Time dilation: a question

I'm having a little trouble understanding something which should be simple.

A quote from the wiki:

Time dilation would make it possible for passengers in a fast-moving vehicle to travel further into the future while aging very little, in that their great speed retards the rate of passage of on-board time. That is, the ship's clock (and according to relativity, any human traveling with it) shows less elapsed time than the clocks of observers on Earth. For sufficiently high speeds the effect is dramatic. For example, one year of travel might correspond to ten years at home. Indeed, a constant 1 g acceleration would permit humans to travel as far as light has been able to travel since the big bang (some 13.7 billion light years) in one human lifetime. The space travelers could return to Earth billions of years in the future. A scenario based on this idea was presented in the novel Planet of the Apes by Pierre Boulle.

Why would the people aboard a spaceship on coming back to earth find the people on earth have aged more than themselves? You could just as easily say that the earth was moving away from the spaceship at high speed... so why can't the opposite be true? Why can't the spaceship come back having aged more than the people on earth? Seems to me they would cancel each other out. Not counting the gravitational effects of time dilation of course.

It is because you are essentially 'squeezing' time inside the aperture of the time dilation 'wormhole'. While the rest of the galaxy experiences 'Normal' time the people inside the wormhole are experiencing faster time (but at a normal rate to them )

The same sort of time effect occurs near neutron stars. If one were to park a ship close to a neutron star and 'open the window' and throw an analogue clock at it, all the time watching the face of the clock, one would notice that the seconds hand would tick slower and slower and slower until when the clock is nearest the star, the clock would appear to stop. Yet if one were to travel WITH that clock that person would not see any slowing down of the hands. This is time dilation present due to the massive gravitational effect of the neutron star.

[edit on 15/10/2008 by Kryties]

The astronauts sent into space currently, return a few seconds younger than they would be had they stayed on earth.

Basically time is relative to the speed at which you are traveling,
to dumb it down a little i'll explain this way,

The person traveling at seriously high speeds is moving through time faster than those people still living on earth, which means by the time the person travelling at seriously high speeds lands, the people on earth wouldve aged much more greatly (or even just by a few seconds) more than the passenger of the high-speed transportation.

The reason it happens to astronauts is because they are actually travelling at incredibly high speeds while in orbit around the earth, Their travelling speed (even when stationary) is no longer measured relatively to the speed at which they physically move, but by the speed they are traveling due to their orbiting of the earth (by gravity)

another somewhat shaky example would be to imagine a garden hose. as you squeeze the end of it, the water seems to come out faster and with more force even though it's a constant rate of flow.

it's sort of a bottleneck effect

i'm not really explaining it correctly, i'm just trying to give you some sort of mental picture.

Originally posted by Kruel
I'm having a little trouble understanding something which should be simple.

Um. Not really all that simple.

Why would the people aboard a spaceship on coming back to earth find the people on earth have aged more than themselves? You could just as easily say that the earth was moving away from the spaceship at high speed... so why can't the opposite be true? Why can't the spaceship come back having aged more than the people on earth? Seems to me they would cancel each other out. Not counting the gravitational effects of time dilation of course.

Since you are omitting gravitational effects, you are talking about Special Relativity. When you are talking about Special Relativity you must consider frames of reference. Your example leaves that part out and in so doing, causes the apparent paradox which you noticed.

Special relativity deals with an inertial frame of reference. While accelerating away from Earth the ship is not in an inertial frame of reference. When the ship slows, stops, and changes direction to head back to Earth it is no longer in an inertial frame of reference. It is while under acceleration, when the Earth and the ship are in different frames of reference that the dilation effects are unbalanced. The Earth is not accelerating but the ship is, they are in different frames of reference. It is not the velocity that causes the differences in time flow but the acceleration of the ship.

Not really all that simple.

[edit on 16-10-2008 by Phage]

Hi Phage,

Well done for mentioning frames of reference, but unfortunately your physics is a little muddled.

You are correct that there are two causes of time dilation, gravitational time dilation and Velocity time dilation.

Gravitational time dilation is apparent when two objects in the same frame of reference are at different gravitation potentials. Time in the same frame of reference, travels more quickly when at a lower gravitational potential. An earlier poster mentioned neutron stars, well it's true that near to a neutron star you would experience time dilation, but this is not specific to neutron stars, just far more measureable due to the massive gravitational field. Right now, sitting in front of this monitor my time dilation is being affected by the monitors gravitational pull, we just couldn't measure it!

Velocity time dilation is a function of special relativity, correct. But this is not only apparent when velocity is changing. To simplify the maths time dilation is often calculated when acceleration is equal. Using Lorentz equations can get pretty complex but they can be used to calculate time dilation when an object is under uniform acceleration throughout the period of measurement.

My main point here is that time dilation is all about frame of reference. Take two spaceships travelling away from earth in formation at near light speed (or relatavistic velocity). In comparison to time on earth, the people on board will age far slower, ie if they were to return, they may only be a few years older, however everyone they know on earth will have died a long time ago. However, relative to the people on the other spaceship that travelled with them they will be the same age.

For further references search google for The Hafele–Keating experiment .

Cheers

Robbie

Originally posted by stratsys-sws

Velocity time dilation is a function of special relativity, correct. But this is not only apparent when velocity is changing. To simplify the maths time dilation is often calculated when acceleration is equal. Using Lorentz equations can get pretty complex but they can be used to calculate time dilation when an object is under uniform acceleration throughout the period of measurement.

I muddled myself trying to come up with what I did. I was about to edit my post last night, right when the site went down for maintenance. I'm already up to my eyebrows so forgive me if I'm getting myself deeper.

Between acceleration phases the relative velocities between Earth and the ship are the same. There is no absolute motion. This is the paradox. The Earth is "moving away" from the ship just as fast as the ship is moving away from Earth. Because of the velocity there is time dilation but it is a symmetric effect between the ship and Earth. From the Earth's point of view, time slows down on the ship. From the ship's point of view, time slows down on Earth. These changes in time flow are due to velocity but are equal so there is no net difference.

I did not mean to say that velocity does not cause dilation, it does. While the ship is accelerating, it leaves the frame of reference so the symmetry of the dilation effect is lost.

[edit on 16-10-2008 by Phage]

Originally posted by stratsys-sws
Take two spaceships travelling away from earth in formation at near light speed (or relatavistic velocity). In comparison to time on earth, the people on board will age far slower, ie if they were to return, they may only be a few years older, however everyone they know on earth will have died a long time ago. However, relative to the people on the other spaceship that travelled with them they will be the same age.

And if the earth traveled away from the spaceships instead of the spaceships traveling away from the earth, would the people on the spaceships be long dead when it returned? It shouldn't matter. It's all relative. What makes the earth so special that it experiences more in less time when they're both going the same speed relative to each other?

I found this of interest in the Hafele-Keating experiment:

In a frame of reference at rest with respect to the center of the earth, the clock aboard the plane moving eastward, in the direction of the earth's rotation, is moving faster than a clock that remains on the ground, while the clock aboard the plane moving westward, against the earth's rotation, is moving slower.

Is this the answer? Spin of the earth?

reply to post by Kruel


The spin of the earth causes different velocities for the two aircraft. It's really only relevant to this experiment.

There is some question about the validity of the Hafele-Keating results.

Here is one of several articles which discuss it.
www.cartesio-episteme.net...&KPaper.htm


[edit on 16-10-2008 by Phage]

[edit on 16-10-2008 by Phage]

Originally posted by Phage
The Earth is "moving away" from the ship just as fast as the ship is moving away from Earth. Because of the velocity there is time dilation but it is a symmetric effect between the ship and Earth. From the Earth's point of view, time slows down on the ship. From the ship's point of view, time slows down on Earth. These changes in time flow are due to velocity but are equal so there is no net difference.

So in effect, wouldn't this mean that upon the spaceship and the earth returning to each other, one group would not have aged more than the other? Not counting gravitational dilation. And would the earth's rotation make a difference?

Originally posted by Kruel

So in effect, wouldn't this mean that upon the spaceship and the earth returning to each other, one group would not have aged more than the other? Not counting gravitational dilation. And would the earth's rotation make a difference?

The earth's rotation doesn't matter in this case.

The difference in aging still occurs because the spaceship is not always in the same frame of reference as the Earth. The acceleration phases of the ships flight "solve" the paradox.

The whole idea is almost too weird to deal with but it is real.

[edit on 16-10-2008 by Phage]

Originally posted by Phage
The difference in aging still occurs because the spaceship is not always in the same frame of reference as the Earth. The acceleration phases of the ships flight "solve" the paradox.

Or:

"The acceleration phases of the earth's flight "solve" the paradox."

See what I did there? It's all relative. That's why I'm having difficulty understanding why one would age more than the other.

reply to post by Kruel


It is only relative when both the ship and the earth are in the same frame of reference. The relative velocities change but the earth does not accelerate. The astronauts on the ship feel the accelerations, the accelerations happen to the ship and its passengers. No one on earth feels anything different for the whole period of time, the earth does not accelerate.

There is no paradox because there are parts of the journey when they are not in the same frame of reference.

[edit on 16-10-2008 by Phage]

reply to post by Phage


Thanks, I understand now.

Ok, another question...

If both the occupants of the spaceship and the earth see each other moving slower when they're at high speed relative to each other, when do the occupants of the earth appear to speed up from the ship's point of view? (to support the reason for the earth occupants aging faster)

Does this happen at acceleration? Is deceleration any different than acceleration in this case?

reply to post by Kruel


Of course, the occupants of the spaceship cannot "see" anything happening on earth. But apart from that, yes, the faster aging for the person on earth happens only during the acceleration phases of the flight. This is a gross simplification. It involves "checking" both clocks at particular points, not really the overall flow. And if you ask me anything else I will get a huge headache.

Note that in physics, the term acceleration includes any change in velocity. So slowing down, speeding up and turning are all accelerations. The direction of the acceleration doesn't matter.

[edit on 16-10-2008 by Phage]

Originally posted by NGC2736
Pick up the wrong end of a hot fireplace poker, and a second seems like ten years. Pick up the right woman, and ten years can seem like a second.

Here is a true relative perspective and one we so often forget. The philosophy behind this thought is the reason why I don't wear a watch, I find I spend less time anticipating the end of my work day and more time focusing on the present.

reply to post by Kruel

Kruel, this does not have a simple answer and I don't fully understand relativity, just some of the parts involved. The most important thing is to understand the meaning of the words we are using.

Acceleration is the rate of change of velocity, positive acc.=increased speed & negative acc.=decreased speed. Think of acceleration as energy or a force acting on an object.

Velocity is maintaining speed or motion without change, this is momentum or conservation of energy from a force. Everything that is made of matter has velocity, or in other words, everything is in motion in some way.

Inertia is the resistance to acceleration.

Time can be difficult to understand, simply put, it is measuring the motion of the Earth in relation to the Sun, Moon and fixed stars. Earth has more then one measurable length of year, most common is the tropical year (Gregorian calendar) measured at vernal equinox which is 365 1/4 days (365.242190 on average). There is also the sidereal year (365.25636), anomalistic (365.25964) and Gaussian year (365.25689). The Moon's orbital times are equally confusing but still relate to how we measure time, it is difficult to measure motion when there is no fixed point.

Inertial frame of reference is the observers point of view. Earth is the most common POV for our measurement of time.

In a velocity time dilation (non-accelerating), ie. two ships moving away from each other at a combined light speed, each observer will see the others clock as motionless even though they are both keeping accurate time. Think of it this way, as you move away from the other ship at the speed of light the image (reflected light) of that clocks time will not update and appear frozen. There is no way to confirm this without a faster then light communication.

Acceleration time dilation & Gravitational time dilation are similar in that both observers would agree that one clock runs slower then the other, gravity and acceleration both slow down time.
I don't understand this next part but as we stand on Earth we experience a 1g force (weight) and even though we don't appear to be moving, the effects are identical to inertia. An object in free fall appears to be accelerating towards the Earth but it doesn't feel any G force or inertia, it's weightless. The 'Feeling' from inertia is in the opposite direction of acceleration. It's as though the Earth is accelerating outward and thus the inertia we feel is inward, towards the center. This is why inertia and gravity are considered "Fictitious Forces".

If we maintained a constant 1g acceleration away from Earth the inertia would simulate Earth's gravity and time dilation. The problem is that our ever increasing speed also increases the inertial mass. When a force accelerates an object some of the energy is used to overcome inertia, as the speed increases the inertial mass increases. The more inertial mass there is the more energy we will need to overcome it and this increases until all the force/energy is converted directly into inertial mass and we can no longer accelerate, infinite energy = infinite inertial mass.

Of the books I have read about relativity and space time I found that Einstein himself was best able to explain his theories to a layman like myself.
Relativity; The Special and General Theory.