Length Contraction - Real or an illusion
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That's a great question and I hope someone can tackle it nicely.
There are two ways one can us S.R. to solve problems - either time dilation or length contraction. I am confident that both can not be used together, unless they both are tweaked to accommodate one another. So, for instance, if you want to know what the time differences between an Earth clock and the space traveler's clock in the travel time to Proxima Centauri then either the time dilation equiation is used or the length contraction is used.
There have been numerous experiments that seem to demontrate that time dilation is real. GPS requires SR time adjustments be used to get accurate results. along with GR to address the gravity differences. But, can we say that it's time that is "really" changed or could it be that the length of travel should be considered the key to the differences? I don't know.
The flattening of an object is part of the length contraction effect, so particle physicists might notice this effect to explain certain behaviors. Apparently, this has been discovered and it may indeed be due to the length contraction in SR. Or, could time alteration somehow create the same effect? I wish I knew.
..... now, you may ask is there movement on a TV screen. If your answer is NO, you are right, there is no movement on a TV screen playing a program. Just a lot of still images which our eyes in combination with our brains cannot resolve and see the whole scene as movement.
Therefore, movement on a TV screen is an illusion.
If you look at it more closely, the frame repetition of a moving scene on a TV is much much slower than the speed of light.
Hence, the image of an object, I would say much faster than the frame repetition on a TV screen would be distorted. Imagine how distorted an image would be travelling extremely fast or very close to the speed of light.
From a non-mathematical approach length contraction is an illusion. [ you have heard of creative accounting, ...well, this is creative physics ] Objects do not appear in their true form. They are perceived differently because our brains do NOT perceive or collect all the information and at a rate which a stationary observer relative to the moving object, in other words moving with the object at the same speed.
With respect to time dilation, an explanation is offered at this URL:
View: https://www.youtube.com/watch?v=yuD34tEpRFw
.
The bottom line is the faster one moves away from the clock the slower the hands of the clock appear to move ( hence, the author calls it time dilation) with respect to a stationary observer, say very close to the clock.
What if I move between two such tower clocks, at the same time, one behind me and the other in front of me. Can I say, I am experiencing time contraction as I move towards the clock in front of me?
Now, imagine the situation where there are no clocks, only space and no lights or objects as reference, would I be experiencing TIME DILATION and TIME CONTRACTION at the same time.
Therefore, movement on a TV screen is an illusion.
If you look at it more closely, the frame repetition of a moving scene on a TV is much much slower than the speed of light.
Hence, the image of an object, I would say much faster than the frame repetition on a TV screen would be distorted. Imagine how distorted an image would be travelling extremely fast or very close to the speed of light.
From a non-mathematical approach length contraction is an illusion. [ you have heard of creative accounting, ...well, this is creative physics ] Objects do not appear in their true form. They are perceived differently because our brains do NOT perceive or collect all the information and at a rate which a stationary observer relative to the moving object, in other words moving with the object at the same speed.
With respect to time dilation, an explanation is offered at this URL:
.
The bottom line is the faster one moves away from the clock the slower the hands of the clock appear to move ( hence, the author calls it time dilation) with respect to a stationary observer, say very close to the clock.
What if I move between two such tower clocks, at the same time, one behind me and the other in front of me. Can I say, I am experiencing time contraction as I move towards the clock in front of me?
Now, imagine the situation where there are no clocks, only space and no lights or objects as reference, would I be experiencing TIME DILATION and TIME CONTRACTION at the same time.
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To put it a different way, why TIME DILATION is an ILLUSION as well.
The author of the clip claims Einstein came to this conclusion -time stands still at the speed of light- or -slows down progressively as the speed approached the speed of light.
By constructing a thought experiment based on the movement of the hands of a tower clock.
Lets analyse his thought experiment more closely. I will begin with the same clock, that is, the clock has an hour hand, a minute hand and a second hand.
The experiment would last for 10 minutes and I will be in a space ship cable of travelling faster than light speed.
On account of 12 O'clock midday, the experiment commences. I
I instantly travel at 70% of the speed of light and I look back at the images of the clock and take note how far apart is the second hand is moving with respect to the time interval if I was standing next to the clock.
Naturally, I would come to the conclusion, as I crank up the speed of my spacecraft, the time appears to be slowing down.
To the point when my velocity is at the speed of light, I would be seeing the same image, as if the clock had stopped. But I know, that is not correct.
So, I may come to the conclusion, at the speed of light time stands still.
Because, my space ship is capable of traveling faster than the speed of light. I crank up my speed again and travel all the way to the first image of the clock when the experiment started, reverse direction and travel into the clock images, initially, at sub-light speed (normal space, they call it in science fiction literature) and progressively, crank up my speed to the speed of light.
What would I observe now ?
Well, the minute and second clock hands going backwards at a faster and faster rate.
Another conclusion, the faster I move the faster the time progresses, can I call it dilation. Obviously, not, time is not dilating. let call it something else, time contraction.
Two diametrically opposite events dilation and contraction. Is that correct ?
Surely, someone as brilliant as Einstein, would have considered the reverse situation as I did or are we missing something.
The author of the clip claims Einstein came to this conclusion -time stands still at the speed of light- or -slows down progressively as the speed approached the speed of light.
By constructing a thought experiment based on the movement of the hands of a tower clock.
Lets analyse his thought experiment more closely. I will begin with the same clock, that is, the clock has an hour hand, a minute hand and a second hand.
The experiment would last for 10 minutes and I will be in a space ship cable of travelling faster than light speed.
On account of 12 O'clock midday, the experiment commences. I
I instantly travel at 70% of the speed of light and I look back at the images of the clock and take note how far apart is the second hand is moving with respect to the time interval if I was standing next to the clock.
Naturally, I would come to the conclusion, as I crank up the speed of my spacecraft, the time appears to be slowing down.
To the point when my velocity is at the speed of light, I would be seeing the same image, as if the clock had stopped. But I know, that is not correct.
So, I may come to the conclusion, at the speed of light time stands still.
Because, my space ship is capable of traveling faster than the speed of light. I crank up my speed again and travel all the way to the first image of the clock when the experiment started, reverse direction and travel into the clock images, initially, at sub-light speed (normal space, they call it in science fiction literature) and progressively, crank up my speed to the speed of light.
What would I observe now ?
Well, the minute and second clock hands going backwards at a faster and faster rate.
Another conclusion, the faster I move the faster the time progresses, can I call it dilation. Obviously, not, time is not dilating. let call it something else, time contraction.
Two diametrically opposite events dilation and contraction. Is that correct ?
Surely, someone as brilliant as Einstein, would have considered the reverse situation as I did or are we missing something.
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That looks like Newtonian physics where photons take longer to reach the ship when the ship moves away. But, upon returning, the clock would do the opposite and appear to speed up.
Also, those on Earth would see the ship clock ticking slower, so both would claim the other clock is equally slower.
SR (Special Relativity) has the returned ship having experienced less time, hence the “Twin Paradox”. I don’t understand it, admittedly, There is some sort of symmetry break due to the acceleration experience for the spaceship.
Also, those on Earth would see the ship clock ticking slower, so both would claim the other clock is equally slower.
SR (Special Relativity) has the returned ship having experienced less time, hence the “Twin Paradox”. I don’t understand it, admittedly, There is some sort of symmetry break due to the acceleration experience for the spaceship.
The thought experiment was based on the same procedure as adopted by Einstein and his subsequent reasoning, time slows down at faster and faster speeds. nothing else, nothing more.
We are looking it from the Einstein point of view with the tram car (spaceship) moving now towards the clock, as the speed gets higher and higher, time would speed up not slow down.
To eliminate acceleration out of this thought experiment, you can compare the time interval between a stationary observer at some reasonable distance away from the clock as opposed to the time interval which would be observed if moving at the speed of light towards the clock, that is, at constant speed.
So how can the concept of time dilation can be sustained.
We are looking it from the Einstein point of view with the tram car (spaceship) moving now towards the clock, as the speed gets higher and higher, time would speed up not slow down.
To eliminate acceleration out of this thought experiment, you can compare the time interval between a stationary observer at some reasonable distance away from the clock as opposed to the time interval which would be observed if moving at the speed of light towards the clock, that is, at constant speed.
So how can the concept of time dilation can be sustained.
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Yes, that is an appearance issue. Assuming a fixed travel speed both ways, I think the rate each clock appears to slowdown with greater separation will equal the same each sees on the return trip. Yet when they meet the spaceship clock will be different.
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I think, you may be getting confused. It is a simple experiment, nothing complicated. Both experiments are of, as you put it, an appearance issue.
Two identical clocks, tower clocks, separated by a suitable distance, showing identical time with Albert Einstein in between them in his tram car.
The experiment begins with Einstein moving from the left tower clock and going to the right tower clock.
Einstein takes notice of the left clock, the clock behind him, only and calls it Time Dilation, however, does not consider the effects of his experiment in moving towards the right clock.
Einstein is well known for his one sided physics. To the point, he would later, change his mind to some of his major inventions. In the mean time, he is awarded the Nobel price for the explanation of the photoelectric effect, which later is proven not correct.
What happens with the Left clock automatically happens with the Right clock just the effects of time are diametrically opposite.
This experiment shows time dilation and time compression or contraction occurring at the same time. In other words, the time from the clock on the left slows down and time from the clock on the right speeds up.
However, both are an attempt to explain real time ( Nature Time ), ...... but both are INCORRECT.
Two identical clocks, tower clocks, separated by a suitable distance, showing identical time with Albert Einstein in between them in his tram car.
The experiment begins with Einstein moving from the left tower clock and going to the right tower clock.
Einstein takes notice of the left clock, the clock behind him, only and calls it Time Dilation, however, does not consider the effects of his experiment in moving towards the right clock.
Einstein is well known for his one sided physics. To the point, he would later, change his mind to some of his major inventions. In the mean time, he is awarded the Nobel price for the explanation of the photoelectric effect, which later is proven not correct.
What happens with the Left clock automatically happens with the Right clock just the effects of time are diametrically opposite.
This experiment shows time dilation and time compression or contraction occurring at the same time. In other words, the time from the clock on the left slows down and time from the clock on the right speeds up.
However, both are an attempt to explain real time ( Nature Time ), ...... but both are INCORRECT.
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Catastrophe
"Science begets knowledge, opinion ignorance.
"Length Contraction - Real or an illusion"
Obviously an illusion (or at least, guess what, relative) otherwise the real object would be jumping up and down in size in reality depending on who was observing. Perception is relative.
Obviously an illusion (or at least, guess what, relative) otherwise the real object would be jumping up and down in size in reality depending on who was observing. Perception is relative.
The assumption he made, IIRC, is that he would envision what the receding tower clock time would be relative the his, initially, synchronized clock if it were instantaneously placed next to his. He realized the two would have different times, so that objects that move will experience less time expended than before they began moving.
He had his finger in a lot of pies and the quantum "pie" was the one he seemed to have the greatest issue with. What inventions, however, were you thinking about?
Disproven?
Huh?
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"... so that objects that move will experience less time expended than before they began moving..."
The above statement hold true overall in moving from A to B. You will be covering the distance in less time. Any images leaving after you, will not be seen by you. However, images coming towards you, will be seen but may be distorted.
Consider this. Your are standing in a field. Two highly trained and accurate marksmen, one on your left and the other one on your right, aim their rifles at you and fire at the same time.
You run away from the bullet (light ray) on your left, this means you put more distance between this bullet (light ray) and yourself, you call this bullet time delay or bullet time dilation.
In running towards the right, your reduce your distance to the bullet and the time before the hit is reduced or bullet time compression.
Gravity is the first that came to mind, as in gravity is the curvature of spacetime. The photoelectric effect has not been disproven only the explanation offered by Einstein has been disproven.
There is neither compression nor dilation of real / natural time.
In fact, if you run away you will experience more time expended with respect to the bullet coming from your left, not less - it will take longer for the bullet to reach you.
As a general fact:
As an aside for more discussion:
The Cosmos is expanding at a faster rate than the speed of light, the only current explanation is that SPACE is expanding, hence, why the distance between galaxies gets greater and greater because anything that has mass cannot travel faster than the speed of light (current thinking).
The above statement hold true overall in moving from A to B. You will be covering the distance in less time. Any images leaving after you, will not be seen by you. However, images coming towards you, will be seen but may be distorted.
Consider this. Your are standing in a field. Two highly trained and accurate marksmen, one on your left and the other one on your right, aim their rifles at you and fire at the same time.
You run away from the bullet (light ray) on your left, this means you put more distance between this bullet (light ray) and yourself, you call this bullet time delay or bullet time dilation.
In running towards the right, your reduce your distance to the bullet and the time before the hit is reduced or bullet time compression.
Gravity is the first that came to mind, as in gravity is the curvature of spacetime. The photoelectric effect has not been disproven only the explanation offered by Einstein has been disproven.
There is neither compression nor dilation of real / natural time.
In fact, if you run away you will experience more time expended with respect to the bullet coming from your left, not less - it will take longer for the bullet to reach you.
As a general fact:
- Everyone or everything in the cosmos is in a state of motion. Nothing is standing still or motionless.
- There is no present, our present is the past. We usually refer to events as Past, Present or Future, this is ok for ordinary conversations, howerver, from a physics point of view, that is, reality, only 2 types of events exist: Past and Future.
As an aside for more discussion:
The Cosmos is expanding at a faster rate than the speed of light, the only current explanation is that SPACE is expanding, hence, why the distance between galaxies gets greater and greater because anything that has mass cannot travel faster than the speed of light (current thinking).
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Another very interesting URL about whether time dilation is an illusion or not:
Search Summary:
"Time Dilation is real insofar as it tells us the rate at which things happen in one reference frame relative to another. However, it is based on clock time, which is virtual. Real time doesn't, and never will vary. Things can still happen at different rates in different reference frames measured against real time. The rate of time itself is constant. Everything we see is some form of optical ... "
The details and the discussion:
"It is said that we can verify time dilation by flying a very accurate clock on a fast jet or spaceship and prove that it registers less time than the clocks on earth. However, the clocks on earth would be moving relative to the clock on the spaceship, and since time always dilates and never goes faster regardless of the direction of relative motion, the clocks on earth should register less time than the clock on the spaceship.
Is this true? Whenever there is a fast-moving object such as a rocket do all clocks on earth really become slow?
If the rocket with the clock landed after moving at relativistic speed, would its clock and the earth's clock again show the same time since during its travel both appeared slow to each other?
Or is all this just an illusion, ie. the clocks just appear to be slow to each other but in actually run at normal speed, and neither is behind when the rocket actually lands?
special-relativity time-dilation
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edited Feb 6 '14 at 11:58
Emilio Pisanty
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khushro
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Time dilation is real and is measured every day. For example the lifetime of a muon produced in the lab at low velocity is 2.2μμs. However the lifetime of muons generated by high energy cosmic rays in the atmosphere is around 11μμs. Their lifetime is extended by their high speed.
Calculating the time dilation of a plane flying around the Earth is complex because you have to take into account the gravitational time dilation as well as the plane's speed. However it has been done, and indeed experiment shows that if you put an atomic clock in a plane you do indeed measure the predicted time dilation.
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answered Feb 4 '13 at 9:10
John Rennie
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You're referring to what is commonly known as the twin paradox. The Wikipedia page provides several different ways of analysing the situation, but one way to look at it is this -
When the clock on the spaceship leaves earth, it'll experience an acceleration (even if it's a really small acceleration for a long time, or a huge acceleration for a small time) to reach relativistic speeds, and will experience an acceleration again when it has to turn around to return to earth.
Special relativity only claims that inertial frames of reference are equivalent. Since one clock experiences acceleration, and hence is in a non-inertial frame momentarily, the two situations aren't equivalent.
If you view the time dilation due to acceleration as a gravitational acceleration (principle of equivalence) and then do the calculations, the results obtained for both the clock on the earth as well as the clock on the spaceship agree. Again, the wikipedia link contains in detail this argument, as well as other arguments.
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answered Feb 4 '13 at 9:42
Kitchi
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In your reasoning, you're tacitly making an assumption of a universal time, or you're at least having doubts owing to this assumption. Your central issue seems to be that two relatively moving inertial observers would each see each other's clocks moving slower to one another. This only leads to a logical contradiction if one assumes that there is one, universal time measurement, which would be shown to be strictly less than itself by your argument. Your implied argument is perfectly sound; it's the assumption of universal time that is wrong. But time is relative: each inertial observer has their own observed time, and simulteneity is relative.
So what of the case where the observers meet up again at a common spacetime point, so they have different elapsed times since they synchronized watches at the former time when they were together? Well, they have different paths through spacetime, and their clocks measure the lengths of those paths. Inertial observers fare geodesics through spacetime, which means their path is of maximal length compared to neighboring paths (this is a slightly unwonted aspect of Lorentzian geometry: in Euclidean / Riemannian geometry we deal with geodesics that are minimum length paths) - so the stay-at-home, always inertial twin in the twin "paradox" ages most. This difference between measured pathlengths joining the same two spacetime events is overwhelmingly experimentally verified, as several of the other answers show. Look up the Hafele Keating experiment, or the Rossi-Hall experiment cited in John Rennie's Answer. You might also find some insight into a more experimentally-oriented conception of time, as I discuss here.
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answered Nov 30 '16 at 1:13
Selene Routley
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A loop-closing scenario comes out symmetrical only when both participants have traveled the same spatial distance from departure to rejoining.
In an Earth-and-a-spacecraft-on-a-relativistic-journey scenario the spacecraft travels a much longer spatial distance than the Earth.
According to special relativity for the clock that has traveled a longer spatial distance less proper time will have elapsed, as seen when comparing clock times on rejoining.
The comparison on rejoining is a direct comparison, so it's clear and unambiguous. It's actually unhelpful to try an visualise what will be observed during the journey; because of transmission delays those raw observations are not a good perspective on what is happening.
It may be a surprise for you that difference in spatial distance traveled matters in special relativity. You may figure 'in space any individual spacecraft cannot count how many miles it has traveled'. And that is the case: any individual spacecraft cannot count its own mileage. More forcefully, no such individual mileage exists. However, special relativity does imply that you can always evaluate difference in spatial distance traveled. That difference in spatial distance traveled must be thought of as something that is relative between the two participants.
The relativity of special relativity is not a sweeping any motion is relative to something else. There is room for structure and buildup, giving rise to non-symmetrical scenario's, such as the one you ask about.
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answered Feb 4 '13 at 21:08
Cleonis
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No, time dilation is real. The universe does not follow the theory we assumed in the past where objects follow the Newtonian laws and light travels at a fixed speed in one frame of reference. If it did, an object going away from us would appear to have a slower system when it doesn't really.
It has been confirmed by observation that at each point in space-time, the universe locally follows special relativity. Special relativity predicts that time dilation is real. According to special relativity, there exists a frame of reference that each system dilates in time by a factor of 11−v2c2√11−v2c2 and contracts in length by a factor of 11−v2c2√11−v2c2. According to this answer, if you're travelling at constant velocity, there's no way to tell that you're moving in that frame of reference. You can consistently assume from your observations that you are not moving and what you deduce from the assumption that you are not moving is said to have actually happened in your frame of reference. You can sometimes make observations that wouldn't be possible at all if light travelled at a fixed speed and matter followed Newton's laws.
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answered Nov 23 '17 at 21:06
Timothy
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Time dilation is based on clock time.
Clock time is the time we measure with when doing experiments. It is also the time according to our eyes.
In clock time, the speed of light is constant.
Most of Physics study is done using clock time.
Things age, and happen more slowly when the local rate of Clock Time is slower that the rate of Clock Time it is being compared with.
STR and GTR are theories based on on Clock Time
Clock Time theories tell us what we will see and what is happening according to our clocks.
There is an underlying time that natural activity is synchronised to - real time. One that is constant everywhere, and in which energy and momentum are independent and the speed of light is variable. It's rate is something like the rate of Earth's rotation, but without the fluctuations. A theory based on this interpretation of time can complement GR quite nicely.
Time Dilation is real insofar as it tells us the rate at which things happen in one reference frame relative to another. However, it is based on clock time, which is virtual.
Real time doesn't, and never will vary. Things can still happen at different rates in different reference frames measured against real time. The rate of time itself is constant.
Everything we see is some form of optical illusion.
UPDATE I will support this with an example
Jack and Jill live on the top and bottom floors of a skyscraper, respectively
When Earth rotates through one revolution, the rotation starts and stops at exactly the same time for Jack and Jill
If Jack, Jill, the skyscraper and Earth were immortal, a trillion revolutions of Earth would start and stop at exactly the same time for all four of them. They would experience every revolution, every radian and every microarcsecond together. They would all rotate together in real time.
Jack would age round 20 minutes more than Jill and his clock would be advanced by just over twenty minutes compared with Jill's clock (based on experimental Gravitational Time Dilation data and assuming a 150m skyscraper)
This example highlights the following points:
answered Mar 29 at 11:11
Alan Gee
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"time dilation is a true physical phenomenon" As A body accelerates closer to the speed of light, all of its extremities and intrinsic energies expend velocity, "for example" if a star where traveling near the speed of light then the orbiting moon or planet would have to slow down in order to keep up with the star, or in this case, hands on a clock slowing not to exceed light speed for the rate it is travelling.
.................."
Search Summary:
"Time Dilation is real insofar as it tells us the rate at which things happen in one reference frame relative to another. However, it is based on clock time, which is virtual. Real time doesn't, and never will vary. Things can still happen at different rates in different reference frames measured against real time. The rate of time itself is constant. Everything we see is some form of optical ... "
The details and the discussion:
"It is said that we can verify time dilation by flying a very accurate clock on a fast jet or spaceship and prove that it registers less time than the clocks on earth. However, the clocks on earth would be moving relative to the clock on the spaceship, and since time always dilates and never goes faster regardless of the direction of relative motion, the clocks on earth should register less time than the clock on the spaceship.
Is this true? Whenever there is a fast-moving object such as a rocket do all clocks on earth really become slow?
If the rocket with the clock landed after moving at relativistic speed, would its clock and the earth's clock again show the same time since during its travel both appeared slow to each other?
Or is all this just an illusion, ie. the clocks just appear to be slow to each other but in actually run at normal speed, and neither is behind when the rocket actually lands?
special-relativity time-dilation
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edited Feb 6 '14 at 11:58
Emilio Pisanty
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khushro
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I will add up to what mentioned below, that if you are using GPS in your mobile phone, then be informed that it uses time corrections due to this deletion ! so you need no super jets to experience that
– TMS Feb 4 '13 at 17:54
@TMS Nice example.. – Schrödinger's Cat Feb 4 '13 at 19:57
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Time dilation is real and is measured every day. For example the lifetime of a muon produced in the lab at low velocity is 2.2μμs. However the lifetime of muons generated by high energy cosmic rays in the atmosphere is around 11μμs. Their lifetime is extended by their high speed.
Calculating the time dilation of a plane flying around the Earth is complex because you have to take into account the gravitational time dilation as well as the plane's speed. However it has been done, and indeed experiment shows that if you put an atomic clock in a plane you do indeed measure the predicted time dilation.
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answered Feb 4 '13 at 9:10
John Rennie
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And, gravitational time dilation would beat relativistic time dilation because spaceships move very slowly. – Schrödinger's Cat Feb 4 '13 at 20:26
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You're referring to what is commonly known as the twin paradox. The Wikipedia page provides several different ways of analysing the situation, but one way to look at it is this -
When the clock on the spaceship leaves earth, it'll experience an acceleration (even if it's a really small acceleration for a long time, or a huge acceleration for a small time) to reach relativistic speeds, and will experience an acceleration again when it has to turn around to return to earth.
Special relativity only claims that inertial frames of reference are equivalent. Since one clock experiences acceleration, and hence is in a non-inertial frame momentarily, the two situations aren't equivalent.
If you view the time dilation due to acceleration as a gravitational acceleration (principle of equivalence) and then do the calculations, the results obtained for both the clock on the earth as well as the clock on the spaceship agree. Again, the wikipedia link contains in detail this argument, as well as other arguments.
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answered Feb 4 '13 at 9:42
Kitchi
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In your reasoning, you're tacitly making an assumption of a universal time, or you're at least having doubts owing to this assumption. Your central issue seems to be that two relatively moving inertial observers would each see each other's clocks moving slower to one another. This only leads to a logical contradiction if one assumes that there is one, universal time measurement, which would be shown to be strictly less than itself by your argument. Your implied argument is perfectly sound; it's the assumption of universal time that is wrong. But time is relative: each inertial observer has their own observed time, and simulteneity is relative.
So what of the case where the observers meet up again at a common spacetime point, so they have different elapsed times since they synchronized watches at the former time when they were together? Well, they have different paths through spacetime, and their clocks measure the lengths of those paths. Inertial observers fare geodesics through spacetime, which means their path is of maximal length compared to neighboring paths (this is a slightly unwonted aspect of Lorentzian geometry: in Euclidean / Riemannian geometry we deal with geodesics that are minimum length paths) - so the stay-at-home, always inertial twin in the twin "paradox" ages most. This difference between measured pathlengths joining the same two spacetime events is overwhelmingly experimentally verified, as several of the other answers show. Look up the Hafele Keating experiment, or the Rossi-Hall experiment cited in John Rennie's Answer. You might also find some insight into a more experimentally-oriented conception of time, as I discuss here.
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edited Apr 13 '17 at 12:39
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answered Nov 30 '16 at 1:13
Selene Routley
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For the rocket to depart from Earth and later return is a scenario in which a loop is closed.
A loop-closing scenario comes out symmetrical only when both participants have traveled the same spatial distance from departure to rejoining.
In an Earth-and-a-spacecraft-on-a-relativistic-journey scenario the spacecraft travels a much longer spatial distance than the Earth.
According to special relativity for the clock that has traveled a longer spatial distance less proper time will have elapsed, as seen when comparing clock times on rejoining.
The comparison on rejoining is a direct comparison, so it's clear and unambiguous. It's actually unhelpful to try an visualise what will be observed during the journey; because of transmission delays those raw observations are not a good perspective on what is happening.
It may be a surprise for you that difference in spatial distance traveled matters in special relativity. You may figure 'in space any individual spacecraft cannot count how many miles it has traveled'. And that is the case: any individual spacecraft cannot count its own mileage. More forcefully, no such individual mileage exists. However, special relativity does imply that you can always evaluate difference in spatial distance traveled. That difference in spatial distance traveled must be thought of as something that is relative between the two participants.
The relativity of special relativity is not a sweeping any motion is relative to something else. There is room for structure and buildup, giving rise to non-symmetrical scenario's, such as the one you ask about.
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answered Feb 4 '13 at 21:08
Cleonis
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No, time dilation is real. The universe does not follow the theory we assumed in the past where objects follow the Newtonian laws and light travels at a fixed speed in one frame of reference. If it did, an object going away from us would appear to have a slower system when it doesn't really.
It has been confirmed by observation that at each point in space-time, the universe locally follows special relativity. Special relativity predicts that time dilation is real. According to special relativity, there exists a frame of reference that each system dilates in time by a factor of 11−v2c2√11−v2c2 and contracts in length by a factor of 11−v2c2√11−v2c2. According to this answer, if you're travelling at constant velocity, there's no way to tell that you're moving in that frame of reference. You can consistently assume from your observations that you are not moving and what you deduce from the assumption that you are not moving is said to have actually happened in your frame of reference. You can sometimes make observations that wouldn't be possible at all if light travelled at a fixed speed and matter followed Newton's laws.
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edited Aug 27 '18 at 22:08
answered Nov 23 '17 at 21:06
Timothy
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Time dilation is based on clock time.
Clock time is the time we measure with when doing experiments. It is also the time according to our eyes.
In clock time, the speed of light is constant.
Most of Physics study is done using clock time.
Things age, and happen more slowly when the local rate of Clock Time is slower that the rate of Clock Time it is being compared with.
STR and GTR are theories based on on Clock Time
Clock Time theories tell us what we will see and what is happening according to our clocks.
There is an underlying time that natural activity is synchronised to - real time. One that is constant everywhere, and in which energy and momentum are independent and the speed of light is variable. It's rate is something like the rate of Earth's rotation, but without the fluctuations. A theory based on this interpretation of time can complement GR quite nicely.
Time Dilation is real insofar as it tells us the rate at which things happen in one reference frame relative to another. However, it is based on clock time, which is virtual.
Real time doesn't, and never will vary. Things can still happen at different rates in different reference frames measured against real time. The rate of time itself is constant.
Everything we see is some form of optical illusion.
UPDATE I will support this with an example
Jack and Jill live on the top and bottom floors of a skyscraper, respectively
When Earth rotates through one revolution, the rotation starts and stops at exactly the same time for Jack and Jill
If Jack, Jill, the skyscraper and Earth were immortal, a trillion revolutions of Earth would start and stop at exactly the same time for all four of them. They would experience every revolution, every radian and every microarcsecond together. They would all rotate together in real time.
Jack would age round 20 minutes more than Jill and his clock would be advanced by just over twenty minutes compared with Jill's clock (based on experimental Gravitational Time Dilation data and assuming a 150m skyscraper)
This example highlights the following points:
- Clock Time is not the same as Real Time
- Things age more quickly at higher altitudes in Real Time
- Clock Time is variable
- Real Time is constant
- The Speed of Light is variable in Real Time
- The Speed of Light is constant in Clock Time
- Time Dilation applies to Clock Time not to Real Time
answered Mar 29 at 11:11
Alan Gee
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"time dilation is a true physical phenomenon" As A body accelerates closer to the speed of light, all of its extremities and intrinsic energies expend velocity, "for example" if a star where traveling near the speed of light then the orbiting moon or planet would have to slow down in order to keep up with the star, or in this case, hands on a clock slowing not to exceed light speed for the rate it is travelling.
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"time dilation is a true physical phenomenon" As A body accelerates closer to the speed of light, all of its extremities and intrinsic energies expend velocity, "for example" if a star where traveling near the speed of light then the orbiting moon or planet would have to slow down in order to keep up with the star, or in this case, hands on a clock slowing not to exceed light speed for the rate it is travelling. "
I f you manage to accelerate at the speed of light (a) the hands of the clock would not be moving at all and (b) because of the accelation not because of the velocity all the clock hands would line up, ontop of each, with one fixed point, the axle at the center of the clock face where they are held, normally, provided this axle will not self destruct during the acceleration process.
I f you manage to accelerate at the speed of light (a) the hands of the clock would not be moving at all and (b) because of the accelation not because of the velocity all the clock hands would line up, ontop of each, with one fixed point, the axle at the center of the clock face where they are held, normally, provided this axle will not self destruct during the acceleration process.
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