# The Naked Scientists Forum

### Author Topic: Does light have inertia?  (Read 17881 times)

#### Farsight

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##### Does light have inertia?
« Reply #25 on: 29/11/2009 15:48:35 »
Lightarrow: a mirror isn't so clear cut, because the wavelength isn't reduced.

I don't ignore all that detail PhysBang, not at all. You do, just as you ignore what people say and contribute only spoiler bile. And as for getting banned from "serious forums" time and time again, it isn't true. Care to provide an example to back up your claim? Oh, and I don't think anybody will be impressed by your insinuation that this isn't a serious forum.

#### Bored chemist

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##### Does light have inertia?
« Reply #26 on: 29/11/2009 18:35:17 »
One of the joys of mossbauer spectroscopy is that it relies on the momentum of (gamma ray) photons.
As far as I can tell the difference between inertia and momentum is simply a point of view.
If a car hits me is it my inertia that causes the damage, or the car's momentum?
photons definitely carry momentum as witnessed by spectroscopy, the compton effect and photon pressure.
I don't see how it can avoid having inertia.

#### lightarrow

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##### Does light have inertia?
« Reply #27 on: 29/11/2009 18:57:25 »
Lightarrow: a mirror isn't so clear cut, because the wavelength isn't reduced.
Did you mean the wavelenght or the frequency? Anyway, if you also want to reduce the frequency it's very simple, you make the mirror recede at speed v with respect to the EM wave:
f' = f(c-v)/(c+v).
« Last Edit: 29/11/2009 19:14:38 by lightarrow »

#### yor_on

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##### Does light have inertia?
« Reply #28 on: 29/11/2009 19:02:50 »
As for photons accelerating :)

That's a 'nono'

As for direction changes? Well, what we see after that direction change is in fact not our 'original photon', ah, mainstream seen (if 'photons' now are 'traveling' at all?)

#### PhysBang

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##### Does light have inertia?
« Reply #29 on: 30/11/2009 00:11:56 »
I don't ignore all that detail PhysBang, not at all. You do, just as you ignore what people say and contribute only spoiler bile.
Yes, how awful for people that I warn them that you tell them your own pet theories as if they were scientific gospel, theories that even you admit are rejected by mainstream scientists. And indeed, you admit that you ignore the important details, because you admit that you have done none of the relevant mathematics!
Quote
And as for getting banned from "serious forums" time and time again, it isn't true. Care to provide an example to back up your claim?
http://forum.richarddawkins.net/viewtopic.php?f=9&t=91298&p=2261402#p2261402
http://www.bautforum.com/forum-rules-faqs-information/30979-baut-banned-suspended-posters-log-23.html#post1628425
http://www.bautforum.com/forum-rules-faqs-information/30979-baut-banned-suspended-posters-log-23.html#post1632224
Quote
Oh, and I don't think anybody will be impressed by your insinuation that this isn't a serious forum.
I didn't say that this wasn't a serious forum. They may ban you, too, but banning you is not a pre-requisite for being a serious forum.

#### Farsight

• Sr. Member
• Posts: 396
##### Does light have inertia?
« Reply #30 on: 30/11/2009 01:08:48 »
Quote from: Lightarrow
Did you mean the wavelenght or the frequency? Anyway, if you also want to reduce the frequency it's very simple, you make the mirror recede at speed v with respect to the EM wave:
f' = f(c-v)/(c+v).
Sorry, I meant frequency. I prefer Compton scattering because it's just a free electron and a photon, it's cleaner.

Quote from: yor_on
As for photons accelerating, That's a 'nono'. As for direction changes? Well, what we see after that direction change is in fact not our 'original photon'
I don't like photons accelerating either. But what can you do? Re original photons, if all you've got is a photon and a free electron, there's no bond there to absorb and re-emit the photon.

Thin gruel, Physbang. A partial bar on Dawkins for mentioning a book (the irony!), and a temporary suspension on Baut for not answering questions, despite answering about two hundred. I contribute to physics discussions. Might be an idea if you did too, instead of being a stalker and a troll intent on spoiling them.

#### Mr. Scientist

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##### Does light have inertia?
« Reply #31 on: 30/11/2009 15:33:46 »
Quote from: Lightarrow
Did you mean the wavelenght or the frequency? Anyway, if you also want to reduce the frequency it's very simple, you make the mirror recede at speed v with respect to the EM wave:
f' = f(c-v)/(c+v).
Sorry, I meant frequency. I prefer Compton scattering because it's just a free electron and a photon, it's cleaner.

Quote from: yor_on
As for photons accelerating, That's a 'nono'. As for direction changes? Well, what we see after that direction change is in fact not our 'original photon'
I don't like photons accelerating either. But what can you do? Re original photons, if all you've got is a photon and a free electron, there's no bond there to absorb and re-emit the photon.

Thin gruel, Physbang. A partial bar on Dawkins for mentioning a book (the irony!), and a temporary suspension on Baut for not answering questions, despite answering about two hundred. I contribute to physics discussions. Might be an idea if you did too, instead of being a stalker and a troll intent on spoiling them.

Bolded by me.

Why farsight? The idea of a photon accelerating works very well with the equivalances set by relativity... I think it's noval.

#### Farsight

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##### Does light have inertia?
« Reply #32 on: 30/11/2009 17:20:19 »
Because if you contrive multiple Inverse Comptons so that the photon ends up travelling in its original direction albeit it with a increased frequency, that means you've accelerated it. But it's still going at the same speed. It's acceleration Jim, but not as we know it!

#### Mr. Scientist

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##### Does light have inertia?
« Reply #33 on: 30/11/2009 17:44:14 »
Because if you contrive multiple Inverse Comptons so that the photon ends up travelling in its original direction albeit it with a increased frequency, that means you've accelerated it. But it's still going at the same speed. It's acceleration Jim, but not as we know it!
Because if you contrive multiple Inverse Comptons so that the photon ends up travelling in its original direction albeit it with a increased frequency, that means you've accelerated it. But it's still going at the same speed. It's acceleration Jim, but not as we know it!

There is always the mathematical relative speed law of motion. Acceleration of a photon is no more incomplete than having a vacuum accelerate at speeds faster than c.

#### Pmb

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• Physicist
##### Does light have inertia?
« Reply #34 on: 02/12/2009 06:32:12 »
As for photons accelerating :)

That's a 'nono'

As for direction changes? Well, what we see after that direction change is in fact not our 'original photon', ah, mainstream seen (if 'photons' now are 'traveling' at all?)
That is incorrect in general. A photon can accelerate when its moving in a gravitational field. This is due to an modified coordinate speed and a change to the time it takes the photon to move.
« Last Edit: 02/12/2009 06:34:30 by Pmb »

#### yor_on

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##### Does light have inertia?
« Reply #35 on: 02/12/2009 18:37:08 »
Maybe I was unclear.

A photon will not accelerate inside a perfect vacuum.

But you can use a laser in another medium, for example a plasma, and then witness a laser frequency upshifting that is equivalent to photon acceleration.

But the main thing here is that its speed won't change no matter what you do to it, and no matter what 'medium' you do it in as far as I know, the only thing you will change is its frequency/energy relative your frame of reference.

#### Luke Daniel Borel

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##### Re: Does light have inertia?
« Reply #36 on: 29/10/2016 20:48:21 »
Usually "inertia" refers to "inertial mass" and a beam of light has zero mass.

For me to understand this question better, I would need to know this; when someone fires a high power laser beam, does a force push back on the device that fired the laser beam, in a direction opposite of the direction of the laser beam.

#### evan_au

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##### Re: Does light have inertia?
« Reply #37 on: 29/10/2016 23:01:54 »
Quote from: LukeDaniel
when someone fires a high power laser beam, does a force push back on the device that fired the laser beam, in a direction opposite of the direction of the laser beam.
Yes.
This was recently answered on the podcast: http://www.thenakedscientists.com/HTML/podcasts/qotw/show/20161024-1/

#### The Naked Scientists Forum

##### Re: Does light have inertia?
« Reply #37 on: 29/10/2016 23:01:54 »