Starship Asterisk*

by **ledelarosa** » Sun Feb 22, 2009 4:22 pm

Thanks Chris (most recently) and everyone else... this was fun and informative

by **Chris Peterson** » Sun Feb 22, 2009 5:40 am

ledelarosa wrote:By chance I came back to this discussion after a couple of years. Now, Qev, you managed to confuse me some more. Photons don't have any "rest" mass, but they have momentum, you say. Isn't momentum mass times velocity? What would you plug in for "mass" in the momentum equation for a photon, since its momentum is non-zero?

No, that is not a complete definition of momentum. It is the definition given by classical mechanics only, which is only valid under certain conditions. Under relativistic mechanics, the momentum of a massless particle is defined as H/lambda, or Planck's constant divided by the wavelength of the particle (photon in this case). This is also equal to E/c.

by **ledelarosa** » Sun Feb 22, 2009 4:59 am

Qev wrote:
harry wrote:Hello all

Photons have mass.

Phontons can be deflected by electromaganets, permanent magnets, deflected by water, unable to escape a black hole and so on.
Photons have no rest mass. They do carry momentum and energy, however, since they can never be at rest, and mass and energy are relativistically equivalent.

By chance I came back to this discussion after a couple of years. Now, Qev, you managed to confuse me some more. Photons don't have any "rest" mass, but they have momentum, you say. Isn't momentum mass times velocity? What would you plug in for "mass" in the momentum equation for a photon, since its momentum is non-zero?

by **Qev** » Fri May 05, 2006 3:13 pm

harry wrote:Hello all

Photons have mass.

Phontons can be deflected by electromaganets, permanent magnets, deflected by water, unable to escape a black hole and so on.

Photons have no rest mass. They do carry momentum and energy, however, since they can never be at rest, and mass and energy are relativistically equivalent.

Also note that the paths of photons are not deflected by electric or magnetic fields, since photons carry no electric charge; electric and magnetic fields do not couple with each other, but with charges. I believe a magnetic field can alter the polarization of light, however.

by **harry** » Fri May 05, 2006 11:13 am

Hello all

Photons have mass.

Phontons can be deflected by electromaganets, permanent magnets, deflected by water, unable to escape a black hole and so on.

by **Qev** » Fri May 05, 2006 7:05 am

Photons have no rest mass, meaning, if you could somehow stop one from moving (an impossibility) it would have zero mass.

Mind, under relativity, gravity doesn't deflect light because it has mass, but rather because it bends spacetime. If spacetime is curved, then the shortest paths between two points also become curved (known as geodesics), and these are the paths that light follows around massive objects.

by **ledelarosa** » Thu May 04, 2006 4:16 pm

Thanks guys for the replies... starnut's answer prompted another question. Starnut claims that photons have no mass. "Gravity can bend light" starts this APOD pic: http://antwrp.gsfc.nasa.gov/apod/ap040217.html . The only way gravity can bend light is if "light" has mass (something with no mass should not be affected by gravity!). Am I wrong?
Cheers

by **BMAONE23** » Mon May 01, 2006 2:09 pm

>>>>>>>>>>>>Witticism Warning<<<<<<<<<<<<<<<

Space may not be a perfect vaccuum but being there without a space suit would certainly suck

by **randall cameron** » Sun Apr 30, 2006 7:11 am

As I recall, in interstellar space they estimated something like a few hundred particles (atom/ions or molecules) of hydrogen and helium per cubic meter, and a few particles of dust, except in nebulae and other "cloud" structures. Intergalactic space is estimated to be even more of a void, with an average density for the known universe of around one atom per cubic meter.

Of course this changes dramatically in close proximity to stars or other bodies, thus the ability to use solar sails to drive interplanetary probes where energy economy is more important than acceleration.

Maybe not a strict vacuum, but close enough for most purposes, i.e., not a lot of drag or pressure below cosmic scales.

Randall

by **harry** » Sun Apr 30, 2006 2:13 am

Hello All

Not even on earth can we create a pure 100% vacuum because of subatomic paticals that go through all.

So relatively speaking we do get a vacuum out in space.

by **Fitzsy** » Thu Apr 27, 2006 7:11 am

Hi Starnut,

That's not completely correct. Comets have two primary tails seen visually. The ion tail is formed from gas molecules being ionised in the head of the comet and then being swept back by the magnetic field in the Solar wind, as you said. In colour photographs of bright comets ion tails are generally blue, because ionised carbon monoxide reflects light in the blue part of the spectrum.

The other tail is the dust tail, which in fact is caused by photon pressure on the microscopic dust grains. In colour images the dust tail is generally whiteish, as dust particles reflect all wavelengths of light.

Although one has to be careful interpreting any processed pseudo-colour image, it's probably that the tails in the 73P image are dust tails formed by photon pressure.

by **starnut** » Wed Apr 26, 2006 4:40 pm

When we speak of vacuum in space, we are talking about relative to Earth's atmosphere, but you are correct that there is no such thing as pure vacuum up there. Space is filled with anything from sub-atomic matters to gas and dust molecules to meteorites to planets to stars. The comet's tail is created by the solar wind, the stream of particles such as protons and electrons, from the sun. I am not sure if photons also contribute to the effect as they have no mass, but they do contribute to heating the comet and the resulting outgassing of water and dust.

by **ledelarosa** » Wed Apr 26, 2006 2:40 pm

As a kid growing up in the space-minded 80s, I remember hearing about the "vacuum of space." Today's APOD http://antwrp.gsfc.nasa.gov/apod/ap060426.html, however, seems to suggest otherwise. I only have high-school physics, but seeing how the smaller fragments are lagging behind tells me there's no such thing as a vaccuum in space. On earth, friciton against the wind (wind resistance?) causes the smaller fragments to fall behind... you see that whenever you stay up til 3am and catch some "world's awesomest fantabulous crashes" TV show.

So now for you more-physics-oriented guys out there: is the effect seen in the picture because of solar wind? is it friction against photons?

Starship Asterisk*

No vacuum? (2006 April 26)

Post a reply

Expand view Topic review: No vacuum? (2006 April 26)

Re: momentum without mass??

Re: momentum without mass??

momentum without mass??

photons without mass?

No vacuum? (2006 April 26)