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# Point of view

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I’ve read that some of the furthest objects we can see are moving away from us at near relativistic speeds.  Doesn’t that change their mass and ours?  Think about particles that our sun spits out at most of the speed of light, how are they not moving faster than light from the point of view of objects farthest away from us?  Does point of view determine what we experience?

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The speed of light does not depend on the motion of an observer.  (Special Theory of Relativity.)  Counter-intuitive, but true.

Doug.

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I must admit that I have never liked the term "speed of light" which suggests distance /time when its mostly used as an absolute limit as in C.....

Alan

Edited by Alien 13

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The distant objects are not moving through space. The distance between them and us is increasing because space is expanding.

The measurement of time and distance are  not absolute but are dependent on the motion of the observer (special relativity) and distribution  of mass ie gravitational field (general relativity)

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If I’m understanding this  it appears as if you are saying that the speed of light is not actually a speed but a constant with some of the qualities of a velocity.  Interesting .  Difficult to wrap your head around but makes sense  sort of.  Matter does become energy sometimes either the exceeding of the speed of light is part of it, or there is another avenue that we don’t know.  I’m still not clear on how the expansion of the universe does not produce velocities between point A and point B.  If it does produce velocities then my question about point of view might have some validity.   Particles leaving the sun have velocities when they leave the sun.  Those velocities change depending on where you are.  Speed is relative which means if I’ve got it right means that the mass of those particles changes based on where they are measured from.

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The speed of light locally is always measured to be constant i.e. c. It is a velocity.

In general over large distances or strong gravity where spacetime curvature becomes significant you cannot do the normal vector addition of velocities as it depends on the path you take.

In a locally flat region of spacetime velocity is not a unique quantity as it is always relative to something.  I currently have a velocity of zero relative to the sofa I am sitting on but a speed of order 0.8c relative to the muon about to pass through me.

The metrical expansion of space does impart a recresional velocity between non gravitationally bound systems (e.g  distant galaxies) but all we can observe is the red shift in the light we receive from them.

Regards Andrew

Edited by andrew s
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51 minutes ago, Michael Kieth Adams said:

If I’m understanding this  it appears as if you are saying that the speed of light is not actually a speed but a constant with some of the qualities of a velocity.  Interesting .  Difficult to wrap your head around but makes sense  sort of.  Matter does become energy sometimes either the exceeding of the speed of light is part of it, or there is another avenue that we don’t know.  I’m still not clear on how the expansion of the universe does not produce velocities between point A and point B.  If it does produce velocities then my question about point of view might have some validity.   Particles leaving the sun have velocities when they leave the sun.  Those velocities change depending on where you are.  Speed is relative which means if I’ve got it right means that the mass of those particles changes based on where they are measured from.

Perhaps wrongly, I think of it like this. I'd welcome Andrew's view on the matter!

Edwin Hubble observed that distant galaxies are receding from us at a speed which increases linearly with their distance. One explanation would be that we were at the centre of this expansion but this is rejected out of respect for the Copernican priniciple and because their is no evidence to support the idea. So cosmologists prefer the idea that every galaxy would observe what we do: distant galaxies are receding from them as they are from us. Now they cannot all be at the centre of the universe's expansion because, for one thing, this would attribute to other galaxies conflicting directions of recession.  And none of these galaxies feels as if it's been accelerated into movement at all.

So recession caused by the expansion of the universe is quite unlike recession caused by acceleration, which must have a direction. There is no acceleration of the separating galaxies because they are moving apart not because they have been accelerated but because the space between them is expanding. This means that their mass does not increase and their recession velocity can greatly exceed the speed of light.

One version of the 'surface of a balloon' analogy represents galaxies as coins glued onto the surface of the balloon so that, being stuck, they cannot 'move' in the normal sense. But when you blow into the balloon they none the less move away from each other, each considering itself to be at rest. (The dimension containing the distance between the centre of the balloon and its surface does not exist in the analogy.)

Olly

Edited by ollypenrice
Typo
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52 minutes ago, Michael Kieth Adams said:

If I’m understanding this  it appears as if you are saying that the speed of light is not actually a speed but a constant with some of the qualities of a velocity.  Interesting .  Difficult to wrap your head around but makes sense  sort of.  Matter does become energy sometimes either the exceeding of the speed of light is part of it, or there is another avenue that we don’t know.  I’m still not clear on how the expansion of the universe does not produce velocities between point A and point B.  If it does produce velocities then my question about point of view might have some validity.   Particles leaving the sun have velocities when they leave the sun.  Those velocities change depending on where you are.  Speed is relative which means if I’ve got it right means that the mass of those particles changes based on where they are measured from.

We call it speed of light because historical reasons. Proper term would be speed of light in vacuum, or modern day description - speed of causality. True speed of light varies with medium it is traveling through.

Yes, mass varies depending on a reference system in which it is measured. This is true even for "massless" particles like photons. Granted, they don't have "mass" in classical sense, but do they have momentum, and that momentum depends on frequency / wavelength. One observes different wavelength depending on reference system, so "mass" changes in that case as well.

Length changes with reference system, and so does time.

Everything changes with reference system except for speed of causality.

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Hi @Olly I think your view is correct.

In modern parlance accelerating a particle increase its energy but its mass stays the same. (Mass being reserved for rest mass in old parlance.) But, as you say the metrical expansion of space does not increase the energy of matter imbedded in it.

Regards Andrew

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13 minutes ago, vlaiv said:

We call it speed of light because historical reasons. Proper term would be speed of light in vacuum, or modern day description - speed of causality. True speed of light varies with medium it is traveling through.

Yes, mass varies depending on a reference system in which it is measured. This is true even for "massless" particles like photons. Granted, they don't have "mass" in classical sense, but do they have momentum, and that momentum depends on frequency / wavelength. One observes different wavelength depending on reference system, so "mass" changes in that case as well.

Length changes with reference system, and so does time.

Everything changes with reference system except for speed of causality.

Yes the speed c is for light its speed in a vacuum. It is the speed for all massless particles. That is with zero rest mass. Relativistic mass is now not generally referred to as one speaks of the energy of the system with the energy momentum tensor being the invariant under a change of reference frame.

Causality is more complex when quantum entanglement is taken into account. "Naive causality " is the term used to refer to the fact that relativity limits the exchange of "actionable" information to the speed of light.

Regards Andrew

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3 minutes ago, andrew s said:

Causality is more complex when quantum entanglement is taken into account.

Ah lovely

Can you expand on that?

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2 minutes ago, vlaiv said:

Ah lovely

Can you expand on that?

Too complex for a short post but simply is you create two entangled photons and allow them to separate in directly opposite directions. Then if you measure say the polarisation of one to be up then measuring the other will be correlated to that measurement even though no light ray could have connected the two measurement events. You can change the exact measurement made at the last possible instant but the correlation persists.

So called spooky action at a distance. These effect have been experimentally verified.

Regards Andrew

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24 minutes ago, andrew s said:

In modern parlance accelerating a particle increase its energy but its mass stays the same. (Mass being reserved for rest mass in old parlance.) But, as you say the metrical expansion of space does not increase the energy of matter imbedded in it.

Is this a recent change? and does it now mean that particles with mass can achieve the speed of light....

Alan

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2 minutes ago, Alien 13 said:

Is this a recent change? and does it now mean that particles with mass can achieve the speed of light....

Alan

It is a change of nomenclature not physics.  Particles with mass cannot reach the seed of light just as before. Its inertia increases with its increasing energy.

Regards Andrew

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Yes.  Increase in mass (or relativistic mass) is generally taken to mean an increase in the "amount of stuff" but is actually an increase in the inertia.

Doug.

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13 minutes ago, andrew s said:

Too complex for a short post but simply is you create two entangled photons and allow them to separate in directly opposite directions. Then if you measure say the polarisation of one to be up then measuring the other will be correlated to that measurement even though no light ray could have connected the two measurement events. You can change the exact measurement made at the last possible instant but the correlation persists.

So called spooky action at a distance. These effect have been experimentally verified.

Regards Andrew

Yes, I'm aware of all of that, but don't see how it is connected to causality. You can't use that to establish causal connection in the same way you can't use it to send information (causality and information transfer are just the same thing).

On the other hand, that brings us to very interesting paradox that has not been resolved, at least to my knowledge it's not.

That would be same setup, entangled pair produced and separated and then measured by Alice and Bob. Nothing strange so far, but given enough separation between measurement events, there exist reference frame in which A measurement is performed first, and also exists reference frame in which B is performed first.

Now that is a paradox - decoherence at A could cause correlated measurement at B, but also in another reference frame, decoherence at B would cause correlated measurement of A. Only solution to this is that A and B were predetermined even if in principle there was no information about outcome (no hidden variables) - which would mean that we live in "deterministic" world - although we have probability of events - that probability arises because we don't have knowledge of what is to happen, and not because there is no true determinism.

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Did not quantum uncertainty put the final nail in the coffin of Newtonian determinism?

Doug.

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@vlaiv the two measurement are causally connected in the sense that the setting of one measurement effects the results seen by the other even when they are space like separated.

Paradox or not it is what happens!

Regards Andrew

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10 minutes ago, cloudsweeper said:

Did not quantum uncertainty put the final nail in the coffin of Newtonian determinism?

Doug.

Not sure if there is straight forward answer to that. In sense, yes. Newtonian determinism implies that there is only one system trajectory allowed by the laws of physics. Quantum Uncertainty is telling us that there is no single trajectory of a system allowed by physical laws. This is not because laws of physics are incomplete (can't predict trajectory), but rather because nature acts as if all trajectories are possible, but one is taken. In the same exact setup, laws of physics would again predict all possible trajectories and assign them "probability" and again system would follow one of "allowed" trajectories but you can't know in advance which one. In that sense there is no Newtonian determinism. Laws of physics are complete in sense that they describe possible trajectories and assign "probability" to them, and show that you can't even in principle know which trajectory will system follow in advance out of possible trajectories.

Here I use term trajectory in Newtonian sense. It is in fact case that system always follows one trajectory (which we can view as being sum of classical trajectories) but the measurement is one that you can't give exact answer to - it is probabilistic in nature.

If we follow up on that, we need to understand what sort of probability are we talking about - or rather interpretation of probability. It is closest to ensemble interpretation of probability - do experiment enough times and limit of occurrence of certain measurement when number of measurements approach infinity will start to approach said probability.

There is another interpretation of probability (well there is more than just two, but let's focus on that one for a second) - that is propensity interpretation of probability. System has property of being inclined to act more one way than another.

Above thought experiment with entangled pair and different reference frames goes against this interpretation of probability. It is more in line with "lack of knowledge" (even in principle) of how the system would behave. We have just enough knowledge not to know exact particular measurement but to characterize it to a certain degree - likelihood of occurrence over repeated experiments (not to be confused with hidden variables).

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Not convinced about quantum entanglement, this was Schrodinger's cat scenario !

How can you possibly prove that the state of something does not already exist ?

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2 minutes ago, andrew s said:

@vlaiv the two measurement are causally connected in the sense that the setting of one measurement effects the results seen by the other even when they are space like separated.

Paradox or not it is what happens!

Regards Andrew

I would argue that we assign term causally connected to these events due to our experience - local forward time flow. We say that something is causally connected if there is certain order of events and there is information transfer between events in our frame of reference.

In our mind we equate that to mundane term - A caused B.

Forget physics for a moment, and just look at logical systems.

A => B & B=>A

(A implies B and B implies A)  leads to A <=> B (A equivalent to B )

I would say that above logical expressions are quite like saying:

A caused B but B also caused A - A and B were bound to happen both.

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Just now, Kev M said:

Not convinced about quantum entanglement, this was Schrodinger's cat scenario !

How can you possibly prove that the state of something does not already exist ?

You may not be but it has been experimentally demonstrated.

Regards Andrew

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Just now, vlaiv said:

I would argue that we assign term causally connected to these events due to our experience - local forward time flow. We say that something is causally connected if there is certain order of events and there is information transfer between events in our frame of reference.

In our mind we equate that to mundane term - A caused B.

Forget physics for a moment, and just look at logical systems.

A => B & B=>A

(A implies B and B implies A)  leads to A <=> B (A equivalent to B )

I would say that above logical expressions are quite like saying:

A caused B but B also caused A - A and B were bound to happen both.

That is now termed naive causality to differentiate it from the causality associated with quantum entanglement.

Regards Andrew

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1 minute ago, Kev M said:

Not convinced about quantum entanglement, this was Schrodinger's cat scenario !

How can you possibly prove that the state of something does not already exist ?

I'm not sure if you understand both Schrodinger's cat and entanglement.

Schrodinger's cat has more to do with measurement "paradox" - since resolved. Currently accepted theory is one of decoherence.

Idea behind the cat is question on why do we measure / see / observe only definite state of system, but QM tells us that system exists and evolves in superposition of states. Why can't we see that superposition. Dead Cat / Alive Cat sort of thing. Answer to that is in decoherence - once quantum system stars interacting with environment it "looses phase" that enables superposition. Phase correlation "leaks" into environment, and you are left with states that are not in superposition. We still have problem of measurement - how particular state is "selected" - and currently accepted answer is: random with certain probability.

Entanglement on the other hand is different sort of effect - it is much like two QM systems forming one large QM system that behaves as one entity, regardless of how far constituents of that system are. There is a bonus to this - larger system behaves with certain correlation permitted by laws of physics - usually conservation law.

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5 minutes ago, andrew s said:

That is now termed naive causality to differentiate it from the causality associated with quantum entanglement.

Regards Andrew

Still don't get it, how is causality associated with quantum entanglement different from "naive causality". We still naively believe that measurement at A causes correlated value of measurement B. As shown above, both A can cause B and at "the same time" (no not real time, figure of speech) - B causes A.

This is actually deeper than paradox would suggest. As each A and B measurement interact with environment thus leak information into environment via decoherence, or become entangled with environment on some level - which one becomes entangled first? There is no first one - A is first and B is first - depending on your reference frame. This entanglement propagates in environment, but again which one is "first" - or if we accept that both are first, we need to accept that on some level reality can depend on reference system (beyond things that we know depend on reference system like length, time and so), or that we live in predetermined universe.

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