Galaxy rotation rates explained without Dark Matter

[ajb]

I doesn't matter if the observer knows he is time dilated or not...

Because with respect to himself he is not time dilated!

 

Thinking of just special relativity for a moment, every inertial observer is time dilated (as you put it) with respect to any and all other inertial observers (assuming causally connected etc). Thus time dilation cannot be a an intrinsic thing to a given inertial observer.

 

The same is true of gravitational time dilation. You need another observer, but you may choose this to be an observer at infinity for the Schwarzschild metric and similar.

 

 

 

...he will always measure light to travel at the same speed - as his time slows by exactly the same amount as does the speed of light in his reference frame.

But not as the observer measures his proper time. The speed of light is c as locally the observer can be considered as an inertial observer and that the laws of physics reduce to that of special relativity. Mathematically this is almost equivalent to the fact that space-time is a smooth manifold.

[Mordred]

Thank you granpa you can see the point I am trying to make.

 

I doesn't matter if the observer knows he is time dilated or not, he will always measure light to travel at the same speed - as his time slows by exactly the same amount as does the speed of light in his reference frame.

 

Every physical process is determined by the rate at which waves (light or matter) propagate through a region of space, so time will slow down by exactly the same amount as does the propagation speed of light.

 

All quanta - be it light, electrons, positrons or neutrinos are wave functions traveling through the space-time medium (aka energy field), so all will follow the geodesics. Light will bend, and matter will gravitate as a result of the slowed propagation speed in a region where the field is more dense.

Do you understand the difference between a change in wavelength as opposed to velocity?

 

In a medium the speed of light is less than c. This isn't an observer effect. You as the observer watch light slow down in a supercooled medium. You have precisely the same reference frame as that of the supercooled medium. It has insufficient mass for time dilation.

 

This isn't the case in spacetime. The speed of light isnt slowed down according to any observer. Due to length contraction its wavelength changes. Not its propogation speed.

[granpa]

An observer knows whether he's deep within a gravity well or not.

[Mordred]

An observer knows whether he's deep within a gravity well or not.

Does he? What about the equivalence principle and inertia.? Assume the observer is in a closed box. Does he know the difference unless there is an acceleration change

[granpa]

I didn't say anything about a closed box

[Mordred]

You didn't have to. Its one of the basic thought experiments of Relativity

[Declan]

That is because light slowing in a medium (such as water) is a process of absorption and re-emission. See my paper on the real cause of Fresnel Dragging:

 

http://gpcpublishing.com/index.php?journal=gjp&page=article&op=view&path%5B%5D=386

[ajb]

An observer knows whether he's deep within a gravity well or not.

So what?

 

For sure he can calculate Doppler shifts and time dilations etc with respect to other observers.

[Declan]

 

To granpa:

 

Yes anti matter falls downwards in a gravitational field too.

[ajb]

 

 

You didn't have to. Its one of the basic thought experiments of Relativity

He does not know if he just performs non-gravitational experiments and for sure locally he does not know. But we can assume he can do some 'non-lcoal' experiments.

[granpa]

The point I was making is that being in a gravity well (and therefore experiencing gravitational time dilation) is not relative. It is absolute.

You either are or you are not

Edited July 3, 2016 by granpa

[Mordred]

He does not know if he just performs non-gravitational experiments and for sure locally he does not know. But we can assume he can do some 'non-lcoal' experiments.

Yes but he needs some form of reference. Even if it is an accelerometer.

Otherwise he would have no means of knowing if he is moving at a constant velocity or at rest.

Edited July 3, 2016 by Mordred

[ajb]

Even if it is an accelerometer.

Sure, so lets assume that

[Mordred]

Lol sure I know your aware of the principle of equivalence. With the accelerometer if the elevator is moving at constant velocity. The inside observer would not be able to distinquish between gravity or inertia.

 

This is one aspect not particularly definable by medium properties.

Edited July 3, 2016 by Mordred

[ajb]

The point I was making is that being in a gravity well (and therefore experiencing gravitational time dilation) is not relative. It is absolute.

You either are or you are not

The 'amount' is relative, you need to fix some comparison obsever. We could pick the observer at infinity for this - or any other obsever.

 

Otherwise it is a bit empty.

 

 

The inside observer would not be able to distinquish between gravity or inertia.

Locally yes, I totally agree.

 

 

In not quite so small regions the observer could measure tidal forces.

 

Or just look of his box and see that great big star near by!

Edited July 3, 2016 by ajb

[Mordred]

Lol provided he can see outside the box. Which is why I specified a closed box.

 

Grandpa the belief that antimatter had anti gravity effects was previous to our understanding of antimatter. Some very old literature still describes antimatter as opposing gravity.

 

Today we know this is false as we can create antimatter at the LHC. We also measured the Earth being bombarded with antimatter.

Edited July 3, 2016 by Mordred

[ajb]

Lol provided he can see outside the box. Which is why I specified a closed box.

The only thing one can do - that does not require the global background - is to locally look for geodesic deviation by examining test particles near by in freefall. So, either we need big enough box so that tidal effects are measurable or we say that he can look outside the box, but not very far!

 

But you are totally correct that inside a small box that one cannot look out of (do non-local experiments) there is no way to detect gravity - or really distinguish it from inertia.

[Mordred]

I fully agree with that. I was looking into finding a simple enough coverage on geodesics for Declan. I was hoping that Master-Geodesics would help. Unfortunately like a lot of papers it skips a lot of pteliminary details that one would find in a textbook such as Walds "General Relativity". Scott Dodelsons "Moderm Cosmology" didn't particularly help either.

 

One of the hassles when your not sure if the target audience understands Langrene density or the connection coeficient.

[ajb]

I was looking into finding a simple enough coverage on geodesics for Declan.

I always suggest the lecture notes by Carroll. They have a good mix of physical ideas and mathematics.

[Mordred]

If your thinking his lecture notes on General Relativity. I would agree with you. Its an easier read than Wald and more specific than Scott Dodelson.

 

"Lecture notes on General Relativity" Sean Carroll.

https://arxiv.org/abs/gr-qc/9712019

Edited July 3, 2016 by Mordred

[Declan]

Seriously if an atomic clock was placed on the surface of a huge planet where the gravitational potemential caused a reasonable time dilation and another placed in a group-synchronous orbit by one observer who then stopped looking at them. Then a hundred years later a different observer brought the two clocks together - the clocks would show a different time, regardless of whether an observer was looking at them or not. The gravitational time dilation effect is real and absolute and does not depend on the observer. Thus the speed of light must be slower in the reference frame on the surface of the planet, otherwise an observer would measure light speed to be higher relative to his clock.

 

Group-synchronous should read geo-synchronous (auto-correct error)

[ajb]

Then a hundred years later a different observer brought the two clocks together - the clocks would show a different time...

Yes, we all agree with this.

 

 

The gravitational time dilation effect is real...

Again, we all agree.

 

...and absolute...

In what sense absolute?

 

...and does not depend on the observer.

It does depend on the clock that you have in orbit, as this is one of the clocks you wish to compare. So in this sense gravittaional time dilation is not absolute and does depend on the clocks you compare.

 

 

Thus the speed of light must be slower in the reference frame on the surface of the planet, otherwise an observer would measure light speed to be higher relative to his clock.

The observer on the surface measures the speed of light to be c as all non-gravitational physics reduces to special relativity locally. The same is true of all observers that are in freefall.

 

So you are just wrong on this... or at least you are not being very clear. If we use other coordinates then we may have the speed of light not being equal to c. For example, even on Minkowski space-time we can consider non-inertial coordinates, say light cone-coordinates. Here the speed of light is not c.

[Declan]

What has special relativity got to do with it? The observer on the planet's surface is stationary yet he measures the speed of light to be the same even though his time is running slowly. If the speed of light was invariant then he would measure the speed of light to be higher than it should be.

[swansont]

If the speed of light was invariant then he would measure the speed of light to be higher than it should be.

 

 

Perhaps you can ponder the nonsense of such a statement. If the speed of light was invariant, how can the observer measure anything but the invariant value?

[Declan]

As I have already said: light speed slows by the same amount as the observer's rate of time, thus all measurements he makes will still give the normal speed of light.

 

You have clearly never built a computer model of space with light moving through it in a region with dilated time, otherwise you would realize the incoherence and impossibility of what you are saying.