Suppose that I am looking in my telescope to a distant star

[michel123456]

Now please correct me:

The experiment shows that the Earth 2,5 sec ago was the same object that it is today, but at another position in spacetime.

That is kind of trivial. It simply says that the Earth has moved.

 

The experiment does not show whether there "exist" in the past another "past-earth" at T=0. In fact. it shows that such an hypothetical "T0-past-earth" does not exist because it would superpose to today's Earth.

And since we don't see such a superposition happening, it must mean that the "T0-past-earth" does not exist.

What happened is that the Earth changed coordinates in spacetime.

 

Again, it sounds trivial but it is not.

[John Cuthber]

So the Moon & Earth are almost acting like a single object.

I see also that the reflectors on the Moon are not parallel to the surface (they are not horizontal on the Moon)

 

http://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment#mediaviewer/File:ALSEP_AS14-67-9386.jpg

http://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment#mediaviewer/File:Apollo_11_Lunar_Laser_Ranging_Experiment.jpg

The Moon is roughly a sphere.

Any plane is parallel to two points on the Moon's surface.

The reflectors were placed so they faced the Earth., that means they were pretty much parallel to the Moon's surface at the point of the Moon nearest to the Earth.

Of course, in practice, the reflectors were not plane mirrors, but corner cube arrays.

Who cares?

You can bounce a radar signal off the Moon, without needing to put a mirror on it.

Edited November 22, 2014 by John Cuthber

[davidivad]

you are correct.

the earth is the same earth just in different locations.

frames of reference are additive.

this builds up as a value that i would consider a distance.

the question of the thread is a consequence of the answer.

Edited November 22, 2014 by davidivad

[MigL]

I'm beginning to see an 'agenda' to Michel123456's OP.

[Strange]

I see also that the reflectors on the Moon are not parallel to the surface (they are not horizontal on the Moon)

 

That doesn't matter because the reflector consists of an array of retro-reflectors so the light is always reflected back in the direction it came from. (And the dispersion is far greater than the relative motion of the Earth and Moon.)

The experiment shows that the Earth 2,5 sec ago was the same object that it is today, but at another position in spacetime.

 

It doesn't say anything about the Earth being the same object. That is some sort of metaphysical idea that isn't amenable to scientific tests. Rather like the Ship of Theseus or Trigger's Broom.

 

All the experiment does is measure the distance between something we choose to call "the Moon" and something else we call "the Earth". It says nothing about the existential nature of those things (or whether anything exists outside our own thoughts).

 

I think you might be overthinking the whole thing.

[michel123456]

 

 

I think you might be overthinking the whole thing.

Am I?

[Strange]

Am I?

 

Apparently.

[swansont]

How do we reflect light from the Moon?

There is an approximate 2 sec gap with the Moon. That means 4 sec to go forth and back. During those 4 sec the Earth & Moon have moved respectively. How do we catch back the signal? from another location?

 

The light spreads out. The beam is around 6.5 km wide by the time it hits the moon, and the reflection from the corner cube array undergoes a similar expansion on the way back. The moon's orbital speed is 1 km/s, so we're still within the extent of the expanded beam.

[michel123456]

 

Apparently.

 

Do you realize how empty is spacetime if one agrees with me?

 

If I say that the coordinates x=0,y=0,z=0,T=0 contain the Earth

it means that coordinates x=0,y=0,z=0,T=-2000000 do NOT contain the Earth.

 

And if coordinates x=0,y=0,z=0,T=-2000000 do contain the Earth

then

coordinates x=0,y=0,z=0,T=0 do NOT contain the Earth

 

It is not metaphysical.

Because if I am correct, there is a lot (a lot) of coordinates able to contain materials that we are unable to observe. And in this case, the universe has much much more than that we can see.

Edited November 22, 2014 by michel123456

[ACG52]

 

If I say that the coordinates x=0,y=0,z=0,T=0 contain the Earth

it means that coordinates x=0,y=0,z=0,T=-2000000 do NOT contain the Earth.

 

 

The fact that the Earth exists at T=0 does not preclude it from existing at other points of the T continuum.

Edited November 22, 2014 by ACG52

[Strange]

 

Do you realize how empty is spacetime if one agrees with me?

 

If I say that the coordinates x=0,y=0,z=0,T=0 contain the Earth

it means that coordinates x=0,y=0,z=0,T=-2000000 do NOT contain the Earth.

 

And if coordinates x=0,y=0,z=0,T=-2000000 do contain the Earth

then

coordinates x=0,y=0,z=0,T=0 do NOT contain the Earth

 

It is not metaphysical.

Because if I am correct, there is a lot (a lot) of coordinates able to contain materials that we are unable to observe. And in this case, the universe has much much more than that we can see.

 

That makes zero sense.

[Mordred]

Sure it does. Your 4d coordinate system has the Earth at one location. However if you wait say 5 months it will not be at the same x,y and z location. Nor will it have the same time coordinate. The Earth is not a static object in space.

 

In point of fact there is no static objects in space. Everything moves.

 

The speed of light and the speed of information is the same c.

 

If you had a rigid rod. 1 lightyear long. If you move one end of that rigid and solid rod. It will take one light year before the other end moves.

 

So even if the Earth and moon were one connected object. Which it isn't any form of interaction or influence is still limitted to the speed of light. Even the four forces are limitted by the speed of light.

 

So if the sun were to suddenly vanish it would take 8 minutes before the Earth to start drifting away into a straight trajectory. This is what GR teaches us.

 

Google rigid rod GR for more details

http://arxiv.org/abs/1105.3899

 

Here is a technical review on the rigid rod paradox

[Strange]

It is not metaphysical.

Because if I am correct, there is a lot (a lot) of coordinates able to contain materials that we are unable to observe. And in this case, the universe has much much more than that we can see.

 

OK. Maybe I take back my earlier comment.

 

You seem to be saying that we cannot see all of space-time. We can't, for example, see the future. And we can't see things which are so far away that the light hasn't reached us yet. Nor can we see beyond event horizons. And so on.

 

You are right. That is not metaphysical. It is just painfully obvious.

[pzkpfw]

If I look out my window at 1 p.m. and see my cat on the BBQ table, then I look out again at 2 p.m. and see it on the bird bath, that simply means it's location (time and space has changed).

 

It doesn't imply there's a bunch of other cats outside that I just haven't seen.

 

(And any cat so far away that light from it hasn't reached me "yet", will have had no other effect on me.)

Edited November 23, 2014 by pzkpfw

[Ophiolite]

If you had a rigid rod. 1 lightyear long. If you move one end of that rigid and solid rod. It will take one light year before the other end moves.

Just so we are clear: your rigid rod is purely hypothetical, yes? No such thing can exist in reality. The propagation of movement through the rod would be at the speed of sound in the material of the rod and not the speed of light. Agreed?

[swansont]

Just so we are clear: your rigid rod is purely hypothetical, yes? No such thing can exist in reality. The propagation of movement through the rod would be at the speed of sound in the material of the rod and not the speed of light. Agreed?

 

I think it's an ideal rigid rod. The upper limit for information transfer, including compression, is c.

[MigL]

Pzkpfw's cat is at the BBQ at 1pm and will always be there. That point is 'fixed' in space-time.

Pzkpfw's cat is at the Birdbath at 2pm and will always be there. That point is also 'fixed' in space-time.

Space-time events do not move, they are fixed. There is nowhere/nowhen else for them to move to.

The cat follows a world line along the time axis. Actually more of a cat shaped world 'tube'.

We've had this discussion before Michel123456. This is what GR tells us.

 

I'm guessing it wasn't to your satisfaction

[michel123456]

Pzkpfw's cat is at the BBQ at 1pm and will always be there. That point is 'fixed' in space-time.

Pzkpfw's cat is at the Birdbath at 2pm and will always be there. That point is also 'fixed' in space-time.

Space-time events do not move, they are fixed. There is nowhere/nowhen else for them to move to.

The cat follows a world line along the time axis. Actually more of a cat shaped world 'tube'.

We've had this discussion before Michel123456. This is what GR tells us.

 

I'm guessing it wasn't to your satisfaction

Your guess is right.

 

Because a cat is an object that extends in space (about 30cm) it also extends in time.

If your cat "remains in time" then you can calculate that under a specific time interval, the 2 cats should be superposing.

 

 

 

Pzkpfw's cat is at the BBQ at 1pm.

One nanosecond after 1pm you must have 2 cats superposing each other.

The same way as the 2 Earths at 2 time stamps in my diagram.

Is that possible?

[swansont]

 

Pzkpfw's cat is at the BBQ at 1pm.

One nanosecond after 1pm you must have 2 cats superposing each other.

The same way as the 2 Earths at 2 time stamps in my diagram.

Is that possible?

 

No, it's not. Thus, your premise (or straw man, not sure which it is) about how time works is incorrect.

[michel123456]

 

No, it's not. Thus, your premise (or straw man, not sure which it is) about how time works is incorrect.

How does time work then?

Pzkpfw's cat is at the BBQ at 1pm and will always be there. That point is 'fixed' in space-time.

Pzkpfw's cat is at the Birdbath at 2pm and will always be there. That point is also 'fixed' in space-time.

 

 

 

MigL believes that there are many cats along the time line, if I understand clearly his point.

Is that the way time works?

[swansont]

How does time work then?

 

That's an excellent metaphysical question to ponder, if one wished to ponder metaphysical questions. Don't hold your breath waiting for an answer, though. This being in a physics subforum, it's not really on-topic. What would be on-topic is how time behaves, which is described by relativity.

[michel123456]

 

That's an excellent metaphysical question to ponder, if one wished to ponder metaphysical questions. Don't hold your breath waiting for an answer, though. This being in a physics subforum, it's not really on-topic. What would be on-topic is how time behaves, which is described by relativity.

How does time behave then?

 

Are there multiple cats along the time line?

Or is there only one single cat traveling through time and space?

 

I don't understand why it is so difficult to answer this question. When it comes to space only, everyone agrees on what motion is: it is a change of coordinates.

But when it comes to time, then everything changes and people are ready to believe that objects do not change coordinates in time, instead they believe that objects remain on their coordinates.

I find that absolutely grotesque.

[Strange]

Because a cat is an object that extends in space (about 30cm) it also extends in time.

...

Pzkpfw's cat is at the BBQ at 1pm.

One nanosecond after 1pm you must have 2 cats superposing each other.

 

You are contradicting yourself: there is one cat extended in time.

[pzkpfw]

How does time behave then?

 

Are there multiple cats along the time line?

Or is there only one single cat traveling through time and space?

That's the standard idea. There's one cat (in my back yard). It changes position, e.g. BBQ table to birdbath. It also changes time, e.g. it was somewhere at 1 p.m. and it was somewhere at 2 p.m. If it was very lazy, it might stay in the same position, but be there at those two times.

 

I don't understand why it is so difficult to answer this question. When it comes to space only, everyone agrees on what motion is: it is a change of coordinates.

But when it comes to time, then everything changes and people are ready to believe that objects do not change coordinates in time, instead they believe that objects remain on their coordinates.

I find that absolutely grotesque.

Who says that? We all change coordinates in time. I woke up at 6:30 a.m. this morning, now here I am at my PC at 7:23 a.m. My position in time and space has changed.

 

I do note your earlier comment on the size of the cat (30 cm). Position (and physical extent) is measured in 3 dimensions, so the cat (an assembly of atoms, and what they're made of) has extent in height, width and depth. But time has a single dimension, so there's no equivalent "time extent" for the cat. It's somewhere at some time. A nanosecond later, that one cat may be in the "same" place (depending on what measured against) or it might have moved. Its space and time coordinates can have changed. But it's still one cat.

Edited November 23, 2014 by pzkpfw

[michel123456]

That's the standard idea. There's one cat (in my back yard). It changes position, e.g. BBQ table to birdbath. It also changes time, e.g. it was somewhere at 1 p.m. and it was somewhere at 2 p.m. If it was very lazy, it might stay in the same position, but be there at those two times.

 

 

Who says that? We all change coordinates in time. I woke up at 6:30 a.m. this morning, now here I am at my PC at 7:23 a.m. My position in time and space has changed.

 

I do note your earlier comment on the size of the cat (30 cm). Position (and physical extent) is measured in 3 dimensions, so the cat (an assembly of atoms, and what they're made of) has extent in height, width and depth. But time has a single dimension, so there's no equivalent "time extent" for the cat. It's somewhere at some time. A nanosecond later, that one cat may be in the "same" place (depending on what measured against) or it might have moved. Its space and time coordinates can have changed. But it's still one cat.

I like your comment.

I hope you understand the difference between your interpretation of a cat that changes coordinates with this Strange comment:

 

there is one cat extended in time.

The first interpretation is a change of coordinates.

The second interpretation is like an extruded cat.