Expert: James Gort - 12/16/2006

Question
RE:"To answer your first question, all pulsars are neutron stars (currently accepted models!), but not all neutron stars are pulsars."

So.. what characteristics does a neutron star that is not a pulsar have?

So... watches on the train moving at 1/2 the speed of light would be slower relitive to an  observers watch - who is not on the train.  So.. bigger an object, the more space-time is bent which slows time down/ time diliation and gravitational red shift is a quantifiable means in which to measure such phenomena- Cool!!! Now, the red shift is caused by light slowing down- as whitnessed by an observer- the light that hits an observer's eye moves into the red range of the EM spectrum. It that correct?

Thanks :)
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The text above is a follow-up to ...

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So... there are neutron stars that do not rotate, but still expel EM radiation? By inference these neutron stars are not pulsars.

Also... so comparing the earth to the mass at the center of a blackhole- the escape velocity (to leave the earth's surface) is about 25,000 m/s (i think)... so the space-time "moving" around the earth is going this fast.  However the space-time moving around a blackhole is going 300 X 10^6 m/s. Is the space-time just curved or is it really moving?


Thank-you for your time!!!
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The text above is a follow-up to ...

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1. Are neutron stars and pulsars the same?
2. Are black holes a hole in space/time, or is there a super-dense object in there somewhere?
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Hello Adia,

To answer your first question, all pulsars are neutron stars (currently accepted models!), but not all neutron stars are pulsars.  Pulsars are rapidly rotating neutron stars whose magnetic axis is inclined to its rotational axis.  As it rotates, high energy particles and electromagnetic radiation are emitted from one magnetic pole, which is periodically directed towards earth as it rotates (this orientation is pure chance - if it's not directed towards earth, we'd never see it as a pulsar).  So we see a pulsating light in various parts of the spectrum (from radio to x-ray) from the neutron star, giving the name "pulsar".  But it's important to know that pulsars are NOT pulsating stars.

Your second question is a bit more difficult.  A black hole is really curved space-time around a mass.  In spite of popular belief, it doesn't need to be a "massive" object, just massive compared to its size.  Since the force of gravity is inversely proportional to distance, if we can get closer to a mass (if the mass is small in size), then the gravity at the surface of the mass should be greater.  So if a mass is dense enough (denser than a neutron star!), then the surface gravity is so great that light cannot escape its surface.  But what does this actually mean?  It means that space-time is so curved around this object that if one wanted to remain "stationary" on the surface (as measured by a distant observer), then one would have to travel at the speed of light (locally measured)!  Since, according to Einstein, no matter can travel at the speed of light (unless you have an infinite force!), matter can't remain stationary!  Our dense mass must continue to collapse, since no force can keep it from collapsing.  In theory (again, according to known physics), it collapses to a singularity (mathematical point).  This may or may not be true, and it may be untestable.  And we may need to modify the theory of relativity with a quantum theory of gravity to better understand what really occurs deep with a black hole.  So to answer your question, current theory says there's an infinitely dense object within a black hole.  It only appears to have a radius because that's the distance at which light cannot escape its gravitation.  And you can call a black hole a "hole in space-time", because it's infinitely curved space-time.  Or you can image it as "space-time moving at the speed of light" close to the  point mass.

I hope that clarifies it a little.  It's a very complex subject, and one which physics does not yet have all the answers.

Prof. James Gort
    

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Hello Adia,

Yes, a non-rotating neutron star (which could very likely still emit strong EM radiation from its magnetic poles) would not be a pulsar.  However, it's not likely you would find a non-rotating neutron star.  If the original star (before collapse) had any rotation at all, the rotation would speed up greatly after collapse due to conservation of angular momentum.  Most stars do have some intrinsic rotation. In addition, any trapped magnetic field in a non-rotating neutron star would dissipate quickly.  It is thought that the large kinetic energy which rotating neutron stars possess actually powers the magnetic field (loss in KE as it slows is converted to magnetic energy).

For the second part, the escape velocity of the earth is actually closer to 11,200 m/s.  Using strictly Newtonian mechanics, that means if we shrunk the earth to a size of .009m radius, then the escape velocity would be the speed of light, in agreement with Einstein.  (This is the size that the earth needs to be in order to become a black hole).  The difference (and an important difference) is that Newton's escape velocity means that if you fired a projectile at that speed, it would gradually lose energy, and completely stop when it got very far away.  According to black hole theory, you cannot fire a projectile (or a photon) away from the black hole mass (within the Schwarzchild radius) and watch it stop.  In fact, the projectile cannot even start to get away - not even one nanometer away.  It would stop before it even starts!

So to answer your question, you can certainly interpret gravity (and the effects of gravity, like escape velocity) as due to curved space-time around the mass.  That curvature means it's energetically favorable to have masses closer together (depending on the slope of the curvature).  Although "movement of space-time" is also a way to visualize the reality, it is a more difficult concept, since movement implies changes in position with respect to time.  The movement we're talking about is actually measured by a distant observer.  For instance, if we watched (from a distance) a train go by at half the velocity of light, we would see the length of the train decrease in the direction of motion and its mass (as measured by us observers) would increase.  That doesn't mean the train riders would measure the same phenomenon. The difference is due to the relative difference in the movement of space-time.

So space-time is bent around a massive object (i.e., orbits are affected) and slows near a massive object (if we synchronized watches with a person living on a massive planet (compared to earth), then we'd see the watches on that massive planet actually run slower).  On a more realistic level, we can see the light emitted from a massive object shifted towards the red (this effect is called "gravitational redshift) due to time dilation near the object!      

I hope that further explains the phenomenon.  It's always a difficult concept, since it's far removed from our everyday experience.

Prof. James Gort

Answer
Hello Adia,

A pulsar is just the observational evidence for certain neutron stars - ones that are oriented towards earth such that we periodically are in the path of its EM beam.  That orientation is pure chance.  It is thought that most (or all) neutron stars emit this energetic beam.  But we're usually not in the line-of-sight path.  Physically, there's no difference between pulsars and neutron stars.

For the second part, you are correct that the bigger (or more massive) the object, the more space-time is bent and the more time slows (relative to a distant observer).  Gravitational redshift is caused by time dilation in the gravitational field, which causes more time delay between EM "waves" - longer waves mean redder light.  The red shift can also be thought of as not by light "slowing down", but by light losing energy in a gravitational field.  Less energetic photons are shifted towards the red, but according to Einstein, photons can never really "slow down".  The speed of light is a constant, no matter how fast you are travelling.  The way to think about this is in the fast-moving train, if you shone a flashlight in the direction of the moving train, you would measure the beam moving away at c.  A distant observer would see the beam also moving at c, but you (on the train) would be following the photons at 1/2 c.  So why wouldn't you (on the train) measure the light beam at 1/2 c?  Because time has slowed for you (relative to the distant observer)!  So your measurements are different from those made by the distant observer.  Both would measure the light beam at a velocity of c.

Hope that helps!

Prof. James Gort