Why Is Light Speed a Limit for Spacecraft?

[machine_slave]

im having trouble understanding why the speed of light is a speed limit for space crafts

light has an origin. and from what i understand, the doppler effect only changes the wavelength, not the speed of the wave

but Earth is spinning very fast and traveling very fast through space and our solar system is traveling very fast through our galaxy and our galaxy is traveling very fast through our universe, but we feel none of this and can stand on the ground and say "im at rest" because of relativity.

so my hypothetical situation is to imagine that you have 2 infinitely long trains in a vacuum connected to an infinitely long railroad. the view from one train would see the other train mirrored and upside down. if these trains accelerated in opposite directions to 3/4 the speed of light compared to the train track, would a viewer on one train see the other train going faster than light? assuming you could actually see the light

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if the answer is yes then why can't an object accelerate to any speed? if you are in a spaceship traveling near the speed of light and you take your foot off the accelerator, you'll feel like you're not moving at all, and while the universe will look confusing when you look out the window, you and your craft will technically be at rest. so if there is no friction or there are no forces that would cause you to eventually slow down, then couldn't you put your foot back on the accelerator and continue to speed up to near light speed compared to that speed where you were at rest? which would actually look more like twice the speed of light to someone on earth, assuming they could see you

please tell me why i am wrong without hurting my feelings. I am just trying to understand so dumb it down if you can

 

[Andrew Mason]

so my hypothetical situation is to imagine that you have 2 infinitely long trains in a vacuum connected to an infinitely long railroad. the view from one train would see the other train mirrored and upside down. if these trains accelerated in opposite directions to 3/4 the speed of light compared to the train track, would a viewer on one train see the other train going faster than light?

No. You have to apply the Lorentz transformation to work it out but each train would measure the other's relative speed to be less than c.

if the answer is yes then why can't an object accelerate to any speed? if you are in a spaceship traveling near the speed of light and you take your foot off the accelerator, you'll feel like you're not moving at all, and while the universe will look confusing when you look out the window, you and your craft will technically be at rest. so if there is no friction or there are no forces that would cause you to eventually slow down, then couldn't you put your foot back on the accelerator and continue to speed up to near light speed compared to that speed where you were at rest? which would actually look more like twice the speed of light to someone on earth, assuming they could see you

You have to study special relativity to appreciate why objects can approach but never reach speed c.

One has to begin with the principle of relativity:the speed of light is c to all inertial observers. This means that observers in each train measure the same light pulse to travel at speed c. If both measure the speed of the same light pulse to be moving at speed c away from them, they cannot be traveling at speed greater than c relative to each other.

AM

 

[DrGreg]

Ypu may find the following answer that I gave in another thread helpful:

Maybe you are thinking, why can't you travel at 99% the speed of light, then go just 2% faster and find yourself at 101% the speed of light? Well, when you are traveling at 0.99c your notions of distance and time change. To the outside observer it looks like you have only 1% further to go, but from your own point of view you still have 100% to go -- the speed of light relative to you is still 299792458 m/s and you are no nearer to it than when you started. To look at this mathematically, velocities don't "add" as u+v but as

$$\frac{u+v}{1+uv/c^2}$$​

In the example above, your final velocity isn't 1.01c but

$$\frac{0.99c+0.02c}{1+0.99\times0.02}=0.99039c$$​

Another way of looking at this: the usual way to measure speed is to take distance on the observer's ruler divided by time on the observer's clock. But there is another way: take distance on the observer's ruler divided by time on the traveller's clock. This method is called "celerity" (or "proper velocity", a name I don't like). It turns out that the celerity of light is infinite, so if you translate your question from speed into celerity, "why is the universal celerity limit infinite?" it's a bit of a non-question.

You may well ask, "Why don't speeds add up" or "Why is the celerity of light infinite yet its speed is finite?" and most books will send you in a circle: "Because the speed of light is invariant" (the same for all observers), so it hasn't really answered your question. That's just the way the Universe is, who can say why?

 

[TobyC]

so my hypothetical situation is to imagine that you have 2 infinitely long trains in a vacuum connected to an infinitely long railroad. the view from one train would see the other train mirrored and upside down. if these trains accelerated in opposite directions to 3/4 the speed of light compared to the train track, would a viewer on one train see the other train going faster than light?

No, they wouldn't see the other train going faster than light. The velocity transformation when you change to a moving reference frame is different in special relativity to Newtonian physics.

It's to do with the fact that the clocks in the train will go more slowly, and their meter sticks will get shorter, so when they measure the speed of the other train they will get a different result to what you would expect.

 

[machine_slave]

thanks guys. i had a feeling that "special relativity" had something to do with it. it's new to me and will take me a while to understand.

but i wasn't talking about looking back from the train at the other train moving away. i was talking about looking forward at the other train coming towards you. i can imagine there would be a lot of problems with trying to see the other train or detect it in any other way from either direction. and i know that perception of time would change for the passengers. but if you were traveling at half the speed of light toward a light emission, then wouldn't it be extremely blue shifted or something?

if both trains were moving at 3/4 the speed of light (or anything more than half the speed of light) then wouldn't the difference between the trains be greater than the speed of light? and even if you couldn't detect the other train at all, you would know in your head that the other train was moving faster than light compared to your train.

that lorentz transformation page is a little confusing. i'll have to read it over a few more times before it makes total sense. but the part where it said "It is often said that nothing can be accelerated to the speed of light because its mass increases as it gets faster."
so my question is how does speed work in space? 60 miles per hour in a car is a measure of the difference in speed between you and the ground. if you were driving on a conveyor belt that was going 15 miles per hour, your speedometer would still read 60 miles per hour even though you'd really be traveling at 75 miles per hour. so if you were in an empty vacuum in an empty part of space with no visible stars or planets to compare yourself to, how would you gauge your speed? you would have no idea if you were going 0 mph or 1000 mph or light speed

please forgive me if your previous answers already explained this

 

[harrylin]

thanks guys. i had a feeling that "special relativity" had something to do with it. it's new to me and will take me a while to understand.

but i wasn't talking about looking back from the train at the other train moving away. i was talking about looking forward at the other train coming towards you. i can imagine there would be a lot of problems with trying to see the other train or detect it in any other way from either direction. and i know that perception of time would change for the passengers. but if you were traveling at half the speed of light toward a light emission, then wouldn't it be extremely blue shifted or something?

Yes of course.

if both trains were moving at 3/4 the speed of light (or anything more than half the speed of light) then wouldn't the difference between the trains be greater than the speed of light?

That's a matter of reference systems. For your system (fixed to the rails) the difference in speeds (their "relative" speed or "closing speed") is definitely 1.5c. However, if in one of the trains an independent reference system is set up, with that system the other train will be measured to move at a speed less than c.

and even if you couldn't detect the other train at all, you would know in your head that the other train was moving faster than light compared to your train.

No, the train moves at 3/4c while light moves at 4/4c! The maximum speed difference between two light rays that come "head on" is 2c.

that lorentz transformation page is a little confusing. i'll have to read it over a few more times before it makes total sense. but the part where it said "It is often said that nothing can be accelerated to the speed of light because its mass increases as it gets faster."
so my question is how does speed work in space? 60 miles per hour in a car is a measure of the difference in speed between you and the ground. if you were driving on a conveyor belt that was going 15 miles per hour, your speedometer would still read 60 miles per hour even though you'd really be traveling at 75 miles per hour. so if you were in an empty vacuum in an empty part of space with no visible stars or planets to compare yourself to, how would you gauge your speed? you would have no idea if you were going 0 mph or 1000 mph or light speed

That's right. You will need to set up a reference system, with respect to which you define speeds. And independent reference systems that are in relative motion measure speeds differently from each other.

 

[MYRYM]

aren't gamma ray burst moving MUCH faster than the speed of light, at least as it escapes the effect of a black hole ?

i.e. nothing can escape a black hole, yet gamma ray bursts DO.

The best explanation I have heard for the effects of a black hole is that it is eating space at a rate faster than the speed of light, hence even though light is still moving at light speed it is moving backwards because the space it is traveling in is moving towards the black hole faster than the light is moving away from it.

The closer to the black hole space gets the faster it moves, so for ANY energy to escape a black hole doesn't it need to be moving much faster than the space rushing towards the black hole?

If space is moving faster than the speed of light towards the black hole and space is full of matter then the matter is also moving faster than the speed of light, violating the premise that nothing can move faster than light.

so it seems that we have things moving faster than the speed of light towards a black hole as well as away from the black hole.

Or is the concept that the black hole is eating space at a rate faster than light speed incorrect to begin with?

 

[Dale]

aren't gamma ray burst moving MUCH faster than the speed of light, at least as it escapes the effect of a black hole ?

Gamma rays are light, so, by definition, they move at the speed of light.

 

[phinds]

aren't gamma ray burst moving MUCH faster than the speed of light, at least as it escapes the effect of a black hole ?

i.e. nothing can escape a black hole, yet gamma ray bursts DO.

No, they do not. Nothing moves faster than c and nothing escapes from a black hole (absent Hawking radiation, but I'm confident that's not what you're talking about).

Or is the concept that the black hole is eating space at a rate faster than light speed incorrect to begin with?

Yes, it is incorrect. SERIOUSLY incorrect. Nothing moves faster than c. If you don't believe that, they you need to study more physics until you DO understand that.

 

[darkhorror]

thanks guys. i had a feeling that "special relativity" had something to do with it. it's new to me and will take me a while to understand.

but i wasn't talking about looking back from the train at the other train moving away. i was talking about looking forward at the other train coming towards you. i can imagine there would be a lot of problems with trying to see the other train or detect it in any other way from either direction. and i know that perception of time would change for the passengers. but if you were traveling at half the speed of light toward a light emission, then wouldn't it be extremely blue shifted or something?

if both trains were moving at 3/4 the speed of light (or anything more than half the speed of light) then wouldn't the difference between the trains be greater than the speed of light? and even if you couldn't detect the other train at all, you would know in your head that the other train was moving faster than light compared to your train.

that lorentz transformation page is a little confusing. i'll have to read it over a few more times before it makes total sense. but the part where it said "It is often said that nothing can be accelerated to the speed of light because its mass increases as it gets faster."
so my question is how does speed work in space? 60 miles per hour in a car is a measure of the difference in speed between you and the ground. if you were driving on a conveyor belt that was going 15 miles per hour, your speedometer would still read 60 miles per hour even though you'd really be traveling at 75 miles per hour. so if you were in an empty vacuum in an empty part of space with no visible stars or planets to compare yourself to, how would you gauge your speed? you would have no idea if you were going 0 mph or 1000 mph or light speed

please forgive me if your previous answers already explained this

Your main problem is you think that there is a real speed things are going and everyone will agree on it. Speed is ONLY relative, when you say something is moving at 3/4 the speed of light you have to say with respect to what otherwise it's meaningless. When talking about speeds you have to look with respect to something else. There is no special inertial frame of reference.

Lets say you have a spaceship flying away from Earth at 3/4 the speed of light with constant velocity. In the frame of reference of the spaceship your velocity is 0, and the Earth is moving at 3/4 the speed of light. While in the Earth's frame of reference it's velocity is 0, and the spaceship is moving at 3/4 the speed of light. They are both just fine, there isn't anything special about them.

 

[DrGreg]

aren't gamma ray burst moving MUCH faster than the speed of light, at least as it escapes the effect of a black hole ?

The gamma rays come from matter that is spiralling into a black hole and while it is still outside the black hole's event horizon, so it can escape without traveling faster than light.

 

[Janus]

thanks guys. i had a feeling that "special relativity" had something to do with it. it's new to me and will take me a while to understand.

but i wasn't talking about looking back from the train at the other train moving away. i was talking about looking forward at the other train coming towards you. i can imagine there would be a lot of problems with trying to see the other train or detect it in any other way from either direction. and i know that perception of time would change for the passengers. but if you were traveling at half the speed of light toward a light emission, then wouldn't it be extremely blue shifted or something?

if both trains were moving at 3/4 the speed of light (or anything more than half the speed of light) then wouldn't the difference between the trains be greater than the speed of light? and even if you couldn't detect the other train at all, you would know in your head that the other train was moving faster than light compared to your train.

From your perspective in the train, it doesn't matter whether the other train is going away or coming towards you, the velocity addition equation given above still holds. So with both trains moving at 0.75c relative to the embankment, each train would measure the other as moving at 0.96c relative to themselves. The frequency of the light from those trains would be Doppler shifted by a factor of 49 (as measured from the other train), which would shift visible light to the Extreme ultraviolet and the far infrared to visible light.

To give you an idea of why this is, you have to consider the postulates on which Relativity is built. Very roughly: One says that the laws of Physics are the same for everyone and the other says that everyone measures the speed of light relative to themselves as being the same.

So let's say that you are in one train, watching the other come towards you. At some point, it emits a flash of light. For someone on the embankment, the light travels at c (~300,000 km/sec) with the train following behind at 0.75c. If you envision the light as an expanding sphere, the train always stays inside this sphere. The light meets up with you in the other train before the train does.

You also measure the light coming towards you, but at c relative to you. The expanding sphere of light expands from a point that remains a fixed distance from you. In order for the laws of Physics to be the same for both you and the person on the embankment, the other train must remain within this expanding sphere according to youand the light must reach you before the other train does, which means that the other train must travel at less than c relative to you.