Flashlight at the speed of light

[Janus58]

No. For all viewers, (any inertial reference frame) the photons always travel at same C (in vacuum).

What happens is the flashlight for, observers which see the flash light traveling near C and pointed at them, the flashlight is emitting gamma rays as the "blue shift" is extreme.

Any inertial frame measure light moving at c relative to themselves. Thus in the frame from which you measure the flashlight as moving at 0.99999 c, the following happens:

Flashlight passes point A and turns on. After 1 sec the light will have traveled 299,792,458 meters from point A. In that same second, the flashlight will have traveled 299,789,460.1 meters in the same direction, and the front of the flashlight will be only 2997.9246 meters behind the the wavefront of the light. If the observer is in the path of the light he will see it blue-shifted by a factor of 447.2 He will also see events occurring at the flashlight happening at this sped up rate too. So, if the flashlight, by its clock, switched itself on and off at a rate of 1/10 of a sec ( 1/20 of a second on and 1/20 of a sec off), The observer in the path of the flashlight will see blue shifted pulses of light 1/8944 of a sec long with 1/8944 of a sec gaps between them.

 

[Q-reeus]

Aha! But:

What about the double slit experiment? Photons can interfere with each other rather strongly under the right circumstances.

So they can at least interfere with each other then, no?

Warning! Warning! You quoted danshawen. Nice chap in a way. But nice does not equate to right. It was Dirac who first insisted each photon interferes only with itself. Under careful conditions modern quantum optics experiments can demonstrate multi-photon interference - but according to standard QM/QED, that does NOT apply to double-slit experiment or anything remotely similar. Interference pattern from single-photon-at-a-time sources confirm that definitively to most folks satisfaction.

The first linked to article in #35 deals with a rather complex system involving atomic absorption/emission coupling to a glass fiber resonator - hardly relevant to your earlier implicitly in vacuo scenario. Second article is a bit closer in that the scattering interaction (not really interference) is in vacuo. But requires gamma ray photon energies in the center of energy frame where two such meet head on. If an electron-positron pair result, automatically they will have less than c speeds in any frame. So nothing to be gained in a quest for tachyon speeds. Bummer, as you Americans say.

 

[Beer w/Straw]

I meant the sun has been there for a long time and that predictions of photon interaction would have been observed before I was born...

 

[GeoffP]

Warning! Warning! You quoted danshawen. Nice chap in a way. But nice does not equate to right. It was Dirac who first insisted each photon interferes only with itself. Under careful conditions modern quantum optics experiments can demonstrate multi-photon interference - but according to standard QM/QED, that does NOT apply to double-slit experiment or anything remotely similar. Interference pattern from single-photon-at-a-time sources confirm that definitively to most folks satisfaction.

The first linked to article in #35 deals with a rather complex system involving atomic absorption/emission coupling to a glass fiber resonator - hardly relevant to your earlier implicitly in vacuo scenario. Second article is a bit closer in that the scattering interaction (not really interference) is in vacuo. But requires gamma ray photon energies in the center of energy frame where two such meet head on. If an electron-positron pair result, automatically they will have less than c speeds in any frame. So nothing to be gained in a quest for tachyon speeds. Bummer, as you Americans say.

I'm not American. Am I in a quest for tachyon speeds? All right. I read that a tachyon is any particle accelerated beyond lightspeed, so presumably that would be photons also. So while physicists have got photons to interact in a glass tube, they have not done so in vacuo, and in vacuo photons never interact. All right. But in the glass, if they do do this, then is it not theoretically possible beyond the glass, with enough engineering? I apologize: I'm not a physicist.

Also, what are the advantages of in vacuo? Does this just reflect a kind of parallel to in vitro vs. in vivo? I suppose that the idea to "bumper" a photon does require multiple photons. But is this... special or limiting in scope or something?

 

[Beer w/Straw]

You're also ignoring me.

 

[Q-reeus]

I'm not American.

Pardon please. Maybe your avatar somehow suggested it. Whatever. I guess wolves in dog's clothing (or is it the other way round?) are ubiquitous.

Am I in a quest for tachyon speeds?

All right. I read that a tachyon is any particle accelerated beyond lightspeed, so presumably that would be photons also.

My reading of #15.

So while physicists have got photons to interact in a glass tube, they have not done so in vacuo, and in vacuo photons never interact.

Never is not correct. In extreme environments like the core of a newly formed neutron star, gamma-ray photons are basically always scattering off each other. But super rare for that to happen in normal situations. That second linked Wiki article of your #35 explains under what conditions photons do mutually interact.

All right. But in the glass, if they do do this, then is it not theoretically possible beyond the glass, with enough engineering? I apologize: I'm not a physicist.

In another thread was discussed why photons go slower in glass, and one sensible explanation is that in glass you really don't just have a photon but a polariton which involves a collective excitation of many polarized atoms. Interactions between polaritons are possible that in vacuo are never observed.

Also, what are the advantages of in vacuo? Does this just reflect a kind of parallel to in vitro vs. in vivo? I suppose that the idea to "bumper" a photon does require multiple photons. But is this... special or limiting in scope or something?

The medium, whether vacuum or glass, sets the speed of light therein. Interactions owing either to high energy gamma-ray collisions in vacuo, or non-linearity in say glass, redistribute energy (photon frequencies and/or particle production) but cannot alter the fundamental constraint on c set by a given medium.
[Slight correction; dispersion occurs in say dielectric media, which means c is a generally modest function of photon frequency. So it's more accurate to say photons of a given frequency always propagate at c for that frequency in a given media. There is no dispersion in vacuum.]

In yet another thread, Cherenkov radiation came up. Not photons, but charged particles e.g. electrons moving faster than c in say water may be thought of as 'tachyonic' in a limited sense but it's a far cry from a 'genuine' tachyon moving faster than c in vacuo. 'Tachyonic' high-energy electrons in a medium slow down as they emit radiation, whereas a 'real' in vacuo tachyon would speed up. Anyway photons simply refuse to move at anything other than c in a given medium. Massless particles are all like that: https://en.wikipedia.org/wiki/Massless_particle
And I'm burning midnight oil. Bye.

 

[GeoffP]

You're also ignoring me.

Could you explain more about the sun thing, there, Britney? I'm familiar with its work, but not how it applies here.

Edit: ah, I see. Well, I didn't see that. If I'd seen it, I would have commented on it. You see? Anyway, if those interactions are going on, I don't know that anyone has measured them.

Q-reeus: thanks for the post. If c is limiting then I suppose that's it. No Star Trek for anyone; and thus, no green women.

Way to go, joykill.

 

[billvon]

No. For all viewers, (any inertial reference frame) the photons always travel at same C (in vacuum).

Correct. But the source may APPEAR to be moving far faster than the speed of light, due to the very fact that the light coming from the object never travels faster than light. Do the math; at speeds very close to the speed of light, the light from an approaching light source arrives very close to the time of arrival of the object itself. Thus the light observed while the flashlight is a light-year away arrives in a year; the object itself arrives shortly thereafter.

Let's take an example. An approaching object at 99.99% of the speed of light is first observable when it is a light-year away. The light from the object takes a year to reach the observer. The object itself takes a year plus 53 minutes to arrive at the observer. Thus the observer first sees the light from the object (which at that point is a light-year away) 53 minutes before the object arrives. Thus, from the observer's perspective, the object APPEARED to travel a light-year in 53 minutes; and all the light emitted during its travel APPEARS to be concentrated (and blue-shifted) within that 53 minutes.

What happens is the flashlight for, observers which see the flash light traveling near C and pointed at them, the flashlight is emitting gamma rays as the "blue shift" is extreme.

Right.

 

[tashja]

Hi Geoff. Interesting question. I want in, too

I have a stupid question. You can't push a flashlight up to light speed anyway. But could you 'knock' a photon with another photon somehow to push it up to beyond lightspeed? If so, what happens?

Before an interaction, as well as after any interaction with another particle of any type, each *free* photon *in vacuum* moves at the speed of light c. After an interaction the photon's momentum p (a vector) and its energy E=|p|c both change (|p| is the length of the vector). However its velocity v (a vector) which is given by the ratio of momentum to energy, v=cx(pc/E), always satisfies |v|=c, for any momentum before or after the collision.

Itzhak Bars

 

[danshawen]

Has observing the sun done anything?

Yes, the sun is traveling at relativistic speed with respect to cosmological distance galaxies as well, and would be red shifted from their point of view, as they are from ours.

You can't turn it off, and it probably does produce entangled photons in its interior as well, but it may take several million years for one to indirectly emit light from the photosphere. Now if you could entangle solar neutrinos (or if they are already entangled, which is likely), you might have something interesting. Nothing stops them. They fly off in every conceivable direction. But they are not superluminal either.

 

[James R]

Brian:

James you have been greatly missed sometimes I wonder if any member has died yet death is a beautiful thing...hmm I was giving MR something to think about in hopes that's he may have reached a resolution about his questions in the op.

Sorry. Do I know you?

GeoffP:

Although I will say this: hitting a photon 'from behind' with another photon wouldn't be possible unless the first photon is moving slower than the second. And I appreciate that light is a wave, as James reminds us - but isn't it also a particle? Double slit?

It's more of a "wavicle" - something that acts either like a particle or a wave, depending on how you choose to observe it.

 

[Beer w/Straw]

Could you explain more about the sun thing, there, Britney? I'm familiar with its work, but not how it applies here.

You see ,well, the sun emits photons. You see of course,

 

[GeoffP]

GeoffP:

It's more of a "wavicle" - something that acts either like a particle or a wave, depending on how you choose to observe it.

Well, that's unfair. To whom can I protest?

You see ,well, the sun emits photons. You see of course,

I do. I just can't see whether or not the photons are hitting each other to boost some of them up past c. That's what I'm asking, ultimately, in the interests of space exploration.

 

[origin]

I do. I just can't see whether or not the photons are hitting each other to boost some of them up past c. That's what I'm asking, ultimately, in the interests of space exploration.

Even if you assume that a photon can easily hit another photon, since they ONLY travel at c, one photon cannot possibly run into the back of another photon.

 

[GeoffP]

Even if you assume that a photon can easily hit another photon, since they ONLY travel at c, one photon cannot possibly run into the back of another photon.

I agree: I was hoping one could slow one down a little and hi it from behind, maybe. But the approach to c is asymptotic.

 

[Beer w/Straw]

Maybe use your brain for once.

 

[GeoffP]

You know, I don't dislike you. But you leave me in the lurch when you say "well, what about the sun? duh!". I'm talking about photons hitting other photons, or more exactly being made to hit other photons so as to violate c. But my cyclopean friend James R tells me that can't be done.

"Apparently, now light's a 'wavicle'."​

Telling me "hey - the Sun!" doesn't help me, unless photons coming from the sun should hit each other and don't even though they should. And maybe they should, if every burning particle in the sun emits photons simultaneously in all directions. Maybe they do. I've already been told that light just forms waves and that photons avoid each other, maybe because they're all jerks. And maybe I'm a geneticist and don't spend a lot of time with physics. Personally, I think it's an interesting question. Ultimately, we're going to have to violate c at some point, in some way.

 

[origin]

Ultimately, we're going to have to violate c at some point, in some way.

We are? Why? So that we can have a universe like Star Wars or Star Trek? Unfortunately the universe does not care (at least I don't think it cares) what we want. It may be that it is simply not economically or physically feasible to get off this rock and explore other star systems. It would be kind of sad, but it is what it is.

That would also be the answer to the question, "if there are other intelligent beings in the universe where are they?" They are on their home planet unable to visit us.

 

[GeoffP]

Look, is this crankery? It doesn't seem like crankery, although I couldn't say how actually knocking a photon past c with a collision (presumably mitigated artificially by a combination of physical effects exerted on both photons as in slit influences or, say, electromagnetics) would help James Kirk take a tour of alien babes out near Rigel, although I'm certain we all look eagerly forward to that day. And maybe the thread is the wrong place for the question, since it was originally about flashlights. Surely, without regressing to my earlier arguments too much, this is not something beyond our theoretical or practical engineering? Can this really not be done? Are photons, much to my confusion, so ghostly and intangible as to be beyond the reach of mankind?

Come on, boffi- oh I already said that.

 

[Russ_Watters]

Photons cannot collide because they can occupy the same point.