A relatively simple EM wave question...or so I thought.

Alferd Packer · Post by **Alferd Packer** » 2002-10-29 11:42pm

So we're doing EM waves in my General physics class, and a question occurs to me that my book can't answer: how does an EM wave propogate through the vacuum of space?

Exonerate · Post by **Exonerate** » 2002-10-30 12:20am

My memory might be a bit spotty, but I think it's traveling through space-time itself. (Hence, the "wave" part.)

Durandal · Post by **Durandal** » 2002-10-30 01:02am

It's a trick question. Light can exist as either a particle or a wave, depending upon the environment or experiment.

Obviously, a vacuum is not a medium through which a wave can propagate, hence why there is no sound in space. This means that light must be a particle. Also, light can knock electrons around, something a wave cannot do.

However, shining light through a slit will also scatter it, so it must be a wave, as well.

Crom · Post by **Crom** » 2002-10-30 01:09am

Okay, look back in the day they thought the medium light propagated through was this stuff called the "Ether." Etheric theory has since been abandoned and now we believe that light is self-propagating.

It doesn't require a medium, hence it can propagate through a vacuum, neat huh? What happens is that there is a moving magnetic field, which induces a moving electric field, which induces a moving magnetic field, on and on like that forever.

CROM

Exonerate · Post by **Exonerate** » 2002-10-30 01:52am

Light is a particle-wave, meaning it exhibits properties of both categories.

Dooey Jo · Post by **Dooey Jo** » 2002-10-30 05:26am

FYI light (and EM waves) is made out of photons. Photons are mass-less particles that always move at the speed of light (naturally because it IS light). When the photon has little energy, like in radio waves and such, it acts like a wave and when it has much energy, like X-rays and gamma rays, it acts like a particle. That's why gamma rays can knock away electrons from atoms.

I hope that didn't sound too complicated...

SWPIGWANG · Post by **SWPIGWANG** » 2002-10-30 12:20pm

GOD

Wave-particle duality sux thats all I can say :/

Alferd Packer · Post by **Alferd Packer** » 2002-10-30 09:00pm

I asked my professor, and he said something about virtual positron-electron pairs in the quantum foam creating a temporary medium through which the EM wave can propogate. Anyone heard of this, or was he talking out of his butt?

Durandal · Post by **Durandal** » 2002-10-30 09:52pm

Well that's certainly interesting. I've never heard of light traveling through the virtual particle pairs. I guess it could be credible; the guy is a professor (in physics, I'm assuming), after all.

In case you're interested, in the vacuum of space, a particle and its anti-particle analogue spontaneously pop into existence and then annihilate each other within the timespan of 1E-43s (Planck Time). When occurring close to a black hole, one particle can get sucked in to the black hole before it has a chance to annihilate it's virtual anti-particle. So, conservation of energy forces the black hole to lose mass to accommodate the new mass in the universe. This is how black holes decay.

However, my experience in quantum physics has been that light simply exists as both a particle and a wave, depending on the environment. So, just ask your professor, "But, doesn't light's wave-particle duality explain its ability to travel through a vacuum?" If he gives you a more in-depth explanation, then he wins. If not, he was just playing with you. :)