1

That CAN'T be true! / Re: where does Colin's sailing boat, energy and momentum come from ?

« on: 22/08/2021 16:31:55 »

The difference between the apparent (based on the Sun) and real rotations of the Earth is this
https://en.wikipedia.org/wiki/Equation_of_time

2

That CAN'T be true! / Re: where does Colin's sailing boat, energy and momentum come from ?

« on: 22/08/2021 02:00:10 »

Hi everyone.

Sidereal day     vs.    Solar day
   This video draws the usual diagrams and says the usual things to explain the difference.   You'll only need the first 2 minutes but the whole thing is only 3 minutes.

   Now imagine the orbit of the earth around the sun is elliptical and not circular.  The angle between the earth and the sun changes by a variable amount, it's not exactly 1 degree per day it just averages at about this amount.
   Halc was suggesting your (Gem) data measures the solar day not the sidereal day.

Best Wishes.

3

That CAN'T be true! / Re: where does Colin's sailing boat, energy and momentum come from ?

« on: 09/08/2021 21:58:21 »

this gives a momentum pressure of between 3.33 x 10^-6 N/m^2  and 6.67 x 10^-6 N/m^2
depending on full absorption to perfect reflection, of the light energy received.

The Crooke's Radiometer, or "light mill" lets you test this calculation.

If light is perfectly reflected (silvered side), you transfer twice the momentum as the absorption case (black side), which means that the silvered side will move away from the source of light:

In fact, the black side moves away from the source of light.
- This is because the photon carries electromagnetic energy, E=hf. This is absorbed by the black side, heats it up, which heats the adjacent air, and starts the light mill turning
- This force of heated air is greater than the momentum transferred by photons to the mirrored side.

The Light Mill is in a partial vacuum (about 0.1% of atmospheric pressure).
- If you put in in a very good vacuum, with very good bearings, the light pressure is the primary force, and it turns in the other direction.

See: https://en.wikipedia.org/wiki/Crookes_radiometer

4

General Science / Re: How would our weight differ on a revolving to that of a non-revolving earth.?

« on: 26/07/2021 12:45:36 »

The plumb line only hangs vertically at the equator,

For a particular definition of vertical. (essentially: "points to the middle of the Earth").
But if your definition is "perpendicular to the surface of the Earth" then the plumb line is always vertical.
This definition also means that things fall vertically and you can build a vertical tower of bricks without needing cement.

Er, no it doesn't. Gravity (G, in black) acts perpendicular to the earths surface

You have assumed a spherical Earth- often a reasonable start, but not in this thread about the Earth not being a sphere.

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General Science / Re: How would our weight differ on a revolving to that of a non-revolving earth.?

« on: 26/07/2021 12:37:38 »

I was quoting directly from

https://www.sfu.ca/~boal/211lecs/211lec12.pdf

Hmm, OK.

I follow the maths which concludes with
"ε ~ 0.0017 x 57.3 = 0.1 of a degree"
which is the figure I got. But then having arrived at that, they throw in
"because the rotational motion affects both the surface of the Earth as well as the bob, then S remains perpendicular to the Earth’s surface"
without further explanation, which is the bit I don't follow.

6

Just Chat! / Re: Mathematics is a decent science.

« on: 19/06/2021 09:17:47 »

Mathematical proofs are absolute deductions from clearly stated axioms. So mathematical knowledge, in the form of standard formulae and solutions, grows by expansion with no culling.

Scientific proof is closer to "proof spirit" or the German Prufung - a test. Scientific knowledge is the residue of explanatory hypotheses that have not been disproved by experiment, so it grows almost by contraction towards "unified theories" and the like, and scientific data, the measurements that we use for engineering and for investigating the validity of our hypotheses, grows by expansion and refinement.

The bit in between, where we use known mathematics to model our observations and make predictions, is intellectually treacherous: as can be seen from many threads in this forum, it is tempting to believe that the model is the reality. The simplest case of the behavior of light, demonstrates that you sometimes need two very different models to predict the result of a single reality.   

7

Physics, Astronomy & Cosmology / Re: Problems with PopSci articles in GR. Should we stop using one example?

« on: 23/05/2021 16:54:42 »

The sudden disappearance of the sun is much more like an example situation to help visualise something about GR.   It simply doesn't fit the criteria to be considered a "thought experiment" (IMHO).

I agree, but I am pretty ambivalent about it's use.

I've never used it directly myself. In the thread mentioned I was interested in pointing to the latter half of the article in the link and hadn't even noticed that this example had been used - notice he also uses the breaking string example. Therin lies a problem, because even if we don't use it directly, it is so widespread that suggesting someone read such and such an article, website or paper might well direct them to this example, and not all those will be Popsci it will include well respected experts in GR.
Anyway, 'nuff said on this before I get on to misrepresentations of QM!

This is a quality resource and it has kept me occupied for hours. Thank you, thank you.  I think you've already pulled the text that is most relevant to this forum thread and put it directly into your earlier post.
I don't seem to have much disagreement with or from you, Colin.

John is one of those really nice guys who is also an excellent researcher and thinker. You should find something on his site about thought experiments in general. Lots of good material and he’s happy to share.

  Any student wishing to follow this example through more carefully with some Mathematics can find a way to provide that acceleration to the sun.  They can add stiff strings, photon rockets or high momentum incoming ping-pong balls if they need to.  The main thing is that they don't hit a wall that stops them proceeding with the Mathematics as soon as they look at the E.F.E. and realise that the proposed stress-energy tensor isn't even continuous let alone differentiable.

Haven’t had chance to read through all the latest posts, but this point caught my eye.
The objective of this site is outreach to a general public and interested amateurs to try and stimulate an interest in science. As such it is very unlikely that the average questioner will understand tensors, let alone stress-energy tensors. For that reason the level of answer we give is very different from that on say physicsstackexchange.
Take this question which is somewhat typical "How long could humanity survive if the sun went out?" https://www.thenakedscientists.com/forum/index.php?topic=82152.0
(Note: We have also had " What would happen to the earth is the sun suddenly dissapeared")
We don't answer this by starting to discuss nuclear fusion and the technicalities of why the sun cannot suddenly blink out, and the audience expects little maths, just a straight forward answer they can understand.  Obviously some of the questions go a little deeper, but in general we don’t expect to go much beyond school physics in terms of technical understanding.
I don’t say this to put you off your quest, but just to put some perspective for this site.

Can I make one plea on terminology. A gravity wave is one where the returning force on the displacement is gravity eg a water wave. In GR we refer to gravitational waves to differentiate.

I think it is a mistake to characterize gravity as 'action' at all. It's a field, not something that radiates towards us.

I think you, like me, enjoy looking at physics from different perspectives and use the most appropriate at the time.  I remember as an undergrad we were set the exercise of deriving Newton’s laws from GR in the weak field limit, a useful reminder that in the approximations they are still valid.
I assume you are familiar with the work of Fokker, Tetrode & Schwarzschild and the Wheeler, Feynman analysis of this in terms of electrodynamics and particle action. It’s an interesting look at the relationship between field theory and stationary action, bit old but still interesting, particularly coming from action man himself.

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Physics, Astronomy & Cosmology / Re: Problems with PopSci articles in GR. Should we stop using one example?

« on: 20/05/2021 18:51:26 »

Would you care to put a number to the mass of a photon? 

Think you can put a value on the equivalence substituting hf for E in the famous equation

In principle you can derive a relativistic mass for the photon, but this is not often used nowadays.
The m in E=mc² is rest mass, more properly m₀, which for a photon is 0, so E=0 for m₀. The energy of the photon is in the second part of the famous equation E=pc and it is possible to have momentum without mass.
Because all forms of energy (of which mass contains a lot concentrated in a small volume) affect spacetime it is easier to use energy as the common currency rather than mass.

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Physics, Astronomy & Cosmology / Re: Can you measure the one way speed of light without synchronised clocks?

« on: 29/04/2021 10:07:33 »

I'm just trying to unscramble those two links.

Hi all

Is there any method that you can think of that could achieve this? Have a stab at it.

Similar to the sodium lamp, I believe it would be possible to derive the speed of light using the red/blue shift due to interaction of light with a gravitational field, for example
 https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment

http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/gratim.html

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Physics, Astronomy & Cosmology / Re: Is time travel possible?

« on: 12/04/2021 01:45:17 »

Hi all
Halc can I please just clarify your points in regards to;

All clocks are affected by these things. Atomic clocks are in no way special in this way. Paint peels slower relative to a frame in which it moves faster.

In regards to your first point does direction of relative away/towards have any bearing

"Direction of relative" does not parse. If you mean direction of motion, the answer is no, it matters not. In SR, time dilation relative to inertial frame F is a function of your speed relative to F, and speed, being a scalar, doesn't have a direction.

The coordinate rate of clocks can indeed be negative given a non-inertial coordinate system.

are you stating rate of atomic clocks as negative as going in reverse or just slowing down ?.
Because if negative = reverse

I meant reverse. We're talking about a coordinate effect, and one can create a non-inertial coordinate system where some clocks run backwards. Look up the Andromeda 'paradox' as an illustration of this.

are we not setting the scenario of the paint analogy going from peeling/flaking into the wind to.  returning to the painted surface becoming shiny and new then liquid in the brush/sprayer to been back in the paint tin ?

If my coordinate system orders the paint events that way, then yes. It's just a coordinate system.

11

New Theories / Re: Split: Attempts to falsify relativity

« on: 17/03/2021 03:12:26 »

So is it not just an energy wave detector, detecting an effect from a cause ?

It is an energy wave detector. The gravitational waves are a cause by which changes in the mirror distance can be detected. That is a cause producing a detectable effect. We can know when they are caused by gravitational waves and not local disturbances because (1) gravitational waves produce predictable, regular and opposite changes between the two different arms of the detectors, and (2) LIGO has two stations very, very far apart: one in Louisiana and one in Washington state. If both detectors pick up the trademark pattern of a gravitational wave at roughly the same time, then that rules out local vibrations as the cause. VIRGO, a similar gravitational wave detector in Italy, can provide further confirmation of the detection.

12

Physics, Astronomy & Cosmology / How to define impulse of a rotating sphere?

« on: 10/07/2011 19:56:58 »

Thanks.
Then I would like to know average speed of all particles of the sphere(relatively of me).

If
ω = angular speed of rotation
R = sphere's radius

then, if I computed correctly the integral, it should be (3/16)πωR.