Photons have mass!!

GCNRevenger · 2004-03-18 15:43:49

If you are talking about propulsion, you have to use a high enough density of hydrogen molecules to get you some place, and hence they will be close enough together to transfer their momentum to eachother as Sbird has pointed out, where the linear velocity you impart with the laser is converted to random thermal motion... So, what you have is a laser heated rocket engine. Making the density of hydrogen molecules so low that this does not happen is not practical.

In theory, you could probably get the laser to heat the hydrogen dang hot, all the way up until you get hot enough to create a plasma, but I don't know how you will get the hydrogen to absorb that much energy easily.

ERRORIST · 2004-03-18 15:49:58

The vasimr does it.

GCNRevenger · 2004-03-18 15:56:06

Oh and Robert, the burden of proof is generally on the new theory to prove itself, not for the old theory to defend, since the old theory is based on the more established and more concrete knowledge. That isn't to say new things can't be right, but they must show their worth first.

I do happen to know a few things about the structure of matter and such, though I happen to be by profession on the applications end of the spectra and not the theory side. "Classical" particles like protons and neutrons have qualitatively determined volumes and masses even at near-zero velocity that can be extrapolated to have nonzero rest mass, but when you extrapolate for photons their mass should be zero. One of those creepy quantum things... Graeme's link is pretty good but sorrta hard to follow explanation, but anyway until somebody comes up with somthing that contradicts this experimentally, I see no reason to believe photons have any rest mass.

GCNRevenger · 2004-03-18 15:58:21

The VASIMR engine does not use a laser to heat the hydrogen in the engine. It uses microwave radiation from conventional antennas tuned to the hydrogen excitation wavelength to heat and produce the plasma.

ERRORIST · 2004-03-18 15:58:52

If anything takes up volume it has mass.

GCNRevenger · 2004-03-18 16:02:35

That is only true in a sense... a particle can have a finite volume of itself at zero velocity, like a Neutron, but other particles... my favorite the electron which governs my business... does not have any volume at zero velocity. Its one of those things; the rules don't work when stuff gets small.

It does however have a probability density (for those in the know, this is the wave function Psi or Psi Squared) where an electron has a finite probability of exsisting at a given time, and this is a nonzero, real volume.

SBird · 2004-03-18 16:03:18

SHOCK! No pipeline? Who are you and what have you done with ERRORIST!?

OK, this is something that's a bit more practical. I actually wouldn't bother with trying to accelerate the hydrogen. If you're pushing a spaceship, all you have to do is shine a light out the back. Since light has momentum, it will push you forward. Now, all the stuff I mentioned about light being a very weak propulsive force still applies.

HOWEVER, this is actually the basis for a proposed propulsion system called a nuclear photonic drive. Basically, it's a nuclear powered flashlight. You have a nuclear reactor or some other endless power supply and use it to drive a super-powerful light source pointing out the back of your spaceship.

You have to have insanely powerful lights to make this work and have any sort of decent thrust. On the other hand, though, the drive doesn't use any fuel. Also, it uses light as the propulsive medium so you have excellent Isp values.

Here's a wikipedia [http://en.wikipedia.org/wiki/Nuclear_photonic_rocket]link.

The drive would run until your reactor ran out of fuel which could be decades later. This is the single best spacecraft propulsion system ever devised. You only get a few pounds of thrust so you can't use it to get off the Earth but it would be an ideal drive for travelling to other star systems.

You wouldn't want to use a laser since lasers are VERY inefficient. The most efficient way would actually probably be to just run the reactor core REALLY hot on the back of the spaceship at the focus of a parabolic mirror. This way all of the visible and IR photons that are sent out as waste heat are used as thrust. You approach efficiencies approaching 100% this way. One problem is that the the reactor will slowly boil away at those temperatures. However, if you use high temp ceramics and design the reactor so that it doesn't start boiling away down to the reactor core until the reactor is depleted anyway, this is not a problem.

The best part is that we could build this with present technology.

SBird · 2004-03-18 16:08:50

Also, don't forget that a neutron isn't a particle - it's a collection of 3 quarks (2 down and one up, IIRC) that themselves have no volume.

Quantum physics is just wierd. If you try and apply ideas like volume, momentum and other normal concepts to it, you just end up getting in trouble. For example, in current physics models, mass itself is illusory. It makes no sense from a common sense point of view but the equations seem to bear that conclusion out. The same equations predict the behavior of the universe to 20 decimal places so they probably are at least partially true.

ERRORIST · 2004-03-18 16:15:09

Evrything you mention has mass because it takes up space or volume. You may think it doesn't but it does. We just can't measure somthing that small yet.

GCNRevenger · 2004-03-18 16:39:20

Yeah you are correct, though we can't isolate quarks for study except for the breifest of fractions of a second in the giant particle accelerators... Heh, and if they don't have any volume either, then classic particles are probability densities too, and so volume itself is only an aproximation of reality.

No Errorist, some things on the microscale (subatomic) realm don't have volume, and the definition of "volume" gets shakey on the quantum scale... The "big" world rule where all things that have matter have volume doesn't work on this scale, as hard as this is to fathom, and some things like electrons really do have zero volume.

The photonic engine is a neat idea and all, but with the super-high temperatures you run a reactor at like that, even with advanced ceramics it will eventually vaporize. Everything (except maybe polymers or covalent solids, but thats a different story) has a vapor pressure, and heating it up raises it... I worry the reactor would burn off before you got anywhere between stars.

ERRORIST · 2004-03-18 20:05:39

Once we are able to manipulate matter in the future we will be able to isolate these subatomic particles, and perhaps measure them. For now we just don't have that ability. The sun acts like the laser if you think about it. The photons are traveling out in all directions so when they slam into a H2,He or Fe ion from behind they also get pushed outward from the kinnetic energy to great velocities.

GCNRevenger · 2004-03-18 20:23:44

Nooo, infact the behavior of the electron precludes it from having volume, in the traditional sense. It really is zero... macroworld thinking and math just don't work on the quantum scale, they are entirely incompatible.

Although the Sun is big and bright and makes alot of photons, the quantity of photons at a given point from the Sun decreases exponentially with distance... there just aren't that many at this distance from the sun. Furthermore, these are of a variety of energies/wavelengths]

ERRORIST · 2004-03-18 20:34:20

Electrons have mass also. I have proven it. In an electrical wire at the power plant I have seen it move when starting large electrical motors as the current surges through it. So this proves they have mass or the wire would not move. Sort of like turning the water on your garden hose the electrons flow the same way.

GCNRevenger · 2004-03-18 20:45:17

Oh electrons have mass all right, real rest mass and not "apparent" mass like photons, but they still have no volume.

And the moving of wires because of electricity flowing through them is not due to their mass for a variety of reasons...:
~Electron mass is really small, 9.10938188 × 10^(-31)kg, and you would need a gargantuan number to affect the mass visibly.

~Electrons aren't actually being "pumped" through the wire persay, the number of electrons is roughly a constant, electricity is only the motion of electrons... You must have a positive and a negative terminal on the power generator, right? One is for electrons flowing out being pushed by the generator's magnetic field, but the other is for electrons flowing IN to replenish them. The speed at which this occurs is also such that there is essentially no lag of electron "pressure" at the "pump" head when it is started, so this is not a workable explanation.

What you are observing with your generators is probably a magnetic field effect, and is not caused by the mass of electrons.

ERRORIST · 2004-03-18 20:49:47

Electricity is only the motion of electrons... Exactly, For anything to have motion it must have mass.

GCNRevenger · 2004-03-18 20:52:55

I think you are confused with mass and volume?

Electrons have mass, that is, they "weigh" somthing
Electrons do not have volume, they occupy zero space

And not all things that move have mass... light probably doesn't have mass, and it certainly moves.

Edit: Light has no REST mass I should say

ERRORIST · 2004-03-18 20:57:17

Light has rest mass in a black hole because it can not escape.
For an object to weigh anything it must occupy space.

SBird · 2004-03-18 20:58:07

The movement of the high power calbe is actually due to the magnetic fields generated by large currents. A current in a wire generates a circular magnetic field along the length of a wire. This makes the wire jump. I suppose that the electrons moving contributes to some extent but ti's a tiny amount. Electrons have next to no mass and realyl don't flow that far in a wire. I don't have the calculations handy but I recall that the eleactrons in the power main of an average house (ignoring the fact that the AC makes then move back and forth, just total motion) only move a few inches in a year. The contribution of the electron motion is probably on the order of a drop of water compared to the Pacific ocean of the magnetic field.

GCNRevenger · 2004-03-18 21:05:51

In a black hole, the rules don't apply either... even quantum mechanics doesn't work there very well. Nobody knows what goes on inside. Light doesn't really slow down either persay, light always travels at the maximum speed in the medium through which it moves, which is "light speed" in a vacuum (no medium at all). What happens is it loses energy and ceases to exsist, not that its photons are "sucked" inside persay.

And no, the macroscale definition of matter doesn't work when things get small... if it has mass, it doesn't need to have volume. This explains why electrons can be thought of and behave as a probability density and not a descrete particle in atoms and molecules.

Euler · 2004-03-18 21:06:09

Light must have a rest mass of 0. If it had a non-zero rest mass, then it would have infinate apparent mass due to realativity.

GCNRevenger · 2004-03-18 21:08:32

That is a reasonable statement Euler... light having a rest mass would also monkeywrench quite a few other workable theories too I believe, Graeme alluded to this on one of the previous pages.

ERRORIST · 2004-03-18 21:11:40

Gravity holds the light in a black hole.

GCNRevenger · 2004-03-18 21:14:59

We don't know what goes on in a black hole... its a mystery.

SBird · 2004-03-18 21:18:27

Light in a black hole never actually stops. As the photon falls into the singularity, relativity shows that time slows down for the photon. Basically, the photon, from the viewpoint of the rest of the universe, never actually stops, it just keeps going slower and slower. Therefore it never is actually at rest.

Of course, our modern physics could be wrong. We already know that there are major problems with it because of the mismatch between relativity and quantum physics. However, we KNOW that Newtonian mechanics which you are using for your assumptions is broken and badly broken at that. You can't make statements like 'modern physics is srong, light has to have a rest mass'. You can say 'light *might* have a rest mass', though. It almost certainly doesn't, though.

ERRORIST · 2004-03-18 21:19:53

If light occupies space it must have mass even if it is at rest. When light reflects off a mirror at 180 degrees it stops for an instant. Though we may not be able to measure this in time it has to occur.

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#26 2004-03-18 15:43:49

Re: Photons have mass!!

#27 2004-03-18 15:49:58

Re: Photons have mass!!

#28 2004-03-18 15:56:06

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#29 2004-03-18 15:58:21

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#30 2004-03-18 15:58:52

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#31 2004-03-18 16:02:35

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#32 2004-03-18 16:03:18

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#33 2004-03-18 16:08:50

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#34 2004-03-18 16:15:09

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#35 2004-03-18 16:39:20

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#36 2004-03-18 20:05:39

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#37 2004-03-18 20:23:44

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#38 2004-03-18 20:34:20

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#39 2004-03-18 20:45:17

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#40 2004-03-18 20:49:47

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#41 2004-03-18 20:52:55

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#42 2004-03-18 20:57:17

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#43 2004-03-18 20:58:07

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#44 2004-03-18 21:05:51

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#45 2004-03-18 21:06:09

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#46 2004-03-18 21:08:32

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#47 2004-03-18 21:11:40

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#48 2004-03-18 21:14:59

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