# Exploring The Twin Paradox: Understanding Time & Motion

• rczmiller
In summary, the twin paradox involves one twin aging slower than the other due to the effects of time on objects moving at different speeds. The resolution of the paradox lies in the theory of general relativity, which takes into account periods of acceleration. The special theory of relativity only applies to constant motion and does not consider acceleration. There is not an absolute point in space and time as all objects are in constant motion, making it difficult to determine a single reference point. Einstein's theory also states that velocity affects the order of time, as seen in the example of lightning bolts hitting a train at the same time for an observer standing next to the track, but at different times for an observer on the train.
rczmiller
I believe I understand the basics concerning the twin paradox. One twin leaves Earth on a near light speed round trip to a local star. On his return, he is younger than his twin due to the fact that time progresses more slowly the faster an object moves.

Here are my points of confusion: If movement is relative and there is no absolute frame of reference for motion, why would time proceed at different rates for the twins. I read that it is due to the force of acceleration that the twin on the spacecraft feels. However, the twin on Earth would also feel acceleration in the form of gravity. I am sure there is more to this. Would someone mind filling in my gaps of understanding on this issue? Thanks.

The paradox is not that one twin ends up younger/older than the other, but in SR alone BOTH twines should be younger and older than the other. The resolution of this paradox lies in GR, and is due to the periods of acceleration the traveling twin undergoes, and the difference between the time it takes to reach the point at which the twin turns back between the two observers. I don't think the gravitational effect of the Earth is really the issue in this thought-experiment, as you could just as well choose the 'at rest' twin to be in space too.

Twines? No, that's string theory! ;o)

GR isn't necessary to resolve the twins paradox. There've been many threads on the twins paradox and many webpages on the twins paradox and many books that treat it. I'd suggest reading one of them.

--J

Such as...?

rczmiller said:
If movement is relative and there is no absolute frame of reference for motion, why would time proceed at different rates for the twins.

Constant motion is relative, accelerated motion is not. The twin that made the trip had to accelerate at least twice. Relativity only applies to inertial frames of reference, it does not apply when accelerating. This is what distinguishes the traveling twin from the stationary twin.

Thanks for all the replies. I thought I read somewhere that Einstein thought that gravity was accelleration. If this was the case, if the twin on the spaceship never exceeded 1g, then the Earth bound twin would "feel" more accelleration than the one on the trip. I am still trying to come to grips with this, and I will continue to do more reading on the subject.

One thing that is really bugging me is why is one spot in space more special than another when it comes to relativity. There seems like there should be an absolute reference point for space/time. What if the 1971 J.C. Hafele and R.E. Keating experiment used a plane that traveled at 1000mph and traveled at the equater in the opposite direction of the Earth's rotation. Would the atomic clock on the plane lose time in reference to the Earth bound (stationary) atomic clock? The plane would accellerate (apply force) while the Earth bound clock would just sit there. However, if an outside observer (say the Sun) looked at the events, the Earth bound clock rotated around the diameter of the Earth while the plane stayed in the same position (not factoring the Earth's movement of orbit around the Sun).

Since everything is moving (the Earth rotates, the Earth orbits the Sun, the Sun orbit the galaxy, our galaxy is moving, etc.), I am having a hard time understanding why with all these points of references moving in respect to one another, why there is not an absolute point in space/time to base all measurements of relativity against. I guess it is time for more reading on the subject. Thanks to all that replied.

rczmiller said:
I thought I read somewhere that Einstein thought that gravity was accelleration.
He did, but that was the general theory of relativity. The classical twins paradox only cares about the special theory of relativity.

rczmiller said:
One thing that is really bugging me is why is one spot in space more special than another when it comes to relativity. There seems like there should be an absolute reference point for space/time. What if the 1971 J.C. Hafele and R.E. Keating experiment used a plane that traveled at 1000mph and traveled at the equater in the opposite direction of the Earth's rotation. Would the atomic clock on the plane lose time in reference to the Earth bound (stationary) atomic clock? The plane would accellerate (apply force) while the Earth bound clock would just sit there. However, if an outside observer (say the Sun) looked at the events, the Earth bound clock rotated around the diameter of the Earth while the plane stayed in the same position (not factoring the Earth's movement of orbit around the Sun).

Since everything is moving (the Earth rotates, the Earth orbits the Sun, the Sun orbit the galaxy, our galaxy is moving, etc.), I am having a hard time understanding why with all these points of references moving in respect to one another, why there is not an absolute point in space/time to base all measurements of relativity against. I guess it is time for more reading on the subject. Thanks to all that replied.
There are two main points Einstein's special theory of relativity makes: 1) Velocity affects the rate of time. 2) Velocity affects the order of time. The second point is very important to the twins paradox, but often overlooked.

To help in understanding the order of time part, Einstein gives the example of a train moving very quickly over a track. An observer standing next to the track sees two lightning bolts hit the train at the exact same time. Each lightning bolt is the same distance from the observer, in opposite directions, thus the observer states "The lightning bolts hit simultaneously." However, if the train is moving fast enough, and if the observer could "watch" the propogation of the light beam, he would see the passenger sitting at the midpoint of the two strikes move toward the lightning bolt at the front of the train and away from the one at the back of the train. We know based on this observation that the passenger at the midpoint will see the front lightning strike first, but according to the law of propogation of light in vacuo, we know light always travels at the same speed. Thus, the passenger will claim the lightning bolt toward the front of the train actually happened first, because he is at the midpoint and that's the one he saw first. And in fact, both of these observers are valid in their claim; they're just experiencing time in a different order.

Last edited:

## 1. What is the Twin Paradox?

The Twin Paradox is a thought experiment in special relativity where one twin travels at high speeds through space while the other remains on Earth. When the traveling twin returns, they find that they have aged less than their Earth-bound twin, even though they experienced the same amount of time. This paradox highlights the effects of time dilation and the relativity of simultaneity.

## 2. How does the Twin Paradox relate to time and motion?

The Twin Paradox demonstrates the effects of time dilation, which is the slowing down of time for an object in motion relative to an observer. It also illustrates the concept of the relativity of simultaneity, which states that two events that appear simultaneous to one observer may not appear simultaneous to another observer in a different frame of reference.

## 3. What is the significance of the Twin Paradox?

The Twin Paradox challenges our understanding of time and motion and highlights the counterintuitive nature of special relativity. It also has practical applications in fields such as space travel and GPS technology, where the effects of time dilation must be taken into account.

## 4. Can the Twin Paradox be resolved?

Yes, the Twin Paradox can be resolved by considering the effects of acceleration and the fact that the traveling twin experiences two different frames of reference during their journey. This leads to a difference in the aging process and explains why the traveling twin ages less than the Earth-bound twin.

## 5. How does the Twin Paradox impact our understanding of time and space?

The Twin Paradox challenges our traditional understanding of time as a constant and absolute concept. It shows that time is relative and can be affected by factors such as motion and gravity. It also highlights the interconnectedness of time and space, as the traveling twin experiences changes in both during their journey.

• Special and General Relativity
Replies
5
Views
687
• Special and General Relativity
Replies
20
Views
2K
• Special and General Relativity
Replies
13
Views
2K
• Special and General Relativity
Replies
11
Views
1K
• Special and General Relativity
Replies
31
Views
1K
• Special and General Relativity
Replies
12
Views
980
• Special and General Relativity
Replies
31
Views
2K
• Special and General Relativity
Replies
36
Views
3K
• Special and General Relativity
Replies
122
Views
5K
• Special and General Relativity
Replies
115
Views
5K