Jump to content

Solar powered space craft


apurvmj

Recommended Posts

hey guys have you heard/seen about intergalactic space craft propelled by light hitting umbrella like structure of space craft, giving very little push to space craft. And idea is over a long distance it will have speed equal to some % speed of light.

cause I have seen this in discovery channel. How can we explain this?

Edited by apurvmj
Link to comment
Share on other sites

!

Moderator Note

Thread split from here.

apurvmj, when posting in threads, please be mindful of what you are posting an ensure that it actually relates to the topic of the thread. If it doesn't, please feel free to start a new one.

If you disagree with this action or feel the need to comment on it, please feel free to use the report button (the yellow triangle on the bottom left of this post) to alert staff.

Link to comment
Share on other sites

Your question is related to whether light has momentum, which it does. The standard view is that photons are massless -- that the intrinsic mass of a photon is identically zero.

 

It's also good to keep in mind that Discovery Channel has fallen to the sad state of presenting woo as true.

Link to comment
Share on other sites

As D H has said, light has momentum. It's given by p = E/c, meaning that you can exert a force of F = P/c where P is the power of the light. That's for direct emission. For reflection, as with a sail, you gain a factor of 2 from the change in direction.

 

You'll note that P/c does not have any explicit wavelength dependence. For a given power, it doesn't matter if you use lower-energy photons, you'll just have to have more of them.

Link to comment
Share on other sites

apurvmj,

The Solar sail wiki article is pretty good.

 

Edit to add:

 

 

 

I have a collection of papers on my computer related to the topic, a couple books with relevant chapters, and several titles in one of my wish lists; and yet, when considering what to recommend as reading material the wiki page seemed best.

 

If you're still curious I'd recommend relevant papers by Geoff Landis as well as K. F. Long's book Deep Space Propulsion. Plenty of other resources but I think the references and links on the wiki page more than suffice.

 

 

Edited by the asinine cretin
Link to comment
Share on other sites

Except photon particles can travel a speed from 0 to C. Photon does not have no moving state, and always move only the speed of light, C. @@What is a mass? F=ma. At a light we can not change an acceleration. So mass concept is important.

Link to comment
Share on other sites

Except photon particles can travel a speed from 0 to C. Photon does not have no moving state, and always move only the speed of light, C.

Neither of these sentences make sense. Try again, but not in this thread.

 

@@What is a mass? F=ma. At a light we can not change an acceleration. So mass concept is important.

Invoking Newtonian mechanics is not valid when relativistic effects come into play. The mass of a photon is zero. The momentum is not zero. End of story.

 

 

Note well: Discussions regarding the mass of a photon do not belong in this thread. This thread was split from the photon mass thread because the issue raised in apurvmj's post was off-topic to that thread. Similarly, discussions regarding the mass of a photon are off-topic to this thread.

Edited by D H
Link to comment
Share on other sites

Except photon particles can travel a speed from 0 to C. Photon does not have no moving state, and always move only the speed of light, C. @@What is a mass? F=ma. At a light we can not change an acceleration. So mass concept is important.

 

Except that the "real" equation isn't F = ma. It's really F = d(momentum). For the vast, vast majority of stuff, mass is constant so that simplifies out to F = m * d(velocity) = m*a.

 

But if you've got momentum (and photons do), then if you change the momentum, you have a force... Even if you don't have mass in the normal sense of the word.

 

Note too that for objects that have mass that varies (like, say... rockets) if you want to be *accurate* you can't use F=ma. You have to use F = d(momentum).

Link to comment
Share on other sites

Note too that for objects that have mass that varies (like, say... rockets) if you want to be *accurate* you can't use F=ma. You have to use F = d(momentum).

Absolutely not! There is no difference between F=ma and F=dp/dt for constant mass systems. There's a huge difference for variable mass systems, and those who work with variable mass systems almost invariably prefer F=ma over F=dp/dt. Use F=dp/dt and force is no longer invariant. It's instead a frame-dependent quantity.

Link to comment
Share on other sites

I take it that you're talking about Newtonian systems?

Of course. Nobody, well hardly anybody, uses relativity when dealing with rockets. There's no point. The uncertainties in thrust and errors in sensor measurements overwhelm the errors that result from ignoring relativistic effects.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.