Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: RE.Craig on 12/02/2013 15:54:42
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Just a muse, but are photons and other parts of the electromagnetic spectrum pushed or pulled after propagation?
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Neither, at least not by emitters or absorbers of photons. Photons are emitted and absorbed by charged particles, but neither pushed or pulled by them. They travel freely after emission until they hit a particle that absorbs them.
They can be pulled by gravity, which can bend their trajectories or shift their frequency.
Finally, the net effect of many absorptions/emissions as a beam of photons travels through a medium can be approximated by a bending of the beam. This can be roughly modeled as a push or pull on the beam by the medium through which it's moving, but it's just a model--the beam is not being pushed or pulled by a real force nor are the photons being pushed or pulled. (http://en.wikipedia.org/wiki/Hamiltonian_optics)
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Hi,JP. Thanks for the useful info and your ideas. But another question: If a photon which has mass and is travelling at light speed "can" be pulled by gravity, would that "pull" not add a net velocity to the photon?
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It's generally easier to talk about beams or pulses of light than photons, as quantum mechanical photons are odd beasts that don't really behave like little bullets flying along through the universe.
So if we have a pulse of light, it's velocity can change, since velocity includes direction and gravity can deflect it. It's speed doesn't change, since we measure speed locally, meaning as the pulse flies by us. We could sit anywhere along its bent path and measure it to travel at the speed of light. The total time it takes to go between two points will change if you put a large mass in the way, since light travels on a curve and even though it moves at a constant speed along that curve, the total distance it has to travel increases.
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But another question: If a photon which has mass and is travelling at light speed "can" be pulled by gravity, would that "pull" not add a net velocity to the photon?
A photon cannot be pushed or pulled by gravity, since gravity is not a force, according to GR.
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It's generally easier to talk about beams or pulses of light than photons, as quantum mechanical photons are odd beasts that don't really behave like little bullets flying along through the universe.
So if we have a pulse of light, it's velocity can change, since velocity includes direction and gravity can deflect it. It's speed doesn't change, since we measure speed locally, meaning as the pulse flies by us. We could sit anywhere along its bent path and measure it to travel at the speed of light. The total time it takes to go between two points will change if you put a large mass in the way, since light travels on a curve and even though it moves at a constant speed along that curve, the total distance it has to travel increases.
I am confused? What is the difference between velocity and speed?
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Velocity tells you speed and direction. Speed is a number telling you how fast you're going, but not in which direction.
For example, if you're driving your car and look at the speedometer, it tells you speed. If you also have a compass that tells you direction, the combination of direction + speed tells you velocity.
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I thought speed + direction = V for vector?
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No, a vector is something that is specified by a number and a direction. v denotes velocity which is indeed a vector, but it stands as shorthand for velocity.
By the way, someone could complain that speed + direction = V is sloppy notation since you can't add a speed to a direction. Rather velocity is a vector which is described in terms a speed and a direction.
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Just a muse, but are photons and other parts of the electromagnetic spectrum pushed or pulled after propagation?
I’m always fascinated by questions about things being pushed or pulled. I don’t understand why people care so much about it. Push means that you apply a force at a point on the object from behind it while pull means that you do the exact same thing as when using a chain and a bolt on the car. When it’s a rod and you’re grabbing it at the front end its being pulled. If you grab it at the rear end and apply force in the direction you want to go, i.e. towards the end that is furthest away, then you’re pushing it.
This does not hold if the object is a point particle like photons or electrons.
The key to this idea is that there must be a force acting. Any object that is not accelerating does not have a force acting on it. Photons don’t accelerate and are point particles. Therefore there is no sense of the terms push/pull which would allow us to say yes.
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By the way, someone could complain that speed + direction = V is sloppy notation since you can't add a speed to a direction.
Yeah but you can tell from the context that the plus sign is being used as a logical and rather than a summation off terms, right?
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I am confused? What is the difference between velocity and speed?
Speed is defined as the magnitude of velocity.
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The total time it takes to go between two points will change if you put a large mass in the way, since light travels on a curve and even though it moves at a constant speed along that curve, the total distance it has to travel increases.
Could a little confusion creep in here?
In the oft quoted analogy of a long journey on the curved surface of the Earth, the curved line is presented as the shortest distance in space.
However, the curve on which the light travels when a significant mass is close to its path is a geodesic, which, I believe, is defined as the shortest distance between two points in spacetime.
If the geodesic is the shortest distance in space, that must be because space is distorted, and there is no way to take a "short cut" by finding a Euclidean straight line. Difficult for the non-expert to visualise.
If the geodesic is the shortest distance in time, in what sense does the total travel distance increase?
Do we have to think in four dimensions to get our heads round that?
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By the way, someone could complain that speed + direction = V is sloppy notation since you can't add a speed to a direction.
Yeah but you can tell from the context that the plus sign is being used as a logical and rather than a summation off terms, right?
Yeah, I can, but someone who hasn't taken enough math to see vectors might not know the distinction. Or maybe I'm being too pedantic. :p
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Bill, Pmb can probably answer better than me, and possibly even correct me on this, but the (null) geodesic that light travels along describes how it moves in already curved space-time.
As I understand it, if light were moving in across flat space-time in the absence of gravity and we calculated the time it took, and then we were to have it travel between the same two points, but put a big mass in the way, the observed time it took the light to travel that distance would increase, simply because the light is now traveling along more space-time.
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Yeah, I can, but someone who hasn't taken enough math to see vectors might not know the distinction. Or maybe I'm being too pedantic. :p
It’s hard to say that you’re being pedantic when nobody fully comprehends the sure I don’t full understand this environment. There are too many unknowns in the audience. This has been one of the things about Internet tutoring that makes it hard to explain things.
My thirteen odd years of posting in physics forums on the Internet has convinced me that it really doesn't matter. First off, one cannot assume that no matter how good you are at tutoring people online, nobody is able to fully tutor someone with all the drive that is required to drive home a point to someone you don’t know or ha. The main reason being that we don't necessarily and fully understand whom it is that we are tutoring. E.g. Yes. I may know that a person may have read many books for the layman or even, perhaps, for the undergraduate. But I think that in the end will do a pretty good job, don’t you think?
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By the way, someone could complain that speed + direction = V is sloppy notation since you can't add a speed to a direction.
Yeah but you can tell from the context that the plus sign is being used as a logical and rather than a summation off terms, right?
Thank you PMB. Some on these pages have suggested my mathematics is weak, but although it's not great; it got me through electrical engineering.
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Just a muse, but are photons and other parts of the electromagnetic spectrum pushed or pulled after propagation?
Pushing or pulling only applies to situatios where the force is applied to the front or back of an extended object. A photon is a point object so pushing and pulling can't be defined here. Also gravitationa attraction of a body acts on all parts of the body and therefore pushing or pulling also doesn't apply.
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Does pulling really exist, when I couple a cable to say a car to pull it I can interpose a pressure measuring device in the coupling to show that I am really pushing from within the coupling can we say whether the car is being pushed or pulled ?
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Heh, just wrote about that, and the definition of pushing that car becomes extremely complicated to me. What you can do is argue that any 'pull' is a example of some 'bonding' of forces maybe, as if they had became one, instead of two objects. But then the same argument should work for a push. And what is left to define should be what a momentum is.