# Solar powered space craft

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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
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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.

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pls correct me if I'm wrong but my point was related to whether light has mass or not?

Btw I did post my same post again in that topic by mistake,

pls excuse me.

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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.

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Interstellar solar sail/light sail concepts typically involve immense lasers that are basically sci-fi for now. There are currently active projects studying the feasibility of interplanetary solar sails. I assume the solar sail concept is what you're referring to.

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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.

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apurvmj,

The Solar sail wiki article is pretty good.

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
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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.

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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

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alpha2cen,

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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).

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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.

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Absolutely not! There is no difference between F=ma and F=dp/dt for constant mass systems.

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

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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.

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