According to presentism, only the present exists. But according to special relativity, which events occur simultaneously (and thus co-presently) is relative to a frame of reference. So something can both exist and not exist, depending on which frame of reference you are looking from. But this is absurd.
An example: Suppose that from reference R1, events A and B occur simultaneously, and C later. But from reference R2, A occurs earlier, whereas B and C are simultaneous.
Now, suppose event B is presently occuring, and thus the objects it involves truly exist. Do the objects of A and C exist? By R1, A is occurring presently, and thus its objects exist. But by R2, A is in the past, and so its objects no longer exist. Contradiction. Ergo, presentism is false.
Or, put more formally:
1. Simultaneity is relative.
2. Existence is not relative.
3. Therefore, existence cannot depend upon simultaneity.
4. Presentism implies that existence depends upon being simultaneous with the present.
5. Therefore, Presentism is false.
Is there any response left for the Presentist? (Could they deny #4, perhaps?)