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the speed of light question

y0ssar1an22

a question that has puzzled me for a while...

1: car A and car B have a velocity towards each other of 100 kph. they are approaching each other is 200kph.

2: if car A and car B are doing the same, but at the speed of light why are they not approaching each other at (2)(speed of light)?

if the rules of physics hold for scenario 1, why not scarios 2?

is there a simple explanation here?

i dunno, maybe we just don't have an answer?

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Capt'n Midnight

Deleted User wrote: »

Thats the famous. E=MC²

Nope

At really high speeds the kinetic energy is even more than that.

skallywag

Deleted User wrote: »

The surface of an expanding balloon is two dimensional which is itself enclosing a three dimensional object, but the universe isn't embedded within a higher dimension and it also isn't expanding into anything. Also, the expansion of space has no centre like an expanding balloon does.

I agree that it has the weaknesses which you mention if you look at it from a 3D perspective, but I think it works fairly well when you picture yourself on the surface of the balloon let's say, and look at it from a 2D perspective. It can provide a nice mental picture of how galaxies are moving away from each other, but in a way that there is still no 'starting point' for said expansion.

The analogy does indeed break down when we take a step off the surface of the balloon and stand back and look at the balloon in 3D, well, unless you want to go down the rabbit hole road of postulating a 4th spatial dimension that we are currently unaware of (just like you might be completely unaware of the 3rd spatial dimension if you were sitting on the surface of the balloon).

Curious_Case

skallywag wrote: »

I think getting ones head around this is key to understanding what is happening, i.e. it is the space in between galaxies which is expanding, and not the galaxies themselves.

I find the balloon analogy very useful ...

https://www.youtube.com/watch?v=_PVitVku_C0

https://www.youtube.com/watch?v=hDTPvIKw9z8

How about raisins in dough ?

As the bread gets baked and expands, all the raisins move further apart.

I call it my *New World Theory" !!!!!

jelem

blinding wrote: »

Not faster than the Speed of Light.

it will not be easy as light bends, which means you may crash into a few objects on the way.

Capt'n Midnight

Curious_Case wrote: »

How about raisins in dough ?

As the bread gets baked and expands, all the raisins move further apart.

I call it my *New World Theory" !!!!!

No completely wrong.

You should be using J. J. Thomson's plum pudding model.

auspicious

Hmmm.... least not forget a shadow can travel faster than light.
...
...

take everything

The speed of light is constant no matter what reference frame you're in.

That is no matter how fast you're moving you will observe light moving at c (how the speed of light is denoted).

This has profound implications for space, time, velocity, momentum and energy.

Basically imagine observing a spaceship passing you moving at half the speed of light which emits a beam of light in its direction of motion. To reconcile the fact that both of you and the spaceship both observe that beam moving at c, you have to observe the spaceship's time dilate and their length contract.

This has consequences for the spaceship's velocity that you observe. And then consequences for momentum that you observe (which is mass times velocity) to preserve conservation of momentum.
And ultimately energy which is where E equals mc squared comes in. Einstein basically found through all this that particles have rest energy given by this formula and also that photons (light) have momentum and energy even though they have no mass.

The factor that dilates time and contracts length is negligible at everyday speeds so all the above approximates to the Newtonian description of physics (force = mass times acceleration etc).

The speed of light as a limit can be seen from these special relativity equations in different ways.

Curious_Case

Capt'n Midnight wrote: »

No completely wrong.

You should be using J. J. Thomson's plum pudding model.

How dare you!!

You've summarily dismissed my theory and I suspect you haven't even read my paper on it. It was quite well received by my peers (the few of my colleagues who are tolerant enough to allow me sit at their table in the canteen). I do concede, however, that the J. J. Thompson model does yield a more palatable result.

Gregor Samsa

Curious_Case wrote: »

How about raisins in dough ?

As the bread gets baked and expands, all the raisins move further apart.

I call it my *New World Theory" !!!!!

"Your theory of a donut-shaped universe is intriguing, Homer... I may have to steal it."

railer201

There was a young lady named Bright
Whose speed was far faster than light;
She set out one day
In a relative way
And returned on the previous night.

Capt'n Midnight

Curious_Case wrote: »

How dare you!!

You've summarily dismissed my theory and I suspect you haven't even read my paper on it.

Untrue. I keep up on currant events.

the purple tin

Capt'n Midnight wrote: »

Untrue. I keep up on currant events.

Stop raisin Hell you two.

blinding

the purple tin wrote: »

Stop raisin Hell you two.

Next they’ll be talking about hundreds and thousands of Multiverses ! !

Say my name

So are Gamma rays superluminal?

Panrich

[Deleted User] wrote: »

The balloon analogy only allows you to understand why expansion causes objects to get further apart without moving, but besides that it's a pretty bad analogy. The surface of an expanding balloon is two dimensional which is itself enclosing a three dimensional object, but the universe isn't embedded within a higher dimension and it also isn't expanding into anything. Also, the expansion of space has no centre like an expanding balloon does.

It also can’t explain our pending collision with the Andromeda galaxy.

Gregor Samsa

Panrich wrote: »

It also can’t explain our pending collision with the Andromeda galaxy.

You're right about the balloon analogy - it doesn't take the forces (gravity, electromagnetism and nuclear) into account. The dot you draw on the balloon will itself expand when you inflate it, but that doesn't happen with objects in the universe - as you mention, the opposite can happen.

Locally, gravity is more powerful than expansion. That's why the earth (or any other object) isn't expanding - the effect of its mass on space counters the expansion. There's a load of galaxies and dwarf galaxies in the Local Group that are close enough that gravity is bringing them closer together. They'll all eventually collide and merge.

There'll come a point in the future where every galaxy in the universe that is close enough with a neighbor to merge will do so, and from that point on, every galaxy will move away from every other galaxy indefinitely, to the point that they'll be so far apart that light will not even be able to span the distance, and they'll be totally undetectable to each other.

Feisar

I knew I wasn't the smartest tool in the shed but I get by. Fupin' starting to feel like Forest Gump now.

Gregor Samsa

Say my name wrote: »

So are Gamma rays superluminal?

No, they're electromagnetic radiation that travels at the speed of light like all other electromagnetic radiation, including radio waves, microwaves, infra red, visible light, ultraviolet and x rays. Gamma rays have the shortest wavelengths, highest frequencies and largest photon energies of all electromagnetic radiation, but they still travel at the SOL.

Larbre34

The original premise is wrong. If two cars approach each other at 100 kph each, their closing speed might be the equivalent of one car travelling towards a stationary object at 200 kph, but the force of impact is the same as one car hitting that object at only 100kph.

Why? The 'opposite reaction' force of both vehicles cancel each other out.

Mythbusters performed the experiment at a University physics testing ground in the desert, using many examples of the same car, a mid size Honda.

They put a car on a rocket sled and accelerated it towards a flat, reinforced concrete block about the size of a small house. They did it at speed x and at speed 2x, analysing the impact damage outcome for both.

They then accelerated two cars on a collision course, both individually travelling at speed x. When they collided and the damage analysed, it was precisely consistent with one vehicle hitting the block at speed x, not at speed 2x.

cj maxx

Larbre34 wrote: »

The original premise is wrong. If two cars approach each other at 100 kph each, their closing speed might be the equivalent of one car travelling towards a stationary object at 200 kph, but the force of impact is the same as one car hitting that object at only 100kph.

Why? The 'opposite reaction' force of both vehicles cancel each other out.

Mythbusters performed the experiment at a University physics testing ground in the desert, using many examples of the same car, a mid size Honda.

They put a car on a rocket sled and accelerated it towards a flat, reinforced concrete block about the size of a small house. They did it at speed x and at speed 2x, analysing the impact damage outcome for both.

They then accelerated two cars on a collision course, both individually travelling at speed x. When they collided and the damage analysed, it was precisely consistent with one vehicle hitting the block at speed x, not at speed 2x.

I not sure what myth busters did but I was always under the impression that if 2 cars hit at 100 mph each the impact was 2x100
If a car travelling at 100 hits a stationary object the impact is 100
If it hits an object travelling in the opposite direction also doing 100 then it’s 200 afaik
I’ll have to dig out my physics book.
May it’s that there is discernible difference between the destruction of the cars ?

Gregor Samsa

Larbre34 wrote: »

The original premise is wrong. If two cars approach each other at 100 kph each, their closing speed might be the equivalent of one car travelling towards a stationary object at 200 kph, but the force of impact is the same as one car hitting that object at only 100kph.

OP's question was very specifically about the speed of light, not the damage done in a head on collision of cars, so the Mythbusters experiment isn't really pertinent to it.

Larbre34

cj maxx wrote: »

I not sure what myth busters did but I was always under the impression that if 2 cars hit at 100 mph each the impact was 2x100
If a car travelling at 100 hits a stationary object the impact is 100
If it hits an object travelling in the opposite direction also doing 100 then it’s 200 afaik
I’ll have to dig out my physics book.
May it’s that there is discernible difference between the destruction of the cars ?

It isn't. They modelled it many different ways, including small scale under lab conditions using soft clay blocks.

One object, travelling at x, impact is with stationary object is x.
One object travelling at 2x, impact with stationary object is 2x.
Two objects on collision course, each travelling at x, impact is x. The opposite reactions cancel each other out. Why? Twice the energy is transferred to twice the mass.To each vehicle, the other performs as a stationary object. QED for Newtons Third Law.

Mythbusters 'car crash force' on Youtube. The damage is unmistakably different, consistent and clear.

[Deleted User]

Larbre34 wrote: »

The original premise is wrong. If two cars approach each other at 100 kph each, their closing speed might be the equivalent of one car travelling towards a stationary object at 200 kph, but the force of impact is the same as one car hitting that object at only 100kph.

Why? The 'opposite reaction' force of both vehicles cancel each other out.

Mythbusters performed the experiment at a University physics testing ground in the desert, using many examples of the same car, a mid size Honda.

They put a car on a rocket sled and accelerated it towards a flat, reinforced concrete block about the size of a small house. They did it at speed x and at speed 2x, analysing the impact damage outcome for both.

They then accelerated two cars on a collision course, both individually travelling at speed x. When they collided and the damage analysed, it was precisely consistent with one vehicle hitting the block at speed x, not at speed 2x.

No, it isn't. Basic conservation of momentum would tell you that's not the case.

Also, as mentioned above, it has nothing to do with OP's question.

Edit: I just watched the video you are talking about. Not a particularly insightful experiment, they need to do either:

Scenario A: 50mph car vs. stationary wall
Scenario B: 50mph car vs 50mph wall
or
Scenario A: 50mph car vs. stationary car
Scenario B: 50mph car vs 50mph car

A car hitting a wall is not equivalent to a car hitting another car regardless of which is/isn't in motion due to crumple zones etc.

ShatterAlan

Gregor Samsa wrote: »

There’s an interesting phenomenon called time dilation that happens the faster you travel. It’s not something you can notice at human speeds (unless you’re measuring with an incredibly accurate atomic clock), but the faster you go, the slower time goes relative to observers travelling slower than you. If you go at 90% of the speed of light, your perception and experience of time is half of a static observer. You’ll actually age half as slow as them. If you travel at the speed of light, for you, time stops, and you’re therefore bot able to observe anything in the universe. Basically at that speed, for you, nothing else exists except yourself, and time is infinite.

If you have any mass, your mass will also become infinite at the speed of light. It would therefore take an infinite amount of energy to get you there. Your length also reduces in the axis you’re travelling the faster you go. And again, the effect is infinite at the SOL.

So, if you were a solid object of any size, it would take infinite energy to get you to the speed of light, and once you got there, you’d have infinite mass, zero length, time would stop and to you the universe would cease to exist. This clearly means that it’s impossible for anything with mass to travel at the speed of light. It also shows that nothing can travel faster than light (it would take more than infinite energy to do it, which can’t exist, and you’d end up more than infinitely massive and less than zero in length - again, things that just can’t happen)

Fortunately, photons and other particles that do travel at that speed don’t have any mass, so they don’t have to worry about the physical effects.

But particles DO have mass. I didn't an experiment in uni that measured the charge to mass ration of the electron. Unless electrons can't travel at the speed of light.

[Deleted User]

ShatterAlan wrote: »

But particles DO have mass. I didn't an experiment in uni that measured the charge to mass ration of the electron. Unless electrons can't travel at the speed of light.

Yes. Electrons cannot travel at the speed of light precisely because they have mass.

And the experiment you're referring to is J. J. Thomson's experiment.

Gregor Samsa

ShatterAlan wrote: »

But particles DO have mass. I didn't an experiment in uni that measured the charge to mass ration of the electron. Unless electrons can't travel at the speed of light.

Electrons do have mass, and don't travel at the speed of light. Electrons in a copper wire travel very slowly - about 1cm a second, which is about the same as an ant. In a vacuum, at 100v, they can reach just under 1% of the speed of light.

In a particle accelerator with a huge amount of energy applied, they can get up to 0.999999999988c. But as they have mass (albeit miniscule), they'd still need infinite energy applied to actually get them to the speed of light.

Most particles do have mass, but photons and gluons don't. Gravitons, if they exist, wouldn't have mass either. Neutros have so little mass that it's currently experimentally impossible to differentiate between their speed and the speed of light, and since we don't know exactly what their mass is, we can't calculate their actual speed, but it's been shown that they must have some mass, so their speed must be less than c.

Capt'n Midnight

Larbre34 wrote: »

They then accelerated two cars on a collision course, both individually travelling at speed x. When they collided and the damage analysed, it was precisely consistent with one vehicle hitting the block at speed x, not at speed 2x.

Flat concrete blocks don't have crumple zones. The proper test would have been to fling cars into parked cars.

Capt'n Midnight

Deleted User wrote: »

Yes. Electrons cannot travel at the speed of light precisely because they have mass.

And the experiment you're referring to is J. J. Thomson's experiment.

The plum pudding guy not the other Thompson who showed that electrons can move like waves when it matters.

Larbre34

Capt'n Midnight wrote: »

Flat concrete blocks don't have crumple zones. The proper test would have been to fling cars into parked cars.

Forget the cars, look at the lab test on their videos. The clay cylinders proved the theory absolutely. The total force was doubled with the two objects in motion, but the force had to be divided equally across both objects in the collision.

Mellor

railer201 wrote: »

There's a simple explanation that's not simple to understand, for me at any rate. The speed of light is relative to the observer. Each driver would see the other car approaching at the speed of light. A person at the side of the road would see each car approaching also at the speed of light. Time slows down for the drivers on board the cars is how it works. Head bendy stuff.

This is probably the best example in a thread of crazy examples.

The cars are getting close in distance at twice the rate compared to an stationary point. But for the drivers of the cars, time is moving slower (for arguments sake lets say half as slow, but it's non-linear).

standardg60 wrote: »

What i like to ask myself is what is causing our sense of the passage of time. What makes the past the past and the future the future.
If the speed of light is a constant and always has been throughout the universe, then why does anything change.
We see change because we can see things which happen at a different relative speed to us.
Could this not be the case on a universal scale? That different areas of the entire universe are all moving at different speeds of light, therefore causing a difference in time.

You have this weird assumption, that everything in the universe happened instantly, and it's the different speds of light that create time. Which makes absolutely no sense on any scale.

Speed of light is constant. We observe passage of time because things happen at different times.

standardg60 wrote: »

The universe is an expansion of light, and life is dependent on that very expansion.
If the universe ceases to expand, then existence ceases.
It is the difference between the rates of expansion from where we are to the edge which gives rise to time.

Each one of those statements makes no sense and is incorrect.

Curious_Case wrote: »

Ok, but would it be correct to say that the distance between them is reducing @ 2C

C is a measure of speed, not distance, I think that's where people are go wrong with this stuff..