An odd light speed question

[Traab]

Ok, I just read an article about a cosmic blast (http://news.yahoo.com/monster-cosmic-blast-zipped-harmlessly-earth-191349814.html) that we recently saw on telescopes. And in it it talks about the gamma ray explosion moving at the speed of light. My question is, if we got hit by an explosion moving at the speed of light, would both the explosion and the light itself reach us at the exact same instant? Or would the blast wave be moving ahead of the light?

[Heliomance]

Well, gamma rays are light, just of a very short wavelength, so naturally they travel at the speed of light. And no, the blast wave couldn't possibly move ahead of the light - light speed is (as far as we know, and in conventional space) an absolute upper bound on the speed of anything in the universe.

[Spiryt]

Ok, I just read an article about a cosmic blast (http://news.yahoo.com/monster-cosmic-blast-zipped-harmlessly-earth-191349814.html) that we recently saw on telescopes. And in it it talks about the gamma ray explosion moving at the speed of light. My question is, if we got hit by an explosion moving at the speed of light, would both the explosion and the light itself reach us at the exact same instant? Or would the blast wave be moving ahead of the light?

If 'explosion' was moving at the speed of light, then it reach the target about at the same time with light, obviously...

Explosions aren't really traveling even near the speed of light though, no known matter.

Particles being radiated can't either, so only part of that blast that could travel at speed of light are gamma rays, and other electromagnetic radiation.

[Traab]

I suppose a better phrasing would have been, which comes first, the explosion or the light from it. But I was not aware that gamma rays counted as light. So that makes the entire question pointless as its both.

*EDIT* A followup question. Say this blast was pointed right at us. How far away would it be before we could see it with the naked eye? How close to death would we be by that point? Would we even see it before it hits?

[Spiryt]

I suppose a better phrasing would have been, which comes first, the explosion or the light from it. But I was not aware that gamma rays counted as light. So that makes the entire question pointless as its both.

If explosion is 'conventional' - heat, expanding gases etc. then the light/view of it will obviously comes first, though it will be indeed mostly observable in the cosmic scale. :smallbiggrin:

[Tylorious]

The light will always come first, as any matter can't move that quickly.

[rs2excelsior]

Since the gamma rays move at the speed of light, they would hit us at the exact moment we "saw" the "explosion."

[Yora]

Gamma Ray Blasts also come with particles, but those only travel at "almost the speed of light". Depending on how far away it happened, that can be still quite some delay.

The Crab Nebula is only about 5,000 years old and about 4,000 lightyears away, and the supernova that created it was very much visible from Earth. Some arabian astronomers made detailed records of their observation, and at the beginning the light was bright enough so you could read during the night, which I assume means about as bright as a full moon. The light was visible even during the day for months before it dimmed down to become too faint to see with the naked eye.

If a blast of gamma rays would hit the Earth head on, we couldn't see it at all, since they are outside the visible spectrum that human eyes can see, like infrared or ultraviolet radiation, or x-rays. Most cosmic events emit radiation over a wide spectrum, and it would depend entirely on the specific composition. Lots of visible light and very few gamma rays and xrays can be seen with the naked eye with no chance of damage, lots of gamma rays with very few visible light could kill while being invisible.

In terms of danger to life on Earth, the risk is relatively low, since we just happen to have a number of natural shields. Because of the massive iron core that Earth has, we have a very strong magnetic field, which happens to be very efficient at deflecting charged particles, like alpha and beta radiation. Gamma radiation is not particles, but we also have our atmosphere, which scatters the rays and "slows them down", if I am not completely mistaken.
Theoretically, there could be cosmic events that cause a blast which could do damage on Earth, but there aren't really any candidates of sufficient size at a close enough distance to threaten us.

[shawnhcorey]

The light will always come first, as any matter can't move that quickly.

Actually, the neutrinos would come first, that's if they had no mass (and nobody knows for sure). The light and neutrinos was be released at the same time but the neutrinos can pass thru mass as tho it wasn't there. The light would be blocked by the mass in the outer layers and would not be detected until those layers would be blow apart.

[Yora]

Light would be a bit later, because it has to travel a longer distance. Still at the same speed, though. :smallbiggrin:

Physics!

[the_druid_droid]

Actually, the neutrinos would come first, that's if they had no mass (and nobody knows for sure). The light and neutrinos was be released at the same time but the neutrinos can pass thru mass as tho it wasn't there. The light would be blocked by the mass in the outer layers and would not be detected until those layers would be blow apart.

Actually, neutrino oscillation (http://en.wikipedia.org/wiki/Neutrino_oscillation) says that they do have mass, quite definitely (unless you're willing to go out on some pretty unsupported theoretical limbs). We don't know why they have mass yet, but the experimental observation is that they do.

[The Grue]

Light would be a bit later, because it has to travel a longer distance. Still at the same speed, though. :smallbiggrin:

Physics!

Wait, what?

[shawnhcorey]

Light would be a bit later, because it has to travel a longer distance. Still at the same speed, though. :smallbiggrin:

Physics!


Wait, what?

The implosion starts deep within the star, near its core. Some of the energy penetrates the outer layers quickly while visible light remains trapped under them, that is, until they are blown apart. That is why gamma rays can be detected before the star is seen to implode.

[The Grue]

The implosion starts deep within the star, near its core. Some of the energy penetrates the outer layers quickly while visible light remains trapped under them, that is, until they are blown apart. That is why gamma rays can be detected before the star is seen to implode.

Oh I understand that, what made me double-take was the idea that the photons have a longer straight-line distance to travel than the neutrinos.

[shawnhcorey]

Oh I understand that, what made me double-take was the idea that the photons have a longer straight-line distance to travel than the neutrinos.

They don't. In fact, the first photons from the implosion would have a shorter distance since the star was being torn apart and they would not have to climb out of a gravity well that was as deep as the one the neutrinos did.

[hamishspence]

The implosion starts deep within the star, near its core. Some of the energy penetrates the outer layers quickly while visible light remains trapped under them, that is, until they are blown apart. That is why gamma rays can be detected before the star is seen to implode.

Neutrinos can also be detected before the star is seen to explode. In John Gribbin's Stardust (a detailed explanation of how the heavier atoms are formed, and end up in planets) it mentions that, while a proportion of neutrinos are absorbed by the dense shockwave (giving it some extra boost) - the rest arrive over 2 hours before the star is seen to explode.

http://en.wikipedia.org/wiki/Supernova_1987A

The gamma rays produced in the collapse of the core of a Type II Supernova, are converted to electrons and positrons - and those are involved in interactions that produce neutrinos. More neutrinos, in fact, are produced in those interactions, than are produced when the protons in the star's core are converted to neutrons.

[Rockphed]

Actually, the neutrinos would come first, that's if they had no mass (and nobody knows for sure). The light and neutrinos was be released at the same time but the neutrinos can pass thru mass as tho it wasn't there. The light would be blocked by the mass in the outer layers and would not be detected until those layers would be blow apart.

Neutrinos have mass and travel at less than light speed. I read about this almost ten years ago. Looking at Wikipedia, proof was found in 1998.

[Yora]

Oh I understand that, what made me double-take was the idea that the photons have a longer straight-line distance to travel than the neutrinos.
Photons would in fact not be traveling in a straight line. Inside a star, atoms fuse and break up, releasing photons, but the matter is so tightly packed that the photon just crashes into something and gets absorbed, but the same amount of energy is almost immediately released again as a new photon, that travels in a random direction. Since there's really a lot of matter in a star, such an energy package is buzzing randomly around inside the star and it can take a very long time until this random movement brings it to the surface of the star from where it can shot out into space without bouncing back inside the star.
Though I am not exactly sure if physicists would say it's the "same photon".

[shawnhcorey]

Neutrinos have mass and travel at less than light speed. I read about this almost ten years ago. Looking at Wikipedia, proof was found in 1998.

Then it depends on how far away the supernova is which will determine which is detected first. If it is near, you would detect the neutrinos first because they travel at almost the speed of light and the detectable light would not have overtaken them yet. If the supernova is far away, you would detect the light first.

[Rockphed]

I think that our neutrino detectors are not quite good enough to detect neutrinos from supernova that are far enough away for the light to have passed the neutrinos. But that is mostly because I know that we can detect light at single photon intesities, and I know that neutrinos are detected at 1 in a million, at most, levels.

[hamishspence]

Then it depends on how far away the supernova is which will determine which is detected first. If it is near, you would detect the neutrinos first because they travel at almost the speed of light and the detectable light would not have overtaken them yet. If the supernova is far away, you would detect the light first.

Supernova 1987A had the neutrinos detected first - by about 2 hours- and was in the Large Magellanic Cloud, some 168,000 light years away.

[shawnhcorey]

I think that our neutrino detectors are not quite good enough to detect neutrinos from supernova that are far enough away for the light to have passed the neutrinos. But that is mostly because I know that we can detect light at single photon intesities, and I know that neutrinos are detected at 1 in a million, at most, levels.

Neutrinos detectors do not detect neutrinos. They detect their decay, that is, when they break apart into more easily detectable quanta. And, of course, the neutrino is no more. The biggest problem for neutrinos detectors is separating the quanta of neutrino decay from all the other quanta that may appear in the detector.

[LCP]

Neutrinos detectors do not detect neutrinos. They detect their decay, that is, when they break apart into more easily detectable quanta. And, of course, the neutrino is no more. The biggest problem for neutrinos detectors is separating the quanta of neutrino decay from all the other quanta that may appear in the detector.

Speaking as a physicist working on a neutrino detector... neutrinos don't decay. What detectors "see" is the interaction of neutrinos with other matter. You're right that the original neutrino doesn't survive this interaction, but it's most definitely not a decay.

[Kato]

Speaking as a physicist working on a neutrino detector... neutrinos don't decay. What detectors "see" is the interaction of neutrinos with other matter. You're right that the original neutrino doesn't survive this interaction, but it's most definitely not a decay.

As another physicist I would like to point out pretty much anything we detect is the interaction of anything with anything else. If the stuff we look for doesn't interact with anything, we can't very well find it :smalltongue:


That said... Considering neutrinos do/can travel near the speed of light I wonder whether it is still able to arrive three hours before the light unless the massive neutrino emission occurred considerably earlier than the light emission.

[shawnhcorey]

That said... Considering neutrinos do/can travel near the speed of light I wonder whether it is still able to arrive three hours before the light unless the massive neutrino emission occurred considerably earlier than the light emission.

Yes. The implosion starts deep within the star but it takes many hours for it to break the surface. The neutrinos are released at the beginning but the light is not until the surface broken up.

[LCP]

As another physicist I would like to point out pretty much anything we detect is the interaction of anything with anything else. If the stuff we look for doesn't interact with anything, we can't very well find it.

The point was that it's a direct neutrino interaction with a target, not a decay (and subsequent detection of the decay products interacting). Neutrinos are too light to decay on their own.


That said... Considering neutrinos do/can travel near the speed of light I wonder whether it is still able to arrive three hours before the light unless the massive neutrino emission occurred considerably earlier than the light emission.

Shawnhcorey has it pretty much right. The imploding star is very dense. Neutrino emission starts at the same time of photon emission, in the centre of the implosion. Neutrinos only interact via the weak force, and so stream freely out of the star with little scattering. Photons interact via the EM force, and so scatter much more strongly inside the dense medium. The time delay between the arrival of the neutrino front and the photon front comes from the difference in how long it takes them to escape the star. At least, that's the case to the best of my memory.

EDIT: Checking back, Yora said exactly the same thing :smallredface:

[hamishspence]

There's also two sources of neutrinos- those produced in the initial transformation from protons to neutrons- and those produced as the collapsed core shrinks further- from 100km to about 10km diameter in about 10 seconds.

[Rockphed]

I think that this week's xkcd what-if (http://what-if.xkcd.com/) is relevant to this discussion. Also, a lethal dose beam of neutrinos sounds like a perfect death-ray.

[nedz]

EDIT: Checking back, Yora said exactly the same thing :smallredface:

I think that's called being neutrino'd :smallbiggrin:

Incidentally, one thing I've always wondered, where do all the neutrinos go ?
Now that might be a silly question, but they have been being produced at an enormous rate since the BB and they don't tend to react with anything much.

[Ravens_cry]

I think that this week's xkcd what-if (http://what-if.xkcd.com/) is relevant to this discussion. Also, a lethal dose beam of neutrinos sounds like a perfect death-ray.
Anything that could be killed by that would likely be killed by what produced said beam.

I think that's called being neutrino'd :smallbiggrin:

Incidentally, one thing I've always wondered, where do all the neutrinos go ?
Now that might be a silly question, but they have been being produced at an enormous rate since the BB and they don't tend to react with anything much.
They just . . . go, spreading ever outward, almost entirely unimpeded by matter, to which they pass through as cosmic ghosts.

[Kato]

Incidentally, one thing I've always wondered, where do all the neutrinos go ?
Now that might be a silly question, but they have been being produced at an enormous rate since the BB and they don't tend to react with anything much.


They just . . . go, spreading ever outward, almost entirely unimpeded by matter, to which they pass through as cosmic ghosts.

Yeah, I guess so... though, they do have some mass so a bunch of them is likely trapped in the odd black hole, I think. But the vast majority of them, as the vast majority of light that never hit any object just flies away into the nothingness of the universe.

[hamishspence]

Light photons also get trapped in the odd black hole, even though they don't exactly have mass,

[Yuki Akuma]

You don't need mass to get trapped in a black hole. Beyond the event horizon, it is literally impossible to escape - there isn't a direction to travel in that would take you outside the black hole. Every direction leads to the black hole.

Gravity warps space, remember.

(Well it's literally impossible to escape using classical physics. I'm sure you could teleport from inside a black hole to outside of one.)

[shawnhcorey]

Well it's literally impossible to escape using classical physics. I'm sure you could teleport from inside a black hole to outside of one.

That's called quantum tunnelling (http://en.wikipedia.org/wiki/Quantum_tunnelling).

[Yuki Akuma]

That's called quantum tunnelling (http://en.wikipedia.org/wiki/Quantum_tunnelling).

Yes it is! Well done.

[nedz]

Also: Hawking radiation (http://en.wikipedia.org/wiki/Hawking_radiation)

[Axiomatic]

Also: Hawking radiation (http://en.wikipedia.org/wiki/Hawking_radiation)

That just means that, once you've been sliced in half by the event horizon, the bit that was above the event horizon is going to radiate away.

[Heliomance]

That just means that, once you've been sliced in half by the event horizon, the bit that was above the event horizon is going to radiate away.

I still don't understand how that's an actual measurable phenomenon, what with the facts that 1) the particle-antiparticle pairs involved are so tiny that the margin of error on their positioning to split is REALLY DAMN SMALL, and 2) the escape velocity a tiny fraction of a tiny fraction of a micron away from the event horizon is REALLY DAMN HIGH. With the combination of those, I would have thought that the amount of particles actually escaping would be so small as to be indistinguishable from the background noise.

[nedz]

I still don't understand how that's an actual measurable phenomenon, what with the facts that 1) the particle-antiparticle pairs involved are so tiny that the margin of error on their positioning to split is REALLY DAMN SMALL, and 2) the escape velocity a tiny fraction of a tiny fraction of a micron away from the event horizon is REALLY DAMN HIGH. With the combination of those, I would have thought that the amount of particles actually escaping would be so small as to be indistinguishable from the background noise.

I'm not sure that it has actually been measured, but then we haven't observed a black hole either closely or directly.

Known Black holes occur in the center of galaxies where star density is fairly high, certainly much higher than hereabouts. So one would expect there to be a significant amount of infalling matter of various kinds, though mainly whole stars.

[shawnhcorey]

Apparently, it hasn't been directly observed but some experince seem to indicate it's possible. See Experimental observation of Hawking radiation (http://en.wikipedia.org/wiki/Hawking_radiation#Experimental_observation_of_Hawk ing_radiation).