Gravity and the "bending" of light

[JamesF]

I've been doing some fairly heavy-duty reading on relativity and related physics this holiday and whilst lying in bed last night started wondering how it explained the apparent bending of light around large stars.

I've come to the conclusion that the photons are always travelling in a straight line (inasmuch as they can be said to be travelling at all), but that the distortion of space-time caused by a large mass makes that path appear to be curved to an external observer.

Does this sound right, or have I got the wrong end of the stick? I have to admit that whilst some of this stuff appears easy to talk about, actually building a conceptual model to get a grasp of the subject is several orders of magnitude more difficult.

James

[ChrisKnipe]

No that's right mass bends space-time and light travels in a straight line on the curved space time, so the external observer sees a lensing effect.

[Capricorn]

Yep, mass distorts space-time, light simply ends up traveling in a curve therefore through the "medium" that is the distorted space-time. The get a gravitational lens you need a damn big mass and you have to be at the right place.

[Theinvoker]

Probably the best comparison in everyday terms is air travel. A flight from London to New York doesn't go in a straight line which we normally perceive as the shortest route, but instead follows a path which allows for the Earth's curvature.

In relativity this is known as a null geodesic.

[welshdai]

love to read through the physics section on here. hopefully it will keep me sharp for teaching!

[JamesF]

One bit I still can't get my head around is where gravity/gravitons fit into the picture.

Einstein says, as far as I understand it, that "mass distorts space-time". My understanding is it's the distortion of space-time that results in the apparent attraction of two masses to each other. That being the case, is "gravity" just the word we use to describe the effect of space-time distortion, and if so, why do we need fundamental particles such as gravitons to "transmit" gravity? Are they the means by which mass distorts space-time?

For bonus points, if gravitons are mass-less "particles" travelling at the speed of light, are they also affected by space-time distortion and do they get diffracted around other large masses?

I really should try to stop thinking about this before I disappear up my own wave-particle duality.

James

[Wildwood]

I highly recommend The Fabric Cosmos by Brian Greene. It explains all of this in a no nonsense way.

[JamesF]

I'm currently working my way through The Elegant Universe. I might have to have a break from particle physics after that and read something a bit lighter. I was thinking Kant, or Nietzsche.

James

[Wildwood]

I wouldn't bet on it ;-). I thought it would be a good idea about a year ago to find out what all this quantum physics 'stuff' was all about by buying myself 'a' book. Many, many books later I'm still learning and still wondering.

Mrs Wildwood has since perfected the "I still have no interest in the Double Slit Experiment" look.…

[JamesF]

Hah

"Rage, rage against the duality of the light".

James

[redgiantjohn]

I'm currently working my way through The Elegant Universe. I might have to have a break from particle physics after that and read something a bit lighter. I was thinking Kant, or Nietzsche.

James

I've just finished this book an amazing read,I must admit a few of the things went over my head a bit but the stuff about the curled up dimensions and string theory were fascinating probably the best book I've read this year:)

[redgiantjohn]

I wouldn't bet on it ;-). I thought it would be a good idea about a year ago to find out what all this quantum physics 'stuff' was all about by buying myself 'a' book. Many, many books later I'm still learning and still wondering.

Mrs Wildwood has since perfected the "I still have no interest in the Double Slit Experiment" look.…

I'm exactly the same,started with a brief history of time now onto superstring and M theory its been quite a journey lol and I've tried talking to people about the double slit experiment but all I get is blank expressions,or "how can it go through both slits?"

[Wildwood]

I would also highly recommend that if you haven't already, that you read and listen to anything and everything by Richard Feynman.

His lectures were lengendary and are available to download as audio and / or video if you know where to look........

Horizon also did two programs about him in the 80's which are well worth watching. Had he taught me physics at school instead of the person who did I may have followed a different career path.

[redgiantjohn]

I would also highly recommend that if you haven't already, that you read and listen to anything and everything by Richard Feynman.

His lectures were lengendary and are available to download as audio and / or video if you know where to look........

Horizon also did two programs about him in the 80's which are well worth watching. Had he taught me physics at school instead of the person who did I may have followed a different career path.

Thanks for the info ill check him out

I agree with you,when I was at school I used to like science but the physics teacher was horrible and didn't make it fun at all we didn't learn about newton or einstien and to be honest I don't recall much of anything that I learnt in physics, but since I got into the subject about 6 months agoi find it so amazing and exciting reading from anything to rotating strings to the many world theory,its pretty much lifting the lid on how the universe works,and if I get this excited about physics now (I'm 30) I just wondered what would have happened if my physics teacher was more encouraging,I'm not saying I would have become a physicist I'm far too thick for that but you never kno:)...

[JamesF]

I'd agree on Feynman. I've found everything I've read or seen involving him interesting. He can come across a little blunt and perhaps immodest sometimes, but I don't think that detracts from what he has to say and it's perhaps as much a reflection of the times he lived in as anything else.

James

[lw24]

One bit I still can't get my head around is where gravity/gravitons fit into the picture.

Einstein says, as far as I understand it, that "mass distorts space-time". My understanding is it's the distortion of space-time that results in the apparent attraction of two masses to each other. That being the case, is "gravity" just the word we use to describe the effect of space-time distortion, and if so, why do we need fundamental particles such as gravitons to "transmit" gravity? Are they the means by which mass distorts space-time?

For bonus points, if gravitons are mass-less "particles" travelling at the speed of light, are they also affected by space-time distortion and do they get diffracted around other large masses?

I really should try to stop thinking about this before I disappear up my own wave-particle duality.

James

Hi,

Well, firstly, a graviton is a hypothetical particle (we don't even know if it exists). But, anyway, it is my understanding that a graviton is essential for gravity to be present in the quantum world, as particles exchange gravitons, in the same way they exchange protons (both massless particles).

Also, from what I have read, gravitons are also affected by space time distortion, but there aren't any visible effects.

Clear Skies,

Luke

[JulianO]

Hi,

Well, firstly, a graviton is a hypothetical particle (we don't even know if it exists). But, anyway, it is my understanding that a graviton is essential for gravity to be present in the quantum world, as particles exchange gravitons, in the same way they exchange protons (both massless particles).

I think you mean photons (as protons definitely have a mass).

Mass/Energy bend space, causing the bending of light and other phenomena, and I believe the graviton is the theoretical particle that propagates the bendiness of space.

Gravity also travels at the speed of light, or at least the effect of a mass does. If the sun were to vanish suddenly, the earth would continue in orbit for about 8 minutes or so, until the ripples of the absence of its gravity made it out this far, then it would go shooting off in a straight line. It would be the gravitons that propagated this effect, just as we'd still see the sun up to that 8 minutes, as the photons that show us its light would still be arriving.

[lw24]

I think you mean photons (as protons definitely have a mass).

I do mean photons

[ollypenrice]

I'd agree on Feynman. I've found everything I've read or seen involving him interesting. He can come across a little blunt and perhaps immodest sometimes, but I don't think that detracts from what he has to say and it's perhaps as much a reflection of the times he lived in as anything else.

James

To be fair, he didn't have much to be modest about!

I find him wonderful to read. When discussing the utterly bizarre nature of the quantum world and why nature does these things, he just says, 'I have no idea.' I see that as modest, really.

Olly