Late_Cretaceous

Supposedly they have some mass, therefor they cannot travel at the speed of light. Yet, neutrinos have been detected from the 1987a supernova explosion. If they travel slower then light, how is it that these particles were detected from something over 100KY away?

Friar Bellows

If they do have some mass, it's very very small. So they can travel at speeds very very close to that of the speed of light. SN1987a actually occurred over 100,000 years ago. We received the neutrinos at roughly the same time we recieved the light from the supernova (i.e. early 1987). In fact, my memory is hazy on this issue, but we may have received the neutrinos first, because they escaped earlier from the supernova than the photons did, and therefore had enough of a headstart on them to reach the Earth first.

tronvillain

Okay, that's pretty funny.

Coragyps

Neutrinos do get a headstart, because they get out of the core of a supernova without having to "fight" their way out like photons do - all that messy matter, and shock waves, etc. - and much (maybe most?) of the light is from excitation of matter in the outer edges of the erstwhile star anyway. The delay can't be too much, though, as the shock wave moves at a decent fraction of the speed of light.
On a slightly related issue - my hazy memory is that one of the three neutrino observatories that detected 1987A saw it a considerable time before the other two. Is this a) haze only b) true, and since explained or c) a coverup by the Multinational Particle Physicist - Evilutionist Conspiracy (TM)?

Edited after a short Google search: The neutrino burst from 1987A was detected in Ohio, Japan, and Russia 3 hours before the light was first observed. This is approximately as predicted a few months before the event. The detector that I remember as having the anomalous detection was the LSD = liquid scintillation detector (?) under Mont Blanc, maybe an Italian effort.

[ March 19, 2002: Message edited by: Coragyps ]</p>

Tim Thompson

Late: If they travel slower then light, how is it that these particles were detected from something over 100KY away?

Well, the speed isn't really related to the distance. Even if neutrinos only zip along at 0.1c, they can still cover 100,000 LY (in a mere 1,000,000 clock-years). But anything less than c is just fine, for any rest mass &gt;0. So they could zing along at 0.999999999999999999999999999999999999999999999999 99999999999999999999999999999999999999999999999999 999c, and get here at just about the same time as the light!

The way to constrain the neutrino mass by the detection from <a href="http://zebu.uoregon.edu/~soper/StarDeath/sn1987a.html" target="_blank">SN1987a</a> is to look at the spread in arrival times of the neutrinos (you don't look at the difference between the neutrino & light arrival because you don't really know what it as, and even if you did, the neutrinos & light aren't emitted at the same time anyway, so your answer will always depend on your ability to computer model the internal guts of a supernova explosion). If neutrinos have no rest mass, they will all arrive at the same time (the neutrinos are all created & emitted within roughly a time span of 0.1 seconds). If neutrinos have a nonzero rest mass, then they will arrive at different times, depending on their energy. <a href="http://www.astro.ucla.edu/~wright/neutrinos.html" target="_blank">According to Wright</a>, the measured spread in arrival times translates into "less than 20 ev" (the should really be "less than 20 ev/c^2, which translates into "less than 3.565x10^-32 grams", which compares to an electron rest mass of about 0.910x10^-27 grams, so we are talking less than a millionth of an electron mass - tiny). According to <a href="http://hepwww.ph.qmw.ac.uk/epp/lectures/lecture24/lecture24.html" target="_blank">this lecture</a>, the neutrino rest mass would be 3.8 ev/c^2, a lot smaller still. No matter how you cut it, the SN1987a neutrino observations appear to constrain the neutrino mass to be really small, and constrain the mass rather more strongly than any other experiments I am aware of.

On the other note, neutrinos are all created in the collpase event, which only lasts about 1/10th of a second (that's how long it takes the core to collapse from roughly Earth size to roughly Rhode Island size). The neutrinos that get out, do it at once, but a significant number are trapped by the overdense envelope and go into reheating the explosion. The light is emitted a long time later (like a few hours), when the shock wave finally breaks through the outer layers of the star. Only about 1% of the emitted energy comes out as photons, the rest is neutrinos, and most of the explosive energy is lost in the mechanism of exploding, and heating the remnants to temperatures on the order of 100,000,000,000 Kelvins.

Supernovae are loud & hot.

• <a href="http://www.seds.org/billa/twn/top.html" target="_blank">The Web Nebula</a> - Bill Arnett's neat collection of images & info on nebula. Not relevant, really, but I put it here any way because it's interesting.

Boro Nut

I depends on the price you are selling them for, and demand is very seasonal. My advice is to stick to electrons, which are available on a sale or return basis.

Boro Nut

Shadow Wraith

LOL @ Boro Nut. Good one .

Anyways, like Tim said, neutrinos must be extremely light to go so fast over such a long distance. While no mass for a neutrino has been measured as of yet, they would have to be no more than the 20ev limit he mentioned, assuming they do have mass.

jpbrooks

How could neutrinos (or any other particle, for that matter) travel faster than light? Wouldn't this involve assigning the particle a negative rest mass? This just seems counterintuitive.


I ask this because given the observations cited above, the neutrinos would had to have been emitted for a long period of time before the light in order for them to have been detected on earth before the light was detected. But why would that long delay occur?

[ March 25, 2002: Message edited by: jpbrooks ]</p>

Friar Bellows

They can't.

No. Imaginary, maybe, but not negative.

Why "long"? If neutrinos are travelling really really close to the speed of light, then they don't need a long head-start in order to reach Earth before the photons. In fact this delay is only a few hours. The reason these neutrinos are so fast is that they carry most (~99%) of the awesome energy of a supernova. That spectacular light show which makes a supernova rival the brightness of its host galaxy contains only a small fraction of the energy that the neutrinos carry.

Coragyps

jpbrooks - In the case of Supernova 1987A, at least, the neutrino burst was observed about 3 hours before the light was first detected - but the light wasn't under continuous observation. A rough 3 hour delay had been predicted, though, because the neutrinos came straight out of the star core - no delay, as they don't interact with matter much. The light we saw wasn't even emitted until after the core collapse itself, because the shock wave from the actual pop had to travel through dense starstuff, at only a fraction of the speed of light, out to the outer shell of the star before any visible light was emitted. So the neutrinos only had to travel very nearly the speed of light to beat the light here.

IIRC, a photon takes many thousands (maybe millions?) of years to get from the core to the surface of the sun - it gets scattered zillions of times, and doesn't have a star-destroying shock wave to speed things up.