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Physics, Astronomy & Cosmology / Re: What's the difference between general and special relativity?

« on: 09/12/2015 13:26:52 »

"What's the difference between general and special relativity?"

The essential difference is that spacetime have not to be warped by masses or energy, to apply special relativity. If it is, general relativity have to be used.
So, for example, in absence of massive objects or others which can warp spacetime, we can use SR even to treat accelerating spaceships, as in the "twin paradox".

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Physics, Astronomy & Cosmology / Re: What would happen if you switched on a laser pointer in space?

« on: 09/12/2015 07:48:48 »

let's put some numbers.
beam's power: P = 3 mW
pointer's mass: m = 10 g
batteries time of continuously work: t = 1000 s
(these values are invented, I believe they are reasonable).

The force on the pointer (assuming its line of action passes through the pointer's centre of mass so it can only translate) due to the beam's momentum is:
F = P/c
so the acceleration is:
a = P/(m*c) =~ 10^-9 m/s²
The final speed acquired is then: v = a*t = 10^-6 m/s.

Let's compute the time it would need to cover 1 astronomical unit, that is ~ 1.5*10¹¹ m:
1.5*10¹¹/(10^-6*365*86400) =~ 4.8*10⁹ years.

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Physics, Astronomy & Cosmology / Re: What would happen if I connected a pipe from the atmosphere into space?

« on: 05/12/2015 13:26:37 »

What would be the possible outcomes if a connecting pipe is kept on the boundary of atmosphere and space with one end in atmospheric layer and the other in the space..

A lot of flying collisions with the pipe  []

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Physics, Astronomy & Cosmology / Re: Does the mass of a system depend on its enthalpy or its free energy?

« on: 05/12/2015 13:21:30 »

But what about the solar wind bringing +ions into our atmosphere and the balancing E=mc^2 equivalent from sunlight?

You can use that equation only if the irradiated body stays still and this happens, in absence of other forces, if the radiation is isotropic.
This said, *if you can neglect variations of pressure and volume* of the body, the body's mass increase is simply given by ΔH, that is by the heat it absorbs (because in this case ΔU = ΔH).

In general, the body's mass increase can also be due to (mechanical or other kind) work made on it: you increase the gas' mass even if you simply compress it adiabatically.

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Physics, Astronomy & Cosmology / Re: Does the mass of a system depend on its enthalpy or its free energy?

« on: 30/11/2015 08:01:32 »

I was inspired by this response in another thread (I added bold for emphasis for the purposes of my question):

...At the same time, using planet-wide temperature readings from NASA and other sources, one can see that the planet is warming up, or gaining thermal energy. This is coming from the Sun, and if energy is added to a system, then since E=mc^2, there is a corresponding increase in mass. Based on present data, this is probably adding a small amount of mass to the planet each year, of the order of 200 tonnes or so.

...

At the same time, the Earth is losing heat energy from its core as radioactive elements decay. Based on estimates of how much energy exits in this way, the mass loss is trivial though at about 16 tonnes per year...

Is the energy contribution to Earth's mass derived from the enthalpy (ΔH) of the system, or does it depend on the free energy (ΔG) of the system?

Recall: ΔG = ΔH – TΔS

My intuition says it has to be ΔH = Δmc², but if I'm wrong, I'll have to rethink some of my understanding of entropy and information.

I would say it's neither one nor the other but that it's ΔU, variation of internal energy. If volume and pressure doesn't vary, then it corresponds to ΔH (H = U + PV → ΔH = ΔU + VΔP + PΔV); but in general it's ΔU.

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Physics, Astronomy & Cosmology / Re: How do I explain general relativity to a six year old?

« on: 29/11/2015 15:44:28 »

When adding the speed of two protons, where v₁=v₂=c(1-x):
Total speed = (v₁+v₂)/(1+v₁v₂/c²)
=c((1-x)+(1-x))/(1+(1-x)(1-x))
=c(2-2x)/(2-2x+x²)
≈c-2c/x²

There is a little mistake here, the result should be ≈  c - cx²/2

≈ c - 2 mm/day

≈ c - 1 mm/day is my result, with your data.

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Physics, Astronomy & Cosmology / Re: How do I explain general relativity to a six year old?

« on: 27/11/2015 16:07:09 »

"How do I explain General Relativity to a 6 year old?"

This way:
"Grow, study physics at university, theoretical physics specialization, and then you will know something of General Relativity".

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Physics, Astronomy & Cosmology / Re: Is it possible that photons have mass and that the spd of light is not absolute?

« on: 16/11/2015 15:20:10 »

Transverse and longitudinal components of the propagating and evanescent waves associated to radially-polarized nonparaxial fields

Found, thanks. I'll have a look but don't know if I'm able to understand anything.

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Physics, Astronomy & Cosmology / Re: Is it possible that photons have mass and that the spd of light is not absolute?

« on: 16/11/2015 15:09:22 »

In other words, yes, mass and energy are equivalent.

And if you write "mass and energy are equivalent" anyone understands what I've written?
Or maybe understands something that he/she would use in wrong equations, as almost everyday happens in forums and at school?   []

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Physics, Astronomy & Cosmology / Re: Is it possible that photons have mass and that the spd of light is not absolute?

« on: 16/11/2015 15:01:24 »

Sorry that link is about the longitudinal component not the transverse component. Sorry for the miscommunication.

I can't open it, don't know why. Is there a copy in Arxiv.org? Which is the title of the paper?

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Physics, Astronomy & Cosmology / Re: Is it possible that photons have mass and that the spd of light is not absolute?

« on: 16/11/2015 14:51:51 »

Any beam of light with a finite diameter has a longitudinal component in addition to the normal transverse components.

Longitudinal "component" of what?

The longitudinal component being by definition an oscillation in the direction of travel which is by definition perpendicular to all transverse components.

An "oscillation" of what?

There is nothing about wave mechanics in general that prevents a wave with both longitudinal and transverse components from existing and traveling in a straight line at the known propagation speed of the medium through which it is traveling.

Are you talking of a light beam in the void or else?

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Here is a paper on the transverse component of the light. It is an oscillation of the electric field and can occur in any medium.

diposit.ub.edu/dspace/bitstream/2445/43553/1/602108.pdf

No, I was asking you about the "longitudinal" component.

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Physics, Astronomy & Cosmology / Re: Is it possible that photons have mass and that the spd of light is not absolute?

« on: 16/11/2015 14:48:21 »

Hello, Photons travelling through space are neutral, I like to call them a ''convertual particle'',  they have neutral mass until they hit something which by thermodynamics then converts the energy into positive  mass.
Of cause I am not a scientist, and science will tell you I am wrong.

more than anything, I would say it's nonsense.

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Physics, Astronomy & Cosmology / Re: Is it possible that photons have mass and that the spd of light is not absolute?

« on: 16/11/2015 14:45:58 »

A photon is a wave packet,

Not proved.

otherwise known as a quantum of energy.

"Wave packet" and "quantum of energy" are two different things.

Light doesn't propagate continuously like water from a firehose. It propagates in discrete lumps like baseballs from a pitching machine.

Actually you can only say that light behaves as lumps of energy in interactions but you cannot say it does it "in propagation". You can find the "lumps" when photons interact with a photographic plate, for example, but you cannot say the same is true from source to detector (photons are really mysterious things).

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Physics, Astronomy & Cosmology / Re: Is it possible that photons have mass and that the spd of light is not absolute?

« on: 16/11/2015 14:37:49 »

Any beam of light with a finite diameter has a longitudinal component in addition to the normal transverse components.

Longitudinal "component" of what?

The longitudinal component being by definition an oscillation in the direction of travel which is by definition perpendicular to all transverse components.

An "oscillation" of what?

There is nothing about wave mechanics in general that prevents a wave with both longitudinal and transverse components from existing and traveling in a straight line at the known propagation speed of the medium through which it is traveling.

Are you talking of a light beam in the void or else?

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Physics, Astronomy & Cosmology / Re: Is it possible that photons have mass and that the spd of light is not absolute?

« on: 16/11/2015 14:14:21 »

Just for the record, I am not a scientist, just somebody interested in this stuff.

Same for me.

Most people here are familiar with the idea of the photon as a "force carrier." That basically means that it sort of like a piece of information. When an excited atom emits a photon, that photon carries a piece of electromagnetic information with it. It tells another particle it deflects, for instance, "deflect this direction," or if it is absorbed, it says, "oscillate at this frequency." The thing the photon is "carrying" is a message about electromagnetic force. I think it's useful to consider the photon as "carrying" mass, as well, or at least information about how much mass it will have once "bound" to a system.

No, mass is a different thing which has nothing to do with the concept you are expressing here.

Less people seem to be familiar with the concept of "binding energy." Without delving into minutiae about relativistic mass,

Better you don't talk of "relativistic mass", in fact.

rest mass and a lot of complicated jargon, suffice it to say that mass and energy are equivalent according to the laws of physics.

They are not, in fact photons have non-zero energy but zero mass.

You can get energy from mass,

Actually it's not true, but I admit I grew up with this idea too.

as in combustion or fusion, or you can get mass from energy

It's incorrect the same: the system mass doesn't change (as well as its energy).

as in the case of particle accelerators where kinetic energy is turned into new particles. In short, mass and energy are interchangeable.

It's not so simple.

The interesting thing, though, is that photons, while being massless as a free travelling photon in space, actually contribute a tiny amount of mass when you isolate their oscillation at a location in space and they become "bound" to that system.

Please let's not talk of "oscillation" in the case of photons. About "mass contribution", it's the same for every form of energy acquired by a system *which is still* in the frame of reference considered. If a photon is absorbed by an atom which then recoils, the energy acquired by the atom doesn't result in its mass increase only, but in its kinetic energy too. For this reason mass and energy are two different things.

My own personal statement of mass/energy equivalence is this: Energy is unbound mass that travels through space, and mass is bound energy that occupies a location in space.

Maybe, depending on what you mean with "unbound" and with "location in space". The central concept is that the system is *still*, that is stationary, that is its total momentum is zero. Then its mass is simply its total energy (a part a factor c^2). So a system's mass is "the system's energy in a frame of reference where the system is still" (where "still" means total momentum = 0, in the case of a system of more than a single mass point or more than a single rigid body). The fact the system must be still means that mass and energy are totally different concepts: mass requires state of motion (= 0) in its definition, energy doesn't !

No comment for the rest of what you write, because it's quite nonsense to me.
Regards.

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Physics, Astronomy & Cosmology / Re: Is it possible that photons have mass and that the spd of light is not absolute?

« on: 16/11/2015 13:36:30 »

Is it possible that photons have mass and that the speed of light is not absolute?

It' possible but it's not true at the present state of experimental results. If you look in the Particle Data Group site you find a superior limit for photon mass (I don't remember the value in this moment, I think around 10^(-50) grams).

“IF” I understand the theory correctly, it was originally assumed that nothing can travel faster than the speed of light, and that since photons just happen to travel at the speed of light they must have no mass.
But, isn't this a circular .? By definition, the speed at which photons travel is the speed of light.

It's not circular because there is however a limit speed (maximum speed of signals, which is the one used in relativity and called "c"). If photons had mass, they wouldn't travel at that maximum speed but a little bit less.

Since light (photons) can be influenced by gravity, magnetism, and other things, is it possible that photons actually do have mass and that the particles that travel faster than photons (faster than the speed of light) simply have less mass?

It's possible but in physics, when at present state something has zero mass because it's lower than the experimental precision, we simply say "it has zero mass" without specifying "for what concerns our present knowledge", because it's always implicit.

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Physics, Astronomy & Cosmology / Re: How can light have energy if it has no mass?

« on: 31/10/2015 18:30:45 »

The second equation involves momentum which is the objects mass x velocity and you said the mass is 0.

The photon carries momentum - this is the mechanism behind the solar sail, with its deployment recently tested in space by the Planetary Society. http://sail.planetary.org/ The momentum of a photon is p=h/λ. See: http://en.wikipedia.org/wiki/Radiation_pressure#Radiation_pressure_by_reflection_.28using_particle_model:_photons.29

What do you mean by a Photon carries ''momentum''? Do you mean when a Photon hits something it applies a force?

Yes and there is no need to consider photons, that is a quantum description: a classical description of the electromagnetic field (Maxwell equations, Lorentz force) is enough: when light hits an object, it makes a force on it, this is for ex. the principle of solar sails, as evan_au wrote:
https://en.wikipedia.org/wiki/Solar_sail

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Physics, Astronomy & Cosmology / Re: How can light have energy if it has no mass?

« on: 31/10/2015 08:10:22 »

  Please could someone explain why the second equation is right? The second equation involves momentum which is the objects mass x velocity and you said the mass is 0. Therefore you are saying E² is still 0 and so is E ∴ contradicting everything you just said. E=mc² and E²=m²c⁴ + (pc)² don't work when equating energy of light. Don't use E=mc² and E²=m²c⁴ + (pc)² when finding energy of light, use E=hf and f=hc / λ, where h is plancks constant, f is frequency of the photon and lamda is the wavelength of the photon to work energy out from its wavelength, NOT MASS.

Momentum, in general IS NOT m*v! It's m*v ONLY for particles of non zero mass AND at low speeds.

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Physics, Astronomy & Cosmology / Re: Does quantum mechanics (QM) require an absolute time frame?

« on: 24/10/2015 12:35:09 »

That can't possibly mean that the same scenario calculated using relativistic and non-relativistic QM would give different answers, and that they would both be right; or can it?

Certainly non-relativistic QM gives incorrect answers for particles' descriptions at relativistic speeds or however where relativity is required; I can't make specific examples because I'm not an expert.

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Physics, Astronomy & Cosmology / Re: What happens at the center of mass of two identical colliding black holes?

« on: 24/10/2015 12:28:49 »

1.  Why would a black hole be attracted to a black hole?

Why it shouldn't be?

angular momentum is not existence of space so is irrelevant anyway.

I'd rather say that it's this phrase to be irrelevant.

and two black holes cant merge it would be against physics

No, it would be against physics if they wouldn't do it (at zero impact parametre, of course).

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