame observer is watching a space ship made of glass traveling 0.9C. There is a stationary (same as the Earth frame) photon emitter that will emit a photon parallel to the glass ship forward and backward as the ship passes going 0.9C. There is also one inside the glass space ship that will emit a photon forward and backward. When the space ship lines up with the observer and the outside photon emitter they go off in unison. The Earth observer sees the outside photon emitter’s photons departing from each other at 2C or each at C in opposite directions. Now inside the ship the Earths observer, observes the forward emitters photon going the speed of light (relative ship frame= 0.1C=C). He sees the rear emitters photon going 0.8C forward (relative ships frame 0.1C=C backwards).
So to the Earths observer the photons are both going forward at "C" and 0.8C respectively.

Do we give up light is measured to be invariant in every frame or give up the idea that the speed of light is the same in every direction within a frame?
They are mutually incompatible statements for a photon.



Happy Holidays

The Earth observer sees both sets of photons ( the ones emitted by the outside emitter and the ones emitted by the ship emitter) travel at c in opposite directions, with the ones headed towards the rear of the ship arriving at the rear before the ones headed for the the front arrive at the front of the ship. It makes no difference from which emitter the photons came from.

For the Observer on the ship, both sets of photons reach the ends of the ship simultaneously. That is what light speed being invariant and the speed of light being the same in every direction in every frame means. They are not incompatible.

[Qzit]

Hi Janus,

The Earth observer sees both sets of photons ( the ones emitted by the outside emitter and the ones emitted by the ship emitter) travel at c in opposite directions, with the ones headed towards the rear of the ship arriving at the rear before the ones headed for the the front arrive at the front of the ship. It makes no difference from which emitter the photons came from.

For the Observer on the ship, both sets of photons reach the ends of the ship simultaneously. That is what light speed being invariant and the speed of light being the same in every direction in every frame means. They are not incompatible.

You are suggesting there is a different reality for each frames observer.

If you are suggesting it’s because of the limit of not being able to measure the one way speed of light that I would understand. The round trip is the same for each I agree.

If it is a different reality I would like to know the reason. (Unless it is just your understanding of a postulate). If there is a physical explanation please in that case I would be very appreciative.


If there is no explanation than we must conclude that the one way speed of light is not measured to be the same in all frames.

[Janus]

Hi Janus,



You are suggesting there is a different reality for each frames observer.

No,just different perspectives of the same "reality"

If you are suggesting it’s because of the limit of not being able to measure the one way speed of light that I would understand. The round trip is the same for each I agree.

If it is a different reality I would like to know the reason. (Unless it is just your understanding of a postulate). If there is a physical explanation please in that case I would be very appreciative.


If there is no explanation than we must conclude that the one way speed of light is not measured to be the same in all frames.

The best simple explanation is that measurements of time and space are frame dependent. To use an analogy, imagine a group of people standing in an open field all facing in different directions. If you were to ask them to point North, they will all point in the same direction (assuming they all know the direction of North). If you place two objects on that field and ask them what the North-South distance between them is, they will all give you the same answer. This is an example of frame independence. No matter what direction the people are facing with respect to each other, they will agree on these measurements.

If you now ask them all to point left, they will all point in different directions, as left is defined by the direction they are facing. If you ask them the left-right distance between the two objects, they will give different answers. Some will even say that object one is to the left of object two while other will say that it is the other way around. This is an example of frame dependence. the measurements do depend on how the people face with respect to each other.

What Relativity says is that measurements of space and time are more akin to the second example.

In Relativity, there is something akin to the straight line distance between the two objects in the example. This is frame independent. Everyone agrees to this even though they might not agree as to the Left-Right and Front-Back components of this distance. In Relativity, it is called the space-time interval and represents the separation of event in space and time. This interval is something everyone will agree on. What different frames will disagree upon (such as in your spaceship example) How much of the separation is in time and how much is in space.

One way to look at it is that we measure time and space we are only measuring "slices" of a larger reality, and that different frames are looking at different slices.

[DaleSpam]

You think the contraction is a physical effect on the space ship and the space ship is being reduced to very near zero length.

I think it is a visual effect that we see zero length with no physical effect on the space ship. Meaning it remains the same physical length or slightly larger in the very near "C" ship as it is in the resting frame ship.I would certainly say it is a physical effect since "physical" means "of or pertaining to physics" and length contraction is part of the Lorentz transform which definitely pertains to physics.

It is not, however, a visual effect. There are several relativistic visual effects, such as relativistic abberation, relativistic Doppler shift, and Terrell-Penrose rotation. Length contraction is what is measured after properly accounting for these visual effects in a given frame. It is not what is seen visually nor with a high speed camera.

I would again recommend that you work out the two calculations I suggested above. I think that you will find it educational.

[Qzit]

One way to look at it is that we measure time and space we are only measuring "slices" of a larger reality, and that different frames are looking at different slices.

On this we agree but it still does not explain why the Earth observer sees the photon hit the back of the ship first and the front second while the ship observer sees them hit simultaneously. If the Earth frame sees a difference in the relative time (to hit the back and front of the ship) than inside of the space ship there has to be a difference in the one way speed of light. There is no way to get around that. From an observer in the center of the ship the one way speed of light is different. Not that the space ship frame could measure it but the reality is the speed within the ship is c+v and c-v. The round trip is “C” in both directions. The Earth observer is the only one that can measure the one way direction of light. That measurement is c+v and c-v for closing and departing distances. So in a moving frame light can cover a different distance in the same amount of time. This would explain why atomic clocks lose Nano seconds going from east to west.

[DaleSpam]

On this we agree but it still does not explain why the Earth observer sees the photon hit the back of the ship first and the front second while the ship observer sees them hit simultaneously. If the Earth frame sees a difference in the relative time (to hit the back and front of the ship) than inside of the space ship there has to be a difference in the one way speed of light. There is no way to get around that.This is incorrect. The distance traveled by the light is different for the forward and backward paths in the Earth frame, and that distance is different than the distance traveled by both paths in the ship frame. The speed of light is the same, and the various distances are different. Again, you should work out the math to convince yourself of this. If you need help there are many on this form who will be willing.

The Earth observer is the only one that can measure the one way direction of light.What makes the Earth observer so special?

[Qzit]

Hi DaleSpam,

This is incorrect. The distance traveled by the light is different for the forward and backward paths in the Earth frame, and that distance is different than the distance traveled by both paths in the ship frame. The speed of light is the same, and the various distances are different. Again, you should work out the math to convince yourself of this. If you need help there are many on this form who will be willing.


Let's see you are saying the photon physically hits the back of the ship before the one in front hits in the Earth frame. Then they physically hit at the same time in the ships frame. Do you understand how absurd that sounds? The photon has to be in two places at the same time.

Math is no guarantee that you are correct. While the math does contract the visual object the same as your view. You actually believe the contraction is physical. Does the math prove it’s physical?

[DaleSpam]

Let's see you are saying the photon physically hits the back of the ship before the one in front hits in the Earth frame. Then they physically hit at the same time in the ships frame. Do you understand how absurd that sounds? Yes, I do. That is why the relativity of simultaneity is the single most difficult concept for students to grasp in learning special relativity. It goes very strongly against our non-relativistic intuition.

Math is no guarantee that you are correct ... Does the math prove it’s physical? The math is a guarantee that what I am saying is consistent with SR and that what you are saying is not consistent with SR. Then experiments prove that SR is correct in the domain where it is tested. The math is necessary, but not sufficient, which is why we do experiments too.

In any case, my repeated suggestions that you do the math are not for the purpose of proving my point, they are simply to help you learn. If I were only interested in proving my point then I would do the math myself. But you will learn more and faster if you do it.

[nitsuj]

Hi DaleSpam,



Let's see you are saying the photon physically hits the back of the ship before the one in front hits in the Earth frame. Then they physically hit at the same time in the ships frame. Do you understand how absurd that sounds? The photon has to be in two places at the same time.

Math is no guarantee that you are correct. While the math does contract the visual object the same as your view. You actually believe the contraction is physical. Does the math prove it’s physical?


The first postulate is the laws of physics are always the same no matter the observer, as long as they're inertial.


Consider that time is merely a measurement, like length is a measurement. Your intuition is right, just gotta tune it a bit more physicsy. It would be absurd for someone right next to (inertial with) the experiment to believe the photons didn't hit simultaneously. (im assuming this is the scenario) That observer is in the "same" spacetime, specifically frame of reference as the experiment.

Said differently, considering another observers FoR doesn't make it "reality". Only your own observation does. I think causality is the final nail in that.

In another, more crude way, conscious observers are special in that we can in-vision what would happen in the other FoR. That cannot "physically" be considered a/the "reality". I know nothing of LET but this conscious observer reasoning would be getting off on the wrong foot (with either theory).

I couldn't think of an equation for that. :smile:

[DaleSpam]

The first postulate is the laws of physics are always the same no matter the observer, as long as they're inertial. I have never seen the first postulate written that way. It is always written in terms of reference frames, not observers.

[Dmytry]

This question is in regard to special relativity.

From my rudimentary understanding, concerning the twin paradox, if one twin leaves traveling near the speed of light and returns, he will find himself younger than his twin who stayed behind. Hence, the effect of time dilation is permanent.

However, I have never read anywhere that the traveling twin's length will also be permanently adjusted due to length contraction.

How is it that one Lorentz transformed aspect remains while the other one vanishes upon the traveling twin's return?

I apologize if this question has been asked before. If so, and you know where to find the responses, please point me in the right direction.

Thank you!

~Dylan
The twin that returns, however, will - upon return - be aging at the same rate as the twin that stayed home. The clock that the twin took with him in a voyage will be counting off time length of 1 second, in a second, when he's back home. The length of the travelling twin's sleep, or the length of his heartbeat (ignoring the senile insomnia, ha ha, and other effects of aging on the staying twin) will not be affected permanently, just as the length of his body was not.

The age is like a position, not like length interval.

[Qzit]

Hi nitsuj,

The first postulate is the laws of physics are always the same no matter the observer, as long as they're inertial.

Thank you. That is what I am trying to get across. Relativity of simultaneity is just the distance light travels for each observer. When you say the photon hits the back of the ship first for the Earth observer that is the reality and proves that the one way distance in the space ship frame that light travels is different than the return trip distance light travels. There does not need to be an observer for reality of physical position in space. The reality is the photon hits the back of the ship in both frames by the same physical position in space. The problem is the MMX experiments was the two way distance light travels. No matter what frame you are in the two way distance will make it appear light travels the same distance/time. If the math you use takes the distance traveled and divided by ½ for the one way speed you will be wrong because the travel out is always adjusted by the return travel. An orthogonal way to prove my point would be with Atomic clocks. You are on the equator and have three atomic clocks synchronized. One stays stationary one is taken in a plane to the west and one to the east around the world. When they return the clock that flew west loses time compared to the stationary one and the one that flew east gains time compared to the stationary one. Einstein said atomic clocks can be used to measure the speed of light. This experiment proves the distance light traverses a moving frame is different than a frame at rest. There was a second experiment I remember reading about using four atomic clocks. Four clocks were synchronized in New York three were taken to the North Pole. One was brought back to New York and the other two were taken to San Francisco. The one brought back to New York stayed synchronized. Out of the two that went to San Francisco one went to New York and the one in New York went to San Francisco. New York to San Francisco lost 14 Nano seconds San Francisco to New York gained 14 Nano seconds. This also proves the one way distance traversed/time interval is different. Wake up the MMX experiments were flawed that Einstein used for Relativity. That does not negate Relativity or simultaneity of relativity. It only proves that simultaneity of Relativity is visual and not physical.

The MMX experiments were not flawed just the math used for the one way distance of traversed light that was assumed.

If you took either clock back to their origin they would be synchronized again.

[DaleSpam]

Qzit, pleas learn to use paragraphs to separate your thoughts. A little bit of organization will help communication a great deal.

When you say the photon hits the back of the ship first for the Earth observer that is the reality I have asked this before, but why? What is it about the Earth observer that makes them so special that their reference frame represents "reality"? Do you believe like Copernicus' detractors that the Earth is at the center of the universe or otherwise occupies some priviledged position?

The reality is the photon hits the back of the ship in both frames by the same physical position in space.Again, this is not true in any frame, we have already discussed this.

Wake up the MMX experiments were flawed that Einstein used for Relativity. That does not negate Relativity or simultaneity of relativity. It only proves that simultaneity of Relativity is visual and not physical.Can you provide any mainstream scientific reference which supports your claim?

I would like to remind you that this forum is not for anti-relativity rants, it is for learning mainstream physics. When you signed up for your account you agreed to the rules that prohibit overly speculative posts. If you have an anti-relativity agenda you had best look elsewhere.

[Qzit]

Hi DaleSpam,

You and I agree on Relativity being correct but we disagree on what that means. In order to stop getting confused let’s look at everything from the photons perspective. Where do you disagree?
1. The photons minimum and maximum speed are the same in every frame and that = invariant speed of light.
2. Our visual perspective depends on where we observe an object.
3. The photon perspective is the only reality of its own position in space.
If we can agree on that we made progress. Now before you read with a challenging attitude try and follow your logic with the three things we agree on not where we disagree.
You are riding a photon and you can know the exact position of another photon instantaneously. This removes relativity of simultaneity.
Lets go through this thought experiment again because the math is different from the reality of the photon.
A space ship is traveling ½ the speed of light. Two photons side by side are traveling an intersect course with the space ship. When the two photons reach the front of the space ship one reflects back to an observer at rest in line with the traveling photons and the ship. The second photon reaches the back of the space ship and reflects back to the same observer at rest. While the photon was moving from the front to the back the ship moved ½ the speed of light forward intersecting a contracted position relative to the length of the ship. We visualize the photon covering 75% and the ship 25%. This is a contraction of the returned photons measurement from the front to the back that the observer at rest sees. (1-v^2/C^2)=1-25=75 just what the observer at rest sees for the returned photon distance. Now because we think light travels the same speed relative to the ships frame we take the square root and get a contraction of 0.866 just about half + the time dilation increase of the ships length by gamma. Once again the MMX experiments are affecting reality of position. The MMX experiment was not wrong but the conclusions of the experiment were incorrect.

The Earth frame is the only place you can measure the one way speed of light in a space ship. The returned light from the front and back of the ship will be the same and if you incorrectly divide that by 1/2 you are mistaken when the photon actually hit the back and front of the ship.

[DaleSpam]

Do you have any mainstream scientific references to support your interpretation?Where do you disagree?
1. The photons minimum and maximum speed are the same in every frame and that = invariant speed of light.
2. Our visual perspective depends on where we observe an object. I agree here.

3. The photon perspective is the only reality of its own position in space.I disagree here. A photon doesn't have a valid perspective. See:

http://www.physicsforums.com/showthread.php?t=511170

And even if it did no reference frame represents the "only reality" of anything. Your claim that one inertial frame is more valid than another is a direct contradiction of the principle of relativity.

[Janus]

Hi DaleSpam,



Let's see you are saying the photon physically hits the back of the ship before the one in front hits in the Earth frame. Then they physically hit at the same time in the ships frame. Do you understand how absurd that sounds? The photon has to be in two places at the same time.



The point is that "at the same time" means something different for the Earth frame than it does for the Ship frame. Events that the two frames can agree happen at the same time are those that happen at the same point (for example, the light hitting one end of the ship and the reading on the clock at that end of the ship at that instant.)

For demonstration here is the classic train example for the Relativity of Simultaneity.

First, events according to the embankment frame:

Here the lightning flashes originate when the ends of the train are adjacent to the red dots on the embankment and the embankment observer is at the midpoint between the two.

The embankment observer sees both flashes at the same time, and being halfway between the lightning strikes concludes that they occurred at the same time. Notice how the flashes reach the train observer at different times.

http://home.earthlink.net/~jparvey/sitebuildercontent/sitebuilderpictures/trainsimul1.gif

Now the thing to keep in mind is that in this frame, the train is length contracted, and it it this length contraction that allows it to just fit in between the two dots on the embankment.

Now the same scenario according to the Train frame. In this frame, the train is its proper length and it is the embankment which is length contracted. Now the train is longer than the distance between the two dots. The front of the train hits the right red dot before the rear reaches the left dot. The lightning strikes must originate when the ends of the trains are aligned with the dots, otherwise you will have a contradiction between the frames.

http://home.earthlink.net/~jparvey/sitebuildercontent/sitebuilderpictures/trainsimul2.gif

This means that in a very real sense, the lightning strikes occur at different times in the train frame. If the light from those flashes expand outward at c relative to the train, the train observer will see each flash at different times (just as he did according to the embankment frame. ). The reverse argument is that since he sees the flashes at different times, is halfway between the ends of the train(where the lightning strikes occurred) and the light from the strikes approaches him at the same speed from both ends (the speed of light is invariant), then the lightning strikes occurred at different times.

You will also note that in the train frame, the flashes also reach the embankment observer at the same time. Further, if you compare the two animations, you will note that the train observers position with respect to the embankment when each flash reaches him is the same in both as is the embankment observer's position with respect to the train when the flashes reach him.

Thus there are no true contradictions between the frames, only a disagreement as to what is simultaneous when it come to events that are separated by some distance. Note that this is not just a matter of one observer being further from the source and seeing it later, because each observer accounts for his distance from the source to determine when the flash really occurred. It is a very real difference in the simultaneity of events separated by distance.

This is length contraction and the Relativity of Simultaneity working together. If you were to place clocks at the red dots, the ends of the trains and with each observer (with the clocks in each frame synchronized in that frame), you could include time dilation into the mix, and find that everyone will agree as to what times were on any two clocks when those clocks passed each other, or when the light from either strike hit them. They will not agree as to the synchronization of clocks between frames (each frame claiming their own clocks as being in sync while the other frame's clocks are not), nor as to the relative clock rate between frames. (each will consider the clocks in the other frame as running slow.)

This is what Special Relativity is about, there are real differences in time and space between inertial frames.

[Qzit]

DaleSpam.

Janus has a depth of understanding that comes from his thinking about Relativity and not just a surface interpretation of what he read. To truly understand Relativity you have to think about what is happening from the different perspectives. Once Janus said in the Earth frame the photon will hit the back of the ship first we no longer had a disagreement in our understanding. He agrees the distance traversed in the forward direction is different than the distance traversed in the rear of the ship for the one way path of a photon. When we find out new information we have to challenge old conclusions with the new information. I agree that the observer in the middle of the ship will measure the distance to be the same for the forward and backward photons return trips. Were they reflected at the same time? NO!! You were arguing that each photon hit the ends of the ship twice because of your belief that contraction is physical. Janus understands Relativity of simultaneity correctly.

Now the second issue we disagree on. Time dilation for the speeding space ship is an increase in volume and decrease in density. This once again takes thinking it out for understanding. You say time dilation is contraction and I say dilation is expansion. This can be proven by photons invariance to a frame. The speed of light can be used as an anchor because it travels through all frames like they were not there the photons speed does not vary from frame to frame. If you contract the distance in a frame you have to speed a clock up to match the shorter distance. If you expand the distance you have to slow the clock down to give the photon enough time to traverse the same distance. Only this way does the clock match the distance in the rest frame of all frames. If your clock is faster in a frame your measuring stick is shorter. If your clock is slower than your measuring stick is longer. This way you measure the speed of light in every frame to be invariant although it does take longer to traverse a dilated frame.

[DaleSpam]

Janus has a depth of understanding that comes from his thinking about Relativity and not just a surface interpretation of what he read. To truly understand Relativity you have to think about what is happening from the different perspectives.I agree on both counts. Janus is quite knowledgeable, and to understand relativity you do have to think about what is happening in different reference frames and recognize that each is equally valid.

Once Janus said in the Earth frame the photon will hit the back of the ship first we no longer had a disagreement in our understanding. He agrees the distance traversed in the forward direction is different than the distance traversed in the rear of the ship for the one way path of a photon. When we find out new information we have to challenge old conclusions with the new information. I am glad you have changed your mind and now agree with me, on this point at least.

You were arguing that each photon hit the ends of the ship twice because of your belief that contraction is physical.I certainly never said any such nonsense.

Time dilation for the speeding space ship is an increase in volume and decrease in density.Please provide a mainstream reference to support this assertion.

This once again takes thinking it out for understanding. You say time dilation is contraction and I say dilation is expansion. This can be proven by photons invariance to a frame. The speed of light can be used as an anchor because it travels through all frames like they were not there the photons speed does not vary from frame to frame. If you contract the distance in a frame you have to speed a clock up to match the shorter distance. If you expand the distance you have to slow the clock down to give the photon enough time to traverse the same distance. Only this way does the clock match the distance in the rest frame of all frames. If your clock is faster in a frame your measuring stick is shorter. If your clock is slower than your measuring stick is longer. This way you measure the speed of light in every frame to be invariant although it does take longer to traverse a dilated frame.You claim that you believe that relativity is correct. If so, then you believe that the Lorentz transform is correct (even if you disagree with my interpretation of it). However, what you are saying is incompatible with the Lorentz transform itself.

I challenge you once again to actually do the math. Unfortunately, it looks like you will have to do so on your own now.

[nitsuj]

I have never seen the first postulate written that way. It is always written in terms of reference frames, not observers.

:redface: Opps.

I always thought of an observer as being the same as a FoR.

Pretty clear there can be more then one observer in a FoR.

An important distinction.

[dfaullin]

Wow! I wish I would have kept following up on my post. I just thought it was kind of over. I learned alot from reading through the posts. Thank you for having the conversation. I wanted to especially thank George for performing that calculation for me on page 2 (post 25 or so).

Amazing that after 105 years of the existence of Special Relativity, it is still a mind warp and debatable as to the true nature and effects implied by the theory.

[ghwellsjr]

Wow! I wish I would have kept following up on my post. I just thought it was kind of over. I learned alot from reading through the posts. Thank you for having the conversation. I wanted to especially thank George for performing that calculation for me on page 2 (post 25 or so).

Amazing that after 105 years of the existence of Special Relativity, it is still a mind warp and debatable as to the true nature and effects implied by the theory.
You're welcome, Dylan. (It was post #26.)