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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/sec˛ 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.

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New Theories / Re: How will buckyballs fired at a double slit behave?

« on: 17/06/2019 19:12:31 »

Atoms bouncing off each other collapses their waves.

That doesn't even parse.

There's something you need to understand.
It has already been pointed out, but you have missed it.
Imagine making a grating with a gap between the "wires" that's a little bit smaller than the molecules you are using.

Classically, no particles will get through it.

QM and the uncertainty principle means that a few will. They will quantum tunnel through.

Here's the bit you don't understand.
If you take the grating and hammer it until the wires are squashed flat and there are no gaps so it becomes a metal foil then repeat the experiment...

More atoms will get through the foil even though it no longer has gaps in it. (In some circumstances)

Do you understand that?
 The probability of tunneling is related to the thickness of the barrier.
Hammering it flat makes it thinner and so it's more likely that atoms will tunnel through.

Obviously, with the grating destroyed, there's no diffraction pattern.

And the (smaller number of) atoms that went through the grating before  you hammered it flat would also show no clear diffraction pattern because, at that level, the atoms are not going through the gaps.

It's the same , classically, with light
You know the equation
d sin θ =  λ
Where lambda is the wavelength and d is the spacing
you can rewrite that as

sin θ =  λ /d
Well, if you make d smaller than lambda then you are trying to find an angle where sin theta is more than 1, but that's impossible.

Diffraction doesn't work if the wavelength is bigger  than the grating's spacing.
And, returning to QM, a particle can't be "smaller" than the associated wavelength.

So, you can't sensibly calculate a diffraction pattern for the the experiment you have proposed.

So there's no way to say whether the particles would follow it or not.
And, as has been pointed out before, you also can't measure it because, in practice big  things don't go through small holes.

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Physics, Astronomy & Cosmology / Re: Can tunneling work with gravitational potential wells?

« on: 15/06/2019 12:34:43 »

Tunneling amplitude is proportional to e^{–width of barrier}

https://en.wikipedia.org/wiki/Quantum_tunnelling

even empty space can be viewed as a barrier through which to tunnel.

Electric potentials within molecule-scale systems can have very high gradients, so the distances involved are small.

For gravitational wells, gradients are typically not very strong (except in the hypothetical cases mentioned by evan_au), so the distances are likely to be such that tunneling can only happen once every million universe lifetimes or so (not actually calculated, but you get the idea)

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Physics, Astronomy & Cosmology / Re: Can tunneling work with gravitational potential wells?

« on: 15/06/2019 11:50:07 »

In the quantum world, everything is a bit fuzzy, if you look closely enough.

One could imagine that if we ever get a self-consistent theory of quantum gravity, that there might be a bit of quantum uncertainty about what is inside or outside an event horizon, a (hypothetical) micro-black hole, or a (hypothetical) cosmic string.
See: https://en.wikipedia.org/wiki/Cosmic_string

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New Theories / Re: How will buckyballs fired at a double slit behave?

« on: 14/06/2019 18:12:09 »

How will buckyballs fired at a double slit behave?
Presumably the same way they did last time.

"In 1999, the double-slit experiment was successfully performed with buckyball molecules "
from
https://en.wikipedia.org/wiki/Double-slit_experiment

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New Theories / Re: How will buckyballs fired at a double slit behave?

« on: 14/06/2019 03:53:18 »

Mass has little to do with how big a slit something needs to get through... (so the proposed experiment can neither confirm nor challenge the notion that waves can have mass--spoiler alert, waves can have mass)

...but there is a somewhat nifty way to see that mass is important for calculating tunneling barriers for particles (which depends on the wave function treatment of the particles):

We can measure the rates of chemical reactions, and see how much they change when hydrogen atoms are switched for deuterium (chemically identical isotope, but with twice the mass). This is called the kinetic isotope effect (or KIE). Sometimes a hydrogen atom has to tunnel from one place to another for the reaction to occur, and sometimes (especially when the reaction is very cold) this is the rate determining step, which means that we can directly measure how long (on average) it takes for protons to tunnel from x to y, and compare with hoe long it takes deuterons to tunnel from x to y. When this tunneling is the rate determining step, the KIE is very large, sometimes even slowing down by a factor of 100 when doubling the mass of the tunneling particle. This means that mass is an important property of a quantum object, even when considering it as a wave.

https://en.wikipedia.org/wiki/Kinetic_isotope_effect#Tunneling

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Physics, Astronomy & Cosmology / Re: Is it possible to predict a quantum transition (and block it)??

« on: 09/06/2019 15:24:15 »

You can certainly change the probability of a transition.
Putting a fluorescent molecule near a mirror changes the fluorescence half life.

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Physics, Astronomy & Cosmology / Re: What would happen if a human freezes to absolute zero temperature?

« on: 02/06/2019 19:15:23 »

What would happen if a human freezes to absolute zero temperature?
Death

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Physics, Astronomy & Cosmology / Re: How does gravity exert its influence infinitely?

« on: 28/05/2019 19:07:16 »

Now if electromagnetic energy has a gravitational field, photons travelling through otherwise unoccupied space will tend to clump together (since gravity always sucks) , so the intensity of light received from distant stars will increase with distance, by self-focussing. And there will be stars out there that we can't see because their self-focussed beam isn't pointing in our direction.

Really?

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Technology / Re: When will driver-less cars become a reality?

« on: 25/05/2019 09:05:51 »

So far, driverless (or at least "driver-assist") vehicles work well on motorways and similar controlled, predictable environments. Things go wrong when you have to make a choice.

Dog in the road. Stop, drive round it (which way?), blow horn, or slow down and drive straight at it? The last is probably the safest as the dog will recognise the threat and move, but you don't know which way. But a bull or a ram is quite likely to hold its ground to defend its family, so your car needs to recognise species (Been there! We stopped, and the ram charged at the car!)

Arrive at destination, turn off the main road onto a building site with temporary signs that have been changed since yesterday. Can your car read handwritten "office closed - report to traffic marshal for parking"? Is the truck in front of you stopped to allow other traffic to pass, parked, or unloading?

Motorways aren't a problem anyway. People die on side streets, city roads, country lanes and work areas (including supermarket car parks), where programmed vehicles tend to get confused.

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Technology / Re: When will driver-less cars become a reality?

« on: 22/05/2019 18:11:42 »

But you just did!

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Physics, Astronomy & Cosmology / Re: Is spacetime always understood in relation to the speed of light (em radiation)?

« on: 19/05/2019 16:26:18 »

Einstein argued that if you were inside a box you would not be able to tell if you were in a freely falling frame in a gravitational field or in an inertial frame. In an accelerating frame an object would be emitting electromagnetic radiation. So does the free falling object in the box emit electromagnetic waves? Does the radiation increase with increasing speed? If it emits no radiation due to acceleration then it can be considered truly inertial. In which case Einstein is correct. Which is it?

https://en.wikipedia.org/wiki/Paradox_of_radiation_of_charged_particles_in_a_gravitational_field

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Physics, Astronomy & Cosmology / Re: Is earth moving at close to the speed of light?

« on: 25/04/2019 08:59:42 »

Since motion is relative then in some frames of reference we can be considered to be moving close to the speed of light. Are we actually moving at close to the speed of light?

The question is incomplete. You need to ask "are we moving near the speed of light in as measured in frame S." where S is specified by the person asking the question. In our frame (sitting on earth) the answer is no.

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Physics, Astronomy & Cosmology / Re: Is earth moving at close to the speed of light?

« on: 24/04/2019 12:45:52 »

But we are! We see red and blue shifts out there, which means that, simultaneously, other observers see us as red or blue shifted, and all shades between.