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Faster-than-light
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<p>Faster-than-light (superluminal or supercausal) <a href="/facts/Superluminal_motion/hAPdgkbr">travel</a> and <a href="/facts/Superluminal_communication/HTCP4ht5">communication</a> are the conjectural propagation of <a href="/facts/Matter/osyc04UN">matter</a> or <a href="/facts/Information/3LdrzXtJ">information</a> faster than the <a href="/facts/Speed_of_light/sKSQwdNx">speed of light</a> in vacuum (c). The <a href="/facts/Special_theory_of_relativity/vsFFc63E">special theory of relativity</a> implies that only particles with zero <a href="/facts/Mass_in_special_relativity/aRloR0nW">rest mass</a> (i.e., <a href="/facts/Photons/mFNhAEzA">photons</a>) may travel <i>at</i> the speed of light, and that nothing may travel faster. </p><p>Particles whose speed exceeds that of light (<a href="/facts/Tachyon/bqvlsfTa">tachyons</a>) have been hypothesized, but their existence would violate <a href="/facts/Causality_(physics)/bkoXXhU3">causality</a> and would imply <a href="/facts/Time_travel/3rmwahY9">time travel</a>. The <a href="/facts/Scientific_consensus/DrTWaQIy">scientific consensus</a> is that they do not exist. </p><p>According to all observations and current scientific theories, matter travels at slower-than-light (subluminal) speed with respect to the locally distorted spacetime region. Speculative faster-than-light concepts include the <a href="/facts/Alcubierre_drive/pFAEq0lT">Alcubierre drive</a>, <a href="/facts/Krasnikov_tube/FEK4EWHV">Krasnikov tubes</a>, <a href="/facts/Wormhole/VtlJwany">traversable wormholes</a>, and <a href="/facts/Quantum_tunnelling/WU4NMrKx">quantum tunneling</a>. Some of these proposals find loopholes around general relativity, such as by expanding or contracting space to make the object appear to be travelling greater than <i>c</i>. Such proposals are still widely believed to be impossible as they still violate current understandings of causality, and they all require fanciful mechanisms to work (such as requiring <a href="/facts/Exotic_matter/bUf3MrSK">exotic matter</a>). </p>