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

[rogerthat1945]

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.

Not this tired old argument.

You cannot `go back in time`; as time is a measuring stick (and just like the `temperature` of `something measured` is not altered by changing the numbers on the side of the gauge); and as its only as tangible as a shadow.

You would also have to reverse every atom of the universe at the same time; for it to be truly as though going back in time. You have more likelihood of slowing down time by stopping atoms moving.

Be realistic sometimes.

 

[rogerthat1945]

Consider it a different way. Given we invent some new, infinite source of energy, how would we get YOU even close to the speed of light? Because you are so extremely fragile, you can only tolerate so much acceleration. So, it will take more than your lifetime just to get you up to a fraction of the speed of light without squishing you into a thin paste. Then there's the time it would take to decelerate you before you reached your destination. Inertia is a huge limiting factor to the use of any current form of propulsion or most any material or technology we have. In order for you to actually explore the stars just in our neighborhood, you would have to either invent a way to live an extremely long time or find an entirely different way to transverse space that doesn't require acceleration. Warp Drive is an invention of science fiction. We currently have the slightest of hints that space can be warped. Perhaps a thousand years from now, someone will find a way. Or, maybe not.

What you suggest is not the problem. You can create conditions in which velocity does not effect matter within a defined space. If you know what you are doing.

 

[Monsterboy]

I think Majid is not asking about rocket science, but about why anything with a mass cannot reach the speed of light. That is, why there is a physical restriction to reach such speed. And I always thought it had to do with E=mc², not exhaust velocity or what not (which is an initial restriction, but not the ultimate restriction).

I think you are right , what the OP asked is that , if any object( spacecraft in this case) is accelerated continuously in space without running out of fuel , why can't that object reach or exceed the speed of light ?

I found this when I googled http://physics.about.com/od/relativisticmechanics/f/SpeedofLight.htm
So, according to this link , you can travel at the speed of light but you will need infinite amount of energy to do so.

The universe itself has finite amount of energy (first law of thermodynamics) hence you can't travel at the speed of light.
I hope someone with a background in this subject validates the information in this link.

 

[phinds]

I found this when I googled http://physics.about.com/od/relativisticmechanics/f/SpeedofLight.htm
So, according to this link , you can travel at the speed of light but you will need infinite amount of energy to do so.

That would better be interpreted as "because there is no such thing as an infinite amount of energy, objects with mass cannot travel at the speed of light".

 

[slatts]

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.

For related reasons, hyperbolae are common in two-dimensional representations of the multiverse: Some good examples are on the site "Next Step Infinity".

 

[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