# Special relativity?

Hi,

Does anyone have a reference to an explanation of special relativity for ignorami? I just can't understand it, specifically the twins in the spaceship example.

Basically twins are born on earth. A spaceship takes one of the twins and travels near the speed of light away from earth. At some point it turns around and returns to earth. The twin on earth appears to have aged much faster than the twin on the space ship.

I can't understand this. Isn't time (whatever time is) constant? I mean our biology progresses at the same rate, regardless of what speed you happen to be traveling at?
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Here's a basic explanation that gets to time dilation right away:

http://www.squidoo.com/relativity_explanation

And no, time is not constant.  You could think of traveling through spacetime as a constant, and because you're not used to moving fast, that really amounts to moving through time at a near-constant rate.  But if you're moving through spatial dimensions faster you're not moving in the direction of time as fast.
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>  Isn't time (whatever time is) constant?
Only within a particular reference frame.

The twin situation involves at least 3 different reference frames.
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During the time of Lord Kelvin, there existed two major problems on physics. One dealt with dark-body radiation, which would eventually be resolved by quantum mechanics, and the Michelson-Morley experiment, which would be resolved by quantum mechanics.

The Michelson-Morley experiment was an attempt to find the ether, the physical construct that allowed light to propagate. Light was known to be an electromagnetic wave and waves needed a physical medium to propagate. Sound needs air, seismic waves need some form of matter, so it wasn't that big a stretch. Just what this represented physically was a great source of interest. Once these two problems were solved, scientists believed they would have a complete picture of the universe; a unified understanding so to speak.

Unfortunately, the M-M experiment caused more problems than it solved. The experiment was designed to detect the speed at which the Earth moves through the ether by measuring the differences in the speed of light in different directions. The experiment provided a null result which meant there was no difference in the speed of light in ANY direction. Something had to be wrong and most, if not all, sought to find some way to modify Maxwell's theory of electromagnetism to fix this problem.

In 1905, Einstein, a virtual unknown, came up with a solution. Einstein was essentially a failure, having been unable to secure an academic position upon graduation and having to find employment as a patent clerk but what he proposed was revolutionary. Space and time were fluid but the speed of light was a constant. As for time not being constant, it doesn't just apply to the tick-tock movement of a clock but it also applies to biological processes.

Einstein published his theory in that same year and explained the consequences of his ideas. This was his most cited paper and it contained no references. One of the more compelling ideas was that as an object traveled near the speed of light, we will see its clock slow down. But this theory is symmetric, so an observer traveling in this object looking at us, will also see our clock slow down.

So does this mean that the twin paradox points to some flaw in the theory? Far from it. The key to understanding is the concepts of symmetry. Special relativity applies to objects traveling at uniform speeds i.e. they are not accelerating. So for one twin to get to near light speeds, he/she must accelerate to attain that speed and thereby breaking the symmetry. This means that we can tell which twin is going to physically experience this time dilation.

As the twin travels at this uniform speed, both twins will see time move slower when they look at each other. If the traveling twin was to instantaneously turn around, the other twin will see the time taken to be time dilated as well. So the earth bound twin sees the turn take longer than it actually occurred.

So when the space faring twin return, she actually observes that she is, in fact, younger than her sibling who is now well into retirement. This happens because of the breaks in symmetry that occurs when the space-faring twin accelerates both at the beginning of the journey and when she turn around.

The Queen song, 39, that appeared on "A Night at the Opera" deals with this effect and paradox. Brian May, who wrote this song, is an astrophysicist.

Hope that helps,
David
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There is more nonsense written about the special theory of relativity than almost any other physical idea. There are a large number of good books describing SR on any level you wish. I think you would enjoy and profit by one, but what level would you like.
A VERY good book (written for a freshman physics course) is
Spacetime Physics (2nd ed recommended) by Taylor and Wheeler. See your library.
The twin and other paradoxes are covered
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If you find the the mathematics convolutes things, then Bertrand Russell wrote a great introductory book on relativity which contained no mathematics, it's called 'ABC of Relativity'.
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>> As the twin travels at this uniform speed, both twins will see time move slower when they look at each other. If the traveling twin was to instantaneously turn around, the other twin will see the time taken to be time dilated as well. So the earth bound twin sees the turn take longer than it actually occurred.

1. When does the aging pocess change for the space twin? Only during the accelerations? During the constant speeds? Or both? If both which has the most impact on the aging process?

>> So when the space faring twin return, she actually observes that she is, in fact, younger than her sibling who is now well into retirement.

2. There appears to be an illusary aspect to the time dilation since both twins see the other's clock as slowing down. Consider two twins docked far from our solar system. They take off in opposite directions each approaching the speed of light at same acceleration; and then they turn off engines for a year of their own time. They turn around and return to dock at their original starting point.
2a. Isn't it true that in every stage, acceleration or not, the twins each see the other's clock as slowing down?
2b. When they are moving away from each other, when a tick-tock occurs in one ship, the other ship receives the tick, but then receives the tock a bit longer than its own tick-tock, since when the tock was sent, the ship was further away from the other ship; and the tock takes even longer to reach the other ship since it continues to move further away.
2c. When they are moving towards each other, then even though the twins see each other's time as slower than their own, doesn't the tock from one ship arrive sooner after the tick arrived than on the receiving ship since the when the tock is sent, it is closer to the other ship than when its tick was sent; and also that the receiving ship is rushing towards the transmitted tock?
2d. Are the questions in 2b and 2c just an illusary aspect of time - like doppler shifts?

3. After the twins both dock, are they surprised that they are both the same age?
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Although there can be a change in the aging process for the space twin during acceleration, that is not relevant to the paradox
More relevant is that since the space twin occupies different reference frames during the outward and inward legs of the journey, their notion of "now" that is attached to that frame changes, which changes their measurement of the "current" age of the earthbound twin.
That the notion of "future" and "past" can be different for people who take different paths through space-time is not unlike the way the notion of "ahead" or "behind" can be different for people who's paths face different directions in space.
When the space twin turns around, the earth twin changes from being far "behind" from one point of view, to being far "ahead" from a different point of view.  This does not mean that the earth twin suddenly moved a great distance, or that either point of view is incorrect.

there are both illusory and real aspects of special relativity

if the twins have a proper understanding of relativity, they will no be surprised.
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Hi,
One of the key things that you need to understand is the concept of space-time and frames of reference. So the twins share the same frame of reference when they are together. But once one twin separates and goes her own way, the time they experience is going to be different. This was the fundamental revolution that Einstein introduced. Only when the come back together, and into the same frame of reference, do they see the difference in their relative experiences of time.

So to answer your first question. The differences they experience in time occurs when they move apart and continues to be different once they remain apart. Acceleration doesn't come into this as Special Relativity only deals with uniform motion; there is no acceleration. So there is no way to really answer the question using SR. What the acceleration does highlight is a separation in the twins' frame of reference and hence the differences in experiences in time.

So now you have the scenario where they are both moving away from each other.

Yes, they will see each other's clock tick slower.

Not sure what you're asking in part b, but yes. That is what Time Dilation means.

As far as c goes, yes, it is possible for a tock to arrive before a tick. That deals with simultaneity. If we assume that the tick and tock are two seperate events, it is possible to move a frame of reference to reverse the observed order of events. No. It does not violate causality.
(Please, do not ask me to write on this. A chapter just explaining it may be necessary. If you need further clarification I'll do my best"

As to d. no it's not illusary. It is reality. The key concept that you have to get is that time is not static or the same for different observers. So it is not an illusion. It's real and how the universe works.

and for the last question. No they will be of different ages. As they are moving away from each other at a uniform speed, they will experience a time dilation. How much depends on the relative speed of the frame of reference they return back to. The situation that you are describing involves more than two frames of reference; there are three and possibly more. So the math is going to be a little more involved.

Believe it or not, what you are describing HAS been performed in 1971. They flew two atomic clocks in opposite directions and they observed time shifts that was predicted by SR. The rotation of the earth had to be taken into account and not just the relative speeds to each other. Only when did the two clocks come back to earth, and in the same reference frame, were they able to compare the time differences.

I hope that helps and hasn't confused you more.
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Hi Photon0137,
Thanks for your response. But I'd like to clarify something about Scenario 2. In it both twins start off talking to each other close by undergoing no acceleration. They agree on a shared itinerary to follow:
a) Then they both accelerate in opposite directions at constant same acceleration to a specified speed
b) They turn off engines for an agreed amount of local time
c) They turn around (in a shared plan - a loop at same speed)
d) They approach each other undergoing no acceleration for an agreed amount of local time
e) They decelerate at constant same acceleration meet at the starting point at zero speed

Firstly, I never meant to infer that a tock would arrive before a tick since the speeds are always < c.

In a, b, d, e, it would appear that both twins would see each their twin clock as slower than their own. Right?

And the shared itinerary requires same acceleration, speed, and deceleration all for agreed upon specified local clock time.

From symmetry, it would seem to me that when the twins reached their rendezvous, that their clocks and their age would be identical. So, all the slower clock that they saw would appear illusory.

Furthermore, when they were approaching each other, the tick tocks should appear to be faster than their local clock due to Doppler shifting. Whether the received tick tocks is faster or slower than SR time dilation is not clear yet. As they approach there is a blue shift so the tick tocks should appear faster than expected. And in the initial departure phase, the tick tocks should appear even slower than expected SR results, maybe?

I was thinking that some of the perceived tick tock increases or decreases may be due to SR and also may be due to Doppler shifting.
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> In a, b, d, e, it would appear that both twins would see each their twin clock as slower than their own. Right?
correct
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> From symmetry, it would seem to me that when the twins reached their rendezvous, that their clocks and their age would be identical.
correct
> So, all the slower clock that they saw would appear illusory.
If you want to call a different perspective "illusory"
You might call Doppler effects  "illusory"
But time dilation is real, regardless of any illusion.
Just as one person can measure a certain interval as 5 steps forward, and another person might measure the same interval as 4 steps forward, 3 steps left.
Neither measurement is illusory, they are just made from different perspectives.

We are used to this kind of perspective change, with rotations between "forward" and "left"
but rotations between "future" and "left" are less familiar because at ordinary speeds, everyones "future"
is so close to the same direction that we do not notice the differences.
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But in d) They approach each other so the tick tock received would appear to be arriving at faster rate than when they were in b) where they were moving away from each other. Right?? (just like blue shift when a star is traveling towards the receiver and red-shift when traveling away)
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I just saw post http:#31838721
My post http:#31838823 was in response to http:#31837874
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> But in d) They approach each other so the tick tock received would appear to be arriving at faster rate than when they were in b) where they were moving away from each other. Right??
Yes, this is the Doppler effect.
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OK, thanks!
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> there is no way to really answer the question using SR.
if your answer needs to know the precise age of the twin down to the pico second based on their exact acceleration schedule then yes, you need to apply GR.
but SR answers the question of which twinis older just fine.
WHat you need to understand is that when you are moving with a different velocity, you are in a different reference frame, which can use a different notion of which direction is "future" and which direction is "left" than the reference frame you may have been in when you were moving with a different velocity.

You can avoid the issue of acceleration completely by having 3 different people,
if the outgoing twin never turns around, but instead keeps going and passes someone headed in the opposite direction
a triplet headed to the left can synchronize watches with an earth triplet when they pass
then the left going  triplet can synchronize watches with a right going triplet when they pass each other,
and finally, the right going triplet can synchronize watches with the earth triplet when they pass.
no acceleration is involved, but the total time elapsed on the left trip plus the right trip is less than the time elapsed on earth.

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