Why can't we reach to Speed of light at Space?

[phinds]

When I was trying to understand why the speed of light couldn't be reached (except by light itself, which is unique by traveling in space but never in time), what helped me was a drawing of the hyperbola, a geometric curve that I'm sure Wikipedia describes pretty well. The curve starts out almost as well-rounded as a circle, but gradually flattens out until it's very nearly a straight line. However, it never actually becomes a straight line, even at infinity. With some unknown propulsion system, some sort of spaceship might hypothetically come EXTREMELY close to the speed of light, but, like the perfectly straight line that you might wish that NEARLY-flat curve would become, it can NEVER get there.

This very common concept is called "asymptotically approaching"

 

[DaveC426913]

Assume you ride a spaceship which has a constant small acceleration of 1g. Inside this rocket you feel the same as you feel your weight on earth.
With this constant acceleration you will reach (and pass) the speed of light in few years and will be able to return home before getting old.

This is false on both counts.

In a nutshell, you will measure yours craft as approaching - but never reaching - the speed of light. What you will notice however, is that objects (such as planets and stars) will flatten along your line of travel, as will the distances between them. After a few years, you will be passing star after star, all squashed like pancakes, and all near each other along your line of travel.

 

[Gbl911]

Actually as far as I know that one something reaches the speed of light space and time bend to keep it a constant so theoretically it is impossible to go past it.

 

[phinds]

Actually as far as I know that one something reaches the speed of light space and time bend to keep it a constant so theoretically it is impossible to go past it.

No, it's impossible to REACH it, not just go past it.

 

[AgentSmith]

thanks all...but none of you friends understood my question... let's give you a clearance about what is across my mind...first i am not from an english country and miss-spelling may occur in my post...i have a basic information about physics and not involved with equations and formulas second suppose we launch a spacecraft with primary speed (for example 1000km/h or mils/h)...in the space where no friction exists, we turn the engine ON and naturally the speed must be increased as in our example to 1100 km/h...we turn the engine OFF...now the speed must be 1100km/h...after a while we turn the engine ON and our speed reaches to 1200km/h...we turn the engine OFF...turn it ON again and so on...we continue this manner for 1 year...2 years..3 years and more(suppose our spacecraft has enough fuel to last all the years)...after many years theorically we must reach to the speed of light...but why we can't reach? this is my question...

Majid1986:
Your question involves an argument based on Newtonian mechanics, the type we are used to in everyday life. You are correct that according to Newton the speed would get faster and faster. But Einstein showed that the speed of light could not be reached because you would need an infinite amount of energy to reach the speed of light. I have avoided any equations because you said your knowledge of physics did not extend to equations. But you really cannot understand physics without knowing some basic equations.
If you could just keep burning the rocket fuel and go faster and faster, you could reach any speed you wanted as long as you had fuel. But you do not have an infinite fuel supply, so there is some limit on how fast you can go even according to Newton's laws, correct? Einstein showed that this limit is the speed of light. There has to be a speed limit, and light speed is the limit.
If there are any terms or words here that you do not understand, just ask and we can explain them to you.
Please do not think I am talking down to you, I am just trying to make as few assumptions as possible.

 

[DaveC426913]

If you could just keep burning the rocket fuel and go faster and faster, you could reach any speed you wanted as long as you had fuel. But you do not have an infinite fuel supply, so there is some limit on how fast you can go even according to Newton's laws, correct? Einstein showed that this limit is the speed of light.

Agent Smith, this is a misleading argument. It implies that the reason we cannot reach the speed of light is a limitation of the propulsion. This is not so.

Even a "magical" propulsion system that had unlimited thrust, unlimited fuel and unlimited time will never reach c.

In fact, it is the laws of the universe itself that prevent us from reaching the speed of light. The nature of the limit is time dilation.

 

[AgentSmith]

Agent Smith, this is a misleading argument. It implies that the reason we cannot reach the speed of light is a limitation of the propulsion. This is not so.

Even a "magical" propulsion system that had unlimited thrust, unlimited fuel and unlimited time will never reach c.

In fact, it is the laws of the universe itself that prevent us from reaching the speed of light. The nature of the limit is time dilation.

Please read the reply carefully. It says that even according to Newton we could not reach any speed desired, since no one can have an infinite supply of fuel.This is a true statement, meant to respond the poster's relative lack of physics knowledge. (Magical propulsion systems do not exist). Throwing equations and advanced physics concepts at him does no good.
Then I say that Einstein showed we could not reach lightspeed. This too is a true statement, based on physical law. So I did not imply what you said I did.

 

[DaveC426913]

Please read the reply carefully. It says that even according to Newton we could not reach any speed desired, since no one can have an infinite supply of fuel.This is a true statement, meant to respond the poster's relative lack of physics knowledge. (Magical propulsion systems do not exist). Throwing equations and advanced physics concepts at him does no good.
Then I say that Einstein showed we could not reach lightspeed. This too is a true statement, based on physical law. So I did not imply what you said I did.

Indeed, they are both true statements; I did not say otherwise. It is the mashing together of two unrelated limits that is misleading.

'Einstein showed us that the speed of light is the limit' is a trivially true statement, i.e. it is a given, even by the OP's acknowledgment. So it does not explain what causes the limit. And because you went straight from 'not enough fuel is the limit' to 'speed of light is the limit' the connection, it implies the limits one is analogous to the other. A paragraph break between the two concepts, and perhaps a bit more explanation of Einstein's concept would avoid the ambiguity.

 

[AgentSmith]

Indeed, they are both true statements; I did not say otherwise. It is the mashing together of two unrelated limits that is misleading.

'Einstein showed us that the speed of light is the limit' is a trivially true statement, i.e. it is a given, even by the OP's acknowledgment. So it does not explain what causes the limit. And because you went straight from 'not enough fuel is the limit' to 'speed of light is the limit' the connection, it implies the limits one is analogous to the other. A paragraph break between the two concepts, and perhaps a bit more explanation of Einstein's concept would avoid the ambiguity.

Yes, you are correct, I should have separated the two ideas. But not enough people realize that even Newtonian mechanics prohibits unlimited speed. Regarding explaining Einstein's STR, OP had a poor understanding of physics and almost none of the math by his admission. I was trying to address things at his level. I guess I could have mentioned the mass increase as one approaches c. Then I would have to explained why. Most of these type questions seem to end up with a continuing series of "why?". Sorry for my original petulant-sound reply. Its just that too many respondents to these posts by beginners start throwing around Lorentz contraction equations and relativity of simultaneity that go right over their heads.

 

[DaveC426913]

But not enough people realize that even Newtonian mechanics prohibits unlimited speed.

But it doesn't.

In a hypothetical Newtonian universe, a craft could accelerate forever. A Bussard ramjet or laser sail would be sufficient to give it the "magical" quality of unlimited propulsion. The limit is one merely of engineering.

Most of these type questions seem to end up with a continuing series of "why?". Sorry for my original petulant-sound reply. Its just that too many respondents to these posts by beginners start throwing around Lorentz contraction equations and relativity of simultaneity that go right over their heads.

In my opinion (and I've answered a lot of these questions), there really isn't a simpler explanation. There is quite simply no layperson's counterpart to the dilatory/contractive effects of relativity. Until they grasp it, they will never understand why it is the way it is. And that causes them to keep asking the same questions like "What if I just accelerated forever? Why would I stop accelerating once I reached c?"

I agree that the formulae do not help most laypeople. I am a layperson myself and not a formulae person. That's why I try so hard to explain as best as possible the concepts without referencing them.

 

[abdullahi abass]

Here's one thing you should consider.No matter how fast the spaceship goes, its passengers would always measure the speed of light to be exactly c, the same velovity would be recorded by a stationary body. It is therefore theoretically impossible to travel faster than something and still measure the speed to be 3.0*10^8m/s faster than you. So uses time dilation and length contraction to correct the velocity of the spacecraft so its always less than the speed of light to all observers in any reference frame. (by imagination, traveling faster than the speed of light would be in fact traveling back in time.) :-)

 

[phinds]

Here's one thing you should consider.No matter how fast the spaceship goes, its passengers would always measure the speed of light to be exactly c, the same velovity would be recorded by a stationary body

You might want to give some thought to what you mean by "a stationary body".

It is therefore theoretically impossible to travel faster than something and still measure the speed to be 3.0*10^8m/s faster than you.

Huh ?

So uses time dilation and length contraction to correct the velocity of the spacecraft so its always less than the speed of light to all observers in any reference frame.

I think it would be better to say that one uses the Lorentz Transformation, but basically I don't disagree w/ you.

 

[DaveC426913]

It is therefore theoretically impossible to travel faster than something and still measure the speed to be 3.0*10^8m/s faster than you.

Huh ?

It's convoluted, but sound logic. How can you travel faster than c, if c is always 300,000 km/s faster than you?

 

[phinds]

It's convoluted, but sound logic. How can you travel faster than c, if c is always 300,000 km/s faster than you?

OK, it is logical, as you say, but it's a weird way to look at why you can't travel at c and personally I don't find it helpful (but that's just old grumpy me )

 

[thetexan]

I'm reading this thread with interest.

Isn't one of the proposed propulstion methods for long distance and long term rocketry an ion rocket?

I seem to recall that it works on the principle of a continuous tiny push by the force of a very small ion discharge and that this tiny push, over a long period of time, provides tiny accelerations that eventually add up to huge velocities.

If I am on a ship at .99 c won't my tiny ion push add a tiny acceleration to my already fast ship and push it a little faster?

tex

 

[phinds]

I'm reading this thread with interest.

Isn't one of the proposed propulstion methods for long distance and long term rocketry an ion rocket?

I seem to recall that it works on the principle of a continuous tiny push by the force of a very small ion discharge and that this tiny push, over a long period of time, provides tiny accelerations that eventually add up to huge velocities.

If I am on a ship at .99 c won't my tiny ion push add a tiny acceleration to my already fast ship and push it a little faster?

tex

By a trivial amount, yes, but you'll never get to c. Even a HUGE additional acceleration won't get you to c. Even a HUGE additional acceleration for a HUGE amount of time won't get you to c. You just won't get to c.

 

[DaveC426913]

al

If I am on a ship at .99 c won't my tiny ion push add a tiny acceleration to my already fast ship and push it a little faster?

tex

Running the drive for x time might get the ship to, say 99.9% of c (as observed from an external viewpoint such as Earth). Running it for 2x time will get it to 99.99% of c, running it for 4x time will get it to 99.99% c etc.

You must account for the fact that each increment of x also increases the time dilation factor, slowing down events aboard the ship (as observed from Earth). By the time your ship is nearing c,the time aboard the spaceship is getting slower and slower (again, from Earth). So, from Earth's viewpoint, the drive aboard the ship is putting put less and less thrust. By the time Earth sees the ship moving 99.999999999999% of c, they are seeing a ship that is spitting out mere photons per second, so - accordingly - very little acceleration.Events aboard the ship are very different. You of course cannot observe yourself traveling at speed; as far as you are concerned, you are stationary and the universe is whizzing past you. In this case, stars - and the distances between them - become compressed. You look at a star (which would look like a pancake if you could view it from a different angle is approaching you at less than c, but it's a lot closer than you expected it to be.

 

[thetexan]

We talk about the theoretically impossible attainment of c but, as stranded earthlings, can't we be very happy with .99c or even .90c, or even .8c. That will get us a long way toward exploring the universe.

In other words, even if we can't attain c isn't a substantial fraction of c useful?

tex

 

[DaveC426913]

...can't we be very happy with .99c or even .90c, or even .8c.

Sure!
Just as happy as we were with automobiles that do 10 mph ...
and airplanes that do 50mph ...
and jets that do Mach .9 ...
and ...Humans: "some" outta be enough.

 

[Nugatory]

OK, it is logical, as you say, but it's a weird way to look at why you can't travel at c and personally I don't find it helpful (but that's just old grumpy me )

Keep trying, and you'll learn to like it

Light always moves at c relative to me, so if I and a light signal leave from the same point at the same time, the light signal will always be in front of me and moving farther away. Now if that light signal is also moving at c relative to an observer back at our departure point, and I'm not catching up to it and am in fact falling farther behind every second... What does that tell us about my speed relative to that observer? And there you have the light speed limit, with not a single equation - what fun!
(Seriously, kidding aside, if you ever want to try explaining relativity to a bunch of kids who haven't yet met algebra, this way really is a lot of fun).

The other nice thing about this way of explaining it is that it slips the notion of absolute speed into the trash while no one is looking.

 

[thetexan]

How DO we measure our speed? Let's say we are traveling close to c? How do we know that? We would have to measure our velocity relative to something which would not be accurate since, as stated earlier, is dependent on the relative velocity of the thing we are measuring against. Or, we could keep track with an accelerometer and a computer could keep track, but that acceleration is even effected by space/time I believe.

So If I am traveling at close to c, how do I know. More importantly, how does the little ion push know and how does the mass of the spaceship know. Why wouldn't the continuous force of the ion force cause a contiuous acceleration causing a continuous increase in speed...relative to SOMETHING?

TEX

 

[DaveC426913]

How DO we measure our speed? Let's say we are traveling close to c? How do we know that?
We would have to measure our velocity relative to something which would not be accurate since, as stated earlier, is dependent on the relative velocity of the thing we are measuring against.

Precisely. That is why it is called relativity.

There is no such thing as absolute speed. It is all relative to something. Something of your choosing.

We might, for convenience sake, measure it with reference to our starting point. So, we can say how fast we are moving wrt Earth.

Note that this works both ways. From our spaceship (which we observe as stationary), Eartgh is moving away from us at high speed. We see it highly compressed and time dilated.

So If I am traveling at close to c, how do I know. More importantly, how does the little ion push know and how does the mass of the spaceship know. Why wouldn't the continuous force of the ion force cause a contiuous acceleration causing a continuous increase in speed...relative to SOMETHING?

That's the thing. The dilation of time, length and mass are only relative to some reference point of our choosing.

So, I could be moving at .9999c relative to Earth, and appear highly compressed, yet a spaceship pacing me - or leisurely passing me at 100mph - will see me normally, with no relativistic effects.

 

[phinds]

So If I am traveling at close to c, how do I know.

You ARE traveling close to c. You are also traveling at 1/2 c and at 1/10 c and not at all. As Dave explained, it all depends on the frame of reference used to decide your velocity.

 

[rootone]

We talk about the theoretically impossible attainment of c but, as stranded earthlings, can't we be very happy with .99c or even .90c, or even .8c. That will get us a long way toward exploring the universe.

In other words, even if we can't attain c isn't a substantial fraction of c useful?

tex

Even if we could attain a substantial fraction of c, a round trip to the nearest solar system, taking into account that we have to first accelerate then decelerate on both legs of the journey is going take around 20 years - and as far as I know, the Alpha Centuri system isn't thought likely to include planets which could be habitable anyway.
Based on the present knowledge we have of exoplanets, although admittedly that knowledge is still very much in it's infancy, we would need to be going at least 3 times further to discover any planets off great interest.

There is also the problem that when traveling at such a velocity, a collision with a microscopic grain of dust would release an amount energy comparable to thermonuclear detonation.

 

[thetexan]

So, let's say I, in my close to c spaceship, pass three observers. One is traveling close to my speed in my direction with a difference of 100 mph. I pass another standing on a planet in a relatively...there's that word again...stationary position. And a third in a spaceship going the other direction at near c.

Now, as I understand it two things are happening.

As to actual relative speeds...

I pass the first with an actual difference in speed of about 100 mph. I actually pass the observer on the planet at near c. As to the third, I actually pass him at near 2c, actual theoretical velocity.

Now to the second thing.

The first observer observes me pass him at about 100 mph since relativistic effects are minimal at those speed differences. The second has considerable relativistic observational warpages but thinks I'm going near c. The third doesn't observe the actual near 2c velocity difference. To both me and the other guy, we both will never observe the other going greater than c. To each of us the most we can hope to observe is each other traveling away from each other at no more than c.

Is that close to correct?

tex

 

[phinds]

to say you "pass him at 2c" is not correct. You see him moving towards you at near c and he sees you moving towards him at near c. If you have a 3rd observer sitting at the point where you pass and he is "stationary" in that he sees each of you moving at near c, then he will see your combined recession speed away from each other as near 2c because this is a recession speed not an actual velocity. You can NEVER see one other single object moving at > c and only massless particles will move at c relative to you.

Also, the 100mph is not correct. Suppose the other guy is going at c- 10mph. If you were able to pass him at 100pmh they you would be going c+90 mph, which cannot be done. You have to do the Lorentz Transforms on each, using the same base observer for each (the observer that you think sees the two of you going at 100mph relative to each other)

 

[thetexan]

"Also, the 100mph is not correct. Suppose the other guy is going at c- 10mph. If you were able to pass him at 100pmh they you would be going c+90 mph, which cannot be done. You have to do the Lorentz Transforms on each, using the same base observer for each (the observer that you think sees the two of you going at 100mph relative to each other)"

Let's say I am in a spaceship in intergalactic space. I don't know if I'm moving or not. I can't tell. Up ahead there appears to be a spaceship coming toward me at about 10 mph tail first. I deduce that is probably me approaching him from the rear. I don't like traveling in a flight so I decide to accelerate and pass him quickly and get several miles in front of him. So I hit the afterburners. But nothing happens. I look out the back and all 6 engines are lit and seem to be working but I'm still only slowly gaining on the other guy at 10 mph.

I suddenly realize that I can't accelerate because I'm already at lightspeed.

Is that what you're telling me?

tex

 

[Drakkith]

I suddenly realize that I can't accelerate because I'm already at lightspeed.

Is that what you're telling me?

No, because you aren't at lightspeed. Relative to the ship you are overtaking, you are moving at 10 mph, so when you hit your engines you will accelerate just like normal. With a 'magic' engine that uses no fuel, you can accelerate forever and you will still never reach lightspeed.

 

[phinds]

"Also, the 100mph is not correct. Suppose the other guy is going at c- 10mph. If you were able to pass him at 100pmh they you would be going c+90 mph, which cannot be done. You have to do the Lorentz Transforms on each, using the same base observer for each (the observer that you think sees the two of you going at 100mph relative to each other)"

Let's say I am in a spaceship in intergalactic space. I don't know if I'm moving or not. I can't tell. Up ahead there appears to be a spaceship coming toward me at about 10 mph tail first. I deduce that is probably me approaching him from the rear. I don't like traveling in a flight so I decide to accelerate and pass him quickly and get several miles in front of him. So I hit the afterburners. But nothing happens. I look out the back and all 6 engines are lit and seem to be working but I'm still only slowly gaining on the other guy at 10 mph.

I suddenly realize that I can't accelerate because I'm already at lightspeed.

Is that what you're telling me?

tex

No, I most emphatically am not telling you you are at c, I'm telling you that no matter how much, or for how long, you accelerate you cannot reach c. If you are going c-80 mph, say, then almost no amount of acceleration is going to increase you speed by a noticeable amount and if he is going at c-10 you aren't likely to catch up to him without a staggering amount of fuel. Again, look at the Lorentz Transforms for you and the other ship relative to the observer whom you think sees you going at c-80 and him at c-10. I've said that twice now. It really is the only way you're going to see the effect in detail.

 

[Drakkith]

If you are going c-80 mph, say, then almost no amount of acceleration is going to increase you speed by a noticeable amount and if he is going at c-10 you aren't likely to catch up to him without a staggering amount of fuel.

Make sure you're clear about whose frame of reference you are observing all this from. From his own frame there's only a 70 mph difference between the two, which is easily made up by a short burst from his engines.

 

[phinds]

Make sure you're clear about whose frame of reference you are observing all this from. From his own frame there's only a 70 mph difference between the two, which is easily made up by a short burst from his engines.

Hm ... that doesn't seem right. Are you sure about that? That implies that increases in speed are very easy at near light speed and I just don't think that's right. I DO get your point about the frame of reference, but it still feels wrong.

 

[thetexan]

Let's deal with actual theoretical velocity.

Let's say both ships are side by side. Let's say that I accelerate to 100 mph faster than him. Then he accelerates so that he takes a turn being faster and passes me up and is now 100 mph faster than me. We continue this forever, each of us accelerating past the other. We have no frame of reference other than ourselves. We have no idea how fast we are actually going relative to some distance reference point. But by now we must be going 4 or 5 c actual velocity thru space. Now, no one on any frame of reference can tell that and, in fact, would swear that we are only going c. But we have to traveling thru space at a very hyper light speed. Between the two of us we only see velocity differences of 100 mph. To the distant observer he sees two spaceships traveling away at c.

So it seems clear to me the ship can actually travel faster than light. It also seems clear to me that no one can prove it because it requires the rules of frames of reference to apply. But if the two ships keep accelerating past each other, it seems to me to be a matter of simple math to add up the accelerations until it adds up to greater than light. I can't say how fast, but I CAN say that we accelerated past each other to a 100 mph difference 100 gillion times and that adds up to faster than light. Whether someone in another frame of reference can tell, who cares. Me and my other pilot know, between ourselves, that we must be going faster than c. In fact, wouldn't a simple accelerometer do this? No matter how fast I travel, in my frame of reference on my ship, I can accelerate faster. That I can continue to accelerate has already been acknowledged in a previous post.

Here's the rub. So what if I'm traveling at 5c?! When I want that to mean something, for example, I want to get somewhere faster than c then I have to calculate that speed reference to the destination. It is the point where I have to take into account my velocity reference my destination I suddenly realize that, hey, all of this accelerating has accomplished nothing. I calculated that I must be traveling at 5c due to adding up the accelerations. But when I calculated my ETA at my destination I can't use the 5c velocity. My frame of reference to the destination and visa versa only sees 1c at best.

Now I'm confused...

tex

 

[phinds]

No, relative to each other you are barely moving but to get your velocity "through actual space" as you put it is only meaningful in relation to an observer and AGAIN, for the several-th time in this thread I say this: you can only get your speed relative to that observer by doing the Lorentz Transforms.

 

[thetexan]

Lorentz transformations apply to all speeds 10 mph, 1000 mph, c. At 100 mph the difference, to my frame of reference, should be negligible. But that's beside the point. The point is that no matter how fast I travel in my ship I can accelerate to a faster speed. I don't need to know what the speed is to know that I must have accelerated. And that includes up to some fantastic velocity.

You can't even talk about c without a frame of reference. Light speed compared to what? But I can say that I, in my ship, can continue to accelerate to some faster velocity. Doesn't it follow that if that's true that actual speed can be faster than 186,282 mps. (and yes I know that even speaking of mps is a relative term.) And, the limiting factor is not some number that represents c, the limiting factor is our ability to measure that speed relative to a frame of reference. This may be splitting hairs but it seems to me that acceleration is additive to unlimited speed, in a mind experiment theoretical way, but its measurement, which must always be in the context of a frame of reference is limited to 186,282 mps.

tex

 

[Orodruin]

Lorentz transformations apply to all speeds 10 mph, 1000 mph, c. At 100 mph the difference, to my frame of reference, should be negligible. But that's beside the point. The point is that no matter how fast I travel in my ship I can accelerate to a faster speed. I don't need to know what the speed is to know that I must have accelerated. And that includes up to some fantastic velocity.

You can't even talk about c without a frame of reference. Light speed compared to what? But I can say that I, in my ship, can continue to accelerate to some faster velocity. Doesn't it follow that if that's true that actual speed can be faster than 186,282 mps. (and yes I know that even speaking of mps is a relative term.) And, the limiting factor is not some number that represents c, the limiting factor is our ability to measure that speed relative to a frame of reference. This may be splitting hairs but it seems to me that acceleration is additive to unlimited speed, in a mind experiment theoretical way, but its measurement, which must always be in the context of a frame of reference is limited to 186,282 mps.

tex

Even if you accelerate with a proper acceleration of 1g for the age of the universe, you will not reach the speed of light relative to any other massive object. This is ultimately related to how relativistic addition of velocities work. If you are already traveling close to the speed of light, adding a some velocity relative to your current rest frame (which is what you would measure as proper acceleration) will only give you a teeny tiny bit more velocity in the frame of the other object (with respect to which you are almost moving with the speed of light) and so your acceleration in that object's frame will be much smaller than your proper acceleration.