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## Photon mass

A long time ago, when I was just coming out of my Physics class examination at college, I had kind of an "inspiration" ; after long calculations, I came out with a sentence which sumarized quite well my thought:
"If photons had a mass, time travelling would be possible".

I don't remember how I arrived to this final sentence; I just remember it was related to how mass is related to energy and speed in Relativity Theory.
But I am not able to rebuild the path which led me to the sentence. I remember I noticed it down on a piece of paper I had in muy pocket... but it got lost years ago.
Now it came back to my mind upon reading a thread in this forum.

Can anybody help me to "track back" my "intuition"?

2. They do not., and you can not...
Truth is I can not help you put together what thoughts led you down this path.
but if this helps... Photons do have energy but not once you stop one. A search through this forum will reveal much on this subject.

3. Originally Posted by jumpjack
Can anybody help me to "track back" my "intuition"?
Just a guess, but you thought: according to GR, mass increases with speed. So if the photon has a mass, and goes at the speed of light, then GR doesn't work for it. Consequently, there could be ways for other things to overcome the same barrier.

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Originally Posted by Jens
Just a guess, but you thought: according to GR, mass increases with speed. So if the photon has a mass, and goes at the speed of light, then GR doesn't work for it. Consequently, there could be ways for other things to overcome the same barrier.
yes, that was about mass growth upon speed growth.
As photons have the "maximum speed" (light), they have "minimum mass" (zero).

What's the GR formula which relates mass to speed? That one with the square root...

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Something is coming back from memory (and from google... ).
"Time dilation": the faster you move, the slower the time "moves", right?
So, for particles moving at light speed, time should be... stopped?!?
No body can be brought to light speed, except photons, which actually are NOT bodies having no mass.

Continuing googling&thinking...

6. I have seen this before. Its silly and dangerous. Stop this 'thinking' you will hurt your self.
For a considerable period we have known that mater can not travel at C. The energy required for that to happen are not available in this universe.
Light photons are not mater. What more do you need to know?
It might be nice if you were to know some thing that could change the fact. You do not. and any thing else in regard to this question is so far away from the mainstream of science I think its ATM all the way to the fiction books.

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Originally Posted by astromark
I have seen this before. Its silly and dangerous. Stop this 'thinking' you will hurt your self.
For a considerable period we have known that mater can not travel at C. The energy required for that to happen are not available in this universe.
Light photons are not mater.
Actually... yes & no. As you know, they behave both like waves & bodies, at the same time.

And Pure Thinking widens mind!

8. ## No and no.

Originally Posted by jumpjack
Actually... yes & no. As you know, they behave both like waves & bodies, at the same time.

And Pure Thinking widens mind!

Information expands the mind. Thinking is a process of thought building ideas and expression. Processing the information known is thinking. Fiction is a work of the imagination.
The description of light waves is demonstrated well as is the particle stream also accurate descriptions of the manor of light photons at work... Bodies was the wrong word. It is not my intention to squash your enthusiasm. From ideas such as these does come some advancement. I will be glad to withdraw and apologize if you ever actually crack this...Mark.

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Another "clue": light slowed down in Bose-Einstein condensate:
http://newton.ex.ac.uk/aip/physnews.415.html

Maybe it does not apply to my "study"... or maybe yes?
Who knows?

All relativity theory is based on light speed to be constant and maximum possible; now it looks like it is NO MORE constant?!? A photon can "pass over" another photon while "running" toward a "target"??
Does time flowe faster or slower for the slower photon w.r.t. the faster one?
Are the photons "aged", or doesn't the time pass for them, as they travel exactly at light speed??

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New intuition
A photon has negative mass.
Being "pushed" up to light speed, its mass increases... till it reaches 0: that's why photons appear not to have a mass!

ok, my brain is burning out...

11. Originally Posted by jumpjack
yes, that was about mass growth upon speed growth.
thatīs a very popular fallacy. "Relativistic mass" doesnīt exist, mass doesnīt "grow upon speed growth". Energy does

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## Pesky Moles

OP, yours is the best example (this week) of ideas that are like the pop-up mole game at the carnival. If photons have any rest mass, it causes so many problems to pop-up in other areas of physics , and by implication cosmology, that you'll never get them all beaten down.

But they're still fun ideas to kick around. Just like (shudder) FTL travel, it makes great fiction, but it's not going to happen in the real world and universe.

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JumpJack, as Astromark mentioned, a search through this forum will reveal much, including many threads where you'll find all the information on this you seek.

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Originally Posted by John Mendenhall
OP, yours is the best example (this week) of ideas that are like the pop-up mole game at the carnival. If photons have any rest mass, it causes so many problems to pop-up in other areas of physics , and by implication cosmology,
... for example explaining where all that damned hidden "dark matter" and "dark energy" are?

Calling something "dark" because you can't see it but it "must" exist is weird...

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## Pelted With Dark Matter

Originally Posted by jumpjack
... for example explaining where all that damned hidden "dark matter" and "dark energy" are?

Calling something "dark" because you can't see it but it "must" exist is weird...
Yes, I know. I don't particularly care for DM. but it's the best explanation available so far. Just think of it as a place holder while the theorizing and observing continues. If there is a lot of non-interacting and invisible mass distributed around real matter, then the galaxy rotation curves and most other problems are taken care of. But I wish there was a more direct detection . . .

16. Calling something "dark" because you can't see it is weird? Do you have something more apt in mind?

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Originally Posted by jumpjack
...after long calculations, I came out with a sentence which sumarized quite well my thought:
"If photons had a mass, time travelling would be possible."
A photon's rest mass is zero. Since EM energy also exerts both a gravitational attraction, and imparts momentum into any object into which it's absorbed, or from which it's reflected, it has a "mass-equivalent" (my term, for lack of seeing it described elsewhere).

The energy of a photon is proportional to it's frequency, given by:

E = h * f = h * c / λ

where h is Planck's contant, λ is the wavelength, and c is the speed of light.

Similarly the photon's momentum is given by:

ρ = E / c = h * f / c= h / λ

Thus, to find a photon's mass-equivalence:

E = h * f = h * c / λ = m * c2

Solving for m, we get:

m = h * f / c2 = h * c / (λ * c2)

Reducing:

m = h * f / c2 = h / (λ * c)

Interestingly, from this we can solve for f and obtain the relationship between f, λ, and c:

f / c2 = 1 / (λ * c)

or

c2 / f = λ * c

Thus,

f = c2 / (λ * c)

and

f = c / λ

I'm thinking about re-terming "mass-equivalence" to "energy-mass equivalence," as any equivalence is due to the energy of the particle, not it's mass (of which there is none).

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This reminds me of Roger Penrose's newish idea about how the universe will one day have zero matter because it will all have decayed into radiation. His theory is that at that point the universe loses track of time and the BB process repeats again.

http://news.bbc.co.uk/1/hi/programme...lk/4631138.stm

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Ok, I think we should "attack" my assertion from another direction: what does prevent us from being able to travel in time?
Physics laws, as far as I know, do NOT deny possibility of time travels: it's just a matter of inverting "time-arrow", "time direction", or whatelse you wanna call it.
But actually we DON'T travel in time. Why?
What do I need to go back to yesterday?

What would it happen to a body pushed up and over light speed, if just it was possible? I remember some "urban legends" talking about over-light speed is required to travel in time: does this assertion have some (distorted) scientific origin?

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I just had a flash. I have to think about it, but in the meantime...

Time slows down as much as speed increases, up to light speed, when time stops.
Over lightspeed, time should go backward.
It's not possibile to cross light speed due to mass growth with speed; infact, photons, which HAVE light speed, does NOT have mass, thus they can travel at light speed.
But if they HAD mass, and as they DO travel at light speed, then it IS possibile to increase body speed up to light speed, thus stopping body's time.

Ok, it's not exactly "time travelling", but at least this "means" you can also stop your time/car, rather than just staying on it while running:I just invented... brakes!
Is it possibile to invet "back-gear"?

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Originally Posted by jumpjack
Ok, I think we should "attack" my assertion from another direction:
what does prevent us from being able to travel in time?
You need to answer some much more basic questions, first:
What do you mean by "travel in time"? If you travelled to, say,
July 1, 2008, where would you go? Where is July 1, 2008 located?
In what direction would you travel in order to reach it?

-- Jeff, in Minneapolis

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Originally Posted by mugaliens
E = h * f = h * c / λ = m * c2
E is not equal to m * c2 . That is only true for particles with rest mass.

All particles must obey E2 = p2c2 + m2c4 . For a photon, we declare 0 rest mass because it would have infinite momemtum for actually travelling at c. Now we set m=0 and get
E = pc for light

I think we should be extremely careful when talking about a mass equivalence for a photon. I restrict myself to speaking of its momentum only.

edit: fixed quote formatting
Last edited by alainprice; 2008-Jul-07 at 09:24 PM.

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Originally Posted by mugaliens

E = h * f = h * c / λ = m * c2

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Just because photons do not have mass does not mean there is a problem
comparing the energy of a photon with the energy of an equivalent mass.
Energy comes in a variety of forms. Mass is one, photons are another.
The relationship between them is shown by E=h*f and E=mc^2. Energy
is energy, whether it is in the form of photons or bricks. Just don't expect
photons to behave like bricks, and vice-versa.

-- Jeff, in Minneapolis

25. [QUOTE=alainprice;1277341]
Originally Posted by mugaliens
E = h * f = h * c / λ = m * c2
[\QUOTE]

E is not equal to m * c2 . That is only true for particles with rest mass.

All particles must obey E2 = p2c2 + m2c4 . For a photon, we declare 0 rest mass because it would have infinite momemtum for actually travelling at c. Now we set m=0 and get
E = pc for light

I think we should be extremely careful when talking about a mass equivalence for a photon. I restrict myself to speaking of its momentum only.
And what is p?

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p=h/λ=E/c=hf/c

27. Originally Posted by What Max
p=h/λ=E/c=hf/c
from mugaliensī post:
All particles must obey E2 = p2c2 + m2c4

isnīt that: Eē= mēvēcē+mēc4

Then again from mugaliensī post:
For a photon, we declare 0 rest mass

ergo: Eē=0 ???

And as I said somewhere else: whatīs the meaning of "rest mass"? Whatīs the meaning of "relativistic mass"? Thereīs no rest mass, and no relativistic mass, thereīs only mass, invariant, independent of the frame of reference
And yes, for a photon E=h*f
But why do we need any mass-considerations concerning photons?

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its best to forget classical momentum (p=mv)

29. yes, p = momentum... Something like light (photons) has energy even without mass, as to what prohibts time travel, that's up to debate I believe that the laws of physics don't allow it, even if we don't know that now... For if it were there would be several pardoxes that would result...

30. Originally Posted by jumpjack
Time slows down as much as speed increases
...Relative to a 'stationary' observer. Relativity requires that you specify the frame of reference of your measurement. A traveller moving close to c would feel nothing strange about time in her spaceship.

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