I Speed of Light & Bullet

[FRANKENSTEIN54]

TL;DR

The speed of light is like the speed of a bullet !

Everything has the speed it's moving at , at the time it's measured . Light always leaves its Source at the speed called "c" .
That doesn't mean its Relative speed is always "c" for all observers .

 

[FactChecker]

TL;DR Summary: The speed of light is like the speed of a bullet !

Everything has the speed it's moving at , at the time it's measured . Light always leaves its Source at the speed called "c" .
That doesn't mean its Relative speed is always "c" for all observers .

That is the whole point. Experiments of fantastic accuracy showed that its relative speed is always "c" for all observers in inertial reference frames. It was very surprising. It's not just a property of light. In fact, the local geometry of space-time does not allow a relative speed greater than "c".
The most famous experiment was the Michelson-Morley experiment. The result surprised them so much that they considered it a failure and Michelson kept trying to prove otherwise for years. (I think Morley did also.) But one result was Einstein's Special Relativity theory, and the proof of that includes the success of the atomic bomb and the GPS system.

 

[DaveC426913]

That doesn't mean its Relative speed is always "c" for all observers .

It does mean that.

If you get in your rocket and try to chase a laser beam, no matter how fast you go, you will always measure the laser beam as moving at c relative to you.

 

[FRANKENSTEIN54]

That is the whole point. Experiments of fantastic accuracy showed that its relative speed is always "c" for all observers in inertial reference frames. It was very surprising. It's not just a property of light. In fact, the local geometry of space-time does not allow a relative speed greater than "c".
The most famous experiment was the Michelson-Morley experiment. The result surprised them so much that they considered it a failure and Michelson kept trying to prove otherwise for years. (I think Morley did also.) But one result was Einstein's Special Relativity theory, and the proof of that includes the success of the atomic bomb and the GPS system.

I guess it's what you mean by "relative speed" . Let's say a Bullet is moving through space at some constant speed , would all observers measure its speed as that constant , no matter what speed relative to the bullet they were moving at ?? The same should apply to light if it also moves at a "constant speed" .

 

[DaveC426913]

I guess it's what you mean by "relative speed" . Let's say a Bullet is moving through space at some constant speed , would all observers measure its speed as that constant , no matter what speed relative to the bullet they were moving at ??

A bullet and light are not the same thing.
Bullet: has mass; is made of fermions.
Light: has no mass; is made of bosons.
Among a few other differences.

The same should apply to light if it also moves at a "constant speed" .

What do you mean by "should"? Are you aware that physics has no obligation to make sense to you?

 

[Dale]

say a Bullet is moving through space at some constant speed , would all observers measure its speed as that constant , no matter what speed relative to the bullet they were moving at ??

No. That is precisely what distinguishes the speed of light from the speed of bullets.

 

[DaveC426913]

FRANKENSTEIN54: this is not a site to expound upon your personal ideas. If you'd like to have this thread to remain open, you'd do well to rephrase your concerns as something akin to "I haven't done a lot of reading, so I don't know why light does not behave like bullets. What am I missing?"

 

[Orodruin]

Light: has no mass; is made of fermions.

The photon is a boson. Irrelevant to the discussion, but it is.

As to OP: No. You cannot deduce this by logical reasoning alone. You need to perform experiments and experiments show that the speed of light is different and does not work like the speed of bullets.

 

[FRANKENSTEIN54]

It does mean that.

If you get in your rocket and try to chase a laser beam, no matter how fast you go, you will always measure the laser beam as moving at c relative to you.

Yes , but what about another observer ?? You can always "measure" anything as going at the speed it's going, no matter how fast you are moving .

 

[FRANKENSTEIN54]

FRANKENSTEIN54: this is not a site to expound upon your personal ideas. If you'd like to have this thread to remain open, you'd do well to rephrase your concerns as something akin to "I haven't done a lot of reading, so I don't know why light does not behave like bullets. What am I missing?"

Oh , ok

 

[FRANKENSTEIN54]

A bullet and light are not the same thing.
Bullet: has mass; is made of bosons.
Light: has no mass; is made of fermions.
Among a few other differences.


What do you mean by "should"? Are you aware that physics has no obligation to make sense to you?

By "should" I mean it should make sense with physics itself . And generally "physics" should make sense , since it's trying to explain something .

 

[FRANKENSTEIN54]

No. That is precisely what distinguishes the speed of light from the speed of bullets.

It has a speed . What makes its speed have a magical property that other speed don't have ?

 

[DaveC426913]

The photon is a boson. Irrelevant to the discussion, but it is.

fer cryin out loud what a gaff. I think I over-edited my post, flipping one part but not the other.

Photons are bosons.
Bullets are made of fermions.
Fixed original post.

 

[DaveC426913]

It has a speed . What makes its speed have a magical property that other speed don't have ?

Science tries to avoid the 'why' questions; that's for philosophers. Science tries to model and predict 'what' nature does. And Einstein's postulates about the invariance of the speed of light work incredibly well.

The device you are currently using to read this probably has a GPS system in it. That system corrects for Einsteinian relativity. If Einstein were wrong about the speed if light's invariance, your device's GPS would not work.

It does work. Therefore Einstein's theory is predictive. There are no comparable theories that predict this phenomenon of nature.

By "should" I mean it should make sense with physics itself .

It does. In fact, physics wouldn't make sense if light were not invariant.

And generally "physics" should make sense ,

It does but it's not easy. It takes work to learn enough for it to make sense. You have some reading to do.

since it's trying to explain something .

Well, it's trying to explain nature - something not in its control. If nature says the speed of light is invariant, there's nothing physics can do to change that for your benefit.

 

[Histspec]

TL;DR Summary: The speed of light is like the speed of a bullet !

Everything has the speed it's moving at , at the time it's measured . Light always leaves its Source at the speed called "c" .
That doesn't mean its Relative speed is always "c" for all observers .

That's the emission (or ballistic) theory of light. While it is consistent with the Michelson-Morley experiment, it was refuted by many other experiments that use moving light sources.

 

[PeroK]

It has a speed . What makes its speed have a magical property that other speed don't have ?

The speed of light in a vacuum is the invariant speed associated with the spacetime of our universe. There are two main alternatives: the classical "Newtonian" space and time; and, relativistic spacetime. There is no logical way to decide between them, as they are both logically consistent. So, as @Orodruin says, you must do some experiments to determine which spacetime we have in our universe.

Perhaps the simplest single experiment is to try to accelerate an electron beyond the speed of light. If we live in a Newtonian universe (as I guess you believe), then there should be no problem to accelerate an electron beyond the speed of light. A modern particle accelerator could give an electron enough kinetic energy to be travelling at 10-100 times the speed of light. But, of course, such an experiment shows that an electron cannot be accelerated to the speed of light and beyond. This is completely consistent with relativity and completely inconsistent with Newtonian physics. This experiment decides the issue.

Here is an old video of this experiment being done at MIT in 1962:

 

[FRANKENSTEIN54]

No. That is precisely what distinguishes the speed of light from the speed of bullets.

Well , the bullet or the light doesn't change their speed , whether I'm moving or not .

 

[FRANKENSTEIN54]

The speed of light in a vacuum is the invariant speed associated with the spacetime of our universe. There are two main alternatives: the classical "Newtonian" space and time; and, relativistic spacetime. There is no logical way to decide between them, as they are both logically consistent. So, as @Orodruin says, you must do some experiments to determine which spacetime we have in our universe.

Perhaps the simplest single experiment is to try to accelerate an electron beyond the speed of light. If we live in a Newtonian universe (as I guess you believe), then there should be no problem to accelerate an electron beyond the speed of light. A modern particle accelerator could give an electron enough kinetic energy to be travelling at 10-100 times the speed of light. But, of course, such an experiment shows that an electron cannot be accelerated to the speed of light and beyond. This is completely consistent with relativity and completely inconsistent with Newtonian physics. This experiment decides the issue.

Here is an old video of this experiment being done at MIT:

Yes I agree that light has a certain speed . A bullet flying through space at some constant speed , also has a certain speed . Any observer , moving or not , does not affect the speed of either light or the bullet .

 

[PeroK]

Yes I agree that light has a certain speed . A bullet flying through space at some constant speed , also has a certain speed . Any observer , moving or not , does not affect the speed of either light or the bullet .

This is a forum to learn about physics. If you have no interest in learning, then it's pointless posting. Repeating your own personal theories is not allowed, in any case.

 

[Orodruin]

Any observer , moving or not , does not affect the speed of either light or the bullet .

Yes it does. That misconception violates not only special relativity but also Galilean relativity on which Newtonian physics is based. The speed of an object depends on the observer. If you are travelling in a train your speed relative to another passenger is zero but your speed relative to someone on the ground is not.

 

[FRANKENSTEIN54]

Science tries to avoid the 'why' questions; that's for philosophers. Science tries to model and predict 'what' nature does. And Einstein's postulates about the invariance of the speed of light work incredibly well.

The device you are currently using to read this probably has a GPS system in it. That system corrects for Einsteinian relativity. If Einstein were wrong about the speed if light's invariance, your device's GPS would not work.

It does work. Therefore Einstein's theory is predictive. There are no comparable theories that predict this phenomenon of nature.


It does. In fact, physics wouldn't make sense if light were not invariant.


It does but it's not easy. It takes work to learn enough for it to make sense. You have some reading to do.


Well, it's trying to explain nature - something not in its control. If nature says the speed of light is invariant, there's nothing physics can do to change that for your benefit.

I guess my point is , ANYTHING moving through space at some distinct speed , has that speed . And any moving or stationary observer does not affect or change that speed . Relative Measurements of the speed can produce different "Relative speeds" . But those are only "Relative Measurements of speed" . Light will hit you quicker the faster you move towards it , just like a bullet will hit you quicker the faster you move towards it . The distance is becoming shorter and "time of impact " is becoming sooner . Measurements can only be done in instants of time .

 

[Ibix]

Any observer , moving or not , does not affect the speed of either light or the bullet .

You may measure the speed of anything with respect to some reference object that you regard as stationary (the reference object is often left out of everyday speech, but is usually the Earth's surface). If you change your mind about the reference object (e.g., pick the car you are driving in instead of the Earth) all your measured speeds can change, except things travelling at the speed of light.

Assuming your two reference objects aren't moving very fast with respect to each other, things you measured to be noving near $c$ won't change speed as much as things moving slowly.

 

[Nugatory]

Light will hit you quicker the faster you move towards it , just like a bullet will hit you quicker the faster you move towards it .

That's a very natural assumption based on our experience with relative velocities that are small compared with $c$: I can throw a rock at a speed of 100 km/hr relative to myself, I'm standing in an open truck driving down the road at 100 km/hr, I throw the rock straight ahead, we expect the rock to be moving at 100 km/hr relative to the truck and 200 km/hr relative to someone standing at the side of the road - the speeds just add, right? And then I do the same thing with a flashlight which is "throwing" light straight ahead at speed $c$ relative to me holding the flashlight, we expect the light is moving at speed $c$ relative to me and $c$+100 km/hr relative to the person standing on the side of the road, right?

No. It turns out that speeds don't add that way. If something is moving at speed $u$ relative to me, and I am moving at speed $v$ relative to you, its speed $w$ relative to you is not $w=u+v$ but instead $w=\frac{u+v}{1+uv/c^2}$. If you play with this formula a bit you will see two interesting things:

First, if either $u$ or $v$ is equal to $c$, then no matter what the value of the other one is, the value of $w$ comes out to be $c$ - the light is moving at speed $c$ relative to both of us even though we are moving relative to one another.

Second, for any of the speeds that we're used to experiencing the difference is too small to measure in a any normal way (it would be a good exercise to do the calculation for my example above with $u=v=100 km/hr$). That's why it took so long to realize that the $w=u+v$ is not exactly correct, just a really really good approximation for speeds that are small compared to $c$.
(As a historical note, the physicist Fizeau was able to measure this discrepancy in the mid-1800s, but the signifcance of this measurement was not recognized until Einstein developed special reativity a half-century later).

 

[Ibix]

I guess my point is , ANYTHING moving through space at some distinct speed , has that speed .

...relative to something that you need to specify.

And any moving or stationary observer does not affect or change that speed .

...unless it's the object you chose as "at rest".

Relative Measurements of the speed can produce different "Relative speeds" .

Correct.

But those are only "Relative Measurements of speed" .

...and there is no other kind.

Light will hit you quicker the faster you move towards it

... according to a measure by which you are moving, but not according to any other.

The distance is becoming shorter and "time of impact " is becoming sooner .

Again, this depends who's doing the measurements and what they regard as "at rest". The way you are phrasing this, you appear to be adding your speed compared to some reference to light's speed. That's fine for some purposes, but it isn't the speed of light that changed - you just added something extra.

 

[Orodruin]

The main issue here is that OP has a very common intuitive picture of how things should work (and it is not even consistent with Galilean relativity) and while it may be self-consistent, it does not agree with experiment and must therefore be rejected. It is simply not how things work.

 

[Ibix]

The main issue here is that OP has a very common intuitive picture of how things should work (and it is not even consistent with Galilean relativity) and while it may be self-consistent, it does not agree with experiment and must therefore be rejected.

I think a lot of the pop-sci one line descriptions of relativistic phenomena, like "time slows down if you travel near the speed of light", actually contribute to that view. As most readers of this thread are aware that kind of statement really needs a list of caveats of textbook length, but these get left out of non-technical discourse. Without them, that kind of sentence can read as supportive of the notion of absolute velocity which, as you note, has been known to be inconsistent with reality since Galileo.

 

[jbriggs444]

Relativistic velocity addition is one way to explain how light can have the same speed relative to every inertial reference frame. But the velocity addition formula itself is a hard swallow for an intuitive armchair physicist. As hard as letting go of the idea of absolute simultaneity.

It's not what you don't know that gets you into trouble. It's what you know for sure that just ain't so.

 

[Dale]

Well , the bullet or the light doesn't change their speed , whether I'm moving or not .

OK, so this statement is false, but part of the reason that it is false is terminology. Physicists use words with very specific technical meanings.

Two important technical terms are "coordinates" and "inertial frame". Coordinates are a set of labels that we use, a kind of address book for locating events in space and time. We use coordinates as a mental tool to make calculations easier. An inertial frame is a particular type of coordinates where the laws of physics are particularly simple to calculate because a lot of things (Christoffel symbols) are zero.

You have an intuitive picture that a bullet is something in the real world travelling on a real path doing its own real thing. That real path exists in the real world, and it doesn't depend on our mental model of it. In particular, its real path doesn't depend on our coordinates. We can imagine all sorts of things about it, but none of our imagination changes any real thing about the bullet. That is correct, but that is not what the word "speed" refers to.

The concept that you are thinking about is called the four-velocity. The four-velocity is a geometric quantity that describes the motion of the bullet and it does not depend on our imagined coordinates. Different people can imagine different coordinates, and they will nevertheless agree on the geometric four-velocity. They will say that it has different "components" with respect to their chosen coordinates, but they will agree that the different components are just their different labels for the same geometric thing.

Speed is a different thing. Speed is relative, you must always specify the thing that you are measuring the speed relative to. Geometrically, speed is a kind of "angle" (in a technical sense it is an angle in time rather than in space) between the four-velocity of the object whose speed is being measured and the four-velocity of the reference relative to which it is being measured. Usually, the thing that is used as a reference is a coordinate system, and often specifically an inertial frame. You cannot measure an angle without defining the two sides between which the angle is measured. Same with speed, the reference is part of the specification of speed.

Now, finally we come back around to your statement. You said "the bullet or the light doesn't change their speed , whether I'm moving or not". So here, you have a description of some speeds, the speed of the bullet relative to moving you and the speed of the bullet relative to stationary you. These two speeds are different. You also have a description of the speed of the light relative to moving you and the speed of the light relative to stationary you. These two speeds are the same.

So "the light doesn't change their speed, whether I'm moving or not" is correct. But "the bullet doesn't change their speed, whether I'm moving or not" is wrong. It does. Speed is relative. It is the four-velocity that is the relevant non-relative geometric quantity. Everything has its own four-velocity that is what it is regardless of the reference, whether it is a bullet or light. But the speed of light is different from the speed of a bullet in that the speed of light is the same for all references.

 

[FactChecker]

The equation for the relative speed of a bullet and light are identical, but the numbers and velocity are extremely different. The bullet relative speed is very slow compared to light but it is not exactly the simple addition of speeds. The difference is so small that nobody would notice. By the time you get close to light speed, the correct equations are so different from simple addition of speeds that it is impossible to ignore. And by the time you get to the speed of light, "c", it has reached the limit and any change of the observer speed makes no difference at all.

 

[Herman Trivilino]

By "should" I mean it should make sense with physics itself . And generally "physics" should make sense , since it's trying to explain something .

That's one of the things you learn when you study physics. Things that make sense aren't always true, and things that are true don't always make sense.

You have a fundamental misunderstanding of natural science in general and physics in particular. Physics is a study of phenomena. Often it provides an explanation or description. That explanation or description may or may not make sense to you, but that doesn't matter. All that matters is that the description matches observation.

In the case you are discussing it just so happens that it does make sense. You just have to study it further. The idea has been around for almost 120 years. It made sense then and it still makes sense now.

If you could devise a bullet that traveled at speed $c$ all observers, regardless of their speed relative to each other, would observe its speed to be $c$. It makes perfect sense to me, lots of other people; and more importantly to the thousands of technicians, engineers, and scientists working in hundreds of places across the globe it is an everyday fact of life.