# Light Bulb Propulssion?

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#### CAllenDoudna

##### Guest
Got a question and the experts seem to disagree: If it's true that the tiny thrust of an ion engine will eventually add up to a considerable speed and if it's true that a sheet of aluminum foil could be used as a sail and sunlight used as wind then we should be able to screw a lightbulb on the back of a spaceship and use the lightbulb as our engine. It would be vastly cheaper and a lot more reliable than an ion engine and could be used in intersteller space beyond the range of sunlight. While your little 60-watt lightbulb would do, a bank of stadium lights would build up speed faster. LEDs do not change the concept.

B

##### Guest
Newton's laws of motion

Third Law: The mutual forces of action and reaction between two bodies are equal, opposite and collinear. This means that whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction. This law is sometimes referred to as the action-reaction law, with F called the "action" and −F the "reaction".

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#### rockett

##### Guest
If you think "lightbulb propulsion" would work, there are a number of perpetual motion machines you can find plans for on the internet too. :lol:

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#### DarkenedOne

##### Guest
CAllenDoudna":2nnx5egp said:
Got a question and the experts seem to disagree: If it's true that the tiny thrust of an ion engine will eventually add up to a considerable speed and if it's true that a sheet of aluminum foil could be used as a sail and sunlight used as wind then we should be able to screw a lightbulb on the back of a spaceship and use the lightbulb as our engine. It would be vastly cheaper and a lot more reliable than an ion engine and could be used in intersteller space beyond the range of sunlight. While your little 60-watt lightbulb would do, a bank of stadium lights would build up speed faster. LEDs do not change the concept.
It is true that a light bulb would generate thrust if mirrors were used to get that light to travel in one direction. However the thrust generated would take an enormous amount of time and an enormous amount energy to actually get anywhere.

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#### MeteorWayne

##### Guest
It is true that a light bulb would generate thrust if mirrors were used to get that light to travel in one direction. However the thrust generated would take an enormous amount of time and an enormous amount energy to actually get anywhere.

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#### neutrino78x

##### Guest
:roll: @ people who think it is somehow impossible to do this. Yes, it would work. The problem is that the ship would move very slowly. Even with really bright sources, like a nuclear photonic rocket, the thrust is small, like an ion engine, and takes an unreasonable amount of power to generate even that small thrust. You can even take a big piece of metal and fling antimatter into it, and generate your heat and light that way. Again, unreasonable amount of input to get a really small thrust.

Ion engines have small thrust too, but don't require an unreasonable power input to get that small thrust.

So, I'm sorry, I don't agree with people who are claiming that Newton's third law somehow means you can't have a photon rocket. You can have one, it just isn't very economical.

--Brian

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#### CAllenDoudna

##### Guest
When you speak of an enormous amount of energy to achieve only a small thrust I suppose you are referring to the fact only 10% of the energy is converted to light by the incadessant bulb and focus only on that 10% visible light. But the other 90% also radiates, merely at a different frequency. Also, LEDs turn 80% of their energy into light.

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#### vattas

##### Guest
Sorry but I also do not see why "lightbulb" propulsion wouldn't work... Any light source produces slight thrust, doesn't it? And if it's directional it should be able to move you spacecraft.

I'm not talking about using it in practice, since about the only way to get some useful thrust would be to use matter/antimatter annihilation.

To OP - there are thousands of other ways how to use electricity needed to power your light bulb to get much more considerable thrust. Ion propulsion being one of these.

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#### dryson

##### Guest
Third Law: The mutual forces of action and reaction between two bodies are equal, opposite and collinear. This means that whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction. This law is sometimes referred to as the action-reaction law, with F called the "action" and −F the "reaction".
So if body F ( starship trying to accelerate to light speed) engages the engines then the reaction between the forward velocity of the ship within a gravitational field would produce a force - F (the mechanics of gravity creating a weight upon the mass of the atoms and molecules of the ship's component's) that would create an equal magnitude of force in neither direction until such time that the ships forward velocity was able to overcome the gravitational pull exerted by gravity upon the ships components.

The faster a ship travels in gravity means that each EM wavelength is encountered at a faster rate of velocity. Which then equals more weight being created by gravity within the atom(s) that causes the ship to be attracted back towards the source generating the EM field.

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#### origin

##### Guest
dryson":3sprdw0q said:
So if body F ( starship trying to accelerate to light speed) engages the engines then the reaction between the forward velocity of the ship within a gravitational field would produce a force - F (the mechanics of gravity creating a weight upon the mass of the atoms and molecules of the ship's component's) that would create an equal magnitude of force in neither direction until such time that the ships forward velocity was able to overcome the gravitational pull exerted by gravity upon the ships components.

The faster a ship travels in gravity means that each EM wavelength is encountered at a faster rate of velocity. Which then equals more weight being created by gravity within the atom(s) that causes the ship to be attracted back towards the source generating the EM field.
So, what you are saying, in a more succinct form is: adlkqpoerifqe'fic-9f[0tiqi3t!

B

##### Guest
The Third Law of Motion might make a little more sense if in this application you replace the light bulb with a hand grenade. You are exchanging the gentle push with a more energetic one, but the motion is utterly identical.

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#### vattas

##### Guest
Hm, so you state that if you exploded grenade near the wall, wall won't get any "thrust"?
Lightbulb with reflector will get it's thrust both from photons reflected off reflector and photons, radiated by the bulb in opposite direction.
Of course, photons hitting reflector won't create any thrust. But reflected photons will.

If that's not true, what did I miss?

T

##### Guest
The lightbulb will be pushed in one direction, the sail in the other. Since in this situation both are attached to the ship, there is no gain.

A better idea that is often floated around is to have an earth-orbit laser that fires at the sail.

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#### vattas

##### Guest
OK, the question is - does the photon, leaving light source, creates any thrust?
As light can transfer momentum, I assume, that upon leaving light source it pushes it in the other direction. Is that right?

BTW, I'm just interested in theoretical aspects of this. Some posters here, whom I highly respect for their knowledge, posted that "lightbulb propulsion" is not even theoretically possible. I feel that it's not the case. Prove me wrong.

B

##### Guest
vattas":2ckf1tvl said:
Hm, so you state that if you exploded grenade near the wall, wall won't get any "thrust"?
Lightbulb with reflector will get it's thrust both from photons reflected off reflector and photons, radiated by the bulb in opposite direction.
Of course, photons hitting reflector won't create any thrust. But reflected photons will.

If that's not true, what did I miss?
Lets try it this way; Rather that a regular light bulb or a hand grenade you use a spot light or a claymore mine. Your spotlight/mine is directed at the reflector only. When you turn on the light or trigger the mine you create pressure in the opposite direction the explosion is directed. Just because you have something on the front of your ship catching the pressure doesn't mean you can overcome the pressure pushing backwards.
Now, if you place the light bulb/explosive device at the rear of your ship, you can direct the pressure in the opposite direction of travel. This fits the Third Law perfectly & is actually applied on every rocket in use today. It is also the basis of the Orion Drive.

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#### vattas

##### Guest
Lets try it this way; Rather that a regular light bulb or a hand grenade you use a spot light or a claymore mine. Your spotlight/mine is directed at the reflector only. When you turn on the light or trigger the mine you create pressure in the opposite direction the explosion is directed. Just because you have something on the front of your ship catching the pressure doesn't mean you can overcome the pressure pushing backwards.
Well, now everything is clear. I was afraid that in earlier post you were stating that lightbulb with reflector (== spotlight) will not produce any thrust and I was wondering how that could be.
And I never suggested that blowing at your own sail (situation that you described above) will get you anywhere. Well, actually, it may. Some say, that if there's no wind, blowing at the sail may call it. Or whistling. But you risk to get storm this way.

BTW, OP did not suggested "blowing at your own sail" either. It's true, that if you placed "lightbulb" at the end of your ship, it will create thrust, because some photons will bounce off you ship's end to right direction. However, as many have pointed out, it's probably many milions times more effective to use sail with stationary laser, ion propulsion or something else.

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#### strandedonearth

##### Guest
vattas":1l2iyk3o said:
To OP - there are thousands of other ways how to use electricity needed to power your light bulb to get much more considerable thrust. Ion propulsion being one of these.
The primary advantage of a photon drive over ion or other electric drives is that it needs no propellant for reaction mass. Any beam powerful enough to produce measurable thrust would make a devastating weapon in close, or a good long-range comm laser

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#### EarthlingX

##### Guest
There might be a 'small' problem with energy requirements for such propulsion. Basically it sounds like inverted solar sail, if this gives you any idea.

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#### csmyth3025

##### Guest
I couldn't help but notice when I was reading this thread that at the top of the page there were three Google ads for lamps and light bulbs and one for cemetery plots.

Someone in their targeted ads department must have a wry sense of humor.

Chris

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