# Thread: CLOCK ON A SPACESHIP

1. I am assured by semi scientific friends that a clock returning to earth after a space journey will indicate less elapsed time than one which remained on earth during the duration of the space journey.
The main points of the scenario are:-
1) the wind up clocks are, for the purpose of the exercise, 100% accurate and set
to indicate the same time before the spaceship departs.
2) the spaceship, which travels at near the speed of light, is outward bound from
earth for 30 minutes and inward bound for the same time. Hence , the
spaceship clock is away from the from the clock remaining on earth for 60
minutes.
3) in these circumstances I am assured that when the clocks are compared
the spaceship clock will have recorded less elapsed time than the one which
remained on earth.
4) I am aware of time dilation phenomena. However, I cannot understand the
nature of any practical/mechanical intervention which could cause the spaceship
clock to slow down ie tick/run more slowly than the one which remained on
earth.
I should be grateful if someone could answer my question.

2.

3. It's not a mechanical phenomenon. The clock doesn't slow down in a mechanical sense. Less time elapses, in a very real/fundamental sense, for the traveling clock. It's not at all intuitive.

I'm no expert, but basically, you can only compare clocks when they are at the same place. Accelerating one clock sent it along a separate 4D path than the one left behind. When you get them back together, you can see the differences in the lengths of those paths by comparing the clocks.

4. Are you saying that when the spaceship returns to earth both clocks will show the same time eg 2.0 o' clock ?

5. Originally Posted by David Mellor
Are you saying that when the spaceship returns to earth both clocks will show the same time eg 2.0 o' clock ?
No, that's not what he's saying. The problem you describe is also known as the twin paradox.The twin paradox isn't actually a paradox when one looks at the effects that special relativity predicts. It only seems to be a paradox to those who don't understand special relativity.

You can find a description of the twin paradox and the answer to your question in the Wikipedia article here:

Chris

6. You might also look up Hefele-Keating experiment.

There's also a nice overview here: http://sheol.org/throopw/sr-ticks-n-bricks.html

And, here... You can see it happens even in our normal velocity cars (just so tiny as to be disregardable): http://blogs.scienceforums.net/swansont/archives/6

7. My thanks to you all for attempting to answer my question. I looked at the suggested articles but they do not seem to deal with my specific question or if they do I lack comprehension.

In my scenario there is only one observer and he is the spaceship pilot who sets the two clocks to exactly the same time on the launch pad. One clock is left on the launch pad. The pilot boards the spaceship with the second clock and blasts off into space returning to earth after a one hour flight at near the speed of light. On landing he compares the two clocks and finds that the spaceship clock has lost time such that it appears that the clocks have been apart for less than one hour.
My difficulty is understanding what mechanical, physical, gravatational force or whatever caused the spaceship clock to tick more slowly than the clock that remained on earth.

David Mellor

8. Originally Posted by David Mellor
My thanks to you all for attempting to answer my question. I looked at the suggested articles but they do not seem to deal with my specific question or if they do I lack comprehension.

In my scenario there is only one observer and he is the spaceship pilot who sets the two clocks to exactly the same time on the launch pad. One clock is left on the launch pad. The pilot boards the spaceship with the second clock and blasts off into space returning to earth after a one hour flight at near the speed of light. On landing he compares the two clocks and finds that the spaceship clock has lost time such that it appears that the clocks have been apart for less than one hour.
My difficulty is understanding what mechanical, physical, gravatational force or whatever caused the spaceship clock to tick more slowly than the clock that remained on earth.

David Mellor
There is no "mechanical mechanism' tht causes one clock to "tick slower" than the other.

Time is a local concept. In general relativity it makes no sense to compare "time here" with "time there".

Clocks measure the proper time of their world line. The proper time of a clock that undergoes acceleration is less than the proper time of a non-accelerated clock, if one compares world lines with common start and end points.

9. Originally Posted by David Mellor
My thanks to you all for attempting to answer my question. I looked at the suggested articles but they do not seem to deal with my specific question or if they do I lack comprehension.

In my scenario there is only one observer and he is the spaceship pilot who sets the two clocks to exactly the same time on the launch pad. One clock is left on the launch pad. The pilot boards the spaceship with the second clock and blasts off into space returning to earth after a one hour flight at near the speed of light. On landing he compares the two clocks and finds that the spaceship clock has lost time such that it appears that the clocks have been apart for less than one hour.
My difficulty is understanding what mechanical, physical, gravatational force or whatever caused the spaceship clock to tick more slowly than the clock that remained on earth.

David Mellor
To repeat what Dr. Rocket has already said, nothing acts on the clock to slow it down.

What happens is that the clock on the space ship and the clock on the Earth just measure different times. One of the first things you have to do in order to start to grasp Relativity is to learn to think about "time" in a different way.

10. To make it easy. Let us just see how it would have looked if u could see both clocks at the same time. You would realise that there is nothing forcing te clock to run slow. Its the basic speed of the most fundamental of particles that seem to transfer information at a slower pace.
its easier to get if we make the clock simpler. let the clock be made of to parallel plates between which a photon is bouncing back and forth(as in b/w mirrors.
now u will see that in the clock thats in the airplane the photon will move faster as we know that the speed of light is less when under infuence of gravity.
Its not just light but every particle since all particles show wave nature(which is ofcourse diffult to observe as the mass of particle increases.

11. Light doesn't move at a different speed under the effects of gravity.

I think what'll happen is you'll see the mirrors closer together or farther apart than you were expecting, but I'm not particularly confident in that. Maybe someone else who knows more about this can chime in.

12. Originally Posted by David Mellor
My thanks to you all for attempting to answer my question. I looked at the suggested articles but they do not seem to deal with my specific question or if they do I lack comprehension.

In my scenario there is only one observer and he is the spaceship pilot who sets the two clocks to exactly the same time on the launch pad. One clock is left on the launch pad. The pilot boards the spaceship with the second clock and blasts off into space returning to earth after a one hour flight at near the speed of light. On landing he compares the two clocks and finds that the spaceship clock has lost time such that it appears that the clocks have been apart for less than one hour.
My difficulty is understanding what mechanical, physical, gravatational force or whatever caused the spaceship clock to tick more slowly than the clock that remained on earth.
It's really simple, much simpler than you think, but to understand it you have to go back to basics. Instead of employing mechanical clocks, use parallel-mirror light clocks. Now look at the Simple inference of time dilation due to relative velocity. The hypotenuse is the light path, the base is the speed as a fraction of c, and the height gives the Lorentz factor. It's basically just Pythagoras' theorem, and the bottom line is that the second clock shows a lower reading because it did a lot of moving out and back through space. The light-path between its parallel mirrors was like this /\/\/\/\/\/\/\ instead of this |, so it "ticked" at a slower rate.

The next step is to appreciate the relevance of pair production where we make subatomic particles out of light. There's also annihilation, which is the inverse:

You're made out of subatomic particles, which have spin. Think of them as "light going round and round". When you move a particle, the light path is no longer circular, but more like a stretched-out spring. Look at it from the side and it's like this: /\/\/\/\/\/\/\/\/\. So matter is affected just like the light between the parallel mirrors.

13. Originally Posted by Farsight
Originally Posted by David Mellor
My thanks to you all for attempting to answer my question. I looked at the suggested articles but they do not seem to deal with my specific question or if they do I lack comprehension.

In my scenario there is only one observer and he is the spaceship pilot who sets the two clocks to exactly the same time on the launch pad. One clock is left on the launch pad. The pilot boards the spaceship with the second clock and blasts off into space returning to earth after a one hour flight at near the speed of light. On landing he compares the two clocks and finds that the spaceship clock has lost time such that it appears that the clocks have been apart for less than one hour.
My difficulty is understanding what mechanical, physical, gravatational force or whatever caused the spaceship clock to tick more slowly than the clock that remained on earth.
It's really simple, much simpler than you think, but to understand it you have to go back to basics. Instead of employing mechanical clocks, use parallel-mirror light clocks. Now look at the Simple inference of time dilation due to relative velocity. The hypotenuse is the light path, the base is the speed as a fraction of c, and the height gives the Lorentz factor. It's basically just Pythagoras' theorem, and the bottom line is that the second clock shows a lower reading because it did a lot of moving out and back through space. The light-path between its parallel mirrors was like this /\/\/\/\/\/\/\ instead of this |, so it "ticked" at a slower rate.

The next step is to appreciate the relevance of pair production where we make subatomic particles out of light. There's also annihilation, which is the inverse:

You're made out of subatomic particles, which have spin. Think of them as "light going round and round". When you move a particle, the light path is no longer circular, but more like a stretched-out spring. Look at it from the side and it's like this: /\/\/\/\/\/\/\/\/\. So matter is affected just like the light between the parallel mirrors.

You are a menace to newbies and a laughing stock to people who actually understand physics.

14. Forgive me but I am still at a loss to understand what it is phyically that makes the space ship clock tick more slowly than the one that remained on earth.
I am not concerned about what the pilot sees when he looks at the clock. Also, in my scenario the time shown by each clock is not compared until the spaceship clock has returned to earth.
I cannot comprehend how the speed of light , light itself, or the speed of the space vehicle can slow down the rate at which the spaceship clock actually ticks.

David Mellor

15. Originally Posted by David Mellor
Forgive me but I am still at a loss to understand what it is phyically that makes the space ship clock tick more slowly than the one that remained on earth.

David Mellor
It is the nature of time itself that is reflected in the differences registered by the two clocks. NOTHING "makes the space ship clock tick more slowly than the one that remained on earth".

There is no such thing as a universal time.

Neither clock "ticks more slowly" than the other. The very word "slowly" is predicated on some concept of time.

BTW please ignore Farsight's posts. He is a well-known internet crank, who spreads misinformation.

16. Another way of saying that is: nothing happens to the clocks, but the moving one simply takes less time to get from point A to point B.

17. Originally Posted by David Mellor
Forgive me but I am still at a loss to understand what it is phyically that makes the space ship clock tick more slowly than the one that remained on earth.
I am not concerned about what the pilot sees when he looks at the clock. Also, in my scenario the time shown by each clock is not compared until the spaceship clock has returned to earth.
I cannot comprehend how the speed of light , light itself, or the speed of the space vehicle can slow down the rate at which the spaceship clock actually ticks.

David Mellor
Let's try a little analogy:

Say you have two men walking at the same pace in the same direction on a featureless plain. This is the equivalent of the Earth clock and Ship clock sitting next to each other ticking at the same rate.

Now one man (man 2) turns and starts walking in slightly different direction at the same pace. (This is the equivalent of the ship taking off from Earth.)

Now, how do things appear to man 1? As he continues to walk, he will notice that man 2 starts to fall behind him. IOW, man 2 is making less progress in the direction that man 1 is walking then man 1 is. (This is the equivalent of the earth observer noting that time on the ship is running slower than his own).

At the same time, man 2 notices the same thing, that man 1 is the one falling behind. (The pilot on the ship will note that it is the Earth clock that is running slow.)

Now man 2 turns again so that he is heading back to intersect with man 1's path.
(the ship turns around and heads back toward Earth. )

From man 1's perspective, man 2 is still falling behind after making the turn. (the Earth observer still notes that the ship time runs slow)

From man 2's perspective, as he turns, man 1 goes from being behind him to being in front of him. Think of yourself as facing East with an object North by Northwest of you. It will be behind your left shoulder. Now you turn to face Northeast. Now the same object will be to the front and the left of you.
After the turn, he will still note that Man 1 is making slower progress.
(From the ship's perspective, time on the Earth "jumps forward", and after the turn is made, he will note that Earth time runs slow. )

Man 2 intersects man 1's path and turns to match walking direction with man 1. (the ship stops when it returns to Earth.

From man 1's perspective, man 2 reaches a point directly behind him and then starts walking behind him staying a constant distance away. (Less time has passed for the ship than has for Earth.)

From man'2 perspective, he reaches the path of man 1 and then starts walking at the same speed and direction but behind man 1. (More time has passed on the Earth than has for the pilot.)

The above illustrates what I meant went I said that you have to learn to think about time differently.

We tend to think of time as something that is fixed, like the direction North. Everyone agrees, no matter which way they are facing, which direction North is in.
If something travels North slower than another object, something must be physically making it move slower.

Time however, is more like the direction forward. If you ask a bunch of people to point forward, they will point in different directions depending on the relative directions they are facing. Whether or not something is moving "forward" slower than another, also depends on who you ask, and what direction they are facing and can be just because it travels in a different direction. (just like our two men in the example. )

18. Originally Posted by David Mellor
Forgive me but I am still at a loss to understand what it is physically that makes the space ship clock tick more slowly than the one that remained on earth. I am not concerned about what the pilot sees when he looks at the clock. Also, in my scenario the time shown by each clock is not compared until the spaceship clock has returned to earth. I cannot comprehend how the speed of light, light itself, or the speed of the space vehicle can slow down the rate at which the spaceship clock actually ticks.
It's because of the electromagnetic interaction, and because clocks are made of electrons etc. If you move a clock in a forward direction at some significant fraction of the speed of light, its internal electromagnetic interactions can't occur as quickly as previously. If they did, then when you add in the forward motion, they'd be occuring faster than light. Hence everything within the clock happens slower, so the clock goes slower. What I was saying about the light clock is something like what Janus was saying, but one man is walking straight back and forth on a 1m line at 300,000,000 m/s, and the other man zigzags back and forth across a 1m band at 300,000,000 m/s out into the desert then back. At the end of the experiment the second guy has gone back and forth fewer times.

19. Originally Posted by Farsight
It's because of the electromagnetic interaction, and because clocks are made of electrons etc. If you move a clock in a forward direction at some significant fraction of the speed of light, its internal electromagnetic interactions can't occur as quickly as previously. If they did, then when you add in the forward motion, they'd be occuring faster than light. Hence everything within the clock happens slower, so the clock goes slower. What I was saying about the light clock is something like what Janus was saying, but one man is walking straight back and forth on a 1m line at 300,000,000 m/s, and the other man zigzags back and forth across a 1m band at 300,000,000 m/s out into the desert then back. At the end of the experiment the second guy has gone back and forth fewer times.
I think I see what you are trying to say here, but it is important to point out that the clock doesn't go "slower". It's "internal electromagnetic interactions" occur at the same speed as they ever did. The clock ticks away the seconds at the same rate as it ever did. But the path the clock is on takes less seconds to travel along, when compared to the clock that remained "at rest" in relation to it.

20. I know that Einstien's prediction that the clock on the space ship will show less elapsed time than the one which remains on earth has been proved correct. However, I am beginnig to wonder if anyone really understands exactly why this should be the case.

To return to my scenario. When the space ship returns to earth the pilot looks at the earth bound clock which tells him that he has been away from earth for 60 minutes as we record time on earth. When he looks at the space ship clock it indicates that his journey lasted for less than 60 minutes. Something caused the space ship clock to lose time and to do that its mechanism must have run slower than the earth clock.

21. Originally Posted by David Mellor
I know that Einstien's prediction that the clock on the space ship will show less elapsed time than the one which remains on earth has been proved correct. However, I am beginnig to wonder if anyone really understands exactly why this should be the case.

To return to my scenario. When the space ship returns to earth the pilot looks at the earth bound clock which tells him that he has been away from earth for 60 minutes as we record time on earth. When he looks at the space ship clock it indicates that his journey lasted for less than 60 minutes. Something caused the space ship clock to lose time and to do that its mechanism must have run slower than the earth clock.
That's the problem - nobody understands why. Yet.

All we know is that it seems as if space and time are intrinsically linked, such that motion through space, relative to something else, changes your motion through time, relative to that something else. It is not about something acting on the mechanism in the clock, it is about the clock, with nothing acting upon it, measuring less time.

22. Originally Posted by David Mellor
I know that Einstien's prediction that the clock on the space ship will show less elapsed time than the one which remains on earth has been proved correct. However, I am beginnig to wonder if anyone really understands exactly why this should be the case.

To return to my scenario. When the space ship returns to earth the pilot looks at the earth bound clock which tells him that he has been away from earth for 60 minutes as we record time on earth. When he looks at the space ship clock it indicates that his journey lasted for less than 60 minutes. Something caused the space ship clock to lose time and to do that its mechanism must have run slower than the earth clock.
No, you are still trying to think about it as something physically acting on the clock to make it run slow. This is not what is happening. Clocks moving relative to each other simply measure time differently. In the same way, measuring sticks will measure distances differently.

For instance, if the distance that the ship traveled was 540,000,000 km as measured by the Earth, it will be shorter distance as measured by the ship. So if it took 60 minutes to travel 540,000,000 km at the speed of the ship according to the Earth, it will take less than 60 minutes for the ship to travel the shorter distance by its measurement at the same speed. The ship clock shows less time because, according to it, the trip was shorter in distance.

23. Originally Posted by Janus
No, you are still trying to think about it as something physically acting on the clock to make it run slow. This is not what is happening. Clocks moving relative to each other simply measure time differently. In the same way, measuring sticks will measure distances differently.
Just to emphasize this point... Length dilation and contraction do not imply you have a problem with the ruler, nor that you have some force causing it to shift its measuring properties. Length itself is changing. While it's more difficult to comprehend, it's the same with time. The problem is not with the clock, nor is there any force acting on the clock. Time itself is dilated or contracted relative to other observers.

24. Originally Posted by David Mellor
I know that Einstien's prediction that the clock on the space ship will show less elapsed time than the one which remains on earth has been proved correct. However, I am beginnig to wonder if anyone really understands exactly why this should be the case.
Lots of people understand precisely why the two clocks show different times.

The reason has been explained to you. You apparently choose to not understand. You are not alone. Farsight also does not understand. The other folks bin this thread do understand.

Why not listen to those that do underrstand ? Better yet, read a book.

Originally Posted by David Mellor
To return to my scenario. When the space ship returns to earth the pilot looks at the earth bound clock which tells him that he has been away from earth for 60 minutes as we record time on earth. When he looks at the space ship clock it indicates that his journey lasted for less than 60 minutes. Something caused the space ship clock to lose time and to do that its mechanism must have run slower than the earth clock.
Wrong.

Each clock registered the correct proper time for its world line. Proper time is the only time that any clock, of any type, is capable of measuring. That is the story, the whole story and nothing but the story. Now go read a good physics book and learn about proper time.

Ignore Farsight. Or become just as nuts as he is.

25. Originally Posted by SpeedFreek
I think I see what you are trying to say here, but it is important to point out that the clock doesn't go "slower". It's "internal electromagnetic interactions" occur at the same speed as they ever did. The clock ticks away the seconds at the same rate as it ever did. But the path the clock is on takes less seconds to travel along, when compared to the clock that remained "at rest" in relation to it.
There is of course a symmetry as per the twins "paradox", wherein if I'm moving through space relative to you, you might assert that my clock's going slower than yours whilst I might assert that your clock's slower than mine. We could talk about world lines and angles etc. But if we both agree that I'm the twin who did the moving, and then we agree that I'm the one who came back time-dilated looking younger than you, then we have to then agree that my clock was ticking slower than yours. If it wasn't, both our clocks would have recorded the same number of seconds.

The crucial point to appreciate with all this is that your second and my second is defined by the motion of light in our respective "frames". See the NIST caesium fountain clock, on this, and look at wikipedia for the definition of the second:

"Since 1967, the second has been defined to be the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom".

In the NIST clock, lasers and a microwave cavity are employed to cause hyperfine transitions, which are electron “spin flips”. These are electromagnetic in nature, as are the emitted microwaves. There’s a peak frequency to them which is found and measured by the detector. But, and this is the important bit: frequency is measured in Hertz, which is defined as cycles per second, and the second isn't defined yet. The peak frequency is 9,192,631,770 Hertz by definition. It's like counting 9,192,631,770 microwaves and saying "OK, that's a second". And if I was carrying one of these clocks on my round-trip, it would be going slower than your stay-at-home identical clock. I don't notice it because my nerve impulses and internal chemical reactions are "going slower" too. And of course I use the motion of light to define my second (and metre) which I then use... to measure the speed of light.

26. Originally Posted by David Mellor
I know that Einstien's prediction that the clock on the space ship will show less elapsed time than the one which remains on earth has been proved correct. However, I am beginning to wonder if anyone really understands exactly why this should be the case.
They do. As I was saying, look at the simple inference of time dilation due to relative velocity. It talks about a parallel-mirror light clock. The hypotenuse is the light path, the base is the speed as a fraction of c, and the height gives the Lorentz factor straight out of Pythagoras' theorem. When this clock is moving fast through space, its tick rate is reduced. The NIST clock I mentioned above is an atomic clock, but it employs electromagnetic phenomena, so in the wider sense it's a light clock too. So again the tick rate is reduced. Ditto for a quartz wristwatch. Your thinking works through nerve impulses, which are electrochemical in nature, so your thinking is similarly affected, as are chemical processes within your body and electromagnetic processes within your machines. They all go slower.

Originally Posted by David Mellor
To return to my scenario. When the space ship returns to earth the pilot looks at the earth bound clock which tells him that he has been away from earth for 60 minutes as we record time on earth. When he looks at the space ship clock it indicates that his journey lasted for less than 60 minutes. Something caused the space ship clock to lose time and to do that its mechanism must have run slower than the earth clock.
Yes, it did. If it had run at the same rate it would show 60 minutes too. If anybody challenges you on this replace the clocks with tape reels, one of which has recorded on 60 metres of tape, the other 50 metres. Then ask if the tape reels were rolling at the same rate. NB: Don't pay any attention to Dr Rocket. He takes a perverse pleasure in insulting people rather than offering sincere considered responses.

27. The most fundamental questions are often the most difficult to answer. The clock is a simple machine and therefore it is beyond dispute that if it is to show less elapsed time than the clock which remains on earth at least two mechanical things must occur. It must tick more slowly or for each tick the hands must move forward less than they do on the earth clock. I am assuming that logic applies in space as it does on earth. The question is what is the nature of the physical intervention which causes one or both of these things?

28. Originally Posted by David Mellor
The most fundamental questions are often the most difficult to answer. The clock is a simple machine and therefore it is beyond dispute that if it is to show less elapsed time than the clock which remains on earth at least two mechanical things must occur. It must tick more slowly or for each tick the hands must move forward less than they do on the earth clock. I am assuming that logic applies in space as it does on earth. The question is what is the nature of the physical intervention which causes one or both of these things?
There are a lot of legitinate internet sites (and books) that explain Special Relativity. One place to start is: http://en.wikipedia.org/wiki/Special_relativity

Concerning the issue of the so-called "twin paradox" (about which much of this thread has concerned itself), you might also want to study the Wikipedia article on it here: http://en.wikipedia.org/wiki/Twin_paradox

Both of these articles contain information, references and links that can provide you with more in-depth explanations than can be found in the relatively short replies typical of this thread. An added benefit is that they don't contain the sort of personal squabbles and pet theories that some of the posts in this thread illustrate.

Chris

29. Originally Posted by David Mellor
The most fundamental questions are often the most difficult to answer. The clock is a simple machine and therefore it is beyond dispute that if it is to show less elapsed time than the clock which remains on earth at least two mechanical things must occur. It must tick more slowly or for each tick the hands must move forward less than they do on the earth clock. I am assuming that logic applies in space as it does on earth. The question is what is the nature of the physical intervention which causes one or both of these things?
No. That is not beyond dispute. There could simply be less ticks for it to tick off. For one to "tick more slowly" you have to have a universal time to compare the two to, which doesn't exist.

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