501

Physiology & Medicine / Re: Is there a food that’s more nutritionally dense than physically dense?

« on: 03/09/2019 15:43:12 »

Is there any food that’s more nutritionally dense than it is physically dense? Meaning, if you eat 100 g of it before going to bed, you’ll wake up 200 g heavier (or at least 101 g heavier?)

Mass cannot just be created from nowhere.  You can consume 100g of hypothetical super food with 10,000 calories in it and you will mass exactly 100g more until you take on additional mass in the form of say air or water.

So I might gain more than 1g by eating a gram of salt, but that's because it makes me thirsty and I retain more of the water I drink to rebalance my salinity level.

502

Physics, Astronomy & Cosmology / Re: Does the Fermi paradox suggest that superluminal speeds are unachievable?

« on: 15/08/2019 12:03:41 »

While pondering the existence and consequence of faster than light particles, I wondered if tachyons are real and useful then we can travel the galaxy in years rather than in millennia.

How would the reality of tachyons help you (or another civilization) get anywhere fast if you're not made of them?

However, as Enrico Fermi pointed out in his famous lunch time paradox, we have seen zero proof of alien contact. So, either advanced extraterrestrials don't exist or exceeding light speed in space travel is impossible. Any thoughts on this?

You suggest the only solution for the Fermi paradox is the light speed limitation.  Here we are such an advanced civilization and the fastest speed we've been able to achieve is under 0.0001c.  OK, I think we got a ship to go faster than that, but only by dropping off a cliff, not actually having the ship do it under its own power.  The speed of light hasn't exactly been a barrier to us yet.
Contact with aliens doesn't necessarily entail travel.  If they were out there on every 1000th planet, we'd notice.

503

Physiology & Medicine / Re: Why Is It Easier To Complete A Puzzle After A "time-out" ?

« on: 13/08/2019 15:45:34 »

As a sheepy during my morning commute on the train, not only do I listen to the Beach Boys "Sheep Sounds" ewesing my "BLEATS"  but I also do word puzzles !...I have found that when I become stuck on a puzzle and then put it away till the commute home that I can then instantly comBLEAT the once offending puzzle from the morning with ease !

I get the same effect.  I suspect certain assumptions act as mental blocks pulling WOOL over your eyes, and giving it a rest allows these blocks to evaporate.  Looking at the problem anew after a time forces one to reassess what's going on, and often that assessment sees new things that were assumed not there before.  Something like that.

Somewhat related:  I often can't think of a name of a word, person or movie or something, and the harder I try, the more impossible the task of recollection.  Later on, I casually need the same word and it pops effortlessly to mind.  The explanation I give above seems not very relevant to that effect, but both involve setting the problem aside.

Sleeping on it works because your ability to think is severely impaired if you need sleep.  I learned in school not to cram all night, but to get proper sleep before an exam and possibly review the material an hour before the test.

504

Physics, Astronomy & Cosmology / Re: Do we go round in circles?

« on: 10/08/2019 14:16:49 »

I’ve put together a few questions, and made some tentative moves towards possible answers, or where they might be found.  Still a long way to go, but comments would be appreciated.

I put in some comments, but have not read the whole thread.
 

1. If infinity is not a number, how can you subtract anything from it?

It isn't meaningful to subtract a number from a not-a-number.  'Infinity' is more of an adjective, meaning 'without limit'.  Much confusion arises when its syntactic usage as a noun leads one to treat it as a number.

2. If the Universe is infinite, it contains an infinite number of galaxies.  How does one define an infinite number?

Again, it simply means there is no limit to the number of galaxies.

3. If one is subtracted from an infinite number of objects, is the remainder still an infinite number?  If not, what is it?

Per point 1, it is not meaningful to do addition and subtraction with 'without limit'.

4. Would an infinite number of (identical) objects contain all the examples of that object that could exist?

If they're identical, how is it not one object?  This sort of gets into the law of identity.
Perhaps you mean something like 'just because there are infinite points along a line doesn't mean that there are not other points that do not fall on that line.  So there are examples of points not in that set, but I'd not call any of the points 'identical' since they're all at different places on the line.

5 Is “absolute infinity” (sensu, Cantor) amenable to mathematical manipulation?

6.

The Absolute Infinite (symbol: Ω) is an extension of the idea of infinity proposed by mathematician Georg Cantor.
It can be thought as a number which is bigger than any conceivable or inconceivable quantity, either finite or transfinite.

How could this concept be expressed without referring to infinity as “a number”?

Since it isn't a number, not sure what you're asking.  It's expressed as Ω, but that's not your question. One infinity isn't larger than another since they're not numbers.  Perhaps it means 'highest cardinality', but cardinality isn't an expression of the magnitude of something.  Only numbers have magnitude.

I'd not trust wiki on this.  They call it a number, but any number is finite.  I think a proper mathematician would not word the description this way.

An infinite universe is possible only if the mean density of matter in the universe vanishes.

Why?  Simple explanation, please.
...
I'd really appreciate some guidance with #23.  I'm trying to tie up loose ends.

Since nobody replied to this, I'll just say that the mean density of matter in the universe is typically presumed to be the same as it is in the parts we see (cosmological principle).  yor-on's statement would only apply to a model where there is infinite space, but there is finite matter, presumably all clumped nearby.  I know of no such model that is seriously considered.

505

Technology / Re: Can we use physics to detect when people look at security devices?

« on: 10/08/2019 02:09:38 »

Responders are making assumptions that have noting to do with the post.

The responses mostly seem to be on topic, although there is a responder spouting nonsense.

For simplicity I will call a device that uses the double split experiment to detect if someone has percieved the device a "quantum perception switch" or QPS.

What exactly do you suppose this QPS does?  If it watches the door of the room, then it detects the person directly, and the double slit setup seems to not even be required.  So it is watching only the double-slit setup.  Exactly what would distinguish the experiment running with or without an observer?  You can win a Nobel prize if you can answer that.

If I put a QPS attached to the ceiling in the middle of a secure room would it send security a message any time somebody enters the secure room?

Only a Schrodinger's box is secure.  I'm willing to posit one, even though the best one ever built barely managed to contain an object just large enough to see without instruments.

Would a cat activate the QPS?

Would a camera actiate the QPS?

Depends on what it is set up to detect.  You haven't told anyone this.

506

Physics, Astronomy & Cosmology / Re: What is the nature of dark matter around a black hole?

« on: 05/08/2019 12:03:47 »

What is the nature of dark matter around a black hole? Does it fall in? Is there dark matter in black holes? Is it repelled?

If it (a specific bit of it) is aimed quite perfectly, it will fall in.  More likely it picks up speed as it falls and exits the system similar to the way a comet leaves.   Maybe it orbits, or maybe it moves too fast to orbit.
It cannot be slowed or repelled since there is no way in interact with dark matter.  That's what makes it dark.

507

Physics, Astronomy & Cosmology / Re: why are some heavenly bodies tilted on their axis and not others?

« on: 24/07/2019 11:51:59 »

Our moon is tide locked with Earth, and thus has lost most original spin/tilt it might have had.
Looking it up, the moon seems to have an inclination (tilt) of about 1.54° relative to the ecliptic and an inclination of its orbit of about 5.15°.

The inclination is typically fairly random and the odds of it being exactly zero is pretty remote.

508

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.

509

New Theories / Re: The possibility of coming to life again after death

« on: 19/07/2019 15:42:09 »

What I mean life doesn't continue forever,  it is discrete periods of living and dying, each period starts and ends, in a way that I won't feel nor remember its continuity but I still exist and feel life itself forever.
It is just the continuity of enjoying things eternally  without feeling eternity.

You should just adopt a more mainstream philosophy that meets these goals.  Try eternalism, which lets you do just that: continuity of enjoying things eternally, but without all the funny need to posit an implausible immaterial experiencer or the need to posit a cyclic universe that somehow produces the same identical configuration of matter each time.

510

Just Chat! / Re: An eternal quandary

« on: 18/07/2019 21:49:39 »

Normally not much, but being its primary defense, I suppose it would chuck plenty of wood at the bear that's with you.

511

Physics, Astronomy & Cosmology / Re: How can galaxies collide if they're moving away all the time/

« on: 18/07/2019 04:10:56 »

Of course variances can be generated by gravity ie bigger mass affecting smaller mass, but to have galaxys vectors significantly altered would take alot of mass.  Perhaps the blackhole and where it goes to is capable of pivoting galaxies ?

Black holes seem to play no significant role.  The central black hole of our galaxy is only about 3% of the mass of the galaxy.  The (relatively nearby) great attractor is on the order of hundreds of times as massive, plenty to pull us and Andromeda together.  There is another even larger mass (called the Shapley attractor) beyond that.  I think that's it, and beyond that scale, mass distribution becomes more or less homogeneous.

512

Technology / Re: What is the impact force of dropping a 40kg weight from waist height?

« on: 14/07/2019 20:11:33 »

Floor loadings are complicated.
The lab where I work has a reinforced concrete floor and it's rated for something like 3 tonnes per square metre. (If I remember rightly).

But, I am 75 Kg or so, and I don't go through the floor, even though, as I walk, most of my weight is carried on a few square cm of my toes.

They had airplane cabin floors rated (local) for 100 tons per square meter, but these women would go on with perhaps 50kg mass and heels that were one sq cm which works out to half a million pascals, about 2.5 times the rate on the deck.  They'd punch holes in them.  They had to ban the shoes on some airlines.

Say its a strip under my shoes an inch by 4 inches that holds up my weight as I walk.
That's about 1/400 of a square metre, so it should hold 1/400 of 3 tonnes
3000 Kg/ 400 gives 7.5 Kg and I'm about 10 times that.

As you point out below, your 3 tons/meter is more of a larger scale average.  The airplane obviously isn't going anywhere if actually loaded to 1% of that rated average pressure.

Ther are, in effect, two different "maximum" floor loads.
There's one where the floor supports fail, and there's one where the material fails locally.

Indeed, and there can be even more ratings, all at different scales.

513

Technology / Re: What is the impact force of dropping a 40kg weight from waist height?

« on: 14/07/2019 19:49:51 »

Any small attempt to explain my mistakes would be gratefully received.

OK....

So you need to know the stopping distance in order to calculate the stopping distance?!!

The question was about computing force.  Measuring/computing stopping distance is one way to begin to approach this.  Nobody suggested needed to know X in order to calculate X.

The stopping distance is misleading.  The floor is going to distort some distance, and the object hitting it will also distort.  You seem aware of this in your comment below.

The smaller the distance for deceleration, the larger is the force, so if the weight stops in 0.5 millimeters the 1Kg becomes 2,000Kg or 19620 Newtons.

For a point mass decelerating uniformly over half a mm, 19620 Newtons under Earth gravity, yes, but deceleration is not uniform over that distance.  Force will vary, and thus hit a max value probably somewhere near the bottom of the distortion on the floor, and that max value may be above 19620 Newtons, or possibly less depending on what exactly the 0.5 mm measures.

Also remember that the suggested 1mm stopping distance is shared between the weight and the floor. Probably the steel weight will deform (and spring back) more than the concrete.

Yes, the object will continue to change its center of gravity even beyond the point where the floor reaches its maximum distortion.  The stopping distance is thus not a nice clean value like 0.5mm.  It's a dynamic process.
Is the 0.5 mm a measure of max floor distortion, or is it a measure of the distance traveled by the center of gravity of the falling object from the moment of first contact to the moment of reaching the lowest point?  If the former, the max force is possibly less than 20k Newtons, and if the latter, it is definitely more.

One good point is that the force is distributed over an area, so if the contact area between your weight and the floor is say 10 square millimeters the actual floor load is 1/10 of that of a 1 millimeter area.

The change of area does not change the load (force) at all.  The area is pretty irrelevant to a computation of force and has more to do with pressure.  The question did not mention pressure (which is measured Pascals, not Newtons), even if pressure is more likely to damage a floor than is force.
Like the force computation, the max pressure computation cannot be computed by force/area.  That would only be the average pressure, not the max pressure reached presumably near the middle given a round mass hitting a flat floor.

The gym that I use has thick rubber mats in areas were weights are present, these would be a help.

That would serve to reduce both force and pressure, yes.  It reduces the former by increasing stopping distance, and it reduces the latter by increase of contact area.

I understand that given the stopping distance you can calculate the force required to decelerate the falling weight. Syphrum gave an example where the stopping distance is one mm, but surely this distance depends on the characteristics of the materials absorbing the impact?

Sure, a more spongy material will have a longer stopping distance, so a 1 Kg nerf ball dropped from a meter will exert far less force on the floor.

"how do you know the stopping distance?"

You don't.  You might be able to compute it if you were a materials engineer, but I'm not one of those.  Best to just measure it.

514

Physics, Astronomy & Cosmology / Re: How can galaxies collide if they're moving away all the time/

« on: 07/07/2019 19:43:13 »

I saw a video of simulated collision of Andromeda and our galaxy. how could Andromeda collide with our galaxy if everything is moving away from everything else ? Are some galaxies moving more quickly and so catch up?

On a large scale, all galaxies are moving apart in general, but there are always local variations due to gravitational influence of nearby mass.  The nearby great attractor plays a role in keeping the local cluster(s) somewhat together, resulting in commonplace collisions of galaxies near each other.  Andromeda is a large such example, but there are smaller collisions that will happen before we get to Andromeda.

515

Physics, Astronomy & Cosmology / Re: Will all moving objects with mass generate gravitational waves?

« on: 14/06/2019 15:34:33 »

If accelerated objects emit gravitational waves, and gravity is said to have an infinite reach, is it fair to say that all space would contain gravitational wave energy, coming and going in all directions, from a potentially infinite history of the relative motion of celestial objects?

All space does contain gravitational wave energy simply because all space is near accelerating objects.  But gravity waves travel at light speed, so a given event can only reach as far as the event horizon, just like light.
Gravity on the other hand is a field and does not propagate at some speed, so gravity has no finite bound to its effect.
Information can be transmitted via gravity waves, but not via gravity.

516

Physics, Astronomy & Cosmology / Re: Impossible question about our Solar System, can anyone answer? Please help.

« on: 08/06/2019 00:51:28 »

If all of the planets in our solar system are gravitating towards the sun, and have been for billions upon billions of years. Then that would mean that planet Earth would have been in the same position (the same distance from the sun) that Mars is now

This doesn't follow.  Earth is accelerating towards the sun at all times, meaning Earth's trajectory is bent, from the straight line out of the solar system it would otherwise take, into the circular orbit that it actually takes.  All other factors not withstanding, that would make for each planet returning to the same spot after each rotation.

There are other factors, and the two making us drift imperceptibly away are stronger than the two making us drift even more imperceptibly closer.
Look at the moon's orbit for the best example.  Its average orbital distance is increasing by almost 4 cm per year, despite always 'gravitating towards Earth'.  That outward drift is due to gravitational tidal forces, a gravitational effect.

517

Physics, Astronomy & Cosmology / Re: Can Special Relativity be derived from Newtonian Mechanics?

« on: 07/05/2019 02:53:02 »

Am I correct in thinking acceleration is to do with general relativity and not special relativity? Special relativity deals with none accelerating objects does it not.

Special relativity deals with acceleration just fine.  General relativity introduce the equivalence principle with allowed the properties of gravitational fields to be (locally) described in terms of accelerating reference frames.
Newton's equations did not cap speed of mass at c.  F=ma.  If you continue to apply force to an object, the acceleration will continue up to any beyond c.  With SR, acceleration (not proper acceleration) drops off and that same force just drives up m after a while.  They do this in particle accelerators where getting a particle to near light speed is easy, but then they just add mass to the thing until up to the limits of the magnetic field to hold it in its path.

Using simple algebra and tracing the path of a photon between two moving mirrors, will result in time dilation for an observer in a stationary reference frame.

Reference frames are not stationary.  Things are stationary relative to them.
That said, I see what you are getting at.  Newton probably wouldn't consider the light device to be a clock since without knowing its speed, there is no way to tell how far the light needs to travel.  Not like there is any real light clock anyway.  How would you get it to work?  You can't measure a photon going between mirrors, counting cycles.  A photon can be measured but once.

518

Physics, Astronomy & Cosmology / Re: How do we measure the circumference of a black hole?

« on: 12/04/2019 12:42:51 »

To answer the second question first, space is bent anywhere around an object with mass, so the space is not Euclidean around them.  You can take a measuring stick and count off the meters of say Earth's circumference and get its proper distance.  You can then divide that value by 2π and compute the Earth's Schwarzschild radial distance, which will be a bit less than the proper radius you would similarly measure with the stick.
The difference between the two is more and more the more dense the object.  With a black hole, the proper distance cannot be measured since there is no way to do it with a stick and the computed value is meaningless.

So to answer the first question, the circumference of the black hole is a limit of the proper circumference as the radius approaches the event horizon.  You can't actually do it directly at the event horizon since there is no way to put a measuring stick there.  The Schwarzschild diameter is just that circumference /π, and that is the only meaningful value we have to work with.  There is no diameter more real than that.  Mathematically, there is an infinite or undefined 'distance' to the center (computed different ways).  It isn't a proper distance since there is no way to orient the stick along the line we wish to measure.  It isn't really spatial distance inside the event horizon.

519

Physics, Astronomy & Cosmology / Re: What would air and water do on a rotating space craft?

« on: 29/03/2019 11:53:48 »

The Avalon is an amazing space craft that uses rotation to simulate gravity. We know that air and water in a plane will move with the plane. What would air and water do on a rotating space craft? What would really to the water if somebody jumped into the Avalon's swimming pool?

Water would be 'down' just like water on Earth which is attracted to Earth by gravity.  So a spinning object would have 'down' feel like the outside of the craft and 'up' be towards the center of rotation.  The closer to the edge you get, the stronger the artificial gravity.  Thus there would be no 'gravity' at the center.

The ship would have to have a huge radius for air pressure to change much from one place to another.  I climb a 4 km mountain and the air is noticeably thinner, so if the spinning craft is several km in diameter and air is not restricted by decks from flowing anywhere inside, then the pressure at the center might be somewhat lower than at the edges.

520

Physics, Astronomy & Cosmology / Re: How does time at the Event Horizon differ for approaching vs Distant Observers?

« on: 20/03/2019 18:03:53 »

I re-read the OP, and if the event horizon is never crossed, then yes, the black hole eventually fades with Hawking radiation and the observer there (who cannot exist any longer since he has also faded away with the same radiation) will experience in no time a sort of finite discontinuity.
So let's say the black hole lives only for a billion years.  As I observe the universe as I fall in, outside time speeds up and abruptly jumps a nice finite billion years, and it turns out I never left the universe at all.  There isn't even a gravity well here anymore.  The black hole is gone, and my clock ticks at the same rate as the patient clock watching all this from a distance, except we're a billion years out of sync now.

So the answer is that the time of the rest of the universe is finite, not infinite.  The infinite answer is only a mathematical idealization which assumes black holes don't evaporate.