Can we see it?

[Spyman]

I made a little gif to see the actual motion of such an object X of my previous diagram. I am still wondering if such a thing exist or is possible...

According to current models and observations it is not possible for a physical object to move slower through time than us in our frame of reference.

 

If such an object would exist it would be beyond our horizon and therefor not observable and impossible to interact with.

 

[EDIT] I don't think the "mass-time surface" is adding anything but confusion in the diagrams.

[michel123456]

According to current models and observations it is not possible for a physical object to move slower through time than us in our frame of reference.

 

If such an object would exist it would be beyond our horizon and therefor not observable and impossible to interact with.

 

[EDIT] I don't think the "mass-time surface" is adding anything but confusion in the diagrams.

 

The red spot in the gif is not moving slower, it is moving after us at the same speed. But to consider the gif diagram, we first have to clear the question between line & dot. The MT surface is there only to make an argument against the line concept. If the line is accepted, I think the MT surface must be taken under consideration. If the line is not accepted, the MT surface vanishes.

 

From your previous statement (Spyman)

I prefer to view the world lines as the path of the objects which they follow as they travel through time, from the past towards the future, precisely like "moving dots", (...)

Personally I don't think objects are like a long spear penetrating time from past to the future or that the original object leaves a separated or duplicated thing, that stays behind, still existing in the past when the object itself continues towards the future.

 

It seems to me you are a "moving dot" supporter, so you don't have to worry much about MT.

 

Iggy must worry.

[Iggy]

Lets consider Iggy is right & objects are lines. As a matter of consequence I suppose then that , if objects "exist" in the past, they have mass "in the past"

 

Yes. The earth had mass yesterday, and the many days before.

 

there is a huge amount of mass hidden in the past (& the future)

 

No, it's the same mass. The earth's mass yesterday is the same mass we see from the earth today. This should be intuitive if you don't obfuscate it.

 

represented by the MT surface in the diagram

 

Your diagram makes no sense to me. Space is for some reason not orthogonal to time, and you've marked off a space-like curve and called it mass, but mass is not proportional to volume so that doesn't work. I really don't understand what that diagram is supposed to show.

 

The red spot in the gif is not moving slower, it is moving after us at the same speed.

 

It is, at the very least, moving faster than light which is a problem. You need to turn your diagram (the one with the moving red dot) into a thought experiment. The red dot is the comet that hit the earth 65 million years ago killing the dinosaurs. Presumably, the earth and the comet would be in the same place on any spacetime diagram 65 million years ago. According to your diagram, the comet would have missed the earth. Is that what happened? Does your diagram translate to reality?

 

For a short description of how a spacetime diagram works: http://www.astro.ucla.edu/~wright/st_diags.htm

 

For a more accurate, but more involved description: http://en.wikipedia.org/wiki/Minkowski_diagram

[michel123456]

Yes. The earth had mass yesterday, and the many days before. (...)

 

No, it's the same mass. The earth's mass yesterday is the same mass we see from the earth today. This should be intuitive if you don't obfuscate it.

 

Since you agree that it is the same mass, you should agree that the Earth is moving through time, and that the object "Earth" is not a line. Or is that simply a vocabulary problem?

 

 

 

Your diagram makes no sense to me. Space is for some reason not orthogonal to time,

in a 2d Space-time diagram, it is. I mean when you represent it on a sheet of paper, Time is a line, and 3d Space is a line orthogonal to time.

 

and you've marked off a space-like curve and called it mass, but mass is not proportional to volume so that doesn't work. I really don't understand what that diagram is supposed to show.

It is intended to show the amount of mass that travel through time. In this diagram, 3d space remains a line (not a surface), time is a line, and Mass is a line. Mass is independent of space.

 

 

 

It is, at the very least, moving faster than light which is a problem.

No it is not. An object moving faster than light has an angle of motion closer to the horizontal. The red spot has an angle closer to the vertical i.e. it moves at speed slower than SOL.

 

You need to turn your diagram (the one with the moving red dot) into a thought experiment. The red dot is the comet that hit the earth 65 million years ago killing the dinosaurs. Presumably, the earth and the comet would be in the same place on any spacetime diagram 65 million years ago. According to your diagram, the comet would have missed the earth. Is that what happened?

 

I guess hundreds of comets have missed the Earth during the last 65 millions years, the one you are talking about reached the target.

 

Does your diagram translate to reality?

That is a good question.

 

For a short description of how a spacetime diagram works: http://www.astro.ucla.edu/~wright/st_diags.htm

 

For a more accurate, but more involved description: http://en.wikipedia.org/wiki/Minkowski_diagram

 

Thank you for the links.

[Spyman]

How did the red spot get behind us on the time axis, if it is moving through time with our speed ?

 

You can have objects missing Earth because they are arriving late but the objects are not allowed to have trajectories where they leave the present. A normal object missing Earth would still be in our present time when it is in our past space location.

 

If you have an asteroid moving one day behind us in time entering the Earts spacetime location one day ago, it would have hit Earth yesterday, but it didn't do that yesterday when we where there, so you end up with a paradox. If it's impossible to change what did happen in our past then such trajectories must be impossible for us to observe too.

 

 

The MT surface don't make any difference since each slice of time can't notice or interact with things outside of their horizon, for every timestamp there would still only be one point mass for each object.

[Iggy]

I guess hundreds of comets have missed the Earth during the last 65 millions years, the one you are talking about reached the target.

 

You need to turn your diagram (the one with the moving red dot) into a thought experiment. The black dot is always in the same place in space. You can call it x=0, y=0, z=0. It is, according to your diagram, never anywhere else in space. It does not move in space.

 

Your diagram would have us believe that another object can occupy that same space (x=0, y=0, z=0), but at some special time where it won't touch the first object, yet the first object is never anywhere but (x=0, y=0, z=0). That is simply not how reality works. If an object, let's say the sun, is always in the same place in space in our coordinate system then no other object can occupy that space (past present or future) without touching it. Your diagram tells us that a meteor could occupy the space at the center of the sun yet not touch the sun simply because it happened yesterday. It makes no reasonable sense.

 

If time is represented by the vertical axis then the object should be a world line.

[michel123456]

You need to turn your diagram (the one with the moving red dot) into a thought experiment. The black dot is always in the same place in space. You can call it x=0, y=0, z=0. It is, according to your diagram, never anywhere else in space. It does not move in space.

 

Right.

 

Your diagram would have us believe that another object can occupy that same space (x=0, y=0, z=0), but at some special time where it won't touch the first object, yet the first object is never anywhere but (x=0, y=0, z=0).

 

You can say that's a point. It is what is happening when you sit on your chair doing nothing. You were there a few seconds ago, but you cannot see yourself a few seconds ago. Time has transported you. In common language, we say that time flows.

 

That is simply not how reality works.

 

We don't know that. Because the other object is not observable. The other (hypothetical) object would sit upon the world line, inside the light cone, totally unobservable for us. It is not more fantastic that what you propose: what you are saying is that the same object occupies the same space at 2 different times (and that we cannot see it, I say that), so that the mass of the object is spreaded through time.

 

 

If an object, let's say the sun, is always in the same place in space in our coordinate system then no other object can occupy that space (past present or future) without touching it. Your diagram tells us that a meteor could occupy the space at the center of the sun yet not touch the sun simply because it happened yesterday. It makes no reasonable sense.

I am not a reasonable person. I just looked at the space-time diagram. And Time is a weird thing. If you want to catch a ball, your hand need to be at the right four coordinates (x,y,z,t). The 3 space coordinates are not enough.

 

If time is represented by the vertical axis then the object should be a world line.

I still disagree, sorry for being so stubborn.

[Iggy]

We don't know that.

Yes, we know the sun was there yesterday. The sun's mass is a conserved quantity over time. It is provable by conservation of energy or Noether's theorem. But, honestly, if you've convinced yourself that the sun was missing yesterday then I suppose conservation of energy won't change your mind.

[michel123456]

Yes, we know the sun was there yesterday. The sun's mass is a conserved quantity over time. It is provable by conservation of energy or Noether's theorem. But, honestly, if you've convinced yourself that the sun was missing yesterday then I suppose conservation of energy won't change your mind.

 

We don't understand each other.

 

I didn't say that the sun was missing yesterday. I simply say that the sun is travelling with us through time. We cannot see the sun yesterday. We can only see the sun today, actually, not today at present time, but today a few minutes ago.

 

You wrote "The sun's mass is a conserved quantity over time." I didn't say anything different.

At my understanding, your position supposes that mass is an increasing value over time, meaning that yesterday's mass "exist" as today's mass.

--------------------------------------

Lets try it another way.

 

Our common ground is:

An object at rest of mass M and coordinates x,y,z, in time.

We have

 

M (0,0,0,1)

 

After a while, we have

 

M (0,0,0,2)

 

So simple

 

We both agree that spatial coordinates didn't change: the object M is not moving.

We both agree that only time coordinate has changed.

We both agree that M is one and only one same object. (I guess)

 

What I say is that M is the same object with other coordinates: the object M has "moved" "position" in time. That's the "moving dot" interpretation.

 

Are you saying something different? (to Iggy)

Edited April 7, 2010 by michel123456

[Iggy]

We don't understand each other.

 

That seems possible. I can explain how a space time diagram works.

 

A normal image shows no motion. In a flip book sort of way, motion can be shown by advancing the position of an object in each frame. In the following animation a ball is thrown into the air. The vertical axis measures height while each frame shows a change in position by advancing the time 1/2 seconds.

 

 

To find the time when the ball is at its peak height you would look at the fifth frame and find T=2. Two seconds after the ball was thrown directly upward into the air it reached its maximum height.

 

A different method of keeping track of time, and one which is very useful when considering relativity, is the space time diagram. Time is treated like another dimension so that an object's position can be shown in both space and time. The above animation could be equivalently diagramed like so:

 

 

The horizontal axis now represents time. A single position on the diagram shows a specific place and time--called an event. The ball is the curve and we can find the time that it reached it's maximum height by following that event down to the time axis.

 

A similar description is given half way down this text: http://www.physicsguy.com/ftl/html/FTL_part1.html.

 

Your moving dot diagram doesn't translate to reality the way the above diagram does. The black dot in your diagram is missing to the red dot. If the black dot were the sun and the red dot a spaceship then the spaceship could literally travel to the center of the sun (or where the sun would be) never finding it or touching it,

 

 

Your diagram depicts the idea that being one day before or after a specific time means that the sun is missing--gone--impossible to touch. Cleopatra, having lived before the black dot, must have had no sun. But, that is clearly not right--we know objects persist thorough time, so it just doesn't make sense.

[michel123456]

The ball is the curve

 

I am in complete disagreement.

 

-----------------------------

 

Your moving dot diagram doesn't translate to reality the way the above diagram does. The black dot in your diagram is missing to the red dot. If the black dot were the sun and the red dot a spaceship then the spaceship could literally travel to the center of the sun (or where the sun would be) never finding it or touching it,

 

No. In order to meet the sun, the spaceship must meet all 4 coordinates. In my diagram, the spaceship missed the sun, lets say a million years.

 

------------------------------

 

Your diagram depicts the idea that being one day before or after a specific time means that the sun is missing--gone--impossible to touch. Cleopatra, having lived before the black dot, must have had no sun. But, that is clearly not right--we know objects persist thorough time, so it just doesn't make sense.

 

No. My diagram depicts the idea that the sun & the black dot are 2 different objects, being at the same place at different times. Why is that bothering you so much?

---

Merged post follows:

Consecutive posts merged

You must think this guy is crazy.....

 

I'll try to resume our positions in a third way.

 

An object M at coordinates (0,0,0,1)

 

After a while the same object at coordinates (0,0,0,2)

 

What you say is that once the object has taken coordinates (0,0,0,1), you mark it with a red cross in the calendar, and no other object will never been able to occupy the same coordinates. Or if invent another object at the same coordinates, they crash together.

 

The same goes for second coordinates (0,0,0,2). Once occupied by the object M, there it is: the square on the chess board has been occupied. Or you mark a second red cross in the calendar. And so on.

 

What I say is that the object M moves from one set of coordinates to another. It means, when the second square of the chess board is occupied, the first is free.

 

I feel like trying explaining motion.

Am I so wrong?

Edited April 7, 2010 by michel123456

[Spyman]

Did you miss my post #80 ?

 

If the Earth 'remains' in the past and someone with a timemachine goes there, what would happen ?

 

A lot of people view time as if they could travel back then they would physically be able to interact with their own old self.

 

Either the timetraveler creates a paradox or the timeline is forced to change, like in the movie Back to the Future.

Edited April 8, 2010 by Spyman
Spelling

[michel123456]

Did you miss my post #80 ?

 

 

Yes, actually I missed it.

---

Merged post follows:

Consecutive posts merged

How did the red spot get behind us on the time axis, if it is moving through time with our speed ?

 

If you look carefully, you will see that the red spot is always one step behind the black one. The horizontal projection of the red spot upon the time axis maintains a standard gap with the black dot.

 

You can have objects missing Earth because they are arriving late but the objects are not allowed to have trajectories where they leave the present. A normal object missing Earth would still be in our present time when it is in our past space location.

That's the question. We can only observe, not the present, but a particular part of the past: the light-cone (our light-cone).

 

If you have an asteroid moving one day behind us in time entering the Earts spacetime location one day ago, it would have hit Earth yesterday, but it didn't do that yesterday when we where there, so you end up with a paradox. If it's impossible to change what did happen in our past then such trajectories must be impossible for us to observe too.

It is impossible to observe, O.K. And you are probably right, it may be a paradox. But in this case, you have to admit that instead of the object I proposed, the Earth is there. And is this case, Earth's mass is distributed in the past, see the MT surface.

 

 

The MT surface don't make any difference since each slice of time can't notice or interact with things outside of their horizon, for every timestamp there would still only be one point mass for each object.

 

Yes. and no. There would be a difference, because a slice of MT in the past can influence other objects upon the light-cone. See object A in Iggy's first diagram.

[Iggy]

I'll try to resume our positions in a third way.

 

An object M at coordinates (0,0,0,1)

 

After a while the same object at coordinates (0,0,0,2)

 

That is essentially correct.

 

What you say is that once the object has taken coordinates (0,0,0,1), you mark it with a red cross in the calendar, and no other object will never been able to occupy the same coordinates.

 

You got your tenses mixed up, but it's essentially correct. It is equivalent to saying: "I occupied the center of central park on December 31, 1999 and no other object can occupy the center of central park on December 31, 1999 without having touched me." In other words, if something occupies (0,0,0,1) then that specific place and time is taken.

 

I've depicted as clearly as I think I can the consequences of having an object be represented as a moving dot along a time axis,

 

 

If the sun were a moving dot as you advocate then we would be very lucky to live in the one time with its light. A more immediate problem for our ancestors would be the missing earth. I'm sorry, Michel. I wish I could say your idea worked.

[Spyman]

How did the red spot get behind us on the time axis, if it is moving through time with our speed ?

If you look carefully, you will see that the red spot is always one step behind the black one. The horizontal projection of the red spot upon the time axis maintains a standard gap with the black dot.

Then when/where was the red spot at the event of the Big Bang ?

 

My point here is that I don't think such objects exists naturally and if they do exist they can't interact with us in any way at all. The only way to find out would be to use a timemachine but that might not be possible either.

So until such objects can be observed or at least explain something in theory, it is more convenient to remove not necessary phenomenon from our models.

 

If you make a thought experiment with a hypothetical timemachine represented by the red spot, that would be fine but assuming that the Universe is filled with objects like that would only seem 'messy'.

 

 

You can have objects missing Earth because they are arriving late but the objects are not allowed to have trajectories where they leave the present. A normal object missing Earth would still be in our present time when it is in our past space location.

That's the question. We can only observe, not the present, but a particular part of the past: the light-cone (our light-cone).

No no no, I don't think thats questionable.

 

We can observe objects and then calculate where they are supposed to be at the present, later on we can observe them again and check if our model is correct. So far we have NEVER observed any object with trajectories leaving or entering our present to/from the past or the future.

 

I don't think timetravel is completely ruled out but those trajectories is certainly not something that normally happens around us.

 

 

If you have an asteroid moving one day behind us in time entering the Earts spacetime location one day ago, it would have hit Earth yesterday, but it didn't do that yesterday when we where there, so you end up with a paradox. If it's impossible to change what did happen in our past then such trajectories must be impossible for us to observe too.

It is impossible to observe, O.K. And you are probably right, it may be a paradox. But in this case, you have to admit that instead of the object I proposed, the Earth is there. And is this case, Earth's mass is distributed in the past, see the MT surface.

If a timetraveler would go back in time and find Earth there as it was, then Earths mass is 'distributed in the past', but it would not make any observable difference on objects in the spacetime diagram.

 

 

The MT surface don't make any difference since each slice of time can't notice or interact with things outside of their horizon, for every timestamp there would still only be one point mass for each object.

Yes. and no. There would be a difference, because a slice of MT in the past can influence other objects upon the light-cone. See object A in Iggy's first diagram.

There is no difference because objects are supposed to influence each other.

 

We are influenced by signals from objects in the past and they in turn are also influenced by signals from objects in their past, which in turn are influenced by signals from objects in their past and so on, there is nothing strange with that and it don't make any difference whether the objects remain in the past or not either, since it is the signals from them that are influencing us, not the objects themself.

 

Let's take a look at Iggy's diagram again and revive what happens:

The star explodes at A. The light first reaches Planet X at B 1.5 million years ago then reaches Earth at C 1 million years ago. Event B is then seen by Earth at D--the here and now.

- At event A a Star goes nova, sending out EM and the reconfiguration of mass sends out new strength of gravity.

- At event B both the EM and the new gravity strength from the nova reaches Planet X.

- At event C both the EM and the new gravity strength from the nova reaches Earth.

- At event D we can observe how the EM lights up Planet X and how the new gravity strength affected Planet X.

 

Now tell me in which of the events would either we or the aliens be able to observe or measure more than one of each of the other objects ?

 

And for the third time what would be different if the objects remain in the past or not ?

 

 

----------

 

If the sun were a moving dot as you advocate then we would be very lucky to live in the one time with its light. A more immediate problem for our ancestors would be the missing earth.

Iggy consider if EVERY objects move through time towards the future and are not allowed to have trajectories where they leave the present, then the lightrays from the Sun will continue to shine on Earth, since both the Sun, the photons and the Earth would travel towards the future. And even though the Earth was there when our ancestors rised and started to walk, today both the remains of our ancestors and the younger Earth has moved on and are here, in the present.

 

The only difference we would notice are if we used a timemachine to go back and look.

 

----------

 

 

From Iggy's last post I think we can recapitulate our standpoints as follows:

 

- I think it's impossible to distinguish whether objects move through time or remains in the past, without a timemachine or other not yet available technologies or knowledge.

(Although I prefer to view objects as "moving dots".)

 

- Michel thinks it is possible to differentiate between the two concepts but doesn't seem to have an idea or be able to explain how that could be done.

(Michel tend to conclude that objects are not "lines", but "moving dots".)

 

- Iggy thinks that it is possible to deduce which is correct of the two concepts and is certain that if something occupies a location sometime then that specific place and time is taken forever.

(Iggy is convinced that objects are "lines".)

 

All three of us agree that a fixed dot in a spacetime diagram is an event and that the objects either are the world line or moving dots that must follow it.

 

 

Your turn...

Edited April 9, 2010 by Spyman
Spelling

[Iggy]

The only difference we would notice are if we used a timemachine to go back and look.

 

You would think that traveling back in time to see if there is a sun and earth would reveal the truth of this. So, I might go back a couple thousand years and stand next to Cleopatra and find out if the sun and earth are there. That would settle the issue for you. But, how is that different from what Cleopatra saw? Can I not take her word on this?

 

The sun was here yesterday and two thousand years ago and we don't need a time machine to prove it. The consequences of this moving dot idea are shown very clearly in the diagram in my last post. The consequences are incompatible with observation.

 

(Iggy is convinced that objects are "lines".)

 

The entire physics community is convinced of that. It is explained in the links I've given through the thread. There is a reason a world line is a line. There is a reason Minkowski made it so. Conservation of energy and all of our human observations demand it.

[toastywombel]

The entire physics community is convinced of that. It is explained in the links I've given through the thread.

 

It is a rather bold statement to say the entire physics community is behind the idea that objects are lines. Furthermore, I looked at your links and they don't imply that.

 

I think you are making the jump in believing that the past tangibly exists somewhere, when in reality all we see is the light that has reflected off of what once was.

[Iggy]

It is a rather bold statement to say the entire physics community is behind the idea that objects are lines.

 

1. an 'object' as it is typically defined is represented as a world-line on a spacetime diagram.

 

2. a world-line is a line.

 

3. the physics community is quite convinced of 1 and 2.

 

Furthermore, I looked at your links and they don't imply that.

 

They say it explicitly.

 

In physics, a world line of an object (approximated as a point in space, e.g., a particle or observer) is the sequence of spacetime events corresponding to the history of the object. A world line is a special type of curve in spacetime. Below an equivalent definition will be explained: A world line is a time-like curve in spacetime. Each point of a world line is an event that can be labeled with the time and the spatial position of the object at that time.

 

For example, the orbit of the Earth in space is approximately a circle, a three-dimensional (closed) curve in space: the Earth returns every year to the same point in space. However, it arrives there at a different (later) time. The world line of the Earth is helical in spacetime (a curve in a four-dimensional space) and does not return to the same point.

 

http://en.wikipedia.org/wiki/World_line

 

I think you are making the jump in believing that the past tangibly exists somewhere

 

That is incorrect. I have not said and not implied that past events exist somewhere (by which I assume you mean some place). I have refuted the idea that world-lines don't exist--that objects can be coherently represented as 'moving dots' on a space time diagram.

 

when in reality all we see is the light that has reflected off of what once was.

 

I agree with that.

[toastywombel]

I apologize for misunderstanding your posts. I attempted to skim through a little to fast. On a totally unrelated small note though,

 

For example, the orbit of the Earth in space is approximately a circle, a three-dimensional (closed) curve in space: the Earth returns every year to the same point in space. However, it arrives there at a different (later) time. The world line of the Earth is helical in spacetime (a curve in a four-dimensional space) and does not return to the same point.

 

I think it important to point out that the orbit of the Earth is not approximately a circle.

[michel123456]

That was a very interesting discussion. I have to admit Iggy has strong arguments.

Now I am more balanced than ever.

 

But:

_ I don't know if it is correct to use a spacetime diagram in such a way Iggy presented it. A spacetime diagram is relative to the observer, it is not an absolute representation. If cleopatra were to make her spacetime diagram, it would not be different from ours, the event "sun" would be at the same position on her diagram, and for her the sun would be travelling through time with her, as it is for us. Does that mean that the sun actually "exist" in the past, that's the question, IMO still unanswered. Nowadays, Iggy made a point and his diagram is clear enough to make me think again (& again).

 

_ Nowhere in the links provided by Iggy there is the sentence "an object is a line". All I read can be interpretated as a trajectory as well. I don't think the concept of trajectory can be confused with the object itself.

 

_you could resume simply the question by a simple statement: the trajectory of an object through time IS (or IS NOT) the object.

 

_at least, the first question of this thread has been answered several times.

Can we see it? the answer is: NO.

 

-----------------------------------------

-----------------------------------------

 

Iggy your last diagram was tricky.

 

You have inserted half of your argumentation into mine to prove I am wrong.

Explaining:

In the moving dot interpretation, there is only ONE dot moving through time. There is no other dot behind with Cleopatra. The second and third dots are part of your interpretation as still "existing in the past".

In the moving dot interpretation, we cannot observe if there is a second hypothetical dot behind us, and if there is such a thing, it is not the Earth but something else.

Edited April 10, 2010 by michel123456

[Iggy]

I have to admit Iggy has strong arguments.

 

Thank you.

 

A spacetime diagram is relative to the observer, it is not an absolute representation.

 

I agree, though I think it might be most accurate to say that the diagram is relative to an observer's reference frame or an observer's velocity--or, the velocity of an observer's reference frame, I suppose.

 

If cleopatra were to make her spacetime diagram, it would not be different from ours, the event "sun" would be at the same position on her diagram, and for her the sun would be travelling through time with her, as it is for us.

 

Right. The event "sun emits photon" (the photon which Cleopatra observes) is in the same position relative to Cleopatra as "sun emits photon" (the one which we observe) is relative to us.

 

But, also, a spacetime diagram shows more than one time. You might say that it shows more than one present, or more than one hypersurface of an observer's present. So, on a single diagram you can represent both Cleopatra's present and our present. You can show the past light cone of both Cleopatra and us--on the same diagram. On such a diagram the sun and the earth and any other object must intersect both light cones to be consistent with observation.

 

Does that mean that the sun actually "exist" in the past, that's the question, IMO still unanswered.

 

We can prove that the sun was here yesterday and in the distant past with conservation of energy or Noether's theorem or we can prove it through direct evidence. Blue-green algae was using photosynthesis billions of years ago.

 

Nowhere in the links provided by Iggy there is the sentence "an object is a line". All I read can be interpretated as a trajectory as well. I don't think the concept of trajectory can be confused with the object itself.

 

But, regardless, either an object's path through spacetime is a curve or a moving dot. I don't know if the universe is really four dimensional in a philosophical sense, but I know that when it is represented that way with a spacetime diagram an object's path cannot be a moving dot. It must be a line or a curve. The two different representations have different observational consequences.

 

In other words, I don't need to argue that "an object is a line", only that it must be represented that way in 4 dimensional spacetime.

 

You have inserted half of your argumentation into mine to prove I am wrong.

Explaining:

In the moving dot interpretation, there is only ONE dot moving through time.

 

In your diagram in post 73 there are two moving dots--a black one and a red one. I don't believe I have misinterpreted your meaning. I believe my diagram follows logically from yours.

 

There is no other dot behind with Cleopatra. The second and third dots are part of your interpretation as still "existing in the past".

 

Again, there is a moving dot behind the black one in your diagram, both representing objects and both moving. You said the difference between the red object and the black object is that the red object was a million years before the black object. Cleopatra was 2,000 years before us, so I think she would belong where I have put her.

 

But, this does reveal the problem. Your diagram has two different kinds of 'before'. The problem is that time is twice represented in the moving dot diagram. The difference from one frame to the next is the advancement of time. The difference from one point along the vertical axis to the next is also the advancement of time. But, time is not a two dimensional construct and can't properly be represented that way to be consistent with observation.

 

If you show an object's trajectory over time as an animation with multiple frames like wikipedia's image here,

 

 

then there is no time axis and there shouldn't be one. On the other hand, if you have a time axis like wikipedia's image here,

 

 

then there is no animated movement of an 'object' or multiple frames showing the movement of an object. The passage of time is shown with the vertical axis. It is a problem to have both the time axis and the animated, moving, object--not simply redundant, but a misrepresentation.

 

I think it's fine to think of a particle as a moving dot, it's fine to diagram it that way, but not on a spacetime diagram.

 

I think it's also fine to make an animated spacetime diagram like I drew here,

 

 

But, we must recognize that the moving dot is an event, not an object. The path of an object (ie particle or material point) through spacetime still must be represented with a line.

Edited April 11, 2010 by Iggy
changed 'dot' to 'object'

[michel123456]

It is quite funny that after such disagreement, we agree on one thing:

 

The path of an object (ie particle or material point) through spacetime still must be represented with a line.

 

As for the rest

In my diagram the red & black dots are different objects.

 

And about Cleopatra, I can't say, but about Tutankhamun, he still belongs to present. You can go to Egypt tomorrow and see his body is there, not in the past only. He travelled through time. like the Earth, he cannot be duplicated in present & in the past. There is only one. I guess you agree with me on that.

 

I could never imagine someone could represent the flow of time as a lined-object. I though it was quite an evidence that we are "moving" through time, since time is considered as "something of the same nature with space". Space & time are made of the same "stuff". The idea that we are travelling was so evident to me that I was totaly surprised with your objections at the beginning of this thread.

 

Maybe due to surprise, I am still unconvinced. It is difficult for me to accept that an object "still exist in the past". But I'll think about it again...

 

On the other hand I agree that the "moving dot" concept is like inserting time twice: it is "motion inside time". And motion has the concept of time already in, so it must be redundant.

 

Having no other argument to provide I will temporarily agree with the line concept.

So you say

 

I have not said and not implied that past events exist somewhere

 

and

 

"I occupied the center of central park on December 31, 1999 and no other object can occupy the center of central park on December 31, 1999 without having touched me." In other words, if something occupies (0,0,0,1) then that specific place and time is taken.

 

Both statements don't seem to be coherent.

Explaining: if the coordinates (0,0,0,1) are taken, there you are.

And there you are in the present reading this post.

So I understand that there are two Iggies, one in the past at coordinates (0,0,0,1), and another one at coordinates (0,0,0,t), in your coordinate system.

[Iggy]

And about Cleopatra, I can't say, but about Tutankhamun, he still belongs to present. You can go to Egypt tomorrow and see his body is there, not in the past only. He travelled through time. like the Earth, he cannot be duplicated in present & in the past. There is only one. I guess you agree with me on that.

 

Right. So, Tutankhamun accomplishes what the red dot never can.

 

 

In that diagram the red and black objects can never interact. They could both persist for billions of years in the same location but never would the one be able to touch the other. In reality objects separated by time need only persist for the duration of that separation and they will be able to interact.

 

If the sun is five billion years old then an object 50 million years ago could not occupy the center of the sun's location without touching it.

 

I could never imagine someone could represent the flow of time as a lined-object.

 

I understand. A world-line is just what happens when time is made a dimension and diagrammed with space. Since the object exists at each point along the time axis a point object represented as a dot in three dimensional space becomes a series of dots making up a line in spacetime. Minkowski introduced the idea in 1908.

 

The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality...

 

A point of space at a point of time, that is, a system of values, x, y, z, t, I will call a world-point. The multiplicity of all thinkable x, y, z, t systems of values we will christen the world... Not to leave a yawning void anywhere, we will imagine that everywhere and everywhen there is something perceptible. To avoid saying "matter" or "electricity" I will use for this something the word "substance". We fix our attention on the substantial point which is at the world-point x, y, z, t, and imagine that we are able to recognize this substantial point at any other time. Let the variations dx, dy, dz of the space co-ordinates of this substantial point correspond to a time element dt. Then we obtain, as an image, so to speak, of the everlasting career of the substantial point, a curve in the world, a world-line, the points of which can be referred unequivocally to the parameter t from -∞ to +∞. The whole universe is seen to resolve itself into similar world-lines, and I would fain anticipate myself by saying that in my opinion physical laws might find their most perfect expression as reciprocal relations between these world-lines.

http://www.spacetimesociety.org/minkowski.html

 

I though it was quite an evidence that we are "moving" through time, since time is considered as "something of the same nature with space". Space & time are made of the same "stuff". The idea that we are travelling was so evident to me that I was totaly surprised with your objections at the beginning of this thread.

 

My opinion was, and still is, that you are mixing up events and objects. For example, you said a couple posts ago "at least, the first question of this thread has been answered several times. Can we see it? the answer is: NO."

 

If "it" refers to an event inside our past light cone then, no, it may not be directly observable. If "it" refers to mass or an object or a material substance then the answer is "yes" we can see it. We can directly observe it.

 

I think a good example is that we cannot directly observe Neil Armstrong walking on the moon (an event inside our past light cone), but we can see the mass of Neil Armstrong and the mass of the moon. The objects are visible--the past event may not be. By conflating events and objects one could come to the conclusion that mass can hide inside our past light cone or that a pair of massive particles in the same location can never come together and touch. Those are good conclusions for events, but bad conclusions for objects.

 

Maybe due to surprise, I am still unconvinced. It is difficult for me to accept that an object "still exist in the past". But I'll think about it again...

 

Believe me, I understand. Your view would be called presentism as opposed to eternalism. But, that's ok. I don't want to argue against presentism or for eternalism, only assure that you diagram spacetime correctly. A moving dot on a spacetime diagram that represents an object is inconsistent with observation regardless of the philosophical arguments.

 

So you say

I have not said and not implied that past events exist somewhere

and

"I occupied the center of central park on December 31, 1999 and no other object can occupy the center of central park on December 31, 1999 without having touched me." In other words, if something occupies (0,0,0,1) then that specific place and time is taken.

Both statements don't seem to be coherent.

Past events don't exist somewhere (by which I mean some place). They existed at a place and time. I existed in the center of central park on December 31, 1999--a place and time--an event--(x,y,z,t).

 

To say that past tangibility exists somewhere would mean to me that I can currently interact with the past event. But, I don't believe that. I can't affect Neil Armstrong walking on the moon. I have not implied that. Past events are not currently tangible.

 

Explaining: if the coordinates (0,0,0,1) are taken, there you are.

And there you are in the present reading this post.

So I understand that there are two Iggies, one in the past at coordinates (0,0,0,1), and another one at coordinates (0,0,0,t), in your coordinate system.

 

Ok, but again, Iggy reading a post is an event. Iggy getting drunk in central park in 1999 is an event. An event is a single place and time, an x, y, z, t, a dot on a space-time diagram. There are two Iggy-events along my world-line and making up my world-line, but that does not imply that there are two Iggies.

 

Since an object is represented with a world-line, saying that there are two objects (e.g. two Iggies) would imply that there are two world-lines--not a single world-line with multiple events. A world-line is, itself, a sequence of events. Neil Armstrong experiencing the moon is an event--one of many that make up the history of that Neil Armstrong object.

 

I should apologize, I meant to keep this post short and not drag out any disagreement. I failed miserably and ended up rambling. Sorry.

Edited April 11, 2010 by Iggy

[michel123456]

To say that past tangibility exists somewhere would mean to me that I can currently interact with the past event. But, I don't believe that. I can't affect Neil Armstrong walking on the moon. I have not implied that. Past events are not currently tangible.

 

Lets slightly disagree on this:-).

 

We are constantly interacting with past event. The entire observable universe is made of past events.

 

What you mean is that we (today on Earth) cannot affect Neil Armstrong walking on the Moon, when you say "I can't affect Neil Armstrong walking on the moon." But another star far away is interacting with N.A. walking on the Moon. A star we can see today.

 

We cannot interact with N.A. because, as you said, "(...)an event inside our past light cone then, no, it may not be directly observable." Speaking for events.

 

You believe that:

I am messing events with objects.

 

I believe that:

You are messing trajectories with objects.

 

Can we, at least agree on something, which was the idea of this thread:

 

the events placed in the inside part of the light-cone are not directly observable.

[Iggy]

To say that past tangibility exists somewhere would mean to me that I can currently interact with the past event. But, I don't believe that. I can't affect Neil Armstrong walking on the moon. I have not implied that. Past events are not currently tangible.

 

Lets slightly disagree on this:-).

 

We are constantly interacting with past event.

 

Yes, we are causally connected to past events in our past light cone. Clearly, they can affect us now. For example, Neil Armstrong walking on the moon has affected the way I write this sentence. Any event where ct > x (where t is time before my present and x is distance from my location) can potentially affect me here and now.

 

We cannot, on the other hand, affect any event outside our future light cone and certainly not any event inside our past light cone which is what I was meaning to convey--that we can't affect the event in question.

 

The entire observable universe is made of past events.

 

Certainly the events we observe existed in our past.

 

What you mean is that we (today on Earth) cannot affect Neil Armstrong walking on the Moon, when you say "I can't affect Neil Armstrong walking on the moon."

 

Yes.

 

But another star far away is interacting with N.A. walking on the Moon. A star we can see today.

 

Every sufficiently old star in Neil's observable universe could conceivably affect him while walking on the moon because they all intersect his past light cone. The moon event could likewise affect any star inside his cosmological event horizon. If the universe is matter or radiation dominated this would include the whole universe (more than just the observable universe). If the future evolution of the universe is dominated by the cosmological constant then it would include only the part of the universe bound by Neil's cosmological event horizon.

 

But, I was referring specifically to my present. No event in my present can affect the Neil Armstrong event. The past is not tangible in that sense. It cannot be changed.

 

We cannot interact with N.A. because, as you said, "(...)an event inside our past light cone then, no, it may not be directly observable." Speaking for events.

 

I was not trying to imply anything about the event's affect on us, only that we (and any other event in our present) cannot affect the event. There may be a different observer out there in our present for whom Neil Armstrong walking on the moon is in their present, like would happen here:

 

 

But, they cannot affect the event. No event in our present can cause or affect an event in our past light cone.

 

I believe that:

You are messing trajectories with objects.

 

Like I've shown, while a trajectory in space can be diagrammed with a moving dot, an object's path through spacetime cannot. Two kinds of diagrams have been mistakenly combined into a kind of spacetime diagram which you will find nowhere in scientific literature that has observational consequences which have been demonstrated false.

 

A world-line is a well established scientific concept and using it doesn't amount to mixing up trajectories and objects.

 

Can we, at least agree on something, which was the idea of this thread:

 

the events placed in the inside part of the light-cone are not directly observable.

 

Yes, I agree, that is usually true in astronomy. You would have to make exceptions where the index of refraction of the medium is greater than unity (like light traveling through air) or where something is observed by some other means than light (like the observations of a bat or a submarine) as those examples would always observe events inside their past light cone. But, for the purposes of astronomy I absolutely agree--we observe the events on the surface of our past light cone.