Posted on 05/18/2021 12:11:52 PM PDT by Red Badger
Faster than light travel is the only way humans could ever get to other stars in a reasonable amount of time. Credit: NASA
The closest star to Earth is Proxima Centauri. It is about 4.25 light-years away, or about 25 trillion miles (40 trillion km). The fastest ever spacecraft, the now-in-space Parker Solar Probe will reach a top speed of 450,000 mph. It would take just 20 seconds to go from Los Angeles to New York City at that speed, but it would take the solar probe about 6,633 years to reach Earth’s nearest neighboring solar system.
If humanity ever wants to travel easily between stars, people will need to go faster than light. But so far, faster-than-light travel is possible only in science fiction.
In Issac Asimov’s Foundation series, humanity can travel from planet to planet, star to star or across the universe using jump drives. As a kid, I read as many of those stories as I could get my hands on. I am now a theoretical physicist and study nanotechnology, but I am still fascinated by the ways humanity could one day travel in space.
Some characters – like the astronauts in the movies “Interstellar” and “Thor” – use wormholes to travel between solar systems in seconds. Another approach – familiar to “Star Trek” fans – is warp drive technology. Warp drives are theoretically possible if still far-fetched technology. Two recent papers made headlines in March when researchers claimed to have overcome one of the many challenges that stand between the theory of warp drives and reality.
But how do these theoretical warp drives really work? And will humans be making the jump to warp speed anytime soon?
This 2-dimensional representation shows the flat, unwarped bubble of spacetime in the center where a warp drive would sit surrounded by compressed spacetime to the right (downward curve) and expanded spacetime to the left (upward curve). Credit: AllenMcC/Wikimedia Commons
Compression and expansion Physicists’ current understanding of spacetime comes from Albert Einstein’s theory of General Relativity. General Relativity states that space and time are fused and that nothing can travel faster than the speed of light. General relativity also describes how mass and energy warp spacetime – hefty objects like stars and black holes curve spacetime around them. This curvature is what you feel as gravity and why many spacefaring heroes worry about “getting stuck in” or “falling into” a gravity well. Early science fiction writers John Campbell and Asimov saw this warping as a way to skirt the speed limit.
What if a starship could compress space in front of it while expanding spacetime behind it? “Star Trek” took this idea and named it the warp drive.
In 1994, Miguel Alcubierre, a Mexican theoretical physicist, showed that compressing spacetime in front of the spaceship while expanding it behind was mathematically possible within the laws of General Relativity. So, what does that mean? Imagine the distance between two points is 10 meters (33 feet). If you are standing at point A and can travel one meter per second, it would take 10 seconds to get to point B. However, let’s say you could somehow compress the space between you and point B so that the interval is now just one meter. Then, moving through spacetime at your maximum speed of one meter per second, you would be able to reach point B in about one second. In theory, this approach does not contradict the laws of relativity since you are not moving faster than light in the space around you. Alcubierre showed that the warp drive from “Star Trek” was in fact theoretically possible.
Proxima Centauri here we come, right? Unfortunately, Alcubierre’s method of compressing spacetime had one problem: it requires negative energy or negative mass.
This 2–dimensional representation shows how positive mass curves spacetime (left side, blue earth) and negative mass curves spacetime in an opposite direction (right side, red earth). Credit: Tokamac/Wikimedia Commons, CC BY-SA
A negative energy problem Alcubierre’s warp drive would work by creating a bubble of flat spacetime around the spaceship and curving spacetime around that bubble to reduce distances. The warp drive would require either negative mass – a theorized type of matter – or a ring of negative energy density to work. Physicists have never observed negative mass, so that leaves negative energy as the only option.
To create negative energy, a warp drive would use a huge amount of mass to create an imbalance between particles and antiparticles. For example, if an electron and an antielectron appear near the warp drive, one of the particles would get trapped by the mass and this results in an imbalance. This imbalance results in negative energy density. Alcubierre’s warp drive would use this negative energy to create the spacetime bubble.
But for a warp drive to generate enough negative energy, you would need a lot of matter. Alcubierre estimated that a warp drive with a 100-meter bubble would require the mass of the entire visible universe.
In 1999, physicist Chris Van Den Broeck showed that expanding the volume inside the bubble but keeping the surface area constant would reduce the energy requirements significantly, to just about the mass of the sun. A significant improvement, but still far beyond all practical possibilities.
A sci-fi future? Two recent papers – one by Alexey Bobrick and Gianni Martire and another by Erik Lentz – provide solutions that seem to bring warp drives closer to reality.
Bobrick and Martire realized that by modifying spacetime within the bubble in a certain way, they could remove the need to use negative energy. This solution, though, does not produce a warp drive that can go faster than light.
Independently, Lentz also proposed a solution that does not require negative energy. He used a different geometric approach to solve the equations of General Relativity, and by doing so, he found that a warp drive wouldn’t need to use negative energy. Lentz’s solution would allow the bubble to travel faster than the speed of light.
It is essential to point out that these exciting developments are mathematical models. As a physicist, I won’t fully trust models until we have experimental proof. Yet, the science of warp drives is coming into view. As a science fiction fan, I welcome all this innovative thinking. In the words of Captain Picard, things are only impossible until they are not.
Written by Mario Borunda, Associate Professor of Physics, Oklahoma State University.
Originally published on The Conversation.
“The three dimensions of length are altered by gravity...”
This is just a nonsense statement. Gravity isn’t “altering” anything. Einstein actually proposed that gravity isn’t even a true force at all, but just an emergent phenomenon created by warping of spacetime. It is mass and energy that warps spacetime (never space) to create the appearance of gravity, NOT the other way around.
It’s analogous to surfing. A wave is moving 40 mph. The actual water is not moving 40 mph, only the wave. But the surfer can ride the wave and be effectively moving at 40 mph, although he appears to be just standing still on top of the moving wave.
The three dimensions of length are altered by MASS-ENERGY just as surely as the fourth dimension of time; in special and general relativity, the only difference between length and time is their opposite signs in the distance metric.
It may be that God created much of the universe to make a special location (Earth) able to sustain life. I'm speaking as an old-earth creationist who on the one hand believes God had a direct hand in creating all things (including breathing life into man -- I'm not a theistic evolutionist who believes God created a system of natural selection and sat back and let nature do all the rest). But at the same time, I also believe God could have put a lot into the design of the Earth so that the Earth itself can have a universe custom made for it to "inhabit" just like the Earth is custom made for life to inhabit.
Here are some possible reasons:
1. God just likes to create stuff. Asking why God created the universe is kind of like asking why God made flowers pretty. Maybe He's just into creating cool stuff.
2. There is a theory that God likes to re-use things He already made. My take on why organisms often seem a lot alike is not because of natural selection, but because they were made by the same Designer. Can't God do the same thing with other stuff? It may be that the cosmic radiation from other galaxies provided the substance -- at just the right mass and energy and timeline -- for our galaxy to form. For our sun to form. For the planets around our sun to form.
3. There are all kinds of things to show that our milky way galaxy was specially made so that one planet within it can support life. For example, most stars in the milky way galaxy have orbits that are highly elliptical. Just by observing our nearest most stars we can see how special our star's circular orbit is (much less volatility wreaked onto the planets that orbit it).
Yes. If you are considering travel as “pushing” yourself to that limit.
The concept of “warp” drives means moving within a pocket of “space time.” The traveler never exceeds light speed—but the bubble bends the space around it.
All very theoretical. I know the back of the envelope description. But they say it might work in theory. Practically? Not in our lives.
For example, in the Schwarzschild metric (the solution to the Einstein field equations outside a spherically symmetric mass distribution) the coefficients of both radial distance and time are different than in the absence of mass: https://en.m.wikipedia.org/wiki/Schwarzschild_metric
You aren’t accelerating the mass, you’re compressing and stretching the warp bubble (like reference frame). Relativity theory does not put limits on the speed the bubble can be manipulated. The occupants of the spacecraft would feel no acceleration, AT ALL.
I don’t doubt a big reveal is coming but Miguel Alcubierre’s theory was published in 1994.
Lentz’ paper was published just this March.
130 years ago, people were saying that about heavier-than-air flight.
Its likely that this technology has been around for 70 years or more but held back from the public domain.
First of all, it’s spacetime; not space. And in either case, second: it is *not* shorthand for “nothing”!
If it were nothing, there’d be total chaos. There’d be no infrastructure there to determine which trajectories are inertial, which paths are trajectories in time, versus curves in space, which directions are temporal versus which ones are spatial, to determine what is congruent to what, which angles are right angles, which points are near which other points, which regions are contiguous to which regions, how many dimensions there are, which paths & motions are smooth versus jagged, the lengths of paths, the durations of trajectories, which speeds are light speed, etc.
That all counts as something. And all that infrastructure is what spacetime is and contains. In fact, everything else can be reduced to it (via Einstein’s field equation). Even the law of inertia is, itself, a consequence of Einstein’s field equation.
So, it’s not only not nothing, it’s everything.
Of course length is altered. But length is different than space. Which is why Einstein always spoke of “length contraction” rather than “space contraction”.
When Einstein talks about length being altered, he is speaking about the apparent change in length, of measuring rods and such, to the observer in a moving inertial frame. He never talks about the actual 3 spatial dimensions in the universe in reality being “warped” as some modern day interpreters of Einstein do. Only the measurements, or our perceptions of length are being altered by motion, that is what Einstein talks about.
Your measurements of length are certainly altered but space itself is not altered. For example, if you travel from here to Alpha Centauri at a relativistic velocity, the length between Earth and Alpha Centauri, to YOU, appears to shorten. However, the space between Alpha Centauri and Earth is not actually changing, as anyone who measures it from either Earth or Alpha Centauri, or any other point in the universe can confirm. Only YOUR measurement is changing.
I’m not disputing that when you do the math that time and the spatial dimensions are interchangeable. However, these are just mathematical conventions. In reality, time and space are clearly different things, even if we can conceptually represent them as interchangeable axes in a coordinate system for our own convenience. Einstein’s entire focus in relativity was solving the problem of SIMULTANEITY in moving systems, which is a temporal problem, not a spatial one. His use of Minkowski space to unify the mathematics of space and time to solve the problem works, but it has the unfortunate side effect of creating this confusion that time and the spatial dimensions are interchangeable IN REALITY, when they are really only interchangeable in this limited mathematical framework.
Otherwise science fiction as we know it would collapse.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.