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I was contemplating the possibility of sending an interstellar probe that can produce results in a reasonable time frame.

For such a mission to be realistic, the spacecraft would have to travel at some (significant) fraction of the speed of light. I calculated that at 0.5c, it would take 14PJ of energy per kilogram. If we assume 500kg probe, that will require 7000PJ of energy.

According to my research, that's about 2000 terrawatts, about half of the yearly electric consumption in the US. Obviously this is enormous, but given that we can somehow provide constant trust without significantly increasing the spacecraft's weight:

Is there any fundamental reason that I'm missing that would make near light speed travel not possible?

Is there any fundamental reason that would make near light speed travel extremely unpractical/unlikely?

Edit: I'm not asking about engineering reasons, but fundamental physics.

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  • $\begingroup$ how to drive so fast? $\endgroup$
    – user46925
    Commented Jan 19, 2016 at 22:24
  • $\begingroup$ The largest man-made nuclear bomb weighed 27,000kg (en.wikipedia.org/wiki/Tsar_Bomba) Unless you're planning to set them all off in the same instant, then the energy from the first one will be spent accelerating the probe plus all of the other bombs. Google for "relativistic rocket equation" $\endgroup$
    – Solomon Slow
    Commented Jan 19, 2016 at 22:34
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    $\begingroup$ Purely engineering reasons. The same reasons why we can't have space elevators, terraforming, transatlantic bridges or Dyson spheres. $\endgroup$
    – Emilio Pisanty
    Commented Jan 19, 2016 at 22:38
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    $\begingroup$ Also, what good is it if your probe flashes past the target star system at 0.5c? If it's going to collect any meaningful data, you'll have to slow it down. Again, consult that rocket equation. Having to slow it down doesn't mean double the size of the rocket: The payload of the rocket that gets everything up to travel speed is the entire rocket that's going to brake the science package back down to normal speed. $\endgroup$
    – Solomon Slow
    Commented Jan 19, 2016 at 22:42
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    $\begingroup$ Well to some observers, we are travelling arbitrarily close to the speed of light. $\endgroup$
    – Kyle Kanos
    Commented Jan 20, 2016 at 3:13

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There is a fundamental reason why it might be very, very difficult (not even mentioning the engineering). As you start approaching the speed of light, it becomes harder and harder to accelerate. At 0.5c, this would definitely become a factor. Accelerating from 1%c to 2%c is much easier than accelerating from 50%c to 51%.

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Research the average atoms of gas and possible dust particles per cubic meter in deep space and the kinetic impact damage of traveling at .5c through that.

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Well fundamnetal physics is pretty solid here, which explicitly says that $$F=\dfrac{dp}{dt}$$ $$p=\gamma(v) m_0v$$ $$\gamma(v)=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$$ $m_0$ is invariant mass.

So theoretically a spacecraft can accelerate upto any $v<c$ in accordance with the laws. The most prime difficulty is only engineering issues like:

  1. Energy source must be huge, reliable, safe, and easy to extract.
  2. Energy density/efficiency of fuel or whatever propulsion system.
  3. Spacecraft materials must sustain such massive acceleration along with i guess occupants?
  4. Cost of the spacecraft.
  5. Interstellar rocks, asteroids, rogue planets?

Currently no such technology exists, neither nuclear or conventional.

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Yes, but what fraction are you talking about?
The most realistic albeit ludicrous idea is using sails to catch nuclear explosions. But achieving only 0.33% lightspeed.

Antimatter catalyzed propulsion assuming sufficient quantities of antimatter can be obtained offer exhaust velocities of .94c which may propel a spaceship to minimum of 0.05 to .25c given enough acceleration time.

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There's no fundamental physical reason. Remember that speed is relative: going with 0.01c according to one frame means going with 0.91c according to another.

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For anything with mass, it would be quite difficult since the speed of light is a limit that cannot be surpassed. On Earth, the The Large Hadron Collider has successfully accelerated protons very close to the speed of light (because the photons have so little mass). For objects without mass (a light spot, for example), it IS possible.

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  • $\begingroup$ Welcome to PSE! "Photons" in the brackets sould be "protons" as well. $\endgroup$
    – Samuel Adrian Antz
    Commented Nov 20, 2022 at 11:19

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