# Thread: About the world within? Effects on Time of Speed of Light Travel.

1. ## About the world within? Effects on Time of Speed of Light Travel.

So, I've been reading Paradox: The Nine Greatest Enigmas in Physics, by Jim Al-Khalili and I'm not satisfied by his analysis of the Paradox of Twins. It raises more questions.

The way this paradox is resolved is by observing there is no paradox. To that end we say that the travelling twin experienced acceleration. But what exactly experienced acceleration?

I'm not interested in the paradox per se. What I'm interested in understanding is exactly what force allows for a system (the travelling twin, for instance) to be considered a single unit and to experience the effect of time dilation as a whole. And if this won't somehow spill out to any outer systems. At the scale of an atom our bodies are made up of as much empty space as the universe. But we implicitly know there's a force binding us into a consubstantial object in our own scale.

Now, the solar system, is it too a system. An outer system, at our human scale. It's entire mass counts with both twins. In a trip to the Oort Cloud, within our Sun's gravitational force, if the travelling twin went there and back a few times at the speed of light, wouldn't his mass (and that of his spaceship) contributed to an infinitesimal degree to an expanded lifetime of the Solar System?

If the two twins were two cells in your body, the travelling cell will remain young. The other cell (and the rest of your body) will age normally. Your body will however benefit from the young cell. Your body (the system) didn't experience the trip, but the system will still benefit from it as a whole. It is younger. It's made up of younger mass.

So... without experiencing any acceleration how come can the solar system become younger by the travelling twin actions?

2. The time dilation is a relationship between coordinate systems and "imaginary clocks," not objects. So it is not tied to the object which is accelerating but actually the coordinate system in which that acceleration is defined. Had you chosen to measure from a different coordinate system you would measure a different degree of effect, however, the physical outcome remains the same.

This is similar and related to the unification of electric and magnetic forces. The degree to which an EM interaction is electrical vs. magnetic actually changes depending on the coordinate system, but it changes in a precise way which conserves the physical behavior of the system, so that different observers all experience a single physical reality. What it is telling us is that our distinction between electrical and magnetic forces is artificial. Same thing with space and time.

3. The problem I have with this paradox is that the "age" part is not well defined. If I get 10 years younger that means that I will live until I am X+10 instead of X. By if I don't know X and cannot know X, this statement is pointless. We could maybe find a way around this by calculating the average life of millions of micro-organisms and just sending them in a trip and back. The point then is to guarantee that Age = F(time) and not Age = F(time, acceleration) as simplified example. In other words that the travel itself won't affect the aging in any way.

Let us try to prove that. We make a valid experiment in in a subject while it is idle and while it is accelerating. We find that the subjects ages the same. The problem with that is that we just invalidated the paradox, the subject is not younger while returning back.
So since we believe on the paradox being accurate, we expect now that the subject is actually younger when it is accelerating. But then we just show that by definition Age = F(time, acceleration). So there is no paradox.
In other words, if we assume that Age = F(t), we accelerate the subject and now see that this equation we observed before isn't true. That just means one thing: our assumption wasn't true. There is still no paradox. If we can prove that Age = F(t), we make the observation and we in fact see now that Age = F(time, acceleration) then we have a paradox, but this is not the case.

More to the point, if you want to talk about the age of a system, provided you can actually define the age of its components, you need to define that. If it is the sum of ages of all components then I would agree that if a component gets younger the whole system gets younger. So I wouldn't be satisfied either if that is the case and I would just say there are more paradoxes spawned. So the solar system will become younger even without experiencing acceleration provided all the previous assumptions are true.

What it is telling us is that our distinction between electrical and magnetic forces is artificial. Same thing with space and time.
I think in the end it is the opposite. Time and space have their definitions regardless of how you measure them. I imagine (as most people I believe) time not being affected by anything else. So if a clock is affected by acceleration I just accept that a clock doesn't count time in general. To say a clock counts time it needs to meet the criteria based on the pure philosophical definition of time I initially had. The fact that a clock used to meet it, but now it doesn't due to the theory of relativity means again one thing: my initial assumption was wrong. Electromagnetic forces on the other hand don't "conflict" on any definition I have put on them initially. That is why you won't see a famous paradox on electromagnetic forces. These are all of course all a matter of my own personal perspective...

4. I think that the best explanation I've ever heard was from Stephen Hawking - "Into the Universe with Stephen Hawking"

Stephen Hawking Time travel - YouTube

5. Mario is well versed in the Master Arthur C Clarke's work - which finely discusses such things through stories- relativity is all i say

6. youtube "Into the Universe with Stephen Hawking"
he has books too

twins is in universe in a nutshell, and before that the grandfather paradox is in brief history of time - IIRC

7. An easy way to imagine this is to combine all 3 spatial dimensions into a single spatial
dimension, which we can represent as an East-West direction. Then we represent time
as a North-South direction. If you move diagonally, say NE, you are moving through both
space and time. If you move dead North, you are only traveling through time, and likewise
if you move dead East, you are only travelling through space.
Our own motion here on earth is analagous to moving almost dead North. We are moving
through time at a relatively high rate, and through space at a relatively low rate. Someone
(like the travelling twin) who is accelerating rapidly is moving through time at a slower rate
and moving through space at a faster rate. This is analagous to moving more Easterly in
the above analogy.

It's important to remember that movement through space is not quite as we imagine it in
our common experience of it. It is a movement through the spatial dimension of spacetime.
If you accelerate, you give up some of your motion through time, your clock ticks more
slowly. This is a real effect and is what is experienced by the travelling twin. The theory
of General Relativity deals with this effect. Also it is not just the clocks that slow down.
The clock is just an indicator. Time has slowed down. The travelling twin notices nothing
different.

If you wanted to consider the age of the universe as an average of the ages of it's various
components, then the travelling twin would reduce the age of the universe compared to if
that twin had stayed home. Spacetime varies over the universe. It is not a single uniform
(flat) field. Everything in it contributes.
The travelling twin is not going to run the clock backwards, it is only going to slow it's rate
forward.

The universe does not contain the matter within it. It is created by it.
"Physical objects are not in space, but these objects are spatially extended" - Albert Einstein

8. Originally Posted by Mario F.
The way this paradox is resolved is by observing there is no paradox. To that end we say that the travelling twin experienced acceleration. But what exactly experienced

acceleration?

I'm not interested in the paradox per se. What I'm interested in understanding is exactly what force allows for a system (the travelling twin, for instance) to be considered a single unit and to experience the

effect of time dilation as a whole. And if this won't somehow spill out to any outer systems. At the scale of an atom our bodies are made up of as much empty space as the universe. But we implicitly know

there's a force binding us into a consubstantial object in our own scale.
The travelling twin can experience a time flow that is different from the "rest" of the universe
because it is travelling along a different path through spacetime. It is not just the direction as
we commonly thing of it. When an object is not acted upon by any external force, it is said
to be in an inertial reference frame. Examples are objects in orbit, objects in freefall, an
object that has been tossed into the air (while it is in flight), or an obect sitting out in deep
space. Even though the object might seem motionless, it is nonetheless travelling through
spacetime. It's path is called a geodesic. Any object travelling along a geodesic is free
of any force like gravity or inertia. It's motion is entirely through time.

True, these objects certainly appear to be moving through space, but each one can perfectly
claim that it is at rest, and it is the rest of the world that is moving. All such objects are governed
equally by the laws of physics. This is given by the principle of relativity.

The stationary twin is in an inertial reference frame (ignoring the gravitational field from the planet
that he might be standing on). He is travelling along a geodesic. You might think of these geodesics
as natural paths, ones that objects will naturally follow unless forced to do otherwise.
The travelling twin though, has been diverted from a geodesic by the forces causing him to accelerate.
He has veered off of the time dimension and is travelling partly along a spatial dimension. He is now
travelling more rapidly along the spatial dimension of spacetime, and more slowly through the temporal
(time) dimension.

This moving object contributes to the universe because it has in fact created part of it. That an object
is spatially extended must also mean that it is temporally extended. They cannot be separated. Just as
cells are bound into a body, the extension of these moving objects are bound into the universe.

The twin paradox is not all that dificult of a paradox, and is usually introduced in the study of special relativity.
It is in the context of the special theory that it seems to form a contradiction. The special theory only deals
with objects and observers in an inertial (non accelerating) reference frame. For such observers, each will
see the other's clock as running more slowly than their own. And there is the paradox: how can each one both
be slower than the other? The paradox is resolved by the fact that the twins are actually not both in inertial
reference frames; only the travelling one is.
Or at least that is one form and resolution of the twin paradox.

Now the two twins could pass each other in opposite directions at uniform speed. Both would be in inertial
reference frames and so neither should experience time dilation. Yet each will observe the other's clock
as running slower than his own. Here I think is the real paradox. Unlike the classic twin paradox, it is possible
for the two twins to meet up again and compare clocks, without either one undergoing any acceleration.
They can't possibly both see each other's clock as being slower than their own. So was it all an illusion?
I believe it isn't. I would describe it as an offset in time. Time didn't slow, it just lagged. Like a clock
that was set 10 minutes slow but still ran at correct speed.

9. Originally Posted by megafiddle
Now the two twins could pass each other in opposite directions at uniform speed. Both would be in inertial
reference frames and so neither should experience time dilation. Yet each will observe the other's clock
as running slower than his own. Here I think is the real paradox. Unlike the classic twin paradox, it is possible
for the two twins to meet up again and compare clocks, without either one undergoing any acceleration.
They can't possibly both see each other's clock as being slower than their own. So was it all an illusion?
I believe it isn't. I would describe it as an offset in time. Time didn't slow, it just lagged. Like a clock
that was set 10 minutes slow but still ran at correct speed.
They cannot though "simultaneously" see each other's clock to make this observation. You always need a specific reference point. Since you are describing the way you see it, what matter is how you count time. Then you can take a clock from both travelers and compare it to yours. In which case you can then check the difference from a specific reference point than the difference between two reference points "at the same time" (same time according to who?).

What you are also counting then is the different measurements in those 3 clocks. If you used an atomic clock then you counted a difference in frequencies of the atoms. I would thus say that it indeed seems like a "slowing down in time" rather than a lagging. Like the atoms actually moved slower compared to the ones on the reference point. If you combine this with general relativity it just shows that both acceleration and gravitational fields affect the atom's movement (I know I am oversimplifying...). Which is a bit easier to comprehend as you can expect gravity to interact with something that has a mass, like an atom, and movement (from acceleration) affects movement (the frequency of the atom).

10. Originally Posted by C_ntua
They cannot though "simultaneously" see each other's clock to make this observation. You always need a specific reference point. Since you are describing the way you see it,

what matter is how you count time. Then you can take a clock from both travelers and compare it to yours. In which case you can then check the difference from a specific reference point than the

difference between two reference points "at the same time" (same time according to who?).
Actually, you can't do that. A separate third observer only has the reality of his own reference frame. One of the points
of special relativity is that twin A has a perfectly valid reference frame from which he can judge reality. What twin A
observes is real, in every sense of the word. Another point of the theory is that the speed of light is constant relative
to any and all observers. The theory itself was not the introduction of these points into physics. That was done by the
principle of relativity for the first point and Maxwell's field equations for the second point. What the theory did was bring
the two together. These two points would be incompatable using Newtonian physics.

Einstein's "bouncing photon" thought experiment illustrates that time must change in order for the speed of light to remain
constant. When twin A observes twin B's clock as running slower than his own, the slower speed of twin B's clock is every
bit as real as the "normal" speed of his own clock. Likewise, when twin B sees twin A's clock as running slower than his own,
that is also a reality for twin B. You can't introduce a separate observor to see what is "really" going on.

Originally Posted by C_ntua
What you are also counting then is the different measurements in those 3 clocks. If you used an atomic clock then you counted a difference in frequencies of the atoms. I would

thus say that it indeed seems like a "slowing down in time" rather than a lagging. Like the atoms actually moved slower compared to the ones on the reference point. If you combine this with general

relativity it just shows that both acceleration and gravitational fields affect the atom's movement (I know I am oversimplifying...). Which is a bit easier to comprehend as you can expect gravity to interact

with something that has a mass, like an atom, and movement (from acceleration) affects movement (the frequency of the atom).
General relativity does have some differences. When you reunite two clocks from inertial and noninertial reference frames,
there will be a difference. More time will have passed in the inertial frame. But there is another difference. You can place
twin A at the surface of a large planet and twin B in a synchronous orbit above him. They do not move in relation to each other,
distance wise. They can observe each other's clocks directly and continuously. Maybe using precise strobes. And twin A's
clock is going to run slower on the planet surface. It is not that time is the same and the atoms are moving more slowly. It
is that time itself has slowed. The atoms are following the laws of physics just as they would anywhere else.

There is a good site with animated illustrations of the "bouncing photon". I will put it up here if I can find it.

My use of the term "lagging" is only from lack of a better one. I think it is clear that two twins who are each in inertial reference
frames will find that their clocks are still synchronized when they meet up again. Since the slowing of twin B's clock is a reality
for twin A, something in addition to slowing must have occured. At this point I don't understand where speeding up of the clock
might have occured, for example. The only other change that you can have besides rate with a clock (or time) is offset.

11. I found the site. This has some good explanations and animations, especially for the bouncing photon clock::

Time Dilation, Length Contraction and Simultaneity (from Einstein Light)

The play, stop, etc buttons are just above each animation image.
You might not see the animations if your security settings are on "high".

Previously I said this:

So was it all an illusion?
I believe it isn't. I would describe it as an offset in time. Time didn't slow, it just lagged. Like a clock
that was set 10 minutes slow but still ran at correct speed.

I think I need to correct that. Time did slow down. But there was an additional change which I called
an offset or lag. Maybe "shift" is a better term.

Also, I have a question myself.

In my own twin paradox, the two twins are travelling in high speed orbits about the same planet,
but in opposite directions. The orbits are slightly tilted from each other so that the twins don't
collide. They pass each other at two points in the orbit, 180 degrees apart. At these two points
they have an opportunity to observe the bouncing photon in the other's ship. The bouncing photon
constitutes the clock. Each bounce is a "tick" of the clock. Each twin sees the other twin's clock
ticking more slowly than his own at each passing. This is not a problem or paradox in itself; it is
exactly to be expected from special relativity. Each twin's observations are their own individual
realities based on their reference frames.

Now at any point, they can decelerate equally to a stop and compare clocks. Because they were
both in inertial reference frames, their two clocks will agree. Both experienced the same acceleration
while stopping, so there experience is symmetrical.

Again this is as must be expected from special relativity (and general relativity for the stopping part).
But how can one twin continue to observe the "moving" clock (the other twin's clock) as moving more
slowly than his own, and then have the two clocks show up exactly in sync when they are compared?

If I am thinking about this correctly, the path of the bouncing photon will always be longer for the "moving"
clock regardless of whether it is receding or approaching. So the "stationary" twin will always see the
"moving" twin's clock as running slower. So something additional must be happening. Could the motion
of the "moving" clock towards the observer compensate for the slowing of the clock in the same way
that the motion would cause a blue shift?

12. Originally Posted by Mario F.
The way this paradox is resolved is by observing there is no paradox. To that end we say that the travelling twin experienced acceleration. But what exactly experienced

acceleration?
Imagine you are out in deep space, just floating there in zero G. Some distance away a tether is anchored
somehow and someone is attached to the free end of the tether and travelling around in a large circle,
restrained by the tether. What would you observe? You would see them travelling in a large circle, off to
Now suppose you are sitting on a rediculously high platform above a planet. Someone is travelling in an
orbit around the planet. What would you observe? You would see them travelling in a large circle, off to

In the first case it is your circulating neighbor that experiences a force, in that case a centrifugal force.
In the second case it is you that experience a force, the gravitational force. Both are accelerating forces
and have the same effect.
In the first case, time will be slower for your circulating neighbor as it is him that was accelerating; he
experienced a force, you did not.
In the second case, time will be slower for you, as you are experiencing a force, the orbiter experiences
no force.

Originally Posted by Mario F.
I'm not interested in the paradox per se. What I'm interested in understanding is exactly what force allows for a system (the travelling twin, for instance) to be considered a

single unit and to experience the effect of time dilation as a whole.
We tend to think of the world in the Newtonian sense, with everything behaving just as it does in our own
limited experience (relativistic effects are only observed at significant speed, ie approaching light speed).
We also tend to think of time as an independant universal constant. In such a world, everything can move
around through space alone, while their motion through time remains fixed and determined by the cosmic
clock.
Also in such a world, one direction or path is no differect than any other. The world would be flat, not flat as
in two dimensional, but flat as in being uniform and the same in all directions and locations. It is not hard to
imagine that objects could move around in such a world with no effect on their age.

We can imagine Newtonian space if we think about a pool table. If you wanted to describe the movement
of the 8 ball across the table surface, you would only need a direction and a velocity. The 8 ball's movement
through time would be independant and unaffected by it's motion through space, in this Newtonian space.
There would be no forces there that acted on time or the 8 ball's age.

In Einstein's model of the universe, time and space are not independant. They are in fact two parts of the
same thing - spacetime. Likewise, the motion of an object through spacetime is always through both space
and time combined.

Back to the Newtonian pool table, a ball can move within two dimensions. It can move along the shorter
dimension of the table (side to side) or along the longer dimension (end to end - pool tables are rectangular).
Now suppose the ball must always be in motion at some constant speed. If you interfere with the ball's motion
and change it's direction, it's movement will change in both dimensions. If it is now moving more in an end to
end direction, it will be moving less in a side to side direction (and vice versa). Since this a Newtonian table,
time is unaffected. In our relativistic world, things appear virtually the same on a pool table, but that is only
because the relativistic effects are miniscule on this scale of things.

It will seem unusual that the ball must always be moving at a constant speed, but that is important to the analogy.
In relativistic space, objects always move at a constant speed. We don't observe that because we only see the
spatial component and temporal components separately. The variations in speed and direction that we normally
see in objects also have an effect on their travel through time (the temporal dimension), but it is unmeasurable.

Now we can imagine a relativistic pool table. Since we need to use one dimension for time, we only have one
dimension left to represent all 3 spatial dimensions. Nonetheless it should work as well, as we are primarily
interested in the time dimension and it's relation to the spatial. We only need one to represent space. Suppose
that the long dimension of the table represents space and the short dimension represents time. Also the ball,
which again represents an object in the universe, must move at some constant speed (on the table). It should
be easy to see that the object could move entirely through space (end to end on the table), or entirely through
time (side to side on the table), or some combination of the two (a diagonal movement).
We, for example, sitting here on earth, would be like a ball moving in a side to side direction, very slightly angled
because of the earth's gravity.

But there is a more important difference between the Newtonian and relativistic tables. The Newtonian table is flat.
The ball has no preference in it's direction, except to continue in it's current direction. The relativistic table though,
has a "grain" or "texture". Left to themselves, balls on a relativistic table will tend to move from side to side only. It
requires some type of force to alter their direction from temporal to spatial (from side to side to lengthwise).

And so it is with spacetime. It has a grain, or network of "preferred directions". These are called geodesics or world
lines. Left to themselves, objects will follow geodesics. The direction of the geodesics lie in the direction of the time
dimension. The spatial dimensions lie at angles to the geodesics. So left to itself, an object will travel at maximum
speed through time and at zero speed through space.

It requires a force to deflect an object from it's movement along a geodesic. This might be a force causing an object
to change it's direction, or a force causing an object to change speed (both are changes in spatial velocity).
Or it might be a gravitational force (which is equivalent to an accelerating force). In all these cases the object is no
longer travelling exclusively along the temporal dimension, while the force is present. Some of it's movment will have
been diverted to the spatial dimension. It will have travelled through time more slowly.

13. Originally Posted by Mario F.
To that end we say that the travelling twin experienced acceleration. But what exactly experienced acceleration?

What I'm interested in understanding is exactly what force allows for a system (the travelling twin, for instance) to be considered a single unit and to experience the effect of time dilation as a whole.
I assume, you can tell how much something is moving, insomuch as time dilation is concerned, by how it's movement compares to the matter who's gravitational field it's most affected by.

In the case of the platform on a planet, and the orbiter of the planet: The platform is moving along with the planet, hence no "movement" / time dilation, but the orbiter is moving contrary to the planet, hence "movement" / time dilation.

14. Originally Posted by Yarin
I assume, you can tell how much something is moving, insomuch as time dilation is concerned, by how it's movement compares to the matter who's gravitational field it's most affected by.
You can only define movement in relation to yourself, your own reference frame. There is no such thing as absolute motion.

Originally Posted by Yarin
In the case of the platform on a planet, and the orbiter of the planet: The platform is moving along with the planet, hence no "movement" / time dilation, but the orbiter is moving

contrary to the planet, hence "movement" / time dilation.
The reference frame is that of the observer on the platform. The movement of the orbiter is in relation to the platform.
The question here is the forces experienced by the various observers. The orbiter experiences none. That is necessarily the case
if he is in orbit. It has nothing to do with rotation of the planet beneath him. The orbiter experiences no general relativistic time dilation.

The forces on the platform would depend on the rotation of the planet. I did not specify a height for the platform; if it were right at
synchronous orbit height, the accelerating forces would be zero. However, I did imply that the orbiter would be moving with respect
to the platform. Therefore the platform could not be at a height that would also place it in zero G like the orbiter. The forces on the
platform then will either be gravitational if it's below synchronous height, or cetrifugal if it is above synchronous height. Either way,
there are accelerating forces on the platform. The observer on the platform will experience general relativistic time dilation.

15. Originally Posted by megafiddle
There is no such thing as absolute motion.
I never claimed anything of the sort.
You write mile long posts, but can't even read my small post. :-/