Why can't you go faster than light

[Homer J Simpson]

[digsys]

From the comments ...
" All those interested in time travel, meet here last Thursday. "
:-)

[tggzzz]

Things going faster than light happens all the time. Here's a picture of one example:


Perhaps you want to modify the thread's title?

[Rerouter]

Faster than light in a vacuum, or exceeding "c" is the hard one, Certain experiments have brought light quanta to a complete stop inside a bose-einstein condensate, at that point every motion you make is faster than "that" light

Exceeding "c", that requires abusing weird physics, like a Alcubierre drive,

[nctnico]

Faster than light is relative. There are analog oscilloscopes which sweep the beam across the screen at speeds faster than light.

[TerraHertz]

Things going faster than light happens all the time. Here's a picture of one example:

Physics would be interesting indeed, if c was a local variable.

https://en.wikipedia.org/wiki/Cherenkov_radiation

[tggzzz]

Faster than light is relative. There are analog oscilloscopes which sweep the beam across the screen at speeds faster than light.

No it isn't, and "everybody" does the latter everyday. Well not with an oscilloscope, but with a torch or headlights. The classic pedagogical example is sweeping a torch across the surface of the moon.

As I'm sure you realise, the limit to do with the speed of light (in a vacuum) c relates to particles. When a spot of light moves faster than c, there are no particles moving faster than c.

[JohnnyMalaria]

Faster than light is relative. There are analog oscilloscopes which sweep the beam across the screen at speeds faster than light.

No it isn't, and "everybody" does the latter everyday. Well not with an oscilloscope, but with a torch or headlights. The classic pedagogical example is sweeping a torch across the surface of the moon.

As I'm sure you realise, the limit to do with the speed of light (in a vacuum) c relates to particles. When a spot of light moves faster than c, there are no particles moving faster than c.

Huh?

What is a spot of light vs. particles of light? How can one but not the other exceed the speed of light?

[Gyro]

To quote the great Sir Terry Pratchett:

“Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it.”

[tggzzz]

Faster than light is relative. There are analog oscilloscopes which sweep the beam across the screen at speeds faster than light.

No it isn't, and "everybody" does the latter everyday. Well not with an oscilloscope, but with a torch or headlights. The classic pedagogical example is sweeping a torch across the surface of the moon.

As I'm sure you realise, the limit to do with the speed of light (in a vacuum) c relates to particles. When a spot of light moves faster than c, there are no particles moving faster than c.

Huh?

What is a spot of light vs. particles of light? How can one but not the other exceed the speed of light?

I'll presume you are asking how a spot of light can move faster than c.

Point a laser at a wall 1m away, and rotate yourself so that the spot moves across the wall at 1m/s. Continue rotating as the same angular velocity, but remove that wall and replace it with a building 1km away; by geometry the spot is moving cross the building at 1000m/s. Now rotate at the same angular velocity but point the beam at the moon 384402km away. The beam will move at 384402000m/s across the moon's surface. c is "only" 299792458m/s. QED.

[JohnnyMalaria]

Things going faster than light happens all the time. Here's a picture of one example:


Perhaps you want to modify the thread's title?

That's a case of going faster the speed of light in a material (water) which is not the same as going faster than c.

[tggzzz]

Things going faster than light happens all the time. Here's a picture of one example:


Perhaps you want to modify the thread's title?

That's a case of going faster the speed of light in a material (water) which is not the same as going faster than c.

Well twigged! Did the big hint in my second sentence help you?

[JohnnyMalaria]

Faster than light is relative. There are analog oscilloscopes which sweep the beam across the screen at speeds faster than light.

No it isn't, and "everybody" does the latter everyday. Well not with an oscilloscope, but with a torch or headlights. The classic pedagogical example is sweeping a torch across the surface of the moon.

As I'm sure you realise, the limit to do with the speed of light (in a vacuum) c relates to particles. When a spot of light moves faster than c, there are no particles moving faster than c.

Huh?

What is a spot of light vs. particles of light? How can one but not the other exceed the speed of light?

I'll presume you are asking how a spot of light can move faster than c.

Point a laser at a wall 1m away, and rotate yourself so that the spot moves across the wall at 1m/s. Continue rotating as the same angular velocity, but remove that wall and replace it with a building 1km away; by geometry the spot is moving cross the building at 1000m/s. Now rotate at the same angular velocity but point the beam at the moon 384402km away. The beam will move at 384402000m/s across the moon's surface. c is "only" 299792458m/s. QED.

No, the spot isn't a physical object. There's nothing moving.

https://en.wikipedia.org/wiki/Lighthouse_paradox#Resolution_of_the_paradox_in_special_relativity

[JohnnyMalaria]

Well twigged! Did the big hint in my second sentence help you?

Honestly, I didn't get the second sentence. I assumed I had come to the party late (sub-c travel) and the title had already been changed (if that's even possible)

[RoGeorge]

Nice video, except it doesn't cover the "Why" part from its title. 

[Vtile]

Things going faster than light happens all the time. Here's a picture of one example:


Perhaps you want to modify the thread's title?

Cherenkov radiation. 

[GlennSprigg]

As a possible 'Twist' on the 'suggestion' by TGGZZZ throwing a philosophical spanner-in-the-works  ,
when talking about the 'moving' SPOT of light, (which of course has no speed as such  ).....
I thought I would throw THIS into the thought process, of those that 'ponder' the interesting..........

As an Elect./Tech., I've always hated it when 'people' think/talk about ELECTRICITY moving at light speed !!!!
NOPE !!!!  There is NO correlation !  Yes, a 'signal' may 'appear' almost instantaneously at the other end
of say a 100 mile long wire, but this has nothing to do with it.  Imagine a TUBE filled with say PingPong
balls, and you 'push' one more in one end. The last one 'pops' out the other end, due to resultant forces !
(I know this is an extremely simplified analogy, which varies due to 'many' atomic/electron effects).

Depending on many various factors, I will 'quote' purely an idealized example for the sake of discussion....
A WIRE, carrying a DC current of say 10-Amps at 100v, (actually the voltage is not relevant), MAY only have
the 'Electrons' in it, (electricity is flow of electrons), moving along the 'wire' at about 1 to 10 Millimetres/Sec !!!
It is the CASCADING effect that produces the 'apparent' speed !!!

In fact, for AC, like in your House, be it 50hz or 60hz, the SAME electrons have been 'vibrating' back and
forth within even a short piece of 'cable' indefinitely, and going nowhere 

[tggzzz]

Faster than light is relative. There are analog oscilloscopes which sweep the beam across the screen at speeds faster than light.

No it isn't, and "everybody" does the latter everyday. Well not with an oscilloscope, but with a torch or headlights. The classic pedagogical example is sweeping a torch across the surface of the moon.

As I'm sure you realise, the limit to do with the speed of light (in a vacuum) c relates to particles. When a spot of light moves faster than c, there are no particles moving faster than c.

Huh?

What is a spot of light vs. particles of light? How can one but not the other exceed the speed of light?

I'll presume you are asking how a spot of light can move faster than c.

Point a laser at a wall 1m away, and rotate yourself so that the spot moves across the wall at 1m/s. Continue rotating as the same angular velocity, but remove that wall and replace it with a building 1km away; by geometry the spot is moving cross the building at 1000m/s. Now rotate at the same angular velocity but point the beam at the moon 384402km away. The beam will move at 384402000m/s across the moon's surface. c is "only" 299792458m/s. QED.

No, the spot isn't a physical object. There's nothing moving.

https://en.wikipedia.org/wiki/Lighthouse_paradox#Resolution_of_the_paradox_in_special_relativity

Please don't assume I am saying things that I am not saying. My statements are correct within their limitations - and it is the limitations that expose the sloppy wording/thinking of some other statements. With that in mind...

The spot is indeed moving!

You can see similar things in the heavens, at a larger scale, e.g. https://www.newscientist.com/article/2131889-weird-energy-beam-seems-to-travel-five-times-the-speed-of-light/

[G7PSK]

I have often wondered if the reason we cannot see dark matter is due to it travelling faster than light. That would then pose the question would we ever be able to detect something travelling faster than light.

[tggzzz]

I have often wondered if the reason we cannot see dark matter is due to it travelling faster than light.

Perhaps for the same reason that we can't detect the luminiferous æther?

[donotdespisethesnake]

I don't know about light, but morons always seem to type responses quicker than knowledgeable people can reply

[Rick Law]

Faster than light is relative. There are analog oscilloscopes which sweep the beam across the screen at speeds faster than light.

No it isn't, and "everybody" does the latter everyday. Well not with an oscilloscope, but with a torch or headlights. The classic pedagogical example is sweeping a torch across the surface of the moon.

As I'm sure you realise, the limit to do with the speed of light (in a vacuum) c relates to particles. When a spot of light moves faster than c, there are no particles moving faster than c.

Huh?

What is a spot of light vs. particles of light? How can one but not the other exceed the speed of light?

I'll presume you are asking how a spot of light can move faster than c.

Point a laser at a wall 1m away, and rotate yourself so that the spot moves across the wall at 1m/s. Continue rotating as the same angular velocity, but remove that wall and replace it with a building 1km away; by geometry the spot is moving cross the building at 1000m/s. Now rotate at the same angular velocity but point the beam at the moon 384402km away. The beam will move at 384402000m/s across the moon's surface. c is "only" 299792458m/s. QED.

No, the spot isn't a physical object. There's nothing moving.

https://en.wikipedia.org/wiki/Lighthouse_paradox#Resolution_of_the_paradox_in_special_relativity

Please don't assume I am saying things that I am not saying. My statements are correct within their limitations - and it is the limitations that expose the sloppy wording/thinking of some other statements. With that in mind...

The spot is indeed moving!

You can see similar things in the heavens, at a larger scale, e.g. https://www.newscientist.com/article/2131889-weird-energy-beam-seems-to-travel-five-times-the-speed-of-light/

The spot is indeed moving, however, the spot is not a physical object but an optical reflection.  What moved is not anything on the surface of the moon, but where the reflection (of your moving light beam) occurred.

This is like shooting a billiard ball at one corner of the table and shoot; then from the same spot, aim another ball at another corner of the table.  The two balls hit the rim and reflect back, one following another.  The point where it hits (contacts the rim) moved from one corner to the other - but the table itself did not move, the table rim did not move.  Nothing else physical moved except where the two balls hit.

You can reduce that example further.  With adjustment for gravity and movement, aim one bullet at the left side of the moon, and then aim another at the right side of the moon.  Visually, the spot where the two bullet hits "moved" from the left to the right faster than light.  The crater formed by the bullet-moon collision on the left did not "move" to the right side.  Nothing moved but the two different bullets hitting two different points.

Most important part of not being a physical object is that it has no mass.  Things with zero mass can move as fast as light.  Photon has zero mass but it does carries momentum.

To move faster than light, back to hunting for tachyons, worm holes, or warp-bubbles...

[JohnnyMalaria]

The spot isn't moving because it doesn't exist. All that exists are the photons being scattered by the surface and reaching the observer. If you turned the light off before pointing it at the second location on the moon and then turned it back on, you wouldn't make the same claim, would you? What about if you have the light flash on and off a couple of times? Or a thousand? Or a million? It would make no difference. What if you had two light sources at the same location? One turned on and pointed at one location on the moon and the other turned off and pointed at another. If you switch the first light off and the second one on would you make the same claim about the spot moving?

And what about a shadow? Can a shadow move faster than light? No.

[tggzzz]

The spot isn't moving because it doesn't exist.

If spots of light don't exist, how come they are used in electronic test equipment?

I remember using spot-reflecting ballistic galvanometers in school, to measure charge. An video example of something you think doesn't exist:

You can still buy them:
http://www.bnrexports.com/laboratory-instruments-equipments-suppliers-india.html?cat_id=106

And what about a shadow? Can a shadow move faster than light? No.

Yes it can.

[hamster_nz]

Have a think about what a (tall) person on the moon would see if the spot moved just over 'c'....

With a really high speed camera i think they would see two spots, both moving away from them - one moving away much faster than the other.

[JohnnyMalaria]

You are assigning a physical attribute to something that is virtual.

A spot isn't a tangible thing and you can't touch a shadow. You can touch the physical object that is being obscured from a light source.

Think what a spot is. It is the scattering of a very large number of photons from a surface (or interface between dissimilar materials) over a concentrated area and that propagate to an observer. A photon leaves the source, travels to the surface and the scattered photon reaches your eye. On a large enough timescale it seems as if the spot is continuous and moves when the light source moves. But if you turn the light source off, the spot disappears. Every time you observe the spot, it is a different set of photons. There's no connection between the spot at t=0 and t=something whether the light is on all the time or just for a fraction of the second at the beginning and end.

[tggzzz]

You are assigning a physical attribute to something that is virtual.

A spot isn't a tangible thing and you can't touch a shadow. You can touch the physical object that is being obscured from a light source.

You'll have to specify what you mean by "virtual" and "tangible" - and check whether you are using them in the same sense as other people.

You can't touch a rainbow, or brockenspectre, or a sun dog, or even the sky. Does that mean they don't exist?

Think what a spot is. It is the scattering of a very large number of photons from a surface (or interface between dissimilar materials) over a concentrated area and that propagate to an observer. A photon leaves the source, travels to the surface and the scattered photon reaches your eye. On a large enough timescale it seems as if the spot is continuous and moves when the light source moves. But if you turn the light source off, the spot disappears. Every time you observe the spot, it is a different set of photons. There's no connection between the spot at t=0 and t=something whether the light is on all the time or just for a fraction of the second at the beginning and end.

Irrelevant.

[JohnnyMalaria]

Irrelevant.

No it's not.

If you are male, have a full bladder, take a whiz outside and pee in one place, you'll get a puddle (unless you're standing over a drain or the edge of a cliff). If you point Percy at a different location, you'll get another puddle. If you wiggle your thingy back and forth then you'll see that the stream of water is actually a series of discrete droplets (single chamber choked flow atomization). If you do it fast enough then the puddle of water will seem to move fast - faster than it should. But each puddle is made by pee droplets emitted in that direction. How the first puddle came about has absolutely no relation with the second. There is no correlation. This is what's irrelevant.

Of course, in this experiment you'd have to be able to pee like a shire horse with tremendous velocity sitting on a merry-go-round and spinning at very high speed. 

(If you're not male then you are missing one of life's wondrous gifts )

[CopperCone]

can you help my dick started to glow blue after I conducted this experiment???

[JohnnyMalaria]

North Pole in January.

[tggzzz]

Irrelevant.

No it's not.

If you are male, have a full bladder, take a whiz outside and pee in one place, you'll get a puddle (unless you're standing over a drain or the edge of a cliff). If you point Percy at a different location, you'll get another puddle. If you wiggle your thingy back and forth then you'll see that the stream of water is actually a series of discrete droplets (single chamber choked flow atomization). If you do it fast enough then the puddle of water will seem to move fast - faster than it should. But each puddle is made by pee droplets emitted in that direction. How the first puddle came about has absolutely no relation with the second. There is no correlation. This is what's irrelevant.

Of course, in this experiment you'd have to be able to pee like a shire horse with tremendous velocity sitting on a merry-go-round and spinning at very high speed. 

(If you're not male then you are missing one of life's wondrous gifts )

Sigh.

You are confusing how fast the drops travel to their landing place (radial velocity) with how fast the landing place moves (tangential velocity).

The radial velocity is, of course, limited by the speed of light in a vacuum.

The tangental velocity is, of course, omega*r where omega is the angular velocity in radians/s, and r is the radius at which the drops land. Since r can be arbitrarily large, the tangental velocity can also be arbitrarily large.

[hamster_nz]

On deeper analysis you will find there is no one 'landing spot'... Each projectile has it's own landing spot determined at the time of release, and heads straight for it, following the laws of motion.

There is no 'tangental velocity'. All projectiles move straight away from the source unless under the influence on an external force.

The location of a landing spot is a position that is a function of time. If you measure the rate of change in position you might get a number greater than the speed of light, but nothing is actually moving that fast.

[tggzzz]

The location of a landing spot is a position that is a function of time. If you measure the rate of change in position you might get a number greater than the speed of light, but nothing is actually moving that fast.

That paragraph is internally contradictory.

[hamster_nz]

The location of a landing spot is a position that is a function of time. If you measure the rate of change in position you might get a number greater than the speed of light, but nothing is actually moving that fast.

That paragraph is internally contradictory.

Not at all.

Can you tell me what is the mass (or energy if you like) of the thing that might be moving at greater than the speed of light?  Does it have any momentum?

I think you will find it has zeo mass, zero energy and zero momentum. It is a nothing - so it can change position as fast as it wants.

[tggzzz]

The location of a landing spot is a position that is a function of time. If you measure the rate of change in position you might get a number greater than the speed of light, but nothing is actually moving that fast.

That paragraph is internally contradictory.

Not at all.

Can you tell me what is the mass (or energy if you like) of the thing that might be moving at greater than the speed of light?  Does it have any momentum?

I think you will find it has zeo mass, zero energy and zero momentum. It is a nothing - so it can change position as fast as it wants.

All that is correct[1] - but it isn't what you wrote!

Einstein didn't state that nothing can travel faster than the speed of light in a vacuum. He did state, to put it in loose modern terms, that you cannot convey information (i.e. particles, waves) faster than the speed of light in a vacuum.

Except, of course, that now measurements relating to quantum entanglement somewhat muddy the waters

[1] Except for the spot of light being "a nothing". Funny how people can and do use "nothings" to make physical measurements

[RoGeorge]

[apis]

Except, of course, that now measurements relating to quantum entanglement somewhat muddy the waters

Sadly no, you can't use entanglement to transmit information faster than the speed of light.

As you say the modern version of nothing travels faster than c is you can't send information faster than c, where c is a physical constant that happens to be equal to the speed of light in vacuum.

[NiHaoMike]

No it's not.

If you are male, have a full bladder, take a whiz outside and pee in one place, you'll get a puddle (unless you're standing over a drain or the edge of a cliff). If you point Percy at a different location, you'll get another puddle. If you wiggle your thingy back and forth then you'll see that the stream of water is actually a series of discrete droplets (single chamber choked flow atomization). If you do it fast enough then the puddle of water will seem to move fast - faster than it should. But each puddle is made by pee droplets emitted in that direction. How the first puddle came about has absolutely no relation with the second. There is no correlation. This is what's irrelevant.

Of course, in this experiment you'd have to be able to pee like a shire horse with tremendous velocity sitting on a merry-go-round and spinning at very high speed. 

(If you're not male then you are missing one of life's wondrous gifts )

Why can't a girl do it?

[Homer J Simpson]

[Vtile]

https://youtu.be/14ToSwTAgT4

QED 

[RandallMcRee]

The spot isn't moving because it doesn't exist.

Nobody can argue with you or make you change your mind, here: you do not understand the english language the same as the rest of the respondents and readers.

Normally, when you say something does not exist that literally means it cannot be perceived. But obviously the moving spot is perceptual. So what *you* say does not make sense to the rest of us!

Think back to the light beam being traced out by the CRT on an analog scope. Folks have already established that no law of physics is broken by a faster than light moving beam. This beam hits the CRT and causes some of its particles to glow. Those particles are not themselves traveling! They just glow in response to the beam. So, very cleverly, we see the path of the beam, travelling at (perhaps) faster than light, using particles (CRT) that are not travelling at all. Our clever ancestors.

Make sense?

[CatalinaWOW]

No it's not.

If you are male, have a full bladder, take a whiz outside and pee in one place, you'll get a puddle (unless you're standing over a drain or the edge of a cliff). If you point Percy at a different location, you'll get another puddle. If you wiggle your thingy back and forth then you'll see that the stream of water is actually a series of discrete droplets (single chamber choked flow atomization). If you do it fast enough then the puddle of water will seem to move fast - faster than it should. But each puddle is made by pee droplets emitted in that direction. How the first puddle came about has absolutely no relation with the second. There is no correlation. This is what's irrelevant.

Of course, in this experiment you'd have to be able to pee like a shire horse with tremendous velocity sitting on a merry-go-round and spinning at very high speed. 

(If you're not male then you are missing one of life's wondrous gifts )

Why can't a girl do it?

She can.  But her aiming apparatus is not as flexible.

[Brumby]

A spotlight, laser, scope trace, etc. does not consist of anything moving in a lateral direction.  In fact, there IS no movement of anything visible in a lateral direction at all.

What is being observed is a series of photons which emanate from different locations at different times.  The distance between any two locations and the time between emissions from those two locations do not combine to create a velocity of any single element of the system.  Anything of this sort being called "motion" is perceived motion - exactly the same as we see every day on television.

[tggzzz]

Anything of this sort being called "motion" is perceived motion

And with that we enter the realm of philosophy, not physics.

[cncjerry]

The inverse of the laser on the moon is looking from one side to the other by turning your head.  Or pointing your willie at one side of the moon then the other, depending on if you have one.  So I have proven that only men can exceed the speed of light with their genitals.

[RoGeorge]

The only problem will be the length: shrinks to zero when approaching the speed of light.
 

[Rick Law]

The only problem will be the length: shrinks to zero when approaching the speed of light.
 

You cannot shrink length gradually to zero.  Length is quantized.  The smallest length measurement is called a Plank Length which is ~1.62x10E-35 meter.  So you can shrink length one quantum (a Plank Length) at a time but you cannot shrinking it in smaller increments.

[JohnnyMalaria]

Think back to the light beam being traced out by the CRT on an analog scope. Folks have already established that no law of physics is broken by a faster than light moving beam. This beam hits the CRT and causes some of its particles to glow. Those particles are not themselves traveling! They just glow in response to the beam. So, very cleverly, we see the path of the beam, travelling at (perhaps) faster than light, using particles (CRT) that are not travelling at all. Our clever ancestors.

Make sense?

No. That's just pure nonsense.

And, as a native of England living in the US, I sleep comfortably at night knowing that the inability to comprehend the language lies with its adopters, not its creators.

Heaven help us all if we get on to the Twin Paradox.

[German_EE]

"Only two things travel faster than light, gossip and starships named Enterprise"

Leonard McCoy MD

[Mr. Scram]

Nice video, except it doesn't cover the "Why" part from its title. 

It does briefly, but the title is fairly clickbaity.

[Mr. Scram]

So some of you are saying that when I have a long row of people and start a Mexican wave by having everyone stand up at the same or almost the same time, the Mexican wave travels faster than light?

[apis]

Can a shadow move faster than light? It can, but you can't use that effect to send information faster than the speed of light. (That is the modern version of the law.)

[Rick Law]

Can a shadow move faster than light? It can, but you can't use that effect to send information faster than the speed of light. (That is the modern version of the law.)

You can't send information faster than light using shadow(s) ---- because any change you make to the shadow can only get to the target with the light that has the shadow - at the speed of light.

[CopperCone]

i think my physics teacher almost brought in a hose to classroom to 'demonstrate' on certain students the whole shadow/laser pointer point moving faster then light if you point away to a distant object

[rfeecs]

So this time he comes up with ways to go faster than light, kinda.  Some of them have been mentioned already in this thread.

https://youtu.be/BhG_QZl8WVY

[Dubbie]

So some of you are saying that when I have a long row of people and start a Mexican wave by having everyone stand up at the same or almost the same time, the Mexican wave travels faster than light?

And how are you going to tell the participants when it is time to stand up? 

[basinstreetdesign]

No it's not.

If you are male, have a full bladder, take a whiz outside and pee in one place, you'll get a puddle (unless you're standing over a drain or the edge of a cliff). If you point Percy at a different location, you'll get another puddle. If you wiggle your thingy back and forth then you'll see that the stream of water is actually a series of discrete droplets (single chamber choked flow atomization). If you do it fast enough then the puddle of water will seem to move fast - faster than it should. But each puddle is made by pee droplets emitted in that direction. How the first puddle came about has absolutely no relation with the second. There is no correlation. This is what's irrelevant.

Of course, in this experiment you'd have to be able to pee like a shire horse with tremendous velocity sitting on a merry-go-round and spinning at very high speed. 

(If you're not male then you are missing one of life's wondrous gifts )

Why can't a girl do it?

She can.  But her aiming apparatus is not as flexible.

Point taken, as it were.

[ruffy91]

So some of you are saying that when I have a long row of people and start a Mexican wave by having everyone stand up at the same or almost the same time, the Mexican wave travels faster than light?

And how are you going to tell the participants when it is time to stand up? 

Give them watches and tell them when to stand up. Where is the problem?

[Dubbie]

Give them watches and tell them when to stand up. Where is the problem?
[/quote]

There is none. That will teach me to comment without reading the post properly.

[Mr. Scram]

Give them watches and tell them when to stand up. Where is the problem?

Alternatively, arrange them in a semicircle and use light or sound to indicate when.