# Are quantum mechanical effects faster than speed of light?

#### kannan_k

For example - When an electron is excited from an inner shell to an outer shell in an atom, it does the quantum jump between the states. Pretty much it disappears from inner state and appears in outer state. Isn't this happening at faster than light? (The velocity is actually infinity for quantum jumps) That means the electron travelled across the distance between the 2 states at faster than light. I know this may not be observable or usable for anything now. But isn't it still a fact that faster than light travel is possible in universe?

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#### Bill_K

That means the electron travelled across the distance between the 2 states at faster than light.
You must be thinking of the "old" quantum theory, in which the electron travels in an orbit, each state at a different radius, and in order to get from one state to the other it has to "jump".

In Schrodinger quantum mechanics, an electron state is represented by a cloud of probability. The states overlap, and a transition from one to another takes place at a single point.

#### Simon Bridge

Homework Helper
But isn't it still a fact that faster than light travel is possible in universe?
There are situation where things can go faster than light - but electron transitions are not one of them.

iirc:
* the expansion of the Universe means that distant objects can be retreating faster than light
* if Alice travels to your left at 4c/5 and Bob travels to your right at 4c/5 then you will observe Alice and Bob's separation growing at 8c/5 > c.
* if you point a laser beam at a screen you get a dot - if you jiggle the laser a little bit, the dot jiggles a bigger bit - how big depends on how far away. Sufficiently far away and the dot moves faster than light.
* some people describe quantum entanglement as involving information travelling faster than light

To go FTL, you need to obey relativity and obey causality.

@BIll_K: I thought the electron "shell"-transitions where basically EM dipole interactions (well, the kind OP is talking about) ... so the time frame is of order 10-23s anyway - so even in the "old QM" it's not faster than light?

I should check - an atom is of the order of an angstrom across ... time for light to cross an atom is 10^-10/3x10^8 ~ 10^-19 > 10^-23 darn!

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#### mfb

Mentor
Dipole interactions with visible light have a corresponding timescale of at least ~500nm/c, which is certainly longer than the time light needs to cross an atom (<1nm/c).
Some transitions have a higher energy, but then the orbitals are smaller, too.

#### San K

* if you point a laser beam at a screen you get a dot - if you jiggle the laser a little bit, the dot jiggles a bigger bit - how big depends on how far away. Sufficiently far away and the dot moves faster than light.
The dot does not move FTL. It's not the same dot (or set of photons).

#### mfb

Mentor
The position of the dot moves. This position is not an object and no information (or anything else) is transferred along the dot, so it does not violate special relativity.

#### Maui

Are you saying that if I shine a laser beam towards Pluto and wait 30 minutes so that it makes a dot, I can move the laser source and the dot will move ftl? Clearly this is impossible and a misconception, as we only have photons that travel at c and their source.

#### Doc Al

Mentor
Are you saying that if I shine a laser beam towards Pluto and wait 30 minutes so that it makes a dot, I can move the laser source and the dot will move ftl?
Sure. So what?

Clearly this is impossible and a misconception, as we only have photons that travel at c and their source.
It's not impossible. The misconception is thinking that a "dot of light" is a single thing that moves. It's better to think of the moving dot as really new dots that are being formed in different spots along the surface, so it looks like something is moving.

#### Maui

Sure. So what?

It's plain wrong.

It's not impossible. The misconception is thinking that a "dot of light" is a single thing that moves. It's better to think of the moving dot as really new dots that are being formed in different spots along the surface, so it looks like something is moving.

False. The dot is made of photons and no photons can move ftl, whatever the mode of their transition from one place to the next.

#### Doc Al

Mentor
Are you saying that if I shine a laser beam towards Pluto and wait 30 minutes so that it makes a dot, I can move the laser source and the dot will move ftl?
Sure. So what?
Let me clarify my response. Shine the laser beam towards Pluto, sweeping it across the surface. When light finally gets there (it will take longer than 30 minutes), the "dot" will sweep across the surface in the time it took you to sweep your laser beam from one side to the other, thus faster than light.

Don't point at Pluto and wait and then move the laser. You'll of course have to then wait for the light to reach the other side.

#### Maui

Let me clarify my response. Shine the laser beam towards Pluto, sweeping it across the surface. When light finally gets there (it will take longer than 30 minutes), the "dot" will sweep across the surface in the time it took you to sweep your laser beam from one side to the other, thus faster than light.

No, the dot will remain in the same place another 30 minutes or so after you move your source. After those 30 minutes, the dot will move across the surface. Remember, we always live somewhat in the past because of c. That's not ftl.

#### Doc Al

Mentor
It's plain wrong.
Perhaps you don't know what is meant by a moving dot?

False. The dot is made of photons and no photons can move ftl, whatever the mode of their transition from one place to the next.
Nothing is really moving faster than light of course, but the "dot" which is not a stable "thing", looks like it's moving faster than light.

If you measured the position of "the dot", it would be "traveling" along the surface of Pluto at faster than light speed. (The moon is a more common example.)

Of course what we call "the dot" is really just a series of reflections of the light as it hits different parts of the surface. No problem in having that series of reflections appear to move faster than light.

#### Maui

Perhaps you don't know what is meant by a moving dot?

Nothing is really moving faster than light of course, but the "dot" which is not a stable "thing", looks like it's moving faster than light.

If you measured the position of "the dot", it would be "traveling" along the surface of Pluto at faster than light speed. (The moon is a more common example.)

Of course what we call "the dot" is really just a series of reflections of the light as it hits different parts of the surface. No problem in having that series of reflections appear to move faster than light.

See my previous post, the dot will not move ftl. It's made of photons and no photons have ever been observed to move ftl. You only have a source, photons and SR. Nothing in your set up allows ftl movement of the dot.

#### Doc Al

Mentor
No, the dot will remain in the same place another 30 minutes or so after you move your source.
Right.

After those 30 minutes, the dot will move across the surface.
Right. At what speed? (That's where the apparent faster than light speed comes in.)

Next time the moon is out, do this: Point your finger at the moon, sweeping across its surface from left to right, which will take a fraction of a second. If you replace your finger with a laser beam, you will have created a dot on the moon's surface (once the light gets there) that will move across the surface in that same fraction of a second it took to move your finger. That's all we're talking about.

#### Maui

Right.

Right. At what speed? (That's where the apparent faster than light speed comes in.)

At c. Photons cannot exceed c.

Next time the moon is out, do this: Point your finger at the moon, sweeping across its surface from left to right, which will take a fraction of a second. If you replace your finger with a laser beam, you will have created a dot on the moon's surface (once the light gets there) that will move across the surface in that same fraction of a second it took to move your finger. That's all we're talking about.

This is a different scenario and I agree that it's at least theoretically possible that if you could extend a SOLID rod from the Earth to Pluto that sweeping it across would cause the tip to move ftl. But that's practically impossible and the laser beam dot is demonstrably not going to move ftl. Especially theoretically.

EDIT: The tip of the rod will not move ftl as the energy required to move the rod as the speed approaches c will be greater than the energy of the Solar system.

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#### mfb

Mentor
Maui: I think you don't understand what Simon Bridge, me and Doc Al mean.

Point a laser pointer towards one edge of the moon, and rotate it so quickly that it points towards the other side of the moon after 1 millisecond. Let's assume that the moon is a flat disk here.
Roughly 1 second after your experiment started, the laser pointer will make a dot at one edge of the moon. 1 millisecond later, the dot is at the other side, and it has been everywhere in between in this millisecond. If you calculate the speed of the dot, you get a speed above c. No photons moved faster than c, but the dot as "point where photons arrive" did.

Edit: Oh, new posts.
This is a different scenario
No, that is the scenario we were all talking about the whole time.
I agree that it's at least theoretically possible that if you could extend a SOLID rod from the Earth to Pluto that sweeping it across would cause the tip to move ftl.
NO! Now that is impossible.
But that's practically impossible and the laser beam dot is demonstrably not going to move ftl. Especially theoretically.
It is possible, both in theory and experiment.

#### Cthugha

False. The dot is made of photons and no photons can move ftl, whatever the mode of their transition from one place to the next.
Oh, come on. This is a standard textbook question even given to people in school regularly to demonstrate the difference misconceptions about what physical objects are. Don't tell me you never heard of it.

Are you saying that if I shine a laser beam towards Pluto and wait 30 minutes so that it makes a dot, I can move the laser source and the dot will move ftl? Clearly this is impossible and a misconception, as we only have photons that travel at c and their source.
And no, this is not what people here are talking about. You shine a laser at Pluto and rotate it. If you rotate it fast enough and track the dots as the position where the light beam hits Pluto versus time, you will see that the dot moves faster than the speed of light. That is no problem as it is not a physical object, carries no information and no single photon ever exceeds c.

edit: Why do other people always write almost the same answers way faster while I am typing? I must be getting really old now.

Mentor

#### Maui

Maui: I think you don't understand what Simon Bridge, me and Doc Al mean.

Point a laser pointer towards one edge of the moon, and rotate it so quickly that it points towards the other side of the moon after 1 millisecond. Let's assume that the moon is a flat disk here.
Roughly 1 second after your experiment started, the laser pointer will make a dot at one edge of the moon. 1 millisecond later, the dot is at the other side, and it has been everywhere in between in this millisecond. If you calculate the speed of the dot, you get a speed above c. No photons moved faster than c, but the dot as "point where photons arrive" did.

Edit: Oh, new posts.
No, that is the scenario we were all talking about the whole time.
NO! Now that is impossible.
It is possible, both in theory and experiment.

You keep saying the dot will move ftl but I am interested in how and why? How specifically, given that the dot is made of a stream of photons striking the surface. It will take more than assertions, give me the technical details.

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#### Maui

And no, this is not what people here are talking about. You shine a laser at Pluto and rotate it. If you rotate it fast enough and track the dots as the position where the light beam hits Pluto versus time, you will see that the dot moves faster than the speed of light. That is no problem as it is not a physical object, carries no information and no single photon ever exceeds c.
#

Same here, only assertions and nothing of technical nature. Are we in the assertions forum?

We already covered why the dot will not move for at least 30 minutes after you move the laser source but maybe you didn't read the thread.

#### Maui

Here's a cute video discussing this from New Scientist TV: One-Minute Physics: How to break the speed of light

(The comments are pretty sad. Every misconception in the book trotted out.)

This video assertion is wrong too. They compare LCD screen dots seemingly moving ftl, but that's because the dots of the LCD screen have as many 'sources' as there are dots on the screen, whereas the laserbeam source is just one and all its photons are bound by the speed of light(and actually moving).

#### Johnahh

Maui,
Let me show you with calculation as you are completely refusing to even think about what everyone is saying.
the radius of the moon = 1737km
if we assume the moon is a flat disc the diameter is 1737x10^3*2 = 3.474x10^6
divide the number by a millisecond (1x10^-3)
we get 3.474x10^9 ms^-1
a magnitude of ten greater than c. so the dot travelled across the moon at that speed therefore the dot exceeded c.

#### DennisN

All photons individually obey the speed limit, no worries . It's the absence of photons (shadows) or the effect of different photons (light spots) that can move faster than light. No single photon exceeds c.

#### Cthugha

Same here, only assertions and nothing of technical nature. Are we in the assertions forum?
Oh, getting personal, are we? This is simple classical rotation. Just calculate the velocity of rotating objects. It is simply omega times r where r is the distance of the axis of rotation to the spot you are interested in. If you make that long enough v will trivially exceed the speed of light. Physical and rigid objects, however, will never get there as the velocity exceeds the speed of sound which will cause them to break. The dot is however no physical object, but just a projection.

We already covered why the dot will not move for at least 30 minutes after you move the laser source but maybe you didn't read the thread.
Of course, the motion will be delayed. Nobody doubted this. This is, however, completely irrelevant to the topic at hand.

You keep saying the dot will move ftl but I am interested in how and why? How specifically, given that the dot is made of a stream of photons striking the surface.
The dot is the point where a single photon hits Pluto. It is NOT a stream of photons. Send a single photon out into some direction, wait a microsecond, then send another single photon out in a different direction equivalent to a rotation by one degree. Wait another microsecond, then send another single photon out into another directed equivalent to rotation by another degree and so on. Now check the difference in photon position after 10, 100, 100 m and so on. The positions will lie on the surface of a sphere. The positions will follow the simple equation for angular velocity given above and it is straightforward to calculate the velocity of the dot. It will obviously increase, if you move further away from the source. If you have a look in 10^20 m instead of 10 m, it will even be faster than the speed of light. Is this a problem? No, because as said before, this dot consist of the difference in arrival positions of different photons each going at c. This is not a physical object. Just do the math. It is just plugging numbers in.

Gold Member
edit: Why do other people always write almost the same answers way faster while I am typing? I must be getting really old now.
Don’t you know...? They use DOTS!! :surprised ()

• Niels & Albert live on opposite side of the moon.
• They communicate through moon-net IPv9 (very fast ~c).
• Niels want to give Albert a nice birthday present on his 135 birthday, next year.
• Niels are going to ‘sacrifice’ his old cat in this experiment, by throwing a birthday cake in his face.
• This ‘glorious event’ will be streamed live to Albert over moon-net.
• Niels has sent me an e-mail to fix the arrangement at the Apache Point Observatory Lunar Laser-ranging Operation.
• Niels has sent Albert an e-mail to also look for “light phenomena” during the “birthday streaming”.
• Niels instructions are that at 14 March 2014 we will send him 10 one second pulses and wait for one minute, and then send 10 new one second pulses, and then instantly point the laser at Albert.
• Niels will throw the birthday cake at the cat at the 20th pulse.
Will Albert get* the laser ‘message’ before he sees the “cat-catastrophe” on the moon-net IPv9 “birthday streaming”?
What will Albert say when Niels claims he has given him an FTL-message as birthday present?

(there are gaps in this story but I hope you know what I mean)

*Of course good old Albert has all fancy equipment needed for timing incoming signals at nanosecond resolution.

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