Flashlight at the speed of light

It could be approaching very fast from far away. If it were only as far away as the Sun, you would have about 20 minutes to get out of its way.

 

Log in or Sign up to hide all adverts.

more boffins:

Please Register or Log in to view the hidden image!

 

Log in or Sign up to hide all adverts.

Right. But you won't see it coming until it is very, very close to you. Say it's launched from the Sun at 99.9999% of the speed of light. If you were in Earth orbit, you wouldn't see it until it was 4 milliseconds from impact. (930 miles away)

 

Log in or Sign up to hide all adverts.

It could be a very large, intense flashlight, and I didn't mean coming directly from the direction of the sun. An instrument like Hubble is sensitive enough to detect an amount of light equivalent to that of a single candle burning at a distance of 100 miles. Mind you, the exposure time to do that may exceed 20 minutes.

I'm just saying, it's not necessarily impossible to dodge something traveling your way that fast. If you plug in actual numbers and time dilation factors you can easily verify this. I already did so using online calculators when this thread began. If it takes 0.1 seconds to turn on a flashlight beam in the rest frame, then even at .99999 the speed of light, it only takes about a second to do so. In the interval of 20 light minutes, there would still be about 20 light seconds left to get out of the way after the beam was detected.

 

If it was running that whole time - yes. However that still might not help you. From your perspective the flashlight is approaching you at many times the speed of light, and you are not seeing its real trajectory - you are seeing its trajectory as it looked a long time ago (relative to where it is now.) So you might see something approaching from the other side of the sun, plot its course, and think "wow, it's going twenty times the speed of light* - but fortunately it will miss me on that trajectory." Then you see its trajectory begin to curve, and at the point at which you realize it is going to hit you, you have only milliseconds to dodge it.

If you have a good estimate of its true speed you can predict its actual trajectory more accurately, of course.

(* - this is, of course, an illusion.)

 

Neither the flashlight nor the photons it emits may exceed the speed of light in a vacuum. Where do you get "many times"? The light emitted by the flashlight is Doppler shifted to the blue end of the spectrum; it doesn't travel faster than c. The photons also do not simply hang around the front of the flashlight until the flashlight itself arrives, unless you have miscalculated or misinterpreted what I suggested. The photons from the perspective of the relativistic flashlight proceed at the speed of light ahead of the flashlight in exactly the same way as they would from any flashlight you picked up and turned on where you are right now. And you and the galaxy we are in are already traveling at relativistic speeds with respect or relative to galaxies that are cosmologically distant. Point the flashlight at those, and the light is red shifted but still travels at c. Point the flashlight at something approaching (like Andromeda) and the light will be blue shifted when it arrives at Andromeda, but still travels at c.

I meant 0.99999000000 times the speed of light. Not faster. Not "many times" faster. It would take an incredible amount of thrust applied over a very long time interval to accomplish pushing something as massive as a flashlight any faster than that, and the limit is still 1.0 c. Converting almost all of the mass of the flashlight into thrust energy leaving it at the speed of light would in fact be required.

Sorry, but this thread seems to have degenerated to about the level of previous threads on the subject. If you don't understand relativity, go back and learn it, or at least learn enough of it to ask better questions. Don't get your relativity lessons from watching old episodes of Star Trek. It doesn't work that way.

 

But billvon is talking about the perspective of the person for which the flashlight is traveling a 0.9999c and is in the path of the flashlight. For him, the light travels at c with respect to him and thus separates from the flashlight at just a fraction of light speed. If the flashlight turns on when it is 10 light min away, the light arrives 10 min later, but the flashlight arrives in 10 min and 0.006 sec later. From the time he first sees the light, he has just that fraction of a sec before the flashlight reaches him.
You can also come to this same conclusion working from the Flashlight frame. The light travels away from the flashlight at c, but the person is rushing towards the flashlight at 0.99999c, so the closing speed between light and person is 1.99999c. In addition, the 10 light min distance as measured by the person is length contracted to 2.683 light sec in the frame of the flashlight so this is the distance between the flashlight and person when the light is fists switched on. This means that the light and person will meet 1.341 sec after the flashlight turns on. At this time the flashlight and person will have closed to 1.341 light secs from each other. It will take the person 1.3417 sec to close the remaining distance to the flashlight (according to the flashlight's clock). The person's clock will be time dilated, and tick off 0.006 sec in that time. So again, by his clock, the person has 0.006 sec from the time the light hits him until he collides with the flashlight.

 

I know they generally don't, but surely there's some fancy way to interact with them that could give them some transient property involving matter, or of a second party element that does have matter, or some bloody thing. I reiterate: come on (physicist) boffins.

 

The light from the front of the flashlight never appears to separate from the flashlight at a FRACTION of light speed to any observer in any state of motion. If it did, that would violate relativity. C is constant to all observers. This is the single assumption relativity makes, and it has been tested scientifically and found to be as binding a law as we know about anything, whether you understand it or not.

You can't "see" a beam of light in advance of its arrival. Once a beam of light has left the flashlight from your position, you nay never "catch up" to it or see any part of it again unless the beam strikes a mirror and travels back along the path from which it came, whatever that means (but then the photon is "bound", whatever that means). The point is, it is wrong to even think of the velocity of the flashlight and its beam to behave like any other velocity vectors with respect to Euclidean vector addition. You can't do that. Nature does not allow it.

 

Insisting on mystical magical 'physics' leads down a dark alley.

 

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

Photons are bosons, and as such, may occupy the same physical space at the same time as other bosons or fermions, whatever that means. This is evidently a very flawed concept, but for the purposes of this thread, it is accurate enough. Fermions, unlike bosons, cannot occupy the same space at the same time, a property that is shared with other particles that have mass with the possible exception of the Higgs boson which is itself responsible for imparting inertial mass to itself, quarks, electrons, W and Z bosons, and their antiparticles.

If you are beginning to see through the veneer of physics that is not understood in any real depth in either domain, you are not hallucinating. Both of these interpretations of photons interacting or not need a lot of maintenance, as does reconciling relativity to explain certain quantum effects.

 

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.

 

Aha! But:

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

I appreciate I'm a geneticist, not a physicist - but there's something about this idea that has a ring of veracity for me. It just seems right, somehow. Can it genuinely not be done in some way? I have never been nearer to picking up a physics book than right now.

 

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?

Although it also seems that this has possibly been done already:

http://www.iflscience.com/physics/quantum-breakthrough-scientists-get-two-photons-interact
https://en.wikipedia.org/wiki/Two-photon_physics

 

Lets say a large fraction of the speed of light. Perhaps a spacecraft transiting to another star.

I think that one of the axioms of special relativity is that the speed of light is the same for all observers. I believe that the beam emitted by the flashlight would seem to be moving at the speed of light to observers on both earth and on the spacecraft. Of course, the observers on earth would think that it's moving more slowly relative to the spacecraft, since the spacecraft is already moving at a large fraction of the speed of light S, so if the beam is traveling at the speed of light C relative to earth, it would seem to earthly observers to only be traveling at C - S relative to the spaceship. But it would still appear to be moving at C when observed by spaceship crew members.

OK, what answer do we have for that seeming contradiction? In special relativity, I believe that the answer is that time wouldn't appear to earthly observers to progress at the same rate on earth and on the spaceship. It would look to earthly observers as if time was slowed down on the spaceship. Everything would still seem perfectly normal on the spaceship though. The second-hand on their watches would still seem to move at the familiar rate relative to them.

I expect that it would look that way to earthly observers if the spaceship could travel at precisely the speed of light. Time would appear to have slowed to a stop on the spaceship.

It might look that way to earthly observers if the spaceship could exceed the speed of light. I guess that's why tachyons, hypothetical superluminal particles, are sometimes imagined (at least in science-fiction) as propagating backwards in time. (Science fiction sometimes has the future sending messages to the past with modulated tachyon-beams.)

 

The flashlight APPEARS to be traveling at many times the speed of light. It is not ACTUALLY traveling at many times the speed of light. It APPEARS to be traveling many times the speed of light since you see the light from the object when it is far away only milliseconds before you see the light from the object when it is right in front of you. That's also why it is harder to dodge. It's not just hard to see it before it is close to you - it is impossible. Only by having some prior knowledge of its course can you have significant warning of its approach.

From the perspective of the flashlight, they travel normally at 1C. But from a stationary observer's perspective they do indeed "bunch up" in front of the flashlight, since they are also traveling at 1C, which is only a tiny amount faster than the flashlight.

 

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.

 

Has observing the sun done anything?

 

Looking...

...it's still there. Am I meant to get more than that?