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# Why Couldn't a Laser be Used to Propel a Spacecraft?

01/26/2013 8:06 PM

could a laser be used for propulsion of a spacecraft? If so, could it be "pumped up" for more power, maybe pumped several million or billion times and push a craft to several times light speed? What if you could exceed light speed by a million times, interesting thought.

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#1

### Re: Why coulden't a laser be used to propel a spacecraft?

01/26/2013 8:37 PM

There's nothing for it to push against. You have no reaction here. Equal and opposite and all.

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#11

### Re: Why couldn't a laser be used to propel a spacecraft?

01/27/2013 11:43 PM

Since there is wave-particle duality, the propulsion force could be the inertial forces of accelerating the light particles. Has anyone ever measured the reaction of a standard laser? I imagine that force would be extremely tiny.

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#44

### Re: Why couldn't a laser be used to propel a spacecraft?

01/28/2013 11:37 AM

The force would be the same as if the equivalent mass were exiting the laser at the same rate (not speed, rate). If this were not true, then neither would be Einstein's mass-energy equivalence relation. Because a force is produced, beaming a laser into empty space will propel the source in the opposite direction just as it does with a conventional rocket motor, except that mass isn't being ejected, it's mass-equivalent is. As far as the Universe cares, mass and energy are indistinguishable.

Moreover, a laser does not need 'something to push against.' For one thing, how would that 'against' be felt back at the laser? It wouldn't, because there is no coupling between the Something and the Laser. The light energy exiting the laser is itself the reaction mass. That's what E = mc^2 means: Energy and mass are completely interchangeable in every way.

Anonymous Poster #3
#105

### Re: Why couldn't a laser be used to propel a spacecraft?

02/25/2013 7:08 AM

"Moreover, a laser does not need 'something to push against.'"

I don't understand.

The laser is typically part of a larger system, and any reaction force will be transferred to that system?

What is this distinction between ejection of mass and ejection of "mass equivalent"? Either way, the system is less massive following the ejection.

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#106

### Re: Why couldn't a laser be used to propel a spacecraft?

02/25/2013 9:23 PM

Laser doesn't need anything to push against because the photons themselves are particles that have mass. Emitting them in space means you move in approximately the opposite direction.

Anonymous Poster #3
#107

### Re: Why couldn't a laser be used to propel a spacecraft?

02/26/2013 4:28 AM

Assuming "you" to be the laser (?), rather fast unless the laser is extremely massive. Red shift and all that

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#108

### Re: Why couldn't a laser be used to propel a spacecraft?

02/26/2013 9:50 AM

Photons have no mass. They do have momentum. They also have energy. They do not experience classical physics acceleration.

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#109

### Re: Why couldn't a laser be used to propel a spacecraft?

02/26/2013 3:03 PM

Yeah, they aren't a classic example of mass. And yeah, they still act like they do have mass. Here's one for you in relation to this. Does a photon exhibit anything like a temperature? We know that a BEC is matter in it's lowest possible energy point, and that we like to get there through an evaporative process. Isn't that exactly what is happening to a given photon or pair flying through space? Maybe a photon exhibits both traits because it really has both requirements.

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#110

### Re: Why couldn't a laser be used to propel a spacecraft?

02/26/2013 4:13 PM

A single photon and temperature?

If we take temperature is a measure of random motion of an aggregate of particles, no single particle will have a temperature.

Aggregates are different. Full black-body radiation has a well-defined temperature, and the even has a (very temperature-dependent) volume specific heat ( http://lamp.tu-graz.ac.at/~hadley/ss1/emfield/quantization_em.php#wavetable ). Mass specific heat seems to me to be too artificial, but would be inversely proportional to temperature.

There is also colour-temperature, but this is mostly a physiological definition.

You could if you wish define the temperature of a single photon as corresponding to the averaged photon energy of black-body radiation at that temperature - but I'm not certain what insights this might provide.

Your thoughts on BEC resemble some of my college lecturer's asides too closely for comfort. I suspect the conclusion to be incompatible with the idea that temperature is meaningful for an individual photon.

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#2

### Re: Why couldn't a laser be used to propel a spacecraft?

01/26/2013 8:42 PM

Sir Arthur Clarke proposed this idea about 50 years ago. Researchers are investigating the idea now. The advantage of this approach is that the craft does not have to carry fuel. For obvious reasons it will never exceed the speed of light.

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#12

### Re: Why couldn't a laser be used to propel a spacecraft?

01/27/2013 11:47 PM

I think it would have to carry fuel, but that fuel might be only energy. Otherwise it would be like perpetual motion.

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#3

### Re: Why coulden't a laser be used to propel a spacecraft?

01/26/2013 9:09 PM

Not likely, because some decimal points are about six places off.

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#4

### Re: Why coulden't a laser be used to propel a spacecraft?

01/26/2013 9:38 PM

Yes they are working on this, it's called " Alcubierre's warp drive"....I wouldn't expect this to come to fruition any time soon....

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#5

### Re: Why couldn't a laser be used to propel a spacecraft?

01/27/2013 1:22 AM

That's not really laser propulsion.

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#6

### Re: Why couldn't a laser be used to propel a spacecraft?

01/27/2013 3:29 AM

It is, maybe, and it isn't, maybe. I guess it comes down to what we count as meaning 'propulsion'. The researcher is proposing the use of ultra-high-energy lasers to distort a small region of spacetime (much the way magnetars do it, I suppose, but with electric fields? And in their case the intense magnetic field makes empty space birefringent, so I don't see how this produce the desired effect); the piece was a little vague on this point and even more so on how this might produce the effect of negative mass in the one direction and positve mass in the other. Not quite the same as ye olde tried-and-true 'squirting something out the back to make it go forward' technique NASA uses but resulting in motion nonetheless - at least on paper. As the laser is the sole agent this case, does this make it count as 'laser propulsion'? I'd say so, but only if we broaden what we mean by 'propulsion' somewhat. Your thoughts?

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#8

### Re: Why couldn't a laser be used to propel a spacecraft?

01/27/2013 7:43 AM

As the Time article quotes Alcubierre:

The only catch: Alcubierre says that, "just as happens with wormholes," you'd need "exotic matter" (matter with "strange properties") to distort space-time.

A laser may one day be able to produce these exotic particles but it sounds to me that the laser will be producing the propulsion. The photons from a laser can be considered to be matter in some discussions. However, photons are far from exotic. Now a laser might become the control signal of the process generating or aligning this exotic matter. The laser will not be causing the propulsion any more than my right foot propels my car while I drive.

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#34

### Re: Why couldn't a laser be used to propel a spacecraft?

01/28/2013 10:42 AM

If you'd use both feet your car would go faster.

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#47

### Re: Why couldn't a laser be used to propel a spacecraft?

01/28/2013 12:07 PM

Affirmative.

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#102

### Re: Why couldn't a laser be used to propel a spacecraft?

02/14/2013 1:13 AM

Well that's just it. Laser is great, but. There is a Japanese engineer that has proposed a jellyfish design concept that I find very interesting, because hydrogen being present all over, in greater or lesser amount. You swim right up your fuel stream, pulsing your solar laser to ignite your fuel during your temperature differential controlled spacejellyfish compression cycle. Get enough V, and you can start jumping from stream to stream more efficiently with gyros.

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#103

### Re: Why couldn't a laser be used to propel a spacecraft?

02/14/2013 7:31 AM

I take it that you mean "accelerate your hydrogen"?

else, what will you use for the other half of the reaction

.

"jumping from stream to stream more efficiently with gyros"

Can you explain?

.

Thanks

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#111

### Re: Why couldn't a laser be used to propel a spacecraft?

03/21/2013 11:21 PM

You ignite your fuel with free laser from space, but some of what you'd eject with that researcher's design would also be just hydrogen, unburnt, as a physical product of heating-cooling, which is what the actual craft is being propelled by even without a laser on. This would be a craft that would gain V very slowly, but be self sustaining.

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#20

### Re: Why couldn't a laser be used to propel a spacecraft?

01/28/2013 6:47 AM

The problem with Alcubierre's and related methods is that the outer edges of the warp field have to propagate faster than the speed of light. I suspect that this means that you cannot simply "turn on and go". In other words, the field itself could only be set up at the speed of light, and if you want to travel a thousand light-years at these speeds, you wold have to start setting up the fields at least a thousand years before your intended arrival date. Also, if you are using light to set up the field, the object that supports the laser will have to cope with the light-momentum, which would be at least 2.2E11 N-s (although you could maybe transmit the same mass in the opposite direction).

IF this is correct, the method could in principle be suitable for setting up a long-term high-speed transportation link, but it would not allow anything akin to "warp drive".

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#21

### Re: Why couldn't a laser be used to propel a spacecraft?

01/28/2013 6:52 AM

P.S.1 An additional reservation: I don't recollect anything in this work that suggests whether structures or information could be preserved in the transition into and out of the accelerated region.

P.S.2. I would be only too pleased to be credibly informed that I was mistaken.

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#51

### Re: Why couldn't a laser be used to propel a spacecraft?

01/28/2013 12:15 PM

"I would be only too pleased to be credibly informed that I was mistaken."

I bet you would, except that your hiding behind your keyboard like some Keyboard Kommando cuts both ways, does it not? Perhaps if we addressed your materials c/o CR4 Admin? Would you find that more convenient, seeming as it does you're afraid to show your avatar?

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#7

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/27/2013 6:55 AM

US and Soviet governments worked on Nuclear power jets and rockets back in the 50's. Flew one jet system for a couple of hours but did not produce thrust. Was quite interesting to read about. Goggle and you can find info on programs. But as far as lasers I am with everyone else, nothing to push against.

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#16

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:12 AM

you know you are a good scientist, so we'd like you to show us more.

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#9

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/27/2013 11:33 PM

E=MxVsquared

Energy equals mass times (speed squared)

Light, laser light, photons, have no mass therefore E equals zero.

bandership

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#13

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 1:29 AM

"Light, laser light, photons, have no mass therefore E equals zero."

Unless they do have mass, just of a very small value which has been disregarded thus far. The (speed squared) will surely make that relevant, not necessarily on Earth where there is much friction, but in the near-vacuum of space it may be different.

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#27

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 9:39 AM

Light has momentum. From the Wiki:

In empty space, the photon moves at c (the speed of light) and its energy and momentum are related by E = pc, where p is the magnitude of the momentum vector p. This derives from the following relativistic relation, with m = 0:

The energy and momentum of a photon depend only on its frequency (ν) or inversely, its wavelength (λ):

where k is the wave vector (where the wave number k = |k| = 2π/λ), ω = 2πν is the angular frequency, and ħ = h/2π is the reduced Planck constant.[16]

Since p points in the direction of the photon's propagation, the magnitude of the momentum is

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#40

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 11:05 AM

Thanks for the "state of the art" response, most informative.

But, it is the general understanding and belief of scientists at the present time.

It is not long ago that electrons were viewed in a similar way - people knew something existed, but proving it, and finding its mass, took many years and advancements in technology.

Even now, the best devices can 'only' measure down to one yoctogram. (If you believe it: while this article has many repeats & rehashes across the net, the date of publication suggests it is still not possible)

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#50

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:15 PM

I'm not certain about the limit to measurement by physical weighing, but a yoctogram sounds rather large.

However, when it comes to standard-model particles and photons we can weigh aggregates of atoms (crystals) and then compare smaller particle masses by observing velocities after collision (for example).

That said, measurements using aggregates of photons with (derivatives of) a Cavendish balance gives an upper bound for the maximum rest-mass for the photon as 7E-17 eV, which is about 3*10-50 gm. Galactic observations point to an upper bound in the order of 1.3*10-60, but these are not as direct....

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#68

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 1:49 PM

I have a rather sneaking suspicion your avatar can only be viewed in the infrared. The writing styles are very similar. Quite, and writing styles are nearly impossible to camouflage. We discussed the unique optical properties of your avatar once before, during my previous incarnation here. As you'll recall I took a short sabbatical to restore my insanity. It keeps life interesting.

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#71

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 2:36 PM

From the article: "They might also diagnose health conditions by identifying proton-scale differences in molecular mass that are markers of disease."

This capability has long existed already in the form of mass spectrometers.

----

"But, it is the general understanding and belief of scientists at the present time."

Yes, and this will always be the case and so what are you saying? Are you proposing that light's momentum is unmeasureable? Far from it! Am I dismissing the idea that light can be used for propulsion? Not at all. Will it be? Who knows? For high-speed craft which attempt to cozy up to c? Probably not (even if it were a viable approach in practical terms), because far more pressing issues exist at such velocities, ones which myself and others have mentioned on this thread. Some of these issues are intractable. The effects of the Lorentz contraction on the wavelengths of ambient EM radiation in our direction of travel being the biggie. Good luck solving that one. In summary, some of the hurdles which have been mentioned:

• Steering such a craft.
• Braking it.
• Avoiding objects in the path of travel. Related to this and one which is sure to be a problem, avoiding dust and gas - both are pervasive in space and any such craft will encounter it, guaranteed. Think relativistic sand-blasting from the atomic scale on up. Not recommended for expensive paint jobs.
• Radiation death - it doesn't matter how clever your propulsion system is. At near lightspeed? You're gonna die, period.
• Efficiency.
• Other considerations - you'll want to keep at least several light-years downwind of the craft's 'exhaust' if you don't want to lose your toupee, and we can forget all about doing this in an atmosphere.
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#74

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:01 PM

"and this will always be the case" - How can you predict that anything discovered in the future will not cause a rethink of the formulae in use today?

For example, the mass of a photon may be 10E-500g, but it would still have some effect and may take centuries to prove.

Or its 'mass' may be unaffected by gravity, so is unmeasurable by current tools.

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#78

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:15 PM

How? For scientists in the future the future will be the 'present time' and so this statement will still be true; hence, 'always'.

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#86

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/29/2013 12:47 PM

The collision issue is confusing (at least to me). What would happen when a space-time distortion travels through space faster than light encounters a particle? The particle would clearly not be perceived within the high-speed zone as travelling faster than light. If the system could work at all (???), could it be that the particle is 'either' non-interacting or tunnels past the distorted region (how to tell the difference)? (And, for that matter, would the travelling space-time distortion appear to be negative in extent?)

Re your perceptive stylistic conclusion: not trying to disguise; rather, not wanting to use... (inconsistency apparently being part of both our stocks-in-trade?)

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#88

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/29/2013 1:17 PM

Faster than light travel should be confusing. It violates one of the most fundamental principles of how we understand the universe. This really is the realm of science fiction. As such, if (when?) this becomes possible nobody now knows how the universe interacts with such a field. So it really is anybodies guess.

My guess is that this faster than light bubble tunnels through space and matter like an electron jumping a band gap. As such the need for a deflector will not be needed. Returning to normal space or entering hyper space might require some form of a deflector.

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#22

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 7:41 AM

"Light, laser light, photons, have no mass therefore E equals zero."

That's not true. Photons have no mass but have an energy... E = h.f where h is the Planck's constant and f the frequency.

The question is that photons move at light speed and therefore cannot have mass. If something with some at rest mass could move at light speed, its mass will become infinity (something with very, very low possibilities).

Kind regards

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#24

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 8:01 AM

How do you think the energy (heat) arrives here from the sun?

The fuller equation would avoid this confusion:
E=m0c2/√(1-(v/c)2)

For light with zero rest mass (m0), the speed of travel gives us 0/0, which can take any value. So it's no surprise that the observed properties of light in free space are entirely determined by a single vector (momentum).

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#54

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:18 PM

"Photons have no mass therefore E = zero" ???

How do you think the Crookes radiometer works?

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#57

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:28 PM

A Crookes' radiometer works by means of the Dark Side of the Vane imparting gas molecules more kinetic energy than does the Light Side of the Vane. That's not photons pushing those vanes around, but gas molecules. The envelope is filled with a gas (air, typically) but at very low pressure.

How to tell? Which direction do the vanes turn? Away from the Dark Side. Go ye and do likewise.

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#62

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:40 PM

Where do you think the energy which heats up the vanes comes from?

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#67

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 1:30 PM

"Photons have no mass therefore E = zero" ???

Certainly not from the mass of photons. If you want to talk energy, then talk energy.

If you wanna talk photons - the basis of laser operation, btw, and this being a thread about lasers and, therefore, photons, specifically - then be clear of which you speak and stop whinging about the consequences of your muddled posts after the fact? Are we clear on this?

You want better replies to your posts? Write better posts.

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#73

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 2:51 PM

A Crooke's radiometer is in a sealed glass bulb for a reason - vacuum. Try again. There is absolutely no need for "any" gas to be present, the radiometer will rotate just as fast in any case, as long as the light intensity is the same.

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#77

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:11 PM

Not so. Sorry.

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#79

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:53 PM

Firstly, if this were the actually the case (and you can make it so without cracking the seal. More on this in a moment), the vanes are rotating in the wrong direction. The vanes rotate with the white side leading, black side trailing. Dead giveaway. If incident photons were the cause of the rotation, the vanes would rotate with black side leading, but they don't. White side leads. Evidently, something else is exerting a pressure on the vanes. If not light, then what? We don't have many options.

Secondly, if you chill a Crooke's radiometer to cryogenic temperatures whilst maintaining the light at constant intensity, the vanes slow down, stop, and then reverse direction (but you must either use a very high-quality radiometer to see this, or (much easier) an intense source of light).

What is going on?

As the radiometer cools gas molecules lose energy when they strike the interior surface of the glass envelope. When the envelope is sufficiently cold, they adsorb to the surface and this takes them out of circulation. At very cold temps, say, a few degrees above absolute zero, the envelope does indeed contain a hard vacuum and so, at this point there is no longer any interference from gas molecules, giving light the floor to itself. Now any momentum imparted to the vanes is from the light itself: twice as much momentum is received by the white side versus the dark side, assuming the sides are 100% and 0% reflective, respectively. The ratio is somewhat less for real radiometers; the actual ratio depending on the quality. What is happening? The photons are absorbed by the dark side and reflected by the light side. Because they're bouncing off the white side, that side gets the greater helping of momentum and so the vanes rotate with black side leading.

Lastly - and something not at all obvious at first glance - when Crooke invented his device, vacuum technology was not capable of producing the hard vacuum we can produce today and so his radiometer by necessity contained trace amounts of gas. Back then it simply couldn't be helped, whereas today its descendants contain trace amounts of gas by design.

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#80

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:53 PM

Sorry but a small amount of gas at about 60 mtorr is required for a radiometer to work. More gas and too much dampening effect. Too little gas and not enough rotational force is generated.

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#104

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

02/15/2013 4:02 PM

A Crookes radiometer works because it is not in a complete vacuum. The dark side heats the few surrounding air molecules and the resulting disturbance causes the vanes to move. Light is not "pushing" the vanes

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#10

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/27/2013 11:35 PM

Like nuclear power,did anybody try laser powered power stations?.

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#18

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:45 AM

Such a concept would be nonsensical.

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#19

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:52 AM

Really?

A laser is only focussed light, after all.

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#28

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:00 AM

"A laser is only focussed light, after all."

Not quite. What makes it 'laser' light is that it is both monochromatic and coherent, having nothing at all to do with its being focused. If focus were the sole criterium, then we have had lasers all around us since the dawn of time - dewdrops on spider webs focus sunlight, for example. No, laser light is single-color and the waves are all in lock-step.

Note that lasers can emit more than one color simultaneously, but each color is monochromatic. Krypton lasers, for example. Moreover, the term 'laser' is an acronym referring to a specific technique for generating the light, but it need not be a laser device which does this. There are natural lasers, dewdrops notwithstanding.

Intense, coherent ultraviolet light is generated in the equatorial region of eta Carinae's homunculus, for instance (around the 'waist' of the hourglass shape below). Were Earth orbiting this region (inside of which are two supermassive, tightly co-orbiting stars, vaguely visible in the image), the planet would be incinerated by the sheer intensity of the UV 'laser' radiation.

To give a sense of scale, the smallest features which can distinguished in the image below are about 10 billion miles across - roughly the size of our solar system. The system itself is about 7,500 light years distant.

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#30

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:34 AM

Maybe this is just a bit over the top - seeing that you were addressing a minimum-length answer to a statement that the concept of laser-powered stations was "nonsensical".

On the other hand, you could justifiably explain why ordinary light radiation can in principle be converted to electricity without need for a thermal-cycle interposer - and why, for the case of laser radiation, this could be made nearly 100% efficient?

But perhaps (other than the focussing issues involved in generating a long-range faster-than-light transportation channel) all this has wandered so far off-topic as to be irrelevant?

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#41

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 11:25 AM

Why are you posting anonymously?

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#53

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:18 PM

Because I'm not sufficiently sure of my ground to post authoritatively.

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#55

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:22 PM

That's no excuse. We all post with some uncertainty, leaning as we all do on others' all-too-often incomplete knowledge, as do they theirs. Heck, half the time I just make up sh*t to see if everyone is still on their toes! Haven't heard any complaints yet (not that I would pay much attention in either case ).

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#58

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:33 PM

But you still post really cool pictures.

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#59

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:33 PM

(i steal them. ssssh! )

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#61

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:40 PM

Hopefully no need to get heavy (~152 atomic units being more than enough?)

Otherwise - each to their own, so long as they aren't offensive;
and maybe my avatar is not quite as anonymous as yours?

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#23

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 8:00 AM

If laser can cut steel why can't it be used to boil water?.

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#26

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 8:50 AM

Well you could boil water with a laser, but it would be a bit more efficient if you just used the "power source" of the laser to generate the heat! Even if you "assumed" there was some way to use a laser for propulsion as the OP suggested, the power source required might just be a bit large?

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#36

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:43 AM

It sounds to me that boiling water can be the result of a byproduct while using the laser for other purposes. So instead of having a specialized power plant, you'd have a facility serving two purposes.

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#38

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:53 AM

SteamPunk warpdrives are all the rage, you know. To wit:

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#42

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 11:30 AM

Hey, where can I get one of those and do I need a license to fly it?

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#43

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 11:34 AM

I think you'll need much more than just a license to fly it.

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#45

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 11:49 AM

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#29

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:28 AM

Who says laser light can't boil water? If the light's wavelength falls in one of water's absorption bands and if the light is sufficiently intense, you can boil the hell out of it. How do you think they make holy water for Pete's sake!

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#31

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:36 AM

I think he's referring to the losses involved in converting the original energy source to coherent radiation...

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#35

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:43 AM

Coulda fooled me. I see no reference to that.

-e

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#33

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:42 AM

P.S. The easiest way to boil water with a laser is to turn the cooling pump off.

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#25

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 8:04 AM

Obviously the laser would need to get its power from somewhere in the first instance. It might be a long way off, but a solar-powered maser or laser in space would be quite an effective way to get concentrated power down to the earth (beam control would be critical...)

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#37

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:44 AM

Why are you posting anonymously? Is your wife lurking in the wings?

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#14

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 2:11 AM

In principle,Objects can be propelled by any light (need not be laser); by means of 'Radiation Preasure' (I/c N/m^2 if absorbed; double if all light reflected by the object). In practice No! read in web about 'Solar Sails'.

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#15

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 2:56 AM

Why not in practice? Solar sails work. Solar sails use a phenomenon that has a proven, measured effect on spacecraft.

Solar radiation imparts something like several grams of force on my roof.

Also, they actually built a solar sail spacecraft.

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#32

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:38 AM

The effects of radiation pressure are all around us, in space. Look at these columns of dust in the Eagle Nebula, for example. The 'columns' are due to star systems in whose radiation-pressure shadows the dust remains. The radiation pressure from nearby young, massive stars has cleared the space between between the columns of gas and dust.

The same happened early on in our own solar system's history. Once the Sun's nuclear fires ignited, the radiation pressure along with the Solar Wind cleared the immediate vicinity of gas and dust, arresting any further planetary formation.

The Eagle Nebula, courtesy of the Hubble Space Telescope:

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#48

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:08 PM

Isn't THAT pretty!

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#69

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 1:52 PM

The Eagle Nebula is one of the most stunning sights in space, bar none.

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#84

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/29/2013 10:26 AM

Is that the one that almost got the best of the Star Ship Enterprise?

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#85

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/29/2013 10:33 AM

Close. It's the one wot killed the series.

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#17

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:42 AM

Yes, actually such a scenario has already suggested and studied by physicists. A very powerful laser is supposed that can give a tiny momentum (or a tiny acceleration) on a spacecraft, in the same way that convetional fuel does. Although this acceleration would be really small, after a sufficient (and rather long) time duration the spacecraft could get a really high speed.

You said: "...and push a craft to several times light speed? What if you could exceed light speed by a million times, interesting thought."

This is basic high-school knowledge: Nothing can exceed (not even reach exactly) the speed of light. Only particles with no rest mass (like photons or gravitons) can travel with the speed of light (and actually they travel only with the speed of light).

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#39

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 10:56 AM

Possibly it's a matter of attitude, which makes all the difference. Perhaps if we found a way to sufficiently motivate those darned particles, they'd get the show on the road. The Universe is a Big Place and we've not got all bluddy day!

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#52

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:16 PM

And they don't always HAVE to travel at the speed of light, light can travel slower if it wants to. Like if it hits an obstruction, like, say, glass.

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#82

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/29/2013 3:32 AM

Hi Yusef1. I read the article but I'm not sure if the light itself is actually slowing down. After all, the light interacts with the Bose-Einstein condensate.

In the case of the light passing through the (e.g.) glass, the light is slowing down due to the continuous absorption and re-transmission by the atoms of the glass. However, in between the atoms it still travels with 300.000 km/sec. So, this is an aparent reduction of the speed of light (not an actual one).

(Something like that, probably, happens also in the case of the Bose-Einstein condensate... ...)

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#83

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/29/2013 9:50 AM
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#46

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:01 PM

For clarification is the OP asking about having a laser on the craft as an engine or having a laser beamed at the craft?

I know of work done using ground based lasers basically focusing on a mirror http://www.lightcrafttechnologies.com/ they were able to launch small mirror craft, although I haven't seen any updates from them in a while. This method requires air however so wouldn't work once actually out in space from my understanding.

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#49

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:09 PM

That thought crossed my mind too. If used as the propellant that beam shooting out the back would be a threat to everyone on the ground for practically the entire time it was in atmosphere.

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#56

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:26 PM

I have actually seen this take place on a TV show, where they used a pulsed laser to lift a lightweight cone from one height to a considerably higher one. The propulsion depends on the energy (Joules) of the laser pulse, the pulse rate, the weight of the "craft", and its interaction with the light pulse.

(1) I don't think this will ever be an economical lift-off vehicle due to (a) inefficiency of lasers as a thermodynamic device, (b) the expense of scaling up a laser system to the enormity required.

(2) I do consider laser propulsion as a suitable acceleration device when the "craft" is already in outer space (low or no atmosphere), and the "station" has relatively unlimited access to power (low solar orbit???)

My question is now that you are traveling at this enormously high rate of speed across the solar system, where is the brake pedal?

By the way, photons (using the particle like characteristic) do have an effective mass (momentum), they just do not have rest mass, since photons do not exist at any velocity less than the speed of light in the medium.

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#60

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:38 PM

I think maneuvering would be an issue anyway. If something is in the path you're screwed. I think in order to provide any breakage, you'd still have to have traditional methods to reverse propulsion, in order to get immediate braking results. Even then, you'd have to start deceleration well in advance.

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#63

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 12:56 PM

He'd have another issue to tackle as well - a Big one - at least in the vicinity of c: radiation death. Space is filled with radiation of all wavelengths. Not particularly intense when you're well away from stars, black holes and such, but a problem nonetheless at very high velocities.

Seems that as he approaches c, the apparent wavelength of even harmless, low-energy microwave photons contracts from his point-of-view in the forward direction. Very, very near c what may look to us as, say, harmless radio waves from some old AM broadcast, would to him appear as high-energy X- or gamma rays impinging on the bow of his craft. He and his craft would literally fry from the radiation. (If Orson Wells had known his broadcast would have that effect as well, I'm sure he would've relented early on).

No problem looking out the stern, however, as what may be gamma rays from our perspective would to him look like benign microwave photons - the exact reverse of what he'd see out the bow. What he sees out the bow is gonna be the real problem, though. All is not lost, however (there's an unobtainium lining in every lead cloud) he'd be the first Crispy Critter to leave the galaxy, just like they did on Star Trek. No ESP powers I'm afraid. Nor delusions of Godhood. That's a load off. Now if we could only convince the Over-Unity Crowd and those other Fringe Elements who occasionally grace our site of their folly, all would be well - although I suspect it's probably easier to just exceed the speed of light millions of times over using lasers.

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#65

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 1:09 PM

Let's give Orson proper credit here. He modulated those radio frequencies. He neither produced them nor stopped their production.

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#66

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 1:11 PM

And it was a good show, besides - or so I've heard. (Personally I'd like a crack at that ominous-looking water tower outside town myself)

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#64

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 1:00 PM

Before we exceed the speed of light, a good detection system will be needed to avoid those small black objects that might get in the path. In fact we will need such a device even if we can attain a small percentage of the speed of light. Think how far ahead we need to "see" an object and then maneuver around it. What response time do we need? Do we need special shields and then to what size of object are we shielded? That detection system will have to be faster than the spaceship.

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#70

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 2:04 PM

What you are proposing is a solar sail http://en.wikipedia.org/wiki/Solar_sail. Only instead of using the power of the sun, use a laser.

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#72

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 2:47 PM

A laser emits a beam of "light". Presumably this beam will be travelling at the speed of light. I don't see how our spaceship will get to several million times the speed of light, using a fuel which only travelling at one times the speed of light.?

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#75

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:04 PM

Head directly to the nail. Nothing exceeds the universal speed limit, unless one considers the warpage of space-time just past the event threshold of a black hole. There the warpage is so high, it is said that not even light escapes. There are some strange beams of photons/particles that are emitted "north" and "south" of the black hole - I was going to say above, but anywhere near a black hole is "above". I remember one astronomic measurement where somehow particles were "communicating" with each other by photons, yet each was traveling away from the other (and also away from the observer) at highly relativistic speed. The impression was that something was going almost 2c... please don't ask me to explain, because I cannot remember enough details.

For 99.99999% of all applications the speed of light is the be-all and end-all.

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#76

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 3:10 PM

Look up 'quantum entanglement' for how such 'communicate'. One of the more bizarre consequences of QM.

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#81

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/28/2013 8:42 PM

Actually photons do have momentum, but it would take a whole lot of them to move a spaceship!

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#87

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/29/2013 12:51 PM

One optical photon would affect the spaceship - but how would you tell?

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#96

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/30/2013 3:20 PM

Momentum of photon p=h/λ. h is Plank's constant = 6.63 x 10-34 m2 kg/sec and λ is wavelength of light photon.

If M is mass of spaceship, change in momentum = p = M * dV;

Change in velocity of spaceship = dV = p/M = h/λM for each photon

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#97

### Re: Why Couldn't a Laser be Used to Propel a Spacecraft?

01/30/2013 3:25 PM

Don't forget the other photons hitting the spaceship.

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