The Naked Scientists
  • Login
  • Register
  • Podcasts
      • The Naked Scientists
      • eLife
      • Naked Genetics
      • Naked Astronomy
      • In short
      • Naked Neuroscience
      • Ask! The Naked Scientists
      • Question of the Week
      • Archive
      • Video
      • SUBSCRIBE to our Podcasts
  • Articles
      • Science News
      • Features
      • Interviews
      • Answers to Science Questions
  • Get Naked
      • Donate
      • Do an Experiment
      • Science Forum
      • Ask a Question
  • About
      • Meet the team
      • Our Sponsors
      • Site Map
      • Contact us

User menu

  • Login
  • Register
  • Home
  • Help
  • Search
  • Tags
  • Member Map
  • Recent Topics
  • Login
  • Register
  1. Naked Science Forum
  2. Non Life Sciences
  3. Physics, Astronomy & Cosmology
  4. Does the energy of a photon affect its path through a gravitational field?
« previous next »
  • Print
Pages: 1 [2]   Go Down

Does the energy of a photon affect its path through a gravitational field?

  • 31 Replies
  • 2704 Views
  • 0 Tags

0 Members and 1 Guest are viewing this topic.

Offline Halc

  • Global Moderator
  • Naked Science Forum King!
  • ********
  • 2217
  • Activity:
    26.5%
  • Thanked: 173 times
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #20 on: 10/01/2020 13:21:12 »
Quote from: PmbPhy on 10/01/2020 06:14:29
Quote from: Halc
The path of a massive object is not a function of its momentum.
Not true. objects with differing momentum move on different geodesics. This holds for all tardyons but not luxons.
Unclear to what 'this' refers.  My statement or your statement only holds for tardyons?

As for luxons (and quantum scale tardyons for that matter), per the principle of locality, any quantum scale objects do not have a position until said position is measured, and hence it is meaningless to speak of its path.

Also, per my reply 8 above, quantum scale objects are subject to gravitational refraction which affects their motion in addition to inertial effects, so I would be open to things like photons taking non-geodesic paths.

Quote
What does "mass more" mean??
Has more (frame dependent) mass, as opposed to having more frame independent proper mass.

Quote
Quote from: Halc
and thus pull the larger object a little closer as it passed, thus bending the path a bit more.
That would violate the principle that the rate of fall of an object does not depends on its mass.
First of all, I didn't say 'fall rate', and the principle should be worded that the acceleration of an object does not depend on its mass.  That much I would not deny.  I said a brick will hit the ground before a feather, not because it accelerates more, but rather because the ground does.

This is not obvious when playing with bricks and feathers, but it does very much become obvious with larger things.  Drop a brick from 385,000 km up (where the moon is) and it will take significantly more time (about 7 more minutes) to fall to Earth than stopping (relative to Earth) the moon in its tracks and letting it fall from that altitude.  This is true even if both are point masses.  This illustrates that your principle above is wrong as worded.
My comment you quote above refers to that difference.
« Last Edit: 10/01/2020 13:50:57 by Halc »
Logged
 



Offline PmbPhy

  • Naked Science Forum King!
  • ******
  • 3903
  • Activity:
    0%
  • Thanked: 125 times
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #21 on: 10/01/2020 15:13:54 »
Quote from: Halc
Unclear to what 'this' refers.
The statement that precedes it.

Quote from: Halc
As for luxons (and quantum scale tardyons for that matter), per the principle of locality, any quantum scale objects do not have a position until said position is measured, and hence it is meaningless to speak of its path.
Same holds for geodesics.

Its unwise to group quantum mechanics into a thread abut generality since there's no theory of quantum gr. Nothing in your posts speaks to QM in this threasd.

Quote from: Halc
This is not obvious when playing with bricks and feathers, but it does very much become obvious with larger things.
A feather will only fall faster than a brick if there's a non-gravitational force acting on it like air pressure. Put the two in a vacuum and they will fall at the same rate. It's an odd thing to watch. :)

A perfect example of a geodesics being a function of momentum is a tardyon moving at constant speed in flat spacetime in an inertial frame. The more momentum the faster the particle will move. Draw the geodesic in the spacetime diagram and it will be a strainght liine whose slope depends on the particles momentum and hence its velocity.


Logged
 

Offline PmbPhy

  • Naked Science Forum King!
  • ******
  • 3903
  • Activity:
    0%
  • Thanked: 125 times
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #22 on: 10/01/2020 15:20:57 »
If quantum mechanics is taken into account then as you said, photon's won't move on a classical trajectory. In fact only position measurements can be taken making moving on a worldline of any kind not meaningful, never mind a non-geodesics world line.


Logged
 

Offline Halc

  • Global Moderator
  • Naked Science Forum King!
  • ********
  • 2217
  • Activity:
    26.5%
  • Thanked: 173 times
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #23 on: 10/01/2020 19:31:28 »
Quote from: PmbPhy on 10/01/2020 15:13:54
Its unwise to group quantum mechanics into a thread abut generality since there's no theory of quantum gr. Nothing in your posts speaks to QM in this threasd.
If there's no quantum GR, then you can't speak of how photons (quantum things) behave under GR.  So there very much is quantum effects in GR theory, and in particular: refraction.  Light refracts if its speed is medium dependent, and under GR gravitational wells, it very much is.

Quote
A feather will only fall faster than a brick if there's a non-gravitational force acting on it like air pressure.
I never suggested a feather falling faster than a brick, nor any suggestion of drag.
To be specific, I described how a brick dropped from a height will hit the ground in less time than a feather independently dropped from the same height.
Quote
Put the two in a vacuum and they will fall at the same rate.
Then stop repeating your assertions and tell me why my explanation of the brick getting there first is wrong.  I computed the difference for the moon falling if that helps.  The example is pretty irrelevant for the photon thing, but you're the one that brought up the principle.

Quote
Draw the geodesic in the spacetime diagram and it will be a strainght liine whose slope depends on the particles momentum and hence its velocity.
It's a function of its velocity in this case and nothing else.  If the particle was twice the mass and thus twice the momentum, it would take the same path.  If you knew only its position and velocity, you'd know its path.  If you knew only its position and momentum, you'd not know its path.  That's what I mean when I say its not a function of momentum.  That momentum doesn't define the line slope.

How long does it take for something to travel 1 km at 100 kg-meters/second of momentum?  Can't answer?  Then it's not a function of momentum.
Logged
 

Offline evan_au

  • Global Moderator
  • Naked Science Forum GOD!
  • ********
  • 9185
  • Activity:
    73%
  • Thanked: 915 times
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #24 on: 10/01/2020 21:30:13 »
Quote from: Halc to PmbPhy
tell me why my explanation of the brick getting there first is wrong
I think PmbPhy was measuring in the frame of reference of the  the Center-of-Gravity of Earth +Brick+Feather.
Halc was calculating in the frame of reference of the Center-of-Gravity of Earth+Brick (or Earth + Feather).

Because (Earth+Brick) has slightly higher total mass than (Earth + Feather), the brick will arrive quicker - by an immeasurably small time.
But not quite as quick as (Earth+Brick+Feather) - also by an immeasurably small time.
Logged
 



Offline PmbPhy

  • Naked Science Forum King!
  • ******
  • 3903
  • Activity:
    0%
  • Thanked: 125 times
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #25 on: 10/01/2020 22:49:39 »
Its common place to speak of photons in GR. When this is done they're treated like luxons. A pulse of radiation which is highly located has energy/momentum related by E = pc, just like a photon does. In Taylor and Wheeler's text "Exploring Black Holes" the authors were warned about using photons in GR by Philip Morrison. Its just easier to speak of photons than luxons because most people don't know what a luxon (or a tardyon for that matter) is.

The question as phrased assumed that he was asking about classical GR, not quantum GR. Its best to answer what the person is seeking to learn rather than what a literal interpretation might yield. If unclear then its best to ask. If you feel like giving the QM its best to state it as such. Since he was speaking about curves followed by photons he had luxons in mind.

Evan et al, see - http://www.newenglandphysics.org/physics_world/cm/two_accel.htm
« Last Edit: 11/01/2020 00:45:14 by PmbPhy »
Logged
 

Offline jeffreyH (OP)

  • Global Moderator
  • Naked Science Forum King!
  • ********
  • 6807
  • Activity:
    0%
  • Thanked: 174 times
  • The graviton sucks
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #26 on: 10/01/2020 23:55:56 »
Quote from: PmbPhy on 10/01/2020 06:28:19
Quote from: jeffreyH on 05/01/2020 01:03:47
That is, does a photon's momentum affect its path through the gravitational field in the same way a massive object's velocity would?
These are two different questions. I'll address the one in the subject line.

The answer is no. Think of it like this - Imagine a particular photon (a given amount of energy) passing by the sun. The amount of deflection depends on the initial conditions of the photon (where it was, the direction it was going). Now imagine the same geodesics but this time another photon  moving side by side with the initial one. The geodesic is still a null geodesic as all are which photons more on. But now the amount of energy is twice as much regardless of how close the particles are moving next to each other, even so close as to be one entity for all practice purposes.

The geodesic a massive object (aka tardyon) moves on depends on the objects initial velocity which can vary.


This is all I wanted. A definitive answer. Thanks Pete. The reason I asked was because F = GMm/r^2 cannot work with a photon since it has no rest mass (m). I was wondering if, since energy has an equivalent mass, the photon's energy could be used to derive this mass term. However, it is moot point. The equivalence principle and the photon's constant speed in vacuum rule out any deviation in the geodesic. Thanks for clearing it up.
Logged
Even the most obstinately ignorant cannot avoid learning when in an environment that educates.
 

Offline Halc

  • Global Moderator
  • Naked Science Forum King!
  • ********
  • 2217
  • Activity:
    26.5%
  • Thanked: 173 times
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #27 on: 11/01/2020 00:26:32 »
Quote from: jeffreyH on 10/01/2020 23:55:56
F = GMm/r^2 cannot work with a photon since it has no rest mass (m).
m is mass in that equation, not proper mass.  A photon has mass per what you said:
Quote
the photon's energy could be used to derive this mass term.
Just so.  If not, light could not move objects.  A lit torch (flashlight) in space will accelerate due to the reaction force against the mass of the photons emitted.

Quote
However, it is moot point. The equivalence principle and the photon's constant speed in vacuum rule out any deviation in the geodesic.
I had questioned that in post 8 since the photon going around a strong gravity well can refract, bending its path from the geodesic.  I agree that the geodesic path is the same for any light-speed particle regardless of energy, but I don't necessarily agree that the photon follows such a path.
Logged
 

Offline PmbPhy

  • Naked Science Forum King!
  • ******
  • 3903
  • Activity:
    0%
  • Thanked: 125 times
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #28 on: 11/01/2020 00:42:09 »
Quote from: jeffreyH
This is all I wanted. A definitive answer. Thanks Pete. The reason I asked was because F = GMm/r^2 cannot work with a photon since it has no rest mass (m). I was wondering if, since energy has an equivalent mass, the photon's energy could be used to derive this mass term. However, it is moot point. The equivalence principle and the photon's constant speed in vacuum rule out any deviation in the geodesic. Thanks for clearing it up.
You're most welcome my friend. A photon is deflected by a gravitating body because it has inertial mass (aka rel-mass). This is how Feynman explains it in his Lectures. Ever read that part? Want the reference?

A photon does not have constant speed in a gravitational field. Einstein proved that in the early history of GR. See

The Principle of Relativity, Lorentz, Einstein, Minkowski, Weyl, Dover Pub. See article which starts on page 99,

Here's a derivation which you may find easy to follow. http://www.newenglandphysics.org/physics_world/gr/c_in_gfield.htm
Logged
 



Offline jeffreyH (OP)

  • Global Moderator
  • Naked Science Forum King!
  • ********
  • 6807
  • Activity:
    0%
  • Thanked: 174 times
  • The graviton sucks
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #29 on: 11/01/2020 11:32:24 »
I would appreciate the link to Feynman. Thanks for the other info.
Logged
Even the most obstinately ignorant cannot avoid learning when in an environment that educates.
 

Offline PmbPhy

  • Naked Science Forum King!
  • ******
  • 3903
  • Activity:
    0%
  • Thanked: 125 times
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #30 on: 11/01/2020 18:52:49 »
Quote from: jeffreyH on 11/01/2020 11:32:24
I would appreciate the link to Feynman. Thanks for the other info.
Sorry. I just recallke4d that I lost that volume. It's in V-II
Logged
 

Offline PmbPhy

  • Naked Science Forum King!
  • ******
  • 3903
  • Activity:
    0%
  • Thanked: 125 times
    • View Profile
Re: Does the energy of a photon affect its path through a gravitational field?
« Reply #31 on: 13/01/2020 17:51:30 »
Evan - If Hacl was referring to the center of mass of a rock and feather then the feather will fall faster than the rock.
Logged
 



  • Print
Pages: 1 [2]   Go Up
« previous next »
Tags:
 

Similar topics (5)

What are "energy" and "work" ?

Started by The ChampBoard Physics, Astronomy & Cosmology

Replies: 108
Views: 54424
Last post 31/12/2018 20:54:40
by yor_on
Can matter in a vacuum create dark energy and dark matter effects?

Started by GoscienceBoard Physics, Astronomy & Cosmology

Replies: 1
Views: 8727
Last post 10/05/2011 23:37:24
by yor_on
What is Kirlian photography and Kirlian Energy?

Started by neilepBoard General Science

Replies: 2
Views: 7395
Last post 27/04/2007 15:02:27
by Karen W.
What really is dark energy and dark matter?

Started by Diogo_Afonso_LeitaoBoard Physics, Astronomy & Cosmology

Replies: 5
Views: 6478
Last post 08/09/2017 12:01:01
by puppypower
What are dark matter and dark energy?

Started by ijazBoard General Science

Replies: 5
Views: 6950
Last post 21/01/2016 12:43:52
by alysdexia
There was an error while thanking
Thanking...
  • SMF 2.0.15 | SMF © 2017, Simple Machines
    Privacy Policy
    SMFAds for Free Forums
  • Naked Science Forum ©

Page created in 0.289 seconds with 61 queries.

  • Podcasts
  • Articles
  • Get Naked
  • About
  • Contact us
  • Advertise
  • Privacy Policy
  • Subscribe to newsletter
  • We love feedback

Follow us

cambridge_logo_footer.png

©The Naked Scientists® 2000–2017 | The Naked Scientists® and Naked Science® are registered trademarks created by Dr Chris Smith. Information presented on this website is the opinion of the individual contributors and does not reflect the general views of the administrators, editors, moderators, sponsors, Cambridge University or the public at large.