# Photon

#### triggernum5

Joined May 4, 2008
216
Ok good then, its all squared away..

#### PatDarden

Joined Aug 23, 2008
4
No I didn't say the mass of the photon is not zero.

I said the rest mass is zero.

What is the definition of a particle "at rest"?
Is that just a theoretical concept or can it actually happen under some circumstances?

/ ... JCD

#### triggernum5

Joined May 4, 2008
216
That doesn't pertain to absolute rest, remember mass is 'relative', so 'at rest' simply means there is 0 velocity in spacial directionsrelative to the measuring observer..
Or, if you understand that, and meant 'is it physically possible for a photon to be at rest with respect to an observer, they have slowed photons down to a few mph through BEC's (Bose-Einstein Condensates), so ignoring the boundary between the condesate, and freer space it is possible, but we could never move as fast as light can through the BEC, or any other medium..

#### PatDarden

Joined Aug 23, 2008
4
Yes, of course. Relative velocity.

Well, it is then that the mass is (as is often inferred from a discussion) that a property of the particle ... or a property of the measurement?

Thanks .. JCD

#### studiot

Joined Nov 9, 2007
4,998
If you want to look deeper than the Lorenz transformations:

Einstein's great insight was to note that certain properties, such as momentum, must calculate the same in all reference systems. This means that certain other properties, such as mass, must vary to accomodate this.

A reference system is simply a co-ordinate system with zero based at an observer. Every observer has a different one. Hence the term (special) relativity.

#### Wendy

Joined Mar 24, 2008
21,929
If a particle is truely massless (which a photon isn't really) would it respond to a gravitational field? I suspect yes, since gravity is bent space/time, but then again...

One of the lovely concepts I've read and thought about is a photon is timeless. All moments are now to it.

#### studiot

Joined Nov 9, 2007
4,998
That's tacyons, Bill.

#### Wendy

Joined Mar 24, 2008
21,929
Not quite. Tachyons are FTL, photons are light speed. This means that their onboard time has stopped relative to ours.

Since photons respond to gravity they have mass. In the book "The Universe and Dr. Einstein" they describe several tons of light falling on a cornfield. Another aspect of the argument is the difference between acceleration and gravity is indistinguishable.

#### studiot

Joined Nov 9, 2007
4,998
I didn't dispute that all travelling photons have mass, or their speed.

I did dispute that they are 'timeless'

Many have tried to use special relativity to propose paradoxes where the order of events are reversed relative to different observers, which obviously cannot be true. The resolution of these 'paradoxes' involves comparing the time that a light signal takes to reach each observer from each event.

Photons carry the signals and very definitely distinguish between event times.

Tachyons on the other hand are credited by scifi authors with possessing the property of containing all signals ever transmitted or ever going to be transmitted. Hence their 'timeless' character.

#### triggernum5

Joined May 4, 2008
216
What Bill is talking about has nothing to do with changing the order of events, or the imaginations of sci-fi writers..
Time is MEANINGLESS from the point of view of a theoretically sentient photon under special relativity.. From its own perspective:
It pops into existance..
Percieves no passage of time..
Realizes it spans the entire universe..
And witnesses all events at the same instant because it never percieves passage of time..

#### BillO

Joined Nov 24, 2008
990

On one side (particle) we have E=mc^2, on the other we have E= h
$image=http://forum.allaboutcircuits.com/mimetex.cgi?\nu&hash=019460512185460887acb4516b093b94$
(wave)

Therefore mc^2= h
$image=http://forum.allaboutcircuits.com/mimetex.cgi?\nu&hash=019460512185460887acb4516b093b94$

So a photon of a given energy as expressed by h
$image=http://forum.allaboutcircuits.com/mimetex.cgi?\nu&hash=019460512185460887acb4516b093b94$
, will have a mass of h
$image=http://forum.allaboutcircuits.com/mimetex.cgi?\nu&hash=019460512185460887acb4516b093b94$
/c^2

Got it?

BTW, from the photon's perspective, they do get from the Sun to Earth in an instant. In fact, to put it in lay terms, photon's would belive they can cross the extent of the universe in an instant.

#### blazedaces

Joined Jul 24, 2008
130

On one side (particle) we have E=mc^2, on the other we have E= h
$image=http://forum.allaboutcircuits.com/mimetex.cgi?%5Cnu&hash=e30feca30f23311e38a8a088afc08c96$
(wave)

Therefore mc^2= h
$image=http://forum.allaboutcircuits.com/mimetex.cgi?%5Cnu&hash=e30feca30f23311e38a8a088afc08c96$

So a photon of a given energy as expressed by h
$image=http://forum.allaboutcircuits.com/mimetex.cgi?%5Cnu&hash=e30feca30f23311e38a8a088afc08c96$
, will have a mass of h
$image=http://forum.allaboutcircuits.com/mimetex.cgi?%5Cnu&hash=e30feca30f23311e38a8a088afc08c96$
/c^2

Got it?

BTW, from the photon's perspective, they do get from the Sun to Earth in an instant. In fact, to put it in lay terms, photon's would belive they can cross the extent of the universe in an instant.
Isn't all information and movement through space-time limited by the speed of light (when I say limited I should probably say always equal to)? As in... even photons need to travel through the universe at the speed of light implying that they can not travel it in an instant?

-blazed

#### BillO

Joined Nov 24, 2008
990
Looking at the Lorenz transform,

x'=x((c-v)/(c+v))^(1/2)
Where c = the speed of light and v = the velocity of the photon through space. We see that measuments (x') the photon makes of space (x) are contracted to 0 since, for a photon, c=v.​

#### beenthere

Joined Apr 20, 2004
15,819
There is a case, called entanglement, where two particles may be induced to carry identical spin states. If one changes state (usually as a result of deliberate observation), the other particle also changes. That change is apparently instantaneous - not limited by light speed.

Link to the Wikipedia article - http://en.wikipedia.org/wiki/Quantum_entanglement.

#### blazedaces

Joined Jul 24, 2008
130
There is a case, called entanglement, where two particles may be induced to carry identical spin states. If one changes state (usually as a result of deliberate observation), the other particle also changes. That change is apparently instantaneous - not limited by light speed.

Link to the Wikipedia article - http://en.wikipedia.org/wiki/Quantum_entanglement.
And this is why quantum mechanics and classical physics have yet to be combined. They say when we can finally combine them we'll have the theory of everything.

According to classical physics, locality is truly absolute... but quantum mechanics shows instances (like the one above) where locality does not hold.

Einstein made many attempts to disprove this, but the more he tried, the more he simply proved it further. Still, there are many theorists out there who believe locality is still holding and these particles are connected in an invisible way, perhaps through different dimensions and such...

But for this argument let's ignore quantum physics for a bit since that'll open up some unnecessary inconsistencies.

As for your response BillO: thanks, that's rather interesting. So from our perspective photons are limited by the speed of light through space, and we are moving through time at our own speed, but from a photon's perspective they aren't moving through time at all, so they can move from one end of the universe to the other in the same instant?

Would that be an accurate description?

#### BillO

Joined Nov 24, 2008
990
Close enough.

Classical Mechanics (CM), Special Relativity (SR) and Quantum Mechanics (QM) are already combined. Think of them as special cases of Quantum Electro Dynamics (QED). Now we just have to combine them with a suitable theory of Gravitation such as General Relativity (GR). However, GR and QM do not get along. Attempts to do this have lead to things like Super String Theory (SST) but the whole mess is very aesthetically unpleasing.

Now throw into that the latest postulates about time, as a traversable dimension, not existing at all and we are in a right mess!

#### steveb

Joined Jul 3, 2008
2,436
There is a case, called entanglement, where two particles may be induced to carry identical spin states. If one changes state (usually as a result of deliberate observation), the other particle also changes. That change is apparently instantaneous - not limited by light speed.

Link to the Wikipedia article - http://en.wikipedia.org/wiki/Quantum_entanglement.
Yes, this is a facinating example of an instantaneous effect. One key thing about this is that this effect can not be used to instantaneously transmit information, matter or energy. You can do an experiment and compare the data later to show that the instantaneous effect happened, but no useful information is instantaneously transferred at the time of the experiment.

Alas, sub-space radio (like in Star Trek) still seems impossible.

#### steveb

Joined Jul 3, 2008
2,436
And this is why quantum mechanics and classical physics have yet to be combined. They say when we can finally combine them we'll have the theory of everything.
Stephan Hawking was able to combine QM and General Relativity in a very limited case at the edge of a Black Hole. Perhaps this is his greatest contribution to Physics. He predicted that Black Holes are not truely black and actually radiate energy (Hawking Radiation).

This is just a glimpse of what mysteries might be unraveled if that theory of everything is established.