# Can Light Travel Faster Than Speed of Light

Svensken
Hey!

If you fire a gun standing still the bullet will travel at a certain velocity.
If you fire a gun sitting in an airplane, the bullet's velocity will be speed of plane plus the the velocity due to the gun.

If you fire a bullet with a light being emitted from it: will the velocity of the light be:
The light plus the velocity of the bullet?

guerom00
No, the speed of light will be the same, whatever frame of reference :)
The reason : when you say that “If you fire a gun sitting in an airplane, the bullet's velocity will be speed of plane plus the the velocity due to the gun.” you simply add the two velocities (the one from the plane plus the one from the bullet). This law (adding the two velocities) is only an approximation when the velocities are small wrt the speed of light; it is incorrect when you have to deal with near-speed-of-light velocities.
When you use the correct composition of velocities law, you see that the speed of light stays the same whatever frame of reference :)

Redsummers
If you fire a bullet with a light being emitted from it: will the velocity of the light be:
The light plus the velocity of the bullet?

No, that could never happen, since photons are massless particles and they will always have that same speed, as for:

$$c = \nu\lambda$$

And you could check the ranges of both the wavelength and the frequency in a electromagnetic radiation table and see how they concur on maintaining the $$c$$ constant. Which, however, must not to be confused with the energy of the photon:

$$E = \frac{hc}{\lambda} = h\nu$$

Ali Inam
the bullet's velocity will travel with the velocity of light, but it won't attain its velocity, will it ? !

Obviously, the speed of light is far too much greater to make the velocity of gun compatible with that of light

Redsummers
the bullet's velocity will travel with the velocity of light, but it won't attain its velocity, will it ? !

Obviously, the speed of light is far too much greater to make the velocity of gun compatible with that of light

I don't think I understand your question, but an object such as a bullet to reach the speed of light, infinite energy would be needed. No matter how fast is the bullet traveling, the speed of light c will be the same for any inertial frames of reference moving with the relative velocity $$V$$. According to the velocity transformations of Lorentz:

$$v' = \frac{v - V}{1 - vV/c^2}$$

So, again you can see from here that if you set $$v = c$$, one can only say that the speed of light is the largest, since for $$V > c$$ would lead you to imaginary numbers as a result, if we regard the other lorentz transformations of the velocity.

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Frame Dragger
I realize this isn't "physics", but the title of this thread is a tautology; light can't travel faster than light, because then light would be traveling that fast.

So, maybe thinking about the issue long enough to formulate a thread title would be a good idea?

As for the basic question you're posing, NO. This is the what SR was about, with light flashes on trains and such. I suggest basic research into first SR, and then GR. Your question is the premise upon which relativity is BASED; that invariant of "bullet", the light ALWAYS moves at local light-speed (c in a vacuum). You can't "add" velocity to light, or subtract it, except insofar as the velocity changes in a given medium, which has nothing to do with your question.

EDIT: This is also a question about Relativity, not Quantum Physics. May I suggest you ask a staff member to move this thread to the appropriate forum? In your case, I would suggest the Acadmic Guidence sub-forum, because you really need to be starting at first principles here. (at least, in terms of Relativity and QM)

ManyNames
Light can travel faster than c in the Casimir vacuum:

http://arxiv.org/abs/hep-th/9810221

This does not lead to causal paradoxes, as explained in detail here:

http://arxiv.org/abs/gr-qc/0107091

The difference are virtual photons. I've not read the link, but i know a fair bit to realize i think this is the type of superluminosity of particles of light. For a short time virtual particles can travel faster than light under the uncertainty principle.

ManyNames
Light can travel faster than c in the Casimir vacuum:

http://arxiv.org/abs/hep-th/9810221

This does not lead to causal paradoxes, as explained in detail here:

http://arxiv.org/abs/gr-qc/0107091

Moreover, whilst it may not lead to causal paradoxes, the Casimir Force does, because it violates the most well-known conservation law of energy; This is the fine line between which is classical, and which obeys the uncertainty principle.

guerom00
I am not aware of the fact that the Casimir force violates “the most well-known conservation law of energy” Could you clarify this point ?

ManyNames
Sure.

The Casimir Force is a direct experimental result of which is believed to be a background energy of virtual particles; also known as the Zero-Point Field. It is from here, energy is released spontaneously and created even in a lab. The conservation in which states energy can only change states and cannot be created is therefore violated, since energy apparently can be created from the virtual vacuum which is a resavior of negative energy - an infinite amount presumably.

guerom00
I know what the Casimir force is since it is my research field
I can assure you that there is no violation of any energy conservation principle

ManyNames
I can assure you there is. It's a well-known fact.

The classical theory states no energy can be created. But we can create energy from the vacuum, from virtual to real.

It stands to reason this is a violation of the classical sense.

guerom00
There is no energy created, don't trust those QED mumbo jumbo
Rather, consider Casimir interaction as usual van der Waals interaction (which it is)

ManyNames
Au contrare;

In a recent discussion on this forum, i repeatedly told a person that there is no general consensus whether van der Waals interaction was literally the case of reality. The ZPF is still highly respected, and one must remember that the ZPF was proposed to exist according to the Rules of Quantum Mechanics long before there was any confirmation of its theoretical evidence in the form of the attractive and repulsive Casimir Force.

One can choose either theory - but the ZPF is still i would estimate, the leader of the pack over this new interpretation, which i am not fond of.

Staff Emeritus
I can assure you there is. It's a well-known fact.

No, it's not. (And it takes quite some chutzpah to argue this point with someone who does this for a living!)

There is no more a violation of conservation of energy than finding a rock on a hilltop and rolling it down.

ManyNames
No, it's not. (And it takes quite some chutzpah to argue this point with someone who does this for a living!)

There is no more a violation of conservation of energy than finding a rock on a hilltop and rolling it down.

Yes there is. Taking for granted that the total energy of the universe is still zero, sporadic appearances of energy from the vacuum which is a creation process goes directly against the idea that energy cannot be created by the classical law.

I'm not going to argue about it. You can agree to disagree if this is what it will consist of. I've studied physics for many years now, and i am positive i have a clear grasp on the subject of the zero-point field.

Homework Helper
Gold Member
There is no violation of conservation of energy in quantum mechanics, and quantum mechanics fully accounts for the Casimir force.

Frame Dragger
Sure.

The Casimir Force is a direct experimental result of which is believed to be a background energy of virtual particles; also known as the Zero-Point Field. It is from here, energy is released spontaneously and created even in a lab. The conservation in which states energy can only change states and cannot be created is therefore violated, since energy apparently can be created from the virtual vacuum which is a resavior of negative energy - an infinite amount presumably.

Holy ****, what have you been reading? That sounds like a nasty pastiche of pop-sci notions and your own suppostions, but it has nothing to do with phyics.

And now you're taking on Vanadium. Wow. I am... wow.

EDIT: Wow, I'm really going to miss ManyNames... he knew how to mix blazing idiocy with semi-science and arrogant disregard so well. There should be a little cage on the forum for "The Banned", where we can all go to blow off steam. Oh, and I love PF staff. Nothing like waking up and having a burr magically vanish from one's paw...

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flashprogram
An example of why some people might get confused with regards to the speed of projectiles fired in near lightspeed vehicles:

Suppose you're in a very very long spaceship, traveling at 99.99999% C. You're at the back end of the spaceship and you fire a laser gun, which travels to the front end and kills the pilot just as it was reaching the destination. According to you the laser reached the pilot at the same time the front end reached the destination, but it took 1 year more for the last end of the ship, from where it was fired to reach the destination. So the gun that was at the same distance as the laser took one year longer to reach the destination than the laser, aka, the laser arrived one year ahead.

The key is that it took one year measured by the clocks inside the ship, but from the outside this is not the time difference that is measured.

Holy ****, what have you been reading? That sounds like a nasty pastiche of pop-sci notions and your own suppostions, but it has nothing to do with phyics.

And now you're taking on Vanadium. Wow. I am... wow.

EDIT: Wow, I'm really going to miss ManyNames... he knew how to mix blazing idiocy with semi-science and arrogant disregard so well. There should be a little cage on the forum for "The Banned", where we can all go to blow off steam. Oh, and I love PF staff. Nothing like waking up and having a burr magically vanish from one's paw...

I think there should be an "Aggressively Clueless Turkeys Hall of Fame" for folks like varga (the dude who kept doubting stuff like synchrotron radiation, and turned out not to even know about Maxwell's equations) and ManyNames.

Frame Dragger
I think there should be an "Aggressively Clueless Turkeys Hall of Fame" for folks like varga (the dude who kept doubting stuff like synchrotron radiation, and turned out not to even know about Maxwell's equations) and ManyNames.

Agreed, and don't worry, I won't forget Varga. I showed him a PICTURE of synchroton light and he didn't even a bat an eyelash. That really is AGGRESSIVE cluelesness as you say.

yoda jedi
Hey!

If you fire a gun standing still the bullet will travel at a certain velocity.
If you fire a gun sitting in an airplane, the bullet's velocity will be speed of plane plus the the velocity due to the gun.

If you fire a bullet with a light being emitted from it: will the velocity of the light be:
The light plus the velocity of the bullet?

On the impossibility of superluminal travel
http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.4960v1.pdf

mrmarley217
Light always has the same speed. If you would point a flashlight to the east (direction of the Earth's spin) a person on the sun may say your light is traveling faster than the speed of light, yet it is relative to the earth.
Something more interesting, if a plane would fly at about 99.999999999999999999% of the speed of light, and one would run to the nose of the plain would that person exceed the speed of light? No, time would be slowed down drastically in order to keep anyone/ anything from exceeding the speed of light. This is not theoretical, this is experimentally shown in the LHC using very short lived molecules, these molecules lasted a lot longer at near the speed of light (don't know exactly how long but it was thousands of times).

Tameas001
Light is relative to itself not the object that emits it.

so an object can be traveling at near the speed of light and light will still travel at its speed.

to light the object is not moving no matter what the objects velocity may be.

so if a spacecraft were moving faster than the speed of light, the light within the craft would still light the inside as if the craft were still.

stevmg
Tevye, in "Fiddler on the Roof" made the same point...

"If I were the Tsar,I would be richer than the Tsar," he boasted.

"How so?" asked a listener, "How could you have more than all the power and wealth of the Tsar?"

"Simple," replied Tevye, "I would have all the wealth and power of the Tsar and ...

I would do a little teaching on the side."