# Questions from dumb engineer about light

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#### crazyant

##### Guest
A question keep on bugging me for sometime, hope someone can help me clearify.<br />Q1. I know light can be bent by gravity, is it possible that we might mistake one star(or galaxy) for serveral different stars( or galaxies) since the light emmit from the star(or galaxy) was bent (by gravity) and reach us by serveral different paths? If it is true then how can we avoid this problem and not to overestimat the size of universe.<br />Q2. I was told the universe is bounded(?) can light be reflected at the boundary of universe like radio wave was reflected at top of atmosphere? <br /><br />Thanks.<br /><br /><br /><br />

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#### heyscottie

##### Guest
Q1: You are describing one of the effects of gravitational lensing. We will occasionally get several images of a distant galaxy if it is lensed by a foreground galaxy or cluster. Instances of this occuring are relatively rare, and are generally detectable when they do occur.<br /><br />Q2: The universe is not bounded in the way you are describing. It does not have an edge in 3-D space.

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#### docm

##### Guest
Light just follows spacetime, and that is what's 'bent' by gravity. It's also the basis for what's called a 'gravitational lens' - almost the effect you describe but very detectable in that the image of the background object is in the form of an arc or circle around the high gravity object. As such it's relatively easy to discern a direct image and one formed by a gravitational lens.<br /><br />Space can be bounded and yet infinite in terms of light's transit through it. <br /><br />One speculation is that this is due to the expansion of the early universe being superluminal - faster than light - meaning light could never catch up with any 'edges' that may exist. It's also possible that space has more than 3 dimensions, allowing it to curve in upon itself and become 'infinite'. Think in terms of a Mobius strip. <div class="Discussion_UserSignature"> </div>

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#### crazyant

##### Guest
Thanks for reply, I am sorry I didn't make Q1 clear enough. Consider the following 2 cases.<br />case1: The light from a star (name it Star-A) 100 years ago and reach my eastern sky now.<br />case2:Light from the same start (Star-A) 200 yeas ago was bent by Star-B, Blackhole-C, ..... and reach my western sky now.<br /><br />Now I can see Star-A both my eastern(100 lightyear away) and western (200 lightyear away)sky now. How can I tell those two star are the same star.<br />

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#### MeteorWayne

##### Guest
Well, your scenario is rather unrealistic. The gravitational lensing is a few milliarcseconds...a papers width from a mile away. I'm not sure, but I don;t believe there's enough concentrated mass in the universe to do the bending (and time delay) you are suggesting. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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#### crazyant

##### Guest
I ask because I don't understand and trying to learn, my following argument may far from truth but please bear with me. <br /><br />If a blackhole be able to suck in the light, then the light is passing very close to blackhole (but not close it enough to be suck in) can make to turn an angle right? <br />

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#### MeteorWayne

##### Guest
Yes, it will be deflected because space time is warped near the black hole.<br />That is exactly what a gravitational lens is, the light is deflected from a straight path by the bending of space-time by the gravity, whether it's a black hole (or more commonly a galaxy or galxy cluster because it has more mass). Even a planet can cause the same effect. So can a rock, though the deflection is too small to measure.<br /><br />A black hole does not "suck in" light unless it gets closer than the event horizon; it which case it can never leave. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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#### crazyant

##### Guest
Thanks all of you to clearify some concepts for me.<br /><br />So, the case I talked about may exist right?<br />suppose there is a blackhole, and a star and earth they form a 60-60-60 triangle, and is 100 lightyear apart.<br />light path 1: star --- />earth<br />path 2: star - />backhole(turned 300 degree)--> Earth<br /><br />Thanks.

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#### Pooua

##### Guest
I think the point is that no gravitational body can bend light as much as you are describing, that is, several degrees. Light only bends less than a degree when going past an entire galaxy. When light bends in this way, it distorts the image of the star, smearing it out. Also, the spectra of stars differ, so if we saw the light from two stars with the same exact spectra, we might be suspicious.

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#### crazyant

##### Guest
Thanks Pooua,<br />--- /> Light only bends less than a degree when going past an entire galaxy. <------<br />But there is billions of galaxies, little by little it may become a 'turn' right?<br />

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#### votefornimitz

##### Guest
Hypothetically, but I doubt for it to travel that distance it would be visible to us....<br /><br />They used what you're describing to support Einsteins theory of relativity....<br />They took a star who's position was know during night, and then measured the position of that star during a solar eclipse...<br />Sure enough, the star's apparent position shifted due to the warping of space time by the Sun...<br />That being said, it would be impossible to notice with the naked eye...<br /><br />I don't have a link atm because I heard it in a podcast, but I'll see if I can find one for you... <div class="Discussion_UserSignature"> <span style="color:#993366">In the event of a full scale nuclear war or NEO impact event, there are two categories of underground shelters available to the public, distinguished by depth underground: bunkers and graves...</span> </div>

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##### Guest
Thought that due to the gravitation and warping of space time of the suns gravity that it bent the light of the star that was behind the sun arond the sun so that it could be viewed and photographed as if it was in front of the sun.<br />Am I reading this wrong the light would still be traveling straight but it would be bent around the sun, so based on this it could never make a right turn it would just be warped around the object that is causing the bend in space time. so it would continue until it got blocked by an object without enough mass to be warped by its gravity<br /> <div class="Discussion_UserSignature"> </div>

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#### jgreimer

##### Guest
Pooua: I think the point is that no gravitational body can bend light as much as you are describing<br /><br />Actually light can be bent by that much passing by a black hole. In fact light can go into orbit around a black hole. However the chances for such an alignment to be so precise for us to witness it are too remote to even consider.<br /><br />Einstein said that in all cases acceleration is indistinguishable from gravity. Since a light beam passing through an accelerating spaceship, perpendicular to the direction of acceleration, would seem to curve in the direction opposite the direction of acceleration, light must also be bent by gravity.

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#### crazyant

##### Guest
Thanks VoteforNimitz and shadow735.<br />I understand the sun make the light bend a little bit part. The question I don't understand is this:<br />Suppose a million years ago some of the light emmited from our sun, was bent again and again while passing serveral blackholes and stars and it come back to my eyes tonight. (The light totally make 360 turns, and take 1 million years). So the star I see 1 million lightyear away actually is our own sun 1 million years ago. Is this possible? If you say the light would be too dim, then I can exchange our sunlight to a super nova to make it brighter. Is this question make sense?<br /><br />

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#### pirated

##### Guest
<img src="/images/icons/laugh.gif" /> <div class="Discussion_UserSignature"> <p>Peace. </p><p><font color="#33cccc">-------------------------------------------------------------------</font> <strong><font color="#993300">I'm a Rock!</font></strong></p><p><font color="#33cccc">Little Johnny was a scientist. Little Johnny is no more. For what he thought was H2O was H2SO4.</font></p> </div>

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#### heyscottie

##### Guest
There's not much problem with what you are saying *theoretically*. There is problem, however, *practically*. As others have suggested, the odds of something like this happening are infinitesimally small. Does this mean it could never happen? Of course not. But if it does, then:<br /><br />1) It is so rare that the errors of knowledge we have would be infinitesimal.<br />2) It would happen for a very brief period of time, as any alignment like this would not last very long at all.

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#### MeteorWayne

##### Guest
Also, any light that made a trip that long would be dimmed by the intergalactic medium to below the point of visibility. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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#### crazyant

##### Guest
Thanks everybody, I don't have enough knowledge to fully understand everything you guys trying to tell me, but I did gain knowledge from the discussion which I appreciated it. The things I learned here:<br />1. Multiple path(light-year appart) of light from one source is possible but not likely.<br />2. We can't tell if "Multiple path" happened or not.<br />3. Light will never be able to reach the edge of universe, even universe is bounded.<br />4. Light can be trapped by blackhole, or bent by gravity.<br /><br />

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#### MeteorWayne

##### Guest
Take one other point. Light is diminished by travel through space, especially within the galaxy and solar sytem where dust concentrations are the highest.<br />A light beam can't bounce around forever; it's not a perpetual motion machine. <img src="/images/icons/smile.gif" /><br /><br />Eventually all the original light is absorbed by what's in between, and spread among the background radiation of the universe. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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#### crazyant

##### Guest
Thanks MeteorWayne.<br /><br />

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#### gammarayburst

##### Guest
If nothing cannot escape a black hole then why do some black holes have light/radiation and or gamma rays coming from their magnetic poles? Is the gravity at the black holes magnetic poles weaker than at its equator? Not likely. Then why do we see jet streams of energy ejecting from a few black holes magnetic poles? Possibly it is the strong magnetic field that effects the light and not gravity.<br /><br />Ever notice how light behaves on the road and appears to be "black" like the pavement is wet, mirage, illusion?

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#### MeteorWayne

##### Guest
The jets eminating from black holes are material and energy from the accretion disk <i> around </i> the black hole. Not from inside the black hole. It is primarily from the magnetic field, and dynamics of rotation.<br /><br />Once anything crosses the event horizon, it don't come back <img src="/images/icons/smile.gif" /><br /><br />Your second point about pavement mirages is completely unrelated as that is cause by very pedestrian (and atmospheric based) refraction of light by the local refractive gradiant of the air.<br /><br />It is caused by the differeing density of hot and cold air.<br /><br />A glass lens, or a pail of water does the same thing.<br /><br />Has nothing to do with black holes. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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#### richalex

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>Possibly it is the strong magnetic field that effects the light and not gravity.<p><hr /></p></p></blockquote>Although black holes have fantasticly huge gravitational fields, it is my understanding that they don't have the strongest magnet fields. Magnetars have more powerful magnetic fields; in fact, the most powerful known in the universe. And, their magnetic fields do absolutely nothing to bend light. Magnetic fields don't bend light, apart from pumping energy into space that then distorts space-time as any matter would. OTOH, gravity certainly bends space-time significantly, and the resulting bending of light has been measured many times.

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