Reading A Brief History of Time

In summary, the author discusses the concept of light as a wave or particle, and how it behaves in different situations. He also discusses the effect of a planet's gravity on the sphere representing light.f
  • #1
I am pleased to have joined and I hope to browse this forum when I have some time.

I have started reading A Brief History of Time for the first time and wanted to express how awesome this book is already (chapter 3 started) and how much it is making me think (and not about work). Chapter 2 has diagrams of cones but I am more comfortable for now thinking in spheres than cones, so I envisage a future light sphere and a past light sphere. Is it possible I could come unstuck at a a later point if I persist?

I am thinking of light bulbs moving away from me but their light is coming towards me, coming towards everything actually, as an expanding sphere. As the bulb moves away someone 'switches it on' and a solid white sphere starts expanding outwards, and as it says in the book regardless of the speed of the bulb or its direction. When someone switches the bulb off a black sphere starts expanding outwards from the bulb representing the lack of light - in the centre of the white sphere if the bulb was stationary or off-centre if it kept moving but still a sphere and still at the same speed as the white sphere. I am hoping I am on track with this..

Light is in waves, but I don't know if that makes it discrete or continuous or something else.

Also, the sphere of white light engulfs a sun at some point and continues. Being a sphere I am thinking it doesn't matter where the sun is in relation to the start of the event (bulb switching on) because all points on the surface are equal? I am having trouble visualising how the sun will distort the shape of my sphere and even harder for me - how it will affect time (or speed? think this might take a while). I also don't know if it matters from what position this engulfment is being observed.

..and wrestling still with the idea that an expanding universe will look the same from one time period to the next, in every direction from any position (if I have read and understood that part correctly - better re-read). Thoroughly looking forward to the rest of the book and can see I won't be finishing it any time soon. Could be the best book I've ever read. That was a bit long, sorry.
  • #2

I didn't read the book, but I listened to the audio version while on a vacation and it is very interesting. It is one of the books which drove me to my degree. I am struggling a bit to understand what exactly your question is, but I can clear up something for you.

Light can be thought of as both a wave and a particle, or as you put it it can be continuous or discrete respectively. For mathematical reasons it is sometimes better to imagine photons as a wave function, but it is also useful to sometimes imagine them as particles. Also this wave-particle duality is a physical reality not just abstract mathematics and cute ways of thinking, discovered with the double slit experiement (it turns out electrons have this duality as well). Here is a description of the wave-particle duality from wiki,–particle_duality.

In regards to the sphere of light being engulfed by a star. It won't neccesarily be engulfed, aside from those photons which directly strike its surface, but they will be bent round the star and that bend will be dependent upon how massive that star is. As I said I am not entirely understanding your question, so if this is not where you were going, please say so.

  • #3

The "cones" in the book are light cones with the time axis as the vertical axis IIRC. His illustrations are all 2+1 geometries (or sometimes 1+1) where he only considers 2 spatial dimensions (like a plane) and 1 time dimension. In that case, light propagates as a cone because it moves out to ever widening area in time.

He does this because it's more or less impossible to illustrate the real case of 3+1 geometry (3 spatial dimensions and time) on one picture (he'd have to show many pictures of a sphere of light growing bigger...but that obscures the points he is trying to make).

You can of course visualize light as moving out as a sphere (which it indeed is in the isotropic case) and the sphere just gets bigger as time goes along. However, you can't really translate this visualization onto 1 picture.
  • #4

Thanks for your answers I appreciate it. I'm trying to visualise the effect of a planets gravity on the light passing by. If the sphere representing light passes by a planet would a hole appear in the surface of the sphere? I envisage this hole growing wider as the sphere expands, leaving behind a 'tunnel' in the sphere leading back to the planet it engulfed.
Would this be accurate in the case of a planet with zero mass, if such a thing existed? Also, the sides of this tunnel would be straight.
If I increase the mass of the planet gradually am I right in thinking the width of the hole on the spheres' surface will shrink? Would the sides of the tunnel still be straight?
Would the hole in the sphere ever close over completely? Could it invert?
If you were looking towards the light source but the planet was directly in between could the mass and light bending properties of the planet actually make the light bend right round enough so you can see the light source? Would it appear as a ring of light? Would it be 'inside out'? Thanks.
Last edited:
  • #5

I think I've understood the expanding universe looking the same in every direction wherever you are now. Awesome how it could be either staying the same or expanding in that sense.
  • #6

Welcome to can be addictive...Hawkings books are great introductions and cause for a lot more questions...

Light is in waves, but I don't know if that makes it discrete or continuous or something else.
Try reading here:

...and similar...light seems to have characteristics of both...try looking for "quantum" on wikipedia...note Wikipedia as a general source and quick reference...sometimes arcane, sometimes very insightful...

and wrestling still with the idea that an expanding universe will look the same from one time period to the next

look up homogeneity and isotropy of the universe in wikipedia...the universe does have large scale uniformity we look to the most distant regions of the universe that we can observe, we see a younger it was billions of years time for life to have evolved, a lot more gas and smaller and newer planets than our galaxy for example...less time for gas to have coalesced to stars,planets,black holes etc...and fewer (flat) spiral form galaxies that take billions of years to form...

Here is a great resource you should bookmark to compare with HAwkings comments:
Last edited:
  • #7

Thanks for the links, will gradually read but it's a lot to take in.
One things I neglected to think of above was the effect of a planets mass on the time taken for light to travel. So I am wondering would the mass of a planet cause the hole in the sphere of light mentioned above to become countersunk around its edges, as it were?
Last edited:

Suggested for: Reading A Brief History of Time