Silly question?

[danielj]

17 hours ago, studiot said:

That's really good news. I think we can build on this mechanical experience.

I have been busy these last week but I see other members have been looking after you.

I would, however like to bring your feet fimly back to ground to feel the gravity.

I think you could benefit from this book. It is one of a kind where the author tries to use physics to explain well known mathematics, rather than the other way round.

So rather than dry mathematical general theorems about polygons try the polygon of least area (p35)

The Mathematical Mechanic  :  Mark Levi :  Princeton University Press.

 

But remember to keep asking questions as you will find much unfamiliar as well as some familiar stuff.

 

Thank you, much appreciated

On 2/15/2025 at 3:49 PM, Genady said:

The measurement by an accelerating observer and by a non-accelerating observer will be different.

I’m just trying visualise some of the mechanics of this, so would it also be true to say that if I (tried) to take a ‘snap-shot’ measurement, with any combination of inertial/accelerating reference frames that in reality there is no way to take a snap-shot, as what was the time frame of the snap shot (by that phrase I mean a static measurement) and no matter how small the time frame, change will have occurred in one or more dimensions?

The questions should  get less stupid as I progress, but I can’t promise 

5 hours ago, KJW said:

The Equivalence Principle basically says that over distances that are sufficiently small for the tidal effect to be negligible, being in a gravitational field is indistinguishable from being in an accelerated frame of reference. This is illustrated by the following diagram:
 

 

However, over larger distances, the gravitational field does differ from being in an accelerated frame of reference due to the tidal effect, which is a manifestation of spacetime curvature that is absence from being in an accelerated frame of reference in flat spacetime.

 

 

What I said about perception of the vertical direction was not about skewing reality, but a consequence of the equivalence principle. I personally discovered the perception of the vertical direction when I was a teenager in an amusement park ride called the "Rotor". I noticed that the people directly opposite me before the start of the ride were very much above me during the ride. I immediately realised that what we regard as up or down is actually a perception that we don't normally notice unless we are in an environment that challenges the notion of up or down.

 

 

So they were above you if you used the ground as your reference frame, but not if you used the rotor itself? Your brain used the ground? 

reading this back I see there being potentially many different frames of reference, equally valid?

Edited February 16 by danielj

[Genady]

26 minutes ago, danielj said:

I’m just trying visualise some of the mechanics of this, so would it also be true to say that if I (tried) to take a ‘snap-shot’ measurement, with any combination of inertial/accelerating reference frames that in reality there is no way to take a snap-shot, as what was the time frame of the snap shot (by that phrase I mean a static measurement) and no matter how small the time frame, change will have occurred in one or more dimensions?

You can take a snapshot of an accelerometer that you hold in your hand. It will show if you accelerate or not.

[danielj]

23 minutes ago, Genady said:

You can take a snapshot of an accelerometer that you hold in your hand. It will show if you accelerate or not.

I see, yes. Thank you. 
I get caught in the details, not always helpful to me. 

[KJW]

2 hours ago, danielj said:

So they were above you if you used the ground as your reference frame, but not if you used the rotor itself? Your brain used the ground? 

Before the ride starts, the people opposite me are at the same height above the ground as I am. During the ride, this doesn't change as the rotation axis of the ride is fixed. But due to the vector addition of the centrifugal acceleration and the earth's gravitational field, my perception of the vertical is no longer the same as the "true" vertical. The people opposite me appear to be well above me even though they remain the same height above the ground as I am. Because the centrifugal acceleration of the ride is about 3g, the perceived vertical direction is about 20° above the horizontal.

 

 

2 hours ago, danielj said:

reading this back I see there being potentially many different frames of reference, equally valid?

Yes, all frames of reference are equally valid in general relativity.

 

 

[danielj]

27 minutes ago, KJW said:

Before the ride starts, the people opposite me are at the same height above the ground as I am. During the ride, this doesn't change as the rotation axis of the ride is fixed. But due to the vector addition of the centrifugal acceleration and the earth's gravitational field, my perception of the vertical is no longer the same as the "true" vertical. The people opposite me appear to be well above me even though they remain the same height above the ground as I am. Because the centrifugal acceleration of the ride is about 3g, the perceived vertical direction is about 20° above the horizontal.

 

 

Yes, all frames of reference are equally valid in general relativity.

 

 

 My take away from this is that I am lacking logical thinking skills, when it comes to physics. In that, I can’t make assumptions and expect to get the right answer. Understanding the question is more important than the answer I think.

thank you

[Phi for All]

20 hours ago, danielj said:

 My take away from this is that I am lacking logical thinking skills, when it comes to physics. In that, I can’t make assumptions and expect to get the right answer. Understanding the question is more important than the answer I think.

thank you

You aren't supposed to be using logic. Those are formal rules used in philosophy and maths.

What you're thinking of is reasoning skills, or critical thinking skills. Those are what we all need to get better at.

Unfortunately, I think what you're referring to is something completely different. What you're "taking away" from this is that your assumptions are based on your intuition, and science has no obligation to make any kind of intuitive sense. There are plenty of explanations that seem at odds with what we think we know. Time is certainly not intuitive, and the laws of physics don't recognize a difference between past and future. And is there anything intuitive about the Placebo Effect? Or Black Holes?

Also, "right answer" is not a scientific concept. Best supported explanations for various phenomena is how theory works. 

[danielj]

5 hours ago, Phi for All said:

and the laws of physics don't recognize a difference between past and future.

Thank you, thats all very helpful. Would you mind expanding the above statement please?

 

Cosmological Principle? 
 at sufficiently large scales, the universe appears as homogeneous and isotropic 

Copied from Mordreds thread ‘Cosmological Principle’

there I go with a guess again 🫣

[studiot]

Just now, danielj said:

Thank you, thats all very helpful. Would you mind expanding the above statement please?

The point about equations is that it does not matter which way round you write them.

If A = B  then   B  = A

 

Most of the laws of Physics are of this form so if Time is involved it does not matter whether time goes forward or backwards, the equations still hold.

 

There is one major law of Physics called the second law of thermodynamics which doe not have the form of an equality (or equation), it is an inequality so

 

If A < B  then going the other way B is greater than A.

Since the second law involves time it cannot be reversed and hold the same form like an equation.

[dedo]

Nice thread.  May check out the references & examples here myself.  In case no one mentioned:

"Space-time for Absolute Beginners" by Goldsmith is nice, short, & has notes in the back listing 40 other books on various related subjects you might also find interesting.  Good luck.

[danielj]

On 2/26/2025 at 11:48 PM, dedo said:

Good luck

Thank you.

 

Ive another question for Studiot, arising from my readings. 
 

I went outside in the evening to take a look at the Stars. When those photons that hit my retina arrive there, they have propagated through space, but in what form have they travelled? Were they emitted as three dimensional waves (spherical waves?)  and then I have collapsed that wave into a single photon by observing (or measuring?)  it? Or am I getting my Classical and Quantum effects mixed up? Also, if I see a distorted star, is that because I’m not viewing the same photon, but one (some?) that have interacted with the earth atmosphere and been absorbed and re-emitted on a slightly different trajectory?

thank you

Edited Monday at 12:28 PM by danielj

[studiot]

Just now, danielj said:

Thank you.

 

Ive another question for Studiot, arising from my readings. 
 

I went outside in the evening to take a look at the Stars. When those photons that hit my retina arrive there, they have propagated through space, but in what form have they travelled? Were they emitted as three dimensional waves (spherical waves?)  and then I have collapsed that wave into a single photon by observing (or measuring?)  it? Or am I getting my Classical and Quantum effects mixed up? Also, if I see a distorted star, is that because I’m not viewing the same photon, but one (some?) that have interacted with the earth atmosphere and been absorbed and re-emitted on a slightly different trajectory?

thank you

The passage of light is affected by many factors as it passes from distant astronomical bodes to your eyes.

Locally to you, yes the atmousphere changes the colour of that light, blocking some colours more than others.

Your own optical instruments, including your eyes, are subject to distortions, called aberrations. these are responsible for most of the 'halo' round the edges.

 

In addition there are dust and gas clouds in deep space that filter and block parts of the light.

The light is not affected by electric or magnetic fields, but it is deflected (bent) by gravity. So sometime the twinkling stars are not actually where you think they are by 'line of sight'.

Yes I advise to use either classical (including relativity) or quantum explanations, but not both together.

I like whichever is easier for the observed results.

[danielj]

5 hours ago, studiot said:

The light is not affected by electric or magnetic fields, but it is deflected (bent) by gravity. So sometime the twinkling stars are not actually where you think they are by 'line of sight'.

If I say that the path of that photon passed close by the Sun before I measured it, then the mass of the sun would cause it to take a curved (longer) path, described in Einsteins General theory, thereby causing that bending effect? And if I used Newtonian physics to try and calculate the same path, it would not agree (with General Relativity) because Newtonian physics does not account for mass/gravity causing deformations in space? Also, with the photon being massless (therefore travelling at c in a vacuum) does this mean that a particle with mass would be affected more or is the curvature of Spacetime the only factor?

[KJW]

The deflection of light by the Sun is twice that predicted by Newtonian gravity. Half of this value (equal to the value predicted by Newtonian gravity) satisfies the equivalence principle and is the result of the gravitational time dilation. The remaining half is due to the curvature of the three-dimensional space. I believe that these two halves are equal because the spacetime trajectory is lightlike.

 

[swansont]

44 minutes ago, danielj said:

Also, with the photon being massless (therefore travelling at c in a vacuum) does this mean that a particle with mass would be affected more or is the curvature of Spacetime the only factor?

The photon takes certain path (null geodesic) because it is massless and travels at c. Massive particles follow different geodesics. But it’s all because of the curvature.

[danielj]

Thank you all

[studiot]

Just now, danielj said:

If I say that the path of that photon passed close by the Sun before I measured it, then the mass of the sun would cause it to take a curved (longer) path, described in Einsteins General theory, thereby causing that bending effect? And if I used Newtonian physics to try and calculate the same path, it would not agree (with General Relativity) because Newtonian physics does not account for mass/gravity causing deformations in space?

Basically yes but it is mass and energy which cause 'deformations' in space, not mass and gravity.

Gravity is the observed result of these 'deformations'.

As swansont says particles with mass (aka massive particles) follow different paths through spacetime so this is a very good question +1

Just now, danielj said:

does this mean that a particle with mass would be affected more or is the curvature of Spacetime the only factor?

Unless you know of a particle of similar mass to the Sun it is likely that the massive particle would be drawn into the Sun by the stronger effect.

It should also be said that we really only notice this deflection effect on light when it allows us to see a star that should be behind or hidden by the Sun.

[danielj]

Thank you, as an aside, I find it very difficult to conceptualise ‘massless’. Perhaps because I think of ‘things’, and they have size/volume/mass. In the case of photons, does this mean that all of its properties are confined to c? Or in effect it is c? A perturbation travelling at c? 

[swansont]

18 minutes ago, danielj said:

Thank you, as an aside, I find it very difficult to conceptualise ‘massless’. Perhaps because I think of ‘things’, and they have size/volume/mass. In the case of photons, does this mean that all of its properties are confined to c? Or in effect it is c? A perturbation travelling at c? 

A perturbation in the electromagnetic field, moving at c, is one way to look at them.

Being quantum particles, classical descriptions don’t always cover all of the behavior.  (angular momentum being one thing that might not be a property that is obvious from the above description)