dgary@ecsvax.UUCP (06/02/84)
For some time I have been reading about rapidly pulsing objects and how this
puts an upper limit on their size. That is, since no signal can propagate
faster than light, the period of oscillation of a body cannot be less than
the light transit time.
I'm confused by this, and I offer a thought experiment to explain why.
Imagine an immense pool of water with a couple of flags at either end.
Suppose I disturb the water in the center of the pool so that waves
cause the flags to move up and down. A distant observer can see that
the flags are moving almost in unison. Can this user then infer that
the pool is of a certain limited size?
Or, more to the point, imagine a spherical body in space light years across.
A signal travelling at less than c but consisting of high-frequency pulses
travels from the center of the body
and reaches the perimeter 'simultaneously' in all directions, causing said
perimeter to pulsate in unison.
I'm not suggesting a mechanism, just asking if this isn't somehow possible.
If we're talking about objects more or less at rest with respect to us,
I don't see how simulaneity considerations really enter into it.
So, does the period of pulsation really place an upper limit on size??
Confusedly,
D Gary Grady
Duke University Computation Center, Durham, NC 27706
(919) 684-4146
USENET: {decvax,ihnp4,akgua,etc.}!mcnc!ecsvax!dgarynather@utastro.UUCP (Ed Nather) (06/03/84)
[] >For some time I have been reading about rapidly pulsing objects and how this >puts an upper limit on their size. That is, since no signal can propagate >faster than light, the period of oscillation of a body cannot be less than >the light transit time. > >Or, more to the point, imagine a spherical body in space light years across. >A signal travelling at less than c but consisting of high-frequency pulses >travels from the center of the body >and reaches the perimeter 'simultaneously' in all directions, causing said >perimeter to pulsate in unison. > >So, does the period of pulsation really place an upper limit on size?? > >Confusedly, >D Gary Grady >Duke University Computation Center, Durham, NC 27706 Your example is just the one usually used to prove the assertion. Yes, it is possible for a central disturbance to propagate outward at less than lightspeed and arrive at a spherical surface "at the same time" but consider what the distant astronomer sees: certainly not a sudden brightening of the whole object at once -- he sees the part nearest to him start to brighten (that light gets to him first), followed by the slightly more distant parts (in a kind of bright ring, if the disturbance was a short pulse) followed by ... Even though the disturbance arrived at the surface of the object everywhere "simultaneously" the observer sees that pulse convolved with the light travel time across the object; if it had a radius of 1 light year (pretty tenuous stuff, I guess) then it would take a year for the observer to see the bright ring finally reach the most distant part of the object -- assuming it to be opaque. If it were transparent, he would then see the bright ring get smaller and smaller, until light from the most distant part of the object reached him -- 2 years after the beginning. That's the argument used by those in favor of the assertion. If the object is approaching you at some substantial fraction of the speed of light, however, then other factors enter which can make a very real difference. -- Ed Nather {allegra,ihnp4}!{ut-sally,noao}!utastro!nather Astronomy Dept., U. of Texas, Austin
elt@astrovax.UUCP (Ed Turner) (06/04/84)
In the example of a source consisting of a central exciter and a peripheral emitter, the duration of the observed pulse will still be of order the light travel time size of the object because of the delays in propogating any signal (pulse) across the emitter. In other words, the observer will see the near side of the source light up at a time D/c (where D is the emitter's diameter) before the light from the far side of the source arrives. By properly phasing the emission from different regions on the emitting surface, observers in some particular direction can see a pulse much narrower than D/c but this will cause other observers to see even longer pulses so the mean will still be of order D/c. Ed Turner astrovax!elt
ntt@dciem.UUCP (Mark Brader) (06/06/84)
D. Gary Grady (ecsvax!dgary) asks why a large body can't change brightness quickly: ... imagine a spherical body in space light years across. A signal travelling at less than c but consisting of high-frequency pulses travels from the center of the body and reaches the perimeter 'simultaneously' in all directions, causing said perimeter to pulsate in unison. Well, suppose the body is N light years across and its center is M light years from us. For simplicity suppose the south pole of the object points at us. Now a pulse travelling symmetrically from the center of the object causes the entire surface to brighten. Fine. But the light from the south pole takes only N/2+M years to reach us, whereas the light from the equator takes M years, and the light from intermediate south latitudes takes intermediate times. Therefore we see the brightening smeared out over N/2 years. That's why. Mark Brader
acscmjm@sunybcs.UUCP (Mike Moroney) (06/06/84)
[ ]Help!!! I'm TRAPPED in a VAX 11-780!!!! >> For some time I have been reading about rapidly pulsing objects and how this >> puts an upper limit on their size. That is, since no signal can propagate >> faster than light, the period of oscillation of a body cannot be less than >> the light transit time. >>I'm confused by this... The upper limit on the size is because the pulsing objects are actually rotating, they appear to pe pulsating from Earth the same way the rotating light on a lighthouse appears to pulse on and off from a distance. THe size limit is because no object can rotate such that any portion of it exceeds the speed of light, c. For example, if an object pulses at 1 pulse per second, it has an upper limit on its size of 29602 miles radius, since if it were larger, its outer edge would rotate at > 2*pi*29602 mi/sec or 186000 mi/s or the speed of light, which is impossible. >> Or, more to the point, imagine a spherical body in space light years across. >> A signal travelling at less than c but consisting of high-frequency pulses >> travels from the center of the body >> and reaches the perimeter 'simultaneously' in all directions, causing said >> perimeter to pulsate in unison. This does not disprove the above, since the actual source is not the object light years across, but the thing in its center which is pulsing. The rest of the object is just responding to the pulses. Mike Moroney ..!rocksvax!sunybcs!acscmjm
matt@oddjob.UChicago.UUCP (Matt Crawford) (06/07/84)
>> >> For some time I have been reading about rapidly pulsing objects >> >> and how this puts an upper limit on their size. That is, since >> >> no signal can propagate faster than light, the period of oscil- >> >> lation of a body cannot be less than the light transit time. >> >>I'm confused by this... >> The upper limit on the size is because the pulsing objects are actually >> rotating, they appear to pe pulsating from Earth the same way the rotating >> light on a lighthouse appears to pulse on and off from a distance. THe size >> limit is because no object can rotate such that any portion of it exceeds >> the speed of light, c. For example, if an object pulses at 1 pulse per >> second, it has an upper limit on its size of 29602 miles radius, since >> if it were larger, its outer edge would rotate at > 2*pi*29602 mi/sec >> or 186000 mi/s or the speed of light, which is impossible. >> >> Or, more to the point, imagine a spherical body in space light >> >> years across. A signal travelling at less than c but consisting >> >> of high-frequency pulses travels from the center of the body >> >> and reaches the perimeter 'simultaneously' in all directions, >> >> causing said perimeter to pulsate in unison. >> This does not disprove the above, since the actual source is not the >> object light years across, but the thing in its center which is pulsing. >> The rest of the object is just responding to the pulses. No, not all pulsating objects rotate. In the case of rotating pulsars, your explanation applies. Nonrotational pulsation still limits the size of the object because of the difference in light travels times for light emitted from the nearest and farthest points on the object. If this time difference is comparable to or greater than the period of pulsation then no pulsation will be observed. The source of a light signal is the last thing it scattered from, which is at or near the surface in the case of stars. ___________________________________________________________ Matt University ARPA: crawford@anl-mcs.arpa Crawford of Chicago UUCP: ihnp4!oddjob!matt