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This is my second post on the subject of why “the speed of light (in empty space)”, more accurately referred to as “the cosmic speed limit”, is so fast. This speed, denoted c, is about 186,000 miles (300,000 km) per second, which does indeed seem quick.

But as I pointed out in my first post on this subject, this isn’t really the right question, because it implicitly views humans as centrally important and asks why the cosmos as strange. That’s backward. We should instead ask why we ourselves are so slow. Not only does this honor the cosmos properly, making it clear that it is humans that are the oddballs here, this way of asking the question leads us to the answer.

And the answer is this: ordinary atomic material, from which we are made, is fragile. If a living creature were to move (relative to the objects around it) at speeds anywhere close to c, it couldn’t possibly survive its first slip and fall, or its first absent-minded collision with a door frame.

Today I’ll use a principled argument, founded on basic particle physics and its implications for atomic physics, to show that any living creature made from atoms will inevitably view the cosmic speed limit as extremely fast compared to the speeds that it ordinarily experiences.

In the next post I’ll try to go further, and suggest that we must travel at even slower speeds. However, doing so becomes increasingly difficult. We potentially have to get into details of materials, of chemistry, of biology real and imagined, and of the specifics of the Earth. Once we enter this territory, the issues become complex, the logic and conclusions may be debatable, and I’m no longer expert enough to be sure I know what I’m doing. Perhaps readers can help me with that stage when we get to it.

By contrast, the logic presented here today is straightforward and impossible to evade. Along the way it will also teach us why nuclear weapons are so terrifying — why the energy hidden inside atomic nuclei is so much larger than the energies we encounter in our daily activities.

There are a lot of elements to the reasoning, so what I’ll give here is an overall sketch. But you’ll find many more details in the hyperlinks to other pages, some on this website and some beyond.

All Atomic Creatures Will View “c” As Fast

The argument is based on two simple observations about atoms, which I’ll combine in two ways to derive facts about energy and speed. All numbers given below will be very approximate, because we don’t need precision to draw the general conclusion, and trying to be more precise would make the argument longer without making it clearer. (See chapter 6.1 of my book for a discussion of physicists’ “rules of precision”.)

1. The Electromagnetic Force Makes Atoms’ Outer Electrons Slow

The electromagnetic force is a moderately weak force by particle physics standards (despite being much stronger than gravity). It has only about 1% of the strength of what we’d consider a reasonably strong force. As a direct result, the electron in a hydrogen atom moves around the atom at a speed v that is far below c — approximately 1% of c. (See also this article and this article.)

The electrons on the very outer edge of any atom (the “valence electrons”) are affected by an attractive electric force from the positively charged nucleus, but are also repelled by the electric force from all the inner electrons, which have negative charge. The net effect is that positive charge of the nucleus is substantially shielded by the negative charge of the electrons, and the resulting force on the valence electrons is not dramatically different from that found in a hydrogen atom. As a result, these valence electrons are somewhat similar to the one electron in hydrogen, and so they too move around at about 1% of c, typically a bit slower. (This “1%” is very imprecise, but it’s never as small as 0.1%.)

Thus for any atom, the outer electrons have

Here the “~” sign means “is very roughly equal to,” indicating purposeful imprecision. From this we can estimate the typical energy required to disrupt the atom — the “first ionization energy“. That energy is the combination of

• the valence electron’s motion energy 1/2 mv2 and
• the “binding energy” that holds that electron to the atom.

The motion energy and binding energy are similar in size. (This is always true for any force that decreases as 1/(distance-squared), including electric and gravitational forces.) As a result, the first ionization energy is close to the valence electron’s motion energy 1/2 mv2. Just as for a hydrogen atom, this energy is always about 1/10000th, or 0.01%, of the internal E=mc2 energy of an electron. In math terms, the fact that v/c ~ 0.01 for a typical valence electron implies that the ionization energy divided by the electron’s internal energy is roughly

Let me just emphasize again that I’m not being precise because precision isn’t needed for this argument.

Aside: Those of you who know a little quantum physics know that v isn’t really defined for an electron, because it’s a wavicle, not a particle, and that electrons don’t really go around their nuclei in orbits, despite the picture of an atom that’s so often drawn. I’m using Bohr’s cartoon of an atom here, which is enough to get the right estimates for what is going on. But we could do things correctly, and avoid ever writing “v,” by just using the ionization energy all the way through the argument. We’d get the same answer in the end.

2. The Strong Nuclear Force Gives Atoms a Big Mass

An atom has a nucleus at its center, consisting of between 1 and about 300 protons and neutrons. The strong nuclear force forms the protons and neutrons by tightly trapping quarks, anti-quarks and gluons. It then subsequently binds protons and neutrons, somewhat more loosely, into nuclei.

The most common materials, which are forged in the early universe and in ordinary stars, run mainly from hydrogen (1 electron and 1 proton) to iron (26 electrons, 26 protons, and typically 30 neutrons). Since neither hydrogen nor helium is suitable on their own for making life forms, as their chemistry is too simple, it’s reasonable to define a “typical atom” as one with roughly 10 or so protons and neutrons in its nucleus. (Carbon usually has 12, oxygen 16, etc.)

Now we need to take note of three different and unrelated aspects of particle physics.

• The strong nuclear force becomes extremely strong at a distance D that is about 1/100000th of an atom’s diameter. This sets the size of protons and neutrons, and assures they have almost the same mass Mp. That mass, about equal to the mass of a single hydrogen atom, is related to D by the formula Mp ~ h/(cD), where c is the cosmic speed limit and h is Planck’s constant, the mascot of quantum physics.
• The value <H> of the Higgs field corresponds to a mass about 250 times larger than that of a proton,
• The interaction of the Higgs field with electrons is tiny, and characterized by a number called “ye” which is about 1/400000.

Combining the last two tells us the electron’s mass, m ~ ye<H>, is very small compared to the majority of known elementary particles.

Taking all three facts together (which are independent as far as we know [none of them can yet be predicted from first principles]), it turns out that

(More precisely, the proton’s mass is nearly that of 1836 electrons, and the neutron’s 1838; but 2000 is close enough for our current purposes.)

Therefore, for a typical atom with mass M ~ 10 Mp , the ratio of the atom’s mass to the electron’s mass m, which is also the ratio of the energy stored inside that atom to the energy stored inside a single electron, is

Again, this all follows from known facts about subatomic particles. It isn’t something that anyone can explain from scratch, but for our purposes, it’s enough to know that it is true. Let’s see what the consequences are.

3. Atoms with Slow Electrons and a Large Mass are Fragile

Combining sections 1. and 2., let’s compare amount of energy that it takes to pull an outer electron off a typical atom (its first ionization energy), which is approximately equal to the motion-energy 1/2 mv2 of the electron, to the energy Mc2 stored inside that atom:

• 
  
  

Thus the amount of energy needed to disrupt an atom is a minuscule fraction of the energy that it carries inside it. Atoms are very fragile indeed!

This now explains why the energies of ordinary life must be so small compared to the energy stored in objects, and why nuclear weapons can draw on so much more energy than we are used to. If the energy per atom involved in ordinary activities, such as catching a ball or jumping up and down, were any more than a tiny fraction of the energy stored inside an atom, many of our atoms would lose one or more electrons right away, instantly ruining our biochemistry and destroying our internal structure. We can only survive because the energy of ordinary life is tiny compared to what the cosmos considers normal.

4. Fragile Atoms Can’t Survive Fast Collisions

A head-on collision between two typical atoms with mass M, each with speed V, will involve energy 1/2 MV2 for each atom. If this collision energy is comparable to or exceeds the energy needed to disrupt either atom, which is about 1/2 mv2 (where as before m is the mass of an electron and v is the speed of a valence electron in the atom) then at least one of the atoms will probably lose an electron. So to avoid this, it must be that

• 
  
  

where in the second line I used the result from section 3. Taking the square root of this line, we find

Thus atoms cannot survive intact in any collision whose relative speed is comparable to or faster than 0.00005 c — about 10 miles (15 km) per second. In any collision of ordinary objects at such a speed, the collisions of their individual atoms will lead to widespread atomic disruption, leaving the objects seriously damaged.

Aside: as noted, I have been imprecise all throughout this argument. The true maximum speed for the survival of typical atomic materials may be somewhat slower than 0.00005 c — but not too much so. Perhaps a reader can suggest a more precise estimate?

5. Life (and Anything Else) Made From Atoms Must Move Gingerly

Therefore, for living creatures to avoid injuring themselves irreparably at the atomic level every time they stub their toe or accidentally bump in to one another, they must travel slowly. Relative to objects in their environment, they dare not travel faster than 10 miles (15 km) per second at all times — much less than a 1/10000th of the cosmic speed limit.

And therefore, when they first discover the existence of c, they will all, without exception, express surprise and amazement at how fast it is — more than 10000 times faster than the motions of their ordinary existence.

Humans and Tardigrades

Now, of course, you and I are restricted to much slower speeds than 10 miles (15 km) per second! Even a collision at 10 feet (3 meters) per second, about 7 miles (11 km) per hour, will hurt a lot, and speeds ten times that would surely be fatal. For us, c isn’t just 10,000 faster than what we’re used to — it’s about 100,000,000 times faster than jogging speed!

So clearly this argument gives an overestimate of how fast we can go. There must be additional issues that force us to move even more slowly than the speeds that would disrupt atoms. This is true, but those constraints are much more complicated. That’s why I decided to begin with this relatively simple and very general argument.

What’s good about this argument is that it applies to all atomic objects. It restricts not only natural life on Earth but all imaginable atomic life anywhere — even artificial life that we or some other species might potentially create.

Many organisms are far stronger than we are, tardigrades most famously among them. (Note that tardigrades are small, but not microscopic.) We’ve been making robotic machines for quite some time that can survive and thrive in environments that would kill us instantly. Current technology can already create simple artificial life forms.

Nevertheless, no matter how good our technology, and no matter how intricate the unconscious process of evolution, we will never encounter or construct complex objects that can remain intact in environments where relative speeds are as high as 1/10000th of c, unless

• they remain forever isolated in deep space, where collisions with any large objects are extremely unlikely, and collisions with atoms are infrequent enough that the resulting surface damage is manageable [which is why our spacecraft, not to mention asteroids and planets, are able to travel safely much faster than Earthly objects do], or
• we someday find a building block stronger than atoms from which a robot or artificial life form can be built [but given what we know about the universe so far, this may forever remain science fiction.]

Here’s an interesting and relevant piece of information: it has been shown that frozen tardigrades can survive collisions with sand at tremendously higher speeds than we humans can handle, but only up to about 0.6 miles (0.9 km) per second — about 1/300000th of c. (This makes it challenging for them to travel successfully between planets and moons across the solar system, where typical relative speeds of large objects and meteors are in the 10 km/second range.) On the one hand, this shows that at least some life forms can survive much more rapid collisions than we can. On the other, they have limitations that are consistent with today’s reasoning.

Could one could create a intelligent creature of a larger size that could match the durability of a tardigrade? That is perhaps doubtful, but we can discuss that after the next post.

Slow is Better

Thus by combining basic knowledge concerning our universe —

• the moderate weakness of the electromagnetic force,
• the great strength and trapping ability of the strong nuclear force,
• the substantial value of the Higgs field,
• the tiny interaction of the Higgs field with electrons,
• the nature of typical atomic nuclei, and
• the behavior of valence electrons in typical atoms —

we learn that any object made from atoms cannot endure collisions at speeds anywhere close to c. And now we know the reason why the cosmic speed limit seems so fast — and why nuclear weapons seem so violent.

The reason is simple: in this universe, only the slow survive.

It is now generally accepted that 66 million years ago a large asteroid smacked into the Earth, causing the large Chicxulub crater off the coast of Mexico. This was a catastrophic event, affecting the entire globe. Fire rained down causing forest fires across much of the globe, while ash and debris blocked out the sun. A tsunami washed over North America – one site in North Dakota contains fossils from the day the asteroid hit, including fish with embedded asteroid debris. About 75% of species went extinct as a result, including all non-avian dinosaurs.

For a time there has been an alternate theory that intense vulcanism at the Deccan Traps near modern-day India is what did-in the dinosaurs, or at least set them up for the final coup de grace of the asteroid. I think the evidence strongly favors the asteroid hypothesis, and this is the way scientific opinion has been moving. Although the debate is by no means over, a majority of scientists now accept the asteroid hypothesis.

But there is also a wrinkle to the impact theory – perhaps there was more than one asteroid impact. I wrote in 2010 about this question, mentioning several other candidate craters that seem to date to around the same time. Now we have a new candidate for a second KT impact – the Nadir crater off the coast of West Africa.

Geologists first published about the Nadir crater in 2022, discussing it as a candidate crater. They wrote at the time:

“Our stratigraphic framework suggests that the crater formed at or near the Cretaceous-Paleogene boundary (~66 million years ago), approximately the same age as the Chicxulub impact crater. We hypothesize that this formed as part of a closely timed impact cluster or by breakup of a common parent asteroid.”

Now they have published a follow up study, having been given access to private seismic data that allows for a detailed 3D analysis of the crater site. This is important because of how scientists identify impact craters. The gold standard is to identify physical evidence of impact, such as shock crystals. There are telltale minerals that can only be formed by the intense sudden power of an impact, or that form when debris is thrown into the high atmosphere while molten and then rains back down. These are conclusive signs of an impact. But there are many somewhat circular structures in the world, and often they may be prematurely declared a crater without solid evidence. So geologists are cautious and skeptical.

For the Nadir crater, which is on the sea floor, we do not have physical evidence. The initial study showed that it has a candidate circular structure, but this was not enough evidence to convince the scientific community. The detailed new analysis, however, is more compelling. First the scientists find that it does have a complete circular structure consistent with an impact basin. Even more significant, however, is that they have documented a “central uplift”, which is a characteristic sign of an impact crater. When asteroids hit they cause a depression and liquify the underlying rock. This causes a shockwave which rebounds, causing the molten rock to uplift in the center of the crater, leaving behind an uplift. This means that a circular basin with an uplift in the exact middle is a signature of an impact. This is solid and convincing evidence, even without the physical evidence of impact crystals. They write:

“Our new study published in Communications Earth & Environment 3 presents this new, state-of-the-art 3D data, revealing the architecture of the crater in exceptional detail and confirms (beyond reasonable doubt!) an impact origin for the crater. This is the first time that an impact structure has ever been imaged fully with high-resolution seismic data like this and it is a real treasure trove of information to help us to reconstruct how this crater formed and evolved.”

Sounds pretty convincing. But this leaves the questions they raised in their study two years ago – did this asteroid hit at the exact same time as the Chicxulub asteroid? If not, how far apart were they? If they did hit at the same time, were they originally part of the same asteroid? Given that there are other candidate craters that date to the same period of time, perhaps the asteroid broke up into multiple pieces that all struck the Earth at the same time.

If these asteroids were not originally part of the same asteroid, then what are the other possibilities? It is possible, although statistically unlikely, that there were simply different independent major impacts within a short time of each other. There is nothing to keep this from happening, and given the history of life on Earth perhaps it’s not that surprising, but it would be a statistical fluke.

The other possibility is that, even if they were different asteroids, perhaps there was some astronomical event that caused multiple chunks of rock and ice to swarm into the inner solar system. This would have caused a temporary period of relatively high bombardment. Perhaps a rogue planet swung by our solar system, scattering material from the Kuiper belt, some of which found its way to Earth.

The authors propose to drill into this structure, to get that physical evidence that would be so helpful. Not only would this confirm its impact status, but we may be able to tell if the chunk of rock that caused the Nadir crater has the same mineral signature as the Chicxulub asteroid. I suspect we could also tell their relative timing. Perhaps we could see the iridium layer from the Chicxulub impact, and see how that relates to the Nadir impact.

We may be able to answer – were the dinosaurs just really unlucky, or was the Chicxulub impact event more devastating than we even realized? Either way I look forward to more scientific investigation of the Nadir crater.

The post Nadir Crater – A Double Tap for Dinosaurs? first appeared on NeuroLogica Blog.

Since the 1970s, astronomers have observed that supermassive black holes (SMBHs) reside at the centers of most massive galaxies. In some cases, these black holes accelerate gas and dust from their poles, forming relativistic jets that can extend for thousands of light-years. Using the NASA/ESA Hubble Space Telescope, a team of astronomers observed the jet emanating from the center of M87, the supermassive galaxy located 53.5 million light-years away. To their surprise, the team observed nova erupting along the jet’s trajectory, twice as many as they observed in M87 itself.

The team was led by Alec M. Lessing, a Stanford University astronomer, and included researchers from the American Museum of Natural History, the University of Maryland Baltimore, Columbia University, Yale University, the SETI Institute, and NASA’s Goddard Space Flight Center. The paper detailing their findings recently appeared in The Astrophysical Journal. Their research was part of a 9-month survey of the M87 galaxy using Hubble’s near-UV Cosmic Origins Spectrograph (COS).

To date, all novae have been observed in double-star systems consisting of a red giant star and a white dwarf companion. In these systems, the outer layers of the red giant are siphoned away by the white dwarf and accreted onto its surface. When the white dwarf has accumulated enough hydrogen, the layer explodes in a “nova eruption,” and the cycle begins again. When the team observed M87 using Hubble’s COS, they found twice as many novae eruptions near the 3000-light-year-long jet than in the galaxy itself during the surveyed period.

A Hubble image of M87 shows a 3,000-light-year-long jet of plasma blasting from the galaxy’s 6.5-billion-solar-mass central black hole. Credit: NASA/ESA/STScI/A. Lessing et al. (2004).

These findings imply that there are twice as many nova-forming double-star systems near the jet or that these systems erupt twice as often as similar systems elsewhere in the galaxy. Another possibility is that the jet is heating the red giant stars in these binary systems, causing them to overflow further and dump more hydrogen onto the dwarf companion. However, the researchers determined that this heating is not significant enough to have this effect. As Lessing explained in an ESA press release:

“We don’t know what’s going on, but it’s just a very exciting finding. This means there’s something missing from our understanding of how black hole jets interact with their surroundings… There’s something that the jet is doing to the star systems that wander into the surrounding neighborhood.

“Maybe the jet somehow snowplows hydrogen fuel onto the white dwarfs, causing them to erupt more frequently. But it’s not clear that it’s a physical pushing. It could be the effect of the pressure of the light emanating from the jet. When you deliver hydrogen faster, you get eruptions faster. Something might be doubling the mass transfer rate onto the white dwarfs near the jet.”

This is not the first time astronomers have noticed increased levels of activity around the M87 jet. Shortly after Hubble launched in 1990, astronomers observed the galaxy’s SMBH using its first-generation Faint Object Camera (FOC). These observations revealed “transient events” that could be evidence of novae, but the FOC’s view was too narrow to compare what was happening between the jet and in the near-jet region. Thanks to the nine-month campaign that relied on Hubble’s upgraded cameras (which have a wider view) and viewed the jet’s environment every five days, the team was able to count the novae along the jet’s trajectory.

Sag A* compared to M87* and the orbit of Mercury. Credit: EHT collaboration

The observations, which were the deepest images of M87 ever taken, revealed 94 novae within the M87 galaxy’s inner region. Said co-author Michael Shara, the Curator of Astrophysics at the American Museum of Natural History:

“The jet was not the only thing that we were looking at — we were looking at the entire inner galaxy. Once you plotted all known novae on top of M87 you didn’t need statistics to convince yourself that there is an excess of novae along the jet. This is not rocket science. We made the discovery simply by looking at the images. And while we were really surprised, our statistical analyses of the data confirmed what we clearly saw.”

These observations confirm that the venerable Hubble still has the capability to reveal new and interesting things about the Universe. In addition, these findings provide an opportunity for follow-up studies to learn more about how relativistic jets could influence star systems extending well beyond their galaxies.

Further Reading: ESA Hubble, The Astrophysical Journal

The post Jets From Supermassive Black Holes Create New Stars Along Their Trajectory appeared first on Universe Today.

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The two Voyager spacecraft have been speeding through space since 1977, powered by decaying chunks of plutonium that produce less and less energy every year. With less electricity available, NASA has decided to shut down one experiment on Voyager 2, the plasma science instrument. This device measures the quantity and direction of ionized particles passing the spacecraft. While Voyager 2 still has enough electricity to support its four other operational instruments, it will likely be down to just one by the 2030s.

NASA said that over the past several years, engineers for the mission have taken steps to avoid turning off any science instruments for as long as possible since the science data collected by the two Voyager probes is unique. As the first spacecraft to reach interstellar space — the region outside the heliosphere – this is currently our only chance to study this region. However, this particular instrument has been collecting limited data in recent years due to its orientation relative to the direction that plasma is flowing in interstellar space.

The 47-year old Voyager 2 is traveling at about 15 km/second (35,000 miles per hour) and is currently more than 20.5 billion km (12.8 billion miles) from Earth. The four remaining science instruments are studying the region outside our heliosphere and include a magnetometer to study the interplanetary magnetic field, a charged particle instrument that measures the distributions of ions and electrons, a cosmic ray system that determines the origin of interstellar cosmic rays, and a plasma wave detector.

The Grand Tour The Grand Tour ‘poster.’ Image: NASA/JPL

The two Voyagers both launched in 1977 (August and September), and their different trajectories were designed to take advantage of a rare geometric arrangement of the outer planets in the late 1970s and the 1980s which allowed for a four-planet tour for a minimum of propellant and trip time. The positions of those planets — which only occurs about every 175 years — took Voyager 2 (which launched first) past the gas giants Jupiter and Saturn, and then its flight path allowed for encounters with the ice giants Uranus and Neptune. It remains the only spacecraft to have visited either of the ice giant planets.

Voyager 1 made flybys of Jupiter, Saturn, and Saturn’s largest moon, Titan. Both spacecraft made incredible discoveries at the distant planets, and the astounding imagery sent back to Earth opened a whole new way of looking at the outer Solar System.

Europa seen during Voyager 2 Closest Approach. Credit: NASA/JPL

Now, they’re in the Voyager Interstellar Mission phase, where their data helped characterize and study the regions and boundaries of the outer heliosphere, and now explores the interstellar medium. Voyager 1 crossed the heliopause and entered interstellar space on August 25, 2012. Voyager 2 entered interstellar space on November 5, 2018, at a distance of 119.7 AU. Both communicate with Earth via the Deep Space Network. It takes nearly a day for one-way communications to reach each spacecraft and another day for data to be sent back to Earth.

Dwindling Power Pellet of Pu-238. RTGs are constructed using marshmallow-sized pellets of Pu-238. As it decays, interactions between the alpha particles and the shielding material produce heat that can be converted into electricity.

Each Voyager 2 is powered by three multihundred-watt radioisotope thermoelectric generators (RTG). At launch, each RTG provided enough heat to generate approximately 157 watts of electrical power, and so collectively, the RTGs supplied the spacecraft with 470 watts at launch, and their power halves every 87.7 years. They were predicted to allow operations to continue until at least 2020, but are still providing enough energy for some data collection and communications. NASA estimates they lose about 4 watts of power each year.

After the twin Voyagers completed their exploration of the giant planets in the 1980s, the mission team turned off several science instruments that would not be used to study of interstellar space. That gave the spacecraft plenty of extra power until a few years ago. Since then, the team has turned off all onboard systems not essential for keeping the probes working, including some heaters. In order to postpone having to shut off another science instrument, they also adjusted how Voyager 2’s voltage is monitored.

The device that was recently turned off, the plasma science instrument, measured the amount of plasma (electrically charged atoms) and the direction it is flowing. In 2018, the plasma science instrument helped determine that Voyager 2 left the heliosphere. Inside the heliosphere, particles from the Sun flow outward, away from our parent star. Since the heliosphere is moving through interstellar space, the plasma flows in almost the opposite direction of the solar particles.

NASA’s Voyager 2 Probe Enters Interstellar Space This illustration shows the position of NASA’s Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Voyager 1 exited the heliosphere in August 2012. Voyager 2 exited at a different location in November 2018. Credit: NASA/JPL-Caltech

When Voyager 2 exited the heliosphere, the flow of plasma into the instrument dropped off dramatically. Most recently, the instrument has been used only once every three months, when the spacecraft does a 360-degree turn on the axis pointed toward the Sun. This limited usage factored into the mission’s decision to turn this instrument off before others.

NASA said the same plasma science instrument on Voyager 1 stopped working in 1980 and was turned off in 2007 to save power.

The post NASA Turns Off One of Voyager 2's Science Instruments appeared first on Universe Today.

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