I need help understanding light.

[RossGr]

trak0rr0kart,

There are many very long, very difficult to follow posts on this topic, the answer to the question is really simple. According GR gravity interacts with ENERGY. That means any form of energy is effected by and effects gravitaional fields. Light is energy therefore it is effected by gravity. Mass of couse is E=mc^2 so it simply is a form of energy, and is therefore effected by gravity.

AKA
Integral @ Physicsforums.com

 

[Fencer128]

Originally posted by: trak0rr0kart
I think I will take your advice and study up more on space-time because my view of it isn't correct and has led me to believe certain things about light and gravity that don't add up. Like you described.. space-time creating a gravity well and the light following it makes sense.. but not because of gravity that causes the light to follow it.. but something else. I am starting to get real curious about space-time now!

Thank you again for the clarification, I must be hard to deal with

It's a difficult topic when you start to ask detailed questions. Don't beat yourself up about it!

Good luck,

Andy

 

[grant2]

Originally posted by: AdvancedRobotics
And do you have a link or something to that experiment? Sounds very interesting; would like to read.

I'm pretty sure it happened sometime in 2001, and was hailed as the next big step to pure optic computing.

 

[grant2]

nm

 

[Smilin]

light will curve around a mass due to space time being curved, not because there is a gravitational attraction between a photon and the mass. Doesn't light follow this curved path because it's the shortest distance between two points in 4 dimensional space? If light went in a straight line while passing by a mass it would have to either lose energy somehow right?

On a somewhat related note: Didn't they recently measure the speed of gravity? I thought there was some experiment recently in which they were able to prove the speed of gravity at least well enough to know that it's not infinite. That kinda blows my mind...if the sun disappeared the earth would continue to go in an eliptical path for another 8.5 minutes.

 

[grant2]

Speed of light = Speed of gravity

even though this older article says it's > 2 x 10^10 x the speed of light.

 

[trak0rr0kart]

Originally posted by: RossGr
trak0rr0kart,

There are many very long, very difficult to follow posts on this topic, the answer to the question is really simple. According GR gravity interacts with ENERGY. That means any form of energy is effected by and effects gravitaional fields. Light is energy therefore it is effected by gravity. Mass of couse is E=mc^2 so it simply is a form of energy, and is therefore effected by gravity.

AKA
Integral @ Physicsforums.com

Your saying that a planet is effected by gravity because it has mass (I agree!)..

..and light is being effected by gravity because it is energy; ok. -> How does that work?? HOW IS light effected by gravity in a GRAVITY LIKE behavior (being bent around the sun, or following the space-time well)? HOW is my question. I have heard most of this stuff before, I'm just trying to figure out HOW it does IT! I just can't settle for an answer along the lines of: it just follows space-time.. . which no one is able to verify so far because I still haven't seen an explination as to WHY it happens. ... I know that it happens now.. BUT WHY. Thanks for your input integral.

 

[RossGr]

Originally posted by: trak0rr0kart

Originally posted by: RossGr
trak0rr0kart,

There are many very long, very difficult to follow posts on this topic, the answer to the question is really simple. According GR gravity interacts with ENERGY. That means any form of energy is effected by and effects gravitaional fields. Light is energy therefore it is effected by gravity. Mass of couse is E=mc^2 so it simply is a form of energy, and is therefore effected by gravity.

AKA
Integral @ Physicsforums.com

Your saying that a planet is effected by gravity because it has mass (I agree!)..

..and light is being effected by gravity because it is energy; ok. -> How does that work?? HOW IS light effected by gravity in a GRAVITY LIKE behavior (being bent around the sun, or following the space-time well)? HOW is my question. I have heard most of this stuff before, I'm just trying to figure out HOW it does IT! I just can't settle for an answer along the lines of: it just follows space-time.. . which no one is able to verify so far because I still haven't seen an explination as to WHY it happens. ... I know that it happens now.. BUT WHY. Thanks for your input integral.

How is mass effeted by gravity? Modern physics cannot, and does not even to attempt, to answer WHY something happens, we can only observe and measure. From our observatations we can create Mathematical models. When we have created a sucessful model we are able to use it to make predictions, when these predictions are later verified through observeation and measurement we proclaim the model a Theory. Few theories have been as throughly verified as Einstein's theories of Relativity. It is in the working of General Relativity from which the interaction of interaction of energy and gravity are predicted. Observations of Gravititional lensing are the observations which have verified this theory. So to sum it up, the effect is predicted by theory, it has been observed and measured, thus verifing the theory.

I would like to think that this simple modification of your basic concept would make it much easier to understand how light can be massless and still follow space-time geodesics.. ie be effected by gravitiy. Gravity is what we call distortions in the fabric of space-time caused by the presence of Energy. Mass is a very concentrated form of energy and causes large distortions.

 

[trak0rr0kart]

"Modern physics cannot, and does not even to attempt, to answer WHY something happens, we can only observe and measure"

I hope your refering to the extremely hard questions we haven't answered yet.. but to say that about everything is a little rough. I believe that we are in the pursuit to finding the why in all problems.. the observation and expirementation helps us in answering the WHY. for example we know a bus would put more force onto a car if it hit it at the same velocitys as a car to car collision. We know that the WHY is because the bus has more force to interact with because it has more mass. F=MA.

ABout the rest of your comment.. You still haven't answered my question as to why the light follows a path, like an object that has mass (gravity like behavior). I know it follows space-time, I know it is energy, I know that gravity interacts with energy..... but why? why does it follow space-time instead of its on its own. You guys can keep responding, but I will keep responding with the same answer until someone gets what I'm trying to figure out.

As always, I appreciate your responses and thank anyone in advance who finally gets what I'm getting at. This might very well be the question that takes another 20 years to answer But answers like "you can't answer that" or "we don't care about the why" etc.. are NOT what I'm looking for.

 

[f95toli]

Then will have to keep looking. I too agree that modern physics can not answer WHY something happens, at least not on a fundemental level.
When it comes to gravity there are a lot of unanswered questions, we do understand why light is affected by gravity but we do not understand why objects with mass is affected, there are clues and the answer might be Higgs bosons. Of course then you can ask WHY Higgs bosons interact with gravity the way they do etc.
WHY is more or less irrelevant in physics (the question is HOW), sometimes you just have to accept that you can not understand something; things just are the way they are.

 

[PrinceXizor]

Your question as to HOW gravity affects space-time is similar to the following questions...to use some of your examples.

What is mass?

Why doesn't greater mass and greater acceleration cause LESS force as opposed to MORE force (i.e why isn't the equation F=1/ma instead F=ma

Why does light travel at a constant speed (instead of infinitely)?

All of these questions don't really have answers per se.

We only know of their existence and how they work by observation. We don't know why they work, they just do. For example, you can demonstrate that F=ma by experimentation, but you can't explain WHY that equation exists in the first place. It just does.

Remember, this is HT, so plz no flamewars based on the next comments...

There are pretty much two theories on the overall HOW of something.

1. Within the overall nothingness of nothing, the eventuality of certain immutable laws of Physics are just there. Things like, Mass affects Space-Time, The speed of light is constant in a vaccum, etc. They are simply immutable facets of the reality of the Universe that sprang into existence for some reason with these laws in place. The Universe IS defined by these immutable laws that exist within it.

OR

2. A Divine being created the Universe establishing all of the relationships and effects of these immutable laws that he designed so that reality as we understand it could take place. Again, these laws are immutable, they simply are, because he chose them to be that way.

In either of these scenarios, there will still be postulates, ideas that cannot be proven scientifically, they can merely be demonstrated to be accurate all of the time. A good example of this is Pythagorean (Planar) vs. Euclidean (Spherical) geometry. There are postulates within each field that form the basis of each that cannot be true within the other. You cannot prove these postulates, they simply are in order for the rest of the proveable things to take place.

P-X

 

[TheInvincibleMustard]

Wow, there's a lot of stuff going on in this article. (BTW, long-time reader of AT, roughly fifth-time reader of forums, first-time poster).

With regards to trak's question about WHY the light behaves that way, well, to put it simply ... there isn't any other way for it to behave. Consider an ant walking on a piece of paper that's about 11" long and 2" wide or so (going from 4-D to 2-D, stick with me here). Please note that as light is travelling, it is not immune to the effects of the fourth dimension, since light != infinite speed, thus light moves unidirectionally through the fourth dimension. Much as the ant has no say in the matter of what direction it's moving in the third dimension on the piece of paper. If you tilt the paper, or curve it this way or that, you're changing the final position of the ant in regards to the third dimension, but the ant has no choice in the matter, it has to stay on the paper. Similarly, the mass (and therfore gravity) of a sun/black hole/my cat's butt/etc. "tilts and curves" the "fabric" of the universe (the paper). The light has no say in the matter, as the "paper" is the only place it can go.

As to WHY the light can't "fall off" the paper (like the ant can, if it's suicidal) ... well, that's an entirely different question.

Feel free to blow holes through this entire analogy, if anyone so desires, as it's 2:45 am where I am right now and it's been a while since I've gone through any of Hawking's material (which makes this seem like a "Dick & Jane" book, but that's beside the point).

 

[Spacehead]

Originally posted by: AdvancedRobotics

Originally posted by: Bowfinger
A couple of nits. First, light travels at a constant speed in a vacuum. It travels at other speeds in other mediums. Second, it is possible for photons to come to a rest. Within the last year or so a group announced that they had successfully stopped photons within an experiment, then released them. In this context, "stopped" meant caused them to stop moving, not absorbed or stored.

Well I meant constant in that the speed of light is constant.

And do you have a link or something to that experiment? Sounds very interesting; would like to read.

Thanks!

According to the new Discover magazine:

In her first experiments in 1999, physicist Lene Hau could slow light to the speed of a bicycle. Now she can stop it entirely for 0.005 seconds.

Here is a link to her work. Not sure how updated it is as i haven't read any of it yet.
Slowing the Speed of Light

 

[RossGr]

Originally posted by: Spacehead

Originally posted by: AdvancedRobotics

Originally posted by: Bowfinger
A couple of nits. First, light travels at a constant speed in a vacuum. It travels at other speeds in other mediums. Second, it is possible for photons to come to a rest. Within the last year or so a group announced that they had successfully stopped photons within an experiment, then released them. In this context, "stopped" meant caused them to stop moving, not absorbed or stored.

Well I meant constant in that the speed of light is constant.

And do you have a link or something to that experiment? Sounds very interesting; would like to read.

Thanks!

According to the new Discover magazine:

In her first experiments in 1999, physicist Lene Hau could slow light to the speed of a bicycle. Now she can stop it entirely for 0.005 seconds.

Here is a link to her work. Not sure how updated it is as i haven't read any of it yet.
Slowing the Speed of Light

A photon travels at c. What is happening in this type of experiment is that a photon is adsorbed by an atom and after a measurable time delay a photon is emitted. This NOT the same as slowing the speed of light in vacumn. Inspite of what the journlest would have you believe.

 

[grant2]

Originally posted by: RossGr
A photon travels at c. What is happening in this type of experiment is that a photon is adsorbed by an atom and after a measurable time delay a photon is emitted. This NOT the same as slowing the speed of light in vacumn. Inspite of what the journlest would have you believe.

An excerpt from one of their papers::

... coherent information initially contained in the laser fields is 'frozen' in the atomic medium for up to 1 ms. The coupling laser is turned back on at a later time and the probe pulse is regenerated ...

Confirmed!

 

[RossGr]

Originally posted by: grant2

Originally posted by: RossGr
A photon travels at c. What is happening in this type of experiment is that a photon is adsorbed by an atom and after a measurable time delay a photon is emitted. This NOT the same as slowing the speed of light in vacumn. Inspite of what the journlest would have you believe.

An excerpt from one of their papers::

... coherent information initially contained in the laser fields is 'frozen' in the atomic medium for up to 1 ms. The coupling laser is turned back on at a later time and the probe pulse is regenerated ...

Confirmed!

read your quote "frozen" in the atomci medium. Those are different words for what I said.