# Tag Info

58

You do, but it's too small to really notice First, it's not correct to say that we don't feel Earth's rotation because it's rotating at a constant speed. Think about driving a car, or riding in an airplane. Whether you're cruising down the road at 90 kph, or soaring through the air at 900 kph, you don't really "feel the speed". However, When you take a ...

56

The light from the Sun spreads, at least initially, in an isotropic fashion into the universe. As it gets further from the Sun, some of that light will interact with the interstellar medium (ISM) and therefore some of the energy emitted by the Sun will be used to excite atoms and molecules or even ionise some atoms. This will be the fate of almost all the ...

48

In a similar way, we could ask... No beams can be exactly 1 meter long. No beams can be exactly straight. The material making up a beam cannot be truly isotropic. So why should we bother calculating the stress in a 1 meter straight beam having isotropic material? Because knowing how to perform this calculation is a building block for doing more ...

47

All models are approximations, we judge a model on how useful it is. Understanding the collapse of a non-rotating star to a black hole gives insight into the nature of gravitational collapse. Much of the physics of collapse does not depend on spin. The formation of an event horizon, for example. Models can be refined, and in this case, considering ...

38

There are three main space weathering processes that will affect the surface of the marble. Cosmic rays, high energy particle from the sun and beyond, will hit the surface. This can change the chemistry of the surface. Solar wind particles, hydrogen and helium, can become implanted in the surface Micrometeoroids will impact the surface, causing small ...

33

You want nature to be frugal and efficient. You want all the energy of the sun to have a purpose. However what you want nature to be like has no bearing on what it is. The light from the sun is a colossal amount of energy in human terms, but very minor in comparison to the rest of the universe. The light that didn't fall onto anything left the solar system ...

28

No more than the observation of light waves disproves quantum mechanics. Light has properties of both a particle and a wave. At low energies, the particle nature of light is hard to detect: radio waves are made of photons, but individual radio wave photons are pretty hard to detect. I'm not sure that we have directly detected individual photons with ...

24

Firstly the speeds are massively different (about 1000 mph (1610 kph) on the equator for Earth's rotation and 70,000 mph (112,654 kph) for the revolution), so the change is not large. Secondly, the green line is far straighter than it appears in your picture (because the orbit is so large) so Earth's motion around the Sun is pretty close to motion at ...

23

A neutron star must have a minimum mass of at least 1.4x solar masses (that is, 1.4x mass of our Sun) in order to become a neutron star in the first place. See Chandrasekhar limit on wikipedia for details. A neutron star is formed during a supernova, an explosion of a star that is at least 8 solar masses. The maximum mass of a neutron star is 3 solar ...

22

The impact of this measurement on the status of quantum gravitation is exactly zero. The proper statement of the incompatibility of general relativity and quantum mechanics is that the quantum field theory of general relativity is not renormalizable. Renormalizability essentially means that the theory is well-defined at all energy scales, which seems like a ...

19

Another question, how can we identify the ripple's origin (let's say that if it's the result from the big bang or another big event)? (I'm just answering this part of the question, as James has already answered the main part about GR vs QM.) LIGO have produced an image which shows their best estimate of where these two black holes were: All they can say ...

18

Of course you would need to specify who the person is - an Olympic athlete? Let us assume so and then you can scale downwards accordingly. So an Olympic high jumper can jump hard enough to raise their centre of gravity about 2m off the ground. Let us assume this is a ballistic problem. The athlete actually gives themselves sufficient upward speed to get ...

17

Another consideration is that the physics that describe a rotating black hole was much harder to develop. The maths describing the Schwarzschild (uncharged, non-spinning) black hole was developed in 1916. This was expanded to charged, non-spinning black holes in 1918 (The Reissner–Nordström metric) It wasn't until 1963 that the Kerr metric for uncharged ...

16

Just to focus on one part of your question. Whilst it might be possible for a neutron star to accrete material, or for two neutrons stars to collide, in order to form black holes, this kind of event must be quite rare (although see below) The distribution of measured neutrons star and black holes masses can be fitted with an estimated true distribution. ...

16

OK, the reason we don't get flung off the surface of the Earth is that the rotational forces are not large enough to do it. Keep in mind that Earth formed because material was pulled together by it's own gravity. If Earth rotated so quickly, that material would be thrown off and Earth, as we know it, would not have formed. That's a bit simplistic, but it's ...

15

That sounds very much like a 22° halo. It's not an astrophysical phenomenon; it results from the refraction of light by ice crystals in Earth's atmosphere. If this is what it is, it should be a fuzzy but regular circle centered on the Moon, with a radius of about 22 degrees. If you hold your fist out at arm's length with the thumb extended, the angular ...

14

The total entropy actually increases, as the molecular cloud shrinks under gravity. It may seem that as the molecules are getting closer, they are more ordered, which means less entropy. That is however only one part of the process. The second (important) part is: when the molecules are closer, they also have higher kinetic energy (since they descended into ...

14

The answer to this is certainly tidal forces, but that doesn't explain the exact mechanism for how tidal forces result in tidal locking, i.e., an orbiting body showing the same face to the central body as it orbits due to the rotation rate and revolution rate being equal. I'll describe this mechanism using the Earth-Moon system so I can be specific, but it ...

12

The Sun will never run out of Hydrogen. This is a common misconception. At this moment the Sun is fusing Hydrogen into Helium. This fused Helium remains at the core until it will reach a critical mass. At this point the core will begin to collapse. This collapse increases the temperature and pressure around the core where Hydrogen is being fused causing the ...

12

If we take neutron star material and somehow transport it somewhere for examination (say the Earth!), the results would be catastrophic. At say a density of $\sim 10^{17}$ kg/m$^{3}$ the neutrons have a number density of $\sim 6\times 10^{43}$ m$^{-3}$ an internal kinetic energy density of $3 \times 10^{32}$ J/m$^{3}$ (calculated using the relevant equations ...

12

The problem with how you're looking at it is that velocities don't cause or result from forces, but accelerations do. Think of Newton's 2nd law, $F = m a$. Circular motion is motion at constant speed but changing direction, this changing direction is a type of acceleration because velocity is a vector (has direction) and acceleration is change in velocity. ...

11

I'm guessing what you see is the moonlight being scattered by the hexagonal ice crystals in cirrus or cirrostratus clouds, which lie at very high altitudes, 5-6 km and above. The light is scattered by roughly 22º, and because of the slight wavelength dependence, the halo actually has rainbow-like colors, although often they are so faint that you just ...

11

File formats tend to be industry/field-specific, with the format, tools, and expectations of the field coevolving to become more dependent on each other over time. JPEG co-evolved with amateur digital photography, PNG co-evolved with the web. Likewise, FITS co-evolved with astronomical data processing, and so is naturally more suited for that purpose than ...

11

The boundedness or otherwise of clusters remains to be established in most cases. The vast majority of clusters become unbound and disperse at a much younger age than the Pleiades. Or they may be born unbound as you suggest. The stars in a cluster have a distribution of velocities and there will always be a tail of high speed stars that will be able to ...

11

There is evidence from Silurian coral fossils that, 430 My ago, there were 420 daily rotations in each annual season, compared to 365 days per year now. If we assume that the year (fixed by mean Earth-Sun distance and Kepler's Laws) hasn't changed much in that interval, but that instead the day has gotten longer due to the Earth-Moon tidal interaction, that ...

11

For a liquid, hydrostatic pressure is $\rho g h$ where $\rho$ is density (this is always the same for all water) g is gravitational acceleration and h is depth. The gravitational acceleration on Europa is 1.3 $\text{m/s}{}^2$ (compared with 9.8 $\text{m/s}{}^2$ on Earth). But on Europa there is 20km of ice floating on the water. As a rough estimate, the ...

11

"Harvest moon" just means the full moon at around the time of the equinox on September 21. The full moon is low in the sky during summer (June and July in the Northern Hemisphere) and high during the winter. So September is a month when the full moon is much higher in the sky than the moon in August. In the past this allowed harvests to continue after dark....

9

Your question presumes that the ISS is beyond Earth's gravity, that it has escaped earth's gravitational pull. This is not correct. All objects with mass in the universe affect all other things with mass in the universe, the effect just gets weaker with distance. So the ISS is feeling the effect of gravity from Earth significantly more than the moon is. The ...

9

No. There is no consensus. The discrepancy between the predicted big bang nucleosynthetic abundance of Lithium 7 and the measured value can be summarised as follows. If we take what we know about the the baryonic mass density of the universe and the Hubble constant, we get a self-consistent picture between the cosmic microwave background, observations of ...

9

No, not really. The first thing is that we know that ${H}$ is far more abundant than other elements or simple molecules in the universe. The next thing is that the 21 cm line comes from a relatively unusual hyperfine splitting, and there just aren't any other sources near that wavelength and in intensity levels that can easily be detected. In most ...

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