Relativity

[steve_bank]

Just thinking out loud.

The metric describes space and time. Distance. Without gravity present space is flat Euclidean? The path a photon takes through space is the distance between two points.

One of validations of relativity was observing a star change position in the presence of a star in the path of the light. The path length of the photons charged in the presence of a star's gravity. Space defined by the path of a photon is warped by gravity.

If I set up a speed trap measuring the speed of photons from a distant star I get C. As a closer star approaches the line of sight photons the speed trap path length increases and measured C drops. Or the clock also slows down due to dilation and C remains constant?

Time dilation has been verified so is the inference all processes including biological vary with dilation?


Comments? I am trying to build a picture before I go out and get a book. Any recommendations for a book?

 

[bigfield]

Your thought experiment isn't complete. How are you measuring the speed of the photons? Are you measuring the time it takes for the photon to travel between two points? Where is the clock you mentioned?

Gravity deflects photons by curving spacetime. In order for a photon to travel between two points on either side of a gravity well, it has to follow a curved trajectory.

So the gravity of the nearby star isn't slowing the light down; it's just forcing it to take a longer route to get from A to B.

Biological processes can be treated the same as one of Einstein's clocks. Astronautson the ISS age slower than people on Earth.

 

[barbos]

If I set up a speed trap measuring the speed of photons from a distant star I get C. As a closer star approaches the line of sight photons the speed trap path length increases and measured C drops. Or the clock also slows down due to dilation and C remains constant?

In general relativity distance and time intervals only defined for infinitesimals. So distance to the star is strictly speaking not defined

Time dilation has been verified so is the inference all processes including biological vary with dilation?

Absolutely

Comments? I am trying to build a picture before I go out and get a book. Any recommendations for a book?

I recommend "Adventures of Huckleberry Finn" by Mark Twain, good read.

 

[phands]

https://www.amazon.com/How-Teach-Relativity-Your-Dog/dp/0465023312

His other books are good as well...the one on QM is really fun.

 

[steve_bank]

Your thought experiment isn't complete. How are you measuring the speed of the photons? Are you measuring the time it takes for the photon to travel between two points? Where is the clock you mentioned?

Gravity deflects photons by curving spacetime. In order for a photon to travel between two points on either side of a gravity well, it has to follow a curved trajectory.

So the gravity of the nearby star isn't slowing the light down; it's just forcing it to take a longer route to get from A to B.

Biological processes can be treated the same as one of Einstein's clocks. Astronautson the ISS age slower than people on Earth.

I am not set up right now to make plots.

A magic photon velocity meter. Photons are traveling in a straight line between two points and a velocity is measured. . A star approaches and the path of photons between the twp points is bent so the photons take more time to travel from point a to point, analogous to walking around a rock in a path.

Veocity between the two points would seem to go down becase of the longer path. But the local clock aslo slows down due to gravitaional dilation.

Locally C to an observer near the sun measures C. A distant observer tracking a photon would see C drop.

Here on Earth C varies with the medium light travels through. It is used in electronics to create a time delay. Point being in the case of the star and nearby photons velocity can be relative to the observed, or so I surmise.

 

[skepticalbip]

It seems that you are assuming Euclidean space is the "real space" so the shortest distance between two points would be a straight line in this sort of space. Relativity assumes a Riemannian space which means that the shortest distance between two points would be a different path than it would be in Euclidean space.

The misunderstanding would be similar to measuring the shortest distance between two points on the Earth using a Mercantor projection map and not understanding why it is different than the actual travel distance over the globe.

 

[bigfield]

Your thought experiment isn't complete. How are you measuring the speed of the photons? Are you measuring the time it takes for the photon to travel between two points? Where is the clock you mentioned?

Gravity deflects photons by curving spacetime. In order for a photon to travel between two points on either side of a gravity well, it has to follow a curved trajectory.

So the gravity of the nearby star isn't slowing the light down; it's just forcing it to take a longer route to get from A to B.

Biological processes can be treated the same as one of Einstein's clocks. Astronautson the ISS age slower than people on Earth.

I am not set up right now to make plots.

A magic photon velocity meter. Photons are traveling in a straight line between two points and a velocity is measured. . A star approaches and the path of photons between the twp points is bent so the photons take more time to travel from point a to point, analogous to walking around a rock in a path.

Veocity between the two points would seem to go down becase of the longer path. But the local clock aslo slows down due to gravitaional dilation.

We measure speed by measuring the time an object takes to traverse a known distance, or vice versa.

v = s/t and v = c, where c is constant. Therefore, increasing s also increases t, the time the photon takes to travel from A to B (in the observer's reference frame, and assuming the observer and the emitter are co-moving).

If the observer has the clock then the observer will never observe any time dilation.
If the photon has the clock then the clock never ticks, because no time passes in a photon's inertial frame of reference.

Locally C to an observer near the sun measures C. A distant observer tracking a photon would see C drop.

Here on Earth C varies with the medium light travels through. It is used in electronics to create a time delay. Point being in the case of the star and nearby photons velocity can be relative to the observed, or so I surmise.

he overall speed of the photon drops below c because the photon is absorbed and re-emitted repeatedly as it passes through the medium. It's basically making a lot of stops along the way, increasing the total time it takes to propagate through the medium without actually reducing the speed at which the photon is travelling. That isn't happening to a photon moving through the vacuum of space.

ETA: It's not absorption/emission in a classical sense, where electrons absorb photons and re-emit them, like in fluorescent tubes. It's something to do with quantum electrodynamics, where the photon is absorbed and then-re-emitted in a superposition of multiple directions.

 

[skepticalbip]

The classical description (as described by Maxwell) would be that the speed of light is determined by the permittivity and permeability of the medium that the light is passing through.

 

[steve_bank]

It seems that you are assuming Euclidean space is the "real space" so the shortest distance between two points would be a straight line in this sort of space. Relativity assumes a Riemannian space which means that the shortest distance between two points would be a different path than it would be in Euclidean space.

The misunderstanding would be similar to measuring the shortest distance between two points on the Earth using a Mercantor projection map and not understanding why it is different than the actual travel distance over the globe.

I would assume in deep space between galaxies space is close to flat unperturbed by gravity.

 

[steve_bank]

Obviously it is not possible to pickup casualty.

I'll probably start with tensors and go from there. Need something to occupy my time.

In my thought experiment to a distant observer the time it takes a photon to travel two points increases due to gravity. Maybe a poor example.

 

[bigfield]

In my thought experiment to a distant observer the time it takes a photon to travel two points increases due to gravity.

That's a better way of describing it than a thought experiment involving a magic photon speedometer.

A distant observer can't observe the photon directly. You need to have a clock at the point of emission and a clock at the point of absorption, then measure the difference between the recorded time of emission and the recorded time of absorption.

 

[skepticalbip]

In my thought experiment to a distant observer the time it takes a photon to travel two points increases due to gravity.

That's a better way of describing it than a thought experiment involving a magic photon speedometer.

A distant observer can't observe the photon directly. You need to have a clock at the point of emission and a clock at the point of absorption, then measure the difference between the recorded time of emission and the recorded time of absorption.

The problem with that is that there would be three different inertial reference frames; the star system emitting the light (the first inertial reference frame), the light being detected in a another star system (the second inertial reference frame), and the observer in another star system (the third inertial reference frame). What are the relative motions of these three systems with respect to each other? What is the time in each system with respect to the other systems?

 

[bigfield]

In my thought experiment to a distant observer the time it takes a photon to travel two points increases due to gravity.

That's a better way of describing it than a thought experiment involving a magic photon speedometer.

A distant observer can't observe the photon directly. You need to have a clock at the point of emission and a clock at the point of absorption, then measure the difference between the recorded time of emission and the recorded time of absorption.

The problem with that is that there would be three different inertial reference frames; the star system emitting the light (the first inertial reference frame), the light being detected in a another star system (the second inertial reference frame), and the observer in another star system (the third inertial reference frame). What are the relative motions of these three systems with respect to each other? What is the time in each system with respect to the other systems?

You're right; it assumes everything is comoving.

 

[Treedbear]

The classical description (as described by Maxwell) would be that the speed of light is determined by the permittivity and permeability of the medium that the light is passing through.

I always wondered how those two parameters are determined. It seems to imply there is a medium to which these properties are attributed. They seem to be somehow analogous to electrical capacitance and inductance. I wish I was good enough at math to be able to understand Maxwell's equations.

 

[steve_bank]

The classical description (as described by Maxwell) would be that the speed of light is determined by the permittivity and permeability of the medium that the light is passing through.

I always wondered how those two parameters are determined. It seems to imply there is a medium to which these properties are attributed. They seem to be somehow analogous to electrical capacitance and inductance. I wish I was good enough at math to be able to understand Maxwell's equations.

Inductance and capacitance are forms of energy storage. It is common to simulate mechanics and heat transfer with resistors, capacities, and inductors. A shock absorber is an example.

Mechanical and electrical resonance are mathematically the same. The underling principles are the same throughout all areas.

 

[Treedbear]

The classical description (as described by Maxwell) would be that the speed of light is determined by the permittivity and permeability of the medium that the light is passing through.

I always wondered how those two parameters are determined. It seems to imply there is a medium to which these properties are attributed. They seem to be somehow analogous to electrical capacitance and inductance. I wish I was good enough at math to be able to understand Maxwell's equations.

Inductance and capacitance are forms of energy storage. It is common to simulate mechanics and heat transfer with resistors, capacities, and inductors. A shock absorber is an example.

Mechanical and electrical resonance are mathematically the same. The underling principles are the same throughout all areas.

Good. So what is the physical basis for permittivity and permeability?

 

[steve_bank]

Inductance and capacitance are forms of energy storage. It is common to simulate mechanics and heat transfer with resistors, capacities, and inductors. A shock absorber is an example.

Mechanical and electrical resonance are mathematically the same. The underling principles are the same throughout all areas.

Good. So what is the physical basis for permittivity and permeability?

The quantum basis for those I would have to look up. As big field said quantum electrodynamics.

In free space vacuum an EM wave is constantly shitimg energy from magnetic to electri fields. You can look at it as voltage and current. There is a measurable free space impedance, 377 ohms. E/H = 377. It is real in an applied sense. An antenna serves to match the output impedance of an amplifier to the free space impedance.

That leads to questions of what space is. And that is beyond me.

 

[skepticalbip]

Obviously it is not possible to pickup casualty.

... Quite true. But then that isn't unusual just as Newtonian mechanics is not possible to pickup casually - both require concerted study to understand.

I'll probably start with tensors and go from there. Need something to occupy my time.

A suggestion, if you don't mind. I think you will have fewer problems in becoming familiar with relativity by not trying to understand it in terms of a Newtonian view. When something is confusing with respect with your current Newtonian understanding, it is best to assume that the confusion is because Newton was wrong.

 

[steve_bank]

Obviously it is not possible to pickup casualty.

... Quite true. But then that isn't unusual just as Newtonian mechanics is not possible to pickup casually - both require concerted study to understand.

I'll probably start with tensors and go from there. Need something to occupy my time.

A suggestion, if you don't mind. I think you will have fewer problems in becoming familiar with relativity by not trying to understand it in terms of a Newtonian view. When something is confusing with respect with your current Newtonian understanding, it is best to assume that the confusion is because Newton was wrong.

Ok. I do not confuse the three mechanics. Well aware of the differences.

Typo. Meant casually.

I will start at the beginning, tensors. The way I learn is part heuristic. As I get into new areas I code and run simulations of problems at increasing complexity.

Tensors alone will take me months to develop a working knowledge. Then more time with relativity itself. Time is what I got lots of.

Thanks all for engaging the thread.

 

[steve_bank]

Adder poking around on the net it would appear to get a basic competence in GR would require the equivalent of 4 or 5 grad courses in math and physics.