Is near light speed achievable?

In summary, if enough fuel is there is it possible to achieve near light speed? Factors preventing this are the amount of energy needed and the mass of the spacecraft.
  • #1
sr241
83
0
assume if enough fuel is there is it possible to achieve near light speed?
what are the factors preventing this?

how we can link relativistic mass to the thrust of rocket at near light speed?

assume mass of spacecraft is constant and enough fuel is there then can chemical rockets can reach near light speed? if not does relativistic mass has anything to do with this

at higher speed does more energy is required to expel gases?

if constant thrust is given is it possible to achieve near light speed?

What is the relativistic version of thrust equation?
 
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  • #2
Hi sr241,

First note that according to relativity, no matter how fast the rocket goes, the people inside always see light going at lightspeed (the constant c). This is elementary relativity.

Second, to the people outside the rocket, the major obstacle to reaching relativistic speeds is the amount of energy needed. Machines such as the LHC are required to send subatomic particles to near lightspeed, so we can't really imagine a realistic machine that could send a macroscopic object to such speeds, much less a human-carrying rocket.
 
  • #3
actually my friend is working on an a rocket engine that doesn't work on mass expulsion

so, if constant thrust is given is it possible to achieve near light speed?

What is the relativistic version of thrust equation?
 
  • #4
sr241 said:
actually my friend is working on an a rocket engine that doesn't work on mass expulsion
Well then it isn't a rocket...

sr241 said:
so, if constant thrust is given is it possible to achieve near light speed?
Yes.

sr241 said:
What is the relativistic version of thrust equation?
Use the Lorentzian transformation to calculate the addition of velocities.

Very simplistically:
If x thrust over time t gets you to .9c, then a greater thrust over more time will get you to .99c, and a still greater thrust over still more time will get you to .999c. No amount of thrust over any length of time will get you to c.
 
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  • #5
sr241 said:
actually my friend is working on an a rocket engine that doesn't work on mass expulsion
At face value that sounds like a violation of the laws of physics.
so, if constant thrust is given is it possible to achieve near light speed?
Um, well yes, of course if we throw the laws of physics out the window we can do whatever we want. But this is a forum where we discuss reality so your question was answered in a realistic way: the answer is no.
What is the relativistic version of thrust equation?
What's the "thrust equation"? I think you may mean the relativistic rocket equations: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]

There may be an automatic calculator out there for it but I'm not sure (you can google).
 
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  • #6
russ_watters said:
At face value that sounds like a violation of the laws of physics.
Russ, there are lots of designs for propulsion that don't involve mass expulsion. Solar sails are an obvious one.

True, he said "rocket", but that is obviously not the word he should have used (since a rocket is defined by mass expulsion). He hould have simply said "a propulsion unit" that does not expel mass.

The fact that his friend is almost surely building a pipe dream does not negate the principle.

russ_watters said:
Um, well yes, of course if we throw the laws of physics out the window we can do whatever we want. But this is a forum where we discuss reality so your question was answered in a realistic way: the answer is no.
The answer is yes. If constant thrust were provided, one would approach light speed. An example of this is a Bussard Ramjet.

Just because you and I are both pretty sure the poster is barking up a whimsical tree doesn't mean we shouldn't give him accurate answers. We just need to qualify them.
 
  • #7
what are the safety hazards in sending a fission reactor to space
 
  • #8
sr241 said:
what are the safety hazards in sending a fission reactor to space

In fact, it has been a popular choice for powering our deep space probes for quite some time now.

So... no hazards that aren't manageable.

(They're not technically nuclear reactors. That would be too heavy. They're radioisotope thermoelectric generators.)
 
  • #9
is fission thermo electric reactors are viable for electric propulsion like MPD or ion drive?
does their weight to energy or weight to thrust ratio allow space mission
 
  • #10
sr241 said:
is fission thermo electric reactors are viable for electric propulsion like MPD or ion drive?
does their weight to energy or weight to thrust ratio allow space mission

sr241, (assuming I have the right thread) please take note of my email.
 
  • #11
Remember that no matter how much one accelerates one is traveling at speed 0 relative to one's own frame and that the speed of light in that frame is just as far away as ever. To be more accurate one should speak of the delta-V of a spacecraft relative to the starting frame.

For a non-relativistic rocket this is dictated by the http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation" [Broken]:
[tex]\Delta V = V_{exhaust} \cdot \ln\left( \frac{m_0}{m_1}\right)[/tex]
where V_exhaust is the exhaust velocity of the ejected reaction mass, and the mass ratio is the initial mass divided by the final mass of the rocket.

There is a http://en.wikipedia.org/wiki/Relativistic_rocket" [Broken]:
[tex]\Delta V = c \cdot\tanh\left( \frac{V_{exhaust}}{c}\ln\left( \frac{m_0}{m_1}\right)\right)[/tex]

To achieve a "delta Vee" approaching light-speed one needs either near-light-speed reaction mass exhaust, light-speed reaction "mass" as in a photonic drive, or external reaction "mass" as with a light-sail or a Bussard ram-scoop.

If one has a photonic drive (eject electromagnetic radiation out the back of your ship via lasers, masers, or a radio antenna) then the exhaust velocity is c and the relativistic delta-V equation becomes:
[tex]\Delta V = c \cdot\tanh\left( \ln\left( \frac{m_0}{m_1}\right)\right)=...= c\cdot\frac{m_0^2-m_1^2}{m_0^2+m_1^2}[/tex]
To achieve a Delta-V of say 50% c you would need to "burn" about 42.3% of your initial mass directly to energy. (say by matter anti-matter annihilation. Many problems there including finding a light-weight means of directing the gamma ray energy emitted by such a reaction in one direction.)

It is useful to look at the light-drive case because any other reaction mass method will be less efficient and so still requires either direct conversion of mass or externally supplied power.

The http://en.wikipedia.org/wiki/Bussard_ramjet" [Broken] idea developed by Robert W. Bussard, assumes we crack fusion power and involves using very large magnetic fields to collect and compress interstellar hydrogen. The hydrogen is then fused for energy and ejected for reaction mass. Since it burns interstellar hydrogen both for reaction mass and energy it has theoretically unlimited Delta V (as a % of c) if one neglects issues of drag vs thrust. Another advantage of this idea is the magnetic scoop would shield the payload from the impact of high energy protons at relativistic speeds since these are what are being diverted for use as fuel.

Another possibility: http://en.wikipedia.org/wiki/Light_sail" [Broken] also do not need to carry fuel but rather use the momentum of reflected light for relative motion and thus theoretically could achieve a delta-V close to c. They of course depend on a source of light and using stars has the added difficulty of their gravity being proportional to their light output at a given distance. Ideally one would push these with a directed beam of electromagnetic radiation, say a bank of orbital mirrors at home focusing sunlight toward the sail, or artificially transmitted EM radiation.

One interesting and potentially viable idea is the http://en.wikipedia.org/wiki/Starwisp" [Broken] concept. It is a microwave "light"-sail pushed by orbital transmitters back home. It can be powered by the very microwaves which are used to push it. This idea seems to me to be the most viable means for us to send probes to the nearest stars.
 
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  • #12
jambaugh said:
One interesting and potentially viable idea is the http://en.wikipedia.org/wiki/Starwisp" [Broken] concept. It is a microwave "light"-sail pushed by orbital transmitters back home. It can be powered by the very microwaves which are used to push it. This idea seems to me to be the most viable means for us to send probes to the nearest stars.

From just that rather short wiki entry alone, it doesn't seem particularly viable to me.
 
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  • #13
DaveC426913 said:
Russ, there are lots of designs for propulsion that don't involve mass expulsion. Solar sails are an obvious one.

True, he said "rocket", but that is obviously not the word he should have used (since a rocket is defined by mass expulsion). He hould have simply said "a propulsion unit" that does not expel mass.

The fact that his friend is almost surely building a pipe dream does not negate the principle.
I have a very good crack-dar, Dave. The fact that he called it a "rocket engine" implies to me that he means a reactionless rocket PMM, not a solar sail.
The answer is yes. If constant thrust were provided, one would approach light speed. An example of this is a Bussard Ramjet.
The Bussard Ramjet is still just theoretical/science fiction.
Just because you and I are both pretty sure the poster is barking up a whimsical tree doesn't mean we shouldn't give him accurate answers. We just need to qualify them.
I gave him the same answer you did, Dave, my qualifications were just different!
 
  • #14
sr241 said:
is fission thermo electric reactors are viable for electric propulsion like MPD or ion drive?
does their weight to energy or weight to thrust ratio allow space mission
It can be done, sure. I think for missions closer to the sun solar panels are typically used, but for something deeper into space, some form of nuclear power would be the way to go.
 
  • #15
please tell me what is the fundamental law or laws that prevents non-mass expulsion type propulsion device? the propulsion device must use energy stored inside spacecraft only.

ignore solar sail or laser propulsion where energy source is from out side.

you must agree that an efficient non-mass expulsion type propulsion device is the next best thing to wormholes.
 
  • #16
http://en.wikipedia.org/wiki/Reactionless_drive
A reactionless drive or inertial propulsion engine (also reactionless thruster, reactionless engine, bootstrap drive, and inertia drive) is any form of pseudoscientific propulsion not based around expulsion of fuel or reaction mass.
In spite of their physical impossibility, such devices have been often proposed in recent history and are a staple of science fiction.

sr241, I have explained why your design won't work. If you would like to get a second opinion, please post the design here and allow others to comment on it. I assure you, it will be the same response.

If they worked, they would be one of the best ways to travel around the universe. But they don't. The "fundamental laws" that prevent such devices are the basics - Newtons laws of motion.
 
  • #17
sr241 said:
please tell me what is the fundamental law or laws that prevents non-mass expulsion type propulsion device? the propulsion device must use energy stored inside spacecraft only.

There is no fundamental law. We just don't have any viable way of doing it.
 
  • #18
DaveC426913 said:
There is no fundamental law. We just don't have any viable way of doing it.

Just for clarity, my post refers specifically to sr241's design itself regarding Newtons Laws, and specifically conservation of momentum.
 
  • #19
russ_watters said:
The Bussard Ramjet is still just theoretical/science fiction.
Anything that is not yet built is theoretical/science fiction. That does not change the answer to the question 'what would happen if you could apply constant thrust'.

I know where you're coming from; I can smell the crackdom too. It's just that my philosophy is to facilitate much as possible (peppered with caveats and warnings) until they violate physics. I don't see the good in shutting people down on principle.

Anyway, that's two takes on the same question. Good for the OP to see differing viewpoints.
 
  • #20
DaveC426913 said:
There is no fundamental law. We just don't have any viable way of doing it.
Dave, I'm not sure if you took jardenjames up on his pm offer to view sr241's "invention", but it was, as I expected, a reactionless propulsion PMM. It violates conservation of momentum.
 
  • #21
russ_watters said:
Dave, I'm not sure if you took jardenjames up on his pm offer to view sr241's "invention", but it was, as I expected, a reactionless propulsion PMM. It violates conservation of momentum.

I did not. There was no need.

Still, IMO, these kinds need our help more than others.
 
  • #23
You are ignoring conservation of momentum.

It doesn't matter how slowly you stop the ball, it imparts an equal reaction force towards the rear as it initially did towards the front. They cancel out.

As per my email, all of the kinetic energy of the ball is converted to heat whether you stop it instantaneously on the back wall of the spacecraft or through your "friction tube". This is independent of momentum. You can't convert the momentum to heat.

The problem with your understanding is that you believe only a quantity of the kinetic energy becomes heat and the rest goes towards a counter-acting force. Thus, the reaction acting against the forward motion is less than the initial forward acting motion, giving a net forward motion. This is incorrect and a flawed understanding of the physics of the system. You have combined two different concepts and drawn an incorrect conclusion.

I recommend reading up on Conservation of Momentum and Conservation of Energy.
 
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  • #24
sr241 said:
here is the link https://sites.google.com/site/exhaustlessnuclearpropulsion/

be specific on why it will not work

I am already half convinced it won't work

No matter how you slow the ball from moving forward, its movement will cause the spaceship to recoil, leaving its centre of mass stationary.

Yes, energy is lost due to friction. But it is not lost from the kinetic enrgy of the craft, it is lost from the device that pushes the ball forward (whatever device or mechanism that might be, it doesn't matter). That mechanism will have to work harder against friction. i.e. it will have to put out more energy (energy lost to heat friction) while the ball will simply move the same amount.

So, ultimately, your device uses a mechanism that works hard to create friction which creates waste heat. None of that has anything to do with the device moving.

In principle: there is absolutely nothing you can do inside the craft to move the craft's centre of mass.
 
  • #25
jarednjames said:
From just that rather short wiki entry alone, it doesn't seem particularly viable to me.

That short wiki entry is not the sum total of information you can find on the topic. I linked to it as an unambiguous definition. You do your own homework.
 
  • #26
jambaugh said:
That short wiki entry is not the sum total of information you can find on the topic. I linked to it as an unambiguous definition. You do your own homework.

That seems a rather aggresively-defensive reaction.

"I propose X as viable. Here's a link."
"Your link is not convincing."
"You do your own homework."
 
  • #27
jambaugh said:
That short wiki entry is not the sum total of information you can find on the topic. I linked to it as an unambiguous definition. You do your own homework.

I have, and from what I've read so far it is rather unrealistic.

This image http://www.joelertola.com/grfx/grfx_img/StarWisp.jpg" [Broken] shows it requires a lens "30,000 miles across" and "100 billion light sensitive processors".

I also refer you to his own paper, equation 35, page 4 where he shows the lens required for a 1km sail is 50,000km with an estimated mass of 50,000 tons. Along with said system requiring "excessively large" amounts of power.

Paper available here: http://www.boomslanger.com/images/starwisp.pdf [Broken]

Your exact words were: "This idea seems to me to be the most viable means for us to send probes to the nearest stars.".

Based on the above, I'd say that is not so.

I'm not exactly sure what I did to deserve that reaction, but I'm pretty sure the rules are that you back up your claims.
 
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  • #28
DaveC426913 said:
That seems a rather aggresively-defensive reaction.

"I propose X as viable. Here's a link."
"Your link is not convincing."
"You do your own homework."

OK, granted. Pardon but I was a bit miffed. I erased my lengthier reply without sending and opted for a terse reply to match the terse dismissal.

Here's the technical paper: http://www.boomslanger.com/images/starwisp.pdf" [Broken]

I wasn't making a claim other than my opinion of comparative viability. The starwisp doesn't need invented technology such as cracking fusion, only some reasonable engineering and hardware design. We could begin tomorrow if we had the will.

The infrastructure could be recycled to home power production from orbit and pay for itself over time. (That may be a stretch but if not the infrastructure could power space based industry and knowledge ALWAYS pays eventually) We could start smaller using wisps to explore e.g. the Oort cloud and probe past the heliopause.

Oh, and when the alien invaders come calling we can roast them in their ships with our microwave weapons battery! :wink:
 
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  • #29
Getting 50,000 tons into space and then the construction of said materials for a 50,000km emitter lens is considered "reasonable engineering"?

What was the cost per kg to get stuff into space?
 
  • #30
jarednjames said:
Your exact words were: "This idea seems to me to be the most viable means for us to send probes to the nearest stars."
Yes I said "most viable", as in better than alternatives (barring certain breakthroughs).

BTW the fresnel lens is not the only option. One can phase correlate the array of transmitters.
(I think... I'll have to do some calculations...but not tonight, classes start tomorrow, delayed by the ice storm and I've got to teach at 8:00am.)

Of course one is talking about a vast engineering project, far grander than anything we've ever done before. But it is feasible within current technology and reasonable extrapolation of engineering.
 
  • #31
NhocCuteGirl said:
so we can't really imagine a realistic machine that could send a macroscopic object to such speeds

See above discussion concerning the "starwisp".

I may not like it, but it's certainly possible for getting a 1km array up to near relativistic speeds.
 
  • #32
jarednjames said:
Getting 50,000 tons into space and then the construction of said materials for a 50,000km emitter lens is considered "reasonable engineering"?

What was the cost per kg to get stuff into space?

Depends on where you find it. If I were lord and master of the Earth and determined to build the thing, my first stage would be to establish a base on the moon and set up materials production. The "lens" is a wire mesh, not a big hunk of plastic. Getting 50,000 tons into space is a matter of lifting 5000 10ton spools.

But I challenge you to suggest a cheaper way to get a probe to the nearest star. Again I said "most" viable, not "super-easy-cheap-why-didn't-Columbus-just-bypass-America-and-go-to-Alpha-Proxima" viable.
 
  • #33
Here's some math for the curious...

Figure the sustained acceleration you need to reach a given % of c (including over 100%) in simple Newtonian physics then take the hyperbolic tangent. That's the relativistic % of c.

Example you boost to what would be c in Newton's universe, that's about 8500 g-hours (almost a year accelerating at 1 g) and you will get to tanh(1)= (about) 0.76 or 76% c as seen by the people watching from your starting frame.

To figure the energy you need, you would take the hyperbolic cosine minus 1 times the mass (the minus 1 subtracts out the initial mass-energy). E.g. cosh(1) = 1.543 so you need about 54% of the payload mass times c^2 in energy to get up to 76% lightspeed.
That's about 48,900 TeraJoules per kg or 48,900 Tera-watt seconds per kg payload...
or 13.6 TW hours per kg or about 19 GW months.

Figure the output of a few industrial scale power plants sustained over a month to accelerate at 10 or 12 gees per kg payload.

That's delivered to the payload. With a beamed source (light or microwaves or whatever) one must figure one or two factors of 10 for losses and beam spreading.

It's not unthinkable but certainly would require we keep the payload size under say 20kilograms and a major industrial presence in space. And .76c may be too ambitious, more like .01c or .005c. Something we could imagine happening in the year 2200. I mentioned we wouldn't need to assume new tech (like cracking fusion) but we'd probably need orbital fusion power plants to get that kind of power output or some other energy production improvement.
 
  • #34
NhocCuteGirl said:
Hi sr241,
First note that according to relativity, no matter how fast the rocket goes, the people inside always see light going at lightspeed (the constant c). This is elementary relativity.
_

c is speed of light in vacuum; that means light slows in a medium like air and water. so in air and water speed of light is going to be less than c, how can you correlate this with the said principle (speed of light is constant).
 
  • #35
Well jambaugh, I concede the point to you.

As un-realistic the prospect of the starwisp appears to me, it really is one of the most viable methods of getting to the nearest stars.

I didn't realize just how 'bad' current propulsion systems were. They're really not up to much when it comes to interstellar travel.

I suppose the scale of the starwisp is a good demonstration of what it takes to get to the nearest star. Even if it is only in a very basic way.
 
<h2>1. Can anything travel at the speed of light?</h2><p>No, according to Einstein's theory of relativity, nothing can travel at the speed of light. As an object approaches the speed of light, its mass increases infinitely and it would require an infinite amount of energy to accelerate it to that speed.</p><h2>2. How close have we come to achieving near light speed?</h2><p>Currently, the fastest spacecraft ever launched by humans is the Parker Solar Probe, which has reached speeds of about 430,000 miles per hour. This is still far from the speed of light, which is about 670 million miles per hour.</p><h2>3. What are the potential implications of achieving near light speed?</h2><p>If we were able to achieve near light speed, it would have significant implications for space travel. It would drastically reduce the time it takes to travel to other planets and potentially allow us to explore further into our universe. However, it would also require advanced technology and safety measures to protect astronauts from the effects of high speeds.</p><h2>4. Is near light speed travel possible in the future?</h2><p>While it is currently not possible with our current technology, it is not impossible to achieve near light speed travel in the future. Scientists are constantly researching and developing new technologies that could potentially make this type of travel a reality.</p><h2>5. How does time dilation play a role in near light speed travel?</h2><p>According to Einstein's theory of relativity, as an object approaches the speed of light, time slows down for that object. This phenomenon is known as time dilation. This means that for an astronaut traveling at near light speed, time would pass slower for them compared to someone on Earth, resulting in them aging slower. This has been observed in experiments with particles traveling at high speeds, and would have significant implications for long-distance space travel.</p>

1. Can anything travel at the speed of light?

No, according to Einstein's theory of relativity, nothing can travel at the speed of light. As an object approaches the speed of light, its mass increases infinitely and it would require an infinite amount of energy to accelerate it to that speed.

2. How close have we come to achieving near light speed?

Currently, the fastest spacecraft ever launched by humans is the Parker Solar Probe, which has reached speeds of about 430,000 miles per hour. This is still far from the speed of light, which is about 670 million miles per hour.

3. What are the potential implications of achieving near light speed?

If we were able to achieve near light speed, it would have significant implications for space travel. It would drastically reduce the time it takes to travel to other planets and potentially allow us to explore further into our universe. However, it would also require advanced technology and safety measures to protect astronauts from the effects of high speeds.

4. Is near light speed travel possible in the future?

While it is currently not possible with our current technology, it is not impossible to achieve near light speed travel in the future. Scientists are constantly researching and developing new technologies that could potentially make this type of travel a reality.

5. How does time dilation play a role in near light speed travel?

According to Einstein's theory of relativity, as an object approaches the speed of light, time slows down for that object. This phenomenon is known as time dilation. This means that for an astronaut traveling at near light speed, time would pass slower for them compared to someone on Earth, resulting in them aging slower. This has been observed in experiments with particles traveling at high speeds, and would have significant implications for long-distance space travel.

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