Question and Answers

Do you have a question about Particle Astrophysics, Astronomy, or Cosmology? Have a look through the previous questions which we've been asked and if you can't find find your answer, ask us!

Ask an Astrophysicist

Question

If the correlation between pressure-temperature means that water boils at a lower temperature as pressure decreases, then, turns to gas as it is superheated, what then happens if it is then subject to vacuum at Kelvin -273, is it then condensed into water then ice? So the bigger question is: how can earth's atmosphere survive in the vacuum that is space?

Answer

Earth's gravity is what holds the atmosphere to it.

In space, any small container of gas will empty immediately, and a cup of water would also boil away quite rapidly.  Unless already supercooled into ice, the molecules in a liquid will have enough energy to escape their weak bonds to
each other, and just fly off into space. Which is why you don't want astronauts' spacesuits to depressurize -- because Bad Things then rapidly result..!

Question

If outside observer observers anything falling in a black hole it will looks like as the time has stretched to almost infinity for the falling matter, the closer it gets to the back hole, and it will look like the matter never actually falls into the balk hole, or it takes very long time to fall into it. So after the black hole is formed initially can we actually observe any mater falling into it or can we say that any new matter has fallen into the back hole up to this point of time? If the time dilation is big enough just outside the event horizon, can we still see all the mater that is headed toward the black hole over the last thousands of years?

Answer

Fascinating question -- because of time dilation which approaches, from a far away observer's viewpoint, infinity as the matter approaches the event horizon of a BH, you intuit correctly that we *cannot* in fact see matter ever 'disappearing' into the BH.  On the other hand the gravitational redshift of the photons also approaches infinity from our point of view there, so we no longer see the matter emit photons, so in some sense it's kind of moot what is happening from our perspective, there.

And from the point of view of the matter falling in, the time dilation works the other way, and the far off observers look very strange, but the BH itself presents no particular issue, and the matter does fall past the horizon and grows the BH (never to be seen again in the Universe, including by Matthew McConnaughey's daughter, as far as we are able to ascertain thus far).

Observationally -- we see BH's of all different masses all over, so we know they do grow as they accrete matter, and further if we had enough time to sit and watch a single one (i.e. millions or billions of years) we know it would also grow in size.  So yes, they do evolve (which accords with our simulations of how they grow in time at e.g. the centers of galaxies and affect galactic structure formation).

It seems strange but -- relativity is kind of weird, and not very intuitive, so, there you go.

Question

I am a molecular biologist and I like science fiction, especially when You read something and say "mmm, that could be..." more than those space fantasy tales which doesn't follow any single known physics law (like Star Wars). So, I found this Korean comic in which the constant of gravity change because of a big bang in another dimension's universe. http://mangafox.me/manga/2015_space_series/c001/13.html.  Then my question is: Could this happen, ? how long could be the time interval between big bangs (if there is any)?

Answer

Well, that's an intriguing question, and there *are* speculative theories in which it happens that some of the fundamental constants of Nature change, including G, but as far as we have been able to test thus far, we are not able to measure any change in any of them right back as close as we can get to the Big Bang.  The idea of a Multiverse in which there are 'other Universes' is also quite speculative at the current time, and has no observational evidence to back it up.

At this point, our simplest model for our own Universe indicates that there will *not* be any other future Big Bangs, but just a steadily increasing expansion of our Universe until everything becomes cold and dark. (But, in case this bothers you -- there have been many twists and turns in our whole cosmological model over time, so I wouldn't take this too seriously yet -- there is much we do not yet understand about the role of Dark Energy in our Universe!)

I couldn't see the comic btw, it tells me it's 'licensed, not available' ?

I'd be curious how they think 'other' Big Bangs would affect G in our own Universe..!

Question

As I understand it, the moon's orbit gets larger by a few inches every year. It seems to me that after a few 1000 years, our tides would be different. Can you elaborate on that. Also, does the earth's orbit around the sun change at all? Also, why does mass have gravity?

Answer

#1. So, I was aware of the change of the Moon's orbit, but had to go look up the numbers to figure this out exactly, found them e.g. here: https://en.wikipedia.org/wiki/Orbit_of_the_Moon.  Which says that in the past, the Moon receded at about 1 inch per year, and the
Earth day used to lengthen at 12 microseconds per year (these are about half the current numbers, but will be illustrative anyway). This means that in a million years, the Earth day will have lengthened just 1.2 seconds, compared to the current day length of 86,400 seconds.

Over a *billion* years, this would be 1200 seconds, which does become appreciable, and yes, we'd expect tides to change significantly over that time, but we don't expect them to over thousands of years.

 #2. The Earth's orbit does have several small effects that change it slightly over time (among them, precession due to Einsteinian gravity), but these are very minor, and unless perturbed by changes the outer massive planets (as may have happened in the early Solar System), we generally expect the Solar System to be quite stable in its current configuration for a long time, possibly until the Sun burns out in 4.5 billion years (so you have a little time to relax, at least).

#3. And, as to why mass has gravity.. hm, you might say for now it's just a postulate that we don't understand at any deeper level. Einsteinian gravity says that all energy and matter curve spacetime, which we interpret as gravity, which then affects all energy and matter in or passing the vicinity of the curvature, and that's essentially our current understanding of it.  But - it works quite well, and is our best current model, anyway!

Question

Why do astronomers talk about looking back into time?

Answer

Light travels very fast, but not infinitely fast.  Therefore, all of the light that we receive on Earth from outer space took some time to get here from wherever it originated.  The farther away it originated, the longer it took to get here.  So for distant objects, we see light that was emitted a long time ago, and therefore, see the object as it existed then.

Question

How do you know that dark matter exists?

Answer

Dark matter cannot be seen directly because it doesn't give off or absorb light, but we know it is there because it interacts gravitationally with the matter that we can see, and we see those effects.  From observing how galaxies rotate, how they move among other galaxies, what happens when clusters of galaxies pass through each other, and the growth of structures over the history of the Universe, we know that there must be much more matter than that which we can see directly, and it must be fundamentally different.

Question

What is dark matter?

Answer

So far, we don't know.  Current leading theories maintain that the dark matter mostly consists of small, neutral particles that have not yet been created in particle physics collider experiments.  We may soon detect such dark matter particles in laboratories on Earth, including in the SuperCDMS experiment.
 

Question

How do you know that dark energy exists?

Answer

Dark energy is accelerating the expansion of the Universe, and by measuring the growth of structure and the distances to very far objects, we know that the Universe's expansion has been accelerating lately.

Question

What is dark energy?

Answer

So far, we don't know.  Many people believe that dark energy may be the so-called a cosmological constant that is uniform throughout space and time, and may result from the vaccum fluctuations predicted by quantum mechanics.  However, this is only a hypothesis at this point.  It is the goal of several KIPAC projects to better understand dark energy.

Question

What is a black hole?  What is it like inside a black hole?

Answer

A black hole is a region of such high gravity that not even light can escape.  Outside of what is called the event horizon, a region surrounding the black hole, it acts like any other large mass which objects can orbit.  Inside the event horizon, all objects and light must fall to the center.  We do not currently know what happens at the center.

Question

How does a black hole form?

Answer

When a lot of matter collapses into a small space, it can become so dense that a black hole forms.  This can happen when a giant star collapses at the end of its life, or if a lot of material is thrown together relatively early in the Universe.

Question

If Gravity is the weakest of the four "forces of nature" and we can observe black holes, is it plausible that the other three forces of nature can result in their own variation of a black hole?

Answer

Gravity is a different force compared to the other three (electromagnetic, weak and strong) forces. The latter three are described by quantum field theories and do not have an analog to a black hole.  A black hole is produced when a high mass is concentrated in a very small space so that its gravity deforms space time so much that nothing can escape from it. The other three forces do not act on space time so they can not form a black hole.

Question

In E=MC2 how does the speed of light influence the conversion of matter into energy? what property of light causes this? On the surface it seems random, but obviously it's not.  Why, when discussing the mass of the Higgs Boson, is the energy used instead instead of the mass? Is it because the units are less awkward?

Answer 

In the physics world, we often express a mass of particle in terms of the equivalent energy, just as you guessed - via E = mc^2.  This is just for convenience, as not to have all those incredibly small numbers (if expressed in grams...).  The two are interchangeable.

Question

How do we know the age of the universe? Isn't it possible that there exists light that's so far away, that it hasn't gotten here yet?

Answer 

We can estimate the age of the universe from several measurements:
1) From measuring the current temperature of the microwave background light which is 2.7 K. We know that this radiation was emitted about 300000 years after the big bang (from theory) and what its temperature was at that time. Now if we assume an expansion rate of the Universe we can calculate the cooling time of the radiation to the current value. The expansion rate of the universe can be derived from supernova explosion measurements in distant galaxies. This gives a good estimate of the age of the universe.

2) From finding far away galaxies. This is indeed only a lower limit for the age of the universe.

3) Measuring the age of stars. This is usually done by looking at many stars in star clusters and results as well in a lower limit of the age of the universe.

Question

If our galaxy was on the edge of what we can currently see as the visible universe, would we be able to see 13.5 billion more lightyears further than we can at our current location? And if so does that support the Big Bang theory? 

Answer

That is a very interesting question. Indeed an observer which is at the edge of our observable universe could see the same distance further. However, he would see parts which we can see and some parts which are not seen by us. His observable universe is formed by his light cone, that means he can observe everything in the distance d=c*t where c is the speed of light and t is the age of the universe. Since he is at the edge of our observable universe he can see our position but in addition he can see other parts in the same distance which are to distant from us to be visible from earth. This can be visualized by drawing two circles of the same radius where the center of one circle is at the radius of the other circle. Everything inside the circle can be seen by an observer in the center and the circles have a large overlap but there is although a significant region only visible to one of the two observers.

Your second question is not so easy to answer in principle the Big Bang theory is compatible with this but it is not really a support for it. The best support for the Big Bang theory is the abundance of the light elements created in the Big Bag nucleosynthesis which is measured with great precision and compatible with the predictions of the Big Bang theory. Further support comes from the microwave background radiation which is caused by an expanding Universe as predicted by the Big Bang theory.

Question

How can, according to Newton's Conservation of energy, antimatter and matter, when they interact, annihilate each other?  Does the energy just transfer to another vessel or is the matter and antimatter truly destroyed? And a far fetched question; is it practical to use some form of energy to compress atoms enough to make them split and expose their inner workings while keeping them trapped in an electric barrier rather than using a particle accelerator?  

Answer

Matter and antimatter indeed annihilate into other energy forms. For example an electron and a positron can annihilate into two photons which takes the energy of the original particles.

Question

If a particle has the rest mass energy of a planck mass or more does it mean that particle must be a black hole or collapse into a black hole?

Answer

The Planck mass and length are defined by the density at which a massive particle would become a black hole according to classical general relativity.  However, at such small scales we know that quantum mechanics is also important, so classical general relativity does not give the correct answer. We did not succeed yet to formulate a unified theory which describes all forces in the same formalism. Such a theory would be needed to make consistent predictions about the properties of particles which enter the regime where the different forces have similar impacts on the behavior of the particle. So we really do not know what happens with a particle that reaches the Planck mass.

Add new comment

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.