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Bill. MC
12th August 2004, 13:45
I am not sure how many there are. Photons, gravitons and manybe neutrinos. The latter of which has much controversy.

IMO The idea of massless particles seems anomalous, if mass and energy are interchangable.

So how exactly were they concluded to be massless?

Chris Mitchell
12th August 2004, 18:41
It's only the rest mass of photons that is zero - and they're never at rest. Anything that has mass when it's standing still can't go at the speed of light - the nearer you get to light speed, the heavier you get, and theortically you'd have infinite mass at light speed.

So photons have energy which is *equivalent* to a given mass, and vice-versa.

A small lump of matter (which has mass) can be converted to a lot of energy - that's how energy is liberated in an atom bomb or a hydrogen bomb; neither of these are perfect converters of mass, however.

If you could convert a teaspoonful of water into energy, the amount of energy released would be E=mc2

m=0.005 (5 grams)
c=300,000,000 m/s

so E would be 450,000,000,000,000 Joules

A 60W lightbulb burns energy at the rate of 60 Joules per second, so the teaspoon of water would keep your bulb glowing for 237,000 years.

Of course, you might need to change the lightbulb before then :D

Bill. MC
12th August 2004, 19:03
518320 In Post 518320, Chris Mitchell said:
It's only the rest mass of photons that is zero - and they're never at rest. Anything that has mass when it's standing still can't go at the speed of light - the nearer you get to light speed, the heavier you get, and theortically you'd have infinite mass at light speed.

So photons have energy which is *equivalent* to a given mass, and vice-versa.

Well done Chris. :applaud: It is so obvious, yet it escaped my attention, but what of these other massless particles, do they also travel at light speed?

Chris Mitchell
12th August 2004, 19:24
I believe the debate is still going on about whether neutrinos have mass or not - they *may* have a very tiny mass, in which case they can't actually go at the speed of light, just very close to it. As for gravitons, I don't know - perhaps our resident science teachers can shed some light on this?

Bill. MC
12th August 2004, 20:17
518323 In Post 518323, Chris Mitchell said:
I believe the debate is still going on about whether neutrinos have mass or not - they *may* have a very tiny mass, in which case they can't actually go at the speed of light, just very close to it. As for gravitons, I don't know - perhaps our resident science teachers can shed some light on this?


Oh you mean The Teacher, Fork Me and Jason Stainer? Well I find that sometimes Rabbit or Memnock surpass them WRT scientific knowledge.

The Teacher
14th August 2004, 00:18
That depends on the field. Admittedly Rabbit knows more than me on physics because my subject is chemistry and I only need to know physics for KS3. However, chemistry I teach at KS4 and KS5.

WRT to the original post, I will find the latest research article and post it up.

The Teacher