21

Physics, Astronomy & Cosmology / Re: How significant is this?

« on: 08/08/2014 13:48:01 »

But if you have two pin holes you can't tell which one it went through and when it went through either of them. If you can't tell when something was at a place then you can't really have said to have measured its position.

Sure you can, but you haven't localized it with the same precision as if you used a single pinhole.  Even if you don't like calling that a measurement, and I can understand why it might not be the best term for it, it's the standard terminology in the field.

22

Physics, Astronomy & Cosmology / Re: How significant is this?

« on: 07/08/2014 16:14:18 »

The filter provided a way of measuring a particle’s position without knowing exactly where it was—without collapsing its wavefunction.

In quantum mechanics I don't think that calling the inference of where the photon is can be said to be a measurement of where it is.

Ok--looking at the PRL paper, this is applying something that's well known in classical wave theory to the fact that photons are also waves.  What's important to remember is that a "measurement" just means that you interact with the wavefunction to change it somehow and extract information from that.  For example, if you put a single pinhole in the photon's path and a camera behind the pinhole, you can tell if the photon went through that pinhole or not by looking for a "click" at the detector.  If instead you put a random array of pinholes (which is basically what they did), you now only know that the photon went through those open pinholes and didn't strike the mask.  In the wave formalism of quantum mechanics, both force the wavefunction to be zero everywhere on the obstructing screen and allow it to remain the same in the open pinholes.  The "weak" part of the measurement is that the single pinhole squeezes the wavefunction down to a tiny point, forcing it to spread in momentum whereas using a mask of tons of pinholes does not squeeze it nearly so much, so momentum doesn't change much.

The lens after the pinhole mask and the screen is just a way of ensuring that particle momentum is measured at each detector pixel.

23

Physics, Astronomy & Cosmology / Re: How significant is this?

« on: 07/08/2014 15:59:40 »

This is an example of hype getting ahead of the science.  I believe there are two things going on here.
The first is weak measurement, which often gets hyped as breaking the uncertainty principle.  It doesn't.  What happens is that one measures an observable, let's say position, of a particle very poorly.  The uncertainty principle says that a sufficiently poor position measurement won't effect the particle's momentum much.

I believe that’s a misunderstanding of the uncertainty principle. Uncertainty has nothing to do with the quality.accuracy of a measurement. You can determine the uncertainty without making any measurements whatsoever and those will be the uncertainty for that observable for that system for that wave function. The uncertainty being the standard deviation of the observable is determined entirely by the wave function. Given the same wave function one calculates both <x> and <x^2> and uses those to determine the standard deviation dx as being dx = sqrt(<x^2> - <x>^2). Likewise you can use the wave function to get dp = sqrt(<p^2> - <p>^2). Note that nowhere has a measurement been made. It can be shown that dp dx >= hbar/2.

I think we're saying the same thing in different ways.  A quantum "particle" always has wavelike properties.  The uncertainty principle is a statement about waves.  A wave can't be arbitrarily narrow in two Fourier conjugate variables.  The limit is precisely the uncertainty principle limit.  This is true of all waves: light, sound, water waves.  It is also true of the wave nature of quantum particles. 

People seem to imbue the quantum version with some magical properties simply because it has the word "quantum" in front of it, but it's a very well understood property of all waves.

24

Physics, Astronomy & Cosmology / Re: How significant is this?

« on: 07/08/2014 14:25:09 »

The article closes by saying "physicists seem to have found a way to get more data with less measurement..." I'm not sure I agree. It would seem to me that they are getting the same amount of information, but now related to more parameters.

This is an example of hype getting ahead of the science.  I believe there are two things going on here.

The first is weak measurement, which often gets hyped as breaking the uncertainty principle.  It doesn't.  What happens is that one measures an observable, let's say position, of a particle very poorly.  The uncertainty principle says that a sufficiently poor position measurement won't effect the particle's momentum much.  No violation of the uncertainty principle.  Then take many identically created particles.  Repeat this poor measurement many times.  Each measurement will differ slightly and be equally poor on its own, but taken together, they tell you about the position of these particles.  Since the uncertainty principle only applies to a single measurement, not to averaging over many, it isn't violated. 

Compressive sensing, which is the other thing they use, is a way of taking many less measurements than you'd traditionally expect by employing what are called priors.  Let's say (as a silly example) you want to describe the contents of a box full of (6-sided) dice.  Your measurement is to roll them all and sum the result.  I do so and tell you that you get a value of "4".  From that single measurement, you know that you have many options: 1+1+1+1, 1+1+2, 2+2, 3+1, 4 (and that's not counting ordering of dice).  A prior would be if I told you that your answer uses the smallest possible number of dice.  Then you would know for certain that you only have 1 die and it's rolled a 4.  This is a silly example, but this type idea is extremely powerful in many real-world applications where we know the types of things we're measuring.  It's been very powerful in MRI/CT imaging where you take a bunch of measurements that look nothing like the final image and process them to get a 3D image of the body.  You need lots of measurements because traditional MRI/CT do not try to use the fact that we know the general structure of the body.  For all they care, you're measuring some random object each time and you need tons of data to figure out its internal structure.  If you instead you use some of what we know about anatomy, you can cut way down on the number of needed measurements. 

So they're basically using what they know about the possible class of solutions to cut way back on the data required compared to the case where they know nothing about the class of solutions.

25

Physics, Astronomy & Cosmology / Re: What is a volt - on a physical level?

« on: 01/08/2014 20:44:29 »

Did you mean physical level is classic level?

Classical physics is any model where we can ignore quantum effects--typically because objects or distances are big enough that properly accounting for quantum effects won't change the answer.  The discussion here has been about classical physics.

26

Physics, Astronomy & Cosmology / Re: What is a volt - on a physical level?

« on: 01/08/2014 18:27:31 »

Current, voltage, potential all happen between atoms, surely is quantum level.

Sure, but it also happens on the classical level, and that's the topic of this thread.

27

Physics, Astronomy & Cosmology / Re: What is a volt - on a physical level?

« on: 01/08/2014 18:06:51 »

No, we're not guessing. 

We're discussing classical electrostatics here, which doesn't include quantum effects.  So long as you don't need precision on the atomic scale, and for most circuits (and for all classical circuits) you don't.  A stationary electron is one which we pretend is perfectly still and located at a point because including corrections for the uncertainty principle would change our answer by such a tiny amount that it wouldn't matter to the problem we're considering.

Of course, if the problem is to model an atom, this quantum effect does matter, which is why we need a non-classical model of the electron in that case.

28

Physics, Astronomy & Cosmology / Re: What's the minimum mass to form a black hole?

« on: 31/07/2014 17:13:37 »

We know the value of C, can we calculate the minimum mass? Searched like 1 to 3 solar mass is enough, don't get it.

How's possible any mini black hole exist?

There's no minimum since all that is required is that all the mass be confined within a certain region of space. All that's required is that the mass be greater than the Planck mass. Theoretically there are micro black holes aka primordial black holes that have been around since the big bang
http://en.wikipedia.org/wiki/Micro_black_hole

Pete's correct.  The reason we don't see small stars forming black holes is that as he says, you need to put enough mass in a small volume to create a black hole.  For smallish stars (below ~3 solar masses), the gravity of the star itself is not strong enough to compress mass to the level needed for a black hole to form.  For large stars, gravity is strong enough to do this.

Of course, if gravitational collapse of a star isn't the mechanism to make a black hole, you can make a smaller one, such as the primordial black holes. 

29

Physics, Astronomy & Cosmology / Re: What is a volt - on a physical level?

« on: 31/07/2014 13:09:13 »

It isn't pressure, but pressure difference. You can remove lots of electrons from a circuit and it will still function fine using the ones that are left, the same pressure differences being generated as before but with a lower pressure at any given point. For this reason, you can't tell what the voltage is just by measuring the electron pressure at one point in a circuit, but you must measure it at two points and see what the difference is (in terms of proportionality, not absolute value).

It can't be simply a pressure because the units are wrong.  Pressure is force/area and potential (voltage) is force*length/charge.  It's a subtle difference, but an important one to really understand what potential is describing.  Pressure can definitely be a useful analogy, but it can also be confusing, particularly when one starts using potential to talk about things other than circuits (such as using it to prescribe an electric field over a region of space).

I'm not convinced that the pressure is higher in thinner wires than fat ones, JP. Are you sure you've got that right? If you picture the -ve end of a battery being connected to a wire, the battery will pump electrons out into the wire until it can't force any more into it, at which point the electron pressure pushing back into it will equal the force from the battery and everything will stop (there's no circuit at this point). If you take that wire away and replace it with a thicker (or thinner) one, the same will happen with electrons being pushed into it until the pressure reaches a level that stops the battery pumping any more in. That pressure should be same regardless of the thickness of the wire (though to make it so you may need to attach another wire of the same thickness and length to the other end of the battery each time to enable it to pick up enough electrons there, bringing the pressure in that wire down to a lower pressure in the process, the same low pressure each time).

You're right in this one.  Having thought about it a bit more, I was thinking of a constant force, not a constant voltage.  The fact that voltage scales per unit charge and charge distributes over the wire keeps pressure constant.

30

Physics, Astronomy & Cosmology / Re: Is there any voltage in between electron and proton in hydrogen atoms?

« on: 30/07/2014 17:45:58 »

Again, please knock off the personal attacks.  I just deleted several posts that went over the line.  If you disagree with each other, please keep it to the science and if you disagree with someone's science, point out the error and back it up with evidence.

-The Mods

31

Physics, Astronomy & Cosmology / Re: Could Quantum Computers be used to find aliens?

« on: 30/07/2014 15:28:27 »

Let's be clear, Dr Karl was "silly", not me, I just wanted clarification of what I thought was the right answer which you have confirmed. Would be interesting to know where he got the idea from...

You said you wanted an expert to chime in and say something like "don't be silly."   

The other guy on the "Since Sliced Bread" podcast mentioned 32 billion potential parallel universes which I thought was an odd number to pluck out of the air, I wonder if this have any basis.

That one I'm not sure of.  There are several theories that speculate that there are multiple universes, but I'm not sure which one is being described here.  I also don't know of any that prescribe a finite number, as they tend to describe an infinite number of multiverses. 

32

Physics, Astronomy & Cosmology / Re: What is a volt - on a physical level?

« on: 30/07/2014 00:13:58 »

To be more precise, voltage is not directly analogous to a pressure, since pressure depends on force per unit area, whereas voltage is in units of energy/charge and does not depend on area.  This is why we can say a battery is measured at 12 V independent of the thickness of the wires we hook up to it.  Whereas if we were measuring pressure, the pressure would increase as we used thinner and thinner wires.  If we fix the diameter of all wires, we could make the analogy, since both energy and pressure depend on force, but there's a more straightforward explanation for voltage that gets at its ties to energy:

The value of voltage at any point isn't useful, but the difference in voltage between two points is useful.  It tells you how much energy is gained (or lost) by an electron in traveling between those two points^*.   Why do we care about energy?  Because it's the currency we use to power light bulbs, motors, etc.  So if we know the energy gained by an electron in traveling from on terminal of a battery to another, we can design a circuit to use this energy to do something useful.  We also, of course, need to know the number of electrons flowing, which is generally characterized by current, since the total energy depends on the energy per electron times the number of electrons.

So what is the closest analogy to water?  Imagine we have a dam in which water flows from a high point to a low point.  Obviously it gains speed as it flows downhill, and this gain in speed is a gain in energy.  We can use this energy to power a turbine (and the water obviously flows more slowly after turning the turbine since we've extracted some energy).  The amount of energy gain per unit water (gram of water or per water molecule) would be akin to voltage.  Obviously if we send it down a pipe, we can change the pressure at any point by changing the pipe diameter, but the energy gain per unit water does NOT depend on pipe diameter.


^* Strictly speaking, as others have said, voltage difference between two points is the work done per unit charge in moving between those two points.  Work in the sense of physics means a change in (mechanical) energy, so work tells you how much energy you must put in or that you gain from doing something.  So what voltage difference tells you is the energy per unit charge that you have to put in to move a charge between two points or how much you gain from a charge moving between two points.

33

Physics, Astronomy & Cosmology / Re: What are the nuclear properties of different isotopes?

« on: 29/07/2014 17:23:44 »

jccc, I've moved your posts including "enertron" theory to the New Theories forum.  You've been asked several times to stop posting your own theories here.  We may restrict your posting rights if you continue to do so.

-The Mods

34

Physics, Astronomy & Cosmology / Re: What is the difference between an electron and a photon?

« on: 29/07/2014 13:13:29 »

This thread is locked since it keeps returning to personal attacks.

35

Physics, Astronomy & Cosmology / Re: Random events

« on: 26/07/2014 23:44:08 »

Are random events related to luck events?

What do you mean by luck events?  The term "luck" has no precise meaning in physics.

36

Physics, Astronomy & Cosmology / Re: What do you think of my physics magic tricks?

« on: 26/07/2014 23:27:01 »

The virus is spreading horribly. Please don't kill the verb "breathe".

(sigh!) It's bad netiquette to point out spelling errors.

The spelling police care not for netiquette! 



But seriously, let's try to keep the discussion on the topic of discussing science and not raise hackles by correcting other users's spalling or grammer.

37

Physics, Astronomy & Cosmology / Re: Could Quantum Computers be used to find aliens?

« on: 26/07/2014 13:32:18 »

Don't be silly.

More specifically

* Dr Karl Kruszelnicki implied that QCs operate by entanglement with all other QCs across space and time possibly including any QCs created by other civilisations in our universe.

This is nonsense and

* Eric Ladizinsky on Since Sliced Bread implied QCs operate by entanglement with all other instances of the same system across all implied parallel universes.

This is potentially true but very very speculative, since we don't have a confirmed theory of multiple universes.  Our computer would be no better at finding aliens in this case since utilizing entanglement to search for something requires a classical information channel open alongside the quantum computer and we can't open classical communications to other universes.

38

Physics, Astronomy & Cosmology / Re: Could Quantum Computers be used to find aliens?

« on: 25/07/2014 14:50:29 »

I'm not sure what the basis is for thinking that a quantum computer operates via entanglement with all other quantum computers, but I know of no way in which that occurs.  One limit on quantum entanlgment is that it cannot send information non-causally, so if we were to invent a quantum computer here on earth, it could only communicate with other quantum computers at the speed of light.  Given the difficulty of maintaining entanglement, I'd be very surprised if it offered any advantage over classical communication which is also limited by the speed of light.

39

Physics, Astronomy & Cosmology / Re: Why don't an atom's electrons fall into the nucleus and stick to the protons?

« on: 23/07/2014 23:30:20 »

I've split a few posts off this thread.  jccc, please keep your posts on topic and don't promote new theories here.  If you want to propose alternative explanations to standard quantum mechanics, the place to do so is in the New Theories forum.

Thanks,
The Mods

40

New Theories / Re: Is mass a field?

« on: 22/07/2014 14:53:06 »

No, mass is not a field, but fields can create mass: the Higgs field being the obvious one, but the strong nuclear force is mediated by a field which gives rise to much of the mass of a proton, for example. 

Since much of your post deals with your own ideas on mass and fields, I've moved it to New Theories.