# Relativity (split from Can relativity be applied to light speed?)

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On 8/13/2019 at 9:59 PM, Ghideon said:

Thanks﻿, good to know that the description was helpful!

﻿ This, and related topics have been discussed by many through the ages. In case you are interested here is a reference with various ideas held by Descartes, Newton, Leibniz and others, https://plato.stanford.edu/entries/spacetime-theories/:

"Since antiquity, natural philosophers have struggled to comprehend the nature of three tightly interconnected concepts: space, time, and motion. A proper understanding of motion, in particular, has ﻿been seen to be crucial for deciding questions about the natures of space and time, and their interconnections. Since the time of Newton and Leibniz, philosophers' struggles to comprehend these concepts have often appeared to take the form of a dispute between absolute conceptions of space, time and motion, and relational conceptions. This article guides the reader through some of the history of these philosophical struggles."﻿

I have a question related to this subject and statement;

Would it be fair to assume that if you are in position xyz and have never experienced a force against you making your momentum 0, could this mean you are actually stationary?

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3 hours ago, MPMin said:

Would it be fair to assume that if you are in position xyz and have never experienced a force against you making your momentum 0, could this mean you are actually stationary﻿?

You would be stationary in your frame of reference. But not in other frames of reference; (x,y,z) is not an absolute point in space according to current models of spacetime. Does this answer your question?

Some questions to use in case you want further discussion and details.
-How did you get to position (x,y,z) without experiencing an accelerating force? Or how did the particles you are made of get to (x,y,z)?
-If you (magically) popped into existence at position (x,y,z) then you would be moving in reference to other observers, how about conservation of momentum?
-If you avoid the points above somehow; what about expansion of the universe? In what coordinate system is your position a stationary position?

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6 hours ago, MPMin said:

I have a question related to this subject and statement;

Would it be fair to assume that if you are in position xyz and have never experienced a force against you making your momentum 0, could this mean you are actually stationary?

No such thing as "actually stationary"; there is only "stationary with respect to something"

There is no physics test you can do to tell whether you are moving or not, in an absolute sense.

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2 hours ago, Ghideon said:

Some﻿ questions to use in case you want further discussion and d﻿etails.
-How﻿﻿ did you get to position (x,y,z) without experiencing an accelerating force? Or ﻿how did ﻿the particles you are made of get to (x,y,z)? ﻿
-If you (magically) popped into existence at position (x,y,z) then you would be moving in reference to other observers, how about conservation of momentu﻿m?
-If you avoid the points above somehow; what about expansion of the universe? In what coordinat﻿e ﻿sy﻿stem is your position a stationary position?﻿﻿
﻿

The Big Bang is theorised to have just magically pop into existence so I’m making the same assumptions there.

I was really just wondering if it was possible to have momentum without a frame of reference?

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Posted (edited)
6 minutes ago, MPMin said:

The﻿ Big Bang is theorised to have just magically﻿﻿ p﻿op into existence so I’m making the same assumption﻿s the﻿re﻿﻿.﻿

Can you point to a reference?

See my last three points regarding Big bang and coordinates.

6 minutes ago, MPMin said:

I was﻿﻿﻿﻿﻿ really just wondering if it was possible to have momentum without a frame of reference? ﻿

You would, at least, have your frame of reference? How would momentum be defined in such a case where no frame of reference exists?

Edited by Ghideon
Missing words

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16 minutes ago, Ghideon said:

Can you point to﻿﻿﻿﻿ a reference? ﻿

The universal origin story known as the Big Bang postulates that, 13.7 billion years ago, our universe emerged from a singularity — a point of infinite density and gravity — and that before this event, space and time did not exist (which means the Big Bang took place at no place and no time).

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42 minutes ago, MPMin said:

The universal origin story known as the Big Bang postulates that, 13.7 billion years ago, our universe emerged from a singularity — a point of infinite density and gravity — and that before this event, space and time did not exist (which means the Big Bang took place at no place and no time).

Nice journalism, but there is no science there.

The singularity simply shows that our theory (ie. the mathematics) no longer applies. It is what happens when you take a naive extrapolation using just General Relativity.

The only scientific evidence we have is that the early universe was very hot and very dense (and very homogeneous).

7 hours ago, MPMin said:

Would it be fair to assume that if you are in position xyz and have never experienced a force against you making your momentum 0, could this mean you are actually stationary?

"Actually stationary" does not mean anything.

Two bodies could have never experienced any force but still be moving relative to one another.

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46 minutes ago, Strange said:

Nice journalism, but there is no science there.

The singularity simply shows that our theory (ie. the mathematics) no longer applies. It is what happens when you take a naive extrapolation using just General Relativity.

The only scientific evidence we have is that the early universe was very hot and very dense (and very homogeneous).

"Actually stationary" does not mean anything.

Two bodies could have never experienced any force but still be moving relative to one another.

Good description, better that the one I was writing.

Note: I interpreted  "Actually stationary" as stationary relative to some kind of absolute space since that was in the context of the old thread. My answer was/is slightly more complicated than necessary since the question referenced my post where an article where older and obsolete models of space-time were mentioned. Older/alternative models could give other answers, but that would not be compatible with current observations AFIK.

2 hours ago, MPMin said:

The Big Bang is theorised to have just magica﻿lly pop into existence﻿ so I’m making the same assumption﻿s there﻿﻿﻿

Since the article you reference does not seem support object magically popping into existing I still think my line of thought regarding coordinates of position xyz is valid. Even if you could pop into existence (you can't) there is no location or time when such a popping into existence could take place and making you absolutely stationary. There is no absolute space at this time and I don't know of evidence that there ever was a time where space could have been absolute. Again, that does not necessary hold for older models in the article I referenced.

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16 minutes ago, Ghideon said:

Note: I interpreted  "Actually stationary" as stationary relative to some kind of absolute space since that was in the context of the old thread.

I interpreted it that way as well. But decided to take a short-cut in the answer!

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22 hours ago, Strange said:

Nice﻿ jou﻿rnalism, but there is no science there.

Isn’t this part of science interpretation of observations, this ‘journalism’ is just another opinion of the same observations

22 hours ago, Strange said:

Two﻿ bodies could have never experienced any force but still be mov﻿ing relative to one another.﻿

That’s why they might actually be stationary, in other words, no momentum.

If an object has experienced a force to make it move surely it has momentum, wouldn’t it make sense to then choose a frame of reference that hasn’t had a force applied to it?

21 hours ago, Ghideon said:

Since﻿﻿ t﻿he article you reference does not seem support object magically popping in﻿﻿to e﻿xisting I still think my line of thought reg﻿arding co﻿ordin﻿﻿ates of posit﻿ion﻿ xyz i﻿s v﻿alid﻿﻿.﻿

This depends on your definition of magic.

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1 minute ago, MPMin said:

Isn’t this part of science interpretation of observations, this ‘journalism’ is just another opinion of the same observations

True, it is just not a very accurate interpretation.

2 minutes ago, MPMin said:

That’s why they might actually be stationary, in other words, no momentum.

If an object has experienced a force to make it move surely it has momentum, wouldn’t it make sense to then choose a frame of reference that hasn’t had a force applied to it?

Apart from the practical impossibility of defining such a frame, there are an infinite number of such frames of reference because, as I said, two things can be moving relative to one a another (even if they started off stationary relative to one another) even if no force is applied to them.

And, even if you could find some frame of reference that has never had a force extorted on it, how useful would it be? What if it was moving at 99.99% of the speed of light relative to Earth? Could we make any use such a frame of reference?

But it doesn't mater, because there is not such frame of reference. It implies there is an absolute zero of momentum, kinetic energy and velocity. All of which can only be defined relative to something else.

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54 minutes ago, Strange said:

But﻿﻿ it doesn't﻿﻿﻿ m﻿ater, because﻿ there is not such frame of reference. It implies ﻿the﻿re is an absolute zero of﻿﻿﻿ momentum, kinetic energy and velocit﻿y.﻿ ﻿A﻿ll ﻿o﻿f which ﻿can only be defined relative to some﻿thing el﻿﻿se.﻿﻿﻿﻿﻿﻿

There is such a frame of reference it’s just impractical to use

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4 minutes ago, MPMin said:

There is such a frame of reference

No, there isn't. (Well, there is. The trouble is there are an infinite number of them.)

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14 minutes ago, MPMin said:

There is such a frame of reference it’s just impractical to use

Quite the opposite. The center-of-momentum frame is often extremely useful. Or the center-of-mass. Or one object being at rest. The thing is that the physics works just as well in any other inertial frame. There is nothing special about any inertial frame, from a physics perspective. You can use whichever one makes the problem easiest to solve.

In any given problem, you may not know the history, so you can't tell if there had been a force applied to the object.

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6 minutes ago, Strange said:

No, there﻿﻿ isn't. (Well, there is. The trouble is there are an infinite﻿﻿ num﻿ber of them.)

That’s why it would be impractical to collectively reference them all at once (but maybe one day it won’t be impractical)

1 minute ago, swansont said:

In any﻿﻿﻿﻿﻿﻿ given problem, you may not know the history, so you can't te﻿ll if there had been a force applied to the object. ﻿﻿

Exactly! And at some point science has to just take a guess and say it just appeared there out of now where like magic.

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1 minute ago, MPMin said:

That’s why it would be impractical to collectively reference them all at once (but maybe one day it won’t be impractical)

But which one would you use? If there are an infinite number of frames of reference, all irrelative motion to one another. How can you "reference them all at once"? What does that even mean?

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4 minutes ago, MPMin said:

That’s why it would be impractical to collectively reference them all at once

In the sense that "wrong" is impractical. Mixing frames leads to wrong answers, as you would be using relative values of parameters incorrectly.

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3 minutes ago, Strange said:

But﻿﻿ which one would you use﻿?﻿﻿ ﻿

That’s the point, I’m not talking about using just one as it’s not relative to the another possible frames of reference so you have to use them all which is impractical.

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7 minutes ago, MPMin said:

That’s the point, I’m not talking about using just one as it’s not relative to the another possible frames of reference so you have to use them all which is impractical.

It is not just impractical, it is meaningless.

You can already use an infinite number of frames of reference (with something having zero momentum in each one).

You would be saying that you, for example, are moving at an infinite number of different speeds in an infinite number of different directions. All at the same time. How is that useful?

So I have no idea what you are suggesting or why.

19 minutes ago, MPMin said:

Exactly! And at some point science has to just take a guess and say it just appeared there out of now where like magic.

No it doesn't. And never does.

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1 hour ago, MPMin said:

This depends on your definition of magic.

True! I need to change use of words:

On 8/15/2019 at 9:26 AM, Ghideon said:

-If you (magically) popped into existence at position (x,y,z) then you would be moving in reference to other observers, how about conservation of momentum?

Maybe this is better:

If you start to exist at position (x,y,z) at time=t then you would be moving in reference to other observers. "Start to exist" in this case means that you, the matter you are created from, did not exist in the universe at time<t.

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Posted (edited)

I have three points nobody seems to have considered, to add to this discussion.

Firstly this idea of  a virgin body that has never felt a force.

2 hours ago, MPMin said:
On 8/15/2019 at 12:29 PM, Strange said:

Two﻿ bodies could have never experienced any force but still be mov﻿ing relative to one another.﻿

That’s why they might actually be stationary, in other words, no momentum. ﻿

If an object has experienced a force to make it move surely it has momentum, wouldn’t it make sense to then choose a frame of reference that hasn’t had a force applied to it?

Within the bounds aof all known mathematics this requires a single body in an otherwise empty universe.

As soon as you introduce more than one body there will always be some sort of force between them.

Secondly   zero momentum could be momentary (and frame dependant), but the requirement has always been zero net force, not zero force.

Thirdly there is no magic in pair production from radiation.

Edited by studiot

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10 minutes ago, studiot said:

I have three points nobody seems to have considered, to add to this discussion.

All good points.

Quote

Within the bounds aof all known mathematics this requires a single body in an otherwise empty universe.

As soon as you introduce more than one body there will always be some sort of force between them.

That is certainly true if you consider gravity a force (a la Newton).

And if you take the GR interpretation, then you get relative motion with no force on the objects (which is the point I was making, albeit not very explicitly).

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54 minutes ago, Strange said:

And if you take the GR interpretation, then you get relative motion with no force on the objects (which is the point I was making, albeit not very explicitly).

It's like, it has no purpose...

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1 hour ago, Strange said:

That is certainly true if you consider gravity a force (a la Newton)

Thank you for the response, but it is not necessary to consider gravity.

Unless the bodies are at exactly the same temperature, there will exist some sort of radiation pressure between them.

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2 minutes ago, studiot said:

Thank you for the response, but it is not necessary to consider gravity.

Unless the bodies are at exactly the same temperature, there will exist some sort of radiation pressure between them.

Another good point.

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