# How To: Break the speed of light in your own backyard

Minute Physics serves up another nifty video.

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1. nehpetsE says:

Ok… So lets say someone planted laser sensors across the surface of the moon, with each sensor attached to its own to (synchronized) accurate clock.  As the laser passes across the surface of the moon, at faster than light speeds, what time does it trigger each of the sensors relative to each other?

1. SamSam says:

The sensors will all be triggered at almost the exact same time. If it took you 1 ms to travel from the sensor on the far left to the sensor on the far right, the sensor on the right will show it as having beed triggered 1 ms after the sensor on the left.

This still doesn’t mean that anything has actually traveled from one sensor to the other faster than the speed of light.

2. eat_it_alive says:

You could, however, blind an airline pilot. So don’t do this.

1. DeanCutlet says:

Boo.  You’re trying to stymie scientific passion with fear-mongering and an authoritarian undertone.

“You could poke someone’s eye out with that pen.  So, don’t use pens.  Just be quiet and wither inside.”

3. Alternate title: how to get people to watch your video by giving it a misleading title

1. Paul Renault says:

Agreed.  It’s like saying: “Think about the moon, now think about Jupiter!  Your mind is travelling faster than the speed of light!  – Whoa, I gotta sit down man, it’s blowing my mind, man!”

4. William Bagilliam says:

That has nothing to do with the breaking speed of light.  Each photon is traveling toward the moon at 186kmph.  When/where the next one hits on the moon’s surface isn’t significant in any way to Relativity.  The ‘beam’ doesn’t really move, and implying that it’s somehow a physically consistent object is misleading.

You may as well ‘break the speed of light’ by shifting your focus from the ground to Sirius.  Your focus would ‘move’ at 8.6 light years per second.

1. In the interest of total accuracy, the speed of light is 186kmps. second, not hour.

1. flagler23 says:

And in the interest of total clarity, the speed of light is 186k miles/s (not km/s).

1. show me says:

And in the interest of more (not total!) clarity and accuracy, the speed of light in a vacuum is 186,282 miles/s.
/Not that I’m trying to be pedantic or anything ;)

1. Chentzilla says:

In “a” vacuum? Like there are different vacuums?

Not to be pedantic, but the article is uncalled for.

2. In “a” vacuum? Like there are different vacuums?

We have a, uh, whatchacall, Hoover.

3. Paul Renault says:

“A” vacuum is where no one hears you scream.

“The” vacuum is what you use to clean up the mess afterwards.

4. Antinous / Moderator says:

“A” vacuum is where no one hears you scream.

“The” vacuum is what you use to clean up the mess afterwards.

Nature couldn’t possibly abhor a vacuum as much as I do. — Phyllis Diller

5. SamSam says:

In “a” vacuum? Like there are different vacuums? Not to be pedantic, but the article is uncalled for.

You are being pedantic, and also wrong.
MW defines “vacuum” as “a space absolutely devoid of matter,” note the indefinite article. Random House Unabridged has the same definite for #1, and #2 is “an enclosed space entirely devoid of matter,” note the article again. American Heritage: “a space entirely devoid of matter.”

Vacuum isn’t the substance, like “water,” it’s the space in which there is no substance. As such, there can indeed be different spaces in which there is no substance.

You certainly can use “in vacuum” if you wish, but it’s completely silly to call others out as being wrong for using “in a vacuum” entirely correctly. And a quick search of journal articles finds many more published papers containing the words “in a vacuum” than “in vacuum.”

But if I remove all the air from like, a jar, and then do the same to another jar, is what’s in the jars any different?
Wouldn’t it be true to say that the vacuum was already there and both jars, and the intervening air, are just stuff that’s in the vacuum?

7. tudza says:

Well, try saying “the vacuum” and see what sort of looks you get.

2. William Bagilliam says:

I meant 186k mph.  Omitting the space was a typo.

1. show me says:

Maybe omitting the space was a typo, but saying 186k mph is still off by a factor of approximately 3600.
/Not being pedantic ;)

5. pablohoney says:

I don’t get this – the light actually isn’t traveling *across* the moon. It’s  just traveling from the laserpointer, and when the angle is adjusted the photons travel from the laser pointer to the new location.  So it certainly might appear that the light is traveling faster than c but that’s not the case.

6. flagler23 says:

For those not familiar with this guy and his youtube videos, he loves to pass off trivial rubbish as insightful and valuable.  In one of his other videos he showed how a divergent infinite series adds to -1, which doesn’t even make sense b/c divergent infinite series don’t obey arithmetical laws.

1. knappa says:

I just watched the video you described. It’s even worse than you say: He sums a non-convergent series with positive terms to get -1.

7. SamSam says:

I remember learning about this in high school physics, and being stumped by the question “if information can’t travel faster than the speed of light, how is it that the spot of a lighthouse can travel between point A and point B… etc etc.”

It took a while to realize that you can’t send a message from point A to point B in this way, so you’re not actually transmitting information between them. And any “trick” you try and pull to send information from A to B in this way (e.g. see what message A is holding up and change the light as you sweep to B) is always going to be slower than the speed of light.

8. When I was a kid I used to wonder about raising my arm and a laser pointer on an infinite plane so that the dot would both ‘race out to infinity’ and yet also simply break the plane as the beam passed the horizontal and started to incline.

1. SamSam says:

It sounds like a nice argument for why an infinite plane would be logically impossible. As you approach nearer and nearer to horizontal, the spot would have to cover more and more ground, and would at the end need to cover an infinitely large amount on distance in an infinitely small amount of time. That is, if you hit the horizontal at time t, the dot would have had to move an infinite distance between t minus n and t, no matter how infinitesimally tiny n is.

9. Peer Bentzen says:

This argument presupposes that when you swipe a beam of light, the swipe is transmitted instantaneously over the whole length of the beam. Which of course is not true.

It’s just like tracking an airplane with a machine gun. When you change your aim, this change has to travel in the new direction all the way up to the airplane with a new round of bullits. And that can only happen with the speed of the individual bullits.
There is absolutely no way that firing a round of bullits in a new direction can influence any already fired bullits what so ever!

1. SamSam says:

This argument presupposes that when you swipe a beam of light, the swipe is transmitted instantaneously over the whole length of the beam. Which of course is not true.

Actually it doesn’t. The dot will still “move” faster than the speed of light.

You’re right that it will take a moment for the dot to start moving — the “wave” of motion will travel towards the moon at the speed of light — but after that the dot will move exactly as described.

In your machine gun example, if you shoot bullets at one side of the moon and then quickly change your angle, say within half a second, to shoot bullets at the other side of the moon, you will indeed have to wait a while for any of the bullets to hit the moon. Once they do, though, they will strafe from one side to the other in that half-second.

1. pablohoney says:

The problem with the laser pointer example and your machine gun example is that the bullets are only “moving” from the gun to the moon. They aren’t moving laterally across the moons surface.

It’s not like the bullet once it reaches the moon moves across the surface.
So you don’t have the situation where the bullet is moving across the surface of the moon (starting from point A to point B).

Let’s say a time index 0:00, the photon (bullet) leave the muzzle of the laser pointer (gun), and for sake of argument hits the surface of the moon 1.5 seconds later. As soon as the first photon leaves the laser pointer, the angle is adjusted so that the next photon departing lands 50km away from the first photon (this is easily done by altering the angle) – because of the distance to the moon is on average 350km, only a minor angular adjustment is necessary for the next photon (which is launched less than 1 second after the first) to land more than 50km away from the first photo (which would land on the moon at approx 0:01.50. If the second photon were launched at 0:00.01, it would land 50km away from the first at 0:01.51. The angle is adjusted slightly again and the third photo launched at 0:00.02, the third photo landing 100km away from the first at 0:01.52.

But the photon (bullet) didn’t travel 100km from the position of the first bullet. It traveled 350km from the laser pointer.  100km in 0.02 seconds would appear to have exceeded the speed of light c, but that’s not what happened here. Since the original photon launched at time index 0:00.00 did not move 100km (it was reflected back to earth showing the dot on the moon).

It’s incorrect to say the photon or bullet that struck the moon first would be moving to a new position.  The photon is reflected back to the viewer (round-trip time of 3 seconds).

I just don’t think this experiment is proving what it thinks its proving. Every time the angle of the laser pointer changes, new photons go to new places on the moon. The photons are not “moving” from one side of the other, the old photons continually get reflected as the red dots we see.

For the experiment to be valid you’d have to be talking about the original photon launched at 0:00.00 which then moved across the moon.

1. SamSam says:

I just don’t think this experiment is proving what it thinks its proving. Every time the angle of the laser pointer changes, new photons go to new places on the moon.

I think you may have misunderstood the point of the video, which was made slightly confusing by its deliberately-hyperbolic title. Neither the video nor anyone else in this thread is saying that anything, including photons, actually moved at the speed of light. Only that location of the point that the light hits “moves” at the speed of light. The video (tries to) make this clear, and agrees with what you’re saying.

As such, both you and I are correct in saying that the stream of photos is like the stream of bullets. Neither the photons nor the bullets move any faster than they are supposed to. Just the location that they are hitting moves. This is what you’re saying, and if you read my post that you were responding to again, you’ll see that this is what I’m saying.

1. Vincentjl says:

“Neither the video nor anyone else in this thread is saying that anything, including photons, actually moved at the speed of light”

But you are wrong! Something does indeed move faster then light, it’s called correlation, and is not the same as causation. This is exactly the kind of reasoning you get into in the very real and very popular experiments where you “teleport” quantum information from one state to another. We have for example 20 km separated electrons that always have exactly opposite spins, however that does not mean that any spin-state information travels faster then light, it simply means that they are correlated.The thing is that people in this thread seem to take issue with the definition of  a “thing” including anything which isn’t completely causal. Which just seems silly. Why isn’t a shadow a thing, why isn’t a light spot a thing, why isn’t a bullet impact point a thing? And what more, if these “whatshouldIcallthem”s aren’t things, then is a photon a thing? Is an electron a thing? Is spin a thing? etc.

The fact of the matter is that the statement that some things can travel faster then light is indeed very true. Information on the other hand cannot, and any thing that carries with it information will be limited by the speed of light.

2. Peer Bentzen says:

Right you are, SamSam!

2. Vincentjl says:

No it doesn’t. The spots position on the surface of the moon will move 20 times faster then the speed of light, even though the current position of the spot will not be where you are pointing until the photons have arrived at light speed.

If you take the same example with bullets, you need to consider the speed of the point of impact,  not the speed of the individual bullets. Basically the difference between group velocity and phase velocity.

Also to the many that are claiming that this is somehow wrong, this is exactly the kind of reasoning used when claiming to have “slowed down” light or even completely stopping it. This all deals with the group velocity never the phase velocity which is constant.

10. Guest says:

Is there any way to apply this is to the speed a bowling ball rolls down a 60 ft. lane?  (I got a bad case of the ‘nines’.)

11. tvugly says:

How to break the speed of light:

Step 1) Turn on laser pointer.

Step 2) Throw laser pointer.

1. asuffield says:

How to break the speed of light:

Step 1) Turn on laser pointer.

Step 2) Throw laser pointer.

I know it sounds clever, but the remarkable thing is that it’s wrong. This is absolutely critical to relativity:

Take two laser pointers. Hold one of them steady, and throw the other so that it’s still pointing in the same direction.

The two beams of light coming out of these two laser pointers are travelling at exactly the same speed, relative to anything you like. When you understand this, you will have a much better grasp of why relativity is so special.

2. tudza says:

That’s the one everyone thinks of and hears the correct answer to.  If you took some level of physics you’d even be asked to do the math.    Wouldn’t work because it wouldn’t surprise anyone.

12. snagglepuss says:

Don’t forget that the outside rim of a vinyl album on a turntable is moving “faster” than the inner edge of the groove near the label – Even though they’re both still doing 33 1/3 rpm’s. Explain THAT, “Minute Physics” Guy.

13. Paul Renault says:

A more mind-blowing everyday example:
Take a pair of scissors, open them, snap the blades shut/together.  The point of contact of  the two blades travels faster than the speed of light…

It still doesn’t mean anything.

And I want to say a ‘hear, hear!’ to asuffield.
As a kid, then teenager, and then young-adult, *I* used to use the “two people on two trains traveling towards each other at 3/4 of the speed of light, each one sees the other go past them at 1 1/2 times the speed of light” uber-bad example…

The problem, tvugly, is that the people on the fast-moving trains experience time passing slower than the ‘observer’ watching both trains.

So as the trains pass each other, the people on the trains see the other train going slower than 3/4 the speed of light.  Their combined speeds don’t add up to the speed of light.

Someone here posted posted the actual relativistic formula for adding velocities some weeks ago.
You can find it here:

14. TimmoWarner says:

Doesn’t the video itself explain that you aren’t really breaking the speed of light?

A lot of people seem to be explaining what the video already said.

1. An easier way for the video to explain that? not being titled “how to break the speed of light”

1. TimmoWarner says:

I take no issue with the posts saying it’s a bad title.

15. technogeekagain says:

Second the reaction: “My god, this bit of stupidity again? It’s a perfect example of Not Even Wrong.”

Explaining _why_ it isn’t what the title claims it to be would be a useful pedagogical exercise. But it’s already been done many times, and it isn’t worth my effort to retype it.

C’mon, BB. Youse guys know better.

1. SamSam says:

I’m surprised by the number of people that didn’t seem to realize that explaining why the notion was wrong *was* the point of the video.

I think what they’re saying is that this is much less amazing than the video makes it out to be.

16. teufelsdrochk says:

HOW TO BREAK THE SPEED OF LIGHT AT HOME:

Open a pair of scissors.

At a point closest to the hinge, the separation between the blades moves faster than ‘c’.

17. telaquapacky says:

If you have a thin nozzle on a garden hose that produces a thin stream, and the pressure is good, so the water stream is moving pretty fast, and then you jerk the nozzle from one side of your back yard to the other very quickly, so what if the interval between water drops reaching one side of your yard and the other is quicker than the speed of the stream coming out of your hose. There is no stream starting at one side of your yard going to the other. Likewise, the light spot on one side of the moon that moves to the other isn’t a stream of light- it’s just a scattering of individual photons, none of which are travelling any faster than the speed of light.