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Just Chat! / Re: The great thing about physics

« on: 26/03/2018 02:58:14 »

LOL no particles were harmed in the making of this experiment - NOT!

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Physics, Astronomy & Cosmology / Re: Laws of thermodynamics

« on: 21/04/2017 17:14:48 »

Higher temperature doesn't mean higher entropy. An increase in entropy over time can also be seen as a loss of the capacity of a system to do work over time (the total energy remains the same, but it is converted from potential energy into kinetic energy). At absolute zero, the entropy is only zero for perfect crystals.

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Physics, Astronomy & Cosmology / Re: Is mass change with speed a flaw in the theory of special relativity?

« on: 11/10/2016 21:23:34 »

Far from being a flaw n the theory, it's an experimental fact.

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Physics, Astronomy & Cosmology / Re: Where does The Water On Comets Come From ?

« on: 14/09/2016 23:04:20 »

Where did the comets get their water from ?

Hydrogen is the most common element in the known universe (excluding unknowns like Dark Matter). It originated in the Big Bang.

Medium-sized stars will burn much of their hydrogen to elements like Carbon, Oxygen or Nitrogen (larger stars produce a lot of silicon, and very large stars produce iron & nickel). When these stars explode in a supernova, they blast much of this matter into space.

So it is not surprising that the protoplanetary disk and Oort cloud from which the Solar System condensed had a lot of compounds of these chemicals.

The cold Oort cloud included a lot of compounds with low boiling points like oxides of Hydrogen (water), oxides of Carbon (CO, CO₂), hydrides of Carbon and Nitrogen (NH₃ ammonia, CH₄ methane and larger hydrocarbons), cyanide (HCN), amino acids as well as O₂, N₂, C_n.

Planets forming in hotter regions contained compounds with higher boiling points, like silicates (SiO_2n) and iron.

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Physics, Astronomy & Cosmology / Re: Is it possible there is a new element hidden inside a Neutron star?

« on: 17/08/2016 23:20:12 »

conventional theory says Neutron stars are extremely compact because the space inside an atom is mostly made from space an it is that what is compressed

The nucleus of an atoms has the protons and neutrons so close they are effectively touching.
In a neutrons star, all the neutrons, and any residual protons present are almost close enough to touch (it is the residual electrons that stop it from collapsing further, into a black hole).
So in one sense, the bulk of a neutron star could almost be considered as one massive nucleus, with a ridiculously high atomic mass.

this element ... has such a strong magnetic pull

Magnetic fields from an atom can be produced by unpaired electrons, or unpaired nucleons (protons or neutrons).
A single atom does not usually produce an enormously large magnetic field from all the electrons being aligned, because:
- In a neutron star (or an ordinary star), many of the electrons are stripped off the atom.
- The Pauli exclusion principle ensures that electrons can't have the same spin in the same orbit
-  it becomes energetically favorable for electrons to flip their spin (canceling the magnetic field), and move into a lower energy state
- Similar logic applies to the spins of nucleons - in a sense, nucleons often form in stable subgroups inside the nucleus with two protons and two neutrons with zero net spin; we often see these groups spat out from radioactive elements as an alpha particle.

It is thought that when a neutron star forms, the magnetic field of the star is compressed into an extremely small volume, and "frozen in" (but at an extremely high temperature). This can produce enormously strong magnetic fields at the surface.

See: https://en.wikipedia.org/wiki/Neutron_star#Giant_nucleus

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Physics, Astronomy & Cosmology / Re: Does time move at the speed of light?

« on: 15/08/2016 03:00:47 »

The term "speed of time" has no logical meaning. The word "time" refers to the passage of events only and when scrutinizing different events, we record the sequence of those events using the standard second. If as you ask: "Does time move at the speed of light?", what are we to make of the theoretical Tachyon who's speed is always faster than c?

If time only moves at the speed of light, then the theoretical existence of the Tachyon wouldn't have any merit.

Considering that the Tachyon is a theoretically recognized particle, every physicist that considers their possible existence should re-evaluate their reasons for investing in such a theory.

Time has no speed my friend, it is only a reference marker between sequential events.....................

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Physics, Astronomy & Cosmology / Re: Why does light take billions of years to cross the Universe?

« on: 13/08/2016 19:21:43 »

You're quite wrong on the size of the visible universe. The diameter of the visible universe is 93 billion light years.

How did you get that figure? The universe is something like 14 billion years old (assuming the big bang theory is right) so light can only at best have traveled 14 billion light years. Call that a radius and you get a diameter of something like 28 Billion light years.

The universe has been expanding quite a bit over those 14 billion years.

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Physics, Astronomy & Cosmology / Re: Why does light take billions of years to cross the Universe?

« on: 12/08/2016 12:36:21 »

There is no valid frame of reference, at light speed, for particles with non zero rest mass. So the question itself will not have a sensible answer in those terms. The speed of light is finite. It takes time for a photon to move position from one point in space to a different point in space.

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Physics, Astronomy & Cosmology / Re: Is it possible that a Black Hole could be made up of Dark Matter?

« on: 03/08/2016 22:33:49 »

The dark matter distribution in galaxies is not uniform. The density appears to increase with radial distance from the galactic cetre. This runs counter to the idea that black holes are made of dark matter.

Why? Is that because you're assuming a uniform distribution of black holes? If so then I'd agree. Nice catch by the way. What I find strange to begin with is that the distribution isn't uniform. This is probably a clue as to the nature of dark matter.

I think it is high likely that black hole distribution is uniform. The most puzzling thing is why dark matter forms halos around the galaxies but does not appear to be distributed between galaxies. If this were the case the galaxies would tend to expand over time. It has a while since I read the text on galaxy formation so I may be in error with these assumptions.

As far as what constitutes dark matter at the particle scale I am not convinced by supersymmetry. At least not by what I have read so far.

Every year you become noticeably better and better at physics. It's a pleasure and an honor to watch you and others here learn.

If anyone wishes to participate in meaningful debate on the subject of physics then at least a basic knowledge of the principles is required. If like me you want to find answers then a more in depth study is required. Plus I just love mathematics.

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Physics, Astronomy & Cosmology / Re: What's The Real Probability Of Life Existing Elsewhere In The Universe?

« on: 03/08/2016 10:38:00 »

Without knowing what was the spark of life on Earth, (assuming life started here and wasn’t transferred from someplace else) can we really surmise it ever happened a second time in the Universe?

Without knowing what was the spark of life on Earth, can we really surmise it didn't happen a second time in the Universe?

That is the problem with only knowing 1 solar system, which only has 1 planet in the Goldilocks zone. It doesn't give a basis to estimate the probability of it happening elsewhere.

But if we find water-soluble life outside the Goldilocks zone (eg Europa), that also changes the statistics.

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Physics, Astronomy & Cosmology / Re: What's The Real Probability Of Life Existing Elsewhere In The Universe?

« on: 03/08/2016 08:38:31 »

It is almost certain that life (i.e. selfreplicating molecules) could evolve on any planet that has liquid water.

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Physics, Astronomy & Cosmology / Re: What would a ball of anti-matter look like? Anti-matter periodic table?

« on: 14/07/2016 04:07:55 »

Is there a possible anti-matter periodic table?

Yes there is, but with current techniques it takes an enormous amount of energy to produce anti-protons, then slow them down enough to combine with anti-electrons to form anti-hydrogen.

The LHC has produced a limited amount of neutral anti-hydrogen gas, and has perfected ways of storing it for at least 15 minutes without it annihilating with nearby "normal matter" atoms.

They are using this anti-hydrogen to confirm their assumption that anti-matter experiences the same gravitational attraction as normal matter.

However, creating elements higher on the anti-periodic table is a real challenge, because while protons and electrons are stable, neutrons are not. So to produce anti-helium, they would need to create anti-neutrons and combine them with the anti-protons before the anti-neutrons decayed (which is expected to take around 15 minutes, on average).

Neutrons are hard to contain and direct (and anti-neutrons are even harder, since they would tend to annihilate with anything that was trying to direct them).

Producing anti-helium with an anti-hydrogen bomb would be even more problematic (as would collecting the anti-helium atoms afterwards).

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Physics, Astronomy & Cosmology / Re: If Energy is neither created nor used up, where did energy come from?

« on: 08/06/2016 15:09:54 »

MODERATOR REQUEST

We appreciate that some of you have 'history' due to contact in other fora and that this will influence your conversations here. We allow a degree of lively discussion but this topic is becoming more personal than science. Please keep your replies on topic.
Thank you

PS - there are a number of members here who are valued for their knowledge of physics and the consistent, high quality of their replies. Some will be intolerant of incorrect or inaccurate science and may seem rather robust in their replies. We would ask both sides to cut the other a little slack, but any poster should not take a lack of response to indicate agreement with their post or that a response is not possible, everyone has the right to ignore.

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Physics, Astronomy & Cosmology / Re: Can we measure the temperature of a photon?

« on: 04/06/2016 13:51:26 »

Temperature is the mean kinetic energy of a large number of particles, so strictly speaking a single photon cannot have a temperature. However the energy of a gamma ray can be anything from a few eV to more than 10²⁰ eV. Radio waves have energy < 0.001 eV.

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Physics, Astronomy & Cosmology / Re: Does the universe have an edge?

« on: 02/06/2016 12:12:55 »

OH! really you are stating your opinion as if it were factual, you do not know if the universe has an edge or not and until you do it is just at best speculation on your part?

Actually, I agree with him. In his defense he did say "according to theory", which is a pretty accurate statement as far as I know. Further, he might been better off saying that the comment was over-reaching in its implication rather than flat-out wrong, but again I agree with him. Almost any analogy can work in-so-far as its intended breadth and depth; however, any analogy can be extended beyond the original intent of the person who made the analogy, and in this extension become false or outright ridiculous. This is a common structure for an argument similar to reductio ad absurdum arguments. "Well if electricity works like water... I should be able to swim in it... but I can't, so therefore this analogy is flawed." This is a classic example of an analogy simply being taken much too far. The most basic principles of electricity do very much resemble water as all the basic components are there. Pressure to voltage, current to rate of flow, resistance to resistance, etc... but even this analogy breaks down past a certain point.

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Physics, Astronomy & Cosmology / Re: What will the James Webb telescope reveal?

« on: 21/05/2016 06:35:06 »

Well, if we knew what it was going to see, there would not be much point in sending it up.  One thing it might do is give us a fuller view of the outer Solar System, including hitherto undiscovered dwarf planets.

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Physics, Astronomy & Cosmology / Is speed of light in a vacuum behaving in same way as speed of sound in air?

« on: 19/05/2016 14:10:40 »

...... If I have it straight, this is simply saying that a wave's speed is dependent on the properties of the medium in which it travels and not on the speed of the craft that caused it......

I've moved this from the other thread as it might get caught up in acrimonious posts.

As Alan says, "Light and sound are very, very different. You can't use one as an analogy of the other."  It's worth spending some time looking at this, so please excuse me if I labour some points which we've been over before.
Imagine a stationary loudspeaker. At low frequencies the cone moves quite slowly compared to the faster movement at higher frequencies, as the air is compressed by the movement it causes a sound wave which propagates at a speed defined by the properties of the medium, and for air between 20Hz-20kHz this speed will be constant relative to the medium.
Let us now move the loudspeaker towards you and imagine a point on the cone which is moving forward as the speaker moves forward, clearly the speed of that point (through the medium) will be that of the cone + the speed of the loudspeaker. However, if you think about it, that increase in speed is equivalent to an increase in frequency - the Doppler effect - even though the wave is still propagating through the air at the speed of sound (you can also think of the movement of the source towards you as causing the wavelength to compress, which means more wave crests/unit time, hence higher frequency). So in this case, the speed of sound relative to the medium is independent of the movement of the source, only the frequency changes.

Is this universally true for all media? Well it certainly isn't for all liquids and gases. With CO2, for example, the speed of sound varies with frequency, and if we look at the example of the moving loudspeaker we can see that a moving source results in increased frequency, hence in CO2 an increase in the speed of sound. So the speed of sound in CO2 depends on the movement of the source. In fact, because air contains a small amount of CO2, air also behaves in this way at frequencies above 28kHz.
So, what if we ignore this and take the special case of audible sound in air, surely this behaves in the same way as light, as suggested. This has been tested by looking at the Doppler effect of light emitted by Hydrogen ions traveling at high speed. They don't follow the classic Doppler  formula of sound in air, but need a relativistic correction eg Ives–Stilwell.

So, how do other analogies hold up? eg Galilean Relativity. Well, in SR it is important to consider the speed of light as measured by an observer in their reference frame, but as we have discussed in previous threads the speed of a wave in a medium, as measured by an observer, varies with the movement of the observer through the medium, or with the movement of the medium relative to the observer. As a reminder for any students: for water: "c=fλ , where c is traditionally used for the wave speed or "celerity". The term celerity means the speed of the progressing wave with respect to stationary water - so any current or other net water velocity would be added to it." http://hyperphysics.phy-astr.gsu.edu/hbase/waves/watwav2.html
" just as the speed of a ship would add to or subtract from the speed of ocean waves as measured from the ship"  http://www.britannica.com/science/relativity
For air, as any school physics student knows, "Two things affect the speed of sound in air
(i) the wind - if the wind is blowing in the same direction that the sound is travelling then the speed of sound is increased." http://www.schoolphysics.co.uk/age11-14/Sound/text/Speed_of_sound/index.html.
Although not often mentioned, the speed of sound measured by an observer moving through the medium also varies with the speed of the observer in the same way as " ...the speed of a ship would add to or subtract from the speed of ocean waves as measured from the ship". This can be easily visualised by considering a train passing through a station, and at the end of the platform is a whistle. To an observer on the platform, in still air, there is no change to the frequency of the whistle, but the observer putting a head out of the train window experiences a wind, hence a change in the observed speed of sound and a change of frequency. Under Galilean Relativity the observer moving through the medium and the medium moving past the observer are equivalent and results in a difference in the measured speed of sound compared to the stationary observer. It is important to note that the speed of sound relative to the medium has not changed. (Aside: I assume you have realised the the calculation - "Because the speed of sound is invariant in a medium it means that it does not obey Galilean transformations, thus the time taken for the sound of the siren to reach car (b) would be 1000/340 = 2.95 secs" - is wrong, and I'm sure you understand why).
There are other differences between the propagation of sound and light:
Because the speed of sound is affected by the movement of the medium, the propagation of sound in a moving medium, eg in a wind, is anisotropic.
There is also a difference with the Doppler effect. If for sound we consider the 2 cases of a stationary observer and a moving source, vs a stationary source and a moving observer, for the same speed of movement the 2 frequency shifts are different. This doesn't happen for light, the shift  only depends on the relative velocity of the source and observer. The reason for the difference is that the speed of sound is locked to the medium whereas the speed of the observer and the speed of light are not.
So, all in all we can see that Alan is right, we cannot draw analogies about the behaviour of light in a vacuum from the behaviour of sound in a medium.

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Physics, Astronomy & Cosmology / Re: In what form is matter inside the black hole

« on: 19/04/2016 22:44:53 »

I think it's fair to say that nobody really knows.

There are still significant theoretical arguments about what happens just outside the event horizon - and that is (at least in theory) accessible to astronomers.

What happens just inside the event horizon is inaccessible without going there - and you can't send a message back.

Inside the event horizon, all light cones point toward the center. Since nothing can exceed the speed of light, all matter must also move towards the center. It is thought that all matter mashes in the center in a singularity of infinite density.

Also see: https://en.wikipedia.org/wiki/White_hole

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Physics, Astronomy & Cosmology / Re: Is there one fundamental energy?

« on: 27/10/2015 08:21:21 »

No.

Energy is a scalar quantity that we have invented, and we observe to be conserved.