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

« on: 19/04/2021 06:36:11 »

It should be fairly easy to calculate the one-way speed of light.  The problem is doing so with any reasonable amount of accuracy.

Here's how I would do it.


SpeedOfLight.gif (9.32 kB . 669x311 - viewed 45605 times)

Light (strong laser) passes through two shaft connected spinning discs, hits a cylindrical (conical) mirror, and is projected onto a wall to record.  Mostly interested in the trailing edge of the light spot.

Light speed  is: 299,792,458 m/s.

So, say the two discs are spaced 10 meters apart, then it will have traversed the gap in about 1/(29,979,245) s

Say the discs are 2 meters in diameter, and spinning at 60,000 RPM (1,000 RPS).  With a circumference of πD, or 6.28 m.

So, your discs are spinning at 6,280 m/s.

Multiply the two, and one gets about 1/4774 m, or .0002094 m  (2x10^-4)

Multiply by 1000, and one is 0.2094 mm

That is well within the range one can discern.

The problem is the accuracy of the reading.  If one uses UV light, one might be able to get down to 100nm accuracy, or about 1x10^-7 m. 

So that gives one about 3 digits of accuracy.  One may be able to add a 4th digit, or even 5th digit by going with a faster shaft speed, longer shaft, larger discs, etc.  Perhaps using X-Rays if they could be reflected on the cylindrical mirror (or mirror pair).

So, the question is would say 3 or 4 digits of accuracy matter?

Now, let's consider some kind of unknown aether or fabric of space(time). 

What is the fastest object known to man?  well, Earth is a good candidate.

The equator is rotating at about 460m/s.  And, even doubling it for the difference of forward rotation vs reverse rotation with respect to the fabric of space during the 24 hour day, and it still isn't fast enough.

Earth's orbital velocity around the sun is about 29,780,000 m/s, or about 1/10,000 the speed of light.  And, still not fast enough to pick up.  Earth's orbital speed around the Milky Way is about 220,000 m/s, and getting close to 1/1000 the speed of light.  And, the Milky Way is moving at about 600,000 m/s.

Now, if our aether or fabric of space is not distorted by gravity, electrical fields, matter, etc, then one can choose a place to orient one's device East/West with respect to the galactic rotation or motion, or North/South with respect to the motion.  This should give comparative velocities, but one is just barely on the cusp of being able to pick it up.

(oops, I got a conversion factor to mm off... but it means it is easier to snag the first digit of the speed of light with the theoretical machine, but still a difficult task to get multiple digits of accuracy).

Ok, thinking about this more, there are issues with the leading edge of the light from the first slot illuminating the trailing edge from the second slot making timing difficult.

One would likely fix that by either adding an adjustible flat mirror to the inbound light, or slightly changing the direction of the source.  Adjust to both maximize the intensity of the output beam as well as maximizing the beam width coming off of the cylindrical mirror.

A concave mirror rather than a convex mirror would also work, but would flip the image.  Parabolic?

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

« on: 18/04/2021 22:54:54 »

Generate a radio signal at a convenient frequency and measure the wavelength at any point, by any means you wish (diffraction, quarter-wave dipole, whatever) . v = fλ by definition.

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

« on: 18/04/2021 15:10:46 »

That's interesting. The Wiki article references the idea that the method can be considered a "doppler shift".
Well, maybe it can or not- I guess it depends on your definitions.

But there's a streetlight at the end of the road and it's not been replaced by LED yet, so it is a sodium lamp.
And I can put a sodium lamp in a car and drive towards the streetlight.
And, in principle, i can allow light from both curses to fall on a photodiode and I will get a voltage from that diode corresponding to the difference in frequencies - one of my local sodium lamp and the other of the doppler shifted streetlight.

Since I know my speed and the beat frequency and the wavelength of the light, I think that's enough to let me calculare teh speed of light.

Equivalently, I can use a radar "speed gun" and a vehicle with a known speed to measure the speed of light.


Does that count as "one way"?

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Just Chat! / Re: Useless factoid of the day

« on: 01/04/2021 02:23:40 »

If the diameter of a circle equals pi then its circumference equals pi squared and its area equals pi cubed. A cubic area! My lord the world makes no sense!

R=½π
Circumference = 2πR = 2π*½π=π²
Area of Circle = πR² = π*(½π)² = ¼π³

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Just Chat! / Re: Can science prove God exists?

« on: 01/04/2020 23:21:14 »

Why so angry about God?

My anger is about the stupidity of people who waste their lives in the pointless worship of something that cannot possibly exist, or if it did, could not be affected by worship. And also towards the disgusting behavior of people who use god as an excuse for their perversions. Good deeds do not need an excuse.

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Physics, Astronomy & Cosmology / Re: Are there half visible galaxies?

« on: 31/03/2020 17:10:26 »

  But that doesn't mean that , if there's someone exactly a mile away, I can only see the tip of their nose and their toes.

I interpreted Jeffrey’s “shouldn't we be able to see only half a galaxy at the boundary of the visible universe?” as meaning something like: If he boundary of the visible universe coincided with the centre line of a galaxy, would this mean that, in principle, only half of the galaxy would be visible, if we had the ability to see it. 

I suspect that, at that distance, any galaxy would appear so small that the point is moot; but Jeffrey’s question still stands.

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Physics, Astronomy & Cosmology / Re: Does the energy of a photon affect its path through a gravitational field?

« on: 05/01/2020 14:40:15 »

That is, does a photon's momentum affect its path through the gravitational field in the same way a massive object's velocity would?

The momentum of a photon does not affect it's path through a gravitational field.  Or another way to say this is that photons of different energies will follow the same geodesic.
Additionally the mass of an object does not affect it's path through a gravitational field.  If you shoot a feather and a cannon ball in a vacuum at the same velocity and angle, they will both follow the same parabolic path.  Mass will not change the path.

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Physics, Astronomy & Cosmology / Re: Can the whole universe be considered a quantum system?

« on: 29/12/2019 13:02:08 »

I think it all depends on what you mean by "the same". Suppose we polarise the unpaired spins of a sample in a homogeneous magnetic field. Then with respect to the field, they are all in the same state, but because they are in different positions in the sample, they are in individual states. If we reverse the field, the spins do not all flip at the same time but follow a relaxation curve.

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Physics, Astronomy & Cosmology / Re: Can the whole universe be considered a quantum system?

« on: 29/12/2019 10:08:00 »

Sean Carroll has discussed similar questions on his Mindscape podcast. As I understand his comments:
A quantum-entangled system coming in contact with an outside particle becomes entangled with the state of that particle.
- But because you generally don't know the quantum state of the outside particle, your quantum system goes into an unknown state.

Extending that, if your carefully controlled quantum-entangled system comes in contact with an uncontrolled outside system, your controlled system becomes entangled with the state of the uncontrolled system, and also enters an unknown state
- That's why quantum states are very delicate and are isolated from all external influences (as far as possible)

can no two electrons in the entire universe share the same quantum state?

I would place a constraint on this:  since a particle cannot influence another particle outside its light cone, I would suggest that the question be rephrased as "can no two electrons in the same light cone can share the same quantum state?".

For a single, isolated atom, the Pauli Exclusion Principle ensures this.
- For atoms that are brought close together (in a gas), the spectral lines are broadened, as the outer electrons adopt very slightly different energy levels
- In a metal, the electrons in the conduction band can take on quite a wide range of energies - the conduction band.

This broadening of energy levels does not apply so much to inner electrons, which are partially shielded from adjacent atoms by the outer electrons.
See: https://en.wikipedia.org/wiki/Spectral_line#Line_broadening_and_shift

Can the whole universe be considered a quantum system?

According to the Big Bang Theory, at one time the universe existed within the same light cone, so there was the possibility that all particles could have been entangled with every other particle in the universe. That produces a quantum state that is too complex for us to understand...

I can't comment on whether the universe actually contained electrons at that time, or perhaps some other, more exotic particles (such as the LHC researchers have been seeking).

So even if we manage to create a controlled quantum state in a 50-odd qubit quantum computer (as Google claims)...
- We must carefully isolate it from the rest of the universe, as any interaction will create an unknown quantum state in our quantum computer   

The dream of quantum computer designers is to develop quantum error correction, so that if the quantum computer does come in contact with (say) 1 stray electron, there will be enough redundant state in the computer to deduce what impact that electron had on the quantum computer, and undo the change, without losing the integrity of the ongoing quantum computation.
- Of course, if the quantum computer comes into contact with a sufficiently complex uncontrolled system, it will overwhelm the  capacity of the error correction, and the quantum computation will need to be restarted.
See: https://en.wikipedia.org/wiki/Quantum_error_correction

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New Theories / Re: Is an Evolutionary Cosmology AND a Complete theory of everything Possible?

« on: 28/12/2019 14:16:04 »

Are you saying that SR has been disproven, and the twin paradox was a hoax?

No, I am saying you do not know what a dimension is and you making up stuff that makes sense to you, but is wrong.

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Physics, Astronomy & Cosmology / Re: What happens when both slits are observed by in the double slit experiment?

« on: 09/11/2019 07:37:47 »

QM theory says no amount of measuring of a system will allow you to predict it.

Be careful how you state this. The test of quantum mechanics is not merely that it explains obvious quantum phenomena like line spectra, but that it scales to classical mechanics for large assemblies. And it does.

The wave function of a billiard ball is negligible in comparison with its apparent diameter, so for all practical purposes (e.g. potting the black) it is adequately predictable. But its structure, mass and elasticity are all calculable from quantum mechanics.

Indeed the "measuring" test isn't fundamental to QM. The classical model of bouncing a photon off an electron sort of illustrates the problem of measurement influencing the measurand, but masks the inherent indeterminacy of Heisenberg's statement that Δp.Δx ≥ h.

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Physics, Astronomy & Cosmology / Re: What happens when both slits are observed by in the double slit experiment?

« on: 08/11/2019 19:10:08 »

When we fire bullets (particles) at a slit they will, as you said, go through one slit or the other and form the 2 slit shadow we expect.

No! There is a calculable probability that a bullet will, in effect,  "go through both slits". As Eddington said, "If a student of physics should fall through the floor and find himself in the room below, he would not be surprised but mildly elated at having observed an extremely rare phenomenon."

That's what I meant by quantum mechanics scaling up to continuum physics at a mesoscopic level. So far, it's only been demonstrated with large molecules like buckyballs, but the maths is sound.

You can work the model two ways: either map an enormous number of parallel universes onto one timeline to get "now", or wait a very long time in one universe for everything to happen. Problem is that once you get beyond 60 carbon atoms, the numbers become ridiculous.

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Plant Sciences, Zoology & Evolution / Re: How do dolphins and whales breathe when asleep?

« on: 08/11/2019 14:35:14 »

A bottlenose dolphin shuts down only half of its brain, along with the opposite eye, while the other half of the brain stays awake at a low level of alertness. The attentive side watches for predators, obstacles etc, it also signals when to rise to the surface for a breath. Every two hours, it will reverse this process, resting the active side of the brain and awaking the rested half. This pattern is often called cat-napping.
Dolphins also enter a deeper form of sleep, mostly at night, called logging because the dolphin resembles a log floating at the surface.

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Physiology & Medicine / Re: How long should a woman abstain from sex after giving birth?

« on: 31/08/2019 01:45:12 »

I think that's the wrong question.
- Having a baby is draining, and caring for a baby is even more draining, even with good partner support - this is a full-time job for many months!

A more important question is how long a man should abstain from pressuring her after a woman gives birth?
- And it depends on how the woman is coping
- She will cope better with good partner support

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Just Chat! / Re: What impact will BREXIT have on Science & Society?

« on: 04/08/2019 11:54:08 »

Was it a majority of pensioners that voted for Brexit, many of whom are now dead a year later?

Not many dead. About 5% per annum.

I voted for Brexit because I'm fed up with paying a membership fee to a club whose primary function is to bankrupt my country. I look forward to a time when I can once again recruit staff on the basis of ability, not nationality.

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Physics, Astronomy & Cosmology / Re: The Ultimate Special Relativity Question to Help Amateurs Understand

« on: 19/07/2019 19:58:15 »

You need to change your topic title per the first reply, and also because the OP has a lot of questions, none of which are the 'ultimate' one.

This scenario has been written to assist in the understanding a multitude of aspects of special relatively for the amateurs of us interested in cool physics (including me). I would be grateful for a physicist with a University degree to answer in detail (sorry no half baked musings on this on please).

While my answers are not half baked musings, I doubt there are any actual relevant physicists on forums like this one.  You want an educated answer, fine, but it's not going to come from a physicist.

Scenario:
C         <-- 100 million light years -->   A          <-- 100 million light years -->    B
                                             1 --> (a = 20 m/s/s until 0.99999c)
                                       2 --> (a = 20 m/s/s until 0.5c)
                                  <-- 3 (a = 20 m/s/s until 0.99999c)   
                                  <-- 4 (a = 20 m/s/s until 0.5c)   

Primary Details:
- Spaceships 1 and 2 set out from observer A towards observer B who is 100 million light years away. Each accelerate at 20 m/s/s (i.e. ~2g) until cruising velocity is made. 1 accelerates to 0.99999c while 2 only accelerates to 0.5c
-Similarly spaceships 3 and 4 set out from observer A to C.

It isn't really stated, but I presume from this description that C, A, and B are locations in some reference frame and that the observers so named are stationary at those locations and the distance given (the 100m LY) is measured in that frame.
The fact that you have accelerating ships kind of gets into accelerated reference frames which is covered better under general relativity (GR), but special relativity also covers it.  It just requires more calculus than would something like instant acceleration scenarios.

The pairs of ships accelerate side by side until the slow ones quit and the faster ones continue to accelerate up to the higher speed.

- Each spaceship also decelerates at 20m/s/s as it approaches the target observer such that they stop when they reach the target.

- 1, 2, 3, 4, A, B, C are all fitted with 6 lasers each, directed to each of the different observers and spaceships. The lasers flash once per 100 seconds for 1 second duration. i.e. on the 99th second the lasers turns on for 1 second. The light is green (wavelength of say 532 nm).

Secondary Details for Auxiliary questions :
- Each laser makes 1 GW of effective light emitted during the 1 second bursts.
- Each spaceship weighs 10 Tonnes excluding fuel. Let's assume each ship has some sort of particle accelerate to use as thrust that can accelerate the particles used as thrust to ~0.9c. Also it is powered by let's say some theoretical antimatter/matter source and the whole system can convert all matter into thrust at 99.9% energy efficiency.

If it exhausts the reaction mass at 0.9c, it is 90% efficient by that metric.  It really doesn't make a different if it is antimatter drive or water rocket like the space shuttle.

Please ignore the expansion of the universe.

It wouldn't be SR if we assumed otherwise.

Q1.a) According to spaceship 1, how long did it take to reach B?

~447222 years

b) According to spaceship 1, when does spaceship 2 arrive at B?

When the ships rejoin, the clock on ship A reads about 100446222 years.

c) describe, inducing a rough timeline, what spaceship 1 observes from the laser lights (i.e. the wavelength, how long between flashes and duration of flashes, the energy intensity) , originating from A, B, 2, 3, 4 as it accelerates to 0.99999c, cruises at 0.99999c, and finally approaches B.

The flashes are all the same at first.
Once everything is up to cruising speed, the light from A is dilated by a factor of about 223.6 and a Doppler factor of 100k, so it blinks around 22.36 million times slower.  Light from B is dilated by 223.6 again, but Doppler is in the other direction, so it blinks 447 times faster.  Frequency shifts are also proportional.

To do one more, the fast ship 3 has a gamma of about 1e5 and a Doppler factor of about 1e10 so it blinks about 1e15 slower.

a) According to spaceship 2, how long did it take to reach B?

I get 173 some million years

b) According to spaceship 2, when does 1 arrive at B?

Frame dependent question since spaceship 2 is not present at that event.

describe, including a rough timeline, what spaceship 2 observes from the laser light originating from A, B, 1, 3, 4 as it accelerates to 0.5c, cruises at 0.5c, and finally approaches B.

At cruise, gamma is 1.1547 and Doppler is a factor of two, at least relative to the lettered objects.  It can be worked out from that.
Ship 1 is moving at .99987c so those two ships see blinking slowed from each other by about 6.2 million

]a) According to observer B, when does 1 reach it?
b) According to observer B, when does 2 reach it?

We haven't stated what the clock at B reads at any particular event, so this cannot be known.  If it is synced to A in that original frame, then ship 1 gets there at year 100001000 and ship 2 at 200MYr (plus about 100 days).  This is pretty trivial arithmetic.

c) Describe, including a rough timeline, what observer B observes from the laser light originating from 1, 2, 3, 4.

Same thing that those ships saw when observing B.
As for timeline, B will observe nothing for 100m years since the departure event takes that long to be seen.  So ship 1 appears to be in flight only the last 1000 years, and ship 2 for the 2nd half of the duration.  All 4 ships appear to get up to cruising speed relatively quickly.  20 m/sec² will definitely get you there.

a) According to observer A, when does 1 reach B?
b) According to observer A, when does 2 reach B?

Assuming the frame of A and that clocks are synced in that frame, all these events are the same answer as B's answer.

c) Describe, including a rough timeline, what observer A observes from the laser light originating from 1, 2, 3, 4.

Same as those that seen by the ships looking at A.

d) According to observer A, how fast are 1 and 3 moving away from each other?

That's a 3 way relation, not particularly defined.  Ships 1 and 3 are moving at .99999999995c relative to each other period.  It's an objective relation, not a frame dependent one.

b) If each laser had  a narrow enough focus and the observers had a large enough receiver to capture all of the light emitted to them from each spaceship, what would the energy levels (GW) look like from each spaceship.

Work it out as a function of energy conservation.

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Physics, Astronomy & Cosmology / Re: Can a test of reciprocal time dilation be constructed from this?

« on: 10/07/2019 23:39:01 »

If that's all it does, it seems to only measure my speed component in the direction the gun is pointed, a fraction of my actual speed if I'm not driving directly towards or away from the gun.

Which is why it is very difficult to defend a charge of speeding against a Doppler radar gun - it can't indicate a higher speed than the target.

Alice could count the rate of rotation of her wheel (I've reduced them to unicycles to save engineering) which is of course how surface vehicles measure speed. Now suppose Bob's vehicle emits a light pulse for each rotation of the wheel. As they approach, Bob appears to be going faster than his measured speed, and as they recede, slower.

Suppose we fit an elastic cylinder to Bob's wheel. As it spins, it stretches, by centrifugal force. At constant speed, its diameter is constant. So Alice can estimate Bob's speed by the diameter of the cylinder, either by looking or
 by asking Bob to look and radio her, with no need for a time base. She could have an identical cylinder on her wheel, and using her own clock, can calibrate the speedometer. I can't think of a relativistic reason why Bob's cylinder should appear to contract perpendicularly to the velocity vector, so she now has two different values for their relative velocity.

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Physics, Astronomy & Cosmology / Re: Is the shift in the wavelength of light cancelled during free fall?

« on: 08/07/2019 12:04:45 »

It is possible that for certain combinations of gravitational field and velocity that these two relativistic effects could cancel each other out.

But if you are in free-fall, then your velocity and position in the gravitational field would be changing, so these relativistic effects would no longer cancel out.

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Physics, Astronomy & Cosmology / Re: Is the shift in the wavelength of light cancelled during free fall?

« on: 08/07/2019 08:47:12 »

True. But the Doppler shift depends on the relative velocity of source and receiver, and the gravitational shift, on their relative gravitational potentials. If both are in free fall towards a point gravitational source but at different altitudes, or even widely separated at the same altitude, all these numbers will be time-variant, so you need to consider the flight time of your photon from source to receiver.

I prefer to avoid "observer" in this forum as some people think physics is bound up with "consciousness" - whatever that is. AFAIK physics preceded anything that might be described as conscious!

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Physics, Astronomy & Cosmology / Re: Where were the electrons before the big bang?

« on: 22/06/2019 15:54:14 »

During the earliest stages of the universe, the fundamental forces were unified into a single force( or at least were all of the same strength).  This means that they did not even exist in forms that they have today.  As the universe cooled, different forces separated out from that mix, one by one. First gravity, then the strong force...   Thus when the "quark soup" was in sway, the electromagnetic force had yet to come into existence as a singular force, so the entity we now call the electron wasn't possible.
To use an analogy, its like when you freeze water.   It doesn't really make sense to ask what "form" or shape the crystal structure of ice took before the water started to freeze, as the crystals depend on the water having the properties to allow them to form.