How close to light speed can we travel?
How close to light speed can we travel?
In reading the adventure of Astro Disastro, you may have noticed that Astro’s speed was almost, but not quite, the speed of light. There’s a very good reason for this. At speeds much less than the speed of light, relativistic effects are unnoticeable (and what fun would that be?), while speeds faster than light are physically impossible.
I’ll show you why. Suppose Astro tried to push his ship all the way up to the speed of light. Well, we’ve already seen that the energy of an object is proportional to that «gamma» factor, which is so prevalent in relativity calculations. But you’ve also learned by now that gamma becomes infinitely large when the speed of an object is equal to the speed of light. So, in order for Astro to accelerate his ship to the speed of light, he would require an infinite amount of energy, which is clearly impossible. So any object with mass can never reach the speed of light, though there’s no limit to how close to the speed of light an object can come. (An object that doesn’t have mass must, in fact, travel exactly at the speed of light, for reasons I won’t go into. But the only objects with no mass are particles of light (called «photons») and maybe neutrinos.)
There are other reasons an object’s speed cannot exceed the speed of light. One of these involves «causality», the relation between cause and effect. Suppose I launch a baseball, and it breaks a window. My throwing the baseball is then the cause of the window breaking. If speeds faster than light were possible, then there could actually be some frames of reference in which the window breaks before the ball is thrown. This leads to all kinds of logical contradictions (especially if someone intercepts the ball in flight, preventing the window from breaking, after the window has already broken!) so we rule out the possibility of things moving faster than light. Furthermore, causality rules out not just objects traveling faster than light, but any kind of communication that travels faster than light. The speed of light, to the best of our knowledge, is an uncrossable barrier.
Now if you’re a science fiction fan, as I am, this may come as bad news. After all, there’s almost certainly no intelligent life in our solar system outside of earth, and the distance between stars is huge. Even the nearest star would take four years to reach at light speed. So without traveling faster than light, it’s pretty much impossible to go flying around the galaxy, meeting alien civilizations, fighting wars for galactic empires, and so forth. Bummer, eh?
On the other hand, it may not be as hopeless as all that, due to length contraction. Suppose you climb into a spaceship and head for a star 10 light years away, at a speed close to the speed of light. From the earth’s frame, the trip has to take at least 10 years. However, to the passengers on the ship, the length is contracted, and so the trip can take less than 10 years. And the closer to the speed of light the ship travels (relative to the earth and the star) the more the length is contracted. (You can also deduce this by considering time dilation.)
To illustrate this, here’s a table showing some travel times to various destinations, at various speeds. Let me explain what all this means. First of all, in order to get serious length contraction, we need to get very close to the speed of light. So I’m considering a trip in which we have a spaceship able to generate a constant acceleration. That is, the people inside the ship experience a continuous acceleration of, for example, 1 g for half the trip, and then turn around and decelerate at 1 g the rest of the way. (I’m almost positive I’ve calculated all this correctly.)
(in g ‘s)
|Top Speed||Earth Time |
|Ship Time |
(in g ‘s)
|Top Speed||Earth Time |
|Ship Time |
The second column gives the distance to the star or galaxy of our destination, in light years. (A light year is the distance light travels in a year: about six trillion miles.) I’ve included calculations for three different accelerations, one smaller, one greater, and one equal to earth’s gravitational acceleration. The 2 g trip would be pretty uncomfortable, and you can probably forget anything much faster than that. The fourth column shows the maximum speed (at the halfway point, just as the ship is turning around to decelerate) as a fraction of the speed of light. The entries near the bottom have more 9’s than I felt like including.
The final two columns give the amount of time the trip would take, first in the earth’s frame of reference, and then in the frame of reference of the spaceship. The difference is important. It means, for example, that if you are on a spaceship traveling to Betelgeuse with an acceleration of 1 g , about 6.8 years will pass on the ship before you get there. (The «ship times» increase very slowly despite the big increases in distance, because the greater the distance, the closer to the speed of light you can get before you have to slow down again, and so the more length contraction you get!) But when you arrive there, over 500 years will have passed on earth. And any message you send to earth when you get to Betelgeuse will take over 500 years to get there, as will any reply. So even if humankind one day spreads out across the galaxies, the different settlements will be very isolated. People on earth won’t be talking to people near Betelgeuse on any regular basis.
Now of course there are enormous technical difficulties to building a spacecraft that can accelerate indefinitely like this. These difficulties may prove insurmountable, and we’re out in fantasy land. But if they’re not insurmountable, and if our species survives long enough to surmount them, then what I’ve just described represents the logistics of far space travel, according to the special theory of relativity.
Of course, many works of science fiction still involve faster than light travel. But they always have to invent some weird concept to go with it, like «warp» or «hyperspace». The bottom line is this: given everything we know about space and time today, traveling faster than light is impossible. But if you like, you can always cling to the hope that some sort of loophole or whole new branch of physics will be discovered that will allow objects to travel faster than the speed of light. Then we can get to work on that big galactic empire. Epilogue
Dave’s relativity page
New technology could enable humans to travel at 7 million MPH
At 1 percent the speed of light, it would take a little over a second to get from Los Angeles to New York.
by Chris Impey
November 22, 2021
Light is fast. In fact, it is the fastest thing that exists, and a law of the universe is that nothing can move faster than light. Light travels at 186,000 miles per second (300,000 kilometers per second) and can go from the Earth to the Moon in just over a second. Light can streak from Los Angeles to New York in less than the blink of an eye.
While 1 percent of anything doesn’t sound like much, with light, that’s still really fast — close to 7 million miles per hour! At 1 percent the speed of light, it would take a little over a second to get from Los Angeles to New York. This is more than 10,000 times faster than a commercial jet.
The Parker Solar Probe, seen here in an artist’s rendition, is the fastest object ever made by humans and used the gravity of the Sun to get going 0.05% the speed of light.
What is the fastest man-made object
Bullets can go 2,600 miles per hour (mph), more than three times the speed of sound. The fastest aircraft is NASA’s X3 jet plane, with a top speed of 7,000 mph. That sounds impressive, but it’s still only 0.001 percent the speed of light.
The fastest human-made objects are spacecraft. They use rockets to break free of the Earth’s gravity, which takes a speed of 25,000 mph. The spacecraft that is traveling the fastest is NASA’s Parker Solar Probe. After it launched from Earth in 2018, it skimmed the Sun’s scorching atmosphere and used the Sun’s gravity to reach 330,000 mph. That’s blindingly fast — yet only 0.05% of the speed of light.
Why even 1 percent of light speed is hard
What’s holding humanity back from reaching 1 percent of the speed of light? In a word, energy. Any object that’s moving has energy due to its motion. Physicists call this kinetic energy. To go faster, you need to increase kinetic energy. The problem is that it takes a lot of kinetic energy to increase speed. To make something go twice as fast takes four times the energy. Making something go three times as fast requires nine times the energy, and so on.
For example, to get a teenager who weighs 110 pounds to 1 percent of the speed of light would cost 200 trillion Joules (a measurement of energy). That’s roughly the same amount of energy that 2 million people in the U.S. use in a day.
Light sails like these seen in an illustration could get us to the stars.
Photon Illustration/Stocktrek Images/Stocktrek Images/Getty Images
How fast can we go?
It’s possible to get something to 1 percent the speed of light, but it would just take an enormous amount of energy. Could humans make something go even faster?
Yes! But engineers need to figure out new ways to make things move in space. All rockets, even the sleek new rockets used by SpaceX and Blue Origins, burn rocket fuel that isn’t very different from gasoline in a car. The problem is that burning fuel is very inefficient.
Other methods for pushing a spacecraft involve using electric or magnetic forces. Nuclear fusion, the process that powers the Sun, is also much more efficient than chemical fuel.
Scientists are researching many other ways to go fast — even warp drives, the faster-than-light travel popularized by Star Trek.
One promising way to get something moving very fast is to use a solar sail. These are large, thin sheets of plastic attached to a spacecraft and designed so that sunlight can push on them, like the wind in a normal sail. A few spacecraft have used solar sails to show that they work, and scientists think that a solar sail could propel spacecraft to 10 percent of the speed of light.
One day, when humanity is not limited to a tiny fraction of the speed of light, we might travel to the stars.
This article was originally published on The Conversation by Chris Impey. Read the original article here.
Faster-Than-Light Travel Could Work Within Einstein’s Physics, Astrophysicist Shows
For decades, we’ve dreamed of visiting other star systems. There’s just one problem – they’re so far away, with conventional spaceflight it would take tens of thousands of years to reach even the closest one.
Physicists are not the kind of people who give up easily, though. Give them an impossible dream, and they’ll give you an incredible, hypothetical way of making it a reality. Maybe.
In a 2021 study by physicist Erik Lentz from Göttingen University in Germany, we may have a viable solution to the dilemma, and it’s one that could turn out to be more feasible than other would-be warp drives.
This is an area that attracts plenty of bright ideas, each offering a different approach to solving the puzzle of faster-than-light travel: achieving a means of sending something across space at superluminal speeds.
Hypothetical travel times to Proxima Centauri, the nearest-known star to the Sun. (E. Lentz)
There are some problems with this notion, however. Within conventional physics, in accordance with Albert Einstein’s theories of relativity, there’s no real way to reach or exceed the speed of light, which is something we’d need for any journey measured in light-years.
That hasn’t stopped physicists from trying to break this universal speed limit, though.
While pushing matter past the speed of light will always be a big no-no, spacetime itself has no such rule. In fact, the far reaches of the Universe are already stretching away faster than its light could ever hope to match.
To bend a small bubble of space in a similar fashion for transport purposes, we’d need to solve relativity’s equations to create a density of energy that’s lower than the emptiness of space. While this kind of negative energy happens on a quantum scale, piling up enough in the form of ‘negative mass’ is still a realm for exotic physics.
In addition to facilitating other kinds of abstract possibilities, such as wormholes and time travel, negative energy could help power what’s known as the Alcubierre warp drive.
This speculative concept would make use of negative energy principles to warp space around a hypothetical spacecraft, enabling it to effectively travel faster than light without challenging traditional physical laws, except for the reasons explained above, we can’t hope to provide such a fantastical fuel source to begin with.
But what if it were possible to somehow achieve faster-than-light travel that keeps faith with Einstein’s relativity without requiring any kinds of exotic physics that physicists have never seen?
Artistic impression of different spacecraft designs in ‘warp bubbles’. (E. Lentz)
In the recent work, Lentz proposes one such way we might be able to do this, thanks to what he calls a new class of hyper-fast solitons – a kind of wave that maintains its shape and energy while moving at a constant velocity (and in this case, a velocity faster than light).
According to Lentz’s theoretical calculations, these hyper-fast soliton solutions can exist within general relativity, and are sourced purely from positive energy densities, meaning there’s no need to consider exotic negative-energy-density sources that haven’t yet been verified.
With sufficient energy, configurations of these solitons could function as ‘warp bubbles’, capable of superluminal motion, and theoretically enabling an object to pass through space-time while shielded from extreme tidal forces.
It’s an impressive feat of theoretical gymnastics, although the amount of energy needed means this warp drive is only a hypothetical possibility for now.
«The energy required for this drive traveling at light speed encompassing a spacecraft of 100 meters in radius is on the order of hundreds of times of the mass of the planet Jupiter,» Lentz said in March last year.
«The energy savings would need to be drastic, of approximately 30 orders of magnitude to be in range of modern nuclear fission reactors.»
While Lentz’s study claimed to be the first known solution of its kind, his paper arrived at almost exactly the same time as another recent analysis, also published in March 2021, which proposed an alternative model for a physically possible warp drive that doesn’t require negative energy to function.
Both teams made contact, Lentz said at the time, and the researcher intended to share his data further so other scientists can explore his figures. Lentz also went on to present his findings to the public through a YouTube livestream.
There are still plenty of puzzles to solve, but the free-flow of these kinds of ideas remains our best hope of ever getting a chance to visit those distant, twinkling stars.
«This work has moved the problem of faster-than-light travel one step away from theoretical research in fundamental physics and closer to engineering,» Lentz said.
«The next step is to figure out how to bring down the astronomical amount of energy needed to within the range of today’s technologies, such as a large modern nuclear fission power plant. Then we can talk about building the first prototypes.»
A version of this article was first published in March 2021.