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According to Einstein, a lot of energy is needed as a spacecraft nears the speed of light.
Therefore,

  1. If Einstein is correct, it will be impossible for humans to travel at the speed of light.

  2. If Einstein is correct, it would be impossible for humans to travel at the speed of light.

What are the differences among the above sentences if both are correct? Because I wonder if it is possible to use "would" in the main clause while the conditional clause is a present tense?

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    For a start, "If Einstein is correct" implies that the speaker thinks he probably is. "If Einstein were correct" suggests a greater degree of scepticism. – Kate Bunting Dec 2 '19 at 10:14
  • What do you think of number 2? Is it a grammatically correct expression? – Suwon Kim Dec 2 '19 at 10:57
  • 1 talks about the future ('will be...') and leaves open the question of whether it was possible in the past. 2 and 3 close off that question. @KateBunting is correct about the difference between 2 and 3. All are grammatically correct. I'd use 2 myself. – simon at rcl Dec 2 '19 at 12:14
  • Thank you for your explanation, but i am still not sure, so let me ask you this way. If Einstein is correct, it would be impossible for humans to travel at the speed of light.-If this sentence is correct, do you mean that it might have been possible, but that it is impossible now? I understand number one and three. but I totally don't understand what number 2 means. Please help me if you don't mind. – Suwon Kim Dec 2 '19 at 13:03
  • For most purposes, #1 and #2 would be understood as having exactly the same meaning. The potential fine distinction is that #1 more strongly implies that humans will try to travel at lightspeed (but they'll won't succeed, because it is/will be impossible). Where #2 implies they might not even try (but if they did, they would fail). It's a confusing context though, because the reason they won't succeed is something "built in" to our current theories, not something where it's meaningful to give it a go and see what happens (we know it can't be done). – FumbleFingers Reinstate Monica Dec 2 '19 at 13:22
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For stating facts that are true use zero conditional statements:

If Einstein was correct, it is impossible for humans to travel at the speed of light.

If Einstein is right, only one thing can happen and it is the fact that "It is impossible for humans to travel at the speed of light."

And that is if you are not planning to explain how modern science might be able to prove that wrong.

Check out this image from grammar.cl for additional help.

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