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/sci/ - Science & Math

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3888947 No.3888947 [Reply] [Original]

As we all should know photons have momentum. Does this mean it would be possible to propel a spacecraft by basically firing a giant laser out of one end? I know that it would be incredibly inefficient, but would conservation of momentum allow it to work?

>> No.3888990

Photons don't have mass; it won't work.

>> No.3888994

>photons
>no mass
>implying newtons forces work on thing without mass

>> No.3889000

>>3888990

you are retarded

also

>implying NASA doesn't already have plans to make a solar-sail powered spacecraft

>> No.3889006

Photons do have mass, just a incredibly tiny amount that has never been detected, same for neutrinos.

>> No.3889010

>>3889006

Einstein disagrees with you

>> No.3889028

>>3889006
lolnope
>>3888947
lolyup it's possible. Reflection of an asteroid an significantly alter its course. One proposed method of asteroid deflection is to blow off the top layer and then the asteroid is pushed more or less.

>> No.3889036

>>3889010
Einstein has been improved upon since his time. Science moves on.

>> No.3889037

If photons had mass, they wouldn't travel light-speed. having ANY mass, means they wouldn't travel light-speed.

Photons have no mass. None. Whatsoever.

>> No.3889043

>>3889006
Neutrinos have mass. It's been experimentally verified.

>> No.3889055

>>3888947
Yes.
>>3888990
They have no mass, but they have momentum.
>>3889006
No.
>>3889043
Yes.

Any questions?

>> No.3889058

Yes, it's possible, but highly inefficient. The concept is called photonic rocket.

See:
http://en.wikipedia.org/wiki/Nuclear_photonic_rocket

>> No.3889062

That idea has been floated by DARPA and NASA before. You build a gigantic laser with miles of solar panels orbiting the sun and have it fire constantly (except when it's behind the sun) at your spacecraft slowly accelerating it to a small percentage of the speed of light.

One of the major benefits to this method is that you do not need to store fuel on your spacecraft (as long as you have no need to return), eliminating a huge amount of the mass requirements for interstellar travel.

>> No.3889085

>>3889062
it should be noted that this is not exactly the same thing as what OP proposed, but similar. Propulsion is not created by the spacecraft, but by the laser being fired at it and hitting its' sails.

>> No.3889088

>>3889058
I could see this working for extremely large generation ships where you're not in a hurry to get to your destination. You would need a lot of power to begin with, and you might as well put the waste heat to productive use.

>> No.3889190

>>3889088
That would be one long boring trip.

>> No.3889293

Nah, you got to deposit the momentum ON the spacecraft.
you would need to shoot lasers AT the spacecraft, or explode nuclear weapons periodically to blast it with a wide spectrum of radiation from the nuclear explosion.

>> No.3889352

I was inspired to do the math of this on the back of an envelope.

You are basically using E=mc^2, but the opposite way around from normal, i.e. using energy to generate a momentum p = E/c = hbar*k. However, with a conventional propellant you get the rest mass for free and only provide the kinetic energy, therefore, that always requires less `energy' (semantics of propellant mass being energy accepted).

>> No.3889368

m = e/(c^2)

>implying that photons don't have mass

>> No.3889371

>>3889190
No, it wouldn't be, at least if the trip went the way every fictional account would suggest.
http://en.wikipedia.org/wiki/Generation_ship#In_fiction

>> No.3889375

It should be noted that ultimately photons are the most efficient way to propel a spaceship. This way, every little bit of energy you put into your propulsion is used to generate a change in momentum, whereas otherwise it can be used to 'create' physical mass:
E^2=p^2+m^2
So creating massless particles, i.e. photons, you get the biggest 'bang for your buck' so to speak

>> No.3889381

>>3889375

>most efficient way to propel a spaceship

this little duck has a learning disability

>> No.3889402

>>3889381
He is correct , though. In a way.

>> No.3889411

yes, and while other propulsion methods require resources, all this requires is energy

>> No.3889430

Solar sails are already in use http://en.wikipedia.org/wiki/IKAROS

>> No.3889436

>>3889371
So it will either end in blinding ignorance or genocide. Nice.

>> No.3889466

>>3889436
Well, there's gotta be adversity in fiction or there wouldn't be anything to hang a plot on.

Of course, there are examples of successful generation ships in stories where they aren't the main plot point.
An extreme example would be the gas-giant-size ship in Marrow by Robert Reed, with a "Hey let's circumnavigate the galaxy with this immense ship we just found. Why? I dunno, for shits&giggles I guess." being a frame withing which the story happens. And they do it way below light speed, with the crew living tens of thousands of subjective years.

>> No.3890899

Why settle for a spaceship propelled by photons when you can have one propelled by FTL neutrinos?