Photons have mass!!

GCNRevenger · 2004-03-18 21:23:52

Yeah that sounds about right...

Another one of the fun things Where Newton Don't Work is that the flow of time itself is not a constant everywhere at the same time. Since velocity is distance traveled per time, the photon appears to slow down, but from the photons' perspective the rest of the universe speeds up, so its velocity is not really reduced, but it depends on who you ask... the photon or the rest of the universe?

SBird · 2004-03-18 21:27:35

Actually, when light reflects off a mirror, it doesn't stop.
If you look at light as photons, the photon is absorbed by the atom and ceases to exist. It is then re-emitted a tiny fraction of a second later. It never actually stops, the absorption process is instantaneous.
If you look at light as a wave, it has no definite position and is reflected by a perterbation on the electron gas of the metal.
In neither case do you have a photon sitting still.

It would probably be worthwhile to read a basic quantum physics book - this will make a lot more sense. John Gribbin wrote a pretty good book called In Search of Schrodinger's Cat and there's lots of other good ones. They're aimed at a non-technical audience so you won't have to worry about being caught up in the math. Just be careful, these books to tend to oversimplify some topics so it's a bit dangerous to try and do serious technical thinking with what you've learned.

GCNRevenger · 2004-03-18 21:33:02

Light probably doesn't have a volume either in the classical sense, this isn't as pressing an issue in the physics world as its mass, so it doesn't have alot of attention.

And Sbird is right about the mirror... for an explanation of why look up "Band Theory" in a general chemistry college textbook, and this will explain the difference between a metal mirror's absortion and the absortion by hydrogen or other elements.

ERRORIST · 2004-03-18 21:40:54

It is then re-emitted a tiny fraction of a second later. Exactly, and in that time frame it has stopped.

GCNRevenger · 2004-03-18 21:44:27

No... what REALLY happens is this...:

1: Photon collides and is absorbed by the metal mirror electrons. Destroyed, obliterated, GONE.
2: The electrons in the metal mirror are excited, that is, they have more momentum and contain more energy then normal.
3: Electrons generally want to store as little energy as possible, so they must give it up somehow and get rid of it.
4: The electrons will produce a brand new photon that is essentially identical, and to maximize energy transfer it releases it opposit of the angle of incidence.

This happens very fast, but between Step 1 and Step 4, there is no photon.

ERRORIST · 2004-03-18 21:56:49

That is correct it happens in the electrons. It is as if it clones itself, and then it readmits the new photon the opposite of the angle of incidence. According to the text books this is what happens.
This is what really happens. The electron acts like a penning trap and holds the electron in it. It heats up from the friction and kinetic energy it creates. Then the electron breaks away from the proton at near the speed of light,and the same photon escapes opposite of the angle of incidence. But what about protons when light hits them?

GCNRevenger · 2004-03-18 22:05:16

Some confusion here I think... the excited electron radiates the excess energy as a new photon. Not an exsisting one trapped inside.

The photon itself is a bundle of energy, not of mass, and 100% of this energy is transferred to the electron and becomes momentum. The original photon ceases to exsist. Now, since the amount of energy per-photon is so small, the actual change in momentum of the electron is not that huge, none of this "flies away at the speed of light" nonsense.

There is also no friction involved, the photon is absorbed with perfect efficency, 100.0000000% energy transfer, nor is there a temperature change since electrons have no temperature, only atoms may have temperature. The electron which was moving slow is now moving faster, and needs to lose the excess energy. In this case, the electron acts as a tiny photon factory, and produces new photons to carry the energy away. For a mirror, these are essentially identical to the photons absorbed.

For Protons, you have a whole new set of issues... first of all, how do you hit one? They are shielded by a wall of electrons.

GCNRevenger · 2004-03-18 22:18:51

Goes like this: schematic...:

Photon/Energy ---> Electron Momentum ---> Photon/Energy

There is no photon inbetween.

ERRORIST · 2004-03-18 22:25:09

Photons are massive it is well documented. If a photon strikes an electron at a 45 degree angle the electron flies away at near the speed of light. This too is well documented.
Since photons are massive there must be fricton as well as temperature change.

ERRORIST · 2004-03-18 22:27:42

Photon/mass------> H2/mass--------->= Mass acceleration

GCNRevenger · 2004-03-18 22:35:24

Photons are massless at zero speed, but they do carry (or rather ARE) substantial energy depending on their wavelength when moving. Perhaps you are talking about the photoelectric effect? The electrons don't go flying away at near light speeds, but they do leave moving pretty quickly. Do you know why this happens?

Friction ONLY occurs when two atoms or groups of atoms are forced and pushed against eachother in close proximity, this has nothing to do with photon interaction.

Second, no mirror or other material is perfectly reflective, and some of the energy absorbed by the electrons may be converted to other forms of energy besides new photons, though this happens predominantly in mirrors. Some of this energy may be converted into motion of the atoms, which would make a temperature change. Electrons and photons don't have a temperature persay, because they don't vibrate like atoms do, you can't say they have any temperature at all.

Euler · 2004-03-18 22:35:31

Friction and temperature are not concepts that are applicable on the sub-atomic scale. The photon has no rest mass. However it is traveling at the speed of light. Due to relativity, an object's apparent mass is equal to its rest mass times (1-v^2/c^2)^(-1/2). Since photons travel at the speed of light, in this case (1-v^2/c^2)^(-1/2)=infinity. 0*infinity is undefined, but in practice the apparent mass of a photon can be found using E=mc^2.

ERRORIST · 2004-03-18 22:47:55

Anything with zero mass does not exist. It is equal to the total and complete vaccum of space with nothing in it at all.No solid, liquid, vapor,plasma or light.

GCNRevenger · 2004-03-18 22:57:50

That isn't true... ENERGY exsists and it is neither of these states of matter, and a form of energy known as electromagnetic radiation, of which light is a part and is manifested as photons/waves... none of which have a rest mass. You cannot think of light as a classic particle, it is not, it isn't a carrier of energy by nature of its momentum rather it IS energy, seperate and apart from conventional matter.

The sum total mass of the universe is also not a constant. The total amount of energy and mass is. Remember E=MC^2? It says that you CAN convert mass into energy, which is what makes nuclear power plants and the Sun go. The apparent binding mass of the nucleus is partially converted into photons/energy in the nuclear reaction.

ERRORIST · 2004-03-18 23:20:19

We will be able to see a photon at rest one day and find out that it is a particle. We just don't have that technology yet.

SBird · 2004-03-19 00:21:18

OK, I've been running some numbers on this nuclear photonic drive and... WOW, it's amazing.

I found some evaporation rates for Tungsten in vacuum at various temperatures and used it to find out what's a practical temperature to run the drive at.

Assume a 100 kg total probe mass. The drive is a small critical mass with a half life on the order of a century although we can moderate the system to try and get a steady level of output for a century, depleting the fuel in the process. The critical core is contained in a spikey, polished and shiny tungsten jacket. The spikes are to mazimize the surface area so that more light is emitted. The jacket has a total surface area of 100 m^2. I think that it's practical to have this whole package weight less than 50 kg. The critical core is controlled to keep the outer surface of the jacket at 2200 K. At this temperature, the evaporation rate is approximately 7*10^-6mg/cm^2 per hour. Assuming our jacket geometry can withstand the loss of 10 mg of material per square centimeter(10 grams of material over the whole jacket), we're looking at about 160 years of active life.

At that temperature, the total photon thrust (assuming perfect mirrors, etc) is about 0.59 N. Assuming that we'll lose some of that thrust as randomly emitted IR re-emitted from imperfect mirrors, let's assume 0.5 N thrust. This gives our 100 kg craft an acceleration of 0.005 m/s^2.

For about 100 years.

Here's a rundown of how our spacecraft does:
1 second: 0.005 m/s, 2.5 mm travel distance.
1 minute: 0.3 m/s, 9 meters travelled.
1 hour: 18 m/s, 32.4 km travelled
1 day: 432 m/s, 18,662.4 km travelled
1 year: 157.5 km/s, 16.5 AU travelled
10 years: 1580 km/s, 1664 AU travelled
100 years: 5.3% c, 2.63 LY travelled

Assuming an matter to energy efficiency of ~0.1%, I calculate that the craft will deplete 13-18kg of radioactives. I'm assuming that this will require multiple core changeouts or some sort of liquid fission core that can be replenished.

If we take our nuclear 'bulb' and split it up into smaller bulbs, we can probaly get another 10-fold increase in surface area. This would require 28 or so bulbs and gives an increase in the thrust to 5 N. If the total mass remains the same, and we assume the same amount of fissible material, we deplete our drive in 10 years since we're now burning the cores 10 times faster to keep the same surface temperature on the jackets. Therefore, now we achieve 5.3%c in 10 years instead but can no longer accelerate after that.
1 year: 1574 km/s, 165 AU
10 years: 5.3% c, .26 LY
100 years: 5.3% c, 5.0 LY

If we start adding more fission fuel, the spacecraft gets heavier but can run the engine longer. Assuming that the weight needed for replacement engine parts and fission fuel weighs about 50 kg/10 year burn, the maximum speeds of the probe increase with longer burn times as follows:
10 years: 5.3% c (100 kg probe)
30 years: 7.88% c (200 kg probe)
100 years: 9.5% c (450 kg probe)
200 years: 10.0% c (1050 kg probe)
1000 years: 10.4% c (10050 kg probe)

Obviously, the practical upper limit to the speed achievable by this system is about 9-10% light speed. Not too bad for a glorified light bulb!

Euler · 2004-03-19 01:34:27

I think you are being a little optimistic about the amount of power you are getting out of your reactor. To get 5N of force using light, you need close to 1 gigawatt of power (assuming an efficient engine). Modern nuclear reactors designed to be used for space need about 10-50 kg/kW.

SBird · 2004-03-19 03:02:34

Curses! You're correct, I had the thrust right but was underestimating the energy flux by several orders of magnitude. It's pretty amazing that it takes a 100 MW of power output to keep a 100 m^2 area at 2200K.

However, I think that it's not too implausible to have a small reactor with that power output. Remember, we're not trying to pull electricity out of the core or control it very much. Basically, we want some sort of short lived, very high energy nuclei that can be counted to decay at a very high rate. Obviously, high power fluxes are possible - as nuclear weapons demonstrate. Is it possible to get a mix of transuranics that can get a 100 MW/second thermal output? Basically it's the mother of all RTG's.

Of course, the problem is that even if such a power flux is possible, you burn through your fissionables quite quickly. Assuming that a 50 kg fissionable core can release 0.05% of it's mass as energy, you can run the engine for only about 6 months before the core runs out. If it were 50 kg of antimatter, you could run the core for 500 years. However, that's unlikely to happen.

I'll need to run through the numbers again but it looks like the fuel fraction just jumped a few orders of magnitude. It also looks as if this is now in the same ballpark as Zubrin's salt water nuclear rocket - just with a slightly better cargo mass fraction. I'll see how the numbers turn out.

ERRORIST · 2004-03-19 07:39:58

Now take those same photons that have exited the spacecraft out the back end, and accelerate H2 with it also out the back end and see what you come up with.

GCNRevenger · 2004-03-19 08:04:34

Errorist, you are not understanding... it is a violation of quantum physics for all particles to have volume and rest mass, and since quantum physics is probably correct, then therefore there must be massless and volumeless particles. This is not a problem of sensing or technology, but a problem of the fundimental mechanisms of the universe, which we think right now say NO, not everything need obey macroscale rules. In fact, the macroscale rules are governed ultimatly by the microscale ones, which is what is really happening, not this big giant universe you can see/touch/measure/etc. All substance and form on our human scale is simply an aggregate, an averaging if you will, over a very large number of microscale particles... They are ultimatly only an empirical estimation thats convienant to work with, and not a description of the true nature of things.

And again, using light to accelerate hydrogen linearly is not practical, because the thrust would be ~0 at low pressures required, would not be very efficent at high pressure because of linear-thermal energy conversion, and it would be insanely difficult to get the hydrogen to efficently absorb the photons and not melt the inside of your engine.

Yeah the photonic drive isn't all its cracked up to be *chuckle* The only reactor i've heard concieved that can produce 100's of megawatts for a reasonable mass is a gas core reactor, but it would have a pretty high fuel burnup rate... And Zubrin's salt water rocket would have way higher thrust (multi-G acceleration!) than photonics.

ERRORIST · 2004-03-19 08:10:45

Look what they just did today. Funny we were talking about it. A neat way to make megawatts of power while using the solar wind at the same time for acceleration.

[http://www.spacedaily.com/news/solarcell-04e.html]http://www.spacedaily.com/news/solarcell-04e.html

GCNRevenger · 2004-03-19 08:24:57

Mmmmm No and No...

The problem with solar pannels for power is that they are limited by the quantity of sunlight in space, so even a 100% efficent system still can't produce multimegawatt powers unless it is many miles wide at Earth and Mars distances. Nor does this solve the problem of pointing them when you have a maneuvering, moving rocket or that if you are in orbit you will need a way to store energy during blackout. You can't simply turn your high-power electric drive on and off every hour or two.

Second, if it is a solar collector, then it isn't a mirror, and hence its light pressure will be nil. Solar cells work by converting the momentum of photons into momentum of electrons, but instead of the electrons reradiating the energy as more photons, instead the excess motion of the electrons IS electricity, and is harvested for electrical applications. There isn't a signifigant momentum transfer from the photon to the atoms of the cell.

ERRORIST · 2004-03-19 08:38:56

Mmmmmm yes and yes,

18 football fields in size should give you 100mws. Pointing them and manevering them is easy with hall thrusters or other types of thrusters such as these.

[http://www.spacedaily.com/news/microsat-04b.html]http://www.spacedaily.com/news/microsat-04b.html

A solar black out will not occur for at least a billion years.Since photons carry mass they will add light pressure.

GCNRevenger · 2004-03-19 08:52:38

Uh, dude, are you realizing what you are saying? Eighteen football fields? Thats... like... alot. Also remember that polymeric solar cells are much less efficent (4x, ~10%) than their silicon cousins, so I think your numbers might be a little off...
At LEO: 714,300sqr meters
At Mars: 2,857,100sqr meters
Jupiter Orbit:Thats alot of zeros!
If you are going to power an electric engine with this, also take into account that your acceleration will drop as you leave Earth, which further inreases trip time.

And when I mean pointing, i'm not talking about by motors or thrusters or whatnot, i'm talking about how you are going to keep the collector pointed at the sun, even when you are thrusting in different directions with the main engine... this will not be an easy feat, and will throw off your center of mass.

And when I mean blackout, I mean when you enter the shadow of the body you are orbiting, which you will have to do several times at least with the engine running to leave orbit, at which time you will have no power to keep your engine cool much less keep it running, requiring some sort fo very nimble ultra-high-energy storage system. Ain't gonna happen.

And solar cells as solar sails, again you are not thinking on the microscale... the photons will carry their energy to the electrons of the cell which becomes electricity, not the momentum of the atom of the cell. Optimizing for light pressure will actually slow you down by cutting the amount of energy produced... a perfect 100% all-spectrum solar cell would be Jet Black.

ERRORIST · 2004-03-19 09:08:05

It will be very easy to point the cells at the sun they do it all the time. All you need are thrusters to keep them in alignment.
During the time of blackout you can swith over to a chemical propulsion rocket.Photons accelerate objects in space when they collide because they have mass.

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Re: Photons have mass!!

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